Abstract:
A method for generating a preamble of an Orthogonal Frequency Division Multiplexed (OFDM) data frame for a multiple input multiple output (MIMO) wireless communication includes determining at least one system condition preamble format parameter. When the system condition preamble format parameter satisfies a first preamble format parameter a preamble having a first preamble format is formed. When the system condition preamble format parameter satisfies a second preamble format parameter, a preamble having a second preamble format is formed. Further, when the system condition preamble format parameter satisfies a third preamble format parameter, a preamble having a third preamble format is formed. The first, second, and third preamble formats differ based upon their lengths, fields, and modulation formats of a high throughput signal field.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present U.S. Utility patent application claims priority pursuant to 35 U.S.C. §120, as a continuation, to the following U.S. Utility patent application which is hereby incorporated herein by reference in its entirety and made part of the present U.S. Utility patent application for all purposes: 
         [0002]    1. U.S. Utility application Ser. No. 11/261,250, entitled “Preamble Formats Supporting High-Throughput MIMO WLAN and Auto-Detection,” (Attorney Docket No. BP4978), filed Oct. 28, 2005, pending, which claims priority pursuant to 35 U.S.C. §119(e) to the following U.S. Provisional Patent Application which is hereby incorporated herein by reference in its entirety and made part of the present U.S. Utility Patent Application for all purposes: 
         [0003]    a. U.S. Provisional Application Ser. No. 60/711,169, entitled “Preamble Formats Supporting High-Throughput MIMO WLAN and Auto-Detection,” (Attorney Docket No. BP4978), filed Aug. 24, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0004]    1. Technical Field of the Invention 
         [0005]    This invention relates generally to wireless communication systems and more particularly to supporting multiple wireless communication protocols within a wireless local area network. 
         [0006]    2. Description of Related Art 
         [0007]    Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof. 
         [0008]    Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network. 
         [0009]    For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard. 
         [0010]    As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. 
         [0011]    In many systems, the transmitter will include one antenna for transmitting the RF signals, which are received by a single antenna, or multiple antennas, of a receiver. When the receiver includes two or more antennas, the receiver will select one of them to receive the incoming RF signals. In this instance, the wireless communication between the transmitter and receiver is a single-output-single-input (SISO) communication, even if the receiver includes multiple antennas that are used as diversity antennas (i.e., selecting one of them to receive the incoming RF signals). For SISO wireless communications, a transceiver includes one transmitter and one receiver. Currently, most wireless local area networks (WLAN) that are IEEE 802.11, 802.11a, 802.11b, or 802.11g employ SISO wireless communications. 
         [0012]    Other types of wireless communications include single-input-multiple-output (SIMO), multiple-input-single-output (MISO), and multiple-input-multiple-output (MIMO). In a SIMO wireless communication, a single transmitter processes data into radio frequency signals that are transmitted to a receiver. The receiver includes two or more antennas and two or more receiver paths. Each of the antennas receives the RF signals and provides them to a corresponding receiver path (e.g., LNA, down conversion module, filters, and ADCs). Each of the receiver paths processes the received RF signals to produce digital signals, which are combined and then processed to recapture the transmitted data. 
         [0013]    For a multiple-input-single-output (MISO) wireless communication, the transmitter includes two or more transmission paths (e.g., digital to analog converter, filters, up-conversion module, and a power amplifier) that each converts a corresponding portion of baseband signals into RF signals, which are transmitted via corresponding antennas to a receiver. The receiver includes a single receiver path that receives the multiple RF signals from the transmitter. In this instance, the receiver uses beam forming to combine the multiple RF signals into one signal for processing. 
         [0014]    For a multiple-input-multiple-output (MIMO) wireless communication, the transmitter and receiver each include multiple paths. In such a communication, the transmitter parallel processes data using a spatial and time encoding function to produce two or more streams of data. The transmitter includes multiple transmission paths to convert each stream of data into multiple RF signals. The receiver receives the multiple RF signals via multiple receiver paths that recapture the streams of data utilizing a spatial and time decoding function. The recaptured streams of data are combined and subsequently processed to recover the original data. 
