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aling with extremely small data packets, setting up the connection for a short trans- mission has a significant impact on the overall connection time and power consumption. To reduce this overhead, control-plane CloT EPS optimization specifies a mechanism to include user data packets within signaling messages. By including the payload in the signal- ing message, the overhead is significantly reduced [11]. As shown in Fig. 6.3, in the control-plane CIoT EPS optimization, the uplink data is trans- ferred from the eNB (CIoT RAN) to the MME. At that point, the data may either be trans- ferred via the S-GW to the P-GW, or to the service capability exposure function (SCEF). The latter is only possible for non-IP data packets. From these nodes, the data is ultimately forwarded to the application server (CIoT services). The downlink data is transmitted over the same route in the reverse direction. However, there is no data radio bearer setup and data packets are instead sent on the signaling radio bearer (SRB). This solution is suitable for transmission of infrequent and small data packets. The SCEF is a new network entity introduced to enable machine-type communications. It is used for delivery of non-IP data over control plane and provides an abstract interface for the network services; that is, authentication and authorization, discovery and access network capabilities. With the 752 Chapter 6 user-plane CloT EPS optimization, the data is transferred in the same way as the conven- tional data traffic; that is, over radio bearers via the S-GW and the P-GW to the application server. Therefore, it introduces some overhead upon setting up the connection; however, it facilitates the transmission of a sequence of data packets. This solution supports both IP and non-IP data transfer [11]. The access network architecture for NB-IoT is similar to that of LTE. The eNBs are connected to the MME and S-GW via S1 interface, with the difference of carrying the NB-IoT messages and data packets. Even though there is no handover defined

, there is still an X2 interface between two eNBs, which enables a fast resume after the UE transitions to the idle state. In the control-plane CloT EPS optimization, the data exchange between the NB-IoT termi- nal and the eNB is performed at RRC level. In the downlink and uplink, the data packets may be piggybacked in the RRCConnectionSetup and RRCConnectionSetupComplete messages, respectively. For larger payloads, data transfer may continue using DLInformationTransfer and ULInformationTransfer messages. In all of these messages, there is a byte array containing non-access stratum (NAS) information, which in this case corresponds to the NB-IoT data packets. As a result, this procedure is transparent to the eNB, and the UE's RRC sublayer forwards the content of the received DLInformationTransfer directly to its upper layer. The dedicatedInfoNAS message is exchanged between the eNB and the MME via the S1-MME interface. For this data transfer mechanism, security on access stratum (AS) level is not applied. Since there is no RRC con- nection reconfiguration, it may immediately start after or during the RRC connection setup or resume procedure. The RRC connection has to be terminated afterwards with the RRC connection release. In the user-plane CloT EPS optimization, data is transferred over the conventional user plane through the network; that is, the eNB forwards the data to the S-GW or receives it from this node. In order to minimize the UE complexity, only one or two dedicated radio bearers may be simultaneously configured. One needs to distinguish between two cases: 1. If the previous RRC connection was released with a possible resume operation indi- cated, the connection may be requested as a resume procedure. If the resume procedure is successful, then the security is established with updated keys and the radio bearer is set up similar to the previous connection. there was no previous RRC connection release with a resume indication, or if the resume request was not accepted by the eNB, the security and radio b

earer have to be reestablished. 6.2.2 Modes of Operation NB-IoT may be deployed as a standalone system using any available spectrum exceeding 180 kHz. It may also be deployed within an LTE spectrum allocation, either inside an LTE Internet of Things 753 channel or in the guard-band. These different deployment scenarios are illustrated in Fig. 6.4. The deployment scenario, standalone, in-band, or guard-band should be transparent to a UE when it is first turned on and scans for an NB-IoT carrier. Similar to existing LTE UEs, an NB-IoT UE is only required to search for a carrier on a 100 kHz raster. An NB-IoT carrier that is intended for facilitating UE initial access and synchronization is referred to as an anchor carrier. The 100 kHz UE search raster implies that for in-band deployments, an anchor carrier can only be placed in certain physical resource blocks (PRBs). For example, in a 10 MHz LTE carrier, the indices of the PRBs that are best aligned with the 100 kHz grid and can be used as an NB-IoT anchor carrier are 4, 9, 14, 19, 30, 35, 40, and 45. The PRB indexing starts from index 0 for the PRB occupying the lowest frequency within the LTE channel bandwidth. In the in-band operation, the assignment of resources between LTE and NB-IoT is not fixed. However, not all frequencies; that is, resource blocks (RBs) within the LTE carrier are allowed to be used for cell connection, rather they are restricted to the values shown in Table 6.1. Fig. 6.5 illustrates the deployment options of NB-IoT in conjunction with a 10 MHz LTE system. The PRB immediately after the DC subcarrier, that is, PRB_25, is centered at 97.5 kHz; that is, at a distance of 6.5 subcarriers from the center of the band. Since the LTE DC subcarrier is placed on the 100 kHz raster, the center of PRB_25 is 2.5 kHz from the nearest 100 kHz grid. The spacing between the centers of two neighboring PRBs above the In-band operation Guard-band operation Standalone operation Multicarrier in-band operation LTE carrier LTE carrier GSM carrier LTE carrier Figur

Definition of channel bandwidth and transmission bandwidth configuration for in-band/guard- band NB-loT operation [1]. DC subcarrier is 180 kHz. As a result, the PRB indices 30, 35, 40, and 45 are all centered at 2.5 kHz from the nearest 100 kHz grid. It can be shown that for LTE carriers of 10 and 20 MHz, there exists a set of PRB indices that are all centered at 2.5 kHz from the nearest 100 kHz grid, whereas for LTE carriers of 3, 5, and 15 MHz bandwidth, the PRB indices are centered approximately 7.5 kHz away from the 100 kHz raster. It must be noted that none of the middle six PRBs of the LTE carrier can be assigned as an NB-IoT anchor car- rier (e.g., PRB_25 of 10 MHz LTE, despite the fact that its center is 2.5 kHz away from the nearest 100 kHz raster). This is due to the fact that LTE synchronization and broadcast channels occupy the resource elements in the middle six PRBs. Similar to the in-band deployment, an NB-IoT anchor carrier in the guard-band is required to have center fre- quency no more than 7.5 kHz distant from the 100 kHz raster. The NB-IoT cell search and initial acquisition are designed for a UE to be able to synchronize to the network in the presence of a raster offset up to 7.5 kHz. Multi-carrier operation of NB-IoT is also sup- ported. Since one NB-IoT anchor carrier is necessary to enable UE initial access/synchroni- zation, the additional carriers do not need to be near the 100 kHz raster grid. These additional carriers are referred to as secondary carriers. As we mentioned earlier, NB-IoT carrier occupies 180 kHz of bandwidth, which corresponds to one PRB in an LTE system [2]. With this selection, the following operation modes are possible (see Fig. 6.4): Standalone: A scenario where currently used GSM frequencies with 200 kHz bandwidth can be utilized for NB-IoT deployment. Guard-band: A deployment scenario which utilizes the unused RBs within an existing LTE carrier's guard-band. In-band: A scenario where PRBs within an existing LTE channel bandwidth are utilized for NB-IoT deploymen

t. Internet of Things 755 In the in-band operation, the assignment of resources between LTE and NB-IoT is not fixed. However, as we mentioned earlier, some of the physical LTE RBs cannot be used for NB- IoT deployment. The NB-IoT system is designed to operate in the following LTE operating bands: 1, 2, 3, 4, 5, 8, 11, 12, 13, 14, 17, 18, 19, 20, 21, 25, 26, 28, 31, 66, 70, 71, 72, 73, and 74 [1]. 3GPP specifications do not specify how to allocate the RBs between LTE and NB-IoT. However, downlink synchronization and paging can only be established on certain RBs. The RBs located at the center of the band cannot be used due to LTE transmission of the downlink synchronization signals and broadcast channel. Owing to capacity limitations, NB-IoT is not designed for 1.4 MHz channel bandwidth. The RBs allocated for a cell connection are referred to as anchor carriers. For the actual exchange of data (in the connected state), other RBs (non-anchor carriers) can be assigned. For an LTE service provider, the in-band option provides the most efficient NB-IoT deploy- ment scenario because if there is no IoT traffic, the PRB(s), available for an NB-IoT carrier, may be allocated to LTE services. Note that NB-IoT can be fully integrated with the exist- ing LTE infrastructure. This allows the base station scheduler to multiplex LTE and NB- IoT traffic in the same spectrum. Fig. 6.6 shows the results of a coexistence study where the impact of NB-IoT uplink transmission in in-band and guard-band modes on an LTE [victim] system throughput is given in terms of cumulative distribution function (CDF) of throughput. The results suggest that there is a negligible impact on LTE operation as a result of NB-IoT deployment [17]. 6.2.3 Protocol Structure The NB-IoT protocol stack is a functionally reduced version of the LTE protocols. The NB-IoT further uses a different bearer structure. SRBs are partly reused from LTE; that is, the SRBO is used for RRC messages transmitted over the common control channel, and the SRB1 is used for transport o

f RRC and NAS messages using the dedicated control channel. However, there is no SRB2 defined for NB-IoT. In addition, a new SRB, the SRB1 is defined, which is implicitly configured with SRB1 using the same configuration, rather without the packet data convergence protocol entity. This bearer type plays the role of the SRB1 until the security architecture is activated, after that SRB 1bis is not used anymore. This also implies that for the control-plane CloT EPS optimization, only SRB1bis is used, because there is no security activation in this mode. The NB-IoT user-plane and control-plane protocol stack are the same as those of LTE with functionalities optimized for NB-IoT sustainable operation (see Fig. 6.7). Chapter 6 Spectrum CDF of throughput LTE_UL NBloT UL Stand-alone Inband Frequency (MHz) SNR (dB) Spectrum CDF of throughput LTE_UL NBloT UL Stand-alone Guardband Frequency (MHz) SNR (dB) Figure 6.6 Example of in-band/guard-band NB-loT operation and uplink coexistence analysis (10 MHz victim LTE system and aggressor NB-loT device). A total of 1000 LTE subframes were used to derive CDF of the throughput 16,17]. NB-loT data transmission on control-plane (low overhead) LTE data transmission on user-plane (IP overhead) User-plane Control-plane Figure 6.7 NB-loT user-plane and control-plane protocol structure [6]. Internet of Things 757 6.3 Physical Layer Aspects 6.3.1 Frame Structure The downlink multiple access scheme of NB-IoT is based on orthogonal frequency division multiple access with the same 15 kHz subcarrier spacing as LTE with the slot, subframe, and frame durations configured as 0.5, 1, and 10 ms, respectively. Furthermore, the slot for- mat in terms of cyclic prefix length and number of OFDM symbols per slot are also identi- cal to those in LTE. In principle, an NB-IoT carrier can occupy one LTE PRB in the frequency domain. Reusing the same OFDM numerology as LTE ensures coexistence with LTE in the downlink. The uplink of NB-IoT supports both multi-tone and single-tone trans- missions. Multi-tone tr

ansmission is based on single-carrier frequency division multiple access (SC-FDMA) with the same 15 kHz subcarrier spacing, slot, and subframe length as LTE. Single-tone transmission supports two numerologies: 15 and 3.75 kHz. The 15 kHz subcarrier spacing is identical to that of LTE, ensuring coexistence with LTE in the uplink. The 3.75 kHz single-tone numerology uses 2 ms slot duration. Similar to the downlink, an uplink NB-IoT carrier uses a total system bandwidth of 180 kHz [2,11]. In addition to the system frames, the concept of hyper frames is further defined, which counts the number of system frame periods; that is, it is incremented each time the system frame number (SFN) wraps. Similar to the SFN, the hyper frame number (HFN) is a 10-bit counter; thus the hyper frame period spans 1024 system frame periods, corresponding to a time interval of approximately 3 hours. Because NB-IoT only uses a single PRB in the downlink, the physi- cal channels are only multiplexed in the time domain, as shown in Fig. 6.8. 6.3.2 Narrowband Primary Synchronization Signal Unlike LTE, the NB-IoT physical channels and signals are primarily multiplexed across time. Fig. 6.8 illustrates how NB-IoT subframes are allocated to different physical channels and signals. The first three OFDM symbols are not used for NB-IoT, because they carry the Even-numbered frame (10 ms) Odd-numbered frame (10 ms) 12 subcarriers (180 kHz) Figure 6.8 NB-loT frame structure [14,25]. 758 Chapter 6 physical control format indicator channel (PCFICH) and physical downlink control channel (PDCCH) in LTE when NB-IoT is operated in the in-band mode. Note that during the time when the UE synchronizes to the narrowband primary synchronization signal (NPSS) and narrowband secondary synchronization signal (NSSS), it may not know the operation mode; consequently, this guard time applies to all modes. In addition, both synchronization signals are punctured by the LTE cell-specific reference signals. It is not specified which of the antenna ports is used for the syn

chronization signals; this may even change between any two subframes. Each NB-IoT subframe spans over one PRB (i.e., 12 subcarriers) in the frequency domain and 1 ms in the time domain. The NPSS and NSSS are used by an NB-IoT UE to perform cell search, which includes time and frequency synchronization, and cell identity detection. Since the legacy LTE synchronization sequences occupy six PRBs, they could not be reused for NB-IoT, thus a new design was introduced. The NPSS is transmitted in the fifth subframe of every 10 ms frame, using the last 11 OFDM symbols in the subframe. The NPSS detection is one of the most computationally intensive operations from a UE perspec- tive. To allow efficient implementation of NPSS detection, NB-IoT uses a hierarchical sequence. The signal itself consists of a single length-11 Zadoff-Chu (ZC) sequence that is either directly mapped to the 11 lowest subcarriers (the 12th subcarrier is null in the NPSS) or is inverted before the mapping process. After successful detection of the NPSS, an NB- IoT UE is able to determine the frame boundaries of a downlink transmission. For each of the 11 NPSS OFDM symbols in a subframe, either p or p is transmitted, where p is the base sequence generated based on a length-11 ZC sequence with root index 5. Each of the length-11 ZC sequence is mapped to the lowest 11 subcarriers within the NB-IoT PRB. The sequence di(n) used for the NB primary synchronization signal is generated from a fre- quency domain ZC sequence according to di(n) = S(1)e-jrun(n+1)/11; n=0, 1, 10, where the ZC root sequence index u=5 and S(1) for different symbol indices l is given as [S(3) S(13)] normal cyclic prefix [2]. The structure of NPSS is illustrated in Fig. 6.9. 6.3.3 Narrowband Secondary Synchronization Signal The NSSS has 20 ms periodicity and is transmitted in the ninth subframe of every other frame and uses the last 11 OFDM symbols. The NSSS spans 132 resource elements and comprises a length-132 frequency domain ZC sequence, with each element mapped to a resource ele

ment. The NSSS is generated by element-wise multiplication between a ZC sequence and a binary scrambling sequence. The root of the ZC sequence and binary scram- bling sequence is determined by narrowband physical cell identity (NB-PCID). The cyclic shift of the ZC sequence is further determined by the frame number. NB-PCID is an addi- tional input parameter SO that it can be derived from the sequence. There are a total of 504 distinct NB-PCID values. The NSSS is transmitted in the last subframe of each Internet of Things 759 0123456789012345678901234567890123456789 Subframe Subframe 5 Slot 0 Slot 1 01234560123456 LTE CRS LTE PDCCH or Unused Unused 1=3,4, 2Nspm Figure 6.9 Structure of NPSS in time and frequency [2,25]. 0123456789012345678901234567890123456789 Subframe Subframe 9 Slot 0 Slot 1 012345601234 LTE CRS LTE PDCCH or unused Figure 6.10 Structure of NSSS in time and frequency domains [2,25]. even-numbered radio frame, as shown in Fig. 6.10. The sequence d(n) used for the NSSS is generated from a frequency domain ZC sequence according to the following: n=0,1,...,131 Ncell n' = n mod131; m=nmod128; q= 760 Chapter 6 The binary sequence bq(m) is given in [2]. The cyclic shift Of in frame number Nf is given by Of = 33/132(nf/2)mod4 [2]. 6.3.4 Narrowband Reference Signals The downlink narrowband reference signal (NRS) consists of known reference symbols inserted in the last two OFDM symbols of each slot for NB-IoT antenna ports 0 and 1, except invalid subframes and subframes transmitting NPSS or NSSS. There is one NRS transmitted per downlink NB-IoT antenna port. In addition to NRSs, the physical layer sup- ports narrowband positioning reference signals. Physical layer provides 504 unique cell identities using the NSSS. It will be indicated to the UE whether it may assume that the cell ID is identical for NB-IoT and LTE systems. In the case where the cell IDs are identical, UE may use the downlink cell-specific reference signals for demodulation and/or measure- ments when the number of NB-IoT antenna ports is th

e same as the number of downlink cell-specific reference signals antenna ports [5]. The NRS time-frequency mapping, shown in Fig. 6.11, is additionally cyclically shifted by NNcell mod6 in the frequency domain. When NRS is transmitted on two antenna ports, then the resource elements that are used for NRS on each antenna portare set to zero on the other antenna port. The narrowband physical broadcast channel (NPBCH) is encoded using a tail-biting convolutional code (TBCC) and QPSK modulated. An important point on the in-band operation concerns N ID Ncell parameter, which may be the same as the PCI for the LTE cell. This is indicated by the opeartionMode parameter in nar- rowband master information block (MIB-NB), which distinguishes between an in-band operation with same PCI as LTE and one whose identities are different. If this parameter is set to true, then NNcell and PCI are the same and the UE may assume that the number of antenna ports is the same as in the LTE cell. The channel may then be inferred from either reference signal set. In that case, LTE cell-specific reference signal (CRS) port 0 is associ- ated with NRS port 0, and CRS port 1 is associated with NRS port 1. One antenna port Two antenna ports Resource element (k, I) Not used for transmission on this antenna port Reference symbols on this antenna port /=6/=0 /=6 /=0 /=6/=0 /=6/=0 /=6/=0 Figure 6.11 NRS subcarrier mapping [2,11,19]. Internet of Things 6.3.5 Narrowband Physical Broadcast Channel The NPBCH carries the MIB-NB. The MIB-NB contains 34 bits and is transmitted over a time period of 640 ms; that is, 64 radio frames, and includes the following information: 4 bits indicating the most significant bits of the SFN, the remaining least significant bits (LSBs) are implicitly derived from the MIB-NB starting point. 2 bits representing the two LSBs of the HFN. 4 bits for the narrowband system information block type 1 (SIB1-NB) scheduling and size. 5 bits indicating the system information value tag. 1 bit representing whether access class barring is

applied. 7 bits indicating the operation mode with the mode-specific values. 11 reserved bits for future extensions. Fig. 6.12 shows the mapping of NPBCH to physical resources. After physical layer base- band processing, the resulting MIB-NB is split into eight blocks. The first block is transmit- ted on the first subframe (SF0) and repeated in SFO of the next seven consecutive radio frames. In SFO of the following radio frame, the same procedure is performed for the sec- ond block. This process is continued until the entire MIB-NB is transmitted. The use of SFO for all transmissions ensures collision avoidance of NPBCH with MBSFN transmissions in LTE, if NB-IoT is deployed in in-band operation mode. The MIB-NB and SystemInformationBlockType1-NB use fixed scheduling. The periodic- ity of MIB-NB is 640 ms in comparison to the periodicity of MIB in LTE, which is 40 ms. The periodicity of SIB1-NB is 2560 ms relative to the periodicity of LTE SIB1 that is 80 ms. The MIB-NB contains the information required to acquire SIB1. SIB1-NB contains the information to acquire other SIBs (see Table 6.2 for a list of NB-IoT SIBs). The broadcast control channel and other logical channels cannot be transmitted in the same subframe. The NB-IoT UE is not required to detect SIB changes in RRC_CONNECTED state. The NPBCH consists of eight independent 80 ms blocks. A block is always transmitted in SFO of a radio frame and then repeated eight times once per radio frame. The NPBCH is not transmitted in the first three symbols to avoid colli- sion with the LTE control channels. The NPBCH symbols are mapped around the NRS and the LTE CRS resources, where it is always assumed that two antenna ports are defined for NRS and four antenna ports for CRS. This assumption is necessary, because the UE obtains the actual antenna port information only after detecting the MIB-NB. The reference signal location in the frequency domain is given by NNcell provided by the NSSS. Although the NNcell may be different than PCI in the in-band operation, its ran

ge is restricted SO that it points to the same frequency locations, thus the CRS cyclic shift in the frequency domain is known to the UE. 762 Chapter 6 Subframe 0 Slot 0 Slot 1 01234560123456 34 bits Channel coding 1600 bits Modulation 800 symbols (QPSK modulation) LTE CRS LTE PDCCH NPBCH NRS Port 0 NRS Port 1 Cinit Cinit Cinit Reinitialization Reinitialization Reinitialization Cinit==NN Subframe 0 112345670 Modulation S,(1)),S,(2),...,S,(8) S2(1),S(2) S2(8) Sg(1),Sg(2),...,S. (8) NPBCH subblock (1) repeats NPBCH subblock (2) repeats NPBCH subblock (8) repeats over 8 \frames over 8 frames over 8 frames bb(0),b,(1),....b.(n) bo(n+1),bo(n1+2),...,bo(n2) bo(n7+1),bo(n7+2),,bo(ng) One MIB transmission Figure 6.12 Processing and resource mapping of NPBCH [2,3,25]. Table 6.2: MIB and SIB specified for NB-loT [8]. Message Content MIB-NB Essential information required to receive further system information SIB1-NB Cell access and selection, other SIB scheduling SIB2-NB Radio resource configuration information SIB3-NB Cell reselection information for intra-frequency, inter-frequency SIB4-NB Neighbor cell-related information relevant for intra-frequency cell reselection SIB5-NB Neighbor cell-related information relevant for inter-frequency cell reselection SIB14-NB Access barring parameters SIB16-NB Information related to GPS time and coordinated universal time (UTC) Internet of Things 6.3.6 Narrowband Physical Downlink Control Channel In NB-IoT, the downlink control information (DCI) is carried by the narrowband physical downlink control channel (NPDCCH). The basic resource allocation unit for NPDCCH is defined as narrowband control channel element (NCCE), which is the basic resource unit (RU) allocated for DCI transport. NPDCCH carries scheduling information for downlink and uplink data channels and further carries HARQ ACK/NACK information for the uplink data channel as well as paging indication and random-access response (RAR) scheduling information. There are two different formats for NPDCCH; that is, Format 0 and Form

