Patent Publication Number: US-2006007898-A1

Title: Method and apparatus to provide data packet

Description:
BACKGROUND OF THE INVENTION  
      In wireless local area networks (WLAN), for example, WLAN systems based on IEEE-802.11-1999 standard, wideband (WB) Orthogonal Frequency Division Multiplexing (OFDM) modulation schemes or duplex time division multiplexing (TDM) modulation schemes may be used. In those systems the data rate and throughput of the WLAN may be increased by increasing a spectrum bandwidth of the transmitted signals or by using several OFDM channels in parallel. The WLAN may include stations that may transmit data packets over a non-stationary frequency-selective shared wireless medium, conventionally referred to in the wireless art as a channel.  
      For example, in some WLAN systems, transmission of data packets may be performed by the stations in-doors. Under these conditions, the signal propagation may include multipath and non-stationary characteristics. The multipath characteristics may be caused by multiple scatters such as walls, ceilings, furniture and other objects in the indoor space, and may result in frequency selectivity of a channel transfer function. Non-stationary characteristics may be caused by motion of scattering objects resulting in a Doppler shift of a received signal frequency. In addition, non-stationary characteristics may be caused by unpredictable behavior of interferences in a band of the received signal. These factors may result in greater Packet Error Rate (PER) and may reduce the throughput performance of wireless network.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanied drawings in which:  
       FIG. 1  is a schematic illustration of a wireless communication system according to an exemplary embodiment of the present invention;  
       FIG. 2  is a block diagram of a station according to an exemplary embodiment of the present invention;  
       FIG. 3  is a schematic illustration of a packet structure according to an exemplary embodiment of the present invention; and  
       FIG. 4  is a schematic illustration of an exemplary time frequency diagram of a transmitted packet over an OFDM channel according to some embodiment of the present invention. 
    
    
      It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.  
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
      In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the present invention.  
      Some portions of the detailed description, which follow, are presented in terms of algorithms and symbolic representations of operations on data bits or binary digital signals within a computer memory. These algorithmic descriptions and representations may be the techniques used by those skilled in the data processing arts to convey the substance of their work to others skilled in the art.  
      Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing” , “sending”, “exchanging” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities within the computing system&#39;s registers and/or memories into other data similarly represented as physical quantities within the computing system&#39;s memories, registers or other such information storage medium that may store instructions to perform actions and/or process, if desired.  
      It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the circuits and techniques disclosed herein may be used in many apparatuses such as stations of a radio system. Stations intended to be included within the scope of the present invention include, by way of example only, wireless local area network (WLAN) stations, two-way radio stations, digital system stations, analog system stations, cellular radiotelephone stations, and the like.  
      Types of WLAN stations intended to be within the scope of the present invention include, although are not limited to, mobile stations, access points, stations for receiving and transmitting spread spectrum signals such as, for example, Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Complementary Code Keying (CCK), Orthogonal Frequency-Division Multiplexing (OFDM) and the like.  
      Turning first to  FIG. 1 , a wireless communication system  100 , for example, a WLAN communication system is shown. Although the scope of the present invention is not limited in this respect, the exemplary WLAN communication system  100  may be defined, for example, by the IEEE 802.11-1999 standard, as a basic service set (BSS). For example, BSS may include at least one communication station, for example, an access point (AP)  110 , a station  120  (STA 1 ) and a station  130  (STA 2 ). In some embodiments, station  120  and station  130  may transmit and/or receive one or more data packets over a communication channel  140  of wireless communication system  100 . The packets may include data, control messages, network information, and the like.  
      Although the scope of the present invention is not limited in this respect, in some embodiments of the present invention wireless communication system may operate under IEEE 802.11a and/or IEEE 802.11g standard and may transmit and/or receive OFDM signals, if desired. In some embodiments of the inventions, station  120  may communicate with AP  110  via a link  125  and station  130  may communicate with AP  110  via a link  135 . In those embodiments, links  125  and  135  may transport OFDM signals, if desired.  
      Although-the embodiments of the present invention are not limited in this respect, the OFDM signals may include data packets of OFDM symbols. One OFDM symbol may consist of orthogonal subcarriers that may be modulated with portions of data of the data packet in accordance with different modulation schemes. Thus, with some embodiments of the invention, the OFDM data packet may be described as a sequence of OFDM symbols. In some embodiments of the invention, the OFDM data packet may be fragmented into one or more fragments, wherein a fragment may include at least one OFDM symbol. The fragments of the OFDM data packet may be separated, for example, by middle-fix training fields, if desired.  
