Abstract:
An apparatus may include an orthogonal frequency division multiplexing (OFDM) physical layer interface. An adaptive subcarrier demodulator or an adaptive subcarrier modulator may be coupled to the OFDM physical layer interface to implement adaptive bit loading (ABL). A decoder or an encoder may be coupled to the adaptive subcarrier demodulator or to the adaptive subcarrier modulator to implement low density parity checking.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims the benefit of U.S. provisional application Ser. No. 60/493,937, filed Aug. 8, 2003, entitled “A High-Throughput Wireless Network Architecture, Apparatus And Associated Methods,” the entire content of which is incorporated by reference herein. 
     
    
     BACKGROUND  
       [0002]     The claimed invention relates to multicarrier communications and, more particularly, to adaptive modulation in multicarrier communications.  
         [0003]     Multicarrier communications may be described as a communications technique in which multiple carriers or subcarriers are used to communicate information. As an example of multicarrier communications, Orthogonal Frequency Division Multiplexing (OFDM) may be described as a communications technique that divides a communications channel into a number of spaced frequency bands. In OFDM, a subcarrier carrying a portion of the user information may be transmitted in each band. In OFDM, each subcarrier may be orthogonal, differentiating OFDM from the commonly used frequency division multiplexing (FDM). An OFDM symbol may include, for example, a multicarrier symbol transmitted simultaneously on the OFDM subcarriers during the OFDM symbol period. The individual symbols on particular subcarriers may be referred to as subcarrier symbols.  
         [0004]     In wireless systems that are susceptible to dynamic or frequency selective fading, such systems may benefit substantially from channel interleavers that interleave at the transmission bit level to break up fading that may be correlated within a transmission symbol. Some OFDM communication systems may also use Adaptive Bit Loading (ABL), which may also be referred to as adaptive subcarrier modulation, where the modulation density may be changed on a subcarrier-by-subcarrier basis based on channel conditions. Techniques such as ABL may complicate the bit interleaving process, however, because each OFDM symbol may contain a different number of bits whenever a new channel adaptation is made (e.g., transmission). It may be difficult to implement an efficient interleaving scheme when the number of bits may vary among OFDM symbols. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more implementations consistent with the principles of the invention and, together with the description, explain such implementations. In the drawings,  
         [0006]      FIG. 1  illustrates an example wireless communication system consistent with the principles of the invention;  
         [0007]      FIG. 2  illustrates an example transmitter consistent with the principles of the invention;  
         [0008]      FIG. 3  illustrates an example receiver consistent with the principles of the invention;  
         [0009]      FIG. 4  illustrates another example receiver consistent with the principles of the invention; and  
         [0010]      FIG. 5  is a flow chart illustrating operation of a transmitter according to an example implementation.  
     
    
     DETAILED DESCRIPTION  
       [0011]     The following detailed description refers to the accompanying drawings. The same reference numbers may be used in different drawings to identify the same or similar elements. Also, the following detailed description illustrates certain implementations and principles, but the scope of the claimed invention is defined by the appended claims and equivalents.  
         [0012]      FIG. 1  is a diagram illustrating an example of a wireless communication system  100  in accordance with one implementation consistent with the principles of the invention. In communications system  100 , a user wireless system  116  may include a wireless transceiver  110  coupled to an antenna  117  and to a processor  112 . Processor  112  in one implementation may include a single processor, or may include a baseband processor and an applications processor, although the claimed invention is not limited in this respect. According to one implementation, processor  112  may include a baseband processor and Medium Access Control (MAC).  
         [0013]     Processor  112  may couple to a memory  114  that may include volatile memory such as DRAM, non-volatile memory such as flash memory, or may include other types of storage such as a hard disk drive, although the claimed invention is not limited in this respect. Some portion, or all, of memory  114  may be included on the same integrated circuit as processor  112 , or some portion or all of memory  114  may be disposed on an integrated circuit or other medium, for example a hard disk drive, that is external to the integrated circuit of processor  112 . According to one implementation, software may be provided in memory  114  to be executed by processor  112  to allow wireless system  116  to perform a variety of tasks, some of which may be described herein.  
