RACH RESOURCE CONFIGURATION FOR MULTIPLE PRACH TRANSMISSIONS

In an aspect of the disclosure, a method, a computer-readable medium, and an apparatus are provided. The apparatus may be a UE. The UE receives RACH occasion (RO) configurations from a base station. The RO configurations indicate associations between one or more SSBs and ROs. The UE selects one SSB from the one or more SSBs received from the base station. The UE determines a number of multiple PRACH transmissions based on a RSRP measurement of the selected SSB. The UE determines a set of RO groups associated with the selected SSB based on the RO configurations and the determined number of multiple PRACH transmissions. The UE transmits multiple PRACHs on a selected RO group from the determined set of RO groups.

BACKGROUND

Field

The present disclosure relates generally to communication systems, and more particularly, to techniques of performing multiple physical random access channel (PRACH) preamble transmissions in a repetition at user equipment (UE).

Background

SUMMARY

In an aspect of the disclosure, a method, a computer-readable medium, and an apparatus are provided. The apparatus may be a UE. The UE receives random access channel (RACH) occasion (RO) configurations from a base station. The RO configurations indicate associations between one or more synchronization signal blocks (SSBs) and ROs. The UE selects one SSB from the one or more SSBs received from the base station. The UE determines a number of multiple PRACH transmissions based on a RSRP measurement of the selected SSB. The UE determines a set of RO groups associated with the selected SSB based on the RO configurations and the determined number of multiple physical random access channel (PRACH) transmissions. The UE transmits multiple PRACHs on a selected RO group from the determined set of RO groups.

DETAILED DESCRIPTION

The base stations102configured for 4G LTE (collectively referred to as Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN)) may interface with the EPC160through backhaul links132(e.g., SI interface). The base stations102configured for 5GNR (collectively referred to as Next Generation RAN (NG-RAN)) may interface with core network190through backhaul links184. In addition to other functions, the base stations102may perform one or more of the following functions: transfer of user data, radio channel ciphering and deciphering, integrity protection, header compression, mobility control functions (e.g., handover, dual connectivity), inter cell interference coordination, connection setup and release, load balancing, distribution for non-access stratum (NAS) messages, NAS node selection, synchronization, radio access network (RAN) sharing, multimedia broadcast multicast service (MBMS), subscriber and equipment trace, RAN information management (RIM), paging, positioning, and delivery of warning messages. The base stations102may communicate directly or indirectly (e.g., through the EPC160or core network190) with each other over backhaul links134(e.g., X2 interface). The backhaul links134may be wired or wireless.

Although the present disclosure may reference 5G New Radio (NR), the present disclosure may be applicable to other similar areas, such as LTE, LTE-Advanced (LTE-A), Code Division Multiple Access (CDMA), Global System for Mobile communications (GSM), or other wireless/radio access technologies.

At the UE250, each receiver254RX receives a signal through its respective antenna252. Each receiver254RX recovers information modulated onto an RF carrier and provides the information to the receive (RX) processor256. The TX processor268and the RX processor256implement layer 1 functionality associated with various signal processing functions. The RX processor256may perform spatial processing on the information to recover any spatial streams destined for the UE250. If multiple spatial streams are destined for the UE250, they may be combined by the RX processor256into a single OFDM symbol stream. The RX processor256then converts the OFDM symbol stream from the time-domain to the frequency domain using a Fast Fourier Transform (FFT). The frequency domain signal comprises a separate OFDM symbol stream for each subcarrier of the OFDM signal. The symbols on each subcarrier, and the reference signal, are recovered and demodulated by determining the most likely signal constellation points transmitted by the base station210. These soft decisions may be based on channel estimates computed by the channel estimator258. The soft decisions are then decoded and deinterleaved to recover the data and control signals that were originally transmitted by the base station210on the physical channel. The data and control signals are then provided to the controller/processor259, which implements layer 3 and layer 2 functionality.

