Abstract:
A method and apparatus are disclosed for transmitting symbols in a multiple antenna communication system. The disclosed frame structure comprises a preamble having a plurality of long training symbols that are transmitted on a plurality transmit antennas. At least a portion of the frame is delayed on at least one transmit antenna. The disclosed frame formats of the present invention are backwards compatible to existing single antenna communication systems. The delay amount, D, can be approximately equal to one OFDM time sample period, T. The delayed version can be obtained by introducing a time delay into a signal on the delayed branch(es) or by cyclically shifting at least a portion of each frame on the delayed branch(es). The entire frame or only the preamble portion of each frame can be delayed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/539,699, filed Jan. 27, 2004, incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to transmission techniques for a wireless communication system, and more particularly, to channel estimation and 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 antenna 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 a channel matrix through training. This is often achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation techniques. It is desirable for multiple antenna communication system 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 an accurate synchronization and channel estimate for the part of the packet that the legacy device needs to understand.  
         [0005]     A number of frame formats have been proposed for evolving multiple antenna communication systems, such as MIMO-OFDM systems. In one proposed frame format, the MIMO preamble is extended to include at least one long training symbol for each transmit antenna, and each transmit antenna sequentially transmits one or more long training symbols, such that only transmit antenna is active at a time. As the transmit antennas are switched on and off, 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. It is therefore desirable to have a continuous transmission from all transmitters once the packet transmission has started to avoid temperature related signal “breathing.” 
         [0006]     In a frame format that continuously transmits signals from all transmit branches, the same long training symbol is typically transmitted by all transmit antennas at the same time (possibly multiplied by a constant value). This scheme, however, may lead to a beamforming phenomenon if the transmitter is not surrounded by many scattering objects. In other words, when multiple antennas transmit the same signal (possible differing by a constant factor), then certain areas in the environment will be better illuminated than others. Depending on the constant factor between the various transmit antennas, these illumination areas will change.  
         [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 a method and system for performing channel estimation and training in a MIMO-OFDM system that does not cause beamforming effects.  
       SUMMARY OF THE INVENTION  
       [0008]     Generally, a method and apparatus are disclosed for transmitting symbols in a multiple antenna communication system. The disclosed frame structure comprises a preamble having a plurality of long training symbols that are transmitted on a plurality transmit antennas. According to one aspect of the invention, at least a portion of the frame is delayed on at least one transmit antenna. According to a further aspect of the invention, the preamble portion of each said frame can be processed by a single antenna communication system. Thus, the disclosed frame formats of the present invention are backwards compatible to existing single antenna communication systems.  
         [0009]     In an exemplary multiple antenna communication system having two transmit branches, at least a portion of a frame is transmitted on a first transmit antenna; and a delayed version of at least a portion of the frame is transmitted on a second transmit antenna. Further delays are employed on each transmit branch for higher order multiple antenna communication systems. The delay amount, D, can be approximately equal to one OFDM time sample period, T.  
