Abstract:
Method, apparatus, and data packet format to implement transmit diversity in a multicarrier environment is disclosed. For diversity transmission operations, space frequency encoding techniques are employed creating distinguishable first and second time domain signals from a multicarrier frequency domain symbol bearing data of interest, which are then broadcast in parallel over first and second transmission units respectively. For diversity reception operations, complementary space frequency decoding is used to recover a corrected multicarrier frequency domain symbol from a time domain signal containing either this symbol, a space frequency modified symbol based on the multicarrier symbol, or a possible partial/complete combination of both. The data packet format includes portions defining a transmission diversity semaphore, a preamble enabling training of a receiver receiving the data packet, and a payload. This payload includes plural data symbol pairs, each defining a first symbol for transmission by a first transmission unit of a diversity transmitter, and a second symbol for transmission by a second transmission unit of the diversity transmitter, the second symbol being derived from the first symbol.

Description:
RELATED APPLICATION 
   This application is a continuation of copending U.S. patent application Ser. No. 10/162,274, filed Jun. 3, 2002, which is incorporated herein fully by reference. 

   TECHNICAL FIELD 
   This application is directed generally to wireless communications, and is specifically concerned with techniques for implementing transmit diversity in a multi-carrier environment. 
   BACKGROUND 
   The past few years has witnessed the ever-increasing availability of relatively cheap, low, power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11a (1999) and IEEE Draft Standard 802.11g (2002) Supplements to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference and collectively referenced as “IEEE 802.11a &amp; 802.11g”, has recently been commercialized with the promise of 54 Mbps+ peak data rates, making it a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps wireless transmission standard. 
   IEEE 802.11a &amp; 802.11g compliant transmission systems achieve their high data transmission rates using a type of multicarrier frequency domain symbol encoding or modulation known as Orthogonal Frequency Division Multiplexing, (“OFDM”). In particular, OFDM encoded symbols mapped up to 64 QAM multicarrier constellation bear the data intended for transmission, though even larger constellations are contemplated to further increase data throughput. Before final power amplification and transmission, these OFDM encoded symbols are converted into the time domain using Inverse Fast Fourier Transform techniques resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR). 
   One concern with the IEEE 802.11a &amp; 802.11g standards is the rather strict power levels compliant transmission equipment must operate within without running afoul of FCC and international intraband and interband interference limits, particularly at lower channels within the 5 GHz band for North American operation. Accordingly, standards compliant transmission equipment designers continue to seek ways to improve reception performance without needing to increase power output of the transmitter. One cost effective technique that has surprisingly not been explored in IEEE 802.11a/802.11g, is the concept of transmit diversity used in single carrier systems. See e.g., S. M. Alamouti, “A Simple Transmit Diversity Technique For Wireless Communications,” IEEE Journal on Select Areas in Communications, vol. 16, no. 8, October 1998, pp. 1451-1458 which is incorporated herein fully by reference. Transmit diversity provides a diversity gain without multiple receiver chains as well as reduced power output from each transmitter since, for a constant total radiated power, the transmitted signals of interest are broadcast over two separately positioned antennas using two different RF transmission pathways at ½ power. This 3-dB reduction for each transmitting amplifier permits use of less expensive and less linear power amplifiers yet retain if not improve overall reception performance. However, in order to properly account for multipath distortion and other artifacts introduced by having dual proximate RF transmitters operating at the same carrier frequency, the receiver must know in advance that it will be operating in a diversity environment and accommodate for these effects. 
   The present high speed data wireless transmission standards in the 802.11 family, including the commercially important IEEE 802.11a &amp; 802.11g standards do not account for transmit diversity. Therefore, it would be advantageous to incorporate transmit diversity in a wireless transmission system that is backwards compatible with the IEEE 802.11a &amp; 802.11g standards, as well as provide for a wireless diversity system capable of operating in multicarrier encoding environments generally. 
   SUMMARY OF THE INVENTION 
   To address these perceived and related shortcomings, the present invention is directed to a method, apparatus, and data packet suitable for implementing transmit diversity in a multicarrier environment. In diversity transmission consistent with the present invention, space frequency encoding techniques are employed to create distinguishable first and second time domain signals from a multicarrier frequency domain symbol bearing the data of interest, which are broadcast in parallel over first and second transmission units respectively. In diversity reception consistent with the present invention, complementary space frequency decoding is used to recover a corrected multicarrier frequency domain symbol from a time domain signal containing either this multicarrier frequency domain symbol, a space-frequency modified symbol based on this multicarrier frequency domain symbol, or a possible incomplete or complete combination of both. 
   Accordingly, diversity transmission in accordance with an embodiment of the invention employs space frequency encoding operable on multicarrier frequency domain symbols which bear the data of interest to provide corresponding modified symbols. These multicarrier frequency domain symbols and their corresponding modified symbols are converted into corresponding time domain counterparts and then transmitted in parallel by first and second RF transmission units. These time domain counterparts may be transmitted at approximately the same time on the same frequency channel, or, alternatively, in a staggered sequence or on dissimilar channels as will be appreciated by those ordinarily skilled in the art. 
