Abstract:
A method of signal processing in a wireless transmit receive unit (WTRU) including multiple input/multiple output (MIMO) functionality. The method includes the WTRU receiving a plurality of simultaneous signals, performing a first process on at least one of the plurality of simultaneous signals, transmitting a feedback signal based on the first process, and performing a second process on at least one of the plurality of simultaneous signals. The first process is a subset of the second process.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional application No. 60/825,977, filed Sep. 18, 2006, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention relates generally to wireless communications. More particularly, a method and apparatus for successive interference cancellation (SIC) for multi-codeword transmissions is disclosed.  
       BACKGROUND  
       [0003]     The Third Generation Partnership Project (3GPP) is an industry group working to improve world-wide wireless communication. Its Long Term Evolution (LTE) project is looking to set standards and guidelines to improve wireless communication systems in the near and long-term future.  
         [0004]     One of the technologies being considered for LTE is Multiple Input/Multiple Output (MIMO). MIMO involves the use of multiple antennas on both the transmitter side and the receiver side. A technology commonly used in MIMO systems is Per Antenna Rate Control (PARC), which is a method for individually adjusting the data rate for each antenna Another technology being considered is multi-codeword transmission. Typically, in an LTE MIMO system, multi-codeword transmission is implemented by transmitting each codeword over a different antenna. Furthermore, hybrid automatic repeat request (HARQ) is an error detection and correction method that is typically used in LTE. In general, error detection information along with an error correction code is encoded with each transmitted data block. The error detection is often a cyclic redundancy check (CRC). Lastly, successive interference cancellation (SIC) may be used in MIMO systems to distinguish between simultaneous signals by successively using one set of interference cancelled signals to process a next set of signals. SIC methods may provide error free signals, but require processing time.  
         [0005]     Combining multi-codeword transmission, PARC, CRC, and SIC in a single wireless transmit receive unit (WTRU) may provide a spectrally efficient and robust product. However, LTE specifies 5-msec user-plane latency. In order to meet this specification, WTRU processing time would need to be about 2 to 3 msec for HARQ reception, processing and acknowledgement/non-acknowledgement (ACK/NACK) generation. The result may be that the WTRU exceeds the requirements for maximum processing time.  
         [0006]      FIG. 1  is a flow chart of a full SIC method in accordance with the prior art. At step  102 , a signal is received by a WTRU. At step  104 , the WTRU selects two HARQ processes for processing. At step  106 , the WTRU checks to see if one of the HARQ processes is a retransmission. If the signal contains a retransmission, at step  108 , the retransmitted HARQ processes are combined. If the WTRU determines that the signal does not contain a retransmitted HARQ process, step  108  is skipped. In either case, the signal is decoded at step  110 , and, at step  112 , a CRC is performed.  
         [0007]     If the HARQ process passes the CRC, at step  114  the ACK/NACK signal is set to ACK, and full SIC is performed at step  116 , removing one of the HARQ components from the signal. If the signal fails, the ACK/NACK is set to NACK at step  118 . The counter is incremented, and the method returns to step  104 .  
         [0008]      FIG. 2  is a continuation of  FIG. 1 . Once the first two HARQ processes are processed as in method  100  of  FIG. 1 , at step  202 , the WTRU determines if the ACK/NACK signal for HARQ( 1 ) is a NACK, and HARQ( 2 ) is an ACK. If not, the ACK/NACK is transmitted to a Node-B in an uplink (UL) signal.  
         [0009]     However, if HARQ( 1 ) generated a NACK and HARQ( 2 ) generated an ACK, HARQ( 1 ) is recoded at step  206  and at step  208 , a CRC is performed on HARQ( 1 ). If the signal passes CRC, the ACK/NACK signal is set to ACK at step  212  and an ACK for HARQ( 1 ) and HARQ( 2 ) is transmitted at step  214 . If HARQ( 1 ) again fails CRC, then a NACK is transmitted for HARQ( 1 ) at step  214 .  
         [0010]     The processing required for method  100  is typically very complex and time consuming. The processing time for a 2×2 multi-codeword transmission with full SIC may be up to twice that as for a single codeword. This processing time may exceed the LTE specified maximum limits. Furthermore, as the number of codewords increases, it becomes less likely that the processing time will be less than the maximum allowed by LTE specifications. Further, in a wireless system using 2×2 PARC MIMO, full SIC processing may increase the WTRU processing time by up to twice that of a receiver without full SIC processing. Therefore, it would be desirable to have a method for multi-codeword SIC processing that will not exceed the timing requirements of LTE.  