The controller/processor259can be associated with a memory260that stores program codes and data. The memory260may be referred to as a computer-readable medium. In the UL, the controller/processor259provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, and control signal processing to recover IP packets from the EPC160. The controller/processor259is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations.

Channel estimates derived by a channel estimator258from a reference signal or feedback transmitted by the base station210may be used by the TX processor268to select the appropriate coding and modulation schemes, and to facilitate spatial processing. The spatial streams generated by the TX processor268may be provided to different antenna252via separate transmitters254TX. Each transmitter254TX may modulate an RF carrier with a respective spatial stream for transmission. The UL transmission is processed at the base station210in a manner similar to that described in connection with the receiver function at the UE250. Each receiver218RX receives a signal through its respective antenna220. Each receiver218RX recovers information modulated onto an RF carrier and provides the information to a RX processor270.

The controller/processor275can be associated with a memory276that stores program codes and data. The memory276may be referred to as a computer-readable medium. In the UL, the controller/processor275provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover IP packets from the UE250. IP packets from the controller/processor275may be provided to the EPC160. The controller/processor275is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations.

New radio (NR) may refer to radios configured to operate according to a new air interface (e.g., other than Orthogonal Frequency Divisional Multiple Access (OFDMA)-based air interfaces) or fixed transport layer (e.g., other than Internet Protocol (IP)). NR may utilize OFDM with a cyclic prefix (CP) on the uplink and downlink and may include support for half-duplex operation using time division duplexing (TDD). NR may include Enhanced Mobile Broadband (eMBB) service targeting wide bandwidth (e.g. 80 MHz beyond), millimeter wave (mmW) targeting high carrier frequency (e.g. 60 GHz), massive MTC (mMTC) targeting non-backward compatible MTC techniques, and/or mission critical targeting ultra-reliable low latency communications (URLLC) service.

A single component carrier bandwidth of 100 MHz may be supported. In one example, NR resource blocks (RBs) may span 12 sub-carriers for each RB with a subcarrier spacing (SCS) of 60 kHz over a 0.25 ms duration or a SCS of 30 kHz over a 0.5 ms duration (similarly, 15 kHz SCS over a 1 ms duration). Each radio frame may consist of 10 subframes (10, 20, 40 or 80 NR slots) with a length of 10 ms. Each slot may indicate a link direction (i.e., DL or UL) for data transmission and the link direction for each slot may be dynamically switched. Each slot may include DL/UL data as well as DL/UL control data. UL and DL slots for NR may be as described in more detail below with respect toFIGS.5and6.

FIG.3illustrates an example logical architecture of a distributed RAN300, according to aspects of the present disclosure. A 5G access node306may include an access node controller (ANC)302. The ANC may be a central unit (CU) of the distributed RAN. The backhaul interface to the next generation core network (NG-CN)304may terminate at the ANC. The backhaul interface to neighboring next generation access nodes (NG-ANs)310may terminate at the ANC. The ANC may include one or more TRPs308(which may also be referred to as BSs, NR BSs, Node Bs, 5G NBs, APs, or some other term). As described above, a TRP may be used interchangeably with “cell.”

The local architecture of the distributed RAN300may be used to illustrate fronthaul definition. The architecture may be defined that support fronthauling solutions across different deployment types. For example, the architecture may be based on transmit network capabilities (e.g., bandwidth, latency, and/or jitter). The architecture may share features and/or components with LTE. According to aspects, the next generation AN (NG-AN)310may support dual connectivity with NR. The NG-AN may share a common fronthaul for LTE and NR.

The architecture may enable cooperation between and among TRPs308. For example, cooperation may be preset within a TRP and/or across TRPs via the ANC302. According to aspects, no inter-TRP interface may be needed/present.

According to aspects, a dynamic configuration of split logical functions may be present within the architecture of the distributed RAN300. The PDCP, RLC, MAC protocol may be adaptably placed at the ANC or TRP.