   Diversity aware reception in accordance with an embodiment of the invention includes utilizing a receiver to receive a time domain signal which may define both a multicarrier frequency domain symbol and a modified symbol based on such multicarrier frequency domain symbol. A conversion unit is employed to generate a composite signal in the frequency domain based on this time domain signal which includes at least an incomplete analog sum of the multicarrier frequency domain symbol and the modified symbol. Thereafter, a space frequency decoder is used to recover a corrected multicarrier frequency domain symbol from this composite signal. 
   A data packet according to an embodiment of the invention includes a first portion defining a transmission diversity semaphore, a second portion adjacent to the first portion and including a preamble to enable training of a receiver receiving the data packet, and a third portion following the second portion defining a payload. Here the payload includes a plurality of data symbol pairs with at least one of the data symbol pairs defining a first symbol capable of being transmitted by a first transmission unit of a diversity transmitter, and a second symbol capable of being transmitted by a second transmission unit of the diversity transmitter, wherein the second symbol is derived from the first symbol. 
   In accordance with these embodiments, a given multicarrier frequency domain symbol or first symbol may conveniently include an OFDM encoded symbol encoded in compliance with at least one of the IEEE Standard 802.11a and IEEE Standard 802.11g Supplements to the IEEE Standard 802.11 (1999) for wireless communications. The corresponding modified symbol or second symbol may conveniently include a re-ordered subcarrier complex conjugate of the given multicarrier frequency domain symbol to enable relatively fast and predictable space frequency encoding and decoding activities. 
   Additional aspects and advantages of this invention will be apparent from the following detailed description of embodiments thereof, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a known PLCP frame format compliant with IEEE 802.11a &amp; 802.11g standards. 
       FIG. 2  is a data packet format according to an embodiment of the invention. 
       FIG. 3  is a simplified block diagram of a transmitter capable of transmitting a data packet formatted in accordance with  FIG. 2 . 
       FIG. 4  is a simplified block diagram of a receiver capable of receiving a data packet formatted in accordance with  FIG. 2 . 
       FIG. 5  is a block diagram of a wireless transceiver incorporating the transmitter and receiver described in  FIG. 3  and  FIG. 4  respectively. 
       FIG. 6  is a flowchart illustrating transmit processing consistent with the transmitter of  FIG. 3 . 
       FIG. 7  is a flowchart illustrating receive processing consistent with the receiver of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   In order to better understand transmit diversity according to the present invention, discussion of the Physical Layer Control Protocol (“PLCP”) frame format used to convey data in packet form (i.e. a type of data packet) in a IEEE 802.11a and/or 802.11g environment is deemed appropriate.  FIG. 1 . illustrates the general organization or format of a PLCP frame according to the IEEE 802.11a &amp; 802.11g standards. The PLCP preamble  110  includes 10 short training symbols in order for an incident receiver to self-adjust the gain of the received baseband signal so that the received signal&#39;s amplitude is within the optimal range for analog-to-digital conversion, recover OFDM symbol timing and initiate coarse carrier signal frequency acquisition as is well known in the art. PLCP preamble  110  also includes two long training symbols following the 10 short training symbols which again allows the receiver to estimate the carrier channel being used, as well as any needed fine frequency acquisition. In essence, the receiver uses these long symbols for fine tuning of the training occurring during the previous 10 short symbols. With this preamble, it takes approximately 16 microseconds to train the receiver after first receiving the frame. 
   Still referring to  FIG. 1 , following the PLCP preamble  110  is the Signal field  115 , which encodes the data rate of the data field  135  portion of the frame, followed by a Reserved bit set to zero under the existing IEEE 802.11a &amp; 802.11g standards, a 12-bit Link field which identifies the number of octets in the frame, a parity bit to ensure that the first 17 bits of the frame Signal field (Rate, Reserved, and Link fields) have even parity and a 6-bit tail which is set to all zeroes. The Signal field  115  forms a first portion of the PLCP header  155  of the frame  100 . As will be discussed in more detail below, for backwards compatibility purposes in accordance with the embodiment of the invention shown in  FIG. 2 , the Signal field  215  of data packet format  200  differs from the Signal field  115  in that the Reserved bit will contain a transmit diversity semaphore indicating to a transmit diversity aware receiver consistent with the present invention (herein “TX diversity receiver”) such as receiver  400  discussed below whether or not transmit diversity has been enabled. The Signal field  115  is convolutionally encoded at 6 megabits per second using Binary Phase-Shift Keying (BPSK) no matter what the data rate the Signal field indicates. 
   A first portion of the data field  135  immediately follows the Signal field  115 . This data field referred to in the figure as DATA 1    120 . More commonly known as the Service field, this field consists of 16 bits, with the first seven bits as zeros to synchronize the descrambler in the IEEE 802.11a &amp; 802.11g compliant receiver and the remaining nine bits reserved for future use and set to zero. Together with the Signal field  115 , the DATA 1 /Service field  120  form the PLCP header  155 . 
   DATA 2    125 , DATA 3    130  . . . DATA N    140  represent the payload or PSDU (“PLCP Service Data Unit”)  150  portion of the PLCP frame. It should be noted that DATA 2    125 , DATA 3    130  . . . DATA N    140  each comprise an OFDM symbol transmitted using BPSK and QAM depending on the chosen data rate as presented in the following table: 
   