      Turning to  FIG. 2 , a block diagram of a station  200  according to some exemplary embodiments of the present invention is shown. Although the scope of the present invention is not limited in this respect, station  200  may include an antenna  210 , a data packet generator  220 , an encoder  230  a modulator  240  a transmitter (TX)  250  to transmit radio frequency (RF) signals, a receiver  260  and a predictor  270 .  
      Although the scope of the present invention is not limited in this respect, antenna  210  may be an omni-directional antenna, a monopole antenna, a dipole antenna, an end fed antenna, a circularly polarized antenna, a micro-strip antenna, a diversity antenna, an internal antenna, or the like.  
      Although the scope of the present invention is not limited in this respect, data packet generator  220  may generate a data packet. An exemplary data packet structure is described in detail below with reference to  FIGS. 3 and 4 . In some embodiments of the invention encoder  230  may encode the data packet with encoding schemes such as, for example, a convolutional encoding scheme, a block encoding scheme, a Low-Density Parity Check (LDPC) encoding scheme, a Reed-Solomon encoding scheme, a turbo encoding scheme, or the like.  
      Although the scope of the present invention is not limited in this respect, modulator  240  may modulate the encoded data packet according to OFDM subcarrier modulation schemes such as, for example, binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature-amplitude modulation (QAM) with different order such as, for example, QAM16, QAM32, QAM64, QAM128, QAM256, etc., differential BPSK (DBPSK), differential QPSK (DQPSK), or the like.  
      Although the scope of the present invention is not limited in this respect, receiver  260 , for example, an OFDM receiver, may receive data packets from communication channel  140 . Predictor  270  may predict long-term characteristics of communication channel  140  based on the information received from at least one of a prefix training field and a postfix training field of the received data packet, although the scope of the present invention is not limited in this respect. In some embodiments of the invention, the data packet may include a middle-fix training field, and predictor  270  may perform for long-term channel prediction by combining the information of the middle-fix training field with information from other fields of the data packet, if desired.  
      Turning to  FIGS. 3 and 4 .  FIG. 3  is a schematic illustration of a structure of a data packet  300 , for example, an OFDM data packet, according to an exemplary embodiment of the present invention, and  FIG. 4  is a schematic illustration of an example of a time-frequency diagram of data packet  300  transmitted over an OFDM channel  400 . Although the scope of the present invention is not limited in this respect, OFDM channel  400  may be a wideband channel and may include at least four  20  MHz sub-channels. In  FIG. 3 , data packet  300  may include training fields that may be used for long-term channel prediction, if desired. Data packet  300  may include a compatibility preamble field  310 , a prefix training field  320 , a PLCP header  330 , which may include bit and power loading (BPL) information, data field  340 , and postfix training field  360 . In some embodiments of the invention data field  340  may be fragmented into two or more fragments, e.g.,  342 ,  346 , separated by at least one middle-fix training field  370 .  
      Although the scope of the present invention is not limited in this respect, modulator  240  may provide similar and/or different modulation schemes to data fragments  342 ,  346 . In some embodiments of the invention, modulator  240  may provide different modulation schemes to data fragments  342 ,  346 . In some embodiments of the invention, encoder  230  may provide similar and/or different encoding schemes and/or rates to data fragments  342 ,  346 . In some embodiments of the invention encoder  230  may provide different encoding schemes and/or encoding rates to data fragments  342 ,  346 , if desired.  
      Although the scope of the present invention is not limited in this respect,  FIG. 4  shows data packet  300  spread over wideband OFDM channel  400 . For example, compatibility preamble field  310  may be spread over sub channels  410 ,  420 ,  430 ,  440 . In addition, channel  400  may include sub-carriers  450 , which are illustrated by thick horizontal lines.  
      Although the scope of the present invention is not limited in this respect, compatibility preamble field  310 , and the prefix, postfix and middle-fix training fields (e.g. fields  320   360  and  370 , respectively), may be used to perform tasks such as, for example, signal detection, channel estimation, timing synchronization, coarse and/or fine frequency offset estimation, channel transfer function estimation, channel variation estimation, long term channel prediction, and the like. In addition, compatibility preamble field  310  may carry plurality of logical functions such as, for example, packet type detection, support of compatibility with legacy devices, possibility of frequency division multiple access (FDMA) mode usage and the like.  
      Although the scope of the present invention is not limited in this respect, prefix, postfix and middle-fix training fields (e.g. fields  320   360  and  370 , respectively) may be used for long term channel prediction, which may include, for example, prediction of channel variation during a delay in transmitting an estimate of channel state information (CSI). For example, a linear prediction method based on autoregressive (AR) modeling of the channel transfer function coefficients may be used for long-range prediction. In this method, the future channel transfer function coefficients may be predicted with minimum mean square error (MMSE) on the base on a number of previous estimates of the channel transfer function.  
