Abstract:
A wireless communication method and system for performing bit-interleaved coded modulation and iterative decoding. The system includes a transmitter and a receiver. The transmitter encodes incoming bits to generate coded bits, punctures the coded bits in accordance with a predetermined puncturing pattern to generate surviving channel bits and stolen bits and interleaves the surviving bits into interleaved surviving bits. The interleaved surviving bits are mapped to channel symbols and the stolen bits are interleaved to generate interleaved stolen bits. At least one of a plurality of antennas is selected to transmit the channel symbols based on the value of the interleaved stolen bits. The receiver receives the transmitted channel symbols, estimates a posteriori probability for both the channel symbols and the stolen bits, and retrieves information of the stolen bits by determining the selected antenna used to transmit the channel symbols.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/771,515 filed Feb. 8, 2006, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is related to multiple-input multiple-output (MIMO) technology in wireless communication systems. More particularly, the present invention is related to a method and system for bit-interleaved coded modulation and iterative decoding.  
       BACKGROUND  
       [0003]     Demand for high data rates has been driving development and standardization efforts for next generation wireless systems, such as evolved universal terrestrial radio access (E-UTRA) and IEEE 802.11n. To achieve high data rates, high-rate channel coding and higher order modulation are needed, which often causes less reliable transmission. One remedy for this is using transmit diversity, such as space-time block code (STBC).  
         [0004]      FIG. 1  is a block diagram of a conventional transmitter  100  employing STBC. Incoming bits  105  are coded by a channel encoder  110 . The output  115  of the channel encoder  110 , (i.e., coded bits), are passed to a puncturer  120 , where some bits are deleted, (i.e., punctured), according to a predetermined puncturing pattern. The deleted bits are often referred to as “stolen” bits  125  and are not transmitted to a receiver in any form. The stolen bits are placed in a data sink  130 , (which is merely a conceptual component), for disposal. The surviving bits  135  are interleaved by an interleaver  140 , to avoid burst errors. The interleaved surviving bits  145  are then mapped to channel symbols  155  by a mapper  150 , such as quadrature phase shift keying (QPSK), or 16 quadrature amplitude modulation (16 QAM), or the like. The channel symbols  155  are finally coded by an STBC encoder  160  and transmitted over the air via antennas  165 . STBC provides full diversity at the symbol level, but it does not provide additional coding gain.  
       SUMMARY  
       [0005]     The present invention is related to a wireless communication method and system for performing bit-interleaved coded modulation and iterative decoding. The system includes a transmitter and a receiver. The transmitter uses multiple antennas to transmit data with coded binary bits divided into two groups: surviving channel bits and “stolen”, (i.e., punctured), bits. The transmitter encodes incoming bits to generate coded bits, punctures the coded bits in accordance with a predetermined puncturing pattern to generate surviving channel bits and stolen bits, and interleaves the surviving bits into interleaved surviving bits. The interleaved surviving bits are mapped to channel symbols and the stolen bits are interleaved to generate interleaved stolen bits. At least one of a plurality of antennas is selected to transmit the channel symbols based on the value of the interleaved stolen bits. The receiver receives the transmitted channel symbols, estimates a posteriori probability for both the channel symbols and the stolen bits, and retrieves information of the stolen bits by determining the selected antenna used to transmit the channel symbols.  
         [0006]     The surviving channel bits are interleaved to avoid burst errors before being mapped into channel symbols and transmitted over the air. To achieve a high data rate, stolen bits are not transmitted over the air as in the prior art. In accordance with the present invention, the stolen bits are used as index to switch between transmit antennas or beams. Thus, information on stolen bits is implicitly passed to the receiver. The receiver uses a soft-in-soft-out (SISO) demapper to retrieve information of stolen bits by estimating which antenna was used to transmit channel symbols. The retrieved stolen bit information is then passed to a SISO decoder, along with surviving channel bit information. The output of the SISO decoder is then fed back to the SISO demapper. The iterative process continues until convergence or a pre-determined iteration number is reached. With more iterations, the reliability of stolen bit information increases, which ultimately improves performance of the SISO decoder. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     A more detailed understanding of the invention 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:  
         [0008]      FIG. 1  is a block diagram of a conventional transmitter employing STBC;  
         [0009]      FIG. 2  is a block diagram of a transmitter configured in accordance with one embodiment of the present invention;  
         [0010]      FIG. 3  is a block diagram of a receiver configured in accordance with the present invention;  
         [0011]      FIG. 4  shows an example of simulation comparison results in accordance with the present invention;  
         [0012]      FIG. 5A  is a block diagram of an orthogonal frequency division multiplexing (OFDM) transmitter configured in accordance with another embodiment of the present invention;  
         [0013]      FIG. 5B  shows an example of an operation performed by a spatial mapper used in the OFDM transmitter of  FIG. 5A ; and  
         [0014]      FIG. 6  is a block diagram of a multiple-input multiple-output (MIMO) transmitter configured in accordance with yet another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     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.  