         [0010]     The delayed version can be obtained by introducing a time delay into a signal on the delayed branch(es) or by cyclically shifting at least a portion of each frame on the delayed branch(es). In one implementation, the entire frame is delayed. In a further variation, only the preamble portion of each frame is delayed and the data portion of each frame is time aligned on each transmit antenna.  
         [0011]     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  
       [0012]      FIG. 1  illustrates an exemplary conventional MIMO-OFDM environment in which the present invention can operate;  
         [0013]      FIG. 2  illustrates a frame format for a conventional SISO OFDM system;  
         [0014]      FIGS. 3A and 3B  illustrate exemplary frame formats for a MIMO OFDM system that provide backwards compatability for an exemplary implementation having two transmit antennas TX- 1  and TX- 2 ;  
         [0015]      FIG. 4  illustrates a beamforming effect that results from the simultaneous transmission of the same symbol from two or more transmit branches in a multiple antenna communication system;  
         [0016]      FIGS. 5A and 5B  illustrates the transmission of the two long preamble segements LP.a and LP.b, respectively, along with the corresponding channel coefficients and receive signals, for the preamble scheme of  FIG. 3A ;  
         [0017]      FIG. 6  illustrates a channel estimation structure that can further process the signals of  FIG. 5  to produce the desired channel coefficients;  
         [0018]      FIG. 7  illustrates a frame format incorporating features of the present invention for an exemplary implementation having two transmit antennas;  
         [0019]      FIG. 8  is a schematic block diagram of a transmitter incorporating features of the present invention;  
         [0020]      FIG. 9  illustrates a frame format incorporating features of the present invention that is a generalization of the preamble format of  FIG. 7 ;  
         [0021]      FIG. 10  illustrates a frame format incorporating features of the present invention for an exemplary multiple antenna communication system having three transmit antennas;  
         [0022]      FIG. 11  is a schematic block diagram of a transmitter incorporating features of the present invention for an exemplary multiple antenna communication system having three transmit antennas;  
         [0023]      FIG. 12  illustrates a channel estimation structure that produces the channel coefficients for a multiple antenna communication system having three transmit antennas;  
         [0024]      FIG. 13  illustrates a frame format incorporating features of the present invention for an exemplary multiple antenna communication system having four transmit antennas;  
         [0025]      FIGS. 14A and 14B  illustrate frame formats where a second signal field SIG 2  is found after the first signal field SIG 1 ;  
         [0026]      FIG. 15  illustrates a frame format incorporating features of the present invention for an exemplary two transmit branch scheme that only delays the preambles and SIGNAL field but not the data;  
         [0027]      FIG. 16  illustrates a frame format incorporating features of the present invention for an exemplary three transmit branch scheme that only delays the preambles and SIGNAL field but not the data;  
         [0028]      FIG. 17  illustrates an alternate scheme for an exemplary multiple antenna communication system having two transmit branches that overcomes the drawback of the delay spread increase in the physical channel;  
         [0029]      FIG. 18  illustrates an alternate scheme for an exemplary multiple antenna communication system having three transmit branches that overcomes the drawback of the delay spread increase in the physical channel; and  
         [0030]      FIG. 19  illustrates a further variation of the present invention where the cyclic rotation is performed only on the short preamble, long preambles and signal field(s). 
     