     
       
             
             
             
             
             
             
           
         
             
                 
             
             
               Data 
                 
                 
               Coded Bits 
               Code Bits 
               Data Bits 
             
             
               Rate 
               Modu- 
               Coding 
               per Sub- 
               per OFDM 
               per OFDM 
             
             
               (Mbps) 
               lation 
               Rate 
               carrier 
               Symbol 
               Symbol 
             
             
                 
             
           
           
             
                6 
               BPSK 
               ½ 
               1 
                48 
                24 
             
             
                9 
               BPSK 
               ¾ 
               1 
                48 
                36 
             
             
               12 
               QPSK 
               ½ 
               2 
                96 
                48 
             
             
               18 
               QPSK 
               ¾ 
               2 
                96 
                72 
             
             
               24 
               16-QAM 
               ½ 
               4 
               192 
                96 
             
             
               36 
               16-QAM 
               ¾ 
               4 
               192 
               144 
             
             
               48 
               64-QAM 
               ⅔ 
               6 
               288 
               192 
             
             
               54 
               64-QAM 
               ¾ 
               6 
               288 
               216 
             
             
                 
             
           
        
       
     
   
   During actual transmission a data scrambler using 127-bit sequence generator scrambles all the bits in the data field  135  to randomize the bit patterns in order to avoid long streams of ones and zeros. 
   Immediately following the payload or PSDU  150  is a tail and pad portion  145  of the PLCP frame. The tail field includes a bit string of 6 “zero” bits to return the convolutional encoder to a “zero” state. (The 6 scrambled “zero” bits in the tail field are replaced by 6 nonscrambled “zero” bits.) Subsequently, a variable length pad field is appended in order to extend the resulting bit string so that the resulting length will correspond to an integer number of OFDM symbols for transmission at the established data rate. 
   In contrast to  FIG. 1 , a data packet format  200  (proposed as an “alternative” PLCP frame format to accommodate transmit diversity according to the present invention) according to an embodiment of the invention will now be detailed with respect to  FIGS. 2 ,  3  and the flowchart of  FIG. 6 . Turning first to  FIG. 2 , the novel packet format  200  begins with a standards compliant PLCP preamble  110 . The PLCP preamble  110  in this embodiment is used for the same receiver training purposes as described above with reference to  FIG. 1 . The Signal field  215 , which immediately follows the standards compliant PLCP preamble  110 , is generally similar to the Signal field  115  described above. However, the Signal field  215  also includes a binary semaphore or flag TXDIV  217 , which occupies the currently Reserved bit of the standards compliant Signal field  115 . A TXDIV=TRUE setting indicates that the data packet follows the diversity data packet format  200 . The advantage of using the Reserved bit ensures that legacy standards compliant transceivers encountering or intercepting a data packet formatted in accordance with the format  200  will correctly train to the PLCP preamble  110  and perceive the Signal field  215 , but ignore it if the TXDIV flag  217  is set, because these legacy devices will perceive such packet as being erroneous and, consistent with existing IEEE 802.11a &amp; 802.11g standards, will not attempt further decoding or recovery operations. At the same time, a TX diversity receiver, such as receiver  400  discussed below, will include appropriate logic to watch for and correctly interpret the TXDIV semaphore  217 . 
   As discussed above, the data packet format  200  of the present embodiment as well as the transmitter  300  and receiver  400  described below attempt to accommodate legacy devices compliant with the IEEE 802.11a &amp; 802.11g standards. To that end, as noted in  FIG. 6 , a preliminary determination is made whether a transmitter incorporating transmit diversity consistent with the present invention (“TX diversity transmitter”), such as transmitter  300  shown in  FIG. 3 , should transmit a frame/packet using the data packet format  200  (Step  610 ). This may be accomplished through appropriate application of user pre-selection, negotiation or handshaking techniques as is well known in the art. Alternatively, a default action would be to assume that the intended receiver is arranged in accordance with transmit diversity techniques consistent with the present invention, and if no acknowledgement is received of the packet transmitted in accordance with  FIG. 2 , the data will be retransmitted in accordance with the known format  100 , and further transmission attempts will remain standards compliant using a single transmission unit such as the first transmission unit  350  of the transmitter  300 , unless overridden in the future through e.g. specific device request or user selection. 