         [0014]     Wireless system  116  may communicate with an access point (AP)  128  (or other wireless system) via wireless communication link  134 . Access point  128  may include at least one antenna  118 . Antennas  117  and  118  may include, for example, a directional antenna or an omni directional antenna, although the claimed invention is not limited thereto. Although not explicitly illustrated in  FIG. 1 , AP  128  may include, for example, a structure that is similar to wireless system  116 , including one or more of a wireless transceiver, a processor, a memory, and software provided in memory to allow AP  128  to perform a variety of functions. In an example implementation, wireless system  116  and AP  128  may be stations in a wireless communication system, such as a wireless local area network (WLAN) system.  
         [0015]     Access point  128  may be coupled to network  130  so that wireless system  116  may communicate with network  130 , including devices coupled to network  130 , by communicating with access point  128  via wireless communication link  134 . Network  130  may include a public network such as a telephone network or the Internet, and/or network  130  may include a private and/or restricted-access network such as an intranet or extranet, or a combination of a public and a private network, although the claimed invention is not limited in this respect.  
         [0016]     Communication between wireless system  116  and access point  128  may be implemented via a WLAN or other wireless network, such as a network which may be compliant with an Institute of Electrical and Electronics Engineers (IEEE) standard like IEEE 802.11a, IEEE 802.11b, IEEE 802.11g and so on, although the claimed invention is not limited in this respect.  
         [0017]     In some implementations, communication between wireless system  116  and access point  128  may be implemented via a cellular communication network compliant with a 3GPP or other cellular standard, although the claimed invention is not limited in this respect.  
         [0018]     According to an example implementation, system  100  may use adaptive modulation, such as ABL, in a multicarrier environment (e.g., OFDM). System  100  may include an encoder/decoder with decorrelating properties as will be described in greater detail below. According to an example implementation, such an encoder/decoder may, for example, be provided in addition to or instead of an interleaving/deinterleaving device, although the claimed invention is not limited in this regard.  
         [0019]      FIG. 2  is a block diagram of a transmitter  200  according to an example implementation. Transmitter  200  may modulate information and may perform other processing on information for transmission, and may be used in one or more elements of system  100 . Transmitter  200  may include a media access controller (MAC)  205 , an adaptive forward error correction (FEC) encoder  210 , an optional adaptive interleaver  215 , an adaptive subcarrier modulator  220 , an OFDM PHY  225  and an amplifier  230 , although the claimed invention is not limited thereto.  
         [0020]     Transmitter  200  may receive information bits or a data block to be transmitted. The information is input to MAC  205 . MAC  205  may perform many tasks, including tasks related to media access.  
         [0021]     FEC encoder  210  is coupled to the output of MAC  205  and may perform FEC encoding or rate coding on the incoming information bits. FEC encoding may involve, for example, adding parity bits to the information bits to generate one or more codewords, although the claimed invention is not limited thereto. The parity bits may allow error detection and/or error correction to be performed at the demodulator or receiver.  
         [0022]     In some implementations consistent with the principles of the invention, FEC encoder  210  may include circuitry and/or processor-implemented code that decorrelates or disassociates bits. One example of a suitable scheme for decorrelating may be a Low Density Parity Check (LDPC) encoder. An LDPC encoder may disassociate and/or decorrelate bits as a natural result of its operation, thereby facilitating the use of an adaptive modulation scheme, such as ABL, in transmitter  200 . For example, an LDPC encoder may perform acceptably with both relatively short (e.g.,˜100) and relatively long (e.g.,˜10 5 ) block sizes that may be produced in an adaptive subcarrier modulation scheme, although the claimed invention is not limited to block sizes within this range.  
         [0023]     In general the FEC block sizes may not be determined by the adaptive subcarrier modulation scheme. Interleaver  215  (or modulator  220 ), however, may match the OFDM symbol size to the FEC block size. The FEC block size may be determined, for example, by the design of the encoding and/or modulating code, or by the size of the data packet.  
         [0024]     Although an LDPC encoder is one example, other encoding schemes that randomize encoded bits may be used. For example, a random or pseudo-random mapper, perhaps implemented via a look-up table, may be used. Also, it should be noted that as used herein “disassociate” is intended to describe separating bits that would otherwise appear together and that may experience, or have experienced, correlated channel fading. Although decorrelation may be considered as one type of disassociation, various schemes other than decorrelation in the strict mathematical sense may be used to disassociate bits.  