at 1. NPDCCH Format 0 occupies only one NCCE, whereas NPDCCH Format 1 occupies two NCCEs. As shown in Fig. 6.13, there are only a few subframes that can be allocated to carry NPDCCH or narrowband physical downlink shared channel (NPDSCH). To reduce UE complexity, all downlink channels use LTE TBCC. Furthermore, the maximum trans- port block size (TBS) of NPDSCH is limited to 680 bits. The NRS is used to provide phase reference for coherent demodulation of the downlink channels. The NRS resources are time and frequency multiplexed with information bearing symbols in subframes carrying NPBCH, NPDCCH, and NPDSCH, using eight resource elements per subframe per antenna port. The NPDCCH structure is depicted in Fig. 6.13. In this example, we show the map- ping for an in-band NB-IoT operation assuming a single antenna port in the LTE cell and two antenna ports in NB-IoT. The parameter lNPDCCHstart, the LTE control region size sig- naled by NB-SIB1 [4], indicates the OFDM start symbol, which helps avoid conflict with the LTE control channel in the in-band operation. For the guard-band and standalone opera- tion modes, the control region size is by default zero, which provides more resource ele- ments for the NPDCCH. In order for the UE to find the control information with reasonable amount of decoding complexity, NPDCCH is grouped into the following search spaces: Subframe Slot 0 Slot 1 NCCE1 NPDCCH format Number of NCCEs LTE CRS LTE PDCCH NPBCH NCCE0 NRS Port 0 NRS Port 1 NPDCCHstart Figure 6.13 NPDCCH structure in time and frequency [2,25]. 764 Chapter 6 Type-1/1A common search spaces used for paging Type-2/2A common search spaces used for random access UE-specific search space A UE is not required to simultaneously monitor an NPDCCH UE-specific search space and a Type-1 or Type-2 NPDCCH common search space. An NPDCCH search space NSK (L',R) at aggregation level (AL) L' E {1,2} and repetition level R E {1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, ,2048} is defined by a set of NPDCCH candidates where each candidate is

repeated in a set of R consecutive NB-IoT downlink subframes excluding subframes used for transmission of system information messages starting with subframe k [4]. Thus each NPDCCH may be repeated several times with an upper limit configured by the RRC signal- ing. In addition, Type-2 common search space and UE-specific search space are configured by RRC, whereas the Type-1 common search space is identified by the paging opportunity subframes. The location of NPDCCH (i.e., the subframes used for transmitting NPDCCH) is determined via a process as illustrated in Fig. 6.14. In case of Type-1 NPDCCH common search space, k = ko, which is determined from locations of NB-IoT paging opportunity subframes. Different radio network temporary identifiers (RNTIs) are assigned to each UE, one for ran- dom access (RA-RNTI), one for paging (P-RNTI), and a UE-specific temporary identifier (C-RNTI) provided during the random-access procedure. These identifiers are implicitly indicated (scrambled) in the NPDCCH CRC. Therefore, the UE has to search for a specific RRC parameter npdcch-Offset-USS (NPDCCH UE-specific search space) RRC parameter npdcch-Offset-RA (NPDCCH Type2-NPDCCH common search space) RRC parameter inpdcch-Offset-SC-MTCH (NPDCCH Type2A-NPDCCH common search space) RRC configured parameter inpdcch-NumRepetitions RRC configured parameter npdcch-Num RepetitionPaging for Type1-NPDCCH common search space RRC configured parameter npdcch-Num Repetitions-SC-MCCH for Type1A-NPDCCH common search space RRC configured parameter npdcch-Num Repetitions-RA for Type2-NPDCCH common search space RRC configured parameter npdcch-Num Repetitions-SC-MTCH for Type2A-NPDCCH common search space RRC parameter npdcch-StartSF-USS (NPDCCH UE-specific search space) RRC parameter inpdcch-StartSF-CSS-RA (NPDCCH Type2-NPDCCH common search space) RRC parameter inpdcch-startSF-SC-MTCH (NPDCCH Type2A-NPDCCH common search space) T = max modT= Offset The value of R is determined by DCI u =0,1, subframe repetition number in DCI (see Subframe ko that 3GPP T

S 36.213 Table 16.6-1/2/3) meets this condition b = u R k Starting subframe b consecutive subframes excluding subframes used for transmission of SI messages Figure 6.14 Search spaces and determination of NPDCCH location [4,25]. Internet of Things RNTI and, if found, decode the NPDCCH. Three DCI formats have been specified for NB- IoT, namely DCI format NO, N1, and N2. When an NB-IoT UE receives a NPDCCH, it can distinguish different formats in the fol- lowing manner. DCI format N2 is implicitly indicated, wherein the CRC is scrambled with the P-RNTI. If the CRC is scrambled with the C-RNTI, then the first bit in the message indicates whether it contains DCI format NO or N1. For the case that the CRC is scrambled with the RA-RNTI, the content is a restricted DCI format N1 including only those fields required for the random-access response. The scheduling delay is included in DCI formats NO and N1; that is, the time interval between the end of NPDCCH and the start of NPDSCH or the start of narrowband physical uplink shared channel (NPUSCH). This delay is at least five subframes for the NPDSCH and eight subframes for the NPUSCH. For down- link transmission via DCI format N2, the scheduling delay is fixed and equal to 10 sub- frames [4] (see Table 6.3). In order to determine whether there is any data sent through NPDSCH to an NB-IoT UE or to detect any uplink grant for NPUSCH, the UE should monitor and try to decode various regions within downlink subframes. There is no information explicitly sent by the network regarding which regions the UE needs to monitor. The UE should monitor all possible regions that are allowed for NPDCCH and attempt to decode the information; that is, blind decoding. However, the UE does not need to decode every possible combination of resource elements within a subframe. There are a certain set of predefined regions in which a PDCCH could be allocated. The UE monitors only those predefined regions which are called NPDCCH search spaces. Since an NB-IoT UE does not know in advance when a DCI

will be sent to it, it must monitor NPDCCH subframes and attempt to decode them, in order to detect a pending downlink trans- mission. The NPDCCH configuration for a UE is primarily defined by how often the UE should start to monitor NPDCCH subframes and the maximum number of subframes that it should monitor. These two parameters are denoted by NPDCCH period T and maximum NPDCCH repetitions Rmax (Fig. 6.15). In the example shown in Fig. 6.16, the NPDCCH period is assumed to start from a subframe at time t = 0. Therefore, the UE starts to monitor Table 6.3: Various NB-loT DCI formats [3]. DCI Format Size (bits) Content Uplink grant NPDSCH scheduling RACH procedure initiated by NPDCCH order Paging and direct indication 766 Chapter 6 Period (T) 12 Assigned repetitions (R = 4) Maximum repetitions (Rmax) 8 Nongap interval Gap interval Start of period = aT Offset: a = 0 UE1: Rmax = 2 UE3: Rmax 8 T= 16 a=3/8 Figure 6.15 Example NPDCCH configuration for an NB-loT UE and the use of an offset to create non- overlapping NPDCCH configurations (18,19]. NPDCCH subframes at the following time instances 0, T, 2T, While there are T sub- frames in one period, the UE monitors NPDCCH subframes for a maximum duration equal to Rmax subframes, alternatively referred to as the non-gap interval. The duration of the remain- ing T Rmax subframes are known as gap interval. Fig. 6.16 further shows that at the start of the second period, the UE has received four repetitions of the NPDCCH, which it would then start decoding to check for any relevant control information. The choice of Rmax, for a given period T, has the following implications. A large Rmax implies higher downlink signal-to-interference plus noise ratio (SINR), resulting in better downlink coverage. A large Rmax further indicates more opportunities to schedule the UE within period T. In heavy net- work loading scenarios, this may decrease the waiting time for the UE to be scheduled. The maximum number of scheduling opportunities per second can be represented by Rmax/T. A trade-o

ff must be made between increasing Rmax value and the increased UE energy consump- tion since the UE receiver needs to be active and monitor NPDCCH subframes for a longer time duration. The beginning of the period may be shifted with respect to t=0 using the off- set parameter a. The number of subframes by which the period is shifted is denoted by aT. Fig. 6.15 shows how different a values can be used to create non-overlapping NPDCCH con- figurations for three UEs having the same value of period T=16. Note that the term non- overlapping means that the non-gap intervals of the UEs do not overlap with each other. As shown in the figure, the NPDCCH periods of UE-2 and UE-3 start two and six subframes Internet of Things Transport block CRC 16 Transport block + CRC TBCC (NPDSCH)/ TC(NPUSCH) Encoded bits Rate matching Rate-matched bits 3GPP TS 36.213 Table Space frequency block code (SFBC) 16.4.1.3-1 3 GPP TS 36.213 Table 16.4.1.5.1-1 (TX diversity) Antenna port 1 Antenna port 0 011110000100110 111000010101100 One resource unit Different scrambling code One cycle NsF=3 NPBCHINPDCCH NPDSCH NPDSCH NPDSCH NPBCH Figure 6.16 NPDSCH processing and mapping procedure [25]. later, respectively, compared to that of UE-1. It must be noted that, in an overlapping configu- ration, if any of the UEs receive one or more NPDCCH subframes, it may partially or fully block the non-gap interval of the remaining UEs, thus blocking the scheduling opportunity of those UEs. Furthermore, the likelihood of this blocking increases with larger Rmax values [4]. 6.3.7 Narrowband Physical Downlink Shared Channel The NPDSCH is the traffic channel and carries SIBs, upper layer data, and RAR messages. The NPDSCH subframe has the same structure as the NPDCCH shown in Fig. 6.16. It starts at a configurable OFDM symbol lNPDCCHstart and is mapped around the NRS and LTE CRS during in-band operation, where the value of parameterl, NPDCCHstart is provided signal- ing for the in-band operation, and is zero otherwise. A maximum TBS of 680 bits is specified for

NB-IoT applications. The mapping of a transport block spans NSF subframes. The trans- port block is repeated providing NRP identical copies using a subframe interleaving mecha- nism for an optimized reception at the NB-IoT UE, as shown in Fig. 6.16. Both values, NSF and NRP, are signaled via DCI. The resulting subframe sequence is mapped to NSFNRP conse- cutive subframes defined for NPDSCH (see Fig. 6.16). For the downlink, there is no auto- matic acknowledgment to a transmission and the eNB indicates this information in DCI. In that case, the UE transmits the acknowledgment using NPUSCH format 2. The associated timing and subcarrier mapping is indicated in this DCI, as well. There is a multi-carrier sup- port for all operation modes, which means that another carrier may be used when the UE is in 768 Chapter 6 the connected state. In the idle state, the UE camps on the NB-IoT carrier from which it received the synchronization signals and broadcast information, that is, the anchor carrier. The UE then waits to receive the paging message or to start system access for mobile- originated data or signaling transmission, both by transmitting a preamble on the associated uplink carrier provided in SIB2-NB broadcast message. The smallest scheduled RU varies depending on the subcarrier spacing and the number of tones. The reliability is obtained through repetitions where in the downlink up to 2048 repe- titions are possible and in the uplink up to 128 repetitions are permissible (see Tables 6.4 and 6.5). The SIB1-NB broadcast message is transmitted over NPDSCH. It has a period of 256 radio frames and is repeated 4, 8, or 16 times. The TBS and the number of repetitions are indi- cated in the MIB-NB. In general, 4, 8, or 16 repetitions are possible, and four TBSs of 208, 328, 440, and 680 bits are defined, respectively. The radio frame on which the SIB1-NB starts is determined by the number of repetitions and NNcell ID The fourth subframe is used for SIB1-NB in all radio frames transmitting SIB1-NB. As the other transmissio

n parameters are fixed, there is no associated indication in the control channel. The SIB1-NB content may only be changed on each modification period, which has a length of 4096 radio frames; that is, every 40.96 seconds. This corresponds to four SFN periods, which is the reason for indication of two LSBs of the HFN in the MIB-NB. If such a modification occurs, it is Table 6.4: Resource allocation in NB-loT [2]. Subcarrier Spacing (kHz) Number of Tones Link Direction Resource Unit Size (ms) Uplink/Downlink Uplink Uplink Uplink Uplink Table 6.5: Uplink resource allocation in NB-loT [2,4]. Subcarrier Subcarriers Slots Duration of Number of Resource Uplink Data Type Spacing (kHz) per RU per RU RU (ms) Elements per RU Uplink data Uplink control information (ACK/NACK) Internet of Things 769 indicated in the NPDCCH using DCI format N2. Although sent over the NPDSCH, the SIB1-NB resources are mapped similar to the MIB-NB shown in Fig. 6.12; that is, exclud- ing the first three OFDM symbols, which is necessary because the UE obtains the start of the resource mapping from SIB1-NB. One of the key features of NB-IoT is coverage enhancement through signal repetition. For typical NB-IoT use cases, the signal-to-noise ratio (SNR) operation point is below 0 dB. Under this operating condition, the channel estimation error is the dominating factor affect- ing the receiver performance. It can be shown that the effective SNR after combining NRP repetitions is given as follows [18]: where Y denotes SNR per transmission, o2 represents the variance of channel estimation error, and NRP is the number of repetitions. 6.3.8 Narrowband Physical Random-Access Channel Similar to LTE, the narrowband physical random-access channel (NPRACH) is used by an unsynchronized UE to inform the eNB of its desire to establish a connection. The random- access procedure is a contention-based and collision-prone mechanism in which the UE selects and transmits a random-access preamble to the base station. The NPRACH preamble consists of four symbol groups, w

ith each symbol group comprising one cyclic prefix and five symbols, as shown in Fig. 6.17. The length of the cyclic prefix is 66.67 us (Format 0) for cell radius up to 10 km and 266.7 us (Format 1) for cell radius up to 40 km. Each symbol, with fixed symbol value 1, is modulated on a 3.75 kHz tone with symbol duration of 266.67 us. However, the tone frequency index changes from one symbol group to another. The waveform of NPRACH preamble is referred to as single-tone frequency hopping. An example of NPRACH frequency hopping is illustrated in Fig. 6.18. To support coverage Preamble format 2048T 5.8192T 8192T 5.81927, 3.75 kHz 67 us (Format 0) 1.333 267 us (Format 1) Figure 6.17 Time domain structure of NPRACH [2]. Chapter 6 Preamble symbol group = n RA SC (i) Frequency location of a random access symbol group = Hopping From 3GPP TS 36.211: The frequency location of NPRACH is determined from RRC parameter nprach- Numsubcarriers Inprach-subcarrier offset-r13 in SIB2 Repetition Repetition Repetition The preambles can be repeated up to 128 times 3.75 kHz Repetition Repetition Repetition num repetitions perpreamble attempt-r13in SIB2 The start of the frame that meets the following: nf mod NPRACH period /10)=0 = Inprach-start time Inprach-start time Inprach-start time 30720N INPRACH+ 30720N NPRACH+ 30720N INPRACH start start start NPRACH period Inprach-periodicity Figure 6.18 Illustration of NPRACH time and frequency domain resource allocation and transmission timing [2,25]. extension, a NPRACH preamble can be repeated up to 128 times. The NPUSCH has two for- mats. Format 1 is used for carrying uplink data and uses the same LTE turbo code for forward error correction. Depending on the coverage level, the cell may indicate that the NB-IoT UE must repeat the preamble 1, 2, 4, 8, 16, 32, 64, or 128 times, using the same transmission power in each repetition. Each of the four groups is made up of a cyclic prefix and four iden- tical symbols. The NPRACH hops among 12 neighboring subcarriers (see Fig. 6.18). The base station

specifies a range for the allowed subcarriers, and communicates both the delay and the permissible range via the SIB to the NB-IoT UE. The UE can choose from 12 subcar- riers. If the UE uses a specific range within the designated subcarriers, this would signal to the base station that the UE supports multi-tone transmission format. Internet of Things 771 The NPRACH resources are separately provided for each coverage enhancement group. They consist of the assignment of time and frequency resources and occur periodically, where an NPRACH periodicity between 40 ms and 2.56 seconds may be configured. Their start time within a period is provided in the system information, whereas the number of repetitions and the preamble format determine their end. In the frequency domain, subcar- rier spacing of 3.75 kHz is utilized. The NPRACH resources occupy a contiguous set of 12, 24, 36, or 48 subcarriers and are located on a discrete set of subcarrier ranges. Depending on the cell configuration, the resources may be further partitioned into resources used by UEs supporting multi-tone transmission for msg3 and UEs that do not support it. The physical random-access preamble is based on single-subcarrier frequency-hopping symbol groups. A symbol group is illustrated in Fig. 6.18, which consists of a cyclic pre- fix of length TCP and a sequence of five identical symbols with total length TSEQ. The pre- amble consists of four symbol groups transmitted without gaps and is transmitted N INPRACH RP times. The NPRACH transmission can only start 30720 NNPRACHTS time units after the start of a radio frame whose frame number satisfies Nf mod(NNPRACH /11 period = 0. Following the transmission of 256(TCP + TSEQ) time units, a gap is inserted [2]. The NPRACH preamble formats, that is, format 0 and format 1, differ in the cyclic prefix length. The five symbols have a duration of TSEQ = 1.333ms, appended with a cyclic pre- fix of TCP = 67 us for format 0 and TCP = 267 us for format 1, making a total length of 1.4 and 1.6 ms, respectively. The

preamble format to be used is broadcast in the system information. The NPRACH starting subcarriers which are allocated to UE-initiated random access are divided into two sets of subcarriers, { 0, 1, ., N NPRACH NYPRACH sc_cont sc_cont NPRACH - 1}, where the second set would indicate the UE support for multi-tone msg3 transmission. The frequency location of the NPRACH transmission is constrained within NRA = subcarriers. Frequency hopping is used within the 12 subcar- riers, where the frequency location of the ith symbol group is given by mod4 = 1,3 RA 1)mod = 0 i mod4 = 1,3 and RRA 1 i mod4 = 2 and <6 mod4 = 2 and f(-1) = 0 Chapter 6 where Ninit denotes the subcarrier selected by the MAC sublayer from {0, NPRACH SC 1}, and the pseudo-random sequence c(n) is specified in [2]. Note that the pseudo-random sequence generator is initialized with Cinit = NNcell ID NPRACH can be transmitted only with a specific timing within a NPRACH period as illus- trated in Fig. 6.18, where Inprach-StartTime and Inprach-Periodicity parameters are config- ured by higher layers via SIB2-NB. The RACH procedure for NB-IoT will be described later in layer 2 aspects. 6.3.9 Narrowband Physical Uplink Shared Channel The NPUSCH transports two types of information: uplink data via NPUSCH format 1 and uplink control information (UCI) via NPUSCH format 2. The latter always uses one subcar- rier and is always BPSK modulated. It carries HARQ ACK/NACK corresponding to NPDSCH, whereas NPUSCH format 1 can use one or more subcarriers. For single-tone /2-BPSK or /4-QPSK modulation is used, while for multi-tone QPSK modulation is used. The NPUSCH can repeat data up to 128 times to improve the link budget. NPUSCH format 1 uses the same LTE turbo code (minimum code rate 1/3) for forward error correc- tion. The maximum TBS of NPUSCH format 1 is 1000 bits, which is much lower than that in LTE. NPUSCH format 2 uses a repetition code for error control (minimum code rate of 1/16). NPUSCH format 1 supports multi-tone transmission based on the same legacy LTE n

umerology. In this case, the UE can be allocated with 12, 6, or 3 tones. While only the 12- tone format is supported by legacy LTE UEs, the six-tone and three-tone formats are intro- duced for NB-IoT UEs, which due to coverage limitation cannot benefit from higher UE bandwidth allocation. Moreover, NPUSCH supports single-tone transmission based on either 15 or 3.75 kHz numerology. To reduce the peak-to-average power ratio (PAPR), single-tone transmission uses /2-BPSK or /4-QPSK with phase continuity between symbols. As shown in Fig. 6.19, NPUSCH format 1 uses the same slot structure as LTE PUSCH with seven OFDM symbols per slot and the middle symbol as the demodulation reference signal (DMRS), whereas NPUSCH format 2 consists of seven OFDM symbols per slot, but uses the middle three symbols as DMRS. The DMRS is used for channel estimation and coherent detection. The smallest unit to map a transport block is the resource unit whose definition depends on the NPUSCH format and subcarrier spacing. For NPUSCH format 1 and 3.75 kHz subcarrier spacing, an RU consists of one subcarrier in the frequency domain, and 16 slots in the time domain; that is, an RU has a length of 32 ms. For the 15 kHz subcarrier spacing, there are four options: 1, 3, 6, and 12 subcarriers over 16, 8, 4, and 2 slots, respec- tively (see Fig. 6.19). For NPUSCH format 2, the RU is always composed of one subcarrier with a length of four slots. As a result, for 3.75 kHz subcarrier spacing, the RU has 8 ms duration and for 15 kHz subcarrier spacing, the RU spans over 2 ms. Uplink multitone Uplink multitone (12x15 kHz) over 1 ms (3x15 kHz) over 4 ms Uplink multitone (6x15 Uplink single-tone (1x15 kHz) over 2 ms kHz) over 8 ms One schedulable resource unit with 15 kHz subcarrier spacing Subframe (1 ms) Uplink single-tone (1x3.75 One schedulable resource unit with 3.75 kHz subcarrier spacing kHz) over 32 ms Subframe (4 ms) 32 ms Uplink single-tone (1x3.75 kHz) over 2 ms (control) Uplink single-tone (1x3.75 kHz) over 8 ms (control) Uplink multitone (12x1