    
     DETAILED DESCRIPTION  
       [0031]      FIG. 1  illustrates an exemplary conventional MIMO-OFDM environment in which the present invention can operate. As shown in  FIG. 1 , an exemplary conventional MIMO-OFDM system  100  comprises source signals S 1  to S Nt , transmitters TX 1  to TX N     t   , transmit antennas  110 - 1  through  110 -N t , receive antennas  115  - 1  through  115 -N r , and receivers RX 1  to RX N     r   . The MIMO-OFDM system  100  transmits separate data streams on the multiple transmit antennas  110 , and each receiver RX receives a combination of these data streams.  
         [0032]     As used herein, the term “SISO” shall mean a system that transmits a single data stream (“signal layer”) into a channel. The term “SISO” may include a system that employs two transmit antennas that transmit essentially the same signal, for example, in a beamforming or transmit diversity type of configuration; and a system that employs multiple receive antennas (such as in a receive diversity/beamforming type of configuration). In addition, the term SISO may be used to describe a system with a single transmitter but multiple receive antennas-. Thus, in our terminology, the term “SISO” captures all systems that have a single transmit layer in the same channel bandwidth, regardless of how the signal is actually generated and regardless of how the receiver “samples” the wireless medium with one or several receive antennas. Similarly, as used herein, the term “MIMO” shall mean a system in which there are multiple transmission layers, i.e., several distinguishable streams are transmitted from different antennas into the same frequency channel. It is noted that there could be one or more receive antennas in various configurations to receive such a MIMO transmission. In typical implementations for rate enhancement, there will be as many receive antennas as transmit antennas, or more receive antennas than transmit antennas.  
         [0033]      FIG. 2  illustrates a frame format  200  for a conventional SISO OFDM system. As shown in  FIG. 2 , the frame format  200  includes a header section  210  and a payload section  220 . The header section  210  includes a “Short Preamble” period  211 , a “Long Preamble period  212  and a signal  213 . The short preamble  211  is generally  8  microseconds long, consisting of 10 Short Preamble symbols in the exemplary 802.11 g/a system. The short preamble  211  is used by a receiver to detect that there is a packet coming in and to adjust the Automatic Gain Control in the radio circuit. The “Long Preamble” period  212  generally contains two Long Preamble symbols in the exemplary 802.11 g/a system. The long preamble  212  is used by a receiver to estimate various parameters, such as channel estimation, frequency offset and timing synchronization. The signal field  213  contains information of the physical layer properties of the frame, for example, to indicate the data rate and packet length to the receiver. The payload section  220  carries the actual information useful to the MAC layer. As shown in  FIG. 2 , the overall duration of transmitting a packet is given by the addition of the header duration T_head plus the data duration T_data.  
         [0034]      FIGS. 3A and 3B  illustrate exemplary frame formats  300 ,  350  for a MIMO OFDM system that provide backwards compatibility for an exemplary implementation having two transmit antennas TX- 1  and TX- 2 . In the examples of  FIGS. 3A and 3B , the first row shows the signal transmitted from the first transmit antenna TX- 1 , whereas the second row is the signal transmitted from the second transmit antenna TX- 2 . As shown in  FIG. 3A , for example, the frame format  300  includes the MIMO Short Preamble SP. 1 , SP. 2  transmitted simultaneously from both antennas TX- 1  and TX- 2 . Thereafter, a first Long Preamble sequence LP.a is transmitted from the first antenna, while the second antenna (TX- 2 ) does not transmit any signal. The second transmitter (TX- 2 ) then transmits a Long Preamble Sequence LP.b while the first antenna TX- 1  does not transmit any signal.  