FIG.4illustrates an example physical architecture of a distributed RAN400, according to aspects of the present disclosure. A centralized core network unit (C-CU)402may host core network functions. The C-CU may be centrally deployed. C-CU functionality may be offloaded (e.g., to advanced wireless services (AWS)), in an effort to handle peak capacity. A centralized RAN unit (C-RU)404may host one or more ANC functions. Optionally, the C-RU may host core network functions locally. The C-RU may have distributed deployment. The C-RU may be closer to the network edge. A distributed unit (DU)406may host one or more TRPs. The DU may be located at edges of the network with radio frequency (RF) functionality.

FIG.5is a diagram500showing an example of a DL-centric slot. The DL-centric slot may include a control portion502. The control portion502may exist in the initial or beginning portion of the DL-centric slot. The control portion502may include various scheduling information and/or control information corresponding to various portions of the DL-centric slot. In some configurations, the control portion502may be a physical DL control channel (PDCCH), as indicated inFIG.5. The DL-centric slot may also include a DL data portion504. The DL data portion504may sometimes be referred to as the payload of the DL-centric slot. The DL data portion504may include the communication resources utilized to communicate DL data from the scheduling entity (e.g., UE or BS) to the subordinate entity (e.g., UE). In some configurations, the DL data portion504may be a physical DL shared channel (PDSCH).

The DL-centric slot may also include a common UL portion506. The common UL portion506may sometimes be referred to as an UL burst, a common UL burst, and/or various other suitable terms. The common UL portion506may include feedback information corresponding to various other portions of the DL-centric slot. For example, the common UL portion506may include feedback information corresponding to the control portion502. Non-limiting examples of feedback information may include an ACK signal, a NACK signal, a HARQ indicator, and/or various other suitable types of information. The common UL portion506may include additional or alternative information, such as information pertaining to random access channel (RACH) procedures, scheduling requests (SRs), and various other suitable types of information.

As illustrated inFIG.5, the end of the DL data portion504may be separated in time from the beginning of the common UL portion506. This time separation may sometimes be referred to as a gap, a guard period, a guard interval, and/or various other suitable terms. This separation provides time for the switch-over from DL communication (e.g., reception operation by the subordinate entity (e.g., UE)) to UL communication (e.g., transmission by the subordinate entity (e.g., UE)). One of ordinary skill in the art will understand that the foregoing is merely one example of a DL-centric slot and alternative structures having similar features may exist without necessarily deviating from the aspects described herein.

FIG.6is a diagram600showing an example of an UL-centric slot. The UL-centric slot may include a control portion602. The control portion602may exist in the initial or beginning portion of the UL-centric slot. The control portion602inFIG.6may be similar to the control portion502described above with reference toFIG.5. The UL-centric slot may also include an UL data portion604. The UL data portion604may sometimes be referred to as the pay load of the UL-centric slot. The UL portion may refer to the communication resources utilized to communicate UL data from the subordinate entity (e.g., UE) to the scheduling entity (e.g., UE or BS). In some configurations, the control portion602may be a physical DL control channel (PDCCH).

As illustrated inFIG.6, the end of the control portion602may be separated in time from the beginning of the UL data portion604. This time separation may sometimes be referred to as a gap, guard period, guard interval, and/or various other suitable terms. This separation provides time for the switch-over from DL communication (e.g., reception operation by the scheduling entity) to UL communication (e.g., transmission by the scheduling entity). The UL-centric slot may also include a common UL portion606. The common UL portion606inFIG.6may be similar to the common UL portion506described above with reference toFIG.5. The common UL portion606may additionally or alternatively include information pertaining to channel quality indicator (CQI), sounding reference signals (SRSs), and various other suitable types of information. One of ordinary skill in the art will understand that the foregoing is merely one example of an UL-centric slot and alternative structures having similar features may exist without necessarily deviating from the aspects described herein.