5 kHz) over 1 ms Uplink multitone (6x15 kHz) over 2 ms Uplink multitone (3x15 kHz) 1 PRB over 4 ms Uplink single-tone (1x15 kHz) over 8 ms Uplink single-tone (1x3.75 kHz) over 32 ms Figure 6.19 NPUSCH time-frequency mapping and uplink slot structure 11,19]. 774 Chapter 6 The signal in time domain is generated by applying an IFFT and inserting a cyclic prefix. For 15 kHz subcarrier spacing, the cyclic prefix is similar to that of LTE, while for 3.75 kHz, the cyclic prefix is 256 samples, corresponding to 8.3 us. For the latter case, a period of 2304 samples (75 us) at the end of each slot remains empty, which is used as a guard interval. For the in-band operation, this guard interval may be used to transmit sound- ing reference signals in the LTE system. Unlike downlink transmission, where HARQ ACK/NACK transmission is configurable, there is always HARQ acknowledgment in the associated downlink slot. The random-access signal s(t) for the ith symbol group is defined as follows [2]: where OSt<IsEQ+Tcp. BNPRACH is an amplitude scaling factor to conform to the transmit power PNPRACH, ko : - NUL /2, Afra = 3.75 kHz, and the location in the frequency domain controlled by the parameter NRA (i). For single-tone NPUSCH carrying uplink shared channel (UL-SCH), the uplink DMRSs are transmitted in the fourth block of the slot for 15 kHz sub- carrier spacing, and in the fifth block of the slot for 3.75 kHz subcarrier spacing. For multi- tone NPUSCH carrying UL-SCH, the uplink DMRSs are transmitted in the fourth block of the slot. The length of the uplink DMRS sequence is 16 for single-tone transmission and is equal to the size (number of subcarriers) of the assigned resource for multi-tone transmis- sion. In the uplink, the DMRS is defined and it is multiplexed with the data SO that it is only transmitted in the RUs containing data. There is no MIMO transmission mode defined for the uplink; consequently, all transmissions use a single antenna port. Depending on the NPUSCH format, DMRS is transmitted in either one or three SC-F

DMA symbols per slot. As shown in Fig. 6.20, the SC-FDMA symbols used for DMRS transmission depend on the subcarrier spacing. The DMRS symbols are constructed from a base sequence multiplied by a phase factor. They have the same modulation as the associated data channel. For 3.75 kHz subcarrier 15 kHz subcarrier 3.75 kHz subcarrier 15 kHz subcarrier spacing spacing spacing spacing Figure 6.20 Resource elements used for DMRS in NPUSCH format 1 and 2 [2]. Internet of Things NPUSCH format 2, DMRS symbols are spread with the same orthogonal sequence as defined for the LTE PUCCH formats 1, 1a, and 1b. 6.4 Layer 2/3 Aspects As in physical layer, there has been a number of changes in LTE L2/L3 operation to enable NB-IoT including introduction of new RRC messages and information elements. Some of the baseline LTE functions are not supported in NB-IoT including public safety notifica- tions, inter-RAT mobility, and security activation for transfer of RRC context information, measurement configuration and reporting, and self-configuration and self-optimization as well as measurement logging and reporting for network performance optimization. The UE in RRC_CONNECTED can be configured, via UE-specific RRC signaling, with a non- anchor carrier for all unicast transmissions. The UE in RRC_IDLE, based on broadcast/mul- ticast signaling, can use a non-anchor carrier for single-cell point-to-multipoint reception. The UE in RRC_IDLE can, based on broadcast signaling, use a non-anchor carrier for pag- ing reception. The UE in RRC_IDLE or RRC_CONNECTED, based on broadcast signaling, can use a non-anchor carrier for random access. If the non-anchor carrier is not configured for the UE, all transmissions occur on the anchor carrier. 6.4.1 HARQ Protocol and Scheduling To enable low-complexity UE implementation, NB-IoT allows only one HARQ process in both downlink and uplink, and allows longer UE decoding time for both NPDCCH and NPDSCH. Asynchronous, adaptive HARQ procedure is adopted to support scheduling flexi- bility. An example is

illustrated in Fig. 6.21. The scheduling commands are conveyed through DCI, which is carried by NPDCCH using aggregation-level (AL)-1 or AL-2 for transmission. With AL-1, two DCIs are multiplexed in one subframe, otherwise each sub- frame carries only one DCI (i.e., AL-2), resulting in a lower coding rate and improved COV- erage. Further coverage enhancement can be achieved through repetition. Each repetition occupies one subframe. DCI can be used for scheduling downlink data or uplink data. In the case of downlink data, the exact time offset between NPDCCH and the associated NPDSCH is indicated in the DCI. Since NB-IoT devices are expected to have reduced com- puting capability, the time offset between the end of NPDCCH and the beginning of the associated NPDSCH is at least 4 ms. In comparison, LTE PDCCH schedules PDSCH in the same subframe. After receiving NPDSCH, the UE needs to send HARQ ACK/NACK using NPUSCH format 2. The resources of NPUSCH carrying HARQ ACK/NACK are also sig- naled in DCI. Considering the limited computing resources in an NB-IoT device, the time offset between the end of NPDSCH and the start of the associated HARQ ACK/NACK is at least 12 ms. This offset is longer than that between NPDCCH and NPDSCH because the size of the transport block carried in NPDSCH can be as large as 680 bits, which is much 776 Chapter 6 NPDSCH ACK/NACK NPDCCH (AL = 2,Rep = 2) (4 resource units) NPUSCH ->4 ms >12 ms ACK/NACK NPDCCH NPUSCH (AL = 2,Rep = 2) NPDCCH >8 ms >3 ms NPDCCH NPDSCH NPUSCH NPDCCH (DCI for NPDSCH) (downlink data) (ACK/NACK) (next transmission) Downlink NPDCCH NPDSCH ->4 ms >12 ms >3 ms repetition repetition NPDCCH NPUSCH NPDCCH (DCI for NPDSCH) (uplink data) (next transmission) Uplink NPDCCH NPUSCH >8 ms >3 ms repetition repetition Figure 6.21 Illustration of the NB-loT HARQ operation [14]. larger than DCI that is only 23 bits long. The DCI for uplink scheduling grant needs to specify which subcarriers are allocated for the UE. The time offset between the end of NPDCCH and the beginning of the a

ssociated NPUSCH is at least 8 ms. Upon completion of the transmission of NPUSCH, the UE monitors NPDCCH to check whether NPUSCH was received correctly by the base station, or a retransmission is required [4,14]. 6.4.2 Physical, Logical, and Transport Channels NB-IoT physical, transport, and logical channel structure are similar to LTE, with the exception that there is no physical uplink control channel in NB-IoT and the UCI content is carried in NPUSCH. In the downlink, NB-IoT provides the following physical signals and channels. Unlike LTE, these NB-IoT physical channels and signals are primarily multi- plexed in time [6]. NPBCH Internet of Things DL-SCH UL-SCH Transport channels Physical channels NPBCH NPDSCH NPDCCH NPUSCH NPRACH Figure 6.22 NB-loT channel structure [6]. NPDCCH NPDSCH The NB-IoT includes the following channels in the uplink: NPRACH NPUSCH NPRACH is a newly designed channel since LTE PRACH uses a bandwidth of 1.08 MHz, which is more than NB-IoT uplink bandwidth (Fig. 6.22). 6.4.3 Cell Search and Random-Access Procedure Synchronization is an important aspect in cellular network operation. When a UE is pow- ered on, it needs to find and camp on a suitable cell. For this purpose, the UE must synchro- nize to downlink symbol, subframe, and frame timing, as well as to the carrier frequency. In order to synchronize with the carrier frequency, the UE needs to correct any frequency offset that is present due to local oscillator inaccuracy, and to perform symbol timing align- ment with the frame structure from the base station. In addition, due to the presence of mul- tiple cells, the UE needs to detect a particular cell on the basis of an NB-PCID. As a result, a typical synchronization procedure consists of determining the timing alignment, correcting the frequency offset, obtaining the correct cell identity, and the absolute subframe and frame number reference. The NB-IoT technology is intended to be used for low-cost UEs and at the same time, to provide extended coverage for UEs deployed in environme

nts with high penetration loss. The low-cost UEs inevitably utilize less expensive crystal oscillators that can have carrier frequency offsets (CFOs) as large as 20 ppm. Deployment of NB-IoT in the in-band or guard-band mode introduces an additional raster offset as large as 2.5 or 7.5 kHz, resulting in even higher CFO values. Despite of the large frequency offset, an NB- IoT UE must be able to perform accurate synchronization at low SNR conditions. Time-frequency synchronization in NB-IoT follows the same principles as in LTE; never- theless, it incorporates improvements in the design of the synchronization sequences in 778 Chapter 6 order to mitigate large frequency offset and symbol timing estimation error issues in low SNR regions. As we mentioned earlier, time-frequency synchronization is achieved through NPSS and NSSS signals. The NPSS is used to obtain the symbol timing and correct the CFO, whereas the NSSS is used to obtain the NB-PCID, and the timing within 80 ms block. For UEs operating at low SNRs, an autocorrelation based on a single 10 ms received segment would not be sufficient for detection, thus for more accurate detection, the UE must coherently accumulate several sequences over multiple 10 ms segments. Due to large initial CFO value, the sampling time at the UE is different from the actual sampling time, the difference being proportional to the CFO. For UEs with limited coverage, more number of accumulations might be required to successfully achieve downlink synchronization. Following the synchronization procedure, the UE proceeds to the acquisition of the MIB. The NPBCH consists of eight self-decodable subblocks, and each subblock is repeated eight times. The design is intended to provide successful acquisition for coverage-limited UEs. Subsequent to symbol timing detection and CFO compensation, in the in-band and guard- band deployments, there is still an additional raster offset, as high as 7.5 kHz, which needs to be compensated. The presence of raster offset results in either overcompensatio

n or undercompensation of the carrier frequency. As a result, the symbol timing drifts in either the forward or backward direction depending on whether the carrier frequency was over- compensated or undercompensated. This may cause a severe degradation in the performance of NPBCH detection. During cell selection, the UE measures the received power and quality of the NRS. These values are then compared to cell-specific thresholds provided by the SIB1-NB. If the UE is in coverage of a cell, it will try to camp on it. Depending on the received NRS power, the UE may have to start a cell reselection. The UE compares this power to a reselection thresh- old, which may be different for the intra-frequency and the inter-frequency scenarios. All required parameters are received from the serving cell; thus there is no need to acquire sys- tem information from other cells. Among all cells fulfilling the cell-selection criteria, the UE ranks the cells with respect to the excess power over a threshold. A hysteresis is added in this process in order to prevent frequent cell reselection, and also a cell-specific offset may be applied for the intra-frequency scenarios. In contrast to LTE, there are no priorities for different frequencies. The UE selects the highest ranked cell which is deemed suitable. In NB-IoT, random-access procedure serves multiple purposes such as initial access when establishing a radio link and scheduling request. Among others, the main objective of ran- dom access is to achieve uplink synchronization, which is important for maintaining uplink orthogonality in NB-IoT. As shown in Fig. 6.23, the contention-based random-access proce- dure in NB-IoT consists of four steps: (1) UE transmits a random-access preamble; (2) the network transmits a RAR message that contains the timing advance command and schedul- ing of uplink resources for the UE to use in the third step; (3) the UE transmits its identity Internet of Things NB-loTUE NB-IoTeNB INPSS,NSSS,NPBCH,SIB1 SIB2 parameters Random access RACH preamble preamb

RAR message MSG 2 inprach-NumCBRA-StartSubcarriers Number of starting sub-carriers allocated to contention based NPRACH response NPDCCH+NPDSCH random access numRepetitionsPerPreambleAttempt Number of NPRACH repetitions per attempt RRC connection Inprach-StartTime NPRACH starting time MSG 3 request (scheduled MSG 3 inprach-SubcarrierMSG3-RangeStart Fraction for calculating starting subcarrier index for the range of NPRACH NPUSCH transmission) subcarriers reserved for indication of UE support for multi-tone msg3 transmission RRC connection setup (contention MSG 4 MSG 4 resolution) NPDCCH+NPDSCH RRC connection NPUSCH setup complete Figure 6.23 NB-loT RACH procedure (these messages are repeated according to the UE coverage enhancement level) [6]. to the network using the scheduled resources; and (4) the network transmits a contention resolution message to resolve any contention due to multiple UEs transmitting the same random-access preamble in the first step. More specifically, upon transmission of the pre- amble, the UE first calculates its RA-RNTI from the transmission time. It then looks for a PDCCH with the DCI format N1 scrambled with the expected RA-RNTI, in which the RAR message is signaled. The UE expects this message within the response window, which starts three subframes after the last preamble subframe and has a coverage enhancement depen- dent length given in SIB2-NB. If the preamble transmission was not successful, that is, the associated RAR message was not received, the UE transmits another RACH preamble. This process is repeated up to a maximum number, which depends on the coverage enhancement level. For the case that this maximum number is reached without success, the UE proceeds to the next coverage enhancement level, if this level is configured. If the total number of access attempts is reached, an associated failure is reported to the RRC sublayer. With the RAR message, the UE receives a temporary C-RNTI and the timing advance command. Consequently, the forthcoming msg3 is already time aligned,

which is necessary for trans- mission over the NPUSCH. Further, the RAR message provides the uplink grant for msg3, containing all relevant data for msg3 transmission. The remaining procedure is performed similar to LTE; that is, the UE sends an identification and upon reception of the contention resolution message, indicating successful completion of the random-access procedure. The network can configure up to three NPRACH resource configurations in a cell in order to serve UEs in different coverage scenarios (based on the measured path loss). In each con- figuration, a repetition value is specified to increase the reliability of random-access pream- ble transmission. The UE measures the downlink received signal strength to estimate its coverage level, and transmits a random-access preamble in the NPRACH resources 780 Chapter 6 configured for that coverage level. To facilitate NB-IoT deployment in different scenarios, NB-IoT allows flexible configuration of NPRACH resources in time-frequency resource grid with the following parameters: (1) periodicity of NPRACH resource and starting time of NPRACH resource; (2) subcarrier offset (location in frequency) and number of subcar- riers (see Fig. 6.23). The UE should indicate its support of single-tone/multi-tone transmis- sion in the first step of random-access procedure in order to facilitate the network's scheduling of uplink transmission in the third step. The network can partition the NPRACH subcarriers in the frequency domain into two non-overlapping sets. A UE can select one of the two sets to transmit its random-access preamble to signal its supports for multi-tone transmission in the third step of random-access procedure. As we mentioned earlier, a set of PRACH resources (e.g., time, frequency, and preamble sequences) is provided for each coverage level. The PRACH resources per coverage level are configurable by the system information. The UE selects PRACH resources based on the coverage level estimated using downlink signal measurements, for example, RSRP [5]

. The UE MAC will reattempt the process at a higher coverage level RACH preamble set, if it does not receive the RAR message after the anticipated number of attempts at a certain level. If the contention resolution is not successful, the UE will continue to use the same coverage level RACH preamble set [2,21]. 6.4.4 Power-Saving Modes Power-save mode (PSM) is a power conserving mechanism in NB-IoT that allows the devices to skip the periodic paging channel monitoring cycles between active data transmis- sions, letting the device enter a sleep state. However, the device becomes unreachable when PSM is active; therefore, it is best utilized by device-originated or scheduled applications, where the device initiates communication with the network. Assuming there is no device- terminated data, an NB-IoT device can remain in PSM state for a long time, with the upper limit determined by the maximum value of the tracking area update (TAU) timer. During the PSM active state, the access stratum at the device is turned off, and the device would not monitor paging messages or perform any radio resource management measurements. In addi- tion, the PSM enables more efficient low-power mode entry/exit, as the device remains regis- tered with the network and its NAS state is maintained during the PSM without the need to spend additional cycles to setup registration/connection after each PSM exit event. Examples of applications that can take advantage of PSM include smart meters, sensors, and any IoT devices that periodically send data to the network. When a device initiates PSM with the net- work, it provides two preferred internal timers (T3324 and T3412), where the PSM time is the difference between these timers. The network may accept these values or set different ones. The network then retains state information and the device remains registered with the network. If a device wakes up and sends data before the expiration of the time interval it Internet of Things Rel-12 power save mode (PSM) eDRX Idle mode sleep-cycle extension

T3412 (timer) T3324 (timer) Up to 9.22s in NB-loT Paging occasions Sleep Sleep (monitoring) (device not reachable) Idle mode power consumption Up to 2.56s Figure 6.24 Illustration of PSM and eDRX operation [21,24]. agreed with the network, a reattach procedure is not required. However, in a similar manner to a radio module that has been powered off, a radio module in PSM cannot be contacted by the network while it is in sleep mode [7,8,21]. One problem with PSM mechanism is the ability to support device-terminated traffic, as the UE is unreachable when it is in PSM state. The device would become reachable by the net- work as the TAU timer expires (Fig. 6.24), which can introduce significant latency for device-terminated traffic. While periodic TAU can be configured to occur more frequently to match the UE's delay requirement, such configurations would result in additional signal- ing overhead from unnecessary periodic TAU procedures and increased device power con- sumption. To address this shortcoming of PSM, the extended DRX (eDRX) was introduced in 3GPP Rel-13. The eDRX is an extension of LTE DRX feature which can be used with NB-IoT devices to reduce power consumption. The eDRX can be used without PSM or in conjunction with it to obtain additional power savings. It allows the time interval during which a device is not listening to the network to be greatly extended. For an NB-IoT appli- cation, it might be acceptable not to be reachable for a few seconds or longer. Although it does not provide the same level of power saving as PSM, for some applications, the eDRX may provide a good compromise between device reachability and power consumption reduction. Fig. 6.24 illustrates the concept and the operation of PSM and eDRX mechan- isms. In eDRX, the DRX cycle is extended up to and beyond 10.24 seconds in idle mode, with a maximum value of 2621.44 seconds. For NB-IoT, the maximum value of the DRX cycle is 10,485.76 seconds [7,21]. 6.4.5 Paging and Mobility Paging is used to notify an idle-mode UE of a pending down

link traffic, to establish an RRC connection and to indicate a change in system information. A paging message is sent 782 Chapter 6 over the NPDSCH and may contain a list of UEs being paged and the information on whether paging is for connection setup or system information has changed. Each UE which finds its identifier in this list would notify the upper layers in order to initialize the RRC connection setup. If the paging message indicates a change in system information, then the UE acquires SIB1-NB to find out which SIBs have been updated. The UE in the RRC_IDLE state only monitors some of the subframes with respect to paging, the paging occasions (POs) within a subset of radio frames and the paging frames (PFs), as shown in Fig. 6.25. If coverage enhancement repetitions are applied, the PO refers to the first trans- mission within the repetitions. The PFs and POs are determined from the DRX cycle pro- vided in SIB2-NB, and the international mobile subscriber identity (IMSI) provided by the universal subscriber identity module (USIM) card. The DRX is the discontinuous reception of downlink control channel that is used to save the UE battery life. DRX cycles of 128, 256, 512, and 1024 radio frames are supported, corresponding to a time interval between 1.28 and 10.24 seconds. Since the algorithm for determining the PFs and POs also depends on the IMSI, different UEs have different POs, which are uniformly distributed across time. It is sufficient for the UE to monitor one PO within a DRX cycle, if there are several POs therein, the paging is repeated in every one of them. As we stated earlier, the concept of extended DRX may be applied for NB-IoT, as well. If eDRX is supported, then the time interval in which the UE does not monitor the paging messages may be considerably extended up to almost 3 hours. Correspondingly, the UE must know on which HFN and on which time interval within this HFN, that is, the paging transmission window (PTW), it has to monitor the paging. The PTW is defined by a start and stop SFN.

Within a PTW, the determination of the PFs and POs is done in the same way as for the non-eDRX. The NB-IoT supports stationary and low-mobility UEs as most of the NB-IoT nodes are sensors and devices that hardly move. Handover is not supported in NB-IoT, thus when an TeDRX Extended sleep Paging transmission window (PTW) DRX cycle Sleep Paging occasion (PO) Figure 6.25 Illustration of the correspondence of PO and PTW. Internet of Things 783 NB-IoT UE moves out of the coverage area of the serving cell, it will experience a radio link failure (RLF). As we mentioned earlier, an NB-IoT UE may support data transport via control plane. The UE may also support data transfer via the user plane, and when it does, the RRC connection reestablishment procedure is supported, which means that after an RLF is detected, the UE attempts to find a suitable cell through cell selection. If the UE finds a suitable cell, it will try to reestablish the connection on that cell and resume the data trans- fer. The RRC reestablishment intends to hide the temporary loss of the radio interface to the upper layers. The RRC reestablishment for a UE only supporting data transfer via the control plane was added in 3GPP Rel-14 [6,8]. 6.5 Implementation and Deployment Considerations The NB-IoT specifications meet a number of challenging requirements including a greater coverage area, longer device battery life, and lower device cost resulting from the small and intermittent data transmissions. The reduced peak data rate requirements make it possible to employ a simple radio and baseband processing in the receiver chain. With half-duplex operation of the NB-IoT, the duplex filter in a typical LTE device can be replaced by a sim- ple switch in addition to fewer oscillators for frequency synthesis. The use of simplified downlink convolutional coding instead of the LTE turbo code would allow a low- complexity baseband decoding process. The main candidate architectures are the zero-IF and low-IF receivers, which combine analog frontends and digital base

band signal proces- sing on a single chip. However, each of these architectures has some structural issues that must be resolved. In the zero-IF receiver, the desired signal is degraded by time variant DC offset caused by local oscillator leakage and self-mixing. In the low-IF receiver, nonideal hardware results in amplitude and phase mismatches between the I and Q signal paths, which results in degradation of the desired signal with leakage from the interference signal. In the generic low-IF receiver architecture, the incoming RF signal in the antenna is filtered by the band selection filter and amplified by a low-noise amplifier. The quadrature demodu- lator down-converts the RF signal to the complex low-IF signal, which is represented by in- phase and quadrature components. The IF signals pass through low-pass filters and then sampled by the analog to digital converters (ADCs). After the ADC sampling and conver- sion, the digitized IF signal is down-converted to the baseband, yielding digital complex signals. Using a moderately low-IF frequency, this architecture can avoid DC offset and 1/f noise issues that frequently arise from zero-IF receivers. However, it also reintroduces image issues. Image cancellation can be achieved after the low-noise amplifier, but requires narrowband filtering, thus significantly increasing the complexity and cost of the device. The latter issue can be addressed by complex mixing and subsequently and filtering techni- ques in the low-IF receiver. 784 Chapter 6 In order to meet the stringent link budget requirements of NB-IoT, low-cost low-complexity single-chip devices have been developed by various vendors [20]. The integration of the power amplifier (PA) and antenna switch simplifies routing by reducing the number of RF components in the frontend. The PA design with a low PAPR is possible with the use of the single-tone transmission technique. This enables the implementation of an RF chip that includes an efficient on-chip PA that may be operated near its saturation point for ma

