Patent Publication Number: US-2005135517-A1

Title: Increasing effective number of data tones in a multi-antenna multi-tone communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority to U.S. Provisional Application No. 60/532,155 filed Dec. 22, 2003, and entitled “Use of Variable Number of Pilot Tones in Multi-Tone and Multiple Transmit Antenna Wireless Local Area Networks,” incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
      Not applicable.  
     REFERENCE TO A MICROFICHE APPENDIX  
      Not applicable.  
     FIELD OF THE INVENTION  
      The present disclosure is directed to communication networks, and more particularly, but not by way of limitation, to increasing the effective number of data tones in a multi-tone, multiple antenna communication system.  
     BACKGROUND OF THE INVENTION  
      In general, communication systems permit data or other signals to be transmitted from a first device to a second device coupled together by a communication channel which may be established wirelessly or over electrical or fiber optical cable. It is generally desirable to maximize data transmission rates within the constraints of the communication system design. Defining a higher rate extension to current wireless local area network (WLAN) systems while retaining as much of the current WLAN structure as possible also is desirable.  
     SUMMARY OF THE INVENTION  
      According to one embodiment, a wireless device is provided that includes host logic, at least two antennas, and network interface logic. The network interface logic is operable to transmit packets comprising symbols containing a plurality of data tones on at least two channels. The network interface logic varies the number of data tones among the symbols. A first channel of packets is transmitted over a first antenna and a second channel of packets is transmitted over a second antenna.  
      In another embodiment, a wireless device is provided that includes one or more antennas and a receiver component in communication with the one or more antennas. The receiver component is operable to receive two channels of packetized information containing symbols that include a variable number of data tones. The two channels include a first channel of packetized information and a second channel of packetized information.  
      In yet another embodiment, a method for wireless communication is provided. The method includes determining a number of data tones to include in a first symbol for a first channel and a second symbol for a second channel. The method provides for forming the first symbol with the determined number of data tones, and forming the second symbol with the determined number of data tones. The method includes transmitting the first symbol on the first channel from a first antenna and the second symbol on the second channel from a second antenna. The method also includes changing the number of data tones to form another symbol for the first channel or the second channel.  
      These and other features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.  
       FIG. 1  illustrates a communication network according to an embodiment of the disclosure.  
       FIG. 2  depicts two communication symbols frameworks for implementing an embodiment of the disclosure.  
       FIG. 3  illustrates an access point and a wireless station in wireless communication over two independent channels according to an embodiment of the disclosure.  
       FIG. 4   a  depicts a first symbol pattern according to an embodiment of the disclosure.  
       FIG. 4   b  depicts a second symbol pattern according to an embodiment of the disclosure.  
       FIG. 5  is a block diagram of a device used in the wireless network of  FIG. 1 .  
       FIG. 6  is a block diagram of a single channel of a transmitter used in the device of  FIG. 5 .  
       FIG. 7  is a block diagram of a single channel of a receiver used in the device of  FIG. 5 .  
       FIG. 8  illustrates an exemplary wireless device suitable for implementing the several embodiments of the disclosure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      It should be understood at the outset that although an exemplary implementation of one embodiment of the present disclosure is illustrated below, the present system may be implemented using any number of techniques, whether currently known or in existence. The present disclosure should in no way be limited to the exemplary implementations, drawings, and techniques illustrated below, including the exemplary design and implementation illustrated and described herein.  
      Communication systems use multiple frequencies over which to transmit data. The orthogonal frequency division multiplexing (OFDM) modulation technique used in the IEEE 802.11 wireless communication standards may employ, for example, 64 frequency tones spaced at intervals of 312.5 kHz. The frequency tones may be referred to as bins or frequency bins. The 64 tones in the 802.11 standards include 48 data tones, 4 pilot tones, and 12 unused tones. A data tone is a tone on which data may be transmitted. A pilot tone is a tone that may contain information used to promote the coherent demodulation of the transmitted signal at the receiver and are not used to convey message data. The 12 unused tones are included to prevent adjacent interference and are not used to convey message data. The IEEE 802.11 wireless local area network (WLAN) channel structures are used as an example to illustrate various embodiments of the disclosure, however, the disclosure and claims that follow should not to be limited to any particular communication standard.  