         [0015]    Heretofore, most systems of this type did not have sufficient flexibility in all aspects of operation, particularly in preamble structure, to satisfy system conditions that change over time. Therefore, a need exists for more flexibility in the operation of such wireless communication systems to adapt to changing system conditions and system requirements. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    The present invention is directed to apparatus and methods of operation that are further described in the following Brief Description of the Drawings, the Detailed Description of the Drawings, and the Claims. Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic block diagram of a wireless communication system in accordance with the present invention; 
           [0018]      FIG. 2  is a schematic block diagram of a wireless communication device in accordance with the present invention; 
           [0019]      FIG. 3  is a schematic block diagram of another wireless communication device in accordance with the present invention; 
           [0020]      FIG. 4  is a schematic block diagram of an RF transmitter in accordance with the present invention; 
           [0021]      FIG. 5  is a schematic block diagram of an RF receiver in accordance with the present invention; 
           [0022]      FIG. 6  is a flow chart illustrating a method for forming a preamble according to an embodiment of the present invention; 
           [0023]      FIG. 7A  is block diagram illustrating a data frame format according to the present invention 
           [0024]      FIG. 7B  is block diagram illustrating a first preamble format according to an embodiment of the present invention; 
           [0025]      FIG. 8  is block diagram illustrating a second preamble format according to an embodiment of the present invention; 
           [0026]      FIG. 9  is block diagram illustrating a third preamble format according to an embodiment of the present invention; 
           [0027]      FIG. 10  is block diagram illustrating LTF fields of preambles according to an embodiment of the present invention; 
           [0028]      FIG. 11  is diagram illustrating rotation of BPSK symbols according to some aspects of the present invention; 
           [0029]      FIG. 12  is block diagram illustrating data fields of an HT-SIG field according to an embodiment of the present invention; and 
           [0030]      FIG. 13  is a flow chart illustrating an embodiment of a method of the present invention for determining a format of an OFDM data frame within a MIMO wireless communication by a wireless receiver. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]      FIG. 1  is a schematic block diagram illustrating a communication system  10  that includes a plurality of base stations and/or access points  12 ,  16 , a plurality of wireless communication devices  18 - 32  and a network hardware component  34 . Note that the network hardware  34 , which may be a router, switch, bridge, modem, system controller, et cetera provides a wide area network connection  42  for the communication system  10 . Further note that the wireless communication devices  18 - 32  may be laptop host computers  18  and  26 , personal digital assistant hosts  20  and  30 , personal computer hosts  24  and  32  and/or cellular telephone hosts  22  and  28 . The details of the wireless communication devices will be described in greater detail with reference to  FIG. 2 . 
         [0032]    Wireless communication devices  22 ,  23 , and  24  are located within an independent basic service set (IBSS) area and communicate directly (i.e., point to point). In this configuration, these devices  22 ,  23 , and  24  may only communicate with each other. To communicate with other wireless communication devices within the system  10  or to communicate outside of the system  10 , the devices  22 ,  23 , and/or  24  need to affiliate with one of the base stations or access points  12  or  16 . 
         [0033]    The base stations or access points  12 ,  16  are located within basic service set (BSS) areas  11  and  13 , respectively, and are operably coupled to the network hardware  34  via local area network connections  36 ,  38 . Such a connection provides the base station or access point  12 ,  16  with connectivity to other devices within the system  10  and provides connectivity to other networks via the WAN connection  42 . To communicate with the wireless communication devices within its BSS  11  or  13 , each of the base stations or access points  12 - 16  has an associated antenna or antenna array. For instance, base station or access point  12  wirelessly communicates with wireless communication devices  18  and  20  while base station or access point  16  wirelessly communicates with wireless communication devices  26 - 32 . Typically, the wireless communication devices register with a particular base station or access point  12 ,  16  to receive services from the communication system  10 . 
         [0034]    Typically, base stations are used for cellular telephone systems and like-type systems, while access points are used for in-home or in-building wireless networks (e.g., IEEE 802.11 and versions thereof, Bluetooth, and/or any other type of radio frequency based network protocol). Regardless of the particular type of communication system, each wireless communication device includes a built-in radio and/or is coupled to a radio. 
         [0035]      FIG. 2  is a schematic block diagram illustrating an embodiment of a wireless communication device that includes the host device  18 - 32  and an associated radio  60 . For cellular telephone hosts, the radio  60  is a built-in component. For personal digital assistants hosts, laptop hosts, and/or personal computer hosts, the radio  60  may be built-in or an externally coupled component. 
         [0036]    As illustrated, the host device  18 - 32  includes a processing module  50 , memory  52 , a radio interface  54 , an input interface  58 , and an output interface  56 . The processing module  50  and memory  52  execute the corresponding instructions that are typically done by the host device. For example, for a cellular telephone host device, the processing module  50  performs the corresponding communication functions in accordance with a particular cellular telephone standard. 
         [0037]    The radio interface  54  allows data to be received from and sent to the radio  60 . For data received from the radio  60  (e.g., inbound data), the radio interface  54  provides the data to the processing module  50  for further processing and/or routing to the output interface  56 . The output interface  56  provides connectivity to an output display device such as a display, monitor, speakers, et cetera such that the received data may be displayed. The radio interface  54  also provides data from the processing module  50  to the radio  60 . The processing module  50  may receive the outbound data from an input device such as a keyboard, keypad, microphone, et cetera via the input interface  58  or generate the data itself. For data received via the input interface  58 , the processing module  50  may perform a corresponding host function on the data and/or route it to the radio  60  via the radio interface  54 . 
         [0038]    Radio  60  includes a host interface  62 , digital receiver processing module  64 , an analog-to-digital converter  66 , a high pass and low pass filter module  68 , an IF mixing down conversion stage  70 , a receiver filter  71 , a low noise amplifier  72 , a transmitter/receiver switch  73 , a local oscillation module  74 , memory  75 , a digital transmitter processing module  76 , a digital-to-analog converter  78 , a filtering/gain module  80 , an IF mixing up conversion stage  82 , a power amplifier  84 , a transmitter filter module  85 , a channel bandwidth adjust module  87 , and an antenna  86 . The antenna  86  may be a single antenna that is shared by transmit and receive paths as regulated by the Tx/Rx switch  73 , or may include separate antennas for the transmit path and receive path. The antenna implementation will depend on the particular standard to which the wireless communication device is compliant. 