ximum output power. While there is a trade-off between an integrated on-chip PA and an external PA, one can analyze the effect of PA nonlinearity on error vector magnitude (EVM), and thus on the NB-IoT uplink coverage, using an RF and baseband cross-domain simulation technique. In that case, the baseband LTE signal is generated, which supports both single- tone and multi-tone transmission. The baseband signal is filtered by two digital filters and fed into the modulator to generate a spectrum centered at the carrier frequency. The signal is then amplified using an amplifier with certain characteristics. The linearity of the PA can be modeled by setting the 1 dB compression point to an appropriate value. After the signal is amplified by the PA, it is demodulated by the receiver to determine the EVM. For single-tone transmission, the EVM values are very small, less than 0.08% for a 3.75 kHz subcarrier spacing and less than 0.9% for 15 kHz subcarrier spacing [16,17]. Therefore, we can conclude that the nonlinearity of PA has slight effect on the EVM for single-tone trans- mission mode. The studies suggest that PAPR is 4.8, 5.7, and 5.6 dB for a signal with 3, 6, and 12 tones, respectively; thus we can conclude that the nonlinearity of PA has unfavor- able effects on EVM for multi-tone transmissions. From this study one can conclude that, in case of single-tone transmission, some of the PAPR reduction circuit inside the chip can be removed which significantly reduces chip design complexity. Considering the key aspects of NB-IoT applications, devices that only support single-tone transmission combined with an on-chip nonlinear PA are much more advantageous in ultra-low-power and low-cost applications. The NPUSCH provides two subcarrier spacing options: 15 and 3.75 kHz. The additional option of using 3.75 kHz provides deeper coverage to reach challenging locations such as inside buildings, where there is limited signal strength. The data subcarriers are modulated using BPSK and QPSK with a phase rotation of /2 and /4,

respectively. Selection of the number of subcarriers for a resource unit can be 1, 3, 6, or 12 to support both single- tone and multitone transmission. The narrowband downlink physical resource block has 12 subcarriers with 15 kHz spacing, providing 180 kHz transmission bandwidth (see Fig. 6.26). It only supports a QPSK modulation scheme. To facilitate low-complexity decoding for downlink transmission in devices, the turbo coding was traded off with TBCC. NB-IoT achieves an MCL 20 dB higher than LTE. Coverage extension is achieved by increasing the reliability of data transmission through increasing the number of repetitions. Internet of Things NB-loT UL Spectrum mask 180 kHz Spectrum mask 878.8 879.28 879.76 880.24 880.72 881.2 Frequency (MHz) Figure 6.26 Example of NB-loT NPUSCH transmission spectrum with 15 kHz subcarrier spacing [16,17]. Coverage enhancement is ensured also by introducing single-tone NPUSCH transmission and /2 - BPSK modulation to maintain close to 0 dB PAPR, thereby reducing the unreal- ized coverage potential due to PA backoff. The single-tone NPUSCH transmission with 15 kHz subcarrier spacing provides a layer-1 data rate of approximately 20 bps when con- figured with the highest repetition factor, that is, 128, and the lowest modulation and coding scheme. The NPDSCH can provide a layer-1 data rate of 35 bps when configured with repe- tition factor 512 and the lowest modulation and coding scheme. These configurations sup- port close to 170 dB coupling loss. In comparison, the LTE network is designed for approximately 142 dB coupling loss. Fig. 6.27 shows an example of NPUSCH format 1 and 2 signal processing stages in order to achieve 144, 154, and 164 dB MCL in the uplink. The information bits can be mapped to NPUSCH format 1 based on LTE uplink SC-FDMA waveform. Furthermore, it depicts the processing stages for achieving 164 dB MCL based on NPUSCH format 2. In the latter case, following the use of /2 BPSK modulation, there are 3840 symbols which are repeated six times, appended with cyclic

prefix and guard time to occupy one slot, requiring 3840 slots in total. For every six slots with data symbols, one slot of DMRS is added. As a result, 640 DMRS slots are added. Overall, it requires 4480 slots, that is, 2240 sub- frames [23]. Chapter 6 24 subframes Channel coding and 24-Bit CRC Subframe Subframe rate matching modulation 2304 Coded 1152 QPSK Information Information bits CRC symbols One subframe carries 48 symbols if 4 subcarriers are NPUSCH processing for 144 dB MCL (NPUSCH format 1) 192 subframes Channel coding and 24-Bit CRC modulation 3x repetition Subframe Subframe rate matching 3072 Coded 1536 QPSK 4608 QPSK Information Information symbols symbols One subframe bits + CRC carries 24 symbols if 2 subcarriers are NPUSCH processing for 154 dB MCL (NPUSCH format 1) Each symbol is mapped to one Channel Coding and 2-BPSK 24-Bit CRC slot using special symbol rate matching modulation repetition pattern 3840 Coded Information 3840 BPSK 26880 data Information bits + CRC symbols symbols + 4480 DMRS 2240 subframes Subframe Subframe One subcarrier per subframe NPUSCH processing for 164 dB MCL (NPUSCH format 2) Figure 6.27 Example NB-loT uplink processing to achieve various MCL values 23] References 3GPP Specifications [1] 3GPP TS 36.104, Evolved universal terrestrial radio access (E-UTRA), Base station (BS) radio transmis- sion and reception (Release 15), June 2018. [2] 3GPP TS 36.211, Evolved universal terrestrial radio access (E-UTRA), Physical channels and modulation (Release 15), June 2018. 3GPP TS 36.212, Evolved universal terrestrial radio access (E-UTRA), Multiplexing and channel coding (Release 15), June 2018. 3GPP TS 36.213, Evolved universal terrestrial radio access (E-UTRA), Physical layer procedures (Release 15), June 2018. [5] 3GPP TS 36.214, Evolved universal terrestrial radio access (E-UTRA), Physical layer measurements (Release 15), June 2018. [6] 3GPP TS 36.300, Evolved universal terrestrial radio access (E-UTRA) and evolved universal terrestrial radio access network (E-UTRAN). Overall de

scription; Stage 2 (Release 15), June 2018. 3GPP TS 36.321, Evolved universal terrestrial radio access (E-UTRA), Medium access control (MAC) pro- tocol specification (Release 15), June 2018. 3GPP specifications can be accessed at the following URL: http://www.3gpp.org/ftp/Specs/archive/ Internet of Things [8] 3GPP TS 36.331, Evolved universal terrestrial radio access (E-UTRA), Radio resource control (RRC); Protocol specification (Release 15), June 2018. [9] 3GPP TR 38.825, Study on NR industrial Internet of Things (IoT) (Release 16), September 2018. Articles, Books, White Papers, and Application Notes [10] Cellular Networks for Massive IoT, Ericsson White Paper, January 2016. [11] J. Schlienz, D. Raddino, Narrowband Internet of Things, Rohde & Schwarz Whitepaper, August 2016. [12] A. Höglund, et al., Overview of 3GPP Release 14 enhanced NB-IoT, IEEE Network, November/ December 2017. [13] M. Chen, et al., Narrow band Internet of Things, IEEE Access, vol. 5, 2017. [14] Y. Wang, et al., A primer on 3GPP narrowband Internet of Things. IEEE Commun. Mag., 55 (3) (2017). [15] J.H. Wu, CAT-M & NB-IoT design and conformance test, Keysight Technologies White Paper, June 2017. [16] Keysight Technologies, the Internet of Things: Enabling technologies and solutions for design and test. Application Note, February 2016. S. Shin, NB-IoT System modeling: simple doesn't mean easy, Keysight Technologies White Paper, August 2016. [18] A. Puschmann, et al., Implementing NB-IoT in software, experiences using the srsLTE library, in: Proceedings of the Wireless Innovation Forum Europe, 2017. [19] B. Schulz, Narrowband Internet of Things measurements, Rohde & Schwarz Application Note, June 2017. [20] Evolution to NB-IoT and LTE-M, Global Mobile Suppliers Association (GSA) Report, August 2017. [21] 5G Americas White Paper, LTE and 5G Technologies Enabling the Internet of Things, December 2016. [22] NB-IoT, A sustainable technology for connecting billions of devices, Ericsson Technology Review, April 2016. [23] 3GPP R1-160085, NB-IoT NB-PU

SCH design, Ericsson, January 2016. [24] 5G Americas White Paper, Wireless Technology Evolution Towards 5G: 3GPP Release 13 to Release 15 and Beyond, February 2017. [25] NB-IoT, ShareTechnote. <http://www.sharetechnote.com/>. CHAPTER 7 Vehicle-to-Everything (V2X) Communications The concept of connected car has recently emerged, which provides new services to drivers via wireless communications that is considered as one of the most distinctive features of next generation vehicles. Vehicles wirelessly connected to other vehicles and pedestrians within proximity can identify the possibility of collisions by exchanging information such as speed, direction and their location. The vehicles can also communicate with a network entity in charge of traffic control SO that they can be informed of weather, and road hazards or receive guidance on the speed and route for traffic flow optimization. The automotive industry is evolving toward connected and autonomous vehicles that have the potential to improve safety and to reduce traffic congestion while reducing the environmental impacts of the vehicles. A key enabler of this evolution is vehicle-to-everything (V2X) communica- tions, which allows a vehicle to communicate with its surroundings, other vehicles, pedes- trians, road-side equipment, and the Internet. Using V2X, critical information can be exchanged among vehicles to improve situation awareness and to avoid accidents. Furthermore, V2X provides reliable access to the vast information available in the cloud. For example, real-time traffic reports, sensor data, and high-definition mapping data can be shared and accessed, which are useful not only for improving today's driving experience, but also will be essential for navigating self-driving vehicles in the future. V2X communications enables the exchange of information between vehicles and between vehicles and other nodes (infrastructure and pedestrians). The 3GPP Rel-12 device-to- device communication served as the basis for the V2X work in Rel-14. In Rel-14, 3GPP spec

ified cellular V2X (C-V2X) communications with two complementary transmission modes: direct communications between vehicles and network communications. 3GPP Rel- 16 focuses on continuation of LTE-based cellular V2X and tries to address advanced use cases. These include vehicle platooning, enhanced vehicle to infrastructure features, extended sensors, advanced driving (to enable semi-automated or fully-automated driving), and remote driving. C-V2X users can benefit from the existing widely-deployed cellular infrastructure. However, since the availability of cellular infrastructure cannot always be guaranteed, C-V2X defines transmission modes that enable direct communication using the sidelink channel over the PC5 interface. 5G NR. DOI: https://doi.org/10.1016/B978-0-08-102267-2.00007-5 © 2019 Elsevier Inc. All rights reserved. 790 Chapter 7 This chapter provides a technical overview of LTE-based V2X standards as of 3GPP Rel-15 and further provides insights into the emerging NR-based V2X technologies in 3GPP Rel-16 that are expected to accelerate the realization of advanced V2X communications and to improve transportation and commute experience. The NR-based V2X is required to enable very high throughput, high reliability, low latency, and accurate positioning use cases beyond the basic safety features provided by LTE-based V2X services. Some of the use cases will involve 5G working in tandem with other technologies, including cameras, radar, and light detection and ranging (LIDAR) 1 The Rel-16 C-V2X includes use cases such as advanced driving catego- ries identified in 3GPP, ranging/positioning, extended sensors, platooning, and remote driving. 7.1 General Aspects and Use Cases The V2X communications enable the vehicles to exchange data with each other and the infrastructure, with the goal of improving road safety, traffic efficiency, and the availability of infotainment services. V2X communications, as defined in 3GPP, consist of four use cases; namely, vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), v

ehicle-to-network (V2N), and vehicle-to-pedestrian (V2P), as shown graphically in Fig. 7.1. It is implied that Periodic location, speed, trajectory, alerts/warnings Vehicle Vehicle Vehicle location/ Wide area traveler motion detection and information, events, local traveler warnings information ("ug" Pedestrian/ eNB/RSU Periodic location, Infrastructure speed, trajectory, alerts/warnings gNB/eNB/RSU Management and Network (Edge cloud) services Application server/ traffic management Figure 7.1 Illustration of various V2X use cases [1,9,13,14]. Light detection and ranging is a remote sensing method used to examine the surface of the earth. Vehicle-to-Everything (V2X) Communications 791 these wireless connections are generally bidirectional; that is, the V2I and V2N also allow the infrastructure to send messages to the vehicles. The V2V and V2P transmissions are typ- ically based on broadcast capability between vehicles or between vehicles and vulnerable road users, for example, pedestrians and cyclists, for providing information about location, velocity, and direction to avoid accidents. The V2I transmission is between a vehicle and a road-side unit (RSU). The V2N transmission is between a vehicle and a V2X application server which is in the cloud. An RSU may be used to extend the range of a V2X message received from a vehicle by acting as a forwarding node. The V2I may include communica- tion between vehicles and traffic control devices near road work sites. V2N may include communication between vehicle and the server via 4G/5G network, such as for traffic operations. There are two prominent V2X technologies available today, which have been designed to operate in 5.9 GHz intelligent transportation systems (ITS) spectrum. The IEEE 802.11p standard, which is commonly referred to as dedicated short-range communication (DSRC) and ITS-G5 in the United States and Europe, respectively, was developed as an extension of IEEE 802.11a standard. IEEE 802.11p specification was completed in 2012 and uses a half-clocked version

of IEEE 802.11a, leveraging a radio technology that is over two dec- ades old and was originally developed for wireless Ethernet cable replacement and not for high-speed mobile applications. IEEE 802.11p performance results indicate its limited range and unreliable large-scale field performance, derived from susceptibility to conges- tion and lack of minimum performance guarantees, which would limit its usefulness. Most of the present-day cars are equipped with several active sensors, including camera, radar, and LIDAR, which compel the V2X wireless sensor to provide longer range and reliabil- ity, especially in non-line-of-sight (NLoS) scenarios where other vehicles and buildings obstruct the vehicle's vision systems. In the meantime, 3GPP has continued to evolve LTE device-to-device (D2D) technology, specified as part of Rel-12, and to optimize it for automo- tive applications in Rel-14, also referred to as C-V2X communications, over proximate radio interface (PC5) or sidelink and/or the LTE Uu interface. The C-V2X incorporates LTE's mobility support and further extends the baseline standard for automotive applications, while learning from IEEE 802.11p issues and shortcomings. The C-V2X includes both direct com- munications and network-based communications. Fig. 7.2 shows various scenarios for sidelink and direct link communications where UEs are located in-coverage and out-of-coverage of a cell. The future transportation technologies will include connected and intelligent vehicles, which can cooperate with each other and a transportation infrastructure that can provide safer and more convenient commute/travel experience. A wide range of use cases that require longer range or higher throughput can be supported with LTE-based/NR-based V2X communica- tions. C-V2X is the technology developed in 3GPP and is designed to operate in the follow- ing two modes: 792 Chapter 7 Direct communication Direct discovery Synchronization ((<000)) Control LTE/NE-Uu LTE/NE-Uu LTE-Uu/NR-Uu Radio interface downlink broadcast (MBSFN/S

C-PTM) eNB/RSU LTE-Uu/NR-Uu Radio In-coverage Partial coverage Out-of-coverage interface uplink unicast Sidelink (PC5) Autonomous LTE/NR PC5 Radio communications interface sidelink broadcast ((<000)) gNB/eNB Autonomous discovery Network-assisted discovery Network-assisted V2X UE communication Figure 7.2 Various V2X scenarios 12-14]. Device-to-device: This mode is sidelink communication between two or more devices without network involvement. Device-to-network: This mode uses the traditional cellular links to enable cloud services as part of the end-to-end solution by means of network slicing architecture for vertical industries. Some key performance metrics for low-latency local communications that are addressed by LTE-based V2X as part of Rel-14 include support of vehicle speeds up to 160 km/h and rel- ative speeds of 280 km/h; extended range to provide the driver(s) with sufficient response time (e.g., 4 seconds); message sizes for periodic broadcast messages between two vehicles/ UEs with payloads of 50-300 bytes and for event-triggered messages up to 1200 bytes; and maximum latency of message transfer of 100 ms between two UEs and 1000 ms for mes- sages sent via a network server [27]. In subsequent releases, more stringent requirements for latency, range, speed, reliability, location accuracy, and message payloads have been specified in order to support more advanced use cases. For example, NR-based V2X sys- tems are expected to provide end-to-end latency between vehicles of less than 5 ms and guarantee more than 99% reliable packet delivery within a short-to-medium range (80-200 m). RSUs are the new network nodes that are part of the LTE-based V2X communications sys- tem. These entities can be co-located with eNBs or operate in standalone mode. The V2I communication mode allows RSUs to monitor traffic-related conditions, such as traffic sig- nals and tolls and subsequently notify the surrounding vehicles. The evolution path for LTE-based V2X is going to exploit the NR features and to provide ubiquitous cover

age as the 5G technologies are designed and deployed. 3GPP evolved V2X (eV2X) has identified several new use cases/applications as follows [1,21,25,26]: Vehicle-to-Everything (V2X) Communications 793 Platooning: In this scenario, vehicles (dynamically) form a platoon while moving together in the same direction. Vehicles in the platoon obtain information from the lead- ing vehicle for managing the platoon. Platooning allows the vehicles to form a tightly coordinated group with significantly reduced inter-vehicle distance, thus increasing road capacity and efficiency. It also improves fuel efficiency, reduces accident rate, and enhances productivity by allowing the drivers to perform other tasks. Vehicles within a platoon must be able to frequently exchange information (e.g., to share information such as vehicle's speed and direction) and to send event notifications such as the intent for braking or acceleration. There are several aspects of platooning that must be sup- ported through reliable V2V communications such as joining and leaving a platoon; that is, to allow a vehicle to signal its intention to join or to leave a platoon at any time while the platoon is active, and to support additional signaling in order to complete join/leave operations; announcement and warning to indicate formation and existence of the platoon SO that nearby vehicles can select to join the platoon or to avoid disruptions to the platoon; steady-state operation group communication to support the exchange of platoon management messages and further to indicate braking, acceleration, road selec- tion, change of platoon leader, etc. Given the small target inter-vehicle distance while the vehicles are traveling at relatively high speed, V2V communication must be able to support reliable and secure message exchange to ensure effective and safe platooning operation. The following are some key V2V communication requirements in order to support platooning [13,14]: 25 ms end-to-end communication latency among a group of vehicles (10 ms for the hi

ghest degree of automation2. 90% message reliability, and 99.99% for the highest degree of automation relative longitudinal position accuracy of less than 0.5 m is required. 10-30 messages per second during broadcasting. Dynamic communication range control to improve resource efficiency given the varying platoon size, and to limit message distribution for privacy reasons. Advanced driving: Vehicle/RSU shares data that is collected through local sensors with nearby vehicles, allowing the vehicles to coordinate their paths and to avoid accidents. Advanced driving enables semi-automated or fully automated driving. Each vehicle shares its intention to change path and speed with vehicles in proximity, thus inter- vehicle distance adjustments are required. The key requirements for communication between two vehicles employing advanced driving mechanisms include: Large system bandwidth to support burst transmission of large data packets 10 ms latency for highest degree of automation 99.99% message reliability for highest degree of automation In an autonomous vehicle scenario, the vehicle's on-board computers are fully capable of performing all driv- ing operations on their own, with no human monitoring or intervention. 794 Chapter 7 Extended sensors: Extended sensors enable the exchange of raw or processed data (e.- g., cameras, radar, LIDAR) gathered from vehicle sensors or live video images among vehicles, road site units, devices of pedestrian, and V2X application servers. The vehi- cles can increase the knowledge of the environment/road conditions beyond of what their own sensors can detect and have a broader and holistic view of the driving condi- tions. The sensor data that a vehicle can share ranges from photo of a road hazard to real-time video stream. The availability of sensor data from multiple separate sources enhances situation awareness of the vehicles and pedestrians, and thus improves road safety. Extended sensors further enable new features such as cooperative driving and precise positioning, which are n

ecessary for autonomous driving. Remote driving: Enables a remote driver or a V2X application to operate a remote vehi- cle for those passengers who cannot drive by themselves or remote vehicles located in dangerous environments. For a case where route variation is limited and routes are pre- dictable, such as public transportation, driving based on cloud computing can be used. High reliability and low latency are the main requirements of this use case. Remote driving enables the remote control of a vehicle by a human operator or by a cloud- based application, via V2N communication. There are several scenarios that can lever- age remote driving, including: Provide a fallback solution for autonomous vehicles. An example is during the ini- tial autonomous vehicle deployment when a vehicle is in an unfamiliar environment and has difficulty navigating. Provide remote driver services to the youths, elderly, and others who are not licensed or able to drive. Enable fleet owners to remotely control their vehicles. Examples including moving trucks from one location to another, delivering rental cars to customers, and provid- ing remotely driven taxi services. Enable cloud-driven public transportation and private shuttles, all of which are par- ticularly suitable for services with predefined stops and routes. Remote driving can reduce the cost of fully autonomous driving for certain use cases because of the less stringent technical requirements (e.g., smaller number of in-vehicle sensors and less computation requirements for sophisticated algorithms). The following are V2X requirements for supporting remote driving: - Data rate up to 1 Mbps downlink and 25 Mbps in the uplink. - Ultra-high reliability at 99.999% or higher [similar to ultra-reliable and low- latency communication (URLLC) use case]. End-to-end latency of 5 ms between the V2X application server and the vehicle. Support vehicular speeds of up to 250 km/h. Remote control will be required when an obstacle blocks an autonomous driving vehi- cle, rendering it unabl

e to decide about a pathway or approach to safely navigate around Vehicle-to-Everything (V2X) Communications 795 it. Examples of obstacles include lanes that are blocked due to a recent accident, double-parked cars not allowing the vehicle to pass without crossing the ingress/egress yellow lines, or unexpected situations where the vehicle is unable to determine a safe action or a way forward. When the vehicle encounters such conditions, it will stop or find a minimum risk position and then will request assistance from a remote control operator to take control and navigate around the obstacle. The remote controller would need to understand the obstacle and determine the path that the vehicle must take. In this case, the controller will utilize the photos or streaming sensor information (e.- g., video, LIDAR, radar) that has been made available by the vehicle. Once cleared of the obstacle, the video stream to the controller will stop and the vehicle reasserts full control toward destination. NR V2X is not intended to replace the services offered by LTE V2X, rather it is expected to complement LTE V2X for advanced V2X services and to support interworking with LTE V2X. From 3GPP RAN technology development point of view, the focus and scope of NR V2X is to target advanced V2X applications. However, this does not imply that NR V2X capability is necessarily restricted to advanced services. It is up to the regional regulators, car manufacturers, equipment vendors, and automotive industry, in general, to deploy the technology suited for their intended services and use cases. NR V2X is planned as 3GPP V2X phase 3 and would support advanced services beyond those supported in Rel-15 LTE V2X. The advanced V2X services would require enhancing the NR baseline system and developing a new NR sidelink to meet the stringent requirements of the new use cases. NR V2X system is expected to have a flexible design to support services with low latency and high reliability requirements, considering the higher system capacity and extended