      Although the scope of the present invention is not limited in this respect, compatibility preamble  310  may be constructed, for example, from 1, 2, 3 or 4 PLCP preambles, which may be transmitted in one, two, three or four 20 MHz sub-channels. The construction of at least one PLCP preamble within compatibility preamble field  310  may be done, for example, according to IEEE 802.11a standard, if desired. In some embodiments of the invention, compatibility preamble field  310  may be divided into a short combined preamble  302 , a long combined preamble  306 , and a combined signal field  308 . In some embodiments of the invention, compatibility preamble field  310  may be used, for example, for energy detection, a packet type detection, a preliminary channel estimation, a timing synchronization, a frequency offset estimation and the like.  
      Although the scope of the present invention is not limited in this respect, short combined preamble  302  may include for example, 1, 2, 3 or 4 short preambles (e.g. as defined by IEEE-802.11a standard) that may be transmitted in one, two, three or four neighboring 20 MHz sub-channels. For example, sub channels  410 ,  420 ,  430 ,  440  may be transmitted substantially simultaneously, if desired. In some embodiments of the invention, channel  400  may be 80 MHz wide and may be divided into one, two, three or four sub channels of 20 MHz, if desired. For example, sub channel  410  may be from 40 MHz to 20 MHz, sub channel  420  may be from 20 MHz to 0 Hz, sub channel  430  may be from 0 Hz to −20 MHz and sub channel  440  may be from −20 MHz to −40 MHz, as is shown in  FIG. 4 .  
      In some embodiments of the invention, short preamble  302  of sub-channel  410  or short preamble  302  of sub-channel  440  may be rotated by 180 degrees relative to other sub-channels (e.g. sub channels  420 ,  430 ) to reduce Peak-to-Average Power Ratio (PAPR), if desired.  
      Although the scope of the present invention is not limited in this respect, long combined preamble  304  may include for example, 1, 2, 3 or 4 long preambles as defined by IEEE-802.11a standard, that may be transmitted in one, two, three or four neighboring 20 MHz sub-channels simultaneously, for example, sub channels  410 ,  420 ,  430 ,  440 , respectively. Long preamble  306  of sub channel  410  or long preamble  306  of sub channel  440  may be rotated by 180 degrees relative to other sub-channels (e.g. sub channels  420 ,  430 ) to reduce the PAPR, if desired.  
      Although the scope of the present invention is not limited in this respect, combined signal field  308  may include, for example, 1, 2, 3 or 4 signal fields, as defined by IEEE-802.11a standard, which may be replicated in one, two, three or four neighboring 20 MHz sub-channels. In some embodiments, signal field  308  in sub-channels  410 ,  420 ,  430 ,  440  may include information that may be used to force other stations to enter the receiving state for the duration of the transmitted packet. This forced operation may protect the data transmission from unwanted interferences from those stations. Signal field  308  of sub channel  410  or signal field  308  of sub channel  440  may be rotated by 180 degrees relative to other sub-channels (e.g. sub channels  420 ,  430 ) to reduce the PAPR, if desired.  
      Although the scope of the present invention is not limited in this respect, it should be understood that in some embodiments of the invention, short preambles  302  and/or long preambles  306  and/or signal fields  308  transmitted on sub-channels  410 ,  420 ,  430 ,  440  may be rotated by any desired angle to reduce the PAPR, if desired.  
      Although the scope of the present invention is not limited in this respect, the prefix, postfix and middle-fix training fields, e.g., fields  320   360  and  370 , respectively, may have, in some embodiments of the invention, substantially the same format. In some embodiments of the present invention, the prefix, postfix and middle-fix training fields, e.g., fields  320   360  and  370 , respectively, may be constructed in accordance with the recommendations of IEEE 802.16 Broadband Wireless Access Working Group, available at http://ieee802.org/16, if desired. However, is some other embodiments of the present invention, other types of preambles may be used, if desired.  
      Although the scope of the present invention is not limited in this respect, prefix training field  320  may be used for wideband (WB) channel estimation, refinement of timing synchronization and frequency offset estimations at the beginning of the packet, and the like. The middle-fix (e.g.,  370 ) and Postfix (e.g.,  360 ) training fields may be provided for channel variation estimation at the middle and the end of the packet, respectively, to allow adaptive fragmentation capability, if desired. In some embodiments of the invention, data packet  300  may be fragmented into two or more fragments separated by middle-fix training field(s)  370 . For example, a fragment of data packet  300  may have BPL information parameters, which may be calculated taking into account long-term channel prediction techniques. The long-term channel prediction techniques may increase overall throughput performance of the system by using longer packets. In some embodiments of the present invention the long-term prediction may be performed to increase the system throughput.  