         [0039]    The digital receiver processing module  64  and the digital transmitter processing module  76 , in combination with operational instructions stored in memory  75 , execute digital receiver functions and digital transmitter functions, respectively. The digital receiver functions include, but are not limited to, digital intermediate frequency to baseband conversion, demodulation, constellation demapping, descrambling, and/or decoding. The digital transmitter functions include, but are not limited to, encoding, scrambling, constellation mapping, modulation, and/or digital baseband to IF conversion. The digital receiver and transmitter processing modules  64  and  76  may be implemented using a shared processing device, individual processing devices, or a plurality of processing devices. Such a processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on operational instructions. The memory  75  may be a single memory device or a plurality of memory devices. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, and/or any device that stores digital information. Note that when the processing module  64  and/or  76  implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory storing the corresponding operational instructions is embedded with the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. 
         [0040]    In operation, the radio  60  receives outbound data  94  from the host device via the host interface  62 . The host interface  62  routes the outbound data  94  to the digital transmitter processing module  76 , which processes the outbound data  94  in accordance with a particular wireless communication standard (e.g., IEEE 802.11, Bluetooth, et cetera) to produce digital transmission formatted data  96 . The digital transmission formatted data  96  will be digital base-band signals (e.g., have a zero IF) or a digital low IF signals, where the low IF typically will be in the frequency range of one hundred kilohertz to a few megahertz. 
         [0041]    The digital-to-analog converter  78  converts the digital transmission formatted data  96  from the digital domain to the analog domain. The filtering/gain module  80  filters and/or adjusts the gain of the analog signals prior to providing it to the IF mixing stage  82 . The IF mixing stage  82  converts the analog baseband or low IF signals into RF signals based on a transmitter local oscillation  83  provided by local oscillation module  74 . The power amplifier  84  amplifies the RF signals to produce outbound RF signals  98 , which are filtered by the transmitter filter module  85 . The antenna  86  transmits the outbound RF signals  98  to a targeted device such as a base station, an access point and/or another wireless communication device. 
         [0042]    The radio  60  also receives inbound RF signals  88  via the antenna  86 , which were transmitted by a base station, an access point, or another wireless communication device. The antenna  86  provides the inbound RF signals  88  to the receiver filter module  71  via the Tx/Rx switch  73 , where the Rx filter  71  bandpass filters the inbound RF signals  88 . The Rx filter  71  provides the filtered RF signals to low noise amplifier  72 , which amplifies the signals  88  to produce an amplified inbound RF signals. The low noise amplifier  72  provides the amplified inbound RF signals to the IF mixing module  70 , which directly converts the amplified inbound RF signals into an inbound low IF signals or baseband signals based on a receiver local oscillation  81  provided by local oscillation module  74 . The down conversion module  70  provides the inbound low IF signals or baseband signals to the filtering/gain module  68 . The high pass and low pass filter module  68  filters, based on settings provided by the channel bandwidth adjust module  87 , the inbound low IF signals or the digital reception formatted data to produce filtered inbound signals. 
         [0043]    The analog-to-digital converter  66  converts the filtered inbound signals from the analog domain to the digital domain to produce digital reception formatted data  90 , where the digital reception formatted data  90  will be digital base-band signals or digital low IF signals, where the low IF typically will be in the frequency range of one hundred kilohertz to a few megahertz. The digital receiver processing module  64 , based on settings provided by the channel bandwidth adjust module  87 , decodes, descrambles, demaps, and/or demodulates the digital reception formatted data  90  to recapture inbound data  92  in accordance with the particular wireless communication standard being implemented by radio  60 . The host interface  62  provides the recaptured inbound data  92  to the host device  18 - 32  via the radio interface  54 . 
         [0044]    As one of average skill in the art will appreciate, the wireless communication device of  FIG. 2  may be implemented using one or more integrated circuits. For example, the host device may be implemented on one integrated circuit, the digital receiver processing module  64 , the digital transmitter processing module  76  and memory  75  may be implemented on a second integrated circuit, and the remaining components of the radio  60 , less the antenna  86 , may be implemented on a third integrated circuit. As an alternate example, the radio  60  may be implemented on a single integrated circuit. As yet another example, the processing module  50  of the host device and the digital receiver and transmitter processing modules  64  and  76  may be a common processing device implemented on a single integrated circuit. Further, the memory  52  and memory  75  may be implemented on a single integrated circuit and/or on the same integrated circuit as the common processing modules of processing module  50  and the digital receiver and transmitter processing module  64  and  76 . 
         [0045]      FIG. 3  is a schematic block diagram illustrating another embodiment of a wireless communication device that includes the host device  18 - 32  and an associated radio  60 . For cellular telephone hosts, the radio  60  is a built-in component. For personal digital assistants hosts, laptop hosts, and/or personal computer hosts, the radio  60  may be built-in or an externally coupled component. 
