Patent Publication Number: US-6912241-B2

Title: Chip-interleaved, block-spread multi-user communication

Description:
This application claims priority from U.S. Provisional Application Ser. No. 60/274,365, filed Mar. 8, 2001, the contents being incorporated herein by reference. 

   This invention was made with government support under ECS-9979443 awarded by the National Science Foundation. The government has certain rights in the invention. 

   TECHNICAL FIELD 
   The invention relates to communication systems and, more particularly, transmitters and receivers for use in multi-user communication systems. 
   BACKGROUND 
   In multi-user wireless communication systems, such as mobile phone networks, wireless local area networks and satellite communications, multiple transmitters and receivers may communicate simultaneously through a common wireless communication medium. One communication format widely used by multi-user systems is Code Division Multiple Access (CDMA), in which the transmitters generate orthogonal waveforms that can be separated by the receivers. More specifically, each transmitter applies one code chosen from a set of orthogonal “spreading codes” to an outbound serial stream of “symbols.” Each symbol represents a discrete information bearing value selected from a finite set (“alphabet”). For example, simple alphabets used by transmitters may be {+1,−1} or {−3,−1,+1,+3}. The application of the orthogonal spreading codes to the symbols produces a set of “chips” for each symbol to be transmitted. The resulting chips are transmitted according to some modulation scheme, such as quadrature phase shift keying (QPSK) modulation. In order to separate signals from multiple users, the receivers isolate the signal of the desired user by matching the signal to the corresponding orthogonal spreading code. 
   When the transmission rate increases, the communication medium can become “frequency selective” in that certain frequencies exhibit significant fading, i.e., significant loss of signal. This property often causes inter-chip interference (ICI) in which the transmitted chips for a particular symbol interfere with each other, destroying the orthogonality of the waveforms at the receiver. By rendering the transmitted waveforms non-orthogonal, ICI can lead to multiple user interference (MUI), in which the receivers are unable to correctly separate the waveforms, eventually leading to data loss and/or bandwidth and power inefficiencies. 
   Various techniques have been developed that attempt to suppress the effects of MUI. For example, various “multi-user detectors” have been developed for separating non-orthogonal user waveforms. These detectors, however, typically use techniques that require knowledge of the characteristics of the current communication medium and that are often complex and expensive to implement in typical mobile communication devices. In addition, alternatives to CDMA have been proposed including multicarrier (MC) spread spectrum based multiple access, e.g., (generalized) MC-CDMA and Orthogonal Frequency Division Multiple Access (OFDMA), where complex exponentials are used as information-bearing carriers to maintain orthogonality in the presence of frequency selective channels. Multicarrier schemes are power inefficient because their transmissions have non-constant magnitude in general, which causes power amplifiers to operate inefficiently. These alternatives can also be very complex and expensive to implement and do not necessarily compensate for channels that introduce significant fading. 
   SUMMARY 
   In general, the described invention provides an efficient technique for maintaining the orthogonality of waveforms in multi-user wireless communication systems, such as systems using the code division multiple access (CDMA) communication formats. Unlike conventional systems in which spreading is performed on a per symbol basis, the “block-spreading” techniques described herein operate on blocks of symbols. Furthermore, the resulting chips are interleaved such that the chips generated from any particular symbol are temporally spaced and separated by “guard” chips. In this manner, the symbol-bearing chips and guard chips are transmitted and received in an alternating format. 
   In one embodiment, the invention is directed to a system in which a transmitter includes a block-spreading unit to form a set of chips for each symbol of a block of information-bearing symbols and to produce a stream of chips in which the chips from different sets are interleaved. A pulse shaping unit within the transmitter generates a transmission signal from the stream of interleaved chips and transmits the signal through a communication channel. A receiver includes a block separator to de-interleave the chips prior to user separation. 
   In another embodiment, the invention is directed to a transmitting device having a symbol-spreading unit to apply a user-specific orthogonal spreading code to each symbol within a block of symbols, thereby forming chips. A buffer stores the resulting chips in an array and pads the chips with “guard” chips. In one configuration the array comprises M columns and K+L rows, where K is the number of symbols per block, L represents the number of guard chips and is a function of the communication channel length, and M represents a maximum number of users. The buffer stores the chips such that each row in the array contains chips generated from the same symbol. A chip-interleaving unit within the transmitting device reads the chips column-wise from the buffer and outputs a stream of chips in which the chips from different sets are interleaved. 