      In some embodiments of the invention, further improved reliability of data packet transmission may be achieved by considering channel variation during bit and power loading calculations and by applying different bit and power loading parameters to the different fragments of data packet  300 , if desired. In addition, prefix training field  320  and/or postfix training field  360  may be used to analyze failure of cyclic redundancy check (CRC), which failure may be caused by errors in a fragment of a received data packet that may result in loss of the fragment. In some cases, such as, for example, fragment loss may be caused by noise, by Dopller shift, or the like.  
      In some other embodiments of the invention, additional training fields may be incorporated in the middle of the packet, e.g. middle-fix training field  370 . For example, middle-fix training field  370 , may be included after at least one predetermined time interval, for example, 1 millisecond (ms) if the packet is longer than a channel coherence time, which may be, for example, 1.2 ms, if desired.  
      Although the scope of the present invention is not limited in this respect, PLCP header  330  may be used both as a collection of parameters needed to demodulate data packet  300  and/or as an additional training field, if desired. In exemplary embodiments of the invention, the spectrum width of channel  400  may be 80 MHz and PLCP header may include up to 4 OFDM symbols. As an example, the information in PLCP header  330  may be encoded by encoder  230  with the a convolutional code with a rate of ½ and may be modulated by modulator  240  with a desired modulation scheme such as, for example, binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) modulation, or the like. In addition, the PLCP header  330  that may be used as additional training field may allow a receiver to perform additional training such as, for example, frequency and phase estimation refinement, channel estimation refinement, and the like.  
      Although the scope of the present invention is not limited in this respect, PLCP header  330  may include the flowing parameters that may be used with WB OFDM WLAN systems. The first parameter may be a BPL information parameter  335 , which may include a modulation types bit to indicate the modulation types per sub-carrier  450  and a power loading bit to indicate the power loading of sub-carriers  450 . In some embodiments, sub-carriers  450  may be grouped into groups with similar modulation types.  
      Although the scope of the present invention is not limited in this respect, the second parameter may be an Overall Transmitted Power Level (e.g. 4 bits) parameter. This parameter may reflect the power level that may be used during transmission of data packet  300 . The power level may be defined, for example, in 3 dB increments down from a maximal value of transmission power level, if desired. This parameter in conjunction with the “Available Tx Power Level” and “Power Request” parameters described below may be used in solving the Near-Far problem known to persons skilled in the art.  
      Although the scope of the present invention is not limited in this respect, an Available Tx Power Level parameter (e.g. 4 bits) may reflect the maximum transmitter power and may be defined in, for example, 3 dB increments. In some other embodiments of the invention, this parameter may be used in a network interface card (NIC), e.g., in a “save power” mode. A packet Duration parameter (e.g., 2 bytes) may reflect the duration of a current packet, e.g., in microseconds (μs), or using OFDM symbols, or any other suitable time-related units.  
      Although the scope of the present invention is not limited in this respect, other parameters may include a Packet Length parameter (e.g. 2 bytes) that may describe the length of a current packet in octets, a Quality of Receiving parameter (e.g. 2 bits) that may be transmitted in a response to a received transmission and may include, for example, four possible values, namely: “Packet Lost” (CRC failed), “Poor” (a relatively large number of errors have been recovered by error correction schemes), “good ” (a relatively small number of errors have been recovered by error correction schemes) and “excellent” (substantially no errors). In addition, a BPL Request parameter (e.g. 2 bits) may be used to request the BPL to be applied during a response transmission. For example, the BPL Request parameter may have values such as, for example, “Transmit robust”, “Use BPL same as in this packet”, “Use BPL same as for previous transmission”, “See MPDU for BPL information”.  
      Although the scope of the present invention is not limited in this respect, a BPL mode parameter (e.g. 1 bit) may select between normal and simplified modes of BPL information exchange, a Power Request parameter (e.g. 4 bits) may request that power level be applied during a response transmission and a Duration Recommendation parameter (e.g. 6 bits) may indicate a recommended duration of the packet in some predetermined units, for example,  200  μs to be applied during a response transmission. In addition, one or more of a CRC parameter (e.g. 1 byte), a Service field parameter (e.g. 1 byte), which may include a scrambler initialization and a Signal Tail parameter (e.g. 6 bits) that may be used for convolutional encoding and/or decoding, may also be implemented into the data packet  300  structure.  
      While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.