Abstract:
Methods and apparatus are disclosed for processing received data in a multiple input multiple output (MIMO) communication system. A multiple antenna receiver can distinguish a MIMO transmission from other transmissions based on the detection of a predefined symbol following a legacy portion of a preamble. A preamble comprises a legacy portion and an extended portion. The legacy portion is comprised of a first long preamble followed by a first signal field and may be processed by both multiple antenna receivers and legacy receivers. The extended portion comprises the predefined symbol following the first signal field from the legacy portion. If the predefined symbol is a second long preamble, a MIMO transmission is detected by performing a correlation on the preamble to detect the second long preamble. If the predefined symbol is a second long signal field, a MIMO transmission is detected by performing a cyclic redundancy check to detect the second long signal field.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to International Patent Application Numbers PCT/US04/21026, PCT/US04/21027 and PCT/US04/21028, each filed Jun. 30, 2004 and incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to wireless communication systems, and more particularly, to techniques for channel estimation, timing acquisition, and MIMO format detection for a multiple antenna communication system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Most existing Wireless Local Area Network (WLAN) systems based upon Orthogonal Frequency Division Multiplexing (OFDM) techniques comply with the IEEE 802.11a or IEEE 802.11g Standards (hereinafter “IEEE 802.11a/g”). See, e.g., IEEE Std 802.11a-1999, “Part  11 : Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification: High-Speed Physical Layer in the Five GHz Band,” incorporated by reference herein. In IEEE 802.11a/g wireless LANs, the receiver must obtain synchronization and channel state information for every packet transmission. Thus, a preamble is inserted at the beginning of each packet that contains training symbols to help the receiver extract the necessary synchronization and channel state information.  
         [0004]     Multiple transmit and multiple receive antennas have been proposed to increase robustness and capacity of a wireless link. Multiple Input Multiple Output (MIMO) OFDM techniques, for example, transmit separate data streams on multiple transmit antennas, and each receiver receives a combination of these data streams on multiple receive antennas. In order to properly receive the different data streams, MIMO-OFDM receivers must acquire synchronization and channel information for every packet transmission. A MIMO-OFDM system needs to estimate a total of N t N r  channel profiles, where N t  is the number of transmit antennas and N r  is the number of receive antennas.  
         [0005]     It is desirable for a MIMO-OFDM system to be backwards compatible with existing IEEE 802.11a/g receivers, since they will operate in the same shared wireless medium. A legacy system that is unable to decode data transmitted in a MIMO format should defer for the duration of the transmission. This can be achieved by detecting the start of the transmission and retrieving the length (duration) of this transmission. A need 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 systems, thus allowing MIMO-OFDM based WLAN systems to efficiently co-exist with SISO systems.  
       SUMMARY OF THE INVENTION  
       [0006]     Generally, methods and apparatus are disclosed for processing received data in a multiple input multiple output (MIMO) communication system. The invention allows a multiple antenna receiver that operates in a shared wireless medium to be backwards compatible with existing IEEE 802.11a/g receivers. A multiple antenna receiver can distinguish a MIMO transmission from other transmissions based on the detection of a predefined symbol following a legacy portion of a preamble. In particular, a preamble according to the invention comprises a legacy portion and an extended portion. The legacy portion is comprised of a first long preamble followed by a first signal field and may be processed by both multiple antenna receivers and legacy receivers. The extended portion comprises the predefined symbol following the first signal field from the legacy portion.  
         [0007]     In two exemplary embodiments, the predefined symbol may be a second long preamble or a second long signal field. In an implementation where the predefined symbol is a second long preamble, a MIMO transmission is detected by performing a correlation on the preamble to detect the second long preamble. In an implementation where the predefined symbol is a second long signal field, a MIMO transmission is detected by performing a cyclic redundancy check to detect the second long signal field.  