   In another embodiment, the invention is directed to a method in which a set of orthogonal spreading codes is applied to a block of information-bearing symbols to form a set of chips for each symbol. Chips from the chip sets are selected in an order that produces a stream in which the chips from different sets are interleaved. A transmission signal is generated from the stream of interleaved chips. 
   In yet another embodiment, the invention is directed to a computer-readable medium having instructions thereon. The instructions cause a programmable processor to apply a set of orthogonal spreading codes to a block of information-bearing symbols to form a set of chips for each symbol. The instructions cause the programmable processor to select chips from the chip sets to produce a stream of chips in which the chips from different sets are interleaved, and generate a transmission signal from the stream of interleaved chips. 
   The chip-interleaved block-spread communication techniques described herein offer many advantages. By interleaving the chips generated from blocks of symbols, the transmitter and receiver are resistant to multi-user interference (MUI), regardless of the underlying frequency selective nature of the communication channel and without using adaptive power control to dynamically adjust the usage of power by transmitter. Because the chips generated from a common symbol are temporally spaced and separated by guard chips, interference experienced during propagation through the channel will cause inter-symbol interference, but inter-chip interference is avoided. Therefore, the spreading codes for the various users may remain orthogonal regardless of channel effects. 
   Other advantages of block spreading include improved bandwidth efficiency, which implies that information can be transmitted at higher rates and that the maximum allowable number of simultaneous users increases. Furthermore, because the waveforms remain orthogonal, the techniques allow receivers to use single-user detectors, which are less complex than multi-user detectors. In other words, the techniques allow receivers to be configured with low complexity detectors that achieve performance equivalent to a set of M single user detectors. The techniques can be easily used with existing code-generation schemes that generate orthogonal spreading codes. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a block diagram illustrating a wireless system in which multiple transmitters communicate with multiple receivers through a channel. 
       FIG. 2  is a block diagram illustrating in further detail the multi-user communication system of FIG.  1 . 
       FIG. 3  is a block diagram illustrating an example embodiment of a block-spreading unit within a transmitter. 
       FIG. 4  illustrates an example mode of operation of the block-spreading unit. 
       FIG. 5  illustrates an example data stream generated by a symbol-spreading unit within the transmitter. 
       FIG. 6  illustrates an example data stream generated by a chip-interleaving unit within the transmitter. 
       FIG. 7  illustrates an example arrangement of the chips stored within a buffer in array format. 
       FIG. 8  is a flowchart illustrating an example mode of operation of communication system in which a transmitter and a receiver communicate using chip-interleaved block-spread communications. 
       FIGS. 9 ,  10  and  11  are graphs illustrating modeled performance estimates of the chip-interleaved block-spreading techniques described herein. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram illustrating a multi-user wireless communication system  2  in which multiple transmitters  4  communicate with receivers  6  through wireless channel  8 . In general, the invention provides an efficient technique for maintaining the orthogonality of waveforms produced by transmitters  4 , thereby suppressing any effects of multi-user interference (MUI)  9  that could otherwise be introduced during transmission through communication channel  8 . 
   Transmitters  4  transmit data using a multi-user spreading scheme referred to herein as “chip-interleaved block-spreading” (CIBS). This scheme can work with existing spreading code generating schemes and, therefore, is backward compatible with a number of conventional multi-user transmission formats including Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM). The former is an example of single-carrier multiple access scheme, while the latter is a multi-carrier scheme. OFDM has been adopted by many standards including digital audio and video broadcasting (DAB, DVB) in Europe and high-speed digital subscriber lines (DSL) in the United States. OFDM has also been proposed for local area mobile wireless broadband standards including IEEE802.11 a , MMAC and HIPERLAN/2. 