       SUMMARY  
       [0011]     a method and apparatus for signal processing in a WTRU are disclosed. The WTRU may include multiple input/multiple output (MIMO) functionality. The method may include, but is not limited to, the WTRU receiving a plurality of simultaneous signals, performing a first process on at least one of the plurality of simultaneous signals, transmitting a feedback signal based on the first process, and performing a second process on at least one of the plurality of simultaneous signals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     A more detailed understanding may be had from the following description of a preferred embodiment, given by way of example and to be understood in conjunction with the accompanying drawings wherein:  
         [0013]      FIGS. 1 and 2  are flow diagrams of a full SIC method for a typical 2×2 MIMO PARC multi-codeword wireless system in accordance with the prior art;  
         [0014]      FIG. 3  shows an exemplary wireless communication system in accordance with one embodiment;  
         [0015]      FIG. 4  is a functional block diagram of the wireless communication system  300  of  FIG. 3 .  
         [0016]      FIG. 5  is a block diagram of a method of hybrid SIC in accordance with one embodiment;  
         [0017]      FIGS. 6 and 7  are flow diagrams of the hybrid SIC method in accordance with one embodiment;  
         [0018]      FIG. 8  is diagram showing an example of the hybrid SIC method in accordance with one embodiment as applied to a typical HARQ signal; and  
         [0019]      FIG. 9  shows a timing diagram for an N-process stop and wait (SAW) HARQ method in accordance with an alternative embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment  
         [0021]     Local SIC, as used herein, includes SIC before decoding or without signal reconstruction via the transmitter channel coding chain. The resulting “soft” ACK/NACK signal is transmitted within the timing requirement to a NodeB. A “soft” ACK/NACK signal means that a NACK result may be reversed to ACK once full SIC is applied after the soft ACK/NACK is transmitted.  
         [0022]     Turning now to  FIG. 3 , there is shown an exemplary wireless communication system  300 , which includes a plurality of wireless communication devices, such as an AP  310  and a plurality of WTRUs  320 , capable of wirelessly communicating with one another. Although the wireless communication devices depicted in the wireless communication system  300  are shown as APs and WTRUs, it should be understood that any combination of wireless devices may comprise the wireless communication system  300 . That is, the wireless communication system  300  may comprise any combination of APs, WTRUs, stations (STAs), and the like. An AP may be a Node-B, a base station, and the like.  
         [0023]     For example, the wireless communication system  300  may include an AP and client device operating in an infrastructure mode, WTRUs operating in ad-hoc mode, nodes acting as wireless bridges, or any combination thereof. However, the wireless communication system  300  may be any other type of wireless communication system.  
         [0024]      FIG. 4  is a functional block diagram of an AP  310  and a WTRU  320  of the wireless communication system  300  of  FIG. 3 . As shown in  FIG. 4 , the AP  310  and the WTRU  320  are in wireless communication with one another. In addition to the components that may be found in a typical AP, the AP  310  includes a processor  415 , a receiver  416 , a transmitter  417 , and an antenna  418 . The processor  415  is configured to generate, transmit, and receive data packets. The receiver  416  and the transmitter  417  are in communication with the processor  415 . The antenna  418  is in communication with both the receiver  416  and the transmitter  417  to facilitate the transmission and reception of wireless data. The antenna  418  may be a plurality of antennas.  
         [0025]     Similarly, in addition to the components that may be found in a typical WTRU, the WTRU  320  includes a processor  425 , a receiver  426 , a transmitter  427 , and an antenna  428 . The processor  425  is configured to generate, transmit, and receive data packets. The receiver  426  and the transmitter  427  are in communication with the processor  425 . The antenna  428  is in communication with both the receiver  426  and the transmitter  427  to facilitate the transmission and reception of wireless data.  
         [0026]      FIG. 5  is a block diagram of a hybrid SIC method in accordance with one embodiment. HARQ( 1 )  502  and HARQ( 2 )  504  are received at a WTRU. The WTRU performs a local SIC  506 . Local SIC  506  is a SIC process that is performed before the HARQ signals  502 ,  504  are decoded. The local SIC function  506  generates a soft ACK/NACK  508  that is transmitted on the uplink  510  to satisfy LTE timing requirements. After a full decoding and SIC process is performed, the soft ACK/NACK  508  may be reversed. The soft ACK/NACK  508  is processed in a full SIC function  512 , producing an updated ACK/NACK  514  that may be used for the next HARQ.  
         [0027]      FIG. 6  is a flow diagram for a hybrid SIC method in accordance with one embodiment. At step  602 , two HARQ processes are received by a WTRU. At step  604 , the WTRU determines if the HARQ processes require full SIC processing. Full SIC processing is required for a new HARQ if a soft ACK/NACK from the previous associated HARQ processing was reversed.  
         [0028]     If full SIC processing is required, the WTRU performs full SIC processing at step  606 . If full SIC processing is not required, the WTRU selects one of the HARQ streams for processing at step  607 . The selected stream is decoded at step  608 . At step  610 , a local SIC process is applied to the non-selected stream. The local SIC process includes removing a contribution of the selected stream from the non-selected stream. At step  612 , the interference cancelled non-selected stream is decoded.  
