Abstract:
Systems and methods for specifying UE power control allocation for simultaneous transmission of PRACH in a secondary serving cell and PUCCH/PUSCH/SRS in a different serving cell in another timing advance group are disclosed. Rules are provided for prioritizing transmission of PRACH and/or other UL channels/signals. Additionally, UE power allocation is controlled for misaligned subframes across different timing advance groups. Latency of UL synchronization for a secondary serving cell is reduced by prioritizing PRACH retransmission.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of the filing date of U.S. provisional patent application No. 61/592,155, filed Jan. 30, 2012, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    The Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access Network (E-UTRAN), are standardized by the 3GPP working groups. An agreed objective of the carrier aggregation (CA) work item in Release-11 of the LTE standard is the support of multiple timing advance commands for uplink (UL) CA in scenarios where UL timing synchronization for a secondary serving cell (SCell) is sufficiently different from that of the primary serving cell (PCell). Examples of such scenarios include simultaneous (parallel) UL transmission to a macro Evolved Node B (eNB) on one carrier frequency and to a lower power node such as a remote radio head (RRH) on a different intra/inter-band carrier frequency. Consequently, it was agreed to partition aggregated serving cells into one or more timing advance groups (TAGs) according to their UL synchronization requirements. For each TAG, only one serving cell is configured to perform the random access (RA) procedure in order to obtain a timing advance (TA) command, which in turn, is applicable to all serving cells in the TAG. The PCell performs the RA procedure for its TAG. A SCell is configured for the RA procedure in a TAG consisting only of SCells, which is hereafter denoted as a “SCell-TAG.” 
         [0003]    Several physical layer aspects need to be considered when a user equipment (UE) receives multiple TA commands for different TAGs. These aspects include:
       the permitted combinations of a Physical Random Access Channel (PRACH) transmission on a SCell and transmission of other UL channels and signals (such as Physical Uplink Control Channel (PUCCH), Physical Uplink Shared Channel (PUSCH), and Sounding Reference Symbol (SRS)) on a different serving cell;   the priorities within a permitted combination pertaining to a PRACH transmission on a SCell and transmission of other UL channels and signals (such as PUCCH, PUSCH, and SRS) on a different serving cell; and   power limitation and power fluctuation during transmission in one subframe due to subframe misalignment between different TAGs.       
 
         [0007]    An LTE Release-10 UE performs random access only on the PCell (i.e., all configured serving cells derive their UL timing from the PCell timing). For LTE Release-11, it has been agreed that when a SCell-TAG is configured:
       there is one timing alignment timer (TAT) per TAG;   non-contention based RA is adopted for a SCell;   when the TAT associated with the PCell expires, all TAT&#39;s are considered expired (i.e., the UE flushes all Hybrid Automatic Repeat Request (HARQ) buffers, clears any configured assignments/grants, and Radio Resource Control (RRC) releases PUCCH/SRS for all configured serving cells; and   when the TAT associated with an SCell TAG expires:
           SRS transmissions in the SCell TAG shall be stopped and the Type  0  SRS configuration is released;   Channel Quality Indicator (CQI)/Precoder Matrix Indication (PMI)/Rank Indicator(RI) reporting configuration for the SCells is maintained; and   Medium Access Control (MAC) flushes all uplink HARQ buffers for the concerned SCells.   
               
 
         [0015]    For LTE Release-10 CA, a UE that is not configured for simultaneous PUCCH/PUSCH transmission transmits uplink control information (UCI) on the PUCCH if it is not scheduled for PUSCH transmission. Conversely, the UE multiplexes Uplink Shared Channel (UL-SCH) data and UCI on the PUSCH if it has an UL grant. On the other hand, if an LTE Release-10 UE is configured for simultaneous PUCCH/PUSCH transmission it can, for example, transmit HARQ-acknowledgement (ACK) feedback on the PUCCH and multiplex CSI with UL-SCH data on the PUSCH in the same subframe. In case the total required power for a subframe would exceed the maximum power capability, an LTE Release-10 UE shall prioritize UL transmission as follows: 
         [0016]    PUCCH&gt;PUSCH multiplexed with UCI&gt;PUSCH without UCI. 