         [0035]     As previously indicated, it is desirable to have a continuous transmission from all transmitters once the packet transmission has started, in order to avoid heating/cooling related signal “breathing,” potentially leading to synchronization and detection errors at the receiver. In the frame format of  FIG. 3A , after transmitting the short preamble SP. 2 , the power amplifier of the second antenna branch TX- 2  will be switched off, and might therefore cool down after achieving a certain temperature during the transmission of SP. 2 . The power amplifier of TX- 2  will then have to be switched on again for transmission of the long preamble LP.b, and the temperature of the amplifier will increase again, while at the same time, the power amplifier of the first antenna TX- 1  will be switched off and will cool down. Generally, heating and cooling leads to “breathing” effects. In other words, the transmitted signal might have slight offsets (in phase or magnitude) from the desired signal. Moreover, due to possible temperature drifts during the preamble, these offsets will change over time.  
         [0036]     It is, therefore, more desirable to continuously transmit signals from all transmit branches. One example for a frame format  350  with continuous transmissions is shown in  FIG. 3B . As shown in  FIG. 3B , the long preamble segment LP.a is transmitted identically from both antennas TX- 1  and TX- 2 , while in the second long preamble segment LP.b, the signal on the second antenna TX- 2  is inverted (e.g., transmitted after pre-multiplying all samples by −1). This scheme, however, may lead to a beamforming phenomenon, discussed below, whenever the transmitter is not surrounded by many scattering objects. A typical wireless Access Point, for example, will typically be mounted high above the ground in an office environment and as such away from many scattering objects.  
         [0037]     The resulting beamforming effect is shown in  FIG. 4 . If two antennas transmit the same signal (possible differing by any constant factor, shown as a factor A in  FIG. 4 ), then certain areas in the environment will be better illuminated, such as point P 2 , than other areas, such as point P 1 . Depending on the constant factor, A, between the two transmitters, these illumination areas will change. For instance, for the frame format  350  of  FIG. 3B , the factor between the two antenna branches during the long preamble LP.a is +1, while for long preamble LP.b the factor is (−1). However, since in an ideal environment the illumination is omnidirectional and temporally constant for reliable coverage, such (time-variant) beamforming effects are generally undesired.  
         [0038]     Moreover, transmission of essentially (apart from a constant factor between the TX antennas) the same signal from several antennas during the Short Preamble will lead to erroneous receive power estimates in the power estimation. This is due to the fact that when essentially the same signal is transmitted from several antennas, there will be a coherent addition of physical channels, while the omnidirectionally radiated (and uncorrelated) MIMO samples in the data portion lead to a power addition. The result is suboptimally adjusted gain settings in the analog amplifier stage (AGC settings), which leads to either increased effective noise levels and/or more distortion.  
         [0039]      FIGS. 5A and 5B  illustrates the transmission of the two long preamble segements LP.a and LP.b, respectively, along with the corresponding channel coefficients and receive signals, for the preamble scheme  300  of  FIG. 3A . It is noted that any noise contributions that will occur are not shown in  FIGS. 5A and 5B , for ease of illustration. Thus, the channel coefficient for the first receiver for long preamble LP.a, as shown in  FIG. 5A , is given by: 
   R   1 . a{k}=LP{k }( H   11 { k}+H   12 } k }),  
 and the channel coefficient for the second receiver for long preamble LP.a is given by: 
   R   2 . a{k}=LP{k }( H   21 { k}+H   22 } k }).  
         [0040]     Likewise, the channel coefficient for the first receiver for long preamble LP.b, as shown in  FIG. 5B , is given by: 
 