         [0029]     A step  614 , the WTRU performs a CRC check on either the full SIC signal or the local SIC signal and an ACK/NACK is generated for both HARQ processes. The ACK/NACK is transmitted on an uplink channel, at step  616 , to satisfy the LTE timing requirements. Simultaneously, both HARQ signals continue to be processed as shown in  FIG. 7 .  
         [0030]     Referring to  FIG. 7 , once the signal has been processed with a local SIC procedure, the WTRU determines at step  702  if, (after the CRC check), one of the HARQ processes generated a NACK. If not, full SIC is not necessary and is skipped at step  714 . If one of the HARQ processes generates a NACK, full SIC is applied to the signal that generated the NACK at step  704 . At step  706 , CRC check is performed again. If the signal passes the CRC check, at step  708 , the corresponding retransmitted signal from a subsequent transmission time interval (TTI) is discarded and an ACK is generated for the uplink transmission. At step  712 , the signal is saved for use in the SIC process for a subsequent TTI. If the HARQ fails at step  706 , the interference cancelled stream is buffered at step  710  and the method returns to  FIG. 6 .  
         [0031]      FIG. 8  is diagram showing an example of the hybrid SIC method in accordance with one embodiment as applied to a typical HARQ signal. Two (2) HARQ processes, HARQ 1   806  and HARQ 2   808  are simultaneously transmitted from a transmitter  802  to a receiver  804 . The receiver  804  decodes the signal, performs a local SIC, and generates the appropriate ACK/NACK for each. In this example, HARQ 1   806  is successfully received (CRC passes) but HARQ 2   808  is not. Therefore, an ACK  812  is sent back to the transmitter  802  for HARQ 1   806 , and a NACK  814  is transmitted for HARQ 2   808 .  
         [0032]     Since the ACK  812  was sent back to the transmitter  802  for HARQ 1   806 , the next HARQ 1   816  is new. Additionally, because a NACK  814  was returned for HARQ 2   808 , the next HARQ 2   818  is a retransmission of the first HARQ 2   808 . Also, the contribution of HARQ 1  is removed from HARQ 2   808  and a CRC is applied to HARQ 2   808 . If HARQ 2   808  passes the CRC, an ACK is returned for HARQ 2   808  and HARQ 2   808  is used for a full SIC that is performed on the next HARQ received by the receiver. In the example shown in  FIG. 8 , the appropriate ACK/NACK  820  is returned for HARQ 1   816 , and an ACK  822  is returned for HARQ 2   818 , which is the retransmitted HARQ 2   808 .  
         [0033]      FIG. 8  is an example of a local SIC process being applied to a HARQ signal and the local SIC process resulting in a NACK. Then, a full SIC process being applied to the same HARQ process after the NACK is returned to the transmitter. The full SIC process results in a change of the NACK to an ACK.  
         [0034]     The next HARQ 1   826  is then transmitted, which may be new if an ACK was returned, or retransmitted if a NACK was returned. Also, a new HARQ 2   828  is transmitted.  
         [0035]     If, however, HARQ 2   808  fails the CRC after a full SIC is applied, a HARQ recombining and local SIC is applied using HARQ 1   806 . Depending upon the outcome of a CRC, an appropriate ACK/NACK  824  is returned to the transmitter  802  for HARQ 1 , and an appropriate ACK/NACK  825  is returned for HARQ 2 . Based on the ACK/NACKs  824 , 825  the next HARQ 1   826  and next HARQ 2   830  may be new, or may be retransmissions. Using this method  800 , an ACK/NACK is returned after each HARQ cycle period, although the receiver may still be processing the HARQ signals after the HARQ cycle period is complete.  
         [0036]      FIG. 9  shows a timing diagram for an N-process stop and wait (SAW) HARQ method  900  in accordance with an alternative embodiment. Each HARQ process takes one TTI. Moving from left to right and top to bottom in  FIG. 9 , a total HARQ cycle period equals a: 1) HARQ TTI  912 ; 2) propagation delay  910 ; 3) WTRU processing time  914 ; 4) second propagation delay  916 ; 5) ACK/NACK TTI  918 ; and 6) Node-B processing time  920 . If N is less than or equal to eight (8), in order to meet the LTE requirement for user plane latency of less than five (5) msec, total processing time for the WTRU and the Node-B should be less than or equal to 2 to 3 msec. In order to implement SIC processing at either the WTRU or the node-B, the hybrid method disclosed herein may be used.  
         [0037]     Although the features and elements are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements. The methods or flow charts provided may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs).  
         [0038]     Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine.  
         [0039]     A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.