         [0046]    As illustrated, the host device  18 - 32  includes a processing module  50 , memory  52 , radio interface  54 , input interface  58  and output interface  56 . The processing module  50  and memory  52  execute the corresponding instructions that are typically done by the host device. For example, for a cellular telephone host device, the processing module  50  performs the corresponding communication functions in accordance with a particular cellular telephone standard. 
         [0047]    The radio interface  54  allows data to be received from and sent to the radio  60 . For data received from the radio  60  (e.g., inbound data), the radio interface  54  provides the data to the processing module  50  for further processing and/or routing to the output interface  56 . The output interface  56  provides connectivity to an output display device such as a display, monitor, speakers, et cetera such that the received data may be displayed. The radio interface  54  also provides data from the processing module  50  to the radio  60 . The processing module  50  may receive the outbound data from an input device such as a keyboard, keypad, microphone, et cetera via the input interface  58  or generate the data itself. For data received via the input interface  58 , the processing module  50  may perform a corresponding host function on the data and/or route it to the radio  60  via the radio interface  54 . 
         [0048]    Radio  60  includes a host interface  62 , a baseband processing module  100 , memory  65 , a plurality of radio frequency (RF) transmitters  106 - 110 , a transmit/receive (T/R) module  114 , a plurality of antennas  81 - 85 , a plurality of RF receivers  118 - 120 , a channel bandwidth adjust module  87 , and a local oscillation module  74 . The baseband processing module  100 , in combination with operational instructions stored in memory  65 , executes digital receiver functions and digital transmitter functions, respectively. The digital receiver functions include, but are not limited to, digital intermediate frequency to baseband conversion, demodulation, constellation demapping, decoding, de-interleaving, fast Fourier transform, cyclic prefix removal, space and time decoding, and/or descrambling. The digital transmitter functions include, but are not limited to, encoding, scrambling, interleaving, constellation mapping, modulation, inverse fast Fourier transform, cyclic prefix addition, space and time encoding, and digital baseband to IF conversion. The baseband processing modules  100  may be implemented using one or more processing devices. Such a processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on operational instructions. The memory  65  may be a single memory device or a plurality of memory devices. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, and/or any device that stores digital information. Note that when the processing module  100  implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory storing the corresponding operational instructions is embedded with the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. 
         [0049]    In operation, the radio  60  receives outbound data  94  from the host device via the host interface  62 . The baseband processing module  64  receives the outbound data  94  and, based on a mode selection signal  102 , produces one or more outbound symbol streams  104 . The mode selection signal  102  will indicate a particular mode of operation that is compliant with one or more specific modes of the various IEEE 802.11 standards. For example, the mode selection signal  102  may indicate a frequency band of 2.4 GHz, a channel bandwidth of 20 or 22 MHz and a maximum bit rate of 54 megabits-per-second. In this general category, the mode selection signal  102  will further indicate a particular rate ranging from 1 megabit-per-second to 54 megabits-per-second. In addition, the mode selection signal will indicate a particular type of modulation, which includes, but is not limited to, Barker Code Modulation, Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), CCK, 16 Quadrature Amplitude Modulation (QAM) and/or 64 QAM. The mode select signal  102  may also include a code rate, a number of coded bits per subcarrier (NBPSC), coded bits per OFDM symbol (NCBPS), and/or data bits per OFDM symbol (NDBPS). The mode selection signal  102  may also indicate a particular channelization for the corresponding mode that provides a channel number and corresponding center frequency. The mode select signal  102  may further indicate a power spectral density mask value and a number of antennas to be initially used for a MIMO communication. 
         [0050]    The baseband processing module  100 , based on the mode selection signal  102  produces one or more outbound symbol streams  104  from the outbound data  94 . For example, if the mode selection signal  102  indicates that a single transmit antenna is being utilized for the particular mode that has been selected, the baseband processing module  100  will produce a single outbound symbol stream  104 . Alternatively, if the mode select signal  102  indicates 2, 3 or 4 antennas, the baseband processing module  100  will produce 2, 3 or 4 outbound symbol streams  104  from the outbound data  94 . 
         [0051]    Depending on the number of outbound streams  104  produced by the baseband module  10 , a corresponding number of the RF transmitters  106 - 110  will be enabled to up convert the outbound symbol streams  104  into outbound RF signals  112 . In general, each of the RF transmitters  106 - 110  includes a digital filter and up sampling module, a digital to analog conversion module, an analog filter module, a frequency up conversion module, a power amplifier, and a radio frequency band pass filter. The RF transmitters  106 - 110  provide the outbound RF signals  112  to the transmit/receive module  114 , which provides each outbound RF signal to a corresponding antenna  81 - 85 . 