   The techniques described herein apply to uplink and downlink transmissions, i.e., transmissions from a base station to a mobile device and vice versa. Transmitters  4  and receivers  6  may be any device configured to communicate using a multi-user wireless transmission including a cellular distribution station, a hub for a wireless local area network, a cellular phone, a laptop or handheld computing device, a personal digital assistant (PDA), a Bluetooth™ enabled device and the like. 
     FIG. 2  is a block diagram illustrating in further detail the multi-user communication system  2  of FIG.  1 . Generally, multiple transmitters  4  corresponding to different users use orthogonal spreading codes to generate transmission waveforms  20  in which the codes remain orthogonal during communication to receiver  6  through channel  8 . Serial to parallel (S/P) converter  12 A of transmitter  4  parses outbound data  23  from a serial data stream of symbols into blocks of K symbols  29 , each symbol representing a discrete information bearing value selected from a finite alphabet. 
   As described in detail below, CIBS unit  14  applies an a user-specific orthogonal spreading code of length M to each of the symbols to produce a set of “chips” for each symbol, thereby spreading the data for each symbol. CIBS unit  14  may, for example, apply a user-specific spreading code of {−1,−1,+1,+1}, in which M equals 4. Next, CIBS unit  14  interleaves the chips to produce data stream  25 . Parallel to serial (P/S) converter  16 A converts the interleaved chip data into a serial bit stream, from which pulse shaping unit  17  forms transmission waveform  20 , which is a continuous time signal for carrying the chip-interleaved, block-spread data through channel  8 . 
   Generally, CIBS unit  14  can be viewed as producing P interleaved chips from blocks of K symbols, where P can be determined as follows:
 
 P=M* ( K+L ).
 
L represents a number of spacing chips, referred to herein as “guard” chips, and is determined by the effective length of channel  8  in discrete time, such as 5, 10 or 15 chips long. M represents the length of the user-specific code, i.e., the maximum number of users that can be supported simultaneously.
 
   Receiver  6  receives incoming data stream  21 , which typically is a function of transmission waveform  20  and noise introduced by channel  8 . Block separator  18  samples data stream  21  and buffers the discrete data by storing the interleaved chips in array form. Block separator  18  selectively outputs blocks of chips that are associated with a common symbol, thereby de-interleaving the chips. 
   Single user detector  22  receives the de-interleaved chips from block separator  18  and applies a conventional matched filter to separates users based on orthogonality and is followed by a single-user decoding scheme to remove channel effects and output the estimated symbols. Parallel to serial converter  16 B converts the stream of symbols produced by single user detector  22  into serial data stream  24 . In this manner, the techniques allow transceivers to be configured with low complexity detectors that achieve performance equivalent to a set of M single user detectors. 
     FIG. 3  is a block diagram illustrating an example embodiment of CIBS unit  14  of transmitter  4 . In this embodiment, CIBS unit  14  comprises a symbol-spreading unit  26  that applies a user-specific orthogonal spreading code of length M to an incoming stream of symbols  29 . As such, chip buffer  27  and chip-interleaving unit  28  may be used with any conventional symbol-spreading scheme that uses orthogonal spreading codes. Symbol-spreading unit  26  applies a user-specific, spreading code of length M to produce a set of “chips” for each symbol, thereby orthogonally spreading each symbol, where M represents the maximum allowable number of simultaneously communicating users. 
   Chip buffer  27  receives and stores the chips in array fashion, where each row of the array stores M chips generated from the same symbol. Chip buffer  27  may be a dedicated hardware buffer, random access memory, or any suitable device for storing the chips. After buffering the K rows of M chips, chip buffer  27  pads the information with L rows of “guard” chips (G), where L indicates the effective length of the channel  8  in discrete time, such as 5, 10 or 15 chips long. The guard chips introduce a guard time of length L that can be made arbitrarily negligible as the block size (K) is increased. In one configuration, the guard chips are null (zero) values. In another configuration, the values are drawn from the same modulation constellation as the current symbol, allowing for constant modulus for the output amplifier of transmitter  4 , thereby increasing power efficiencies. Thus, in one embodiment, chip buffer  27  stores an array having dimensions of [M, K+L], where M represents the maximum allowable number of simultaneously communicating users, K represents the number of symbols in a block and L represents a number of guard chips. Chip-interleaving unit  28  reads and transmits the chips from chip buffer  27  in column-wise fashion, thereby interleaving the chips to produce output stream  25 . 