coverage enabled by the baseline NR system. The flexibility of NR sidelink framework would allow easy extension of NR system to support the future development of further advanced V2X services. More specifically, the NR V2X enhancements include the following areas [11]: Sidelink design: Identify technical solutions for NR sidelink to satisfy the requirements of advanced V2X services, including support of sidelink unicast, sidelink groupcast, and sidelink broadcast; study NR sidelink physical layer structure and procedure(s); study sidelink synchronization mechanism; study sidelink resource allocation mechanism; and study sidelink L2/L3 protocols. Uu enhancements for advanced V2X use cases: Evaluate whether Rel-15 NR Uu and/or LTE Uu interfaces could support advanced V2X use cases and identify enhancements, if any, that are needed to meet advanced V2X use cases. Uu-based sidelink resource allocation/configuration (LTE V2X mode 3 and mode 4): Identify necessary enhancements of LTE Uu and/or NR Uu to control NR sidelink through the cellular network and further identify the necessary enhancements of NR Uu to control LTE sidelink from the cellular network. 796 Chapter 7 RAT/interface selection for operation: Study if additional mechanisms are required to decide whether LTE PC5, NR PC5, LTE Uu, or NR Uu should be utilized for operation. Quality of service (QoS) management: Study technical solutions for QoS management of the radio interface (including Uu and sidelink) used for V2X operations. In-device coexistence: Study the feasibility of the coexistence mechanisms when NR sidelink and LTE sidelink technologies are implemented in the same vehicle for the non-co-channel scenarios such as advanced V2X services provided by NR sidelink while coexisting with V2X service provided by LTE sidelink in different channels. Sidelink operating bands: Sidelink frequency bands include both unlicensed ITS bands and licensed bands in FR1 and/or FR2. The target is to have a common sidelink design for both FR1 and FR2. For groupcast V2X c

ommunication, the following radio-layer enhancements are considered to improve sidelink communication performance in distributed resource allocation mode: Group radio-layer feedback When transmitter node sends a sidelink transmission/message to a group of UEs, the UE that has not successfully received physical sidelink shared channel (PSSCH) sends a NACK on physical sidelink control channel (PSCCH) resource reserved by the transmitter for acknowledgment. Group radio-layer (re-)transmission - If the UEs in a group detect a NACK from at least one of the group members, the UEs can retransmit successfully the received packet on a resource that can be either reserved by the UE which is failed to receive or by the original source of transmis- sion. Note that for groupcast communication, the same principles of sensing and resource selection as well as resource reservation can be reused. The group radio- layer feedback and (re-)transmissions can be developed using the same sidelink channel access and resource selection mechanisms. The latency of V2X systems is categorized into transmission time interval (TTI) dependent and TTI-independent, depending on whether the latency is proportional to or independent of the transmission time interval of the radio air-interface [15]. Each data transmission, that is, control signaling, scheduling, HARQ (re-)transmission, and SO on, consumes at least one TTI. The V2X services can be categorized into three groups: (1) safety-related services, (2) non-safety-related services, and (3) automated driving-related services. The safety-related services handle real-time safety messages, such as warning messages (e.g., abrupt brake warning message) to reduce the risk of car accidents. In this type of services, timeliness and reliability are the key requirements. On the other hand, non-safety-related services are intended to optimize the traffic flow on the road SO that travel time is reduced. Therefore, Vehicle-to-Everything (V2X) Communications Table 7.1: Performance requirements of different V

2X use cases [1]. Data Rate per Communication Use Case V2X Mode End-to-End Latency Reliability Vehicle (kbps) Range Cooperative awareness V2V/V21 100 ms to 1 second 90-95% Short-to-medium Cooperative sensing V2V/V2I 3 ms to 1 second 5-25,000 Short Cooperative maneuver V2V/V21 <3-100 ms 10-5000 Short-to-medium Vulnerable road user 100 ms to 1 second Short Traffic efficiency V2N/V2I > 1 second < 90% 10-2000 Tele-operated driving 5-20 ms > 99% > 25,000 these services enable more efficient driving experience with no stringent requirements in terms of latency and reliability. For the safety-related services, if we consider the frequency of periodic messages (e.g., from 1 to 10 messages/second) and the reaction time of most dri- vers (e.g., from 0.6 to 1.4 seconds), then the maximum allowable end-to-end latency must not exceed 100 ms. In fact, depending on the service type, the latency requirement may even be less than 100 ms (e.g., 20 ms for a pre-crash sensing warning). In addition to these types of services, automated driving-related services are being developed as a key transfor- mation in the automotive industry. These services require more rigorous latency limits, data rates, and positioning accuracy. Therefore, the latency requirements for automated driving- related services are more stringent than those required for safety-related services. For exam- ple, automated overtaking or high-density platooning services have a 10 ms latency require- ment. Table 7.1 lists the V2X use cases and the corresponding latency and data rate requirements [15]. V2V communication is conceptually based on D2D communications that was specified as part of proximity-based services (ProSe) in 3GPP Rel-12/Rel-13. The D2D feature provided public safety UEs the option to communicate directly. As part of ProSe, a new D2D inter- face (designated as PC5 interface or sidelink) was defined, which has been subsequently enhanced for vehicular use cases, specifically addressing high-speed (up to 250 km/h) and high node-density scenarios. Therefore

, some fundamental modifications to PC5 have been made, including additional reference symbols to support high Doppler frequencies associ- ated with relative speeds of up to 500 km/h and high frequency bands (e.g., 5.9 GHz ITS band being the main target) [1,24]. In order to support distributed scheduling, a sensing mechanism with semi-persistent trans- mission has been introduced. The C-V2V traffic from a device is mostly periodic in nature. This was utilized to sense congestion on a resource and estimate future congestion on that resource. Based on this estimation, the resources are reserved in advance. This technique optimizes the use of the channel by enhancing resource separation between transmitters that are using overlapping resources. The design is scalable for different bandwidths. There are 798 Chapter 7 GNSS Timing GNSS Timing Interface Interface Interface Interface Interface Interface Scenario1: Dedicated carrier and Scenario 2: Dedicated carrier and distributed scheduling V2V centralized scheduling V2I and V2V communications communications Figure 7.3 LTE V2X use cases and scheduling options [1]. two high-level deployment scenarios that are currently defined, as illustrated in Fig. 7.3. Both scenarios use a dedicated carrier for V2V communications, meaning that the target band is only used for PC5-based V2V communications. In both use cases, global navigation satellite system (GNSS) is used for time synchronization. In the first scenario, the schedul- ing and interference management of V2V traffic is supported based on distributed algo- rithms (referred to as mode 4) implemented between the vehicles. As we mentioned earlier, the distributed algorithm is based on sensing with semi-persistent transmission. Furthermore, a new mechanism where resource allocation is dependent on geographical information is introduced. Such a mechanism counters near-far effect arising due to in-band emissions. In the second scenario, the scheduling and interference management of V2V traf- fic is assisted by eNBs (referred to

as mode 3) via control signaling over the Uu interface. The eNB assigns the resources that are used for V2V signaling in a dynamic manner. The goal of the next-generation V2X communication is to enable accident-free cooperative automated driving by efficiently using the available roadways. To achieve this goal, the communication system will need to enable a diverse set of use cases, each with a specific set of requirements. The V2X feature was initially introduced with IEEE 802.11p and sup- ported a limited set of basic safety services. IEEE 802.11 group has initiated a new project called IEEE 802.11bd which is tasked to enhance IEEE 802.11p and develop similar fea- tures as NR V2X, while backward compatible to IEEE 802.11p [28]. 3GPP Rel-14 V2X supported a wider range of applications and services, including low-bandwidth safety Vehicle-to-Everything (V2X) Communications Advanced V2X 3GPP C-V2X Rel-15 and NR-V2X Rel-16 (Self-driving vehicles) Longer range, higher density Very high throughput, very high reliability Wideband ranging and positioning Very low latency Enhanced V2X 3GPP C-V2X Rel-14 Network coverage Long range Multimedia services Basic V2X IEEE 802.11p, DSRC ETSITS V2V,V2P,V2I Safety Figure 7.4 V2X technology evolution [13,14,25]. Table 7.2: High-level comparison of attributes for DSRC, LTE V2X, and NR V2X [13,14]. Features DSRC/IEEE 802.11p Rel-14 LTE C-V2X (5G) (Rel-15/16) C-V2X Out-of-network operation Support for V2V Support for safety-critical uses Support for V2P Support for V21 Limited Support for multimedia services Network coverage support Limited Global economies of scale Regulatory/testing efforts Limited Very high throughput Very high reliability applications and high-bandwidth applications such as infotainment. 3GPP Rel-15 and 16 enable even more V2X services by providing longer range, higher density, very high throughput and reliability, accurate positioning, and ultra-low latency. Fig. 7.4 summarizes these features and shows the evolution of V2X technologies and how IEEE 802.11p, LTE- b

ased, and NR-based V2X may coexist over time. Table 7.2 compares DSRC, Rel-14 C- V2X, and 5G C-V2X key features at a high level. The analysis of radio link performance of LTE V2X and IEEE 802.11p indicates that the likelihood of successful delivery of warning messages between two vehicles both equipped with LTE V2X (PC5) is notably greater than utilizing IEEE 802.11p technology under the same test conditions. 800 Chapter 7 5850 5855 5925 US Spectrum allocation (MHz) Channel numbers Channel usage Figure 7.5 DSRC channel arrangement in the United States [13,14]. 7.2 Spectrum Allocation Some DSRC-based systems have been deployed or are expected to be deployed in 5.9 GHz band or in 700 MHz band. As shown in Fig. 7.5, the United States allocation divides the entire 75 MHz ITS band into seven non-overlapping 10 MHz channels, with 5 MHz reserved as the guard band. There is one control channel (CCH), corresponding to channel number 178 and six service channels (SCHs) for the DSRC in this band. The pair of chan- nels (174 and 176, and 180 and 182) can be combined to form a single 20 MHz channel in either channel 175 or channel 181, respectively. Channel 172 is reserved exclusively for critical public safety communications, while channel 184 is reserved for high-power public safety use cases. The remaining channels can be used for non-safety applications. The V2X spectrum demands highly depend on environment (vehicle density, applications, traffic model, etc.), radio access technology, and target communication range and performance metrics. System-level analysis shows that approximately 20-30 MHz of bandwidth is needed in typ- ical highway scenarios for target range of 300-320 m based on LTE V2X sidelink commu- nication [10]. Discussions on ITS spectrum allocation adjustments in low and high bands are in progress to allocate new spectrum for V2X as follows (see Fig. 7.6): Low-band: Potential use of 5.925-6.425 GHz in addition to 5.855-5.925 GHz. High band: Frequency allocation above 6 GHz in Europe may be updated-center fr

e- quency of ITS band may be shifted to 64.80 GHz and extended to 2.16 GHz of band- width (to avoid overlapping with two IEEE 802.11ad channels and to increase ITS bandwidth). In the quest for increased network capacity, reduced cell sizes and the use of wider system bandwidths (whether contiguous or via carrier aggregation) have been actively investigated in the past decade. The use of mmWave spectrum for sidelink communications is believed to have several advantages for V2I communication, wherein transitory, high data rate Vehicle-to-Everything (V2X) Communications 801 dBm/MHz European V2X spectrum allocation 10 MHz 23 dBm/MHz 13 dBm/MHz ETSI EN 302 686 EIRP 43 dBm, Transmit power up to 27 dBm, antenna gain 14-23 dBi Frequency (GHz) ETSI ITS G5A ETSI ITS G5B EU-Wide available safety ETSI ITS G5D Nonsafety applications under ITS Future applications decentralized congestion applications control Figure 7.6 Region-specific ITS spectrum allocation in Europe [24,25]. connection can be established between vehicle and nearby base stations (small cells) to exchange delay-insensitive data (e.g., map updates and infotainment data in the downlink, and collected traffic and sensor information for large-scale traffic monitoring in the uplink); and for directional V2V communication for supporting specific use cases, such as communi- cation between adjacent vehicles in a platoon. Although nmWave communication is very attractive from the data throughput perspective, it creates challenges for the physical layer. Due to high propagation loss (path loss) and its susceptibility to shadowing, the use of mmWave bands is deemed suitable for mainly short range (a few hundred meters) and point-to-point LoS communications. Furthermore, since the Doppler spread is linear func- tion of the carrier frequency, the amount of Doppler spread in the 60 GHz band will be 10-30 X of that in 2-6 GHz band. Radio signals used for communication with the cellular network and between vehicles can also be used for position estimation. The achievable posi-

tioning accuracy can be significantly higher in 5G networks relative to legacy LTE net- works due to higher signal bandwidths and dense network deployments, which enables LoS communication with a high probability. Estimation of the relative position between vehicles is directly useful for certain use cases such as platooning, but can also serve as input to the cellular network in order to improve the estimation of vehicles' absolute position [16]. Global ITS spectrum is currently under further study within ITU-R Working Party 5A. This study is considered essential in improving the safety and efficiency of roads and highways. Specifically, the ITU World Radio communication Conference 2015 (WRC-15) adopted a ITU-R Study Group 5 (SG 5) Working Party 5A (WP 5A): Land mobile service excluding IMT; amateur and amateur-satellite service https://www.itu.int/en/ITU-R/study-groups/rsg5/rwp5a 802 Chapter 7 resolution to include a new agenda item in the WRC-194 to conduct studies on technical and operational aspects of evolving ITS implementation using existing mobile service allocations. 7.3 Network Architecture and Protocol Aspects 7.3.1 Reference Architecture LTE-based V2X operates in two modes: direct connection via sidelink and via the network. In the direct communication, the UE uses the LTE PC5 interface, which is based on 3GPP Rel-12 ProSe. The D2D communication in LTE operates over the sidelink channel in two different modes. In the first mode, the resources are allocated by the network (i.e., eNB). Devices that intend to transmit will send a request to the network for resource allocation, and the network subsequently allocates the resources and notifies the device. This mode requires additional signaling for every transmission, thus increasing transmission latency. In the second operation mode, devices select the resource autonomously. The autonomous mode reduces latency, but some issues related to possible collisions and interference may arise. Optimization of resource allocation procedures has been considered wi

th emphasis on the autonomous mode due to its lower latency. Further enhancements to accommodate high-speed/high-Doppler, high-vehicle density, improved synchronization, and decreased message transfer latency have been studied. This mode is suitable for proximal direct com- munications over short distances and for V2V safety applications that require low latency; for example, advanced driver-assistance systems or situational awareness. This mode can work in and out-of-network coverage (see Fig. 7.2). Network-based communication uses the LTE Uu interface between the vehicle (i.e., the UE) and the eNBs. The UEs send unicast messages via the eNB to an application server, which in turn re-broadcasts them via evolved multimedia broadcast multicast service (eMBMS) for all UEs in the relevant geographical area. This mode uses the existing LTE radio access and is suitable for latency-tolerant use cases (e.g., situational awareness, mobil- ity services). It is possible to deploy the network-based mode in an operator licensed spec- trum, while the direct PC5-based mode can be deployed in unlicensed spectrum. LTE uses unicast and broadcast bearers for data transmission. Broadcast is applicable, if the mobile operator has deployed eMBMS. LTE can complement the short-range communica- tion path for V2X provided by other technologies (e.g., DSRC/IEEE 802.11p). This type of broadcast transmission can potentially cover more vehicles in network coverage because the network can control the broadcast range, which is more suitable for the V2I/V2N type of services. Mobile operators can provide additional value-added services to subscribed drivers including traffic jams/blocked roads further ahead, real-time map/3D building/landmarks, World Radiocommunication Conference 2019 (WRC-19) https://www.itu.int/en/ITU-R/conferences/wrc/2019 Vehicle-to-Everything (V2X) Communications 803 Application server Application Application server server S/P-GW V2X Control S/P-GW function V2X Control V2X Control function function (Pedestrian) Application

(PLMN Application E-UTRAN Application (Stationary) LTE-Uu E-UTRAN LTE-Uu LTE-Uu LTE-Uu (Vehicle) (Vehicle) Application Application V2X Application PLMN A PLMN B Nonroaming V2X network architecture Roaming V2X network architecture Figure 7.7 Roaming/non-roaming network architecture for LTE-based V2X communication 2 V2X Application server S1-MME MB2-C E-UTRAN MBMS GW BM-SC LTE-Uu SGi-mb Figure 7.8 Reference architecture for eMBMS for LTE-Uu-based V2X communication via MB2 2]. updates for the area, or suggested speed. These two operation modes may be used by a UE independently for transmission and reception; for example, a UE can use eMBMS for recep- tion without using LTE-Uu for transmission. A UE may also receive V2X messages via LTE-Uu unicast downlink. V2X application servers (e.g., for traffic management services) in different domains can communicate with each other to exchange V2X messages. The interface between V2X application servers and the methods of the exchange of messages between V2X application servers is out of scope of 3GPP specifications. ProSe discovery feature can be used by a V2X-enabled UE and it is up to UE implementation. The RSU is not an architectural entity, but an implementation option. The high-level roaming and non-roaming reference architec- tures for PC5 and LTE-Uu-based V2X communications are shown in Fig. 7.7. Fig. 7.8 shows the high-level reference architecture with eMBMS for LTE-based V2X com- munication. The V2X application server may apply either MB2 or xMB reference points 804 Chapter 7 when managing eMBMS service-related information via a broadcast multicast service center (BM-SC), where MB2 reference point provides the functionality related to group communi- cation and xMB reference point provides an interface for any content and supports security framework between content provider and BM-SC. In the V2X reference architecture, the V2X control function is a logical function that is used for network-related functionalities required for V2X. It is assumed that there is only one l

ogical V2X control function in each public land mobile network (PLMN) that supports V2X services. If multiple V2X control functions are deployed within the same PLMN, then the method to locate the specific V2X control function, for example, through a database lookup, is not specified by 3GPP. The V2X control function is used to configure the UEs with necessary parameters for V2X communication. It is used to provide the UEs with network-specific parameters that allow the UEs to use V2X service in that PLMN. The V2X control function is also used to configure the UEs with parameters that are needed when the UEs are not served by an LTE network. The V2X control function may also be used to obtain V2X user service descriptions (USDs) SO that the UEs can receive eMBMS-based V2X traffic through V2 reference point from the V2X application server. V2X control function may also obtain the parameters required for V2X communications over PC5 reference point, or from the V2X application server via V2 reference point. The V2X control function in home PLMN can be always reached, if home-routed configura- tion is applied for packet data network (PDN) connection (e.g., the P-GW is part of the home PLMN), when such function is supported by the home PLMN. In the case of local breakout (e.g., the P-GW is part of the visited PLMN), a V2X control function proxy can be deployed by the visited PLMN to support UE to home V2X control function logical connection, if inter-PLMN signaling is required. The UE is not aware of these transactions and it will not know which access point name (APN) can be used for communication with the V2X control function unless the specific APN information is configured in the UE indicating that this APN provides signaling connectivity between the UE and the home V2X control function. The V2X control function of the home PLMN is discovered through interaction with the domain name service (DNS) function [2]. The V2X application server supports receiving uplink data from the UE over a unicast connection and deliv

ering data to the UE(s) in a target area using a unicast and/or eMBMS delivery mechanism as well as mapping of geographic location information to the appropriate target eMBMS service area identities for the broadcast. It is further responsible for mapping from geographic location information to the appropriate [target] E-UTRAN cell global identifier (ECGI), which is used to globally identify the cells. The Broadcast multicast service center is an evolved multimedia broadcast multicast service network entity located in the core network and is responsible for authorization and authentication of content providers, charging, and overall configuration of the data flow through the core network. Vehicle-to-Everything (V2X) Communications 805 V2X application server handles the mapping from UE provided ECGI to appropriate tar- get eMBMS service area identities for the broadcast; provides the appropriate ECGI(s) and/or eMBMS service area identities to BM-SC, which is preconfigured with the local eMBMS information (e.g., Internet protocol (IP) multicast address, multicast source, com- mon tunnel endpoint identifier); and is preconfigured with the local eMBMS IP address and port number for the user plane. The V2X application server is also in charge of send- ing local eMBMS information to the BM-SC; requesting BM-SC for allocation/de-alloca- tion of a set of temporary mobile group identities; requesting BM-SC for activating/ deactivating/modifying the eMBMS bearer; providing the V2X USDs SO that the UE can receive eMBMS-based V2X traffic from V2X control function; providing the parameters for V2X communications over PC5 reference point to V2X control function; or providing the parameters for V2X communications over PC5 reference point to UE [2]. The reference points or network interfaces shown in Fig. 7.7 can be further described as fol- lows [2]: V1: This reference point is between the V2X application in the UE and in the V2X application server, and it is not defined in 3GPP specifications. V2: This reference point is betwe

en the V2X application server and the V2X control function in the operator's network. The V2X application server may connect to V2X control functions belonging to multiple PLMNs. V3: This reference point is between the UE and the V2X control function in UE's home PLMN. It is based on the service authorization and provisioning part of the PC3 refer- ence point. It is applicable to both PC5 and LTE-Uu-based V2X communication and optionally eMBMS and LTE-Uu-based V2X communication. V4: This reference point is between the home subscriber server (HSS) and the V2X con- trol function in the operator's network. V5: This reference point is between the V2X applications in the UEs, and is not speci- fied in 3GPP specifications. V6: This reference point is between the V2X control function in the home PLMN and the V2X control function in the visited PLMN. PC5: This reference point is between the UEs for direct communication over user-plane for V2X service. S6a: In case of V2X service, S6a is used to download V2X service-related subscription information to mobility management entity (MME) during E-UTRAN attach procedure or to inform MME subscription information in the HSS has changed. S1-MME: In case of V2X service, it is used to convey the V2X service authorization from the MME to eNB. xMB: This reference point is between the V2X application server (e.g., content pro- vider) and the BM-SC. 806 Chapter 7 MB2: This reference point is between the V2X application server and the BM-SC. SGmb/SGi-mb/M1/M3: The SGmb/SGi-mb/M1/M3 reference points are internal to the eMBMS system. LTE-Uu: This reference point is between the UE and the E-UTRAN. The V2N and communication via the network represent suitable applications for the net- work slicing. For instance, autonomous driving or safety/emergency services would require URLLC network slice. Meanwhile, some infotainment services or personal mobility ser- vices would require either a best-effort slice or an eMBB. A vehicle can access different slices at the same time, with passengers watchi