         [0052]    When the radio  60  is in the receive mode, the transmit/receive module  114  receives one or more inbound RF signals  116  via the antennas  81 - 85  and provides them to one or more RF receivers  118 - 122 . The RF receiver  118 - 122 , based on settings provided by the channel bandwidth adjust module  87 , down converts the inbound RF signals  116  into a corresponding number of inbound symbol streams  124 . The number of inbound symbol streams  124  will correspond to the particular mode in which the data was received. The baseband processing module  100  converts the inbound symbol streams  124  into inbound data  92 , which is provided to the host device  18 - 32  via the host interface  62 . 
         [0053]    As one of average skill in the art will appreciate, the wireless communication device of  FIG. 3  may be implemented using one or more integrated circuits. For example, the host device may be implemented on one integrated circuit, the baseband processing module  100  and memory  65  may be implemented on a second integrated circuit, and the remaining components of the radio  60 , less the antennas  81 - 85 , may be implemented on a third integrated circuit. As an alternate example, the radio  60  may be implemented on a single integrated circuit. As yet another example, the processing module  50  of the host device and the baseband processing module  100  may be a common processing device implemented on a single integrated circuit. Further, the memory  52  and memory  65  may be implemented on a single integrated circuit and/or on the same integrated circuit as the common processing modules of processing module  50  and the baseband processing module  100 . 
         [0054]      FIG. 4  is a schematic block diagram of an embodiment of an RF transmitter  67 ,  69 ,  71 . The RF transmitter includes a digital filter and up-sampling module  475 , a digital-to-analog conversion module  477 , an analog filter  479 , and up-conversion module  81 , a power amplifier  483  and a RF filter  485 . The digital filter and up-sampling module  475  receives one of the outbound symbol streams  89  and digitally filters it and then up-samples the rate of the symbol streams to a desired rate to produce the filtered symbol streams  487 . The digital-to-analog conversion module  477  converts the filtered symbols  487  into analog signals  489 . The analog signals may include an in-phase component and a quadrature component. 
         [0055]    The analog filter  479  filters the analog signals  489  to produce filtered analog signals  491 . The up-conversion module  481 , which may include a pair of mixers and a filter, mixes the filtered analog signals  491  with a local oscillation  493 , which is produced by local oscillation module  99 , to produce high frequency signals  495 . The frequency of the high frequency signals  495  corresponds to the frequency of the RF signals  492 . The power amplifier  483  amplifies the high frequency signals  495  to produce amplified high frequency signals  497 . The RF filter  485 , which may be a high frequency band-pass filter, filters the amplified high frequency signals  497  to produce the desired output RF signals  91 . 
         [0056]    As one of average skill in the art will appreciate, each of the radio frequency transmitters  67 ,  69 ,  71  will include a similar architecture as illustrated in  FIG. 4  and further include a shut-down mechanism such that when the particular radio frequency transmitter is not required, it is disabled in such a manner that it does not produce interfering signals and/or noise. 
         [0057]      FIG. 5  is a schematic block diagram of each of the RF receivers  75 ,  77 ,  79 . In this embodiment, each of the RF receivers includes an RF filter  501 , a low noise amplifier (LNA)  503 , a programmable gain amplifier (PGA)  505 , a down-conversion module  507 , an analog filter  509 , an analog-to-digital conversion module  511  and a digital filter and down-sampling module  513 . The RF filter  501 , which may be a high frequency band-pass filter, receives the inbound RF signals  93  and filters them to produce filtered inbound RF signals. The low noise amplifier  503  amplifies the filtered inbound RF signals  93  based on a gain setting and provides the amplified signals to the programmable gain amplifier  505 . The programmable gain amplifier further amplifies the inbound RF signals  93  before providing them to the down-conversion module  507 . 
         [0058]    The down-conversion module  507  includes a pair of mixers, a summation module, and a filter to mix the inbound RF signals with a local oscillation (LO) that is provided by the local oscillation module to produce analog baseband signals. The analog filter  509  filters the analog baseband signals and provides them to the analog-to-digital conversion module  511  which converts them into a digital signal. The digital filter and down-sampling module  513  filters the digital signals and then adjusts the sampling rate to produce the inbound symbol stream  95 . 
         [0059]      FIG. 6  is a flow chart illustrating a method for forming a preamble according to an embodiment of the present invention. The operations  600  of the embodiment of  FIG. 6  are described with reference to a MIMO wireless communications system using an OFDM baseband signal format. The principles and teachings of  FIG. 6  would apply to other types of communications systems as well. 
         [0060]    Operation  600  commences with determining at least one system condition preamble format parameter (step  602 ). Operation then includes selecting one of a plurality of preamble formats based upon the preamble format parameter (step  604 ). This operation may include selecting a particular high throughput signal field (HT-SIG field) format. When a first preamble format is selected (step  606 ), operation includes creating/transmitting a preamble according to the first preamble format (step  608 ). One embodiment of the first preamble format will be described with reference to  FIG. 7 . When a second preamble format is selected (step  608 ), operation includes creating/transmitting a preamble according to the first preamble format (step  610 ). One embodiment of the second preamble format will be described with reference to  FIG. 8 . When a third preamble format is selected (step  612 ), operation includes creating/transmitting a preamble according to the third preamble format (step  614 ). One embodiment of the third preamble format will be described with reference to  FIG. 9 . Generally, each preamble format differs from each other preamble format, as will be described further below. Differences among the preamble formats may include differences in preamble lengths, differences in field lengths, differences in the field structure of the preambles, differences in the number of fields of the preambles, differences in the durations of the preambles, and differences in the modulations and/or encodings of the high throughput signal field of the preambles, among other possible differences. 