         [0008]     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  
       [0009]      FIG. 1  illustrates a conventional frame format in accordance with the IEEE 802.11a/g standard;  
         [0010]      FIGS. 2A and 2B  are schematic block diagrams of a conventional transmitter and receiver, respectively;  
         [0011]      FIGS. 3A and 3B  illustrate the transmission of information in SISO and MIMO systems, respectively;  
         [0012]      FIG. 4  illustrates the timing synchronization for the exemplary MIMO system of  FIG. 3B ;  
         [0013]      FIGS. 5A and 5B  are schematic block diagrams of a MIMO transmitter and receiver, respectively;  
         [0014]      FIG. 6  illustrates an exemplary preamble format that may be used in a MIMO system;  
         [0015]      FIG. 7  is a flow chart describing an exemplary receiver parametric estimation algorithm incorporating features of the present invention to process the preamble format of  FIG. 6 ;  
         [0016]      FIG. 8  illustrates an alternate preamble format that may be used in a MIMO system; and  
         [0017]      FIG. 9  is a flow chart describing an exemplary receiver parametric estimation algorithm incorporating features of the present invention to process the preamble format of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  illustrates a conventional frame format  100  in accordance with the IEEE 802.11a/g standards. As shown in  FIG. 1 , the frame format  100  comprises ten short training symbols, t 1  to t 10 , collectively referred to as the Short Preamble. Thereafter, there is a Long Preamble, consisting of a protective Guard Interval (GI 2 ) and two Long Training Symbols, T 1  and T 2 . A SIGNAL field is contained in the first real OFDM symbol, and the information in the SIGNAL field is needed to transmit general parameters, such as packet length and data rate. The Short Preamble, Long Preamble and Signal field comprise a legacy header  110 . The OFDM symbols carrying the DATA follows the SIGNAL field.  
         [0019]      FIG. 2A  is a schematic block diagram of a conventional transmitter  200  in accordance with the exemplary IEEE 802.11a/g standard. As shown in  FIG. 2A , the transmitter  200  encodes the information bits using an encoder  205  and then maps the encoded bits to different frequency tones (subcarriers) using a mapper  210 . The signal is then transformed to a time domain wave form by an IFFT (inverse fast Fourier transform)  215 . A guard interval (GI) of 800 nanoseconds (ns) is added in the exemplary implementation before every OFDM symbol by stage  220  and a preamble of 20 μs is added by stage  225  to complete the packet. The digital signal is then converted to an analog signal by converter  230  before the RF stage  235  transmits the signal on an antenna  240 .  
         [0020]      FIG. 2B  is a schematic block diagram of a conventional receiver  250  in accordance with the exemplary IEEE 802.11a/g standard. As shown in  FIG. 2B , the receiver  250  processes the signal received on an antenna  255  at an RF stage  260 . The analog signal is then converted to a digital signal by converter  265 . The receiver  250  processes the preamble to detect the packet, and then extracts the frequency and timing synchronization information at the synchronization stage  270 . The guard interval is removed at stage  275 . The signal is then transformed back to the frequency domain by an FFT  280 . The channel estimates are derived at stage  285  using the frequency domain long training symbols. The channel estimates are used by the demapper  290  to extract soft symbols, that are then fed to the decoder  295  to extract information bits.  
         [0021]      FIGS. 3A and 3B  illustrates the transmission of information in SISO and MIMO systems  300 ,  350 , respectively. As shown in  FIG. 3A , the SISO transmission system  300  comprises one transmit antenna (TANT)  310  and one receive antenna (RANT)  320 . Thus, there is one corresponding channel, h.  
         [0022]     As shown in  FIG. 3B , the exempary 2×2 MIMO transmission system  350  comprises of two transmit antennas (TANT- 1  and TANT- 2 )  360 - 1  and  360 - 2  and two receive antennas (RANT- 1  and RANT- 2 )  370 - 1  and  370 - 2 . Thus, there are four channels profiles: h 11 , h 12 , h 21  and h 22 . The additional channels makes both timing synchronization and channel estimation more challenging. In order to perform channel estimation, the training preamble of  FIG. 1  needs to be lengthened.  
         [0023]      FIG. 4  illustrates the timing synchronization for the exemplary MIMO system  350  of  FIG. 3B  having four channels h 11 , h 12 , h 21  and h 22 . The exemplary guard interval (GI) should be placed as a window of 800 ns (i.e., 16 Nyquist samples) that contains most of the energy of the impulse responses  410 ,  420 ,  430 ,  440  corresponding to the four channels h 11 , h 12 , h 21  and h 22 . In other words, the guard interval is positioned to find the optimum  64  sample window for the OFDM symbol within the 80 sample window (that most avoids the four impulse responses). For the MIMO case, the guard interval window should be chosen to maximize the total power of all four channels.  
         [0024]      FIG. 5A  is a schematic block diagram of a MIMO transmitter  500 . As shown in  FIG. 5A , the transmitter  500  encodes the information bits and maps the encoded bits to different frequency tones (subcarriers) at stage  505 . For each transmit branch, the signal is then transformed to a time domain wave form by an IFFT (inverse fast Fourier transform)  515 . A guard interval (GI) of 800 nanoseconds (ns) is added in the exemplary implementation before every OFDM symbol by stage  520  and a preamble of 32 μs is added by stage  525  to complete the packet. The digital signal is then converted to an analog signal by converter  530  before the RF stage  535  transmits the signal on a corresponding antenna  540 .  