ng a high-definition movie while a back- ground application detects a road hazard and triggers an emergency message for the cars behind or nearby to slow down or stop to prevent an accident. Fig. 7.9 illustrates this scenario. The slices could come from one device or multiple devices. In the case of one device, 3GPP has defined that a given device can support up to eight different slices with a common access and mobility management function (AMF) for all slices and a session management function (SMF) per slice. In the example shown in Fig. 7.9, there are three network slices attached to the same device sharing the same AMF instance. The first slice is massive machine-type communication, which sends data to the core and the PDN. The second slice offers caching at the edge, while the third slice provides access to an edge V2X application. The C-V2X can operate outside of network coverage using direct communication without requiring provisioning of a universal subscriber identity module (USIM). To enable USIM- less communication, automobile manufacturers will preconfigure the vehicle device with parameters necessary for out-of-network operation, including authorization to use V2X; a list of authorized application classes and the associated frequencies to use; radio parameters for use over the direct link; and configuration for receiving V2X messages via cellular V2X Control Slice function orchestration URLLC: Safety/Self-driving service management (Ultra-reliable and low latency) V2X App URLLC slice NFV MANO ((III) Caching eMBB slice eMBB: Infotainment/Video streaming mMTC slice (Mobile broadband) mMTC: Maintenance/Statistics High-density and low throughput NFVI (Edge cloud) NFVI (Core cloud) Figure 7.9 Example of NR V2X network slicing [13,14]. Vehicle-to-Everything (V2X) Communications V2X App V2X App LTE-Uu (Stationary) V2X UE V2X App V2X App Server Local Gateway (L-GW) V2X UE LTE-Uu Figure 7.10 RSU implementation options [20]. broadcast (i.e., eMBMS). Direct USIM-less communication allows C-V2X to support criti-

cal safety services when network coverage is unavailable or if the vehicle does not have an active cellular subscription. These parameters can also be securely updated by the vehicle manufacturers or the mobile operators. For V2I applications, the infrastructure that includes an RSU can be implemented in an eNB or a stationary UE. Fig. 7.10 shows two RSU implementation options and the func- tional entities in each case. 7.3.2 Sidelink and Radio Access Protocols Sidelink communication is a mode of operation whereby UEs can communicate with each other directly over the PC5 interface. This operation mode is supported when the UE is served by an eNB/gNB or outside of the network coverage. The use of sidelink communica- tion was originally limited to those UEs that were authorized for public safety operation; however, the application of sidelink communication was further extended to V2X services. In order to perform synchronization for out-of-coverage devices, the originating device may act as a synchronization source by transmitting sidelink broadcast control channel (SBCCH) and synchronization signals. The SBCCH carries the most essential system information needed to receive other sidelink channels and signals. In LTE V2X, the SBCCH is transmitted along with a synchronization signal with a fixed periodicity of 40 ms [6]. 808 Chapter 7 When the UE is in network coverage, the content of SBCCH are derived from the para- meters signaled by the serving eNB. When the UE is out-of-network coverage and if it selects another UE as a synchronization reference node, then the system information is obtained from SBCCH transmitted by the reference node; otherwise, the UE uses preconfi- gured parameters. The system information block type 18 (SIB18) provides the resource information for the synchronization signal and SBCCH transmission. There are two precon- figured subframes every 40 ms for out-of-coverage operation. The UE receives the synchro- nization signal and the SBCCH in one subframe and transmits the synchronization signa

l and the SBCCH in another subframe, if it assumes the role of the synchronization node. The UE performs sidelink communication in subframes defined over the duration of side- link control period. The sidelink control period is the time interval during which the resources are allocated in a cell for sidelink control information (SCI) and data transmis- sion. Within the sidelink control period, the UE sends SCI followed by sidelink data. SCI indicates a physical layer identifier and resource allocation parameters; for example, modu- lation and coding scheme (MCS), location of the resource(s) over the duration of sidelink control period, timing alignment, etc. The UE performs transmission and reception over LTE-Uu and PC5 (without sidelink discovery gap) where Uu transmission/reception is the highest priority, followed by PC5 sidelink communication transmission/reception and then by PC5 sidelink discovery announcement/monitoring which is considered the lowest prior- ity. The UE further performs transmission/reception over Uu and PC5 (with sidelink discov- ery gap) starting with Uu transmission/reception for RACH, followed by PC5 sidelink discovery announcement during a sidelink discovery gap for transmission; non-RACH Uu transmission; PC5 sidelink discovery monitoring during a sidelink discovery gap for recep- tion; non-RACH Uu reception; and PC5 sidelink communication transmission/reception [6]. The UE radio protocol structure for sidelink communication consists of user-plane and control-plane protocols. Fig. 7.11 shows the protocol stack for the user plane (the access PC5-U Figure 7.11 User-plane protocol stack for sidelink communication [6]. Vehicle-to-Everything (V2X) Communications 809 stratum protocol stack over PC5 interface), where packet data convergence protocol (PDCP), radio link control (RLC), and medium access control (MAC) sublayers, which are terminated at the other UE, perform similar functions defined for LTE protocols with some exceptions. The user plane corresponding to sidelink communication doe

s not support HARQ feedback; uses radio link control (RLC) unacknowledged mode (UM) and the receiving UE must maintain at least one RLC UM entity per transmitting UE; a receiving RLC UM entity used for sidelink communication does not need to be configured before reception of the first RLC UM protocol data unit (PDU); and robust header compression (RoHC) unidirectional mode is used for header compression in PDCP for sidelink communi- cation. A UE may establish multiple logical channels. In that case, the logical channel ID (LCID) included in the MAC subheader uniquely identifies a logical channel with one source layer-2 ID and destination layer-2 ID combination [7]. The parameters for logical channel prioritization are not configured. The access stratum is provided by ProSe per- packet priority (PPPP)6 of the PDU transmitted over PC5 interface by higher layers. Note that there is one PPPP associated with each logical channel [6]. The user-plane protocol stack and functions are further used for V2X sidelink communication. In addition, for V2X sidelink communication, the sidelink traffic channel (STCH) is used. Non-V2X data (e.g., public safety) is not multiplexed with V2X data transmitted over the resources configured for V2X sidelink communication. The access stratum is provided by the PPPP of a PDU transmitted over PC5 interface by upper layers. The packet delay budget (PDB), which refers to the permissible latency of data packets transported between UE and P-GW, of the PDU can be determined from the PPPP. The low PDB is mapped to the high priority PPPP value. The existing logical channel prioritization based on PPPP is used for V2X sidelink communication [6]. The control-plane protocol stack for the sidelink broadcast channel (SL-BCH) is also used for V2X sidelink communication (see Fig. 7.12). A UE that supports V2X sidelink commu- nication can operate in two modes for resource allocation: (1) scheduled resource allocation, In 3GPP Rel-13, ProSe per-packet priority (PPPP) was introduced to enable QoS differenti

ation across differ- ent traffic streams corresponding to different sidelink logical channels. PPPP has eight values ranging from one to eight, and each PPPP value represents the priority at which the associated traffic should be treated over the sidelink. Each data packet to be transmitted is assigned a PPPP value selected by the application layer. The UE then performs logical channel prioritization such that the transmission of the data associated with higher PPPP is prioritized. PPPP is also applied to transmission pool selection in the case of UE autonomous resource selection. The network can configure one or multiple PPPP for each transmission pool in the list of pools. Then, for each MAC PDU to transmit on the sidelink, the UE selects a transmission pool associated with the PPPP [2]. For downlink to sidelink mapping, which occurs when the relay UE receives traffic from the eNB, it identifies whether the packet should be relayed, by referring to the destination IP address of the packet. The relay UE then assigns a priority value called PPPP to the received packet to be relayed. The prior- ity assignment is based on the mapping information representing the association between the QoS class identi- fier values of downlink bearers and the priority values. The QoS class identifier-to-priority mapping information is provisioned to the relay UE by the network. 810 Chapter 7 which is characterized by the UE's need for radio resource control (RRC) connection estab- lishment to transmit data and the UE's request for transmission resources from the eNB. The eNB schedules transmission resources for transmission of SCI and data. Sidelink semi- persistent scheduling (SPS) is supported for scheduled resource allocation; and (2) UE autonomous resource selection, which is characterized by the UE's self-selection of resources from resource pools and transport format selection to transmit SCI and data. In the latter case, if mapping between the zones and V2X sidelink transmission resource pools is configured, the UE selects V

2X sidelink resource pool based on the zone in which the UE is located. The UE further performs sensing for (re) selection of sidelink resources. Based on sensing results, the UE (re)selects some specific sidelink resources and reserves multiple sidelink resources. The UE can perform up to two parallel and independent resource reservation processes. It is also allowed to perform a single resource selection for its V2X sidelink transmission [6]. A UE does not establish and maintain a logical connection to receiving UEs before one-to- many sidelink communication. Higher layer protocols establish and maintain a logical con- nection for one-to-one sidelink communication, including ProSe UE-to-network relay opera- tion. The control plane functions for establishing, maintaining, and releasing the logical connections for unicast sidelink communication are shown in Fig. 7.12. Sidelink discovery is defined as the procedure used by the UE supporting sidelink discovery to find other UE(s) in its proximity, using LTE direct radio signals via PC5 (see Fig. 7.13). Sidelink discovery is supported regardless of whether the UE is within network coverage or out-of-coverage. The service was originally limited to ProSe-enabled public safety UEs to perform sidelink discovery when they were out-of-network coverage where the allowed operating frequency was preconfigured in the UE, and is used even when UE is out-of- network coverage in that frequency. The preconfigured frequency is the same frequency as PC5 Signaling PC5 Signaling protocol protocol PC5-C Signaling protocol Figure 7.12 Control-plane protocol stack for sidelink broadcast and unicast transmissions [6]. Vehicle-to-Everything (V2X) Communications 811 ProSe protocol ProSe protocol PC5-D Figure 7.13 PC5 interface for sidelink discovery [6]. the public safety ProSe carrier [6]. The AS protocol stack for sidelink discovery consists of only MAC and PHY. The AS layer interfaces with upper layer (ProSe protocol) where the MAC sublayer receives the discovery message from the upper

layer (ProSe protocol). The IP layer is not used for transmitting the discovery message. The MAC layer determines the radio resource to be used for announcing the reception of the discovery message from upper layer and further generating the MAC PDU carrying the discovery message and sending the MAC PDU with no MAC header to the physical layer for transmission in the predetermined radio resources [7]. The content of discovery message is transparent to the access stratum, over which no distinction is made for sidelink discovery models and types; however, higher layer protocols detect whether the sidelink discovery notification is related to public safety. A UE can participate in announcing and monitoring of discovery message in both RRC_IDLE and RRC_CONNECTED states following eNB configuration, where the UE announces and monitors its discovery message subject to the half-duplex constraint. In order to perform synchronization, the UE(s) participating in announcing of discovery messages may act as a synchronization source by transmitting SBCCH and a synchronization signal based on the resource information allocated for synchronization signals provided in SIB19. There are three range classes. The upper layer authorization provides applicable range class of the UE. The maximum allowed transmission power for each range class is signaled in SIB19. The UE uses the applicable maximum allowed transmission power corresponding to its authorized range class. This sets an upper limit on the configured transmit power based on open-loop power control parameters. 7.4 Physical Layer Aspects V2X communications enable information exchange between vehicles and between vehicles and the infrastructures and/or pedestrians. The information exchange will provide the Public safety ProSe carrier is the carrier frequency used for public safety sidelink communication and public safety sidelink discovery. 812 Chapter 7 vehicles with more accurate knowledge of their surroundings, resulting in improved traffic safety. Some efforts were made in r

ecent years to deploy V2X communications using IEEE 802.11p. However, IEEE 802.11p uses a carrier sense multiple access scheme with collision avoidance which may not be able to guarantee stringent reliability levels and network scal- ability as the traffic increases. As an alternative, 3GPP LTE Rel-14 included support for V2X communications. The LTE-based V2X physical layer improves the link budget relative to IEEE 802.11p. In addition, it can improve the reliability, under certain conditions, by adding redundant transmission per packet. The C-V2X standard includes two radio inter- faces. The LTE-based Uu radio interface supports vehicle-to-infrastructure communications, while the PC5 interface supports V2V communications based on LTE-based sidelink com- munications. LTE sidelink (or D2D communication) was originally introduced in Rel-12 for public safety, and included two modes of operation: mode 1 and mode 2 (see Fig. 7.14). Both modes were designed with the objective of prolonging the battery life of mobile devices at the cost of increased latency. Connected vehicles require highly reliable and low- latency V2X communications; therefore, modes 1 and 2 are not suitable for vehicular appli- cations. 3GPP Rel-14 introduced two new communication modes (modes 3 and 4) specifi- cally designed for V2V communications (see Fig. 7.15). In mode 3, the cellular network selects and manages the radio resources used by vehicles for their direct V2V communica- tions. In mode 4, vehicles autonomously select the radio resources for their direct V2V communications. In mode 4, the UE (the vehicle) can operate without cellular coverage; therefore, this is considered the baseline V2V mode since safety applications cannot depend DCI Format 5 on (e)PDCCH with CRC Scrambled with SL-RNTI ((III) ) EPC with preSe function SIB18 Provides reception resource pools V2X UE1 V2X UE2 SCI Format 0 on PSCCH Data on PSSCH according to Information in SCI Format 0 scrambled with group destination ID Figure 7.14 Scheduling transmission resources for

direct communication, mode 1 [18,19]. Vehicle-to-Everything (V2X) Communications 813 Mode 3: eNB Controlled resource selection ((HH)) ((HH)) Mode 4: UE Autonomous resource selection Geo-Area ID1 Geo-Area ID2 Geo-Area ID3 Resource Resource Pool 2 Pool 1 Resource Pool 3 Figure 7.15 Illustration of LTE V2X modes 3 and 4 operation [25]. on the availability of cellular coverage. Mode 4 includes a distributed scheduling scheme for vehicles to select their radio resources and includes the support for distributed conges- tion control. In mode 3, the vehicle reports its location and coordinates to assist the eNB in scheduling, whereas in mode 4, the vehicle location information can be used to restrict side- link resource selection (to enable spatial reuse in a distributed system). The C-V2X utilizes LTE uplink multiple access scheme (SC-FDMA), and supports 10 and 20 MHz channels. Each channel is divided into subframes, resource blocks (RBs), and sub- channels. Subframes are 1 ms long. The resource block is the smallest unit of frequency resources that can be allocated to a user. It is 180 kHz wide in frequency (12 subcarriers with 15 kHz subcarrier spacing). The C-V2X defines subchannels as a group of RBs in the same subframe, and the number of RBs per subchannel can vary. Subchannels are used to transmit data and control information. The data is transmitted in the units of transport blocks (TBs) over PSSCH, and the SCI messages are transmitted over PSCCH. A TB con- tains a full packet to be transmitted; for example, a beacon or cooperative awareness mes- sage. A node that wants to transmit a TB must also transmit its associated SCI, which is also referred to as a scheduling assignment. The SCI includes information such as the MCS that is used to transmit the TB, the number of RBs, and the resource reservation interval for SPS. This information is critical for other nodes to be able to receive and decode the TB; thus the SCI must be correctly received. A TB and its associated SCI must always be trans- mitted in the same s

ubframe. The overall LTE-based V2X sidelink physical layer processing is shown in Fig. 7.16. 814 Chapter 7 Sidelink baseband processing MIB-SL Transport channel Physical signals discovery TB Physical channel and layer-1 signal generation and SCI format 0/1 RF processing processing processing OFDM modulation V2X traffic SL-BCH, SL-DCH, SL-SCH PSBCH, PSDCH, PSCCH, SL-DMRS, PSSS, SSSS PSSCH Code block Demodulation Synchronization segmentation and Scrambling reference signals CRC attachment signals TB CRC attachment Modulation Mapping of symbols to physical resources Transform SC-FDMA Signal Channel coding precoding generation Mapping to Rate matching physical resources Interleaving Figure 7.16 LTE V2X sidelink physical layer processing [17,22]. The C-V2X specifies two subchannelization schemes, as shown in Fig. 7.17: (1) adjacent PSCCH + PSSCH, where the SCI and TB are transmitted in adjacent RBs. For each SCI + TB transmission, the SCI occupies the first two RBs of the first subchannel utilized for the transmission. The TB is transmitted in the RBs following the SCI, and can occupy several subchannels depending on its size. It will also occupy the first two RBs of the fol- lowing subchannels; and (2) non-adjacent PSCCH + PSSCH, where the RBs are divided into pools. One pool is dedicated to transmit only SCIs, and the SCIs occupy two RBs. The second pool is reserved to transmit only TBs and is divided into subchannels. The TBs can be transmitted using QPSK or 16QAM modulation, whereas the SCIs are always transmitted using QPSK. The C-V2X uses turbo coding with normal cyclic prefix. There are 14 OFDM symbols per subframe, and four of these symbols are dedicated to the transmission of demodulation reference signals (DM-RS) in order to improve robustness against the Doppler effect at high speeds. The reference signals are transmitted in the third, sixth, ninth, and twelfth symbol of the subframe. The maximum transmit power is 23 dBm, and the stan- dard specifies a sensitivity-power-level requirement at the receiver of

-90.4 dBm and a maximum input level of - 22 dBm [17]. Vehicle-to-Everything (V2X) Communications Adjacent PSCCH+PSSCH Non-adjacent scheme PSCCH+PSSCH scheme Subchannel in RBs SCI or TB (PSCCH or PSSCH) TB Only (PSSCH) TB Transmission SCI Transmission SCI Only (PSCCH) Subframe 1 ms Figure 7.17 C-V2X frame structure and resource allocation schemes 17]. Vehicles communicate using sidelink or V2V communications in mode 4 and autonomously select their radio resources, independent of cellular coverage. When the vehicles are under cellular coverage, the network decides how to configure the V2X channel and informs the vehicles through the sidelink V2X configurable parameters. The message includes the car- rier frequency of the V2X channel, the V2X resource pool, synchronization references, the subchannelization scheme, the number of subchannels per subframe, and the number of RBs per subchannel. When the vehicles are not within the cellular coverage, they utilize a preconfigured set of parameters to replace the sidelink V2X configurable parameters. However, the standard does not specify a specific value for each parameter. The V2X resource pool indicates which subframes of a channel are utilized for V2X. The rest of the subframes can be utilized by other services. The standard includes the option to divide the V2X resource pool based on geographical areas. In this case, vehicles in an area can only utilize the pool of resources that have been assigned to those areas (see Table 7.3). Sidelink transmissions use the same basic transmission scheme as the uplink; however, side- link is limited to single-cluster transmission for all sidelink physical channels. Furthermore, sidelink uses one symbol gap at the end of each sidelink subframe. For V2X sidelink com- munication, PSCCH and PSSCH are transmitted in the same subframe (see Fig. 7.18). The Chapter 7 Table 7.3: Characteristics of LTE V2X transmission schemes [20]. Transmission Characteristics Uu Interface PC5 Interface Operating frequency All bands specified in 3GPP TS 36.

101 support For Rel-14 range operation with the Uu interface, except band 47 Band 47: Bands for Uu interface when used in combination with 5855-5925 MHz Band 3: Uplink 1710-1785 MHz Downlink 1805-1880 MHz Band 7: Uplink 2500-2570 MHz Downlink 2620-2690 MHz Band 8: Uplink 880-915 MHz Downlink 925-960 MHz Band 39: 1880-1920 MHz Band 41: 2496-2690 MHz RF channel bandwidth 1.4, 3, 5, 10, 15, or 20 MHz per channel 10 or 20 MHz per channel RF transmit power/EIRP Maximum 43 dBm for eNB Maximum 23 or 33 dBm Maximum 23 or 33 dBm for UE Modulation scheme Uplink: QPSK SC-FDMA, 16QAM SC-FDMA, 64QAM QPSK SC-FDMA, 16QAM SC-FDMA SC-FDMA Downlink: QPSK OFDMA, 16QAM OFDMA, 64QAM OFDMA Forward error Convolutional coding and turbo coding Convolutional coding and correction scheme turbo coding Data transmission rate Uplink: From 1.4 to 36.7 Mbps for 10 MHz channel From 1.3 to 15.8 Mbps for Downlink: From 1.4 to 75.4 Mbps for 10 MHz channel 10 MHz channel Scheduling Centralized scheduling by eNB Centralized scheduling or distributed scheduling Duplex method FDD or TDD sidelink physical layer processing of transport channels differs from uplink transmission in two aspects: (1) scrambling in physical sidelink discovery channel (PSDCH) and PSCCH processing is not UE-specific and (2) 64QAM and 256QAM modulation schemes are not supported for the sidelink. The PSCCH is mapped to the sidelink control resources and indi- cates resource and other transmission parameters used by a UE for PSSCH. For PSDCH, PSCCH, and PSSCH demodulation, reference signals similar to uplink DM-RS are transmit- ted in the fourth symbol of the slot in normal cyclic prefix and in the third symbol of the slot for extended cyclic prefix. The sidelink DM-RS sequence length equals the size (num- ber of subcarriers) of the assigned resources in the frequency domain. For V2X sidelink communication, reference signals are transmitted in the third and the sixth symbol of the first slot and the second and the fifth symbol of the second slot in normal cyclic prefix. For PSDCH

and PSCCH, the reference signals are generated based on a fixed base sequence, cyclic shift, and orthogonal cover code. For V2X sidelink communication, cyclic shift for PSCCH is randomly selected in each transmission [6]. Vehicle-to-Everything (V2X) Communications 817 For in-coverage operation, the power spectral density of the sidelink transmissions is deter- mined by the eNB. For measurement on the sidelink, the following basic UE measurement quantities are defined [18,19]: Sidelink reference signal received power (S-RSRP) is defined as the linear average over the power contributions [in (Watts)] of the resource elements that carry DM-RSs associ- ated with physical sidelink broadcast channel (PSBCH), within the six middle-band PRBs of the relevant subframes. The reference point for the S-RSRP is the antenna connector of the UE. The reported value must not be lower than the corresponding S- RSRP of any of the individual diversity branches, if receive diversity is utilized by the Sidelink discovery reference signal received power (SD-RSRP) is defined as the linear average over the power contributions (in [Watts]) of the resource elements that carry DM-RSs associated with PSDCH for which CRC has been validated. The reference point for the SD-RSRP is the antenna connector of the UE. If receive diversity is used by the UE, the reported value will be lower than the corresponding SD-RSRP of any of the individual diversity branches. PSSCH reference signal received power (PSSCH-RSRP) is defined as the linear average over the power contributions [in (Watts)] of the resource elements that carry DM-RSs associated with PSSCH, within the PRBs indicated by the associated PSCCH. The refer- ence point for the PSSCH-RSRP is the antenna connector of the UE. If receive diversity is used by the UE, the reported value must not be lower than the corresponding PSSCH-RSRP of any of the individual diversity branches. Sidelink reference signal strength indicator (S-RSSI) is defined as the linear average of the total received power [in (W