   It should be noted that in accordance with the packet format  200  shown in  FIG. 2 , both the PLCP preamble and the Signal field  215  are contemplated for transmission on the first of two transmission units of a TX diversity transmitter, such as the first one 350 of the transmission units  350  and  360  of the transmitter  300  shown in  FIG. 3 , again for backwards compatibility reasons. In the packet format  200  shown in  FIG. 2 , the notation “TX I” in a given field or portion of the data packet  200  means that this portion will be transmitted on a first transmission unit of a TX diversity transmitter, such as transmission unit  350  shown in  FIG. 3 , whereas the notation “TX 2 ” means that so-labeled portions will be transmitted by a second transmission unit of such TX diversity transmitter, such as transmission unit  360 . 
   Immediately following the Signal field  215  in the packet format  200  is a second training preamble  218 . It should be noted that the second preamble  218  has no analogue in the standards compliant format  100 . This second preamble  218  is contemplated for transmission by a second transmission unit of a TX diversity transmitter (e.g. second transmission unit  360  of the transmitter  300  shown in  FIG. 3 ). The purpose of this second preamble  200  is to permit a TX diversity receiver (e.g. receiver  400 ,  FIG. 4 ) to re-estimate the potentially different signal characteristics between the second transmission unit and the receiver (step  625 ,  FIG. 6 ) relative to the first transmission unit. 
   In the present embodiment, though not required, the second preamble  218  includes two long training symbols in sequence. In particular, the first long training symbol is contemplated to be used by a TX diversity receiver for coarse AGC adjustment to quickly attenuate the received signal broadcast by the second transmission unit of an originating TX diversity transmitter in case there may be strong difference in experienced gain between it and that previously encountered when receiving from the first transmission unit of such transmitter. The second long training symbol of the second preamble  218  is used to help this receiver estimate the channel transfer function from the second transmission unit to the receiving antenna. 
   Referring to  FIG. 2 , immediately following the second preamble is the data field  250  of the data packet  200  of the present embodiment. It should be noted that OFDM encoded symbols presented in the data field are modulated at the rate specified in the Signal field  215  in a manner similar to that specified for the standards compliant Signal field  115 . As shown in  FIG. 2  and as referenced in step  630  of  FIG. 6 , the Service field/DATA 1    120  and its space-frequency analogue DATA 1 N  220  are transmitted twice in succession by both the first and second transmission units of the TX diversity transmitter in parallel. In this embodiment, the data symbol pair DATA 1    120  and DATA 1 ′  220  are transmitted at approximately the same time. The first transmitted pair is used by a TX diversity receiver such as receiver  400  to further refine automatic gain control after the coarse adjustment with reference to the first long training symbol of the second preamble  218  has been performed. The second or retransmitted pair of the Service field  120  and its space-frequency analogue DATA 1 N  220  is actually demodulated according to the space-frequency decoding technique noted below and its contents recovered by this receiver. 
   The actual payload or PSDU portion  255  of the data packet format  200  will now be discussed with reference to  FIGS. 2 ,  3  and  6 . Similar to the known PLCP frame  100 , the present PLCP frame  200  includes the payload transmitted in modulated, OFDM symbol encoded form. It should be noted that OFDM symbols DATA 2    125 , DATA 3    130  . . . DATA N    140 , as well as the Service field/DATA 1    120 , are transmitted by a first transmission unit of a TX diversity transmitter unmodified from their presentation in accordance with known format  100 . However, unlike PLCP frame format  100 , the data packet format  200  also envisions at least substantially parallel transmission of modified DATA 2 N  225 , DATA 3 N  230  . . . DATA N N  240 , as well as the modified Service field or DATA 1 N  220  at approximately the same time as their unmodified counterpart symbols by the second transmission unit of a this TX diversity transmitter. Thus, generally, in the payload  255  of the data packet format  200 , each data symbol pair consisting of an OFDM encoded symbol (e.g. a given OFDM symbol DATA K ) and its corresponding space frequency modified symbol (e.g. DATA K ′) is transmitted at approximately the same time over the first and second transmission units of a TX diversity transmitter. 
   According to  FIG. 2 , assuming the given OFDM symbol DATA K  is to be transmitted in the Data field  250  of a packet conforming to packet format  200  by the transmitter  300 , the first transmission unit  350  would transmit DATA K  unmodified. The modified form of the symbol, DATA K N will be transmitted over the second transmission unit  360  approximately the same time if not concurrently. Herein, DATA K N differs from DATA K  in that the 52 frequency domain subcarrier constellation points forming the OFDM encoded symbol DATA K  are presented in complex conjugate form and are resequenced in pairs. Finally, the magnitude of the first subcarrier constellation point of each pair is multiplied by −1. For example, if DATA K  comprises the following sequence of subcarrier symbols: 
                                                               subcarrier   1   2   3   4   . . . 52           subsymbol of DATA K     A   B   C   D   . . . AZ                        
The subsymbols of DATA K N would be transformed as follows:
 