      Turning now to  FIG. 1 , a communication network  50  is illustrated that is implemented in accordance with a preferred embodiment of the disclosure. As shown, the network  50  comprises at least one access point (AP)  52  configured to be wirelessly coupled to communicate with at least one wireless station  54 . Four wireless stations  54  are depicted in the exemplary network  50 , but in other embodiments either more or fewer wireless stations  54  may be wirelessly coupled to communicate with the access point  52 . The access point  52  includes a wired connection (not shown) to a server or other suitable network device (also not shown) whereby the wireless network  50  is connected to a wired network such as the public data network, for example the Internet (not shown). Additional access points  52  may be included as desired thereby permitting wireless stations  54  to wirelessly access the wired network via any of a plurality of access points  52 .  
      In addition to communicating with the access point  52 , which may be termed infrastructure mode, the wireless stations  54  also may be configured to communicate directly with each other, without the intervention of the access point  52 , which may be termed ad-hoc mode or peer-to-peer mode. The wireless stations  54  may comprise desktop computers, notebook computers, computer-related equipment in general, or any type of device that is desired to be used in a communication network.  
      In accordance with a preferred embodiment of the disclosure, each access point  52  and each wireless station  54  may form packets comprised of multiple OFDM symbols containing data to be transmitted to another device. Each symbol comprises a plurality of data tones, and the device, either the wireless station  54  or the access point  52 , preferably varies the number of data tones among the various symbols that are transmitted to another device. As such, some symbols may comprise more data tones than other symbols comprise.  
      Turning now to  FIG. 2 , a diagram depicts two exemplary symbols  60  and  70 . Symbol  60  comprises 48 data tones  62 , 4 pilot tones  65 , and 12 unused tones  66  for a total of 64 tones. Symbol  70  comprises 52 data tones  72 , no pilot tones, and 12 unused tones  66  for a total of 64 tones. As shown, the number of unused tones may be the same between the symbols  60  and  70 . The frequencies used for the pilot tones in symbol  60  have been recruited to be used as data tones for symbol  70 . Because symbol  70  has more data tones  62  than symbol  60 , symbol  70  advantageously is able to transmit more data than symbol  60 . This provides a higher data transmission rate. As explained above, the pilot tones are used by the receiver of the symbol in the demodulation process. The wireless network  50  generally functions poorly without pilot tones, but in accordance with the preferred embodiment, not every transmitted symbol need have pilot tones. Thus, the data may be transmitted from one device to another in the wireless network  50  as symbols that may or may not have pilot tones.  
      Turning now to  FIG. 3 , a block diagram shows the access point  52  in wireless communication with the wireless station  54 . The wireless station  54  includes two antennas  59 —a first antenna  59   a  and a second antenna  59   b . The access point  52  includes two antennas  68 —a third antenna  68   a  and a fourth antenna  68   b . Two wireless communication channels  69  —a first wireless channel A  69   a  from the first antenna  59   a  to the third antenna  68   a  and a second wireless channel B  69   b  from the second antenna  59   b  to the fourth antenna  68   b —are established between the wireless station  54  and the access point  52 . Independent streams of data, comprised of symbols  60  and/or symbols  70 , may be transmitted over the two wireless communication channels  69 —a first data stream and a second data stream. The two wireless communication channels  69  are bidirectional, and data may flow in both directions through the two wireless communication channels  69 . The two antennas  68  associated with the access point  52  may be located relatively close together, for example about Y 2  wavelength apart. Similarly, the two antennas  59  associated with the wireless station  54  may be located relatively close together, such as about ½ wavelength apart. For example, if the wireless frequency is about 2.5 GHz, ½ wavelength would equate to about or less than 2.36 inches (6 cm) (the exemplary calculation of 2.36 inches is based on the speed of propagation in free space, whereas the communication network  50  may operate in an environment with a reduced speed of propagation and hence exhibit a shorter wavelength).  