atts)] per SC-FDMA symbol observed by the UE only in the configured subchannels over SC-FDMA symbols (1, 2, 6) of the first slot SC-FDMA symbols (0, 5) of the second slot of a subframe. The reference point for the S-RSSI is the antenna connector of the UE. If receive diversity is used by the UE, the reported value should not be lower than the corresponding S-RSSI of any of the individual diversity branches. In the sidelink, there is no HARQ feedback and retransmissions are always performed in a predefined/configured manner. Measurement gaps and sidelink discovery transmission dur- ing a sidelink discovery gap for transmission are of higher priority than HARQ retransmis- sions, and whenever a HARQ retransmission collides with a measurement gap or sidelink discovery transmission during a sidelink discovery gap for transmission, the HARQ retrans- mission does not take place. Vehicles (or the UEs) select their subchannels in mode 4 using the sensing-based SPS scheme specified in Rel-14 [6]. A vehicle reserves the selected subchannel(s) for several consecutive reselection counter packet transmissions. This counter is randomly set between 818 Chapter 7 PSCCH PSSCH PSSCH PSSCH LTE V2X Mode 3 (eNB) Multiple active SPS configurations No UE transmission if no data Different MCS and periods PSSCH LTE V2X Mode 4 (Ad-Hoc) PSSCH Autonomous selection of resource Location and MCS PSSCH Advance resource reservation (SPS) Control data in the same subframe PSSCH D: Data Resource selection window (1s) C: Control PSSCH PSSCH LTE V2X Mode 4 resource allocation 1s Monitoring windows PSSCH Selection of 20% RB with lowest SNR PSSCH Color of resources indicate the SNR (High to low) Figure 7.18 Comparison of LTE V2X modes 3 and 4 [25]. Selection window TCSR-1000 (ms) TCSR-300 (ms) TCSR-200 (ms) TCSR-100 (ms) TCSR (ms) Average sensed RSSI = ( RSSI (Tcsr100 j)) Figure 7.19 Calculation of the average RSSI of a candidate resource [17]. 5 and 15, and the vehicle includes its value in the SCI. After each transmission, the reselec- tion counter i

s decremented by one. When it is equal to zero, new resources must be selected and reserved with probability (1 - p) where the UE/vehicle can set the value of p between 0 and 0.8. New resources also need to be reserved, if the size of the packet to be transmitted does not fit in the subchannel(s) previously reserved. The reselection counter is randomly chosen every time that new resources are reserved. Packets can be transmitted every 100 subframes or in multiples of 100 subframes with a minimum of one packet per subframe. Each UE/vehicle includes its packet transmission interval in the resource reserva- tion field of its SCI. The semi-persistent reservation of resources and the inclusion of the Vehicle-to-Everything (V2X) Communications reselection counter and packet transmission interval in the SCI would help other vehicles to estimate the [potentially] unused subchannels when making their own reservation, resulting in reduced risk of packet collision. Let us assume that a vehicle needs to reserve new subchannels at time Treservation It can reserve subchannels between Treservation and the maximum latency of 100 ms. This interval is referred to as the selection window (see Fig. 7.19). Within the selection window, the vehicle identifies candidate single-subframe resources (CSR) to be reserved by all groups of adja- cent subchannels within the same subframe where the SCI + TB information to be transmit- ted would fit. The vehicle analyzes the information it has received in the previous 1000 subframes before time instant Treservation and creates a list of permissible CSRs. LTE V2X mode 4 provides an option for each packet to be transmitted twice to increase the reliability. The sensing-based SPS scheme randomly selects a CSR from a candidate list for the redundant transmission of the SCI + TB. 3GPP Rel-14 includes a variant of the sensing- based SPS scheme for pedestrian-to-vehicle communications, where pedestrians broadcast their presence using mobile devices. Since the sensing process increases the battery con- s

umption, the standard provides an option to only sense a fraction of the 1000 subframes (1000 ms) before Treservation. The mobile devices can only select CSRs in the sensed sub- frames using the sensing-based SPS scheme. LTE V2X supports two types of sidelink trans- mission: (1) single shot without resource reservation and (2) multi-shot with resource reservation. Both types of transmission follow sensing and resource selection procedure. The timing diagram for LTE V2X mode 4, dedicated sensing and resource selection, is illustrated in Fig. 7.20 with sensing (1000 ms) and resource (re)-selection (up to 100 ms) windows. The concept of sensing and resource reservation is further depicted in Fig. 7.21. Sensing window (1000 ms) Resource selection interval PSCCH PSCCH PSSCH PSSCH (n-a), a = 1000 Resource reselection trigger Resource reservation period at time n (20, 50, 100, 200, 1000) Sensing window (1000 ms) PSCCH and PSSCH Resource Resource reselection reselection trigger n- T2 20 <T2<100 Figure 7.20 Mode 4 UE autonomous sensing and resource selection [25]. 820 Chapter 7 Resource reselection trigger Next packet transmission Selection Control Sensing Sensing window Selection window Figure 7.21 Sensing and resource reservation for collision avoidance in LTE V2X [6]. Vehicles also communicate using sidelink or V2V communications in mode 3. However, the selection of subchannels is managed by the base station and not by each vehicle as opposed to mode 4 (see Fig. 7.18). Mode 3 is only available when vehicles are within the network coverage. 3GPP has defined network architecture enhancements to support V2X. One of these enhancements is the V2X control function that is used by the network in mode 3 to manage radio resources and to provide vehicles with the sidelink V2X configurable parameters. Mode 3 utilizes the same subchannel arrangements as defined for mode 4. Vehicles using mode 3 must also transmit an associated SCI/TB, and the transmission of the SCI/TB must take place in the same subframe. In contrast to mode 4, t

he standard does not specify a resource management algorithm for mode 3. Each operator can implement its own algorithm that should fall under one of two categories: (1) Dynamic scheduling where the vehicles request subchannels from the eNB for each packet transmission, which increases the signaling overhead, and delays the packet transmission until vehicles are noti- fied of their assigned subchannels; and (2) SPS-based scheduling where the base station reserves subchannels for the periodic transmissions of a vehicle. However, in contrast to mode 4, the eNB decides how long the reservation should be maintained and it can activate, deactivate, or modify reservation of subchannels for a vehicle. The vehicle must inform the eNB of the size, priority, and transmission frequency of its packets SO that the eNB can semi-persistently reserve the appropriate subchannels. This information must be provided to the eNB at the start of a transmission, or when any of the traffic characteristics (size, prior- ity, and frequency) change [17]. Vehicles operating in mode 3 can be supported by different cellular operators. To enable their direct communication, 3GPP has defined an inter-PLMN architecture that can support vehicles subscribed to different PLMNs to transmit over different carriers. In this case, vehi- cles must be able to simultaneously receive the transmissions of vehicles supported by other PLMNs on multiple carriers. Therefore, each PLMN broadcasts in the sidelink V2X config- urable parameters the necessary information SO that the subscribed vehicles can receive the packets transmitted by other vehicles supported by different PLMNs. In an alternative sce- nario, the vehicles may be supported by different PLMNs sharing the same carrier, where each PLMN is assigned a fraction of the resources of the carrier. The standard does not Vehicle-to-Everything (V2X) Communications 821 Timing reference radio frame i Sidelink radio frame i (NTA, SL+ NTA offset) Ts seconds Figure 7.22 Sidelink timing alignment [3]. specify how the

resources should be allocated among the PLMNs, but introduces a coordi- nation mechanism (through the V2X control function) between PLMNs to avoid packet col- lisions [17]. The frame timing synchronization is an important consideration. Transmission of the ith sidelink radio frame from the UE starts at (NTA,SL + NTA offset)T seconds before the start of the corresponding timing reference frame at the UE (see Fig. 7.22). The UE is not required to receive sidelink or downlink transmissions earlier than 624T's following the end of a side- link transmission. The parameter NTA,SL differs between channels and signals where NTA,SL = NTA for PSSCH in sidelink transmission mode 1 and 0 otherwise [3]. 7.4.1 Sidelink Physical Resources and Resource Pool In LTE-based sidelink communication, physical layer transmissions are organized in the form of radio frames with duration Tframe = 10 ms, each consisting of 20 slots of duration Tslot = 0.5 ms. A sidelink subframe consists of two consecutive slots. A physical channel or signal is transmitted in a slot and is described by a resource grid of NSLNRB SC subcarriers symbol SC-FDMA symbols. The sidelink bandwidth is equal to the uplink bandwidth NSL = NUL RB if the cell-selection criterion is satisfied for a serving cell having the same uplink carrier frequency as the sidelink; otherwise, a preconfigured value is used. The side- link cyclic prefix is configured independently for type 1 discovery, type 2B discovery, side- link transmission mode 1, sidelink transmission mode 2, control signaling, and PSBCH as well as synchronization signals. The configuration is done per resource pool for discovery, sidelink transmission mode 2, and control signaling. The PSBCH and synchronization sig- nals always use the same cyclic prefix value. Normal cyclic prefix is only supported for PSSCH, PSCCH, PSBCH, and synchronization signals for a sidelink configured with trans- mission mode 3 or 4 [3]. Each resource element in the resource grid is uniquely defined by the index pair 0, , NSLNRB - 1; l =

0, NSL symbol ,1} in a slot where the first and the second indices represent the frequency and time, respectively. The resource elements that are not used for transmission of a physical channel or a physical signal in a slot are set to zero. 822 Chapter 7 A physical resource block (PRB) is defined as Nsymbol or 6 (extended cyclic prefix) con- secutive SC-FDMA symbols in the time domain and NRB SC = consecutive subcarriers in the frequency domain. A PRB in the sidelink consists of NSL symbol X SC resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain [3]. The relationship between the PRB number NPRB in the frequency domain and resource elements (k,1) in a slot is given by NPRB = k/12 A key concept in LTE-based sidelink communication is the resource pool, which defines a subset of available subframes and RBs for either sidelink transmission or reception. Sidelink communication is a half-duplex scheme and a UE can be configured with multiple transmit resource pools and multiple receive resource pools. The resource pools are config- ured semi-statically by RRC signaling. When data is sent using a resource pool, the actual transmission resources are selected dynamically from within the pool using one of the two following modes: Transmission mode 1, where the serving eNB identifies the resources via downlink con- trol information (DCI) format 5 that has to be sent to the transmitting UE. This mode requires the UE to be fully connected to the network; that is, in RRC_CONNECTED state. Transmission mode 2, where the transmitting UE self-selects the resources according to certain rules aimed at minimizing the risk of collision. This mode can be used when the UE is in connected state, idle state, or out of network coverage. There are two types of resource pools: (1) reception resource pools and (2) transmission resource pools. These are either signaled by the eNB for the in-coverage cases, or preconfi- gured for the out-of-coverage scenarios. Each transmission resource pool has

an associated reception resource pool in order to enable bidirectional communication. However, within a cell, there may be more reception resource pools than transmission resource pools, allowing reception from the UEs in the neighboring cells or from the UEs that are out-of-coverage. Fig. 7.23 illustrates the LTE V2X resource pool structure. A sidelink direct communication resource pool is configured semi-statically using layer-3 signaling. The physical resources (subframes and RBs) associated with the pool are parti- tioned into a sequence of repeating hyper frames known as PSCCH periods or alternatively referred to as the scheduling assignment period or sidelink control period. Within a PSCCH period, there are separate subframe pools and RB pools for control (PSCCH) and data (PSSCH). The PSCCH subframes always precede those for PSSCH transmission. This is analogous to the symbol layout of the physical downlink control channel and physical downlink shared channel OFDM symbols within a single downlink subframe, where the control region precedes the data subchannel. The PSCCH carries SCI messages, which describe the dynamic transmission properties of the PSSCH that follows. The receiving UE Vehicle-to-Everything (V2X) Communications 823 Time division resource multiplexing between scheduling and data to frequency division resource multiplexing Rel-12 D2D Resource pool structure Rel-14 LTE V2V Resource pool structure Scheduling assignment pool subchannel Scheduling/data Data pool subchannel Minimum periodicity 40 ms Non-adjacent Adjacent scheduling/ PSCCH PSSCH (Different colors indicate scheduling and data belonging to different UEs) scheduling/data data allocation Resource pool Resource pool allocation Figure 7.23 LTE V2X resource pool structure [13,14]. searches all configured PSCCH resource pools for SCI transmissions of interest. A UE can be a member of more than one sidelink communications group. For PSSCH, the number of current slot in the subframe pool nPSSCH = 2n PSSCH where ie {0,1} is the number of curre

nt slot within the current sidelink subframe PSSCH jmod10, where j is equal to the subscript of IPSSCH for sidelink transmission modes 1 and 2, respec- tively; and ie {0,1} is the number of the current slot within the current sidelink subframe PSSCH = kmod10 in which k is equal to the subscript of ISL for sidelink transmission modes 3 and 4. The last SC-FDMA symbol in a sidelink subframe is used as a guard period and is not used for sidelink transmission [3,5]. 7.4.2 Sidelink Physical Channels 7.4.2.1 Physical Sidelink Shared Channel In LTE-based sidelink communication, the processing of the sidelink shared channel (SL-SCH) follows the procedures for LTE downlink shared channel processing with some differences as follows: (1) data arrives at the channel coding unit in the form of a maximum of one TB in every transmission time interval; (2) in the step of code block concatenation, the sequence of coded bits corresponding to one TB after code block concatenation is referred to as one codeword; and (3) physical uplink shared channel (PUSCH) interleaving is applied without any control information in order to apply a time-first rather than frequency- first mapping, where For SL-SCH configured by higher layers for V2X sidelink, is used [3,5] (Fig. 7.24). 824 Chapter 7 Figure 7.24 Transport block processing for PSSCH [4]. The PSSCH processing begins with the scrambling of the block of bits b(0), b(Nbit where Nbit is the number of bits transmitted on the PSSCH in one subframe. The scrambling sequence generator is initialized with Cinit = n 2014 + nPSSCH29 nssf + 510 at the start of each PSSCH subframe. For sidelink transmission modes 1 and 2, = nSp is the destination identity obtained from the sidelink control channel, and for sidelink transmission modes 3 and 4 = L-1 with p and L equal to the decimal representation of CRC on the PSCCH transmitted in the same subframe as the PSSCH [3]. The output of the scrambling function is modulated using QPSK or 16QAM modulation and layer-mapped assuming a single antenna port [3]. Tr

ansform precoding is then performed with MPSSCH and MPSSCH parameters followed by precoding for single antenna port transmission. The block of complex-valued modulated symbols z(0), z(M'symb are power-adjusted using the scaling factor BPSSCH and sequentially mapped to PRBs on antenna port p starting with z(0) allocated for PSSCH transmission. The resource elements (k, 1) used for the latter frequency-first mapping should not be designated for transmission of reference signals, start- ing with the first slot in the subframe. Resource elements in the last SC-FDMA symbol within a subframe are counted in the mapping process but not used for transmission. If sidelink fre- quency hopping is disabled, the set of PRBs used for transmission is given by NPRB = N'VRB' where N'VRB is given in [3]. If sidelink frequency hopping with predefined hopping pattern is enabled, the set of PRBs used for transmission is given by the SCI associated with a prede- fined pattern such that only inter-subframe hopping is used. The number of subbands Nsb E {1, 2, 4} is configured by RRC signaling. The parameters E {0, ., 110} is given higher layers; ns = PPSSCH where PSSCH is given in [3]; and CURRENT_TX_NB The pseudo-random sequence generator is initialized at the start of each slot with NSS and the initialization value Cinit E {0, 1, 503,510} is determined by hoppingParameter-r12 RRC parameter; the quantity NVRB is replaced by N'VRB; and for sidelink transmission mode 1, = NSL for sidelink transmission mode 2, NUL = MPSSCH_RP where in given Vehicle-to-Everything (V2X) Communications 825 in [5]; the quantity NPRB is replaced by N'PRB; and the physical RB to use for transmission in [3]. 7.4.2.2 Physical Sidelink Control Channel As we mentioned earlier, the SCI messages are transmitted over PSCCH in LTE-based side- link communication. The fields defined in the SCI formats below are mapped to the infor- mation bits ao to aA-1 such that each field is mapped in the order in which it appears in the description, with the first field mapped to the

lowest order information bit ao and each suc- cessive field mapped to higher order information bits. There are two SCI formats for transmission of the SCI content as follows [4]: SCI format 0 carries 1-bit frequency hopping flag; RB assignment and hopping resource allocation [log2[1 SL ( NSL + 1) /2] bits where for PSSCH hopping Nsl_hop most significant bits (MSBs) are used to obtain the value of (i) and - NsL_hop) bits provide the resource allocation in the sub- frame. For non-hopping PSSCH, bits provide the resource allocation in the subframe. The SCI format 0 further includes 7-bit time resource pat- tern; 5-bit modulation and coding; 11-bit timing advance indication; and 8-bit group destination ID. SCI format 1 is used for the scheduling of PSSCH and carries 3-bit priority; 4-bit resource reservation; frequency resource location of initial transmission and retransmis- sion 1)/2) bits; 4-bit time gap between initial transmission and retransmission; 5-bit modulation and coding; 1-bit retransmission index; and a num- ber of reserved bits to adjust the size of SCI format 1 to 32 bits. The reserved bits are set to zero. The block of bits b(0), b(Nbit where Nbit is the number of bits transmitted on the physical sidelink control channel in one subframe are scrambled with the scrambling sequence generator initialized to Cinit = 510 at the start of each PSCCH subframe and then QPSK-modulated. Layer mapping with single antenna port and transform precoding are per- formed on complex-valued modulated symbols. Transform precoding is performed similar to LTE uplink with MRUSCH and MPUSCH parameters replaced by MPSCCH and MPSCCH values, respectively. For transmission on a single antenna port, precoding is defined by where i = 0, 1, 1 and y(0)(i) denotes the transform-precoded com- plex-valued symbols. The block of complex-valued symbols z(0), z(M's -1) are multiplied by BPSCCH in order to adjust the transmit power and are sequentially mapped, starting with z(0), to the PRBs on antenna port p that have been assigned for tran

smission of PSCCH. The latter frequency-first resource mapping must avoid resources that are designated to transmission 826 Chapter 7 of reference signals, starting with the first slot in the subframe. Resource elements in the last SC-FDMA symbol within a subframe are considered in the mapping process but are not used for transmission. The radio resources for direct communication can be selected by the device autonomously or will be scheduled by the network. In case the device has acquired SIB18 and has further a passive connection with the network in RRC_IDLE, the device would select radio resource from the broadcast resource pool in SIB18. Similar to direct discovery case, a UE would have to transit to the RRC_CONNECTED state when no valid (transmit) resource pool are provided by SIB18. In this case, a ProSe UE information indication is sent by the terminal to the network, indicating the intent to use the direct communication capability. In response, the network will assign a sidelink radio network temporary identifier (SL-RNTI) to the device. The network then uses the SL-RNTI and the downlink control channel to assign a transmission grant to the device with the new defined DCI format 5. The DCI for- mat 5 is used for scheduling of PSCCH and contains some SCI format 0 fields that are used for scheduling of PSSCH. The DCI format 5 information fields include 6-bit resource indi- cation for PSCCH; and 1-bit transmit power control command for PSCCH and PSSCH, as well as SCI format 0 fields including frequency hopping flag; RB assignment and hopping resource allocation; and time resource pattern [5]. Similarly, the DCI format 5A is used for scheduling of PSCCH, containing some SCI format 1 fields to schedule PSSCH. The DCI format 5A information fields include 3-bit carrier indicator; the lowest index of the sub- channel allocation to the initial transmission bits as well as SCI format 1 information fields including frequency resource location of initial transmission and retrans- mission; time gap between initial tra

nsmission and retransmission; and 2-bit sidelink index [this field is present only for TDD uplink/downlink configuration 0-6]. When DCI format 5A CRC is scrambled with sidelink semi-persistent scheduling V-RNTI (SL-SPS-V-RNTI ), it would further include 3-bit sidelink SPS configuration index and 1-bit activation/release indication [4]. For sidelink transmission mode 1, if a UE is configured via RRC signaling to receive DCI format 5 with the CRC scrambled by the SL-RNTI, the UE is required to decode PDCCH/ ePDCCH according to the combination defined in Table 7.4. For sidelink transmission mode 3, if a UE is configured by higher layers to receive DCI format 5A with the CRC scrambled by the SL-V-RNTI or SL-SPS-V-RNTI, the UE must decode the PDCCH/ePDCCH according to the combination defined in Table 7.4, and it is not expected to receive DCI format 5A with size larger than DCI format 0 in the same search space that DCI format 0 is defined. The PSCCH carrying SCI format 0 is transmitted in two subframes within the configured resource pool occupying only one RB pair. The 7-bit time resource pattern determines Semi-persistently scheduled sidelink transmission for V2X sidelink communication, which is used for activa- tion, reactivation, deactivation, and retransmission. Vehicle-to-Everything (V2X) Communications 827 Table 7.4: PDCCH/ePDCCH configured by various RNTIs [5]. DCI Format Search Space DCI format 5 For PDCCH: Common and UE-specific by C-RNTI For ePDCCH: UE-specific by C-RNTI DCI format 5A For PDCCH: Common and UE-specific by C-RNTI For ePDCCH: UE-specific by C-RNTI which subframes are used for transmission of PSSCH. A subframe indicator bitmap of vari- able length is defined, where the length of this bitmap depends on the duplex mode; i.e., FDD or TDD, and in case of TDD which UL/DL configuration is used. In case of FDD, the bitmap is 8 bits long. Up to 128 different time resource patterns define how these 8 bits are used. The RB allocation for PSSCH follows the same principles defined for LTE Rel-8 while inter