     FIG. 4  illustrates an example mode of operation of CIBS unit  14  while generating a chip-interleaved block-spread waveform. In this example, input data stream  29  comprises two symbols to be transmitted: {A,B}. Symbol-spreading unit  26  sequentially applies a user-specific orthogonal spreading code  33  of length 4 (M equals 4) to the symbols, to produce chips  21 . In this example, symbol-spreading unit  26  applies a spreading code of {−1,−1,+1,+1} to the first symbol {A} of input data stream  29  to produce chips {−A, −A, A, A}. Next, symbol-spreading unit  26  applies spreading code  33  to the second symbol {B} of input data stream  29  to produce chips {−B, −B, B, B}. 
   Chip buffer  27  receives and stores the generated chips  29  in matrix form. The first two rows of chip buffer  27  store the set of chips generated from symbols A and B, respectively. After storing the chips, chip buffer  27  pads the information with a row of guard chips  38 . Chip-interleaving unit  28  extracts data from chip buffer  27  in column-wise fashion, thereby interleaving the coded symbol information to produce output data stream  25 . Thus, in this example, the maximum number of simultaneous users (M) equals 4, the blocks of symbols that are spread and interleaved (K) equals 2, and the channel length (L) equals 1. 
   It can be seen from data stream  25  that the chips for the symbols  29  are interleaved, in that data stream  25  repeats a pattern of chips that includes a chip generated from symbol A, a chip generated from symbol B, and a guard chip. Within a communication cell, every user has a user-specific spreading code that is orthogonal to the spreading codes of the other users. The chip-interleaved block-spreading process can be viewed as using the user-specific spreading code  33  to create “child” orthogonal codes for each symbol in the block. For example, examination of data stream  25  reveals that the child spreading code for the first symbol A can be viewed as {−1,0,0,−1,0,0,+1,0,0,+1,0,0}. Similarly, the child spreading code for symbol B is {0,−1,0,0,−1,0,0,+1,0,0,+1,0}. 
   By interleaving the chips corresponding to different symbols within a block of symbols, transmitter  4  and receiver  6  are resistant to multi-user interference (MUI), regardless of the underlying frequency selective nature of channel  8  and without using adaptive power control to dynamically adjust the usage of power by transmitter  4 . Interference experience during propagation through the channel may cause inter-symbol interference, but interchip interference is avoided because the chips for a given symbol are spaced in time and separated by guard chips. Therefore, the spreading codes for the various users remain orthogonal regardless of channel effects. 
     FIG. 5  illustrates in more detail data stream  21  generated by symbol-spreading unit  26 . Data stream  21  includes a sequence of chips, designated as C (k,m) , where k identifies a particular symbol within block of symbols and ranges from 0 to K−1, and m identified a particular chip for the user-specific spreading code and ranges from 0 to M−1. As illustrated in  FIG. 5 , symbol-spreading unit  26  outputs M chips for each of the K symbols within a block of symbols. 
     FIG. 6  illustrates in more detail data stream  25  generated by chip-interleaving unit  28 . Data stream  25  includes a sequence of chips, again designated as C (k,m) . Chip-interleaving unit  28  produces data stream  25  such that the chips generated from a common symbol are temporally distributed and separated by guard chips  42 . For example, a first set of generated chips  44 A includes the first chip D (k,0)  generated from each of the K symbols, followed by L guard chips  42 A. The second set of generated chips  44 B includes the second chip D (k,1)  generated from each of the K symbols, again followed by L guard chips  42 B. This pattern repeats until chip-interleaving unit  28  has extracted all M*(K+L) chips from chip buffer  27  and transmitted the chips via data stream  25 . 