      In an embodiment, the wireless station  54  may have a single antenna  59   a  over which the wireless station  54  may receive the first wireless channel A from the third antenna  68   a  and the second wireless channel B from the fourth antenna  68   b . In this embodiment, a transmitter, for example the access point  52 , sends symbols on channel A and channel B concurrently, in the same frequency band. The transmitter sends each of the symbols twice. The transmitter may use various combinations of the symbols to allow simplification in the processing at the receiver. On one channel, for example channel A, the transmitter sends a first symbol and then sends a second symbol while on the other channel, for example channel B, the transmitter sends the negative of the complex conjugate of the second symbol and then sends a complex conjugate of the first symbol. The receiver, the wireless station  54 , receives the symbols sent concurrently on channel A and channel B, discriminating the channel A symbols from the channel B symbols through processing algorithms known to those skilled in the art. While the data throughput rate of this configuration, which may be referred to as a 2×1 configuration, is the same as the 1×1 configuration, the communication mechanism may be more robust for the 2×1 configuration.  
      Turning now to  FIG. 4   a , an exemplary first sequence of symbols  80  sent by the first antenna  59   a  over the first wireless channel A  69   a  and an exemplary second sequence of symbols  82  sent by the second antenna  59   b  over the second wireless channel B  69   b  are depicted. In general, different OFDM symbols are sent on each antenna  59 . The data sent in the data tones of each of the sequences of symbols  80  and  82  is different and individual. The two sequences of symbols  80  and  82  are sent substantially concurrently.  
      In this example, pilot tones are being transmitted from one antenna  59 , for example antenna  59   a , at the same time and at the same frequency that data tones are being transmitted by the alternate antenna  59 , for example  59   b . A receiver  63  in the wireless station  54 , for example, may process the two sequences of symbols  80  and  82  using a generic multiple input multiple output (MIMO) processing algorithm such as zero forcing or minimum mean-square error, and use the resulting estimate of the pilot tone in a pilot tracking algorithm. While in this example, the communication was transmitted by the antennas  68  coupled to the access point  52  and received by the antennas  59  coupled to the wireless station  54 , in another example the communication may be transmitted by the antennas  59  coupled to the wireless station  54  and received by the antennas  68  coupled to the access point  52 .  
      The pilot tone pattern of the first sequence of symbols  80  may be represented as 4-0-4-0 repeating. The pilot tone pattern of the second sequence of symbols  82  may be represented as 0-4-0-4 repeating. According to these pilot tone patterns, 400 data tones are transmitted in 8 symbols, realizing an average of 50 data tones per symbol, an increase over the 48 data tones per symbol when using 4 pilot tones per symbol.  
      Turning now to  FIG. 4   b , an exemplary third sequence of symbols  84  sent by the first antenna  59   a  over the first wireless channel A  69   a  and an exemplary fourth sequence of symbols  86  sent by the second antenna  59   b  over the second wireless channel B  69   b  are depicted. In general, different OFDM symbols are sent on each antenna  59 , and the two sequences of symbols  84  and  86  are sent substantially concurrently. The pilot tone pattern of the third sequence of symbols  84  may be represented as 4-0-0-0 repeating. The pilot tone pattern of the fourth sequence of symbols  86  may be represented as 0-0-4-0 repeating. According to these pilot tone patterns, 408 data tones are transmitted in 8 symbols, realizing an average of 51 data tones per symbol, an increase over the 48 data tones per symbol when using 4 pilot tones per symbol and an increase over the 50 data tones per symbol average depicted in  FIG. 4   a.    