                                                   subcarrier   1   2   3   4   . . . 52       constellation point of DATA K N   −B*   A*   −D*   C*   . . . AY*                    
As shown in  FIG. 3 , a space frequency encoder  315  such as that detailed in the aforementioned Alamouti_reference is used to accomplish this transformation. This transformation is believed necessary in order for a TX diversity receiver to receive and recover both symbols DATA K  and DATA K N presented simultaneously on a common channel/carrier frequency, since in many instances both DATA K  and DATA K N will be perceived as part of a composite signal. As shown in Alamouti, transmission of the same data points A, B, . . . , AZ according to the this space-frequency code can achieve a second order diversity gain with one receive antenna. Thus, the reliability of the transmission is increased.
 
   It should be noted that a corresponding space frequency decoder provided within the receiver  400  in order to recover the OFDM encoded symbol DATA K  from a possible composite signal combining DATA K  and DATA K N. In accordance with the data packet format  200  shown in  FIG. 2 , the space frequency encoding and parallel transmission of DATA K  and DATA K N continues until the end of the Data field  250  has been reached. See e.g., step  637 , step  645  through  655  shown in  FIG. 6  (note that step  655  and  660  are executed in a substantially parallel manner). 
   The data packet format  200  according to the present embodiment of the invention terminates with the tail and pad field discussed above. 
   Turning now to  FIG. 3 , it should be noted that the transmitter  300  receives baseband OFDM symbols for RF transmission from the symbol encoder  520  shown in  FIG. 5 . These OFDM symbols are encoded in the frequency domain as well known in the art in order to present extremely high speed time domain data in a less high speed manner by spreading the data over multiple subcarriers. Each OFDM symbol is presented serially by the symbol encoder  520 , is converted into parallel form so that the entire OFDM symbol can be ascertained and space frequency encoded by the space-frequency encoder  315 . It should be noted that the space frequency encoder  315  in this embodiment only encodes OFDM signals intended for transmission by the second transmission unit  360  when the second transmission unit  360  operates in parallel with the first transmission unit  350 . However it will be appreciated by those of ordinary skill that the space frequency encoder  315  can be used in other encoding schemes and employ other symbol modification formats, as long as the behavior of the possible combinations of x 2 (n) and x 1 (n) within h(n) can be accounted for so that the underlying symbol encoded data can be adequately recovered. 
   It should be noted in the transmitter  300  of  FIG. 3  that both the first and second transmission units  350 ,  360  include conversion units  320  and  335  respectively to transform the OFDM encoded symbols and corresponding modified symbols into the time domain using inverse discrete Fourier transform techniques. Then, within the conversion units  320  and  335 , these time domain counterparts are serialized for cyclic prefix insertion compliant with the IEEE 802.11a and 802.11g standards, as well as other routine overhead preparation and formatting consistent with such standards. The so-converted time domain counterparts are then sent to the RF transmitter  325 ,  340  for IF and RF upconversion and amplified for transmission over antennas  330 ,  345  as signals x 1 (n) and x 2 (n) respectively. 