 R   1 . b{k}=LP{k }( H   11  { k}−H   12 } k }), 
 
 and the channel coefficient for the second receiver for long preamble LP.b is given by: 
 
 R   2 . b{k}=LP{k }( H   21 { k}−H   22 } k }). 
 
         [0041]      FIG. 6  illustrates a channel estimation structure  600  that can further process the signals of  FIG. 5  to produce the desired channel coefficients. Thus, the output of the channel estimation structure  600  is the desired channel coefficients (for an exemplary 2×2 MIMO system). The channel estimation structure  600  processes the R signals of  FIG. 5  using a number of adders  610 ,  630 , and subtractors  620 ,  640 , followed by a multiplication stage  650 , to produce the channel coefficients, H. It is noted that noise contributions would be added to each coefficient, such that the estimate would not be completely identical to the actual coefficient, leading to some degradation.  
         [0042]     According to one aspect of the present invention, a frame format is selected that continuously transmits from all transmit branches, such as the frame format  350  of  FIG. 3B , and the overall frame (preamble and data sections of the packet) transmitted from the second antenna is delayed or otherwise shifted before transmission. A typical delay is given, for example, by one OFDM sample period, such as 50 nanoseconds in the IEEE 802.11a/g standards. In one variation, the sampled data stream on one or more transmit antennas can be constructed by performing cyclic OFDM rotations by the same delay amount, D. For example, on an OFDM symbol-by-symbol basis, time-domain samples are moved from the end of each symbol to the beginning of the symbol.  
         [0043]      FIG. 7  illustrates a frame format  700  incorporating features of the present invention for an exemplary implementation having two transmit antennas. As shown in  FIG. 7 , the exemplary frame format  700  is based on the frame format  350  of  FIG. 3B , wherein the transmission of the frame by the second transmitter TX- 2  is delayed by an amount D. The delay, D, should be matched to the auto-correlation function of the respective (Short and Long) preambles, such that the auto-correlation value is low for the value of delay D compared to the main peak. For the formats and correlation properties of the Preamble sequences given in IEEE 802.11a/g OFDM, a short delay D is advisable.  
         [0044]     One promising example would be for the delay D to correspond to one OFDM time sample period T. In 802.11a/g OFDM, this number is 50 nanoseconds. For a general OFDM system, the OFDM time sample period is given by the inverse of the frequency spacing of two adjacent subcarriers, divided by the maximum number of OFDM subcarriers captured by the FFT. In 802.11a/g, for example, these numbers are 312.5 KHz spacing and 64 subcarriers, where only 52 subcarriers are non-empty. However, a period of 0.5 T, 1.5 T or 2 T can be used in practice, as well. The advantage of using the delay D lies in the fact that during the preamble, from a standpoint of the effective transmit beampattern at any given time instant, the two antennas will no longer simultaneously transmit the same sequence. This randomizes the radiation and appears as an overlay of two independent, thus essentially omnidirectional signal streams. Using delays D smaller than T leads to partially correlated transmission, which may cause some limited beampattern effect for certain propagation environments.  
         [0045]     Among other benefits, the delayed transmission of the frame by the second transmitter allows continuous transmission from both antennas without the heating/cooling phenomena that might cause the signal to “breathe” and thus impair the transmission reliability.  
         [0046]      FIG. 8  is a schematic block diagram of a transmitter  800  incorporating features of the present invention. As shown in  FIG. 8 , data to be transmitted is received from the MAC layer and the signal is constructed in the frequency domain at stage  805 , such as signal encoding and modulation. Thereafter, an inverse fast Fourier transform is performed at stage  810  and a guard interval is added at stage  820 . The MIMO preamble is constructed at stage  825  and combined with the data signal at stage  830 . The frame for the second transmitter is delayed in accordance with the present invention at stage  850 . The digital signal is converted to an analog signal at stage  860  and transmitted at stage  870 .  
         [0047]     Thus, a delay element  850  is introduced at the end of the digital signal processing chain in the second antenna branch TX- 2 , before the signal is converted to analog and forwarded to the RF stage. It is important to note that in the exemplary embodiment this delay is applied across the whole packet including all data symbols, i.e., this delay is not limited to the preamble section of the frame. Also note that the channel estimation in the frequency domain has not changed.  
         [0048]      FIG. 9  illustrates a frame format  900  incorporating features of the present invention that is a generalization of the preamble format  700  of  FIG. 7 . As shown in  FIG. 9 , two parameters K 1  and K 2  are specified. The parameters K 1  and K 2  can generally take any convenient values that allow for a reconstruction (separation) of the channel coefficients using the procedure described above. In the specific implementation shown in  FIG. 9 , the parameter K 1  equals 1 and the parameter K 2  equals −1.  
         [0049]      FIG. 10  illustrates a frame format  1000  incorporating features of the present invention for an exemplary multiple antenna communication system having three transmit antennas. As shown in  FIG. 10 , the second antenna TX- 2  is delayed from the first antenna TX- 1 , and the third antenna TX- 3  is further delayed.  
         [0050]     Using these parameters, it is ensured that each transmit antenna transmits the same power at any time (magnitude of all Ks is 1.0), and that the receiver can separate the channel coefficients in a multi-dimensional manner. The delay D would typically correspond to T or 0.5 T in this implementation. For example, the K coefficients can have the following exemplary values: 
 