In a centralized network, for example with a next generation base station (gNB), and/or coordination through the core network for a group of gNBs, the time/frequency resources for transmission can be well controlled to reduce interference. This means the transmission by UEs is controlled by the network (base stations), and the transmission among gNBs can also be coordinated to reduce interference with each other.

The sidelink is the transmission between UEs. There are generally two cases where the centralized mechanism applies:1. The transmission between a group of UEs is coordinated/determined by the gNB(s).2. The transmission between a group of UEs is coordinated/determined by a master UE, when there is no network. This means a certain level of centralized mechanism may be feasible without a base station.

For a decentralized mechanism, the transmission between a group of UEs may rely on some mechanisms to reduce interference:1. Sensing the time/frequency resources based on RSRP threshold to find out potential resources for transmission. Then select the resources that are not occupied (lower RSRP) for transmission.2. When a UE transmits a signal in a slot, the UE can also deliver some information that the time/frequency resources are occupied. The UE can indicate in the control channel (e.g., Sidelink Control Information, SCI) that the current slot and a set of future slots are being reserved/occupied. Thus, when other UEs decode the SCI, they can know the occupancy status of future slots.

The present disclosure aims to enhance the current RACH framework to support multiple PRACH transmissions in an optimized manner. Specifically, the techniques described infra may address the following aspects of a RACH procedure when a UE is transmitting multiple PRACH repetitions:1) PRACH preambles to transmit for contention based random access (CBRA).2) Time-frequency resources to use for transmitting a set of PRACH repetitions.3) Resource efficiency optimization in the cell while minimizing the RACH delay experienced by the user?4) Minimization of the rate of collision probability between different users attempting PRACH transmissions in the same cell. This includes:(4a) A potential collision between a legacy user (with a single PRACH transmission) and a new user (with multiple PRACH transmissions).(4b) A potential collision between a new user (with multiple PRACH transmissions) and another new user (with multiple PRACH transmissions).

If a UE is configured to transmit multiple PRACH transmissions, a set of RACH occasions needs to be identified based on the detected strongest SSB index. A UE may transmit a PRACH preamble repetition set or bundle. All PRACH preamble transmissions in the same repetition set/bundle are transmitted according to the same SSB index, which has been identified to be the strongest SSB by the UE.

Once the UE determines the index of the strongest SSB in a cell, the UE can determine the set of RACH resources that are allowed to be used for the transmission of the current PRACH preamble repetition set/bundle based on the mapping configuration between RACH occasions and the strongest SSB index.

Furthermore, such mapping association also requires enhancements over the legacy RACH configuration method. Since the UE can transmit multiple PRACH preamble in the same repetition set, a set of RACH resources shall be determined by the UE.

It should be understood that a RACH occasion generally refers to the total number of times or opportunities that RACH is used or can be used within a specific time window. This is usually related to the overall transmission time structure of the system, including a certain number of slots or times allocated for RACH. A RACH resource occasion generally more specifically refers to a wireless resource block that can be allocated to a specific user or data flow within a given RACH occasion. Therefore, a RACH resource occasion can be seen as a more fine-grained resource allocation within a RACH occasion.

FIG.7is a diagram700illustrating a first scheme of PRACH preamble repetition transmissions between a base station702and multiple UEs704-1to704-N. The base station702transmits multiple SSB signals labeled SSB 0, SSB 1, . . . , SSB K to the UEs704-1,704-2, . . . ,704-N. Each SSB signal has a different spatial direction and is associated with a specific index number. The UE measures the received signal strengths of the different SSB signals and determines the index of the strongest SSB signal. The index of the strongest SSB detected by each UE is then used to determine the PRACH resources and preamble sequences that can be used by that UE, based on mapping configurations pre-defined by the base station702.

In this example, the RACH resources configured for the UEs704-1,704-2, . . . ,704-N has a periodicity of 160 ms. In each period of 160 ms, each of the UEs704-1,704-2, . . . ,704-N may select one or more of the configured RACH resources to transmit PRACH preambles as described infra.

