Abstract:
Methods and apparatus are provided for communicating data in a multiple antenna communication system having N transmit antennas. According to one aspect of the invention, a disclosed header format includes a legacy preamble having at least one legacy long training field and an extended portion having at least N additional long training fields on each of the N transmit antennas, wherein one or more of said at least N additional long training fields are comprised of only one OFDM symbol. The extended portion optionally comprises one or more repeated OFDM symbols for frequency offset estimation. In one implementation, the extended portion comprises a first high throughput long training field comprised of two repeated OFDM symbols and N−1 high throughput long training fields comprised of only one OFDM symbol. In another variation, the extended portion comprises N high throughput long training fields comprised of only one OFDM symbol.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/651,346, filed Feb. 9, 2005, incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to multiple antenna wireless communication systems, and more particularly, to preamble training techniques for a multiple antenna communication system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity in next generation Wireless Local Area Network (WLAN) systems. The increased robustness can be achieved through techniques that exploit the spatial diversity and additional gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques. A multiple antenna communication system increases the data rate in a given channel bandwidth by transmitting separate data streams on multiple transmit antennas. Each receiver receives a combination of these data streams on multiple receive antennas. 
         [0004]    In order to properly receive the different data streams, receivers in a multiple antenna communication system must acquire the channel matrix through training. This is generally achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation. It is desirable for multiple antenna communication systems to co-exist with legacy single antenna communications systems (typically referred to as Single Input Single Output (SISO) systems). Thus, a legacy (single antenna) communications system must be able to interpret the preambles that are transmitted by multiple antenna communication systems. Most legacy Wireless Local Area Network (WLAN) systems based upon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”). Generally, the preamble signal seen by the legacy device should allow for accurate synchronization and channel estimation for the part of the packet that the legacy device needs to understand. Previous MIMO preamble formats have reused the legacy training preamble to reduce the overhead and improve efficiency. Generally, the proposed MIMO preamble formats include the legacy training preamble and additional long training symbols, such that the extended MIMO preamble format includes at least one long training symbol for each transmit antenna or spatial stream. 
         [0005]    A number of frame formats have been proposed for evolving multiple antenna communication systems, such as MIMO-OFDM systems. Existing frame formats provide inaccurate estimations for the MIMO systems, such as inaccurate power measurement or outdated frequency offset and timing offset information, or fail to provide full backwards compatibility to the legacy devices of some vendors. In one proposed MIMO frame format, each transmit antenna sequentially transmits one or more long training symbols, such that only one transmit antenna is active at a time. As the transmit antennas are switched on and off, however, the temperature of the corresponding power amplifier will increase and decrease, respectively. Generally, such heating and cooling of the power amplifier will lead to “breathing” effects that cause the transmitted signal to have a phase or magnitude offset, relative to the desired signal. 
         [0006]    It is therefore desirable to have a continuous transmission from all transmit antennas to avoid temperature related signal “breathing.” Thus, in further proposed MIMO frame formats, orthogonality is maintained using cyclic delay diversity (CDD) or tone-interleaving across different transmit antennas. The CDD short training symbol, however, cannot measure the received signal power with sufficient accuracy. Thus, additional backoff is required in the RF chain and additional dynamic range is required in the digitization process. Likewise, the tone interleaved design is not fully backwards compatible with a number of existing 802.11a/g devices that use short training for timing synchronization or use time domain channel estimation. 
         [0007]    A need therefore exists for a method and system for performing channel estimation and training in a MIMO-OFDM system that is compatible with current IEEE 802.11a/g standard (SISO) systems, allowing MIMO-OFDM based WLAN systems to efficiently co-exist with SISO systems. A further need exists for MIMO preamble formats and training techniques that provide reduced preamble overhead. 
       SUMMARY OF THE INVENTION 
       [0008]    Generally, methods and apparatus are provided for communicating data in a multiple antenna communication system having N transmit antennas. According to one aspect of the invention, a disclosed header format includes a preamble having at least one legacy long training field and an extended portion having at least N additional long training fields on each of the N transmit antennas, wherein one or more of said at least N additional long training fields are comprised of only one OFDM symbol. The extended portion optionally comprises one or more repeated OFDM symbols for frequency offset estimation. In one implementation, the extended portion comprises a first high throughput long training field comprised of two repeated OFDM symbols and N−1 high throughput long training fields comprised of only one OFDM symbol. In another variation, the extended portion comprises N high throughput long training fields comprised of only one OFDM symbol. 