      It will be readily appreciated by one skilled in the art that other pilot tone patterns may be employed to achieve other data tone per symbol averages. In practice, the pilot tone pattern may be negotiated between the access point  52  and the wireless station  54  to achieve an optimum throughput of data tones as constrained by the current wireless operating environment or by the operating characteristics of the access point  52  and/or the wireless station  54 . For example, in a noisy wireless environment, the optimum throughput of data tones may be lower than the optimum throughput in a noise-free wireless environment. Because the transmit antennas  59  or  68  are nominally synchronized, transmitting pilot tones in every symbol from both antennas  59  or  68  may unnecessarily duplicate information. The present disclosure contemplates reducing the duplication of pilot tone information to obtain the benefit of increasing data throughput. To reduce the duplication of pilot tone information, when one communication channel  69  transmits a symbol containing pilot tones, the other communication channel  69  may transmit a symbol containing zero pilot tones.  
      Turning to  FIG. 5 , an exemplary block diagram depicts at least a portion of the wireless station  54 . The wireless station  54  comprises a host logic component  53 , a media access control (MAC) component  55 , and a physical component  57 . The media access control component  55  may be said to provide a MAC layer of processing, and the physical component  57  may be said to provide a physical layer of processing. The host logic component  53  couples to the media access control component  55 , and the media access control component  55  couples to the physical component  57 . The host logic component  53  provides the specific functionality of the wireless station  54 . The media access control component  55  receives data from the host logic component  53  and formats the data into packets that conform to the protocol to which the network  50  adheres. For example, the media access control component  55  may form a packet that includes data from the host logic component  53  as well as a preamble and/or header that provides relevant routing information. The physical component  57  couples to the first antenna  59   a  and the second antenna  59   b  through which the wireless station  54  wirelessly communicates with the access point  52 , with other wireless stations  54 , and, possibly, other access points  52  in the network  50 . The physical component  57  receives packets from the media access control component  55  and processes the packets to ensure successful transmission through the wireless network  50 . Packets from other devices, for example the access point  52  or another wireless station  54  in the wireless network  50 , may be received by the physical component  57  and provided to the host logic component  53  through the media access control component  55 . The physical component  57  comprises a transmitter  61  and a receiver  63  which are detailed below in  FIGS. 6 and 7 . While the description above was directed to the wireless station  54 , the description and drawing apply to the access point  52  with the understanding that the third antenna  68   a  and the fourth antenna  68   b  are associated with the access point  52  rather than the first antenna  59   a  and the second antenna  59   b.    
      Turning to  FIG. 6 , an exemplary block diagram of a single channel of the transmitter  61  of the physical component  57  is depicted. The other channel of the transmitter  61  is of identical or similar structure. Each of the two channels of the transmitter  61  generates independent streams of symbols, each stream containing at least partially unique data. The transmitter  61  comprises padding and scrambling logic  100 , forward error correction (FEC) encoder  102 , one or more symbol interleavers  104   a ,  104   b , map to complex numbers logic  106 , map complex numbers to OFDM symbols logic  108 , pilot symbol insertion logic  110 , inverse fast Fourier transformer (IFFT)  112 , cyclic prefix add logic  114 , OFDM symbol append logic  116 , and radio frequency upconverter  118 .  
      The padding and scrambling logic  100  acts as follows. The padding logic adds pad bits at the end of the input data to accommodate encoder tailing and mapping to an integer number of OFDM symbols. The scrambling logic scrambles the data using a packet-specific seed, to ensure that if a retransmission is required, the transmitted packet will not be exactly the same.  
      The forward error correction encoder  102  guards against data loss due to interference and multipath. Any suitable technique for performing forward error correction may be employed. The forward error correction encoder  102  receives the transmitted bit sequence from the padding and scrambling logic  100  as input, computes a corresponding set of coded bits according to a deterministic rule, and outputs the coded bits.  
      The symbol interleavers  104  receive the encoded bits from the forward error correction encoder  102 . In general, an interleaver, for example an interleaver suitable for IEEE 802.11a communications, takes the coded bits that will be mapped to a single OFDM symbol and interleaves, i.e., scrambles, the bits according to a known pattem. For example, for 802.11a communicates at  54  megabits per second (Mbps), there are six coded bits per tone and 48 data tones. As such, there are 6×48=288 coded bits for each OFDM symbol. Therefore, for typical IEEE 802.11a systems, the coded bit stream is divided into blocks of 288 bits, and each 288 bit block is scrambled within itself.  