         [0017]    In other words, PUCCH has the highest priority, followed by PUSCH multiplexed with UCI, and then PUSCH without UCI has the lowest priority. 
         [0018]    Furthermore, PUSCH power is scaled down while the PUCCH power is preserved in order to meet the UE&#39;s maximum transmit power capability PCMAX (i) in subframe i. 
         [0019]    Adopting the LTE Release-10 non-contention based random access, the UE may be scheduled for PRACH transmission on a SCell via a Physical Downlink Control Channel (PDCCH) order. It is assumed that there is no PUSCH/SRS transmission in any SCell of the SCell-TAG because the UE is not UL-synchronized for the SCell-TAG. Therefore, the main issue for further study are the permitted combinations of PRACH on a SCell and PUCCH/PUSCH/SRS in a different TAG(s). 
       SUMMARY 
       [0020]    Embodiments of the invention provide systems and methods for specifying UE power control procedures for simultaneous transmission of PRACH in a secondary serving cell and PUCCH/PUSCH/SRS in a different serving cell in another timing advance group. Rules are provided for prioritizing transmission of PRACH and/or other UL channels/signals. Additionally, UE power allocation is controlled for misaligned subframes across different timing advance groups. Latency of UL synchronization for a secondary serving cell is reduced by prioritizing PRACH transmissions. 
         [0021]    In one embodiment, a UE device receives a Physical Downlink Control Channel (PDCCH) order to initiate a random access procedure on a first secondary serving cell. The UE also identifies that it is scheduled to transmit on the Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH) of a second serving cell, which is in a different TAG with respect to the first serving cell. The UE transmits the PRACH transmission to the first serving cell in a subframe and transmits the PUCCH or PUSCH transmission to the second serving cell in the same subframe. 
         [0022]    The UE may scale a transmission power level for the PUCCH, PUSCH or SRS during the subframe so that a combined transmission power level for the PRACH transmission and the PUCCH, PUSCH or SRS transmission does not exceed a maximum transmission power for the user equipment. 
         [0023]    The UE may determine that the PRACH transmission will be retransmitted in a second subframe. The UE may increase the power level of the PRACH transmission during the second subframe and/or increase the transmission priority value for the PRACH transmission during the second subframe. 
         [0024]    In another embodiment, the UE device may identify a first transmission for a first serving cell during a subframe and identify a second transmission for a second serving cell during the subframe. If the UE identifies a transmission time difference between the first transmission and the second transmission within the subframe, then it scales the transmission power levels for the first transmission or the second transmission or both prior to a start of the subframe so that a combined transmission power level for the first transmission and the second transmission does not exceed a maximum transmission power for the user equipment. A constant transmission power may be assigned to all symbols in the first and second transmissions. The first and second transmissions may be one or more of PUCCH, PUSCH, PRACH, or SRS transmissions. 
         [0025]    In another embodiment, the UE device may identify a first transmission for a first serving cell during a subframe and identify a second transmission for a second serving cell during the same subframe. If the UE identifies a transmission time difference between the first transmission and the second transmission within the subframe, and the combined transmission power of both transmissions in an overlap period within the subframe would exceed the maximum transmission power of the UE, the UE shall scale the transmission power levels for the first transmission or the second transmission or both within the overlap period so that a combined transmission power level for the first transmission and the second transmission does not exceed a maximum transmission power for the user equipment. The first and second transmissions may be one or more of PUCCH, PUSCH, PRACH, or SRS transmissions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Having thus described the invention in general terms, reference will now be made to the accompanying drawings, wherein: 
           [0027]      FIG. 1  is a block diagram of a wireless communication network according to one embodiment. 
           [0028]      FIG. 2  is a high level block diagram of a system that may be used as an eNB or UE according to embodiments described herein. 
           [0029]      FIG. 3A  illustrates a full overlap between PUSCH or PUCCH or SRS in one TAG and PRACH in another TAG in a subframe. 