         [0009]    A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic block diagram of an exemplary MIMO transmitter; 
           [0011]      FIG. 2  is a schematic block diagram of an exemplary MIMO receiver; 
           [0012]      FIG. 3  illustrates a preamble design that is backwards compatible with 802.11a/g legacy devices; 
           [0013]      FIG. 4  illustrates the preamble format of  FIG. 3  in further detail; 
           [0014]      FIG. 5  illustrates a preamble format incorporating features of the present invention; and 
           [0015]      FIG. 6  illustrates an alternate preamble format incorporating features of the current invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The present invention provides preamble formats and techniques for preamble training for MIMO system. The training phase of a MIMO transmission will contain two phases. The first training phase is a legacy training phase particularly suited, for example, to WLAN OFDM legacy systems and the second phase is particularly suited to a multiple antenna system, such as a MIMO system. To overcome the problems in the prior systems, the Automatic Gain Control (AGC) of a receiver will perform one training sequence during the first training phase and the AGC of the receiver will retrain during the second training phase. This will allow the receiver to retrain its power measurements during the MIMO phase in order to ensure accuracy, while also allowing the receiver to be backwards compatible to WLAN systems that are not MIMO based. 
         [0017]      FIG. 1  is a schematic block diagram of a MIMO transmitter  100 . As shown in  FIG. 1 , the exemplary two antenna transmitter  100  encodes the information bits received from the medium access control (MAC) layer and maps the encoded bits to different frequency tones (subcarriers) at stage  105 . For each transmit branch, the signal is then transformed to a time domain wave form by an IFFT (inverse fast Fourier transform)  115 . A guard interval (GI) of 800 nanoseconds (ns) is added in the exemplary implementation before every OFDM symbol by stage  120  and a preamble of 32 μs is added by stage  125  to complete the packet. The digital signal is then pre-processed at stage  128  and converted to an analog signal by converter  130  before the RF stage  135  transmits the signal on a corresponding antenna  140 . 
         [0018]      FIG. 2  is a schematic block diagram of a MIMO receiver  200 . As shown in  FIG. 2 , the exemplary two antenna receiver  200  processes the signal received on two receive antennas  255 - 1  and  255 - 2  at corresponding RF stages  260 - 1 ,  260 - 2 . The analog signals are then converted to digital signals by corresponding converters  265 . The receiver  200  processes the preamble to detect the packet, and then extracts the frequency and timing synchronization information at synchronization stage  270  for both branches. The guard interval is removed at stage  275 . The signal is then transformed back to the frequency domain by an FFT at stage  280 . The channel estimates are obtained at stage  285  using the long training symbol. The channel estimates are applied to the demapper/decoder  290 , and the information bits are recovered. 
         [0019]      FIG. 3  illustrates a preamble design  300  that is backwards compatible with 802.11a/g legacy devices. The preamble design  300  provides a dedicated legacy portion  310  with a signal field for backward compatibility and a dedicated MIMO training portion  320  for performance of the MIMO system. 
         [0020]    For a detailed discussion of an implementation of the preamble design of  FIG. 3  and techniques for employing cyclic delay diversity (CDD) or tone-interleaving across different transmit antennas, see, for example, U.S. patent application Ser. No. 11/043,025, filed Jan. 24, 2005, entitled “Method And Apparatus For Preamble Training In A Multiple Antenna Communication System,” incorporated by reference herein. 
         [0021]    In the exemplary preamble design  300 , the transmitter  100  first transmits the legacy 802.11a/g preamble  310 , for example, using CDD. The legacy preamble  310  permits the performance of packet detection and coarse frequency offset estimation. The results of these two functions are also going to be used in the MIMO transmission. In addition to these two functions, the legacy preamble  310  is used to perform legacy AGC, timing and frequency synchronization and channel estimation, in a known manner. The receiver  200  then decodes the subsequent legacy and HT signal fields. The HT signal field is also transmitted using CDD. As shown in  FIG. 3 , following the legacy and HT signal fields is a MIMO short training field  330  and then the MIMO long training fields. The MIMO short training field  330  is used only to adjust the AGC setting, and the length can be much shorter than the legacy short training field. As illustrated herein, the MIMO short training field includes a 0.8 μs guard interval and a 1.6 μs training symbol for the accurate power measurement. 
         [0022]    It is noted that the preamble format  300  of  FIG. 3  may not use the first Long Preamble in the legacy header for MIMO channel estimation. The dedicated short training symbol  330  allows precise power measurement for MIMO, at the expense of higher preamble overhead (9.6 μs extra). Hence, the preamble format  300  provides low dynamic range requirements (ten bit ADC). The MIMO long training fields of  FIG. 3  can be transmitted on the same frequency grid as the data. Thus, Frequency Domain Channel Estimation (FDE) may be performed. 