         [0025]     Optional interleaver  215  may, or may not, be coupled to an output of FEC encoder  210 . For example, interleaver  215  may interleave bits in various OFDM subcarriers (e.g., either linearly or by interleaving the bits or codewords), although the claimed invention is not limited thereto. Optional interleaver  215  may be omitted in some implementations from transmitter  200 , due to the presence of a randomizing encoder  210 , although the claimed invention is not limited thereto.  
         [0026]     Adaptive subcarrier modulator  220  may be coupled to an output of adaptive interleaver  215 , if present. In the absence of interleaver  215 , modulator  220  may be coupled to FEC encoder  210 , although the claimed invention is not limited thereto. Adaptive subcarrier modulator  220  may adaptively modulate one or more bits onto one or more carriers or subcarriers using a modulation scheme or schemes. For example, subcarrier modulator  220  may modulate bits (e.g., FEC encoded bits) onto a plurality of OFDM subcarriers, although the claimed invention is not limited thereto.  
         [0027]     Adaptive subcarrier modulator  220  may modulate bits using a selected one of a number of modulation schemes. For example, one modulation scheme may be adaptively selected for all OFDM subcarriers based on a detected criteria, such as a detected channel condition or channel conditions. On the other hand, the channel condition or other criteria may be detected for each subcarrier, and then a different modulation scheme may be used for each OFDM subcarrier based on the channel condition for that particular subcarrier, although the invention is not limited thereto. Further, subcarriers may be grouped (e.g., in twos, fours, etc.) into subbands so that modulation schemes need not be individually selected for each subcarrier, but one modulation scheme may be selected for the subband.  
         [0028]     Adaptive subcarrier modulator  220  may use any of a variety of modulation schemes. Some example modulation schemes may include binary phase shift keying (BPSK) which transmits two different symbols (1 bit per symbol), quadrature phase shift keying (QPSK), 8-PSK (8 different symbols encoding 3 bits/symbol), quadrature amplitude modulation (QAM), QAM 16 (16 different symbols to encode 4 bits/symbol), QAM 32, QAM 64, QAM 256, etc. These are just a few example modulation schemes and the claimed invention is not limited thereto. Compared to lower level modulation schemes, the higher level modulation schemes may provide a higher data rate due to a greater number of bits per symbol for higher level modulation schemes. However, it may be more difficult to correctly recover the data at the receiver for higher level modulation schemes due to the increased number of possible symbols. Therefore, according to an example implementation, an adaptive modulation scheme (e.g., ABL) may be used that is based upon a detected channel condition. This may allow a higher level modulation scheme to be used for higher quality channels, and a lower level modulation scheme to be used for a lower quality channel, although the invention is not limited thereto.  
         [0029]     A PHY (e.g., physical layer interface), such as a OFDM PHY  225  may be coupled to an output of adaptive subcarrier modulator  220 . OFDM PHY  225  may generate signals having the appropriate qualities, such as the appropriate voltages, timing, etc. Amplifier  230  may be coupled to an output of OFDM PHY  225  to amplify the signals output from the PHY  225 . The signals generated by amplifier  230  may be transmitted via an antenna over a wireless channel, for example, or over another type of channel.  
         [0030]     MAC  205  may control or coordinate via line  235  the operation of one or more of FEC encoder  210 , adaptive interleaver  215  (if present), and adaptive subcarrier modulator  220 , although the claimed invention is not limited thereto. MAC  205  may also control or coordinate via line  240  a level of amplification performed by amplifier  230 .  
         [0031]      FIG. 3  is a block diagram of a receiver  300  according to an example implementation. Receiver  300  may receive information and may perform other processing after reception, and may be used in one or more elements of system  100 . Receiver  300  may include an amplifier  305 , an OFDM PHY  310 , an adaptive subcarrier demodulator  315 , an optional adaptive deinterleaver  320 , an FEC decoder  325  and a MAC  330 , although the claimed invention is not limited thereto.  
         [0032]     Rx data may be received via a wireless channel through an antenna (not shown in  FIG. 3 ) or other channel and passed to amplifier  305 . OFDM PHY  310  may be coupled to the output of amplifier  305 , and may generate signals having the appropriate qualities (e.g., appropriate voltages, timing, etc.).  