         [0016]     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.  
         [0017]     The features of the present invention may be incorporated into an integrated circuit (IC) or configured in a circuit comprising a multitude of interconnecting components.  
         [0018]     The present invention is related to a method of reusing “stolen”, (i.e., punctured or deleted), bits output from a puncturer and an iterative decoding method. The present invention achieves a similar diversity gain as STBC, but has a better coding gain by retrieving information from stolen bits. The present invention improves robustness of high data rate transmission and has better performance than prior art methods. One of the benefits of the present invention is randomization of co-channel interference to other users since signals are transmitted over different antennas and the pattern of antenna switching is pseudo random to other users.  
         [0019]      FIG. 2  is a block diagram of a transmitter  200  configured in accordance with one embodiment of the present invention. The transmitter  200  includes a channel encoder  205 , a puncturer  210 , a first interleaver  215 , a second interleaver  220 , a mapper  225 , an antenna switch  230  and a plurality of transmit antennas  235 . Although only two antennas  235  are shown in  FIG. 2 , it should be understood that any number of antennas  235  may be used in accordance with the present invention.  
         [0020]     As shown in  FIG. 2 , incoming bits  240  are coded by the channel encoder  205 . The coded bits  245  are passed to the puncturer  210 , where some bits are deleted according to a predetermined puncturing pattern. The surviving bits  250  are interleaved by the first interleaver  215  and mapped to a symbol by the mapper  225 . The stolen bits  255  are interleaved by the second interleaver  220 . It should be noted that the puncturer  210  and the first interleaver  215  may be switched by properly designing the interleaving and puncturing pattern.  
         [0021]     The operation of the antenna switch  230  is controlled by interleaved stolen bits  260  which are output from the second interleaver  220 . For example, a first one of the antennas  235  is used to transmit a current channel symbol when the corresponding stolen bit value is “0” (zero), and a second one of the antennas  235  is used to transmit a current channel symbol when the stolen bit value is “1” (one). The antenna switch  230  may be a physical antenna switch or an antenna beam switch. If an antenna beam switch is used, for example, symbols “x” and “x” are simultaneously transmitted from the first and second antennas  235 , respectively, when the stolen bit value is “0”, and symbols “x” and “−x” are simultaneously transmitted from the first and second antennas  235 , respectively, when the stolen bit value is “1”.  
         [0022]      FIG. 3  is a block diagram of a receiver  300  configured in accordance with the present invention. The receiver  300  includes an antenna  305 , a radio frequency (RF) front end  310 , a SISO demapper  315 , a deinterleaver  320 , a SISO decoder  325  and an interleaver  330 . Signals received by the antenna  305  are processed by the RF front end  310  to generate samples  335 . The samples  335  are fed into the SISO demapper  315 , where a posteriori probability is estimated for both surviving channel bits and stolen bits. The SISO demapper  315  retrieves information of the stolen bits by estimating which antenna of the transmitter  200  was used to transmit the channel symbols. Also fed into the SISO demapper  315  is a priori probability of surviving channel bits and stolen bits, (i.e., the interleaved extrinsic bit probability information outputted by the interleaver  330  in the second iteration and beyond), which are set to zero in the first iteration and are passed from the SISO decoder  325  from the second iteration and beyond. The SISO demapper  315  calculates and outputs extrinsic information  340  of both channel and stolen bits by subtracting a priori probability from a posteriori probability. The extrinsic information  340  is deinterleaved by the deinterleaver  320  and passed to the SISO decoder  325 , where a coding structure is utilized to refine information on both channel and stolen bits.  
         [0023]     Two types of information are output by the SISO decoder  325 : a posteriori probability for information bits  345  which is sent to a decision-making stage  350 , and extrinsic bit probability information  355  which is interleaved by the interleaver  330  and fed back to the SISO demapper  315  for the next iteration.  
         [0024]     Although other implementations are available under similar principle, one example of detailed operation of the SISO demapper  315  is explained hereinafter. For the sake of simplicity, reference to a time index is removed from the following description.  