     FIG. 7  illustrates an example arrangement of the chips within chip buffer  27 . In this arrangement the chips are organized in an array having M columns and K+L rows. Each row within buffer  27  is filled from data stream  21  and stores M chips generated from a common symbol. Chip-interleaving unit  28  read the chips from chip buffer  27  in column wise format in which each column holds K chips, each chip being generated from a unique one of the K symbols, and L guard chips. As discussed above, block separator  18  stores the received chips in a similar array. While receiving the interleaved chips from data stream  21 , however, block separator fills the array column by column, and then outputs the data row by row, thereby de-interleaving the chips. 
     FIG. 8  is a flowchart illustrating an example mode of operation of communication system  2  of  FIG. 1  in which transmitter  4  and receiver  6  communicate using chip-interleaved block-spread communications. Generally, transmitter  4  parses an outbound serial data stream into blocks of K symbols ( 50 ) and applies a user-specific code of length M to each of the symbols within the blocks ( 52 ). This generates a set of M chips for each symbol, which transmitter  4  buffers in array format ( 54 ) and pads with guard chips ( 56 ). 
   After block-spreading the symbols, transmitter  4  extracts the chips from the buffer so as to interleave the chips for the K symbols ( 58 ). The chips that are generated from the same symbol, therefore, are temporally spaced and separated by the guard chips. In this manner, each block of K symbols produces M*(K+L) interleaved chips, where L represents the number of guard chips and M represents the maximum number of users that can be supported simultaneously. The transmitter converts the interleaved chips into a serial bit stream ( 60 ) and outputs a transmission waveform for carrying the chip-interleaved, block-spread data through the communication channel  8  to receiver  4  ( 62 ). 
   Receiver  6  receives the incoming data stream ( 64 ) and stores the interleaved chips in array form ( 66 ). Once all of the chips for a block of symbols have been received and stored, the receiver reads K+L subsets of the stored chips, each subset having M chips that were generated from the same symbol, thereby de-interleaving the chips ( 68 ). Receiver  4  then applies a matched filter ( 70 ) to each subset in order to separate the symbols for the multiple users based on orthogonality, and then applies a single-user decoding scheme ( 72 ) to remove channel effects and output the estimated symbols. Receiver  4  then converts the symbols into serial data ( 74 ). 
     FIG. 9  is a graph illustrating how the described chip-interleaved block-spreading (CIBS) technique described herein can increase bandwidth efficiency as the number of symbols K within a block is increased. In particular,  FIG. 9  illustrates how the maximum number of supported users M increases as K increases from 2 to 16 symbols per block, where the system spreading gain is set to 33. The shift orthogonal CDMA method  80  is illustrated for comparison purposes. As a baseline, the conventional CDMA method used in practice would only allow the number of users to be up to about 25% of the system&#39;s spreading gain, which is approximately 8 users here; thus, conventional CDMA would allow even less users than the shift-orthogonal CDMA shown in FIG.  9 . Plot  82  illustrates modeled results when the chip-interleaved block-spreading technique is applied to a communication channel having a length (L+1) of two chips in discrete time. Plot  84  illustrates modeled results for a communication channel when the channel length equals four chips. As can be seen from  FIG. 9 , the number of supported users approximately doubles the number supported by shift-orthogonal CDMA as the block size is increased from 2 to 16 blocks. 
     FIGS. 10 and 11  are graphs illustrating bit error rate of the CIBS technique compared to conventional multi-user detection schemes, which are often much more complex and expensive to implement.  FIG. 10  illustrates the CIBS technique compared with Direct Sequence (DS) CDMA.  FIG. 11  illustrates the CIBS technique compared with Multicarrier (MC) CDMA. 
   Various embodiments of the invention have been described. The invention provides efficient techniques for maintaining the orthogonality of user waveforms in multi-user wireless communication systems, such as systems using code division multiple access (CDMA). Unlike conventional systems in which spreading is performed on a per symbol basis, the chip-interleaved block-spreading techniques described herein spread blocks of symbols and interleave the resulting chips. The inventive techniques can be embodied in a variety of receivers and transmitters including base stations, cell phones, laptop computers, handheld computing devices, personal digital assistants (PDA&#39;s), and the like. The devices may include a digital signal processor (DSP), field programmable gate array (FPGA), application specific integrated circuit (ASIC) or similar hardware or software. These and other embodiments are within the scope of the following claims.