More specifically, a UE may use multiple RACH resources in a RACH configuration period (e.g., 160 ms) to transmit multiple PRACH preambles (in a bundle) in repetition. The same mapping association between SSB and RACH resources can be reused within the same RACH configuration period. In particular, the UE may determine, at least based on a reference signal received power (RSRP) measurement of the selected SSB, the number of multiple PRACH transmissions.

In addition, the number of RACH occasions (ROs) can be limited for some numbers of multiple PRACH preamble transmissions in a given time instance. For example, for a bundle of 4 PRACH preamble transmissions, there may be 4 available ROs for the 2nd PRACH preamble transmission and only one available RO for the 4th PRACH preamble transmission.

In certain configurations, the UE may determine a subset of PRACH preambles to use based on the number of multiple PRACH transmissions it decides to transmit. For example, the UE may determine to transmit 4 PRACH transmissions, and accordingly determine a subset of preambles configured for 4 transmission PRACH bundles by the base station. Alternatively, the base station may directly indicate the subset of preambles to use based on the PRACH transmission number.

Furthermore, the base station may pre-configure associations between each SSB, each candidate number of multiple PRACH transmissions (e.g. 2, 4, etc.), and a corresponding set of PRACH preambles. Based on the SSB selected and number of transmissions determined, the UE can determine the specific set of preambles to use from the pre-configured mappings.

Additionally, the number of multiple PRACH transmissions may also be pre-configured by the base station through an indication to the UEs (claim14). The UE can determine how many transmissions to use based on this indication, in addition to metrics like the RSRP measurement.

InFIG.7, the horizontal axis is time domain, the vertical axis is frequency domain. In this example, K is 15. Further, the base station702configures the UEs704-1,704-2, . . . ,704-N to transmit initial PRACH preambles corresponding to SSBs 0 to 7 at particular ROs at 0 ms in a RACH configuration period (e.g., 160 ms) and to transmit initial PRACH preambles corresponding to SSBs 8 to 15 at ROs at 10 ms in the RACH configuration period. The UE uses one of these ROs for a single PRACH preamble transmission.

In certain configurations, for multiple PRACH preamble transmissions, the UE transmits the selected PRACH preamble corresponding to a particular target SSB in a bundle of ROs as described infra.

The base station702broadcasts RO configurations that specifies various ROs to be used for PRACH preamble transmissions. In this example, for PRACH preambles corresponding to SSBs 0-7, the base station702configured ROs at 2 ms, 4 ms, and 6 ms in addition to ROs at 0 ms.

In this example, assuming that the UE704-1detects SSB 4 as the strongest SSB, the UE704-1accordingly selects one from a set of PRACH preambles corresponding to SSB 4 to use. The UE704-1may transmit the PRACH preamble (corresponding to SSB 4) at multiple corresponding ROs. In certain configurations, the UE704-1may determine the number of ROs in the bundle by itself. In certain configurations, the base station702may have previously indicated a PRACH repetition configuration specifying the number of ROs in a bundle. In one scenario, the UE704-1may determine that a bundle of 2 ROs should be used. Accordingly, the UE704-1sends the PRACH preamble (corresponding to SSB 4) at the initial two corresponding ROs, which are a RO720at 0 ms and a RO722at 2 ms of the RACH configuration period (e.g., 160 ms). In another scenario, the UE704-1may determine that a bundle of 4 ROs should be used. Accordingly, the UE704-1sends the PRACH preamble (corresponding to SSB 4) at the initial4corresponding ROs, which are the RO720at 0 ms, the RO722at 2 ms, a RO724at 4 ms, and a RO726at 6 ms of the RACH configuration period (e.g., 160 ms).

Further, in this example, the base station702configures ROs for PRACH preamble repetition in time domain only. That is, different ROs at different time points are configured for repeat transmission of a particular PRACH preamble. However, no multiple PRACH preamble transmissions in repetition are configured for the same at time point.