           [0030]      FIG. 3B  illustrates a partial overlap between PUCCH or PUSCH in one TAG and PRACH in another TAG in a subframe. 
           [0031]      FIG. 4  illustrates a subframe offset for a PUCCH transmission in one serving cell with respect to a PUSCH transmission in another serving cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The invention now will be described more fully hereinafter with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. One skilled in the art may be able to use the various embodiments of the invention. 
         [0033]      FIG. 1  is a block diagram of a wireless communication network  100 , which may be an LTE network that utilizes orthogonal frequency-division multiple access (OFDMA) on the downlink and single-carrier frequency division multiple access (SC-FDMA) on the uplink. LTE partitions system bandwidth into multiple orthogonal subcarriers, which may be referred to as frequency tones or frequency bins. Each subcarrier may be modulated with data, control, or reference signals. The wireless network  100  includes a number of evolved Node Bs (eNBs)  101  and other network entities. The eNBs  101  communicate with user equipment devices (UEs)  102 ,  105 . Each eNB  101  provides communication services for a particular geographic area or “cell”  103 , which may be a macro cell, a pico cell, a femto cell, and/or other types of cell. A network controller  104  may be coupled to a set of eNBs  101  and provide coordination and control for these eNBs  101 . 
         [0034]    UEs  102 ,  105  may be stationary or mobile and may be located throughout the wireless network  100 . UEs  102 ,  105  may be referred to as a terminal, a mobile station, a subscriber unit, a station, such as a mobile telephone, a personal digital assistant (PDA), a wireless modem, a laptop or notebook computer, a tablet, and the like. A UE  102  communicates with an eNB  101  serving the cell  103  in which the UE  102  is located. 
         [0035]    A UE  102  may communicate with more than one eNB  101  if the cells  103  of the eNBs  101  overlap. One eNB  101  will be the primary cell (PCell) and the other eNBs  101  will be secondary serving cells (SCell). 
         [0036]      FIG. 2  is a high level block diagram of a system  200  that may be used as an eNB or UE, which may be, for example, eNB  101  or UE  102  in  FIG. 1 . System  200  receives data to be transmitted from an interface  201  at transmit processor  202 . The data may include, for example, audio or video information or other data file information to be transmitted on a PUSCH. The transmit processor  202  may also receive control information to be transmitted on a PRACH, PUCCH, PUSCH, or SRS, from a controller  203 . Transmit processor  202  processes (e.g., encode and symbol map) the data and control information to obtain data symbols, control symbols, and reference symbols. The transmit processor  202  may also perform spatial processing or precoding on the data symbols and/or the control symbols and reference symbols. The output of the transmit processor  202  is provided to a modem  204 . Modem  204  processes the output symbol stream from the transmit processor  202  to obtain an output sample stream that is further processed by converting to analog, amplifying, and upconverting before being transmitted via antenna  205 . In other embodiments, multiple modems  204  may be used to support multiple-input multiple-output (MIMO) transmission on multiple antennas  205 . 
         [0037]    Signals are also received at system  200  on antenna  205  from other devices. The received signals are provided to modem  204  for demodulation. Modem  204  processes the signals by filtering, amplifying, downconverting, and/or digitizing, for example, to obtain input samples. Modem  204  or a receive processor  206  may further process the input samples to obtain received symbols. Receive processor  206  then processes the symbols by demodulating, deinterleaving, and/or decoding, for example. Receive processor  205  then provides decoded data to interface  201  for use by the eNB or UE. Receive processor further provides decoded control information to controller  203 . 
         [0038]    Controller  203  may direct the operation of system  200  in the eNB or UE, such as by adjusting timing and power levels. A memory  207  may store data and program codes for controller  203 , transmit processor  202 , and/or receive processor  206 . Additional components, such as a scheduler  208  may schedule downlink and/or uplink data transmission on one or more component carriers by system  200  (e.g., in an eNB). 
         [0039]    In the present invention a UE is said to be power limited if the UE is scheduled to transmit multiple channels or signals in a subframe and the combined transmission power of all the transmissions within a part or whole of the subframe would exceed the maximum transmission power capability of the UE. 