         [0025]      FIG. 5B  is a schematic block diagram of a MIMO receiver  550 . As shown in  FIG. 5B , the exemplary 2×2 receiver  550  processes the signal received on two receive antennas  555 - 1  and  555 - 2  at corresponding RF stages  560 - 1 ,  560 - 2 . The analog signals are then converted to digital signals by corresponding converters  565 . The receiver  550  processes the preamble to detect the packet, and then extracts the frequency and timing synchronization information at synchronization stage  570  for both branches. The guard interval is removed at stage  575 . The signal is then transformed back to the frequency domain by an FFT at stage  580 . The channel estimates are obtained at stage  585  using the long training symbol. The channel estimates are applied to the demapper/decoder  590 , and the information bits are recovered.  
         [0026]     As previously indicated, a MIMO-OFDM system should be backwards compatible with existing IEEE 802.11a/g receivers. A MIMO system that uses at least one long training field of the IEEE 802.11a/g preamble structure repeated on different transmit antennas can scale back to a one-antenna configuration to achieve backwards compatibility. A number of variations are possible for making the long training symbols backwards compatible. In one variation, the long training symbols can be diagonally loaded across the various transmit antennas. In another variation, 802.11a long training sequences are repeated in time on each antenna. For example, in a two antenna implementation, a long training sequence, followed by a signal field is transmitted on the first antenna, followed by a long training sequence transmitted on the second antenna. A further variation employs MIMO-OFDM preamble structures based on orthogonality in the time domain.  
         [0027]     According to one aspect of the present invention, a parametric estimation algorithm at the receiver, discussed further below in conjunction with  FIGS. 7 and 9 , provides the multiple training needed in a MIMO system to get the improved frequency offset estimation, optimal timing offset estimation and complete channel estimation. Moreover, using the two signaling schemes in this invention, the receiver can effectively detect the MIMO transmission while still maintaining backwards compatibility.  
         [0028]      FIG. 6  illustrates an exemplary preamble format  600  using the long preamble for MIMO signaling. In the preamble format  600  of  FIG. 6 , the first long preamble LP- 1  is sent after the short preamble SP- 1 . SP- 1  consists of 10 identical short training symbols (STS). LP- 1  consists of extended GI (GI 2 ), and two identical long training symbols, LTS- 1  and LTS- 2 . The first signal field, SF 1 , which is the same as the 802.11a/g legacy signal field, is transmitted after the first long preamble LTS- 1 . The Short Preamble STS- 1 , first Long Preamble LTS- 1  and the first Signal field SF- 1  comprise a legacy header  610 .  
         [0029]     Thereafter, the second long preamble LP- 2  is transmitted and then an optional second signal field SF- 2 . The first and second long preambles LP- 1 , LP- 2  are constructed using the 802.11a/g long preamble with a long guard interval of 1.6 μs and two indentical long training symbols, LTS- 1  and LTS- 2 . The long preambles LP- 1 , LP- 2  transmitted from different transmitter antennas at different time are all derived from the 802.11a/g long training symbols. The first signal field SF- 1  transmitted from different antennas is derived in the same fashion as the first long trainig symbol. The MIMO data follows the second signal field SF- 2 .  
         [0030]     The first short preamble SP- 1  is used by both receive branches RANT- 1  and RANT- 2  to perform carrier detection, power measurement (automatic gain control) and coarse frequency offset estimation. The first long preamble LP- 1  is used by both receive branches RANT- 1  and RANT- 2  to perform fine frequency offset estimation, windowed FFT timing and SISO channel estimation. The second long preamble LP- 2  is used by both receive branches RANT- 1  and RANT- 2  to perform MIMO channel estimation, refine fine frequency offset estimation and refine the windowed FFT timing.  
         [0031]     It is noted that in a SISO system, the receiver would expect to receive data after the first signal field SF- 1 . The present invention provides receiver parametric estimation algorithms  700 ,  900 , discussed further below in conjunction with  FIGS. 7 and 9 , respectively, that allow a MIMO receiver  550  to detect whether a second long training preamble LP- 2  will follow the first signal field SF- 1  (indicating a MIMO transmission), without any explicit signaling requirement.  
         [0032]      FIG. 7  is a flow chart describing an exemplary receiver parametric estimation algorithm  700  incorporating features of the present invention. The receiver parametric estimation algorithm  700  processes the preamble format  600  of  FIG. 6 . As shown in  FIG. 7 , the receiver parametric estimation algorithm  700  is initially in an idle mode  710  until a positive carrier is detected on both receive branches. Once a positive carrier is detected, the receiver parametric estimation algorithm  700  performs power measurements and coarse frequency offset (CFO) estimation on both receive branches during step  720 .  