         [0025]     The transmitter  200  of  FIG. 2  transmits channel symbols to the receiver  300  of  FIG. 3 . Referring to  FIG. 2 , let └u 1 ,u 2 , . . . ,u k ┘ be input to the channel encoder  205  of the transmitter  200 , and └c 1 ,c 2 , . . . ,c n ┘ be output of the channel encoder  205 . The output of the channel encoder  205  is punctured and interleaved. Let  t =[t 1 ,t 2 , . . . ,t m ] be interleaved surviving channel bits that are to be mapped to a channel symbol, and let  s =[s 1 ,s 2 , . . . ,s L ] be stolen bits associated with the channel symbol. The mapper  225  of the transmitter  200  maps surviving channel bits into channel symbols, preferably according to Gray mapping x=μ(  t ). A reverse mapping function is also defined as t i =μ i   −1 (x). Similarly, an antenna mapping function is defined as g=η(  s ) and corresponding inverse mapping s i =η i   −1 (g) is defined. Finally, channel symbol subsets X i   0 ={x; μ i   −1 (x)=0} and X i   1 ={x; μ i   −1 (x)=1}, and antenna mapping index subsets G i   0 ={g; η i   −1 (g)=0} and G i   1 ={g; η i   −1 (g)=1} are defined.  
         [0026]     For the sake of simplicity, a receiver  300  with a single antenna  305 , as shown in  FIG. 3 , is explained without losing the essence of the present invention. The extension to multiple receive antenna systems is straightforward. Let H=[h 1 ,h 2 , . . . ,h L ] be a channel response vector, where h l  is channel response from l th  transmit antenna to the receive antenna. A flat channel model is assumed, which is generally valid from narrow band communication and also valid for each subcarrier of OFDM(A) systems. The digitized version of the receiver signal, which is sent to the SISO demapper  315  of the receiver  300 , can be expressed as follows: 
 
 y=h   l   x+γ,    Equation (1) 
 
 where γ is noise term. The SISO demapper  315  calculates a posteriori probability of both channel and stolen bits. A posteriori probability of surviving channel bits is calculated as follows, (after ignoring constant factors):  
                   P   ⁡     (       t     i   ⁢               =     b   ❘   y       )       =       ∑     x   ∈     X   i   b         ⁢           ⁢       P   ⁡     (     y   ❘   x     )       ⁢     P   ⁡     (   x   )             ;     ⁢     
     ⁢     where   ⁢     :               Equation   ⁢           ⁢     (   2   )                       P   ⁡     (     y   ❘   x     )       =       ∑   g     ⁢           ⁢       P   ⁡     (       y   ❘   x     ,     h   g       )       ⁢     P   ⁡     (   g   )             ;     ⁢     
     ⁢     where   ⁢     :               Equation   ⁢           ⁢     (   3   )                   P   ⁡     (       y   ❘   x     ,     h   g       )       =       exp   ⁡     (     -              y   -       h   g     ⁢   x            2         2   ⁢           ⁢     σ   2       ⁢                 )       .             Equation   ⁢           ⁢     (   4   )               
 
         [0027]     P(x) and P(g) in Equations (2) and (3) are a priori probability. Equal probability is assumed in the first iteration and use an extrinsic bit probability from the SISO decoder  325  of the receiver  300  after the first iteration.  
         [0028]     The extrinsic a posteriori probability of surviving channel bits for the second iteration and beyond is as follows:  
                     P   ⁡     (         t     i   ⁢               =       ⁢   b     ;   O     )       =         P   ⁡     (       t     i   ⁢               =     b   ❘   y       )       /     P   ⁡     (         t     i   ⁢               =   b     ;   I     )         =                     ⁢       ∑     x   ∈     X   i   b         ⁢       ∑   g     ⁢           ⁢       P   ⁡     (       y   ❘   x     ,     h   g       )       ⁢       ∏     j   ≠   i       ⁢           ⁢     P   ⁡     (         t   j     =       μ   i     -   1       ⁡     (   x   )         ;   I     )                               ⁢       ∏   l     ⁢           ⁢       P   ⁡     (         s   l     =       η   l     -   1       ⁡     (   g   )         ;   I     )       .                     Equation   ⁢           ⁢     (   5   )               
 
 Similarly, the extrinsic a posteriori probability of stolen bits is calculated as follows:  
                     P   ⁡     (         s     i   ⁢               =       ⁢   b     ;   O     )       =         P   ⁡     (       s     i   ⁢               =     b   ❘   y       )       /     P   ⁡     (         s     i   ⁢               =   b     ;   I     )         =                     ⁢       ∑   x     ⁢       ∑     g   ∈     G   i   b         ⁢       P   ⁡     (       y   ❘   x     ,     h   g       )       ⁢       ∏   j     ⁢           ⁢     P   ⁡     (         t   j     =       μ   i     -   1       ⁡     (   x   )         ;   I     )                               ⁢       ∏     j   ≠   i       ⁢           ⁢       P   ⁡     (         s   j     =       η   i     -   1       ⁡     (   g   )         ;   I     )       .                     Equation   ⁢           ⁢     (   6   )               
 
         [0029]     The extrinsic a posteriori probability of both surviving channel bits and stolen bits are deinterleaved by the deinterleaver  320  and passed to the SISO decoder  325 .  