         [0061]    According to one particular aspect of the embodiment of  FIG. 6 , the first preamble format includes a legacy short training field, a legacy long training field, a legacy signal field, a high throughput signal field having a first duration and modulation, and a high throughput long training field. In such case, the second preamble format includes a legacy short training field, a legacy long training field, a legacy signal field, a high throughput signal field having a second duration and modulation, and a high throughput long training field. Further, in such case, the third preamble format includes a legacy short training field, a legacy long training field, a legacy signal field, a high throughput signal field having a third duration and modulation, and at least one high throughput long training field. 
         [0062]    According to another particular aspect of the embodiment of  FIG. 6 , each of the preamble formats includes a respective modulation format for a high throughput signal field of the preamble. With one particular example of this aspect, the at least one system condition preamble format parameter includes a channel signal to noise ratio (SNR) between a transmitting MIMO wireless device and a receiving MIMO wireless device. With this example, when a relatively higher channel SNR exists, using a relatively higher order modulation for the high throughput signal field. Further, when a relatively lower channel SNR exists, using a relatively lower order modulation for the high throughput signal field. Extending this concept to the three preamble format embodiment of  FIG. 6 , a high throughput signal field of the first preamble format includes one QPSK OFDM symbol, a high throughput signal field of the second preamble format includes two BPSK OFDM symbols that are both rotated by 90 degrees, and a high throughput signal field of the third preamble format includes two BPSK OFDM symbols, one of which is rotated by 90 degrees. Rotation of the BPSK OFDM symbols may be by positive 90 degrees or by negative 90 degrees. 
         [0063]    According to another aspect of  FIG. 6 , when the at least one system condition preamble format parameter indicates that the preamble will be used for clear channel assessment by a non-data-receiving MIMO wireless device, a relatively longer preamble is employed. Further, when the at least one system condition preamble format parameter indicates that the preamble will not be used for clear channel assessment by a non-data-receiving MIMO wireless device, a relatively shorter preamble is employed. According to the present invention, the first preamble format is selected when a highest throughput is required, the second preamble format is selected when long range operations are required, and the third preamble format is selected for maximum backward-compatibility when also performing transmit beamforming. With the third preamble format selected, auto-detection at the receiver is favored. With the preambles of the present invention, best network performance under different system conditions is met while the preambles are distinguishable automatically at the receiver. 
         [0064]    Formation of the preambles according to the present invention makes use of the following definitions:
       Guard interval=cyclic prefix of an OFDM symbol; the last N_guard samples of the IFFT output prepended to the beginning of the first sample of the IFFT output.   N_ss=number of spatial streams (independent data streams that may be sent over the air in the same space, time and frequency band).   N_tx=number of transmitter RF paths.   L-STF=legacy (IEEE 802.11a or 802.11g) short training field, which comprises 10 identical symbols each of 800 nsec duration. The L-STF is typically used for carrier detection, AGC, and coarse carrier frequency offset estimation.   L-LTF=legacy (IEEE 802.11a or 802.11g) long training field, which comprises 2 identical symbols each of 3200 nsec duration, preceded by a 1600 nsec double-length guard interval. The L-LTF is typically used for fine carrier frequency offset estimation, initial sampling frequency offset estimation, initial FFT window placement and channel estimation.   L-SIG=legacy (IEEE 802.11a or 802.11g) signal field, which comprises one symbol of 4 usec duration inclusive of an 800-nsec guard interval. The L-SIG field includes information about the payload physical-layer rate and the frame length.   TX beamforming is a process in which the output of each N_ss element vector corresponding to each OFDM subcarrier index k is multiplied by an N_tx×N_ss element matrix, P(k).   HT-STF=high-throughput short-training field. The HT-STF field is typically used for re-AGC of the received input sequence on a transition from a non-beamformed prefix to a beamformed portion of the frame.   HT-LTF=high-throughput long-training field. The HT-LTF field is used for MIMO channel estimation and may also be used for fine carrier frequency offset estimation, sampling frequency offset estimation and FFT window placement. The HT-LTF field may be 4 or 8 usec in duration with 800 or 1600 nsec guard intervals.   HT-SIG=high-throughput signal field. The HT-SIG field contains MIMO physical-layer rate and length information as well as other possible information about the frame format.   CDD=cyclic delay diversity, which involves cyclically shifting the samples out of the IFFT on any of the streams, where the period of the cyclic shift (the modulus) is the number of points in the IFFT.   The Log-Likelihood Ratio (LLR) of a bit b is given by:       
 
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         [0077]      FIG. 7A  is block diagram illustrating a data frame format according to the present invention. As shown, the data frame  700  includes two streams, stream  1 , and stream  2 . Stream  1  includes a preamble  702  and a data field  704 . Stream  2  includes a preamble  706  and a data field  708  that have been shifted using a time orthogonal shifting format and/or a CCD shifting format. The concepts of  FIG. 7A , as well as those of  FIGS. 7B ,  8 , and  9  may be extended to N streams, as was previously described with reference to  FIG. 3 . 