         [0033]     When the start of the first long training preamble LP- 1  is detected, a fine frequency offset (FFO) estimate and fine timing are performed on receive branches RANTI and RANT 2  and estimates are obtained for the SISO and MIMO channels during step  730 . Thereafter, the first signal field SF- 1  is decoded during step  740 .  
         [0034]     The receiver parametric estimation algorithm  700  then begins processing the received signal on two parallel branches, a MIMO track and a SISO track. On the MIMO track, the long training symbol LTS- 1  is correlated with LTS- 2  in the second long preamble, LP- 2 , during srep  750 . This process corresponds to an autocorrelation with an offset of 64 samples (i.e. 3.2 us). If the correlation exceeds a defined threshold, a MIMO transmission is detected.  
         [0035]     On a parallel SISO track, the received signal is processed in a conventional manner as if it is a SISO payload. If the MIMO track does not detect the start of the second long training symbol LTS- 2  during step  750 , then the received signal is processed as a SISO signal during step  760 . If, however, the MIMO track does detect the start of the second long training symbol LTS- 2  during step  750 , then the received signal is processed as a MIMO signal and program control proceeds to step  770 . In particular, the MIMO transmission is processed during step  770  to refine the fine frequency offsets on both receive branches RANT 1  and RANT 2 . As shown in  FIG. 4 , the optimal timing can only be acquired whan all four channel impulse responses are available, which is only possible after receiving the second long preamble LP- 2 . Hence, the FFT timing window is adjusted on both receive branches RANT 1  and RANT 2  and the MIMO channel estimation is completed. The second signal field SF- 2  is decoded during step  780  and the MIMO payload is processed during step  790 , before program control terminates (i.e., signifying the end-of-packet).  
         [0036]      FIG. 8  illustrates an alternate preamble format  800  that uses a second signal field to signal the MIMO transmssion. As shown in  FIG. 8 , the alternate preamble format  800  changes the order of the second long preamble and second signal field, relative to the preamble format  600  of  FIG. 6 . In the alternate preamble format  800 , the second signal field SF- 2  is transmitted right after the first signal field SF- 1  and the positive decoding of the second signal field SF- 2  is used to signal the MIMO transmission. The Short Preamble SP- 1 , first Long Preamble LP- 1  and the first Signal field SF- 1  comprise a legacy header  8610 .  
         [0037]      FIG. 9  is a flow chart describing an exemplary receiver parametric estimation algorithm  900  incorporating features of the present invention. The receiver parametric estimation algorithm  900  processes the preamble format  800  of  FIG. 8 . As shown in  FIG. 9 , the receiver parametric estimation algorithm  900  is initially in an idle mode  910  until a positive carrier is detected on both receive branches. Once a positive carrier is detected, the receiver parametric estimation algorithm  900  performs power measurements and coarse frequency offset (CFO) estimation on both receive branches during step  920 .  
         [0038]     When the start of the first long training preamble LP- 1  is detected, a fine frequency offset (FFO) estimate and fine timing are performed on receive branches RANT 1  and RANT 2  and estimates are obtained for the SISO and MIMO channels (h 11  and h 21 ) during step  930 . Thereafter, the first signal field SF- 1  is decoded during step  940 .  
         [0039]     The receiver parametric estimation algorithm  900  then begins processing the received signal on two parallel branches. On a MIMO track, the second signal field is decoded during step  950 . A positive CRC check is used to detect the MIMO transmission. On a parallel SISO track, the received signal is processed in a conventional manner as if it is a SISO payload.  
         [0040]     If the MIMO track does not detect the start of the second signal field SF- 2  during step  950 , then the received signal is processed as a SISO signal during step  960 . If, however, the MIMO track does detect the start of the second signal field SF- 2  during step  950 , then the received signal is processed as a MIMO signal and program control proceeds to step  970 . In particular, the MIMO transmission is processed during step  970  to refine the fine frequency offsets on both receive branches RANT 1  and RANT 2 . In addition, the FFT timing window is adjusted on both receive branches RANT 1  and RANT 2  and the MIMO channel estimation (h 22  and h 12 ) is completed. The MIMO payload is processed during step  990 , before program control terminates.  
         [0041]     It is noted that the performance of the receiver parametric estimation algorithms  700 ,  900  can each be optionally improved by performing both the autocorrelation on the second Long Preamble LP- 2  and the cyclic redundancy check on the second signal field SF- 2 .  
         [0042]     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.