         [0023]    The short training field  330  of  FIG. 3  can be constructed in a tone-interleaved fashion. While an OFDM symbol with 12 tones (i.e., 0.8 us long) would be sufficient to provide accurate power across four antennas, a 24-tone OFDM symbol (i.e. 1.6 us long) provides even more accuracy, at the expense of slightly larger overhead. The populated tones are interleaved across the transmit antennas. 
         [0024]    Since the short training symbol  330  is only 1.6 μs long, only 24 tones are used (of 64 total available tones). The indices of those tones are all multiples of four, so that the resulting time domain signal has a period of 1.6 μs. In the exemplary two antenna case, only half of the tones are transmitted on each transmitter antenna, i.e., every other used tone is transmitted on the first antenna and the rest of tones are transmitted on the second antenna. Moreover, this short training symbol  330  can be further shortened to 0.8 μs, which only uses 12 tones, to reduce the overhead. 
         [0025]      FIG. 4  illustrates the preamble format  300  of  FIG. 3  in further detail. The preamble design  300  provides the dedicated legacy portion  310  with a signal field for backward compatibility and a dedicated MIMO training portion  320  for performance of the MIMO system. As shown in  FIG. 4 , the dedicated MIMO training portion  320  includes two OFDM symbols for each MIMO long training field (HT-LTF). Thus, a single MIMO long training field in the exemplary embodiment shown in  FIGS. 3 and 4  comprises a guard interval and two tone interleaved OFDM symbols for a total duration of 7.2 μs (0.8+2*3.2 μs). It is noted that the preamble format  300  generally comprises N additional long training fields on each of N transmit antennas. Thus, the total duration of the preamble is 30.4 μs (8*3+4+2.4 μs)+N*7.2 μs. In a typical four antenna implementation, the preamble duration is 59.2 μs. The channel estimation performance that has been observed for the preamble format  300  having two OFDM symbols for each MIMO long training field is 1.76 dB less than that with an ideal channel estimation (i.e., this demonstrates the nominal channel estimation error). 
         [0026]      FIG. 5  illustrates a preamble format  500  incorporating features of the present invention. The preamble design  500  provides the dedicated legacy portion  510  with a signal field for backward compatibility and a dedicated MIMO training portion  520  for performance of the MIMO system. As shown in  FIG. 5 , the first MIMO long training field (HT-LTF) contains a repeated OFDM symbol  530 - 1 ,  530 - 2 . The motivation for repeating the OFDM symbol is to allow for a re-estimation of the fine frequency offset. However, unlike the format  320 , the “subsequent” N−1 MIMO long training fields do not contain a repeated OFDM symbol. This leads a reduction in the overall length of the preamble. Therefore, the length of the preamble can now be computed as follows: 
         [0027]    For N=1, length=30.4+7.2=37.6 us 
         [0028]    For N=2 and greater, length=37.6+(N−1)*4 us 
         [0000]    where N equals the number of spatial streams (or equivalently transmit antennas). In a typical four antenna implementation, the preamble duration is thus 49.6 μs. Note that by not repeating the OFDM symbol in subsequent MIMO long training fields, the error in channel estimation in the subsequent MIMO long training fields increases from 1.76 dB to 3 dB. 
         [0029]      FIG. 6  illustrates a preamble format  600  incorporating features of the current invention. The preamble design  600  provides the dedicated legacy portion  610  with a legacy signal field for backwards compatibility and a dedicated MIMO long training portion  620  to achieve performance of the MIMO system. As shown in  FIG. 6 , neither the first nor the subsequent MIMO Long Training Fields (HT-LTF) are repeated. This leads to the shortest possible preamble design, whose length is given by (30.4+N*4) us, where N is the number of spatial streams. For preamble format  600 , the increase in channel estimation error is 3 dB over that of an ideal channel estimation. 
         [0030]    The present invention recognizes that shortening each MIMO long training field (HT-LTF) from two OFDM symbols to one OFDM symbol increases the channel estimation error while reducing the preamble overhead. It has been observed that the preamble formats  500 ,  600  having one OFDM symbol per MIMO long training field outperforms the preamble format  400  having two OFDM symbols per MIMO long training field (in the sense of throughput versus SNR (dB)) at high SNR and larger number of spatial streams (N). Meanwhile, at lower SNR, performance of the two formats is comparable 
         [0031]    It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.