   A transmitter logic unit  318  is also shown in  FIG. 3  communicatively coupled to the RF transmitters  325  and  340  as well as the space-frequency encoder  315  in order to carry out the transmit processing outlined above with reference to  FIG. 6 , including transmission of one or more outbound data packets formatted in accordance with format  200 , as well as legacy format  100 . To this end, the transmitter logic unit  318  includes logic to selectively enable at least the RF transmitters  325 ,  340  of the first and second transmission units  350 ,  360 , as well as direct the space-frequency encoder  315  to space-frequency encode outbound OFDM symbols for transmission by the second transmission unit  360  consistent with the packet format  200 . Moreover, the transmitter logic unit  318  will include logic to determine whether the transmitter  300  should operate in transmit diversity or legacy operating modes through recognition of an externally generated and managed TX_DIVERSITY SEMAPHORE indicating whether TX diversity transmission in accordance with packet format  200  should be employed. It should be noted that the transmit logic unit  318  may include any combination of dedicated logic and or circuits, including one or more finite state machines, as well as programmable logic such as ASICs, information processors such as a general purpose microprocessor or a specific purpose processor (e.g. a DSP) may be used to carry out the processing described above, such as that shown in  FIG. 6 . 
   While the embodiment of  FIG. 3  contemplates dual conversion units  320 ,  335  to separately generate the time domain counterparts to the OFDM symbols and corresponding modified symbols, the teachings of the present invention are not intended to be so limited and in fact other arrangements including a single conversion unit may be provided as long as parallel (but not necessarily concurrent) transmission of the time domain counterpart signals can be performed. 
   Transmit diversity receive processing and a receiver  400  according to an embodiment of the invention will now be discussed in more detail with reference to  FIGS. 4 and 7  respectively. The transmit diversity compliant receiver  400  includes antenna  435  to receive, and, if necessary, attenuate a received signal h(n) based on an incident transmitted signal x 1 (n) and/or x 2 (n) transmitted by a TX diversity transmitter such as transmitter  300 . The receiver  400  includes an RF receiver  410  to attenuate and downconvert the received signal. The channel estimator  430  estimates the frequency domain transfer function from the transmitter to the receiver for each subcarrier in the OFDM symbol consistent with the IEEE 802.11a &amp; 802.11g standards. These transfer function estimates are then subsequently used by the conversion unit  415  for coherent data detection. The conversion unit  415  performs the opposite of conversion units  320  and  335  shown in  FIG. 3  by verifying and removing cyclic prefix, converting the received signal into parallel form and performing a discrete Fourier transformation in order to recover a frequency domain composite signal  417 . This signal  417  presents a data packet formatted in accordance with format  200  or the PLCP frame format  100  depending on the operational mode of the receiver  400 . 
   As shown in  FIG. 4 , a receiver logic unit  422  is communicatively coupled to the output of the conversion unit  415  to monitor the frequency domain composite signal  417  to recognize and even decode certain portions of a data packet or PLCP frame, such as the PLCP preamble  110  and the Signal field  115  or  215 . Referring to  FIG. 7 , when the receiver logic unit  422  perceives the “first” preamble  110  within the frequency domain composite signal  417 , it undertakes conventional training procedures utilizing the RF receiver  410  and the channel estimator  430  as described above (step  702 ). The receiver  400  next receives the Signal field  115  shown in  FIG. 1  or  215  shown in  FIG. 2 . The receive logic unit  422  perceives and decodes the Signal field, and if it fails to detect the TXDIV  217  flag being set in the Signal field (step  705 ), the receiver logic unit  422  assumes that a standards compliant PLCP frame according to frame format  100  is being received and conventional reception and processing techniques consistent with the IEEE 802.11a &amp; 802.11g standards will be employed. In such case, the receiver logic unit  422  directs the space-frequency decoder  420  to operate as a pass-through buffer and present the recovered OFDM symbols generated by the conversion unit  415  to the parallel too serial converter  425  with no further modification or decoding. 