Further, in certain scenarios, while the UE704-1determines that the strongest SSB detected is SSB 4, the UE704-2may determine that the strongest SSB detected is SSB 3. In the configuration shown inFIG.7, the ROs corresponding to SSBs 3 and 4 do not overlap at 0 ms and 4 ms, but overlaps at 2 ms and 6 ms of the RACH configuration period (e.g., 160 ms). That is, the UE704-1and the UE704-2may both transmit PRACH preambles at the RO722and at the RO726. Nonetheless, the base station702may configure different orthogonal sets of PRACH preambles corresponding to SSB 3 and SSB 4. Accordingly, the base station702may distinguish one PRACH preamble (corresponding to SSB 3) from the other PRACH preamble (corresponding to SSB 4) when receive them at the RO722and the RO726.

Although the example inFIG.7, ROs for PRACH preamble repetitions are only added at 2 ms, 4 ms and 6 ms between ROs for initial PRACH preambles at 0 ms and 10 ms, the time points and quantities of these ROs for PRACH preamble repetitions are flexible and configurable. For example, ROs may be added at 8 ms for a 5th transmission. Further, the repetition transmission gap between two ROs at different time is configurable. In this example, the repetition transmission gap is 2 ms (e.g., between the RO722and the RO724). In another example, the repetition transmission gap may be another duration such as 3 ms, 4 ms, etc.

Using the techniques as described supra, when the PRACH preambles received at the base station702are not strong enough (e.g., when the UE704-1at the edge of the cell), the base station702can combine the PRACH preambles received at multiple repetition ROs and then decode the combined signals.

In certain configurations, the number of repetition transmissions (e.g., 2, 3, 4, etc.) is preconfigured by the base station702. Otherwise, the base station702may need to determine by itself whether the multiple PRACH preamble repetition is used and the configuration of the repetition. The configuration of the first scheme may improve latency and resource efficiency.

FIG.8is a diagram800illustrating a second scheme of PRACH preamble repetition transmissions. In this example, K is 31 and the base station702transmits SSBs 0 to 31 to the UEs704-1,704-2, . . . ,704-N. For example, within the RACH configuration period (e.g., 160 ms), the base station702may configure initial PRACH preamble transmissions at ROs with an initial transmission gap of 10 ms. As shown, the base station702configures ROs for transmissions of PRACH preambles corresponding to SSBs 0-7 at 0 ms, for transmissions of PRACH preambles corresponding to SSBs 8-15 at 10 ms, for transmissions of PRACH preambles corresponding to SSBs 16-23 at 20 ms, and for transmissions of PRACH preambles corresponding to SSBs 24-31 at 30 ms. In this example, the duration from after 30 ms to 160 ms are not configured for initial PRACH preamble transmissions but may be used for repetition PRACH preamble transmissions.

In this scheme, ROs that may be used for initial PRACH preambles are reconfigured for repetition PRACH preamble transmissions. A UE transmitting a PRACH preamble repetition set would follow the pre-existing mapping association, but such a repetition set could be transmitted across multiple ROs in a RACH configuration period (e.g., 160 ms).

In this example, the ROs corresponding to SSB 0-7 are repeated starting at 40 ms with a repetition transmission gap of 40 ms. In other words, the same ROs corresponding to SSB 0-7 are configured at 10 ms, 40 ms, 80 ms, 120 ms within the RACH configuration period (e.g., 160 ms).

In this example, assuming that the UE704-1detects that the strongest SSB received is SSB 4, the UE704-1may transmit PRACH preamble (corresponding to SSB 4) at ROs820,822,824,826. In certain configurations, the UE704-1can determine the number of ROs (e.g., 2, 4, etc.) in a bundle for repetition PRACH preamble transmissions. In certain configurations, the base station702may configure the RO bundle. For a repetition of 2 transmission, the UE704-1transmits PRACH preamble (corresponding to SSB 4) at ROs820,822. For a repetition of 4 transmission, the UE704-1transmits PRACH preamble (corresponding to SSB 4) at ROs820,822,824,826.