         [0078]      FIG. 7B  is block diagram illustrating a first preamble format according to an embodiment of the present invention. Each of streams  1  and  2  of the first preamble format include L-STF fields, L-LTF fields, L-SIG fields, HT-SIG fields, and HT-LTF 2  fields. With the first preamble format of  FIG. 7B , the L-LTF fields and the HT-LTF 2  fields use time-orthogonal and/or CDD shifting formats for the two streams. With the first preamble format of  FIG. 7B , the HT-LTF 1  field=the L-LTF field. The preamble format of  FIG. 7B  supports straightforward single-stream channel estimation that may be used for decoding the HT-SIG field. The channel estimate for decoding the HT-SIG is a simple SISO (legacy) channel estimate. With the first preamble format of  FIG. 7B , the HT-SIG field is encoded using QPSK modulation using a 64-state binary convolutional code at rate=½. The encoding for this preamble format may employ an IEEE 802.11a convolutional code. Further, the L-SIG may specify a physical layer rate of 6 Mbps. 
         [0079]      FIG. 8  is block diagram illustrating a second preamble format according to an embodiment of the present invention. Each of streams  1  and  2  of the second preamble format include L-STF fields, L-LTF fields, L-SIG fields, HT-SIG fields, and HT-LTF 2  fields. With the third preamble format of  FIG. 8 , the L-LTF fields and the HT-LTF 2  fields use time-orthogonal and/or CDD shifting formats for the two streams. With the first preamble format of  FIG. 8 , the HT-LTF 1  field=the L-LTF field. The preamble format of  FIG. 8  supports straightforward single-stream channel estimation that may be used for decoding the HT-SIG field. The channel estimate for decoding the HT-SIG is a simple SISO (legacy) channel estimate. The HT-SIG field includes two contiguous 4-usec symbols encoded as 90-degree rotated BPSK (i.e., +/−sqrt(−1) instead of +/−1 values) using a 64-state binary convolutional code at rate=½. The encoding for this preamble format may employ an IEEE 802.11a convolutional code. The guard interval of the HT-SIG field is 800 nanoseconds. 
         [0080]      FIG. 9  is block diagram illustrating a third preamble format according to an embodiment of the present invention. Each of streams  1  and  2  of the first preamble format include L-STF fields, L-LTF fields, L-SIG fields, HT-SIG fields, HT-LTF 1  fields, and HT-LTF 2  fields. With the third preamble format of  FIG. 9 , the L-LTF fields, the HT-LTF 1  fields, and HT-LTF 2  fields use time-orthogonal and/or CDD shifting formats for the two streams. With the first preamble format of  FIG. 9 , the HT-LTF 1  field=the L-LTF field. The preamble format of  FIG. 9  supports straightforward single-stream channel estimation that may be used for decoding the HT-SIG field. The channel estimate is for decoding HT-SIG is a simple SISO (legacy) channel estimate. The HT-SIG field includes two contiguous 4-usec symbols. The first symbol is encoded as 90-degree rotated BPSK symbol (i.e., +/−sqrt(−1) instead of +/−1 values) using a 64-state binary convolutional code at rate=½. The guard interval is 800 nsec The second symbol is encoded as an unrotated BPSK (i.e., +/−1) symbol using a 64-state binary convolutional code at rate=½ using as its initial state the final state at the end of the encoding of the first HT-SIG symbol. The encoding for this preamble format may employ an IEEE 802.11a convolutional code. 
         [0081]    The preamble formats of  FIGS. 7B ,  8  and  9 , all assumed that the number of symbol streams equals two, i.e., N_ss=2. With N_ss=1, 3, and 4, the following apply:
       For N_ss=1, HT-LTF 2  is deleted with respect to  FIGS. 7B ,  8 , and  9 .   For N_ss=3 and 4, there are 1 or 2 additional spatial streams and an HT-LTF 3  and an HT-LTF 4 , each of 4- or 8-usec duration and 800- or 1600-nsec guard interval are added.   4-usec total duration and 800-nsec guard interval is preferred.       
 
         [0085]      FIG. 10  is block diagram illustrating LTF fields of preambles according to an embodiment of the present invention. These LTF fields have a Time-Orthogonal Format. Further, a per-subcarrier phase shift (CDD) is applied on the second tx antenna (second stream) to avoid large received power fluctuations due to beamforming. With this per-subcarrier phase shift, 
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         [0000]    where LTRN is some base training sequence and the subcarrier index is k. The per-subcarrier phase shift may be implemented by cyclic shifts. 