K 1 =K 4 =1; 
 
 K   2 = K   6 =exp(− j* 120 deg); and 
 
 K   3 = K   5 =exp(+ j* 120 deg). 
 
         [0051]     The short preambles SP. 1  to SP. 3  could follow any suitable well-known or MIMO-specific short preamble format. For example, SP. 1  to SP. 3  could all follow the 802.11 a/g OFDM standard.  
         [0052]      FIG. 11  is a schematic block diagram of a transmitter  1100  incorporating features of the present invention for an exemplary multiple antenna communication system having three transmit antennas. The transmitter  1100  has the same structure as the transmitter  800  of  FIG. 8 , with the addition of another transmit branch. As shown in  FIG. 11 , data to be transmitted is received from the MAC layer and the signal is constructed in the frequency domain at stage  1105 , such as signal encoding and modulation. Thereafter, an inverse fast Fourier transform is performed at stage  1110  and a guard interval is added at stage  1120 . The MIMO preamble is constructed at stage  1125  and combined with the data signal at stage  1130 . The frame for the second and third transmitters are delayed in accordance with the present invention at stage  1150 . The digital signal is converted to an analog signal at stage  1160  and transmitted at stage  1170 .  
         [0053]     Thus, delay elements  1150 - 2  and  1150 - 3  are introduced at the end of the digital signal processing chain in the second antenna branch TX- 2  and third antenna branch TX- 3 , respectively, before the signal is converted to analog and forwarded to the RF stage. It is important to note that in the exemplary embodiment this delay is applied across the whole packet including all data symbols, i.e., this delay is not limited to the preamble section of the frame. Also note that the channel estimation in the frequency domain has not changed. The individual delays do not necessarily have to be multiples of the same basic delay D.  
         [0054]      FIG. 12  illustrates a channel estimation structure  1200  that produces the channel coefficients for a multiple antenna communication system having three transmit antennas. Thus, the output of the channel estimation structure  1200  is the desired channel coefficients (for an exemplary 3×3 MIMO system). The channel estimation structure  1200  multiplies the received signals R by the corresponding long preamble symbol at stage  1210  and then performs an appropriate matrix inversion at stage  1220 , to produce the channel coefficients, H. It is noted that noise contributions would be added to each coefficient, such that the estimate would not be completely identical to the actual coefficient, leading to some degradation.  
         [0055]     It is assumed in  FIG. 12  that the magnitudes of all long preamble symbols in the frequency domain LP{k} have a unity magnitude, i.e., LP{k} equals +1 or LP{k} equals −1. Thus, for each receive branch  1 ,  2  or  3  and in each subcarrier k, the outputs from the three preamble segments a, b, c, are premultiplied by LP{k} at stage  1210 . Then, a matrix multiplication with the inverse of the K coefficient matrix is carried out at stage  1220 , as shown in  FIG. 12 .  
         [0056]     It is further noted generally that, although the K factor in front of the LP preamble transmitted from the first antenna TX- 1  is arbitrarily chosen to be 1.0 for all preamble segments, these factors could have different values, too. The selection of 1.0 for the first antenna TX- 1  is useful. If K factors other than 1.0 are chosen for the first transmit antenna LP segments, they should appear in the matrix inverse  1220  as well.  
         [0057]     In general, the channel estimation scheme shown in  FIG. 12  can be applied to multiple antenna communication systems having two or four transmit antennas as well. In the former case, the matrix to be inverted and used for the multiplication would be 2×2 dimensional where the first row has unity values and the second row of the matrix has values of K 1  and K 2 , and the functionality turns out to be identical to the channel estimation scheme shown in  FIG. 6 . For the four transmit antenna case, the matrix to be inverted would have a first row of all unity values, a second row with values of K 1 , K 2 , K 3  and K 4 , a third row with values of K 5 , K 6 , K 7  and K 8 ; and a fourth row with values of K 9 , K 10 , K 11  and K 12 . Again, if the K coefficients linked to the first transmit antenna would be different from 1.0, the first row in these matrices would have to carry the corresponding coefficients.  
         [0058]     It should be noted that for the specific example of K parameters used here, the required matrix inverses are very straightforward and essentially given by the K factors (or complex conjugates of them) and possibly a simple overall scaling factor valid for the whole matrix multiplication. This makes the solution straightforward.  
         [0059]     Another extension is possible for a system that has a larger number of receive antennas than transmit antennas. Then, the corresponding channel coefficients could easily be estimated by adding another row of processing blocks carrying out a corresponding pre-multiplication with LP{k} and then applying the matrix inverse. For example, for  FIG. 12 , if there were still three transmit antennas but four receive antennas, a fourth row would be added, combining receive signals R 4   a,  R 4   b,  and R 4   c,  and the coefficients generated after the inverse matrix multiplication would be the desired coefficients H 41 , H 42 , and H 43 . A similar extension would apply to a 2×3 MIMO system.  
         [0060]      FIG. 13  illustrates a frame format  1300  incorporating features of the present invention for an exemplary multiple antenna communication system having four transmit antennas. As shown in  FIG. 13 , the second antenna TX- 2  is delayed from the first antenna TX- 1 , the third antenna TX- 3  is further delayed from the second antenna, and the fourth antenna TX- 4  is delayed relative to the third antenna.  
         [0061]     Using these parameters, it is ensured that each transmit antenna transmits the same power at any time (magnitude of all Ks is 1.0), and that the receiver can separate the channel coefficients in a multi-dimensional manner. The delay D would typically correspond to T or 0.5 T in this implementation. For example, the K coefficients can have the following exemplary values: 
 