         [0040]    There are two clear options when PRACH is scheduled for a SCell-TAG and PUCCH/PUSCH/SRS is scheduled in another TAG: allow or disallow simultaneous transmission of PRACH and PUCCH/PUSCH/SRS. If simultaneous transmission is disallowed, the following observations about prioritizing a specific UL channel/signal may be considered: 
         [0041]    PRACH-only transmission or PRACH transmission prioritized over PUCCH/PUSCH. 
         [0042]    The purpose of the PRACH is to achieve UL synchronization on the SCells associated with the same TAG in a timely manner. This is independent of downlink (DL) transmission on the PDSCH, which relies on HARQ-ACK and CSI feedback, to optimize the DL transmission efficiency and avoid unnecessary re-transmissions. Therefore, there may be significant degradation in DL performance if PUCCH transmission is dropped when it occurs in the same subframe with a PRACH transmission on a SCell. 
         [0043]    Advantages of PRACH-only transmission: it may be argued that the SCell RA procedure is quite infrequent so the impact on UCI transmission is quite small. For example, the impact of dropping a CSI report should be negligible for low mobility scenarios. 
         [0044]    Disadvantages of PRACH-only transmission: While the preceding statement may be acceptable for Frequency Division Duplexing (FDD), it may lead to unacceptable performance for Time Division Duplexing (TDD) where up to M=4 (or M=9) DL subframes need to be acknowledged in one UL subframe. Additionally, the Random Access Channel (RACH) preamble transmission may span multiple subframes, where PUCCH and PUSCH are dropped. 
         [0045]    PUCCH/PUSCH transmission prioritized over PRACH. 
         [0046]    With this option the PRACH can be dropped when it coincides with PUCCH/PUSCH transmission on other serving cells. For PUCCH or UCI on PUSCH this option ensures that there is no degradation in DL performance but it adds some latency to achieving UL-synchronization on the SCell-TAG. Similarly, dropping PRACH in favor of PUSCH ensures there is no degradation in UL PUSCH performance. 
         [0047]    Advantages of prioritizing PUCCH/PUSCH: for non-contention based random access the eNB can avoid collision between SCell PRACH and PUSCH/PUCCH/SRS transmission in a different TAG by scheduling PRACH when there are no DL transmissions requiring HARQ-ACK feedback or PUSCH transmissions. 
         [0048]    Disadvantages of prioritizing PUCCH/PUSCH: increased eNB scheduling complexity when handling PUCCH, PUSCH, aperiodic SRS, and PRACH transmissions. Furthermore, for heavy DL data transmission there is no guarantee that PRACH can be scheduled within a reasonable time period. 
         [0049]    As discussed above, when a UE is scheduled for PRACH and PUCCH or PUSCH or SRS transmissions at the same time, there are advantages and disadvantages to dropping either PRACH or PUCCH/PUSCH/SRS in favor of the other transmission type. In one embodiment, the UE may simultaneously transmit PRACH on a SCell and transmit other channels or UL signals in a different TAG. In alternative embodiments, rules may be defined for prioritizing or postponing a PRACH transmission when it coincides with a PUCCH, PUSCH and/or SRS transmission if the combined transmission power would exceed the maximum transmission power for the UE. 
         [0050]    Various embodiments may use the following proposals for simultaneous transmission of PRACH and PUCCH/PUSCH in a subframe: 
         [0051]    Proposal 1: transmission of both the random access preamble and the PUCCH/PUSCH/SRS. 
         [0052]    If a random access procedure on a SCell is initiated by a PDCCH order in subframe n, then, if requested by higher layers, the UE transmits the random access preamble in the first subframe n+k 2 , k 2 ≧6, where a PRACH resource is available. If there is a PUCCH/PUSCH transmission in another serving cell in the same subframe (i.e., subframe n+k 2 ), the UE shall transmit both the random access preamble and the PUCCH/PUSCH transmission. 