FIG.9is a diagram900illustrating a third scheme of PRACH preamble repetition transmissions. In this example, K is 15. Further, the base station702configures the UEs704-1,704-2, . . . ,704-N to transmit initial PRACH preambles corresponding to SSBs 0 to 7 at particular ROs at 0 ms in a RACH configuration period (e.g., 160 ms) and to transmit initial PRACH preambles corresponding to SSBs 8 to 15 at ROs at 10 ms in the RACH configuration period. The UE uses one of these ROs for a single PRACH preamble transmission.

In this example, the ROs corresponding to SSB 0-7 are repeated after 0 ms and prior to 10 ms with a repetition transmission gap of 2 ms for 3 additional transmissions. In other words, the same ROs corresponding to SSB 0-7 are configured at 0 ms, 2 ms, 4 ms, and 6 ms within the RACH configuration period (e.g., 160 ms). The ROs at each of the repetition time point has the same frequency division multiplexing (FDM) pattern.

In this example, assuming that the UE704-1detects that the strongest SSB received is SSB 4, the UE704-1may transmit PRACH preamble (corresponding to SSB 4) at ROs920,922,924,926. In certain configurations, the UE704-1can determine the number of ROs (e.g., 2, 4, etc.) in a bundle for repetition PRACH preamble transmissions. In certain configurations, the base station702may configure the RO bundle. For a repetition of 2 transmission, the UE704-1transmits PRACH preamble (corresponding to SSB 4) at ROs920,922. For a repetition of 4 transmission, the UE704-1transmits PRACH preamble (corresponding to SSB 4) at the RO920at 0 ms, RO922at 2 ms, RO924at 4 ms, and RO926at 6 ms. The mapping between SSB 4 and these ROs is preconfigured by the base station702.

Similarly, the ROs corresponding to SSB 8 to 15 can be repeated after the initial transmission of PRACH preambles corresponding to those SSBs at 10 ms at 12 ms, 14 ms, and 16 ms. Assuming that the UE704-1has selected a SSB from the SSBs 8 to 15 and the repetition has 4 transmissions, the UE704-1may transmit the corresponding PRACH preambles at 10 ms, 12 ms, 14 ms, and 16 ms.

Referring back toFIG.7, in the first scheme, unlike third scheme, a different FDM pattern from the ROs for initial PRACH preamble transmissions are used for better resource efficiency. In the first scheme, the ROs for repetition PRACH preamble transmissions may be mapped to resources different from the resources of ROs for initial PRACH transmissions. For example, at 0 ms, the ROs corresponding to SSBs 0 to 7 are split across 4 resources, and at 2 ms, the ROs corresponding to the same set of SSBs are across 3 resources. Continuing, at 4 ms, the ROs corresponding to the same set of SSBs are two resources, and at 6 ms the ROs corresponding to the same set of SSBs is fully allocated in 1 resource.

In the first scheme, the mapping between the SSBs and RACH resources may change over time. For example, at 0 ms, SSBs 0-7 may be distributed across 4 RACH resources. But at 2 ms, the same set of SSBs 0-7 is redistributed across only 3 RACH resources, and so on. This allows the base station702to limit the number of available RACH resources for later transmissions in a PRACH repetition bundle, providing a tradeoff between resource efficiency and collision probability. The UE704-1can transmit an initial PRACH preamble corresponding to its detected strongest SSB (e.g. SSB 4) at 0 ms, and then transmit repetitions at 2 ms, 4 ms, 6 ms based on the configured mapping. Thus, the first scheme provides more flexibility in SSB-to-RACH resource mapping over time.

Moreover, in the first scheme, different resource configurations can be considered for the mapping between SSBs and RACH resources.FIG.10is a diagram1000illustrating a first resource configuration for mapping between SSBs and RACH resources. This configuration allows for more ROs1022for repetition PRACH preamble transmissions for UEs. This provides more transmission opportunities for the UEs at the cost of lower resource efficiency.