      In accordance with the preferred embodiments, however, the symbols have a variable number of data tones. For example, some symbols have 48 data tones while other symbols have 52 data tones as explained above. Because of this variation in the number of data tones per symbol, the division of the coded bit stream must vary to correspond with the varying number of data tones. In the example of 48 data tones in some symbols and 52 data tones in other symbols, the 48 data tone symbols will have 288 coded bits. Each 52 data tone symbol, however, will have 6×52=312 bits. As such, two interleavers  104   a  and  104   b  are provided to accommodate 288 coded bits for some symbols and 312 coded bits for other symbols. Any suitable interleaving algorithm can be used for the interleavers  104 .  
      The map to complex numbers logic  106  maps the interleaved bits to complex numbers in accordance with known techniques. The map complex numbers to OFDM symbols logic  108  preferably takes the set of mapped complex numbers and maps the complex numbers onto the data tones. The mapping logic  108  takes into account the number and location of the data tones in the frequency domain.  
      The pilot symbol insertion logic  110  processes the mapped data to add pilot tones in accordance with the pilot tone requirements of the applicable symbol being generated by the transmitter. In the example given above, a symbol with 48 data tones, for example symbol  60 , has four pilot tones, while a symbol with 52 data tones, for example symbol  70 , has zero pilot tones.  
      The inverse fast Fourier transformer  112  converts the bits received from the pilot symbol insertion logic from the frequency domain to the time domain. The cyclic prefix add logic  114  generally duplicates the end portion of the time domain signal and prepends it to the beginning of the time domain signal. The cyclic prefix add logic  114  may be included to enable the frequency domain equalization that may be included in the receiver  63 .  
      The OFDM symbol append logic  116  appends the time domain signals corresponding to each OFDM symbol one after another. Finally, the radio frequency upconverter  118  converts the symbol to an appropriate radio frequency signal for transmission across the wireless network.  
      Turning now to  FIG. 7 , an exemplary block diagram of a single channel of the receiver  63  of the physical component  57  is depicted. The other channel of the receiver  63  is of identical or similar structure. Each of the two channels of the receiver  63  receive independent streams of symbols, each stream containing at least partially unique data. The receiver  63  comprises a radio frequency downconverter  150 , a number of OFDM symbols determination logic  152 , a fast Fourier transform placement determination logic  154 , a fast Fourier transformer  156 , a pilot symbol remove logic  158 , a tracking loops component  160 , a metrics determination logic  162 , an OFDM symbol deinterleaver  164 , a forward error correction decoder  166 , and a padding removal and descrambling logic  168 . The functional units shown in the receiver of  FIG. 7  generally reverse the processes described above and shown in  FIG. 6 .  
      The radio frequency downconverter  150  receives the transmitted radio signal and demodulates the radio signal to recover the transmitted symbol. The number of OFDM symbols determination logic  152  receives the downconverted signal and, from the received signal, determines the number of symbols. The fast Fourier transformer placement determination logic  154  determines a suitable interval in which to take the samples from the received sequence in order to take the fast Fourier transform for that symbol. The fast Fourier transformer  156  converts the signal from the fast Fourier transform placement determination logic  154  from the time domain into the frequency domain.  
      The pilot symbol remove logic  158  removes any pilot symbols that may be present. Symbols that have 48 data tones, such as the symbol  60 , require the four pilot tones to be removed, while symbols having 52 data tones, such as the symbol  70 , do not have pilot tones and thus do not require pilot tone removal. The tracking loops component  160  uses information derived from the pilot symbols to compensate for the cumulative effects of impairments and mismatches such as frequency offset.  