         [0053]    As a separate embodiment, if a CSI report is scheduled for the PUCCH, and there is no HARQ-ACK feedback in the same subframe used for a PRACH transmission on a SCell, then the CSI report is dropped and PRACH is transmitted. As random access on a SCell is rather infrequent, dropping a CSI report is not expected to have a significant impact on DL throughput. 
         [0054]    In case of power limitation for simultaneous PRACH and PUCCH/PUSCH transmissions, prioritization of UL channels/signals is required as in LTE Release-10. In one embodiment, a simple extension to the LTE Release-10 specification is to scale down the PRACH power similar to the PUSCH power. However, it is noted that:
       there is no HARQ retransmission feature for PRACH; and   the received power at the eNB is lower than the higher-layer configured PREAMBLE_RECEIVED_TARGET_POWER. This complicates eNB receiver design in detecting the PRACH transmission. Equivalently, for the same PRACH receiver there is a higher probability of missed detection.       
 
         [0057]    Following the LTE Release-10 specification, if the UE does not detect a random access response (RAR) within the RA response window, a power ramping procedure is followed wherein the UE increments the PREAMBLE_TRANSMISSION_COUNTER by 1 to boost the received power at the eNB as PREAMBLE_RECEIVED_TARGET_POWER=preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1) * powerRampingStep 
         [0058]    For heavy DL transmission, it is likely that PRACH retransmissions will collide with PUCCH/PUSCH transmissions. Therefore, increasing the preamble received target power may not produce a different outcome for PRACH reception and it also increases the UL interference caused by the PRACH retransmission. Incrementing the transmit power was designed for pathloss compensation and not to handle power scaling in case of simultaneous transmission of PRACH and PUCCH/PUSCH/SRS. One possible solution is to prioritize PRACH retransmissions in the case of simultaneous transmission of UL channels/signals. 
         [0059]    Proposal 2: prioritization of PUCCH in case of power limitation for simultaneous transmission on multiple serving cells. 
         [0060]    In one embodiment, PRACH has the same priority as PUSCH transmission without UCI (i.e., PUCCH&gt;PUSCH with UCI&gt;PUSCH without UCI or PRACH). Therefore, if the total transmit power of the UE would exceed {circumflex over (P)} CMAX  (i) , the UE scales the PUSCH or PRACH power, denoted as {circumflex over (P)} channel,c  (i) for a serving cell c in subframe i such that the condition: 
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         [0000]    is satisfied where {circumflex over (P)} PUCCH  (t) is the linear value of {circumflex over (P)} PUCCH  (i), {circumflex over (P)} channel,c  (i) is the linear value of either the PUSCH P PUSCH,c  (i) or the PRACH P PRACH,c  (i). 
         [0061]    In another embodiment, PRACH has the same priority as PUSCH transmission with UCI (i.e., PUCCH&gt;PUSCH with UCI or PRACH&gt;PUSCH without UCI). 
         [0062]    Proposal 3: UE behavior in case of PRACH retransmission. 
         [0063]    If the UE does not detect a RAR within the RA response window for a PRACH transmission on a SCell corresponding to a detected PDCCH order, then the following UE behavior may be specified: 
         [0064]    Event 1: At initial transmission, the UE was not power limited. For the re-transmission, the UE is still not power limited. The UE should ramp up. There is no need to change priority. 
         [0065]    Event 2: At initial transmission, the UE was power limited due to collisions with UL transmissions in other TAGs. For the re-transmission, the UE is not power limited anymore. The UE may transmit at the initial PRACH transmission nominal power. There may be no need for ramp-up. There is no need for changing priority. The UE increments the PREAMBLE_TRANSMISSION_COUNTER by 1 and sets the powerRampingStep to zero. 
         [0066]    Event 3: At initial PRACH transmission, UE was power limited due to collisions with UL transmissions in other TAGs. For the re-transmission, it is still power limited due to collisions with other UL transmissions. Ramping up is ineffective in this case, and the UE should change priority. The UE increments the PREAMBLE_TRANSMISSION_COUNTER by 1 and applies the higher-layer signaled parameter powerRampingStep. The PRACH retransmission is prioritized over all other UL channels/signals (i.e., no scaling is applied to PRACH transmission). 