         [0033]     Adaptive subcarrier demodulator  315  may be coupled to OFDM PHY  310  and may demodulate one or more received signals, for example demodulating one or more OFDM subcarriers. As described above, demodulator  315  may demodulate groups of subcarriers (e.g., two or more) that have been given a common modulation.  
         [0034]     Optional adaptive de-interleaver  320  may be coupled to subcarrier demodulator  315  to de-interleave received signals in some implementations. In other implementations consistent with the principles of the invention, de-interleaver  320  may be omitted, and demodulator  315  may be coupled to FEC decoder  325 .  
         [0035]     FEC decoder  325  may coupled to an output of de-interleaver  320 , if present, to decode codewords into data bits. In some implementations consistent with the principles of the invention, FEC decoder  325  may include circuitry and/or processor-implemented code that disassociates and/or decorrelates bits. In some implementations, even though the bits may be locally disassociated from their received ordering, they may be in some sense “re-associated” to reassemble bits in to an order before operated on by FEC encoder  210 . One example of a suitable scheme for such disassociating may be a Low Density Parity Check (LDPC) decoder. An LDPC decoder may separate (e.g., re-associate) bits that may have suffered correlated fading as a natural result of its operation, thereby facilitating the use of an adaptive modulation scheme, such as ABL, in receiver  300 . Other encoding schemes that de-randomize decoded bits may be used.  
         [0036]     MAC  330  may control the operation of one or more of adaptive subcarrier demodulator  315 , adaptive de-interleaver  320  (if present), and FEC decoder  325 . MAC  330  may perform tasks related to media access to a channel and other tasks. MAC  330  may receive control information from a remote node, for example provided in one or more fields or messages, that may indicate one or more selected transmission parameters. MAC  330  may then control, based on the received control information, various blocks of receiver  300 , including to control adaptive subcarrier demodulator  315  to demodulate according to one or more specified modulation schemes, to control de-interleaver  320  to perform (or not perform) de-interleaving, to control FEC decoder  325  to FEC decode using a selected rate code, etc., although the invention is not limited thereto. The resulting data may be output by MAC  330 .  
         [0037]     Returning to  FIG. 1 , in an example implementation, transceiver  110  may include one or both of transmitter  200  and receiver  300 , and MAC  235  and MAC  330  may include the same MAC for example, although the invention is not limited thereto. Transceiver  110  may include additional blocks or components.  
         [0038]      FIG. 4  illustrates another example receiver  400  consistent with the principles of the invention. Receiver  400  may be considered to be a specific implementation of receiver  300  without a deinterleaver, and may include a receiver  410 , a fast Fourier transform (FFT) module  420 , a slicer  430 , and an LDPC decoder  440 . Some elements of receiver  400  may not be shown for ease of explanation.  
         [0039]     Receiver  410  may include, for example, elements similar in function to the previously-described amplifier  305  and OFDM PHY  310 . Receiver  410  may provide a received signal to FFT module  420 .  
         [0040]     FFT module  420  may demodulate the received signal and provide demodulated constellation symbols in some implementations consistent with the principles of the invention. Slicer  430  may receive the demodulated constellation symbols and may slice them based on the encoding scheme to produce detected coded bits. FFT  420  and slicer  430  may operate together to adaptively demodulate differently-modulated subcarriers, or groups of subcarriers. FFT  420  and slicer  430  may demodulate received symbols that were encoded with an ABL or another adaptive subcarrier modulation scheme.  
         [0041]     LDPC decoder  440  may decode the detected coded bits to produce corrected bits at its output. LDPC decoder  440  may include a number of bit nodes  442  and check nodes  444  arranged to perform the decoding. Bit nodes  442  may be connected to check nodes  444  by a number of edges, the arrangement of which may determine performance of decoder. For example, one LDPC code may be a (2000, 1600) code, with R=0.8. Such a code may have 2000 bit nodes  442  and 1600 check nodes  444 , with four edges per information bit and two per parity bit. There may be 18 edges per check node and a total of 7199 edges in such an LDPC code. Although this is one example of a suitable LDPC code, the claimed invention is not limited thereto.  