      The receiving device, for example the wireless station  54 , needs to know the pattern of how the number of data tones per OFDM symbol varies for each of the two wireless communication channels  69 . There are several techniques to achieve this feature. One technique is for the access point  52  to broadcast a single policy for the entire network  50  in each beacon. A beacon is a packet broadcast by the access point  52  to synchronize the wireless network  50  and to provide other information. The beacon may include an address of the access point  52 , a time stamp, an identification of the service area of the wireless network  50 , traffic delivery metrics, and/or other information. This can be done by utilizing selected bits in the regularly transmitted beacon to select from a finite, predetermined list of policies known to all wireless stations  52 . Alternatively, a protocol for packet headers may be adopted in which the transmitter signals the policy for that packet in the packet header and the receiver decodes the header first and can apply the resulting pattern for the data payload. These techniques are given as examples only. The exact method is not essential for this disclosure.  
      The metrics determination logic  162  derives reliability information for bits at the input to the forward error correction decoder  166  from the received signals of the data tones and from the channel estimates. The OFDM symbol deinterleaver  164  generally reverses the process implemented by the symbol interleavers  104  of  FIG. 6 . The forward error correction decoder  166  decodes the encoded bit stream received from the OFDM symbol deinterleaver  164 . The forward error correction decoder  166  has knowledge of all of the coded bit sequences that can possibly result from the encoding process. The forward error correction decoder  166  preferably keeps a running comparison of the coded bit sequence that is recovered against all known coded bit sequences. The forward error correction decoder  166  retains the best matches and after a certain amount of data has been recovered, the forward error correction decoder  166  makes an estimate of the correct decoded bit sequence.  
      By comparing the bit sequence recovered versus all known transmit coded bit sequences, the forward error correction decoder  166  can predict the value of the original transmitted bit sequence. Provided with this information, the forward error correction decoder  166  can detect and correct an error. The padding removal and descrambling logic  168  reverses the scrambling procedure described above with reference to  FIG. 6 . One or more advantages of the disclosed subject matter are possible. Flexibility is enabled by providing two basic symbol frameworks, for example the form of the symbol  60  and the form of the symbol  70 . As such the frameworks can be mixed and matched to achieve a larger number of possible numbers of data tones per symbol. The second advantage is that of at least partial reuse of the conventional architecture, in that the position and number of pilot tones, if present, is according to the conventional symbol framework.  
      The access point  52  or the wireless station  54  described above may be implemented on any communication device such as is well known to those skilled in the art. An exemplary system  250  for implementing one or more embodiments disclosed herein is illustrated in  FIG. 8 . The system  250  includes a processor  252  (which may be referred to as a central processor unit or CPU) that is coupled to memory devices including a read only memory (ROM)  254 , a random access memory (RAM)  256 , a first transceiver  258  that is coupled to a first antenna  260 , a second transceiver  262  that is coupled to a second antenna  264 , and an input/output (I/O) device  266 . The processor may be implemented as one or more CPU chips.  
      The ROM  254  is used to store instructions and perhaps data which are read during program execution. ROM  254  is a non-volatile memory device. The RAM  256  is used to store volatile data and perhaps to store instructions. The ROM  254  may include flash memories or electrically erasable programmable memory to support updating the stored instructions remotely, for example through an over-the-air interface via the transceivers  258  and/or  262  and the antennas  260  and/or  264 .  
      The transceivers  258 ,  262  and the antennas  260 ,  264  support radio communications. The I/O device  266  may be a keypad and a visual display such as a liquid crystal display to permit entering numbers and selecting functions. Alternatively, the I/O device  266  maybe a keyboard and a touch pad, such as a keyboard and a touch pad of a laptop computer. The processor  252  executes instructions, codes, computer programs, scripts which it accesses from ROM  254  or RAM  256 .  
      The several embodiments of the disclosure describe above each are directed to increasing data throughput by reclaiming pilot tones to use for transmitting data in such a way as to not diminish the ability of receivers to synchronize or remain synchronized.  
      While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.  
      Also, techniques, systems, subsystems and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be coupled through some interface or device, such that the items may no longer be considered directly coupled to each other but may still be indirectly coupled and in communication, whether electrically, mechanically, or otherwise with one another. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.