         [0067]    In an alternative embodiment, other UL channels/signals are dropped in case of collision with a PRACH retransmission and the UE is power limited. 
         [0068]    In another alternative embodiment, transmit power allocation for other UL channels/signals are scaled down in case of collision with a PRACH retransmission and the UE is power limited. 
         [0069]    Event 4: At initial transmission, UE was not power limited. For the re-transmission, the UE is power limited. The UE should ramp up transmit power as in LTE Release-10, but also prioritize PRACH over other UL channels/signals as in Event 3 above. 
         [0070]      FIG. 3A  illustrates a subframe offset for a PUCCH or PUSCH transmission  301  in the PCell with respect to a PRACH transmission  302  in a SCell. Subframe misalignment due to independent timing advances for the PCell and SCell-TAG(s) leads to overlapped SC-FDMA symbol transmissions. SRS transmission  303  in the last symbol of subframe n can also overlap with the PRACH transmission.  FIG. 4B  illustrates a different subframe offset where the PUCCH or PUSCH transmission  304  is in subframe n+1 whereas PRACH transmission  305  is in subframe n. For both illustrations the PUCCH or PUSCH or SRS transmission can be scaled down if the combined power of PRACH and PUCCH or SRS would exceed the maximum transmission power of the UE. 
         [0071]    Power allocation due to subframe misalignment. 
         [0072]      FIG. 4  illustrates a subframe offset for a PUCCH transmission  401  in the PCe 11  with respect to a PUSCH transmission  402  in the SCell. Subframe misalignment due to independent timing advances for the PCell and SCell-TAG(s) leads to overlapped SC-FDMA symbol transmissions. For example, PUSCH  402  begins transmitting at time t 0 ; however, PUCCH  401  does not begin transmitting within the same subframe until time t 1  due to timing misalignment Δt. One possible consequence is a transmit power fluctuation within a subframe. 
         [0073]    In this scenario, the total required transmit power exceeds the maximum UE power capability  403  when both PUCCH  401  and PUSCH  402  are transmitted simultaneously. When PUSCH  402  begins transmitting, the UE transmit power is at a first level  404 ; however, when PUCCH  401  begins transmitting within the same subframe, the required power jumps to a second level  405 , that exceeds the UE&#39;s maximum power level  403 . Following the LTE Release-10 specification, the PUCCH  401  transmission would be prioritized over the PUSCH  402  even though the PUSCH  402  transmission started before the PUCCH  401 . 
         [0074]    It is not desirable to change the PUSCH  402  transmit power within a subframe because this affects the PUSCH receiver design (e.g., QAM demodulation when different SC-FDMA symbols are received with different powers). The relative propagation delay difference between serving cells, including eNB time alignment accuracy, is at most 31.3 μs. Assuming roughly equal DL and UL propagation delays, this timing difference is roughly 50% of an SC-FDMA symbol duration of 66.6 μs. Thus, if the subframe offset Δt is within one SC-FDMA symbol, it should be possible for the UE to adjust the total transmit power across multiple serving cells and channel/signal combinations, including power scaling in the case of power limitation, prior to transmission on any serving cell. It shall also be possible for a different UE to the UE to adjust the transmission power only within a subframe offset or overlap period between the two transmissions. 
         [0075]    The following proposals address the scenario of  FIG. 3  in which overlapping, off-set transmissions exceed the UE&#39;s maximum power. 
         [0076]    In one embodiment, a constant transmission power is allocated to all SC-FDMA symbols for PUSCH/PUCCH transmission in a subframe. 
         [0077]    In another embodiment, when the total transmit power would exceed the maximum transmit power at any point in a subframe, then the power scaling rules are applied prior to transmission of PUSCH/PUCCH/SRS/PRACH on any UL serving cell for the same subframe. 
         [0078]    In another embodiment, when the total transmit power would exceed the maximum transmit power for any period in a subframe, then the power scaling rules are applied for that same period of a same subframe for transmission of PUSCH/PUCCH/SRS/PRACH on any UL serving cell. 
         [0079]    Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions, and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.