         [0030]     While the receiver  300  shown in  FIG. 3  achieves optimum performance, suboptimum receivers may be used. One of the suboptimum receivers may be a decision feedback receiver, where hard decisions on both surviving channel bits and stolen bits are obtained and used in the next iteration as if they were known. It is assumed that channel state information is known to the receiver. However, it is also possible to apply blind detection approach where channel state information is unknown to the receiver.  
         [0031]      FIG. 4  shows an example of simulation comparison results in terms of frame error rate versus a ratio of energy per information bit verses noise spectrum density (EbN0) when comparing the transmitter  200  of  FIG. 2  with the conventional STBC transmitter  100  of  FIG. 1 . In this example, channel codes are rate ½ convolutional code with a polynomial generator. The coded bits are punctured to rate ¾ in accordance with IEEE 802.11a/g. Each data frame has 576 information bits. The interleaver used has a depth of 24. The channel model used is fast flat Rayleigh fading. As illustrated by  FIG. 4 , with more iterations, a receiver would have a better estimation of stolen bits, and achieve better overall performance when a signal is received from the transmitter  200  of  FIG. 2  in lieu of the STBC transmitter  100  of  FIG. 1 . When compared to the prior art STBC transmitter  100  of  FIG. 1 , the second iteration of the transmitter  200 /receiver  300  outperforms at a higher signal-to-noise ratio (SNR) regime. At the tenth iteration, the transmitter  200  of  FIG. 2  outperforms the conventional STBC transmitter  100  by about 2 dB at a 5% frame error rate.  
         [0032]      FIG. 5A  is a block diagram of an OFDM transmitter  500  configured in accordance with another embodiment of the present invention. The OFDM transmitter  500  includes a channel encoder  505 , a puncturer  510 , an interleaver  515 , a serial to parallel (S/P) converter  520 , a plurality of mappers  525 , a spatial mapper  530 , a cyclic shifter/interleaver  535 , a first inverse fast Fourier transform (IFFT) unit  540 , a second IFFT unit  545 , and antennas  550  and  555 . Although only one of each of antennas  550  and  555  are shown in  FIG. 5A , it should be understood that any number of antennas  550  and  555  may be used in accordance with the present invention. Incoming bits are coded by the channel encoder  505 . The coded bits are passed to the puncturer  510 , where some bits are deleted according to a predetermined puncturing pattern. The surviving bits  512  are interleaved by the interleaver  515 . It should be noted that the puncturer  510  and the interleaver  515  may be switched by properly designing the interleaving and puncturing pattern. The interleaved bits  518  are converted into a plurality of parallel substreams  522 A and  522 B, (only two of which are shown by way of example), by the S/P converter  520 .  
         [0033]     In OFDM systems, each substream  522 A and  522 B corresponds to a subcarrier. Thus, the number of substreams is determined by number of subcarriers of an OFDM communication system. Bits on each substream  522 A and  522 B are mapped by the mappers  525  to a respective channel symbol  528 A and  528 B. The stolen bits  538  are fed to the spatial mapper  530  after being cyclic-shifted and interleaved by the cyclic shifter/interleaver  535 . The stolen bits  538  are used to map the channel symbol streams into spatial streams, on a subcarrier by subcarrier basis.  
         [0034]      FIG. 5B  shows an example of an operation performed by the spatial mapper  530  used in the OFDM transmitter  500  of  FIG. 5A . The spatial mapper  530  includes a first substream mapper  532 A and a second substream mapper  532 B. Each substream mapper  532  associates a channel symbol  528  of each substream  522  with a stolen bit  538 . Each channel symbol  528  is mapped to a respective one of the spatial streams according to the value of the stolen bit  538  associated with the channel symbol  528 . A “0” (zero) is inserted in the other spatial stream of that substream.  
         [0035]     As shown in  FIG. 5B , the value of a stolen bit  538 A assigned to the first substream is equal to “1” (one). Therefore, the channel symbol  528 A on the first substream is sent to the IFFT unit  540  of the first spatial stream. In the meantime, a “0” is sent to the second spatial stream as a placeholder on the first substream. Similarly, the value of a stolen bit  538 B assigned to the second substream is equal to “0”. Therefore, the channel symbol  528 B on the second substream is sent to IFFT unit  545  of the second spatial stream, and a “0” is sent to the first spatial stream as a placeholder on the second substream.  