FIG.11is a diagram1100illustrating a second resource configuration for mapping between SSBs and RACH resources. This configuration has fewer ROs1122for repetition PRACH preamble transmissions and is a more resource efficient configuration. This is a more resource efficient configuration, but provides fewer transmission opportunities for the UEs.

Therefore, the base station702can flexibly configure these mapping associations to achieve different trade-offs between resource efficiency and transmission opportunities for PRACH repetitions.

FIG.12is a flow chart12000of a method (process) for preforming multiple PRACH transmissions. The method may be performed by a UE (e.g., the UE704, the UE250). In operation1202, the UE receives, from a base station, random access channel (RACH) occasion (RO) configurations indicating associations between one or more synchronization signal blocks (SSBs) and ROs. In operation1204, the UE selects one SSB from the one or more SSBs received from the base station.

In operation1206, the UE determines, at least based on a reference signal received power (RSRP) measurement of the selected SSB, a number of multiple PRACH transmissions. In operation1208, the UE determines, based on the RO configurations and the determined number of multiple PRACH transmissions, a set of RO groups associated with the selected SSB for transmitting the determined number of multiple PRACH transmission. In operation1210, the UE determines a subset of PRACH preambles based on the determined number of multiple PRACH transmissions. In operation1212, the UE selects a PRACH preamble to be transmitted over the multiple PRACHs on the selected RO group from the subset of PRACH preambles.

In certain configurations, the subset of PRACH preambles is determined based on an indication received from the base station. In certain configurations, the RO configurations indicate a set of PRACH preambles associated with each SSB of the one or more SSBs and each candidate number of multiple PRACH transmissions. The UE further determines one set of PRACH preambles based on the selected SSB and the determined number of multiple PRACH transmissions. The UE further selects a PRACH preamble from the determined one set of PRACH preambles to be transmitted over the multiple PRACHs on the selected RO group. In certain configurations, the UE determines the each candidate number of multiple PRACH transmissions based on an indication received from the base station.

In operation1214, the UE transmits multiple PRACHs on a selected RO group from the determined set of RO groups.

In certain configurations, the RO configurations indicate: a first RO group (RO720, RO722) for a first number (e.g., 2) of multiple PRACH transmissions corresponding to a given SSB (e.g., SSB 4); and a second RO group (e.g., RO720, RO722, RO724, RO726) for a second number (e.g., 4) of multiple PRACH transmissions corresponding to the given SSB, wherein the first number is different from the second number.

In certain configurations, the RO configurations indicate: a first RO group for multiple PRACH transmissions corresponding to a first SSB; and a second RO group for multiple PRACH transmissions corresponding to a second SSB.

In certain configurations, the first RO group (e.g., RO920, RO922, RO924, RO926corresponding to SSB 4) and the second RO group (e.g., RO930, RO932, RO934, RO936corresponding to SSB 1) do not overlap in time domain or frequency domain. The number of multiple PRACHs transmitted on the first RO group and the number of multiple PRACHs transmitted on the second RO group may be the same. In certain configurations, multiple PRACH preambles transmitted on the first RO group and the number of multiple PRACH preambles transmitted on the second RO group may be the same.

In certain configurations, the first RO group (e.g., RO920, RO922corresponding to SSB 4) fully overlaps with the second RO group (e.g., RO920, RO922, RO924, RO926corresponding to SSB 5) in time domain and frequency domain. The number (e.g., 2) of multiple PRACHs transmitted on the first RO group and the number (e.g., 4) of multiple PRACHs transmitted on the second RO group may be different. In certain configurations, multiple PRACH preambles transmitted on the first RO group and multiple PRACH preambles transmitted on the second RO group may be different.

In certain configurations, some but not all ROs of the first RO group (e.g., RO720, RO722, RO724, RO726corresponding to SSB 4) overlap with the second RO group (e.g., RO730, RO732, RO724, RO726corresponding to SSB 7) in time domain or frequency domain.