   If, however, the TXDIV flag  705  is perceived as set by the receiver logic unit  422 , it assumes the inbound data packet presented by the frequency domain composite signal  417  emanates from a TX diversity transmitter and assumes the inbound data packet is formatted in accordance with format  200  ( FIG. 2 ), and begins second transmission unit retraining (steps  710 - 728  of  FIG. 7 ) and recovery of the Service field  120  using space-frequency encoding. Once retraining and Service field recovery are complete, the receiver logic unit begins recovery of the PSDU/payload  255  portion (steps  730  through  750 ) of the data packet. As noted above, within a given timeslice needed to convey the given OFDM symbol DATA K , the recovered composite signal  417  may incorporate a complete or incomplete weighted analog sum of both DATA K  and DATA K N OFDM symbols. Therefore, in transmit diversity mode, this composite signal  417  is then fed to a space frequency decoder  420  for recovery of the “corrected” OFDM symbol of interest (here DATA K ). 
   The stream of recovered OFDM encoded symbols  418  generated by the space-frequency decoder are then sent to the parallel-serial unit  425  and then on to the symbol decoder  550  ( FIG. 5 ) through interface  405 . 
   It should be noted here that the receiver logic unit  422  may include a combination of dedicated logic and circuits configured as an event-driven state machine capable of carrying out the processing described above with reference to  FIG. 7 . Alternatively, the receiver logic  422  may incorporate one or more application specific circuits in combination with one or more specialized or general purpose information processors (e.g. such as a microprocessor, a microcontroller, a digital signal processor, consistent with the present invention) programmed in accordance with e.g. program code statements contained in memory accessible to such information processor(s). 
     FIG. 5  shows a transceiver  500  adapted for wireless data transmission consistent with the present invention as well as IEEE 802.11a &amp; 802.11g standards, incorporating the transmitter  300  and receiver  400 . The transmitter  300  is shown as part of a larger diversity transmitter unit  560  which also provides, through the symbol encoder  520 , conventional OFDM encoding of outbound digital data presented by a network interface, such as the OSI layer 2+ MAC interface  510  shown in  FIG. 5 . Likewise, the receiver  400  forms part of a larger diversity aware receiver unit  570 , which also performs conventional OFDM symbol decoding via decoder  550  in order for recovered inbound digital data to be presented to the data interface  510 . Though not shown in  FIG. 5 , the transceiver  500  may form an operational part of a network interface apparatus such as a PC card or network interface card capable of interfacing with the CPU or information processor of an information processing apparatus such as a desktop or laptop computer, and may be integrated within and constitute a part of such information processing apparatus. This network interface apparatus may alternatively form an operational component of a wireless communications access point such as a base station as will be appreciated by these ordinarily skilled in the art. 
   It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. For example, though the transmit diversity techniques described above with reference to specific wireless data transmission implementations and embodiments interoperable with the IEEE 802.11a &amp; 802.11g standards, the teachings of the present invention may easily encompass other types of wireless transmission, as will become apparent to those ordinarily skilled in the art. In addition, though the embodiments described above contemplate substantially concurrent or parallel transmission of the DATA K  and modified counterpart DATA K ′ symbols, other sequencing of the DATA K  and DATA K ′ symbols may be used, such as staggered sequencing of DATA K  and DATA K ′ symbols in time, so long as a TX diversity aware receiver has the capability of discerning both symbols in accordance with the particular sequencing being utilized. The scope of the present invention should, therefore, be determined only by the following claims.