         [0036]     All of the substreams of a spatial stream, including “0” substreams, are converted into a time domain signal by each of the IFFT units  540  and  545 . The time domain signals are finally transmitted over the air using the antennas  550  and  555 . Although only one of each of antennas  550  and  555  are shown in  FIG. 5B , it should be understood that any number of antennas  550  and  555  may be used in accordance with the present invention.  
         [0037]     The performance advantage over STBC should be similar to narrow band systems. An additional benefit of the present invention in OFDM systems is that a peak-to-average ratio is reduced because in each spatial stream, some of subcarriers will be empty, so effective number of subcarriers is reduced.  
         [0038]      FIG. 6  is a block diagram of a MIMO transmitter  600  configured in accordance with yet another embodiment of the present invention. The MIMO transmitter  600  includes a channel encoder  605 , a puncturer  610 , an interleaver  615 , a mapper  620 , a multiplexer  625 , antenna switches  630  and  635 , and antennas  640  and  645 . Although only two of each of antennas  640  and  645  are shown in  FIG. 6 , it should be understood that any number of antennas  640  and  645  may be used in accordance with the present invention. Incoming bits are coded by the channel encoder  605 . The coded bits are passed to the puncturer  610 , where some bits are deleted according to a predetermined puncturing pattern. The surviving bits are interleaved by the interleaver  615 . It should be noted that the puncturer  610  and the interleaver  615  may be switched by properly designing the interleaving and puncturing pattern. The surviving bits are interleaved by the interleaver  615  and mapped by the mapper  620  into channel symbols. The stolen, (i.e., punctured), bits are fed into the antenna switches  630  and  635 . The symbol stream output by the mapper  620  is then split into multiple, (for example, two substreams as shown in  FIG. 6 ), streams by the multiplexer  625  and transmitted over the air from separate antennas  640  and  645  simultaneously. Different than conventional MIMO systems, stolen bits from the puncturer  610  are used by the antenna switches  630  and  635  to select an antenna  640  and  645  for each data stream.  
         [0039]     In accordance with the present invention, antenna selection is used to add extra redundancy to data transmission in order to improve the data link. However, application of data dependent antenna selection is not limited to data link improvement. When a data link is robust enough and no more extra redundancy is needed, other types of data can be used to control antenna selection, such as security related data, (e.g., digital watermarking, security keys, or the like).  
         [0040]     In the foregoing description, the punctured bits are used to select antennas for maximum code redundancy. However, the antenna selecting bits do not have to be stolen bits, and other data may be used for antenna selection.  
         [0041]     If it is considered that a combined QAM mapper and an antenna selection switch as an inner coder and a convolutional coder as an outer coder, this is a serial concatenated code and turbo decoding applies. Associate antenna selection with a coded bit (preferably stolen bit, but not necessarily). When channel state information is known, the receiver can make an estimation which antenna was used to transmit therefore obtain information on the stolen bit. The added redundancy on the stolen bit will improve decoder performance. When the transmitted bits are unknown, initial estimation on the stolen bit will be unreliable. This is why iterative decoding is necessary. As the number of iteration goes up, the receiver will have more reliable information on the transmitted bits, therefore, improve estimation on the stolen bits. This ultimately improves coding gain.  
         [0042]     Since the stolen bits are random, each codeword will likely have bits transmitted from both antennas, like conventional antenna switching. When combined with channel coding, spatial diversity is achieved.  
         [0043]     It should be noted that the system performance is not significantly affected when the channel state information of two different transmit antennas is similar. The effect is that log likelihood ratio (LLR) of stolen bits will be near zero, and will not affect decoder in either direction. However, when channel state information is significantly different, a reliable measure is achieved on the stolen bits and the greatest improvement in performance is achieved.  
         [0044]     As a logic extension, more coding gain will be achieved when multiple receive antennas are used because of diversity on the stolen bits. It is more likely to observe two different vectors, (multiple receive antennas), than two different scalars, (single receive antenna).  
         [0045]     The present invention can be applied to any wireless communication systems including, but not limited to, time division multiple access (TDMA), code division multiple access (CDMA), OFDM, single carrier-frequency division multiple access (SC-FDMA), MIMO, or the like. The present invention can be applied to both downlink and uplink.  
         [0046]     Although the features and elements of the present invention are described in the preferred 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 of the present invention. The methods or flow charts provided in the present invention 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).  
         [0047]     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.  
         [0048]     A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), a user equipment (UE), a terminal, a base station, a radio network controller, 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.