         [0042]      FIG. 5  is a flow chart illustrating operation of a transmitter according to an example implementation. At  510 , the transmitter receives a data block, which may be a group of information bits. The block of data may be of variable size, although the claimed invention is not limited thereto.  
         [0043]     At  520 , FEC encoder  210  may encode the data to decorrelate the bits therein. FEC encoder  210  may use, for example, an LDPC scheme, which intrinsically decorrelates, to perform such encoding. The LDPC encoding may be performed via dedicated hardware, software/firmware, or some combination thereof. FEC encoder  210  may in other implementations use another error control code that has decorrelation properties.  
         [0044]     At  530 , transmitter  200  (e.g., MAC  205 ) may detect a channel condition or other criteria. The channel condition may provide information to the transmitter  200  that describes the current condition or quality of a channel (or carrier or subcarrier). Various frequency bands or channels may experience various types of interference, noise, selective fading and other conditions which may degrade the quality of a channel, from time to time. Transmitter  200  may detect the channel condition of a particular frequency band or bands using a variety of different techniques, including measuring bit error rate (BER), measuring packet error rate or packet transmission failure rate, measuring signal to noise ratio (SNR) for received signals, exchanging Channel Side Information (CSI) with another terminal, maintaining and updating a channel estimate which may estimate the current condition or quality of a channel, etc. These are just a few examples of how a channel condition may be detected, although the claimed invention is not limited thereto.  
         [0045]     In a multicalrier system, such as an OFDM system, where information is transmitted over multiple carriers or subcarriers, the MAC  205  may measure the channel condition for a number (e.g., one or more) of the different subcarriers, although the claimed invention is not limited thereto.  
         [0046]     At  540 , MAC  205  and/or PHY  225  may select (or adjust) one or more modulation parameters to more efficiently use the OFDM symbols for transmission of the data block and/or to reduce any unused portion(s) of OFDM symbols (OFDM subcarriers). The modulation scheme may be selected or adjusted for one or more OFDM subcarriers, and there may be one modulation scheme used for one or more OFDM subcarriers, possibly even all subcarriers, although the claimed invention is not limited thereto. Alternatively, there may be a modulation scheme selected by MAC  205  and/or PHY  225  for OFDM subcarriers (e.g., adaptive subcarrier modulation or ABL), for example, based upon one or more detected channel conditions for the subcarriers, although the claimed invention is not limited thereto. The same modulation scheme may be applied to a group of two or more subcarriers (e.g., two, three, four, etc.) if such does not harm performance beyond an acceptable level.  
         [0047]     According to an example implementation, different thresholds for the detected channel condition may be used to select different modulation schemes, although the claimed invention is not limited thereto. For example, if the detected channel condition meets a first threshold, then a first modulation scheme may be used for that channel or subcarrier. If the detected channel condition meets a second threshold, then a second modulation scheme may be used for the channel or subcarrier.  
         [0048]     At  550 , the data block may be processed and transmitted according to the selected transmission parameters. This may involve, for example, performing OFDM subcarrier modulation, and then amplifying the data for transmission over a channel, although the claimed invention is not limited thereto. This processing and transmission may be accomplished, for example, by MAC  205  providing control information via line  235  to control adaptive subcarrier modulator  220  to modulate the one or more OFDM subcarriers using the one or more selected modulation schemes, etc., although the claimed invention is not limited thereto.  
         [0049]     Although flow charts are not explicitly presented for receivers  300  and  400 , it will be appreciated that the acts in  FIG. 5  may be substantially reversed to recover the encoded and modulated data. For example, after reception demodulator  315  may perform demodulation on a (sub)carrier-by-(sub)carrier basis (or groups thereof) to functionally undo act  540 . Demodulator  315  may demodulate, for example, in accordance with an ABL scheme. Similarly, FEC decoder  325  may perform LDPC processing, or other decoding with similar decorrelating properties, to decode the coded data received from demodulator  315 .  
         [0050]     The foregoing description of one or more implementations consistent with the principles of the invention provides illustration and description, but is not intended to be exhaustive or to limit the claimed invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.  
         [0051]     Moreover, the acts in  FIG. 5  need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. Further, the acts in this figure may be implemented as instructions, or groups of instructions, implemented in a machine-readable medium.  
         [0052]     No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Variations and modifications may be made to the above-described implementation(s) of the claimed invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.