K 1 =K 5 =K 7 =K 9 =1; 
 
 K   2 = K   12 =exp( j* 90 deg)=− j;  
 
 K   3 = K   6 = K   8 = K   11 =−1; and 
 
 K   4 = K   10 =exp(+ j* 90 deg)=+ j.  
 
         [0062]     The short preambles SP. 1  to SP. 4  could follow any suitable well-known or MIMO-specific short preamble format. For example, SP. 1  to SP. 4  could all follow the 802.11 a/g OFDM standard.  
         [0063]     The preamble formats discussed above have been based on the use of a single SIGNAL field OFDM symbol after the first Long Preamble segment LPa. This SIGNAL field could be as specified by the IEEE 802.11g/a OFDM standard, i.e., using a 6 Mbps mode under all circumstances. In order for the MIMO receiver to obtain additional information on the high-speed transmission of the data field, a second SIGNAL field may be helpful.  
         [0064]      FIGS. 14A and 14B  illustrate a frame format  1400 ,  1450 , respectively, where a second signal field SIG 2  is found after the first signal field SIG 1 .  FIG. 14A  illustrates the 2-Tx MIMO system, where  FIG. 14B  illustrates the example of the 3-Tx MIMO system. Another alternative would be to have the SIG 2  field after the second Long Preamble (not shown in the Figures).  
         [0065]     The concrete delay transmission scheme proposed above, such as in  FIG. 9 , may increase the channel delay spread (in the Preamble). In the 2-Tx MIMO system, such an increase is marginal. However, the delay spread increases as the number of transmitter antennas increases, which would degrade the system performance.  
         [0066]      FIG. 15  illustrates a frame format  1500  incorporating features of the present invention for an exemplary three transmit branch scheme that only delays the preambles and SIGNAL field but not the data. This way, the data transmission does not suffer from the delay spread increase. After the transmission of the preamble and signal field right before the data transmission, a dummy signal F (silence, or cyclic extension of the Long Preamble, or an arbitrary padding signal) of length D is added to the first transmit stream. Then, the MIMO DATA parts are transmitted simultaneously from both antennas. Note that the MIMO receiver needs to know about this delayed/non-delayed transmission differing between the Preamble parts and the DATA parts, in order to properly adjust the synchronization (timing of each OFDM symbol) and channel estimation.  
         [0067]     As shown in  FIG. 15 , two parameters K 1  and K 2  are specified (in a similar manner to  FIG. 9 ). The parameters K 1  and K 2  can generally take any convenient values that allow for a reconstruction (separation) of the channel coefficients using the procedure described above. In the specific implementation shown in  FIG. 15 , the parameter K 1  equals 1 and the parameter K 2  equals −1.  
         [0068]      FIG. 16  illustrates a frame format  1600  incorporating features of the present invention for an exemplary three transmit branch scheme that only delays the preambles and SIGNAL field but not the data. As shown in  FIG. 16 , the frame format  1600  uses two dummy periods F 1  and F 2 .  
         [0069]      FIG. 17  illustrates an alternate scheme  1700  for an exemplary multiple antenna communication system having two transmit branches that overcomes the drawback of the delay spread increase in the physical channel. Rather than delaying the transmission from the second antenna by D, the signal transmitted from the second antenna is cyclically rotated within each symbol (to introduce a rotation delay). In the embodiment of  FIG. 17 , the samples on the second transmit branch are cyclically shifted and corresponding Guard Intervals (GI) are added on transmit branches Tx- 1  and Tx- 2 . As shown in  FIG. 17 , such a cyclic rotation can be achieved, for example, by putting the last samples of duration D of one OFDM symbol (still without guard interval, GI) before the rest of the symbol. Then, the guard interval, reusing the last G samples (0.8 microseconds in 802.11a OFDM) from the newly created symbol (shown as A′ in  FIG. 17 ), is added. This cyclic rotation is done for all OFDM symbols across the whole packet per OFDM symbol including the preambles.  
         [0070]     Note that the Short Preamble in IEEE 802.11a OFDM doesn&#39;t have an explicit Guard interval, so there, the cyclic rotation should be taken across all ten Short Preamble symbols that form the Short Preamble, and no GI subsequent addition step is required. Alternatively, it can be carried out individually per each one of the ten Short Training segments (due to periodicity, there is no difference). For the Long Preamble, the rotation should take place per each one of the two Long Training Sequences or across both at the same time, again there is no difference. Then, the (long) GI should be based on the newly constructed Long Training Sequence. Mathematically, in the OFDM context, where signals are eventually translated back into and treated in the frequency domain at the receiver, such a cyclic rotation is equivalent to delaying the signal transmission by D samples. Therefore, the scheme here has the same inherent decorrelation and beampattern-avoiding benefit of the delayed transmission scheme discussed above, while all transmissions at the TX antennas start at the same moment, in contrast to the non-cyclic approaches from above.  
         [0071]      FIG. 18  illustrates an alternate scheme  1800  for an exemplary multiple antenna communication system having three transmit branches that overcomes the drawback of the delay spread increase in the physical channel. In the embodiment of  FIG. 18 , the sample(s) on Tx- 2  and Tx- 3  are cyclically shifted and corresponding Guard Intervals (GI) are added on transmitters TX- 1 , TX- 2  and TX- 3 . The third antenna simply performs the same cyclic rotation but with 2D samples. Again, the guard intervals to be prepended are based on the rotated symbols.  
         [0072]      FIG. 19  illustrates a further variation of the present invention where the cyclic rotation is performed only on the short preamble, long preambles and signal field(s). The data symbols are not cyclically rotated. The scheme reduces the complexity of the transmitter and maintains the benefit of cyclic rotation. In this scheme, the receiver needs to specifically adjust its synchronization and channel estimation procedure to this delayed/non-delayed difference between the Preamble parts and the DATA parts. For the channel estimation, the Lp{k} multipliers in  FIGS. 6 and 12  for the second antenna should be Lp{k}exp(−j2πDk/N) (N is the size of FFT), and Lp{k}exp(−j4πDk/N) for the third antenna, again assuming that the magnitude of all Lp{k}&#39;s is either +1 or (−1).  
         [0073]     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.