         [0086]    For a time-orthogonal preamble, any constant times a unitary matrix may be used to multiply the legacy long-training symbols. In the previous example, a Walsh-Hadamard matrix was employed. Another example of a rotation matrix that provides the phase shift function is: 
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         [0087]    Note that for the 2-stream case, the upper left-most 2×2 sub-matrix is selected. Further, note that that any group of columns of P HTLTF *P HTLTF   H , where “H” indicates a Hermitian (complex-conjugate) transpose is equal to a constant times an identity matrix. 
         [0088]      FIG. 11  is diagram illustrating rotation of BPSK symbols according to some aspects of the present invention. This encoding is used on each OFDM subcarrier when required to rotate BPSK symbols of the HT-SIG field for a particular preamble format. Note that a rotation of +90 or −90 degrees may be employed depending upon the embodiment. Further, note that with the rotation, the receiving device must be able to sense two-dimensional modulation constellation formats. 
         [0089]      FIG. 12  is block diagram illustrating data fields of an HT-SIG field according to an embodiment of the present invention. With the embodiment of  FIG. 12 , the HT-SIG field data fields include:
       MCS: Modes 0-32 as defined in nSync spec #889-05; Modes 33-127 reserved   20/40: “0”=&gt;20 MHz, “1”=&gt;40 MHz   Length: # Octets in payload (not including SVC field); min Length=1   STC: # Chains used for Space-Time Coding−# spatial streams from MCS   AdvCdg: “0”=&gt;802.11a BCC64, “1”=&gt;Frame uses advanced coding   SGI: “0”=&gt;¼-symbol Guard Interval, “1”=&gt;⅛-symbol Guard Interval   #LTF: Number of LTFs in frame (applicable to channel sounding frames only)   Reserved bits set to all “1s” to avoid a long string of zeros   CRC: CRC-8   Tail: Set to all 0s         
         [0100]      FIG. 13  is a flow chart illustrating an embodiment of a method of the present invention for determining a format of an OFDM data frame within a MIMO wireless communication by a wireless receiver. The method  1300  commences with receiving a data frame that includes a preamble and a data field (step  1302 ). Operation continues with determining a modulation format of a high throughput signal field (HT-SIG field) of the preamble (step  1304 ). In particular, the operation of step  1304  considers the modulation format of a 1 st  HT-SIG field modulation symbol. When the 1 st  HT-SIG field symbol modulation is QPSK (step  1306 ), operation continues with determining a data rate specified in the L-SIG field, e.g. 6 Mbps (step  1308 ). When the L-SIG field does not specify that the data rate is 6 Mbps (or another specific rate), the received data frame is a legacy frame (step  1310 ). However, when the L-SIG field specifies the particular data rate, e.g., 6 Mbps, the received preamble is of the first preamble format and the data frame is of a first type. 
         [0101]    When the 1 st  HT-SIG field symbol modulation is BPSK_rotated (step  1314 ), operation continues with determining the modulation type of the 2 nd  symbol of the HT-SIG field, e.g. BPSK or BPSK_rotated (step  1316 ). When the 2 nd  symbol of the HT-SIG field is BPSK_rotated, the preamble is of the second preamble format and the data frame is of the second type (step  1318 ). However, when the 2 nd  symbol of the HT-SIG field is BPSK, the preamble is of the third preamble format and the data frame is of the third type (step  1320 ). 
         [0102]    Stated generally, the method of the present invention determines: (1) when the high throughput signal field has a first modulation format, determining that the preamble is of a first preamble format and that the data frame is of a first type; (2) when the high throughput signal field has a second modulation format, determining that the preamble is of a second preamble format and that the data frame is of a second type; and (3) when the high throughput signal field has a third modulation format, determining that the preamble is of a third preamble format and that the data frame is of a third type. In determining a modulation format of the HT-SIG field, log likelihood ratios may be employed. 
         [0103]    As one of average skill in the art will appreciate, the term “substantially” or “approximately”, as may be used herein, provides an industry-accepted tolerance to its corresponding term. Such an industry-accepted tolerance ranges from less than one percent to twenty percent and corresponds to, but is not limited to, component values, integrated circuit process variations, temperature variations, rise and fall times, and/or thermal noise. As one of average skill in the art will further appreciate, the term “operably coupled”, as may be used herein, includes direct coupling and indirect coupling via another component, element, circuit, or module where, for indirect coupling, the intervening component, element, circuit, or module does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. As one of average skill in the art will also appreciate, inferred coupling (i.e., where one element is coupled to another element by inference) includes direct and indirect coupling between two elements in the same manner as “operably coupled”. As one of average skill in the art will further appreciate, the term “compares favorably”, as may be used herein, indicates that a comparison between two or more elements, items, signals, etc., provides a desired relationship. For example, when the desired relationship is that signal  1  has a greater magnitude than signal  2 , a favorable comparison may be achieved when the magnitude of signal  1  is greater than that of signal  2  or when the magnitude of signal  2  is less than that of signal  1 . 
         [0104]    The preceding discussion has presented various embodiments for wireless communications in a wireless communication system that includes a plurality of wireless communication devices of differing protocols. As one of average skill in the art will appreciate, other embodiments may be derived from the teachings of the present invention without deviating from the scope of the claims.