repeated to form an extended bitmap (bo, b1, bLPsscH~1) where bj = bjmodNTRP covers entire PSSCH subframe pool. The subframes used for PSSCH transmission are selected by PSSCH values in this extended bitmap to obtain the final subframe set denoted by n1 PSSCH NPSSCH-1 ,PSSCH where NPSSCH value is a multiple of 4, and denotes the num- ber of subframes that can be used for PSSCH transmission in the PSCCH period. This is consistent with the fact that each TB transmitted within this interval will be sent four times using the fixed HARQ redundancy version sequence (0,2,3,1) [5]. 828 Chapter 7 7.4.2.3 Physical Sidelink Discovery Channel In LTE-based sidelink communication, the processing of the SL-DCH follows the downlink shared channel with the following differences: (1) data arrives at the channel coding unit in the form one TB per each transmission time interval; (2) in the step of code block concate- nation, the sequence of coded bits corresponding to one TB after code block concatenation is referred to as one codeword; and (3) PUSCH interleaving is applied without any control information in order to apply a time-first rather than frequency-first mapping such that block b(0), b(Nbit - -1), where Nbit is the number bits transmitted on the PSDCH in one subframe, are scrambled with the scrambling sequence generator initialized with Cinit = 510 at the start of each PSDCH subframe and then QPSK-modulated. The layer mapping, transform precoding, precoding, and the mapping to the physical resources are similar to PSCCH processing described earlier. 7.4.2.4 Physical Sidelink Broadcast Channel Fig. 7.25 shows the processing stages of SL-BCH transport channel in LTE-based sidelink communication. The broadcast channel data in a TB is processed through channel coding module, which includes CRC attachment to the TB, channel coding, and rate matching. Since the latter processing is in the uplink direction of LTE, following the rate matching, LTE PUSCH interleaving is used without multiplexing with control information. A time- fi

rst rather than frequency-first mapping is applied, where Cmux 2 For SL-BCH configured by higher layers for V2X sidelink, The entire TB containing the sidelink broadcast channel is used to calculate the 16-bit CRC. Information bits inclusive of the attached 16-bit CRC are encoded using tail biting convolutional code (i.e., a TBCC with constraint length 7 and coding rate 1/3) and rate matched. The block of bits b(0), , b(Nbit -1), where Nbit is the number of bits transmitted on the PSBCH in one subframe, are scrambled SO that the scrambling sequence generator is initial- ized at the start of every PSBCH subframe with Cinit = NSL and then QPSK-modulated. Layer mapping with single antenna port and transform precoding are performed on Figure 7.25 Physical sidelink broadcast channel processing [4]. Vehicle-to-Everything (V2X) Communications 829 complex-valued modulated symbols. Transform precoding is performed similar to LTE uplink with parameters MPSBCH RB and MPSBCH SC For transmission on a single antenna port, pre- coding is defined by where i = 0, 1, Mayer symb - 1 and y(0) (i) denotes the transform-precoded complex-valued symbols. The block of complex-valued symbols z(0) z(MP symb 1) are multiplied by an amplitude scaling factor BPSBCH in order to adjust the transmit power, and sequentially mapped to PRBs on antenna port p. The PSBCH uti- lizes the same set of RBs as the synchronization signal. The frequency-first mapping to the PRBs would avoid resources designated for transmission of reference signals or synchroni- zation signals, starting with the first slot in the subframe. The resource element index k is determined by k=k-36 + NSLNRB/2, k' 0, 1, ..,71 since the last symbol of the sub- frame is used as a gap and not used for transmission. 7.4.3 Sidelink Physical Signals 7.4.3.1 Demodulation Reference Signals In LTE V2X, the DM-RSs associated with PSSCH, PSCCH, PSDCH, and PSBCH are trans- mitted similar to LTE PUSCH with different parameters and antenna ports. The set of phys- ical RBs used in the mapping proc

ess are identical to the corresponding PSSCH/PSCCH/ PSDCH/PSBCH transmission. As shown in Fig. 7.26, for sidelink transmission modes 3 and 4 on PSSCH and PSCCH, the mapping uses symbols l = 2,5 for the first slot in the Synchronization subframe 1 ms 6 PRBs N subchannels PSCCH PSBCH First slot Second slot Figure 7.26 C-V2X frame structure and location of various physical channels and signals [3]. 830 Chapter 7 subframe and symbols l = 1, 4 for the second slot in the subframe. For sidelink transmission modes 3 and 4 on PSBCH, the mapping uses symbols l = 4, 6 for the first slot in the sub- frame and symbol l = 2 for the second slot in the subframe. For sidelink transmission modes 1 and 2, the pseudo-random sequence generator used for scrambling is initialized at the start of each slot where PSSCH = 0, whereas for sidelink transmission modes 3 and 4, the pseudo-random sequence generator is initialized at the start of each slot where PSSCH mod2 = 0. For sidelink transmission modes 3 and 4, the quantity nx is equal to the decimal representation of the CRC on PSCCH transmitted in the same subframe as PSSCH according to nx = --- with parameters p and L defined in [3]. 7.4.3.2 Synchronization Signals Time and frequency synchronization are important aspects of cellular communication for interference management and mitigation, which are further extended to V2X communica- tion. A V2X-enabled device first needs to determine if it is in coverage of the network. A device is defined to be in-coverage based on signal quality measurement using the RSRP measurement performed on the downlink synchronization signals. When the measured RSRP values are above a specific threshold, the device considers itself in coverage and uses the base station downlink synchronization signals for timing and frequency alignment. This threshold is defined as part of broadcast system information. If the received signal quality measurement falls below the threshold, the device would start transmission of sidelink syn- chronization signals (SLSS) and the

PSBCH. These signals have a periodicity of 40 ms. Assuming the device cannot detect an eNB, due to possibly being out-of-coverage, the device starts looking for SLSS from other devices and performs signal quality measure- ments (S-RSRP) on those synchronization signals. The SLSS comprise a primary sidelink synchronization signal (PSSS) and a secondary sidelink synchronization signal (SSSS). The PSSS and SSSS are both transmitted in adjacent time slots in the same subframe (see Fig. 7.26). The combination of both signals defines a sidelink ID (SID), similar to the physi- cal cell ID transmitted in the downlink. The SIDs are split into two sets. The SIDs in the range of {0, 1, , 167} are reserved for in-coverage, whereas the SIDs {168, 169 are used when the device is out-of-coverage. The subframes to be used as radio resources to transmit SLSS and PBSCH are configured by higher layers and no PSDCH, PSCCH, or PSSCH transmissions are allowed in these subframes. The resource mapping is slightly dif- ferent for normal and extended cyclic prefix. Fig. 7.27 shows the mapping for normal cyclic prefix. In the frequency domain, the inner six RBs are reserved for SLSS and PSBCH trans- mission. More specifically, a physical layer sidelink synchronization identity is represented {0, 1, ., 335}, divided into two sets consisting of identities {0, 1, , 167} and {168, 169, ., 335}. The PSSS is transmitted in two adjacent SC-FDMA symbols in the same subframe. Each of the two sequences di(0), di(61), is used for the PSSS the two SC-FDMA symbols with root index 26 and u=37, otherwise. The sequence d(n) used for the primary synchronization signal is derived from a frequency Vehicle-to-Everything (V2X) Communications 831 Subframe 1 ms DMRS Location for Rel-12/13 PSCCH/PSSCH DMRS Location for Rel-12/13 PSBCH DMRS Location for V2V PSCCH/PSSCH DMRS Location for V2V PSBCH Figure 7.27 LTE V2X sidelink frame structure. domain Zadoff-Chu sequence according todu(n)exp-jun(n+1)/63])n=0,1,...,30 and du(n) = exp[-jru(n + 1)(n + ,61, where U denot

es the Zadoff- Chu root sequence index [3]. The sequence di(n) is multiplied with an amplitude scaling factor of 72/62BPSBCH and mapped to resource elements on a single antenna port in the first slot of the subframe according to ak.1=di(n)Vn= 0,...,61; k=n- NSLNRB/2 with l=1,2 (normal cyclic prefix) and l=0,1 (extended cyclic prefix). The SSSS is transmitted in two adjacent SC-FDMA symbols in the same subframe. Each of the two sequences d;(0),...,d,(61)Vi=1,2 is used for the SSSS. The sequence d(0),.. d(61) used for the second synchronization signal is an interleaved concatenation of two length-31 binary sequences. The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal assuming subframe 0 with and NIZ = NSL/168 for transmission modes 1 and 2, and subframe 5 for transmission modes 3 and 4 [3]. The sequence di(n) is multiplied with the amplitude scaling factor BSSSS in order to adjust the transmit power and mapped to resource elements on a single antenna port in the second slot in the subframe according to =di(n)Vn=0, 61; with l=4,5 (normal cyclic prefix) and l=3,4 (extended cyclic prefix) [3]. 7.5 Layer 2/3 Aspects The LTE layer-2 functions are divided into three sublayers: MAC, RLC, and PDCP. Fig. 7.28 depicts the layer-2 structure for the LTE-based sidelink. In this figure, the service access points (SAP) for peer-to-peer communication are marked with circles at the interface 832 Chapter 7 Radio bearers Security Security Security Security Segmentation Segmentation Reassembly Reassembly SBCCH SBCCH Logical channels Scheduling and priority handling Demultiplexing Multiplexing Packet filtering Transport channels SL-SCH(TX) SL-SCH(RX) SL-BCH(TX) SL-BCH(RX) SL-DCH(TX) SL-DCH(RX) Figure 7.28 Layer-2 structure for sidelink [6]. between sublayers. The SAP between the physical layer and the MAC sublayer provides the transport channels. The SAPs between the MAC sublayer and the RLC sublayer provide the logical channels. The multiplexing of several logical channels (i.

e., radio bearers) on the same transport channel is performed by the MAC sublayer. In sidelink communications, only one TB is generated per TTI. The sidelink specific services and functions of the MAC sublayer include radio resource selection and packet filtering for sidelink communication and V2X sidelink communication [6]. MAC sublayer provides different types of services that are represented by logical channels, where each logical channel is defined by the type of information it conveys. The logical channels are generally classified into two groups: (1) control channels (for the transfer of control-plane information) and (2) traffic channels (for the transfer of user-plane informa- tion), as shown in Fig. 7.29. The SBCCH is a sidelink logical channel for broadcasting side- link system information from one UE to another UE(s). STCH is a point-to-multipoint channel, for transfer of user information from one UE to another UE(s). This channel is used only by sidelink communication-capable UEs and V2X sidelink communication- capable UEs. Point-to-point communication between two sidelink communication-capable UEs is also realized with an STCH. As shown in Fig. 7.29, STCH logical channel can be mapped to SL-SCH transport channel and SBCCH logical channel can be mapped to SL-BCH transport channel. Sidelink Vehicle-to-Everything (V2X) Communications 833 STCH SBCCH Sidelink logical channels SL-DCH SL-SCH SL-BCH Sidelink transport channels Sidelink physical PSDCH PSSCH PSBCH PSCCH channels Figure 7.29 Mapping between sidelink logical, transport, and sidelink physical channels [6]. transport channels include SL-BCH, which is characterized by a predefined transport for- mat; SL-DCH that is characterized by a fixed size, predefined format, and periodic broad- cast transmission, as well as support for both UE autonomous resource selection and scheduled resource allocation by the eNB. It is subject to collision risk due to support of UE autonomous resource selection; however, no collision is expected when UE is allocated dedi

cated resources by the eNB. It further supports HARQ combining, but there is no sup- port for HARQ feedback. The sidelink transport channels further include SL-SCH, which has similar characteristics as the SL-DCH, as well as support for dynamic link adaptation by varying the transmit power, modulation, and coding [6]. In the sidelink, no HARQ feed- back is used and the retransmissions are always performed in a predefined/configured num- ber. Furthermore, the measurement gaps and sidelink discovery transmission during a sidelink discovery gap for transmission are of higher priority than HARQ retransmissions; that is, whenever a HARQ retransmission collides with a measurement gap or sidelink dis- covery transmission during a sidelink discovery gap for transmission, the HARQ retransmis- sion does not happen. One of the functions of the RRC sublayer is broadcasting the system information. Both LTE RRC states, RRC_IDLE and RRC_CONNECTED, support sidelink transmission and reception; sidelink discovery announcement and monitoring; and V2X sidelink transmis- sion and reception. The SystemInformationBlockType18 contains information related to sidelink communication; SystemInformationBlockType19 contains information related to sidelink discovery; and SystemInformationBlockType21 contains information related to V2X sidelink communication. The LTE RAN uses SL-RNTI and SL-V-RNTI to identify sidelink communication scheduling and V2X sidelink communication scheduling, respec- tively [6]. 834 Chapter 7 In order to assist an LTE base station in providing sidelink resources, a UE in RRC_CONNECTED state may report geographical location information to the serving eNB. The eNB can configure the UE to report the complete UE geographical location infor- mation based on periodic reporting via the existing measurement report signaling. Geographical zones can be configured by the eNB or preconfigured. When zones are (pre-) configured, the area is divided into geographical subdivisions using a single fixed reference point; that is, geograp

hical coordinates (0,0), length and width. The UE determines the zone identity by means of modulo operation using length and width of each zone, number of zones in length, number of zones in width, the single fixed reference point, and the geo- graphical coordinates of the UE's current location. The length and width of each zone, num- ber of zones in length, and number of zones in width are provided by the eNB when the UE is in-coverage, and preconfigured when the UE is out-of-coverage. The zone is configurable for both in-coverage and out-of-coverage. In an in-coverage scenario, when the UE uses autonomous resource selection, the eNB can provide the mapping between zone(s) and V2X sidelink transmission resource pools via RRC signaling. For out-of-coverage UEs, the map- ping between the zone(s) and V2X sidelink transmission resource pools can be preconfi- gured. If the mapping between zone(s) and V2X sidelink transmission resource pool is (pre- )configured, then the UE selects transmission sidelink resources from the resource pool cor- responding to the zone where it is presently located. The zone concept is not applied to exceptional V2X sidelink transmission pools as well as reception pools. Resource pools for V2X sidelink communication are not configured based on priority. For V2X sidelink transmission, during handover, transmission resource pool configurations including exceptional transmission resource pool for the target cell can be signaled in the handover command to minimize the transmission interruption. In this way, the UE may use the V2X sidelink transmission resource pools of the target cell before the handover is com- pleted, if either synchronization is performed with the target cell in case eNB is configured as synchronization source or synchronization is performed with GNSS in case GNSS is con- figured as synchronization source. If the exceptional transmission resource pool is included in the handover command, the UE uses randomly selected resources from the exceptional transmission resource pool,

starting from the reception of handover command. If the UE is configured with scheduled resource allocation in the handover command, then it continues to use the exceptional transmission resource pool while the timer associated with the hand- over is running. If the UE is configured with autonomous resource selection in the target cell, it continues to use the exceptional transmission resource pool until the sensing results on the transmission resource pools for autonomous resource selection are available. For exceptional cases (e.g., during radio link failure, during transition from RRC_IDLE to RRC_CONNECTED, or during change of dedicated V2X sidelink resource pools within a cell), the UE may select and temporarily use resources in the exceptional pool provided in serving cell's SIB21 message or in dedicated signaling based on random selection. During Vehicle-to-Everything (V2X) Communications 835 cell reselection, the RRC_IDLE UE may use the randomly selected resources from the exceptional transmission resource pool of the reselected cell until the sensing results on the transmission resource pools for autonomous resource selection are available [6]. To avoid interruption in receiving V2X messages due to delay in acquiring reception resource pools broadcast from the target cell, synchronization configuration and reception resource pool configuration for the target cell can be signaled to RRC_CONNECTED UEs in the handover command. For RRC_IDLE UE, it is up to UE implementation to minimize V2X sidelink transmission/reception interruption time associated with acquisition of SIB21 message of the target cell. A UE is considered in-coverage on the carrier used for V2X sidelink communication, when- ever it detects a cell on that carrier. If the UE that is authorized for V2X sidelink communi- cation is in-coverage on the frequency used for V2X sidelink communication or if the eNB provides V2X sidelink configuration for that frequency (including the case where the UE is out-of-coverage on that frequency), the UE uses t

he scheduled resource allocation or UE autonomous resource selection according to eNB configuration. When the UE is out-of- coverage on the frequency used for V2X sidelink communication and if the eNB does not provide V2X sidelink configuration for that frequency, the UE may use a set of transmission and reception resource pools preconfigured in the UE. The V2X sidelink communication resources are not shared with other non-V2X data transmitted over sidelink. An RRC_CONNECTED UE may send a sidelink UE information message to the serving cell, when it wishes to establish V2X sidelink communication in order to request sidelink resources If the UE is configured by upper layers to receive V2X sidelink communication and V2X sidelink reception resource pools are provided, then the UE will receive sidelink communi- cation on the allocated resources. The reception of V2X sidelink communication on differ- ent carriers or from different PLMNs can be supported by incorporating multiple receivers in the UE. For sidelink SPS, up to eight SPS configurations with different parameters can be configured by the eNB and all SPS configurations can be simultaneously active. The activation/deactivation of SPS configuration is signaled via PDCCH by the eNB. The exist- ing logical channel prioritization based on PPPP is used for sidelink SPS. The UE can pro- vide supplementary information to the eNB which configures the reporting of such information for V2X sidelink communication. The latter information includes traffic charac- teristic parameters (e.g., a set of preferred SPS interval, timing offset with respect to sub- frame 0 of the SFN 0, PPPP and maximum TB size based on the observed traffic pattern) related to the SPS configuration. The UE supplementary information can be reported regard- less of whether SPS is configured. Triggering of UE supplementary information transmis- sion is implementation specific. For instance, the UE can report its supplementary information when a change in estimated periodicity and/or timing offset of pa

cket arrival 836 Chapter 7 occurs. The scheduling request mask per traffic type is not supported for V2X sidelink com- munication. The serving cell can provide synchronization configuration for the V2X side- link carrier. In this case, the UE follows the synchronization configuration received from serving cell. If no cell is detected on the carrier used for V2X sidelink communication and the UE does not receive synchronization configuration from serving cell, the UE follows preconfigured synchronization procedure. There are three possible synchronization nodes; that is, eNB, UE, and GNSS. In case GNSS is configured as synchronization source, the UE utilizes the universal time and the (pre)configured direct frame number (DFN) offset to cal- culate DFN and subframe number. If the eNB timing is configured as synchronization refer- ence for the UE in order to perform synchronization and conduct downlink measurements, the UE follows the cell associated with the acquired frequency when in-coverage. The UE can indicate the current synchronization reference type to the eNB. One transmission pool for scheduled resource allocation is configured, considering the synchronization reference of the UE. For controlling channel utilization, the network can indicate how the UE adapts its transmis- sion parameters for each transmission pool depending on the channel busy ratio (CBR). The UE measures all configured transmission resource pools including the exceptional resource pool. If a resource pool is pre)configured such that a UE always transmits PSCCH and PSSCH in adjacent RBs, then the UE measures PSCCH and PSSCH resources together. If a resource pool is (pre)configured such that a UE may transmit PSCCH and the corresponding PSSCH in non-adjacent RBs in a subframe, then PSSCH resource pool and PSCCH resource pool are measured separately. A UE in RRC_CONNECTED state can be configured to report CBR measurement results. For CBR reporting, periodic reporting and event-triggered reporting are supported. Two reporting events are intr

oduced for event-triggered CBR reporting. In case PSSCH and PSCCH resources are placed non-adjacently, only PSSCH resource pool measure- ment is used for event-triggered CBR reporting. In case PSSCH and PSCCH resources are placed adjacently, CBR measurements of both PSSCH and PSCCH resources are used for event-triggered CBR reporting. Event-triggered CBR reporting is triggered by overloaded threshold and/or less-loaded threshold. The network can configure which of the transmission pools the UE needs to report [6,8]. A UE (regardless of its RRC state) performs transmission parameter adaptation based on the CBR. If PSSCH and PSCCH resources are placed non-adjacently, only PSSCH pool measurement is used for transmission parameter adaptation. However, if PSSCH and PSCCH resources are placed adjacently, CBR measurements of both PSSCH and PSCCH resources are used for transmission parameter adaptation. When CBR measurements are not available, the default transmission parameters are utilized. The exemplary adapted transmis- sion parameters include maximum transmission power, range of the number of retransmis- sions per TB, range of PSSCH RB number, range of MCS, and the maximum limit on channel occupancy ratio. The transmission parameter adaption applies to all transmission resource pools including exceptional resource pools [5,6]. Vehicle-to-Everything (V2X) Communications 837 In V2X sidelink communication, sidelink transmission and/or reception resources including exceptional resource pool are provided via dedicated signaling, SIB21 message, and/or pre- configuration for different frequencies in both scheduled resource allocation and UE autono- mous resource selection scenarios. The serving cell may signal only the frequency on which the UE should acquire the resource configuration for V2X sidelink communication. If multi- ple frequencies and associated resource information are provided, then it is up to UE to select a frequency among the permissible frequencies. The UE does not use preconfigured transmis- sion resourc

e, if it detects a cell providing resource configuration or inter-carrier resource con- figuration for V2X sidelink communication. An RRC_IDLE UE may prioritize the frequency that provides cross-carrier resource configuration for V2X sidelink communication over other choices during cell reselection. If the UE supports multiple transmission chains, it may simul- taneously transmit on multiple carriers via PC5. In that case, a mapping between V2X service types and the suitable V2X frequencies is configured by upper layers. For scheduled resource allocation, the eNB can schedule a V2X transmission on a frequency based on the sidelink buffer status report, in which the UE includes the destination index uniquely associated with a frequency reported by the UE to the eNB in sidelink UE information message [6,8]. The UE may receive the V2X sidelink communication of other PLMNs. The serving cell can directly signal the resource configuration for V2X sidelink communication in an inter- PLMN operation or indirectly via the frequency on which the UE may acquire the inter- PLMN resource configuration. Note that the V2X sidelink communication transmission in other PLMNs is not permissible. When uplink transmission overlaps in time domain with V2X sidelink transmission on the same frequency, the UE prioritizes the latter over the for- mer, if the PPPP of sidelink MAC PDU is lower than a (pre-)configured PPPP threshold; otherwise, the UE prioritizes the uplink transmission over the V2X sidelink transmission. When uplink transmission overlaps in time domain with V2X sidelink transmission in dif- ferent frequency, the UE may prioritize the V2X sidelink transmission over the uplink trans- mission or may reduce uplink transmission power, if the PPPP of sidelink MAC PDU is lower than a (pre-)configured PPPP threshold; otherwise, the UE prioritizes the uplink trans- mission over the V2X sidelink transmission or reduces V2X sidelink transmission power. However, if uplink transmission is prioritized by upper layer or random-access proce