Abstract:
A data channel to transmit data from a transmitter to one or more of a plurality of receivers, each of which intermittently reports to the transmitter its reception quality of signals transmitted by the transmitter. The transmitter transmits the data in frames which include at least one block. Each block includes the same predefined number of traffic symbols, and includes a header portion and a payload portion. The header portion of each block is packaged for transmission in a robust manner, enhancing the probability that each receiver will be able to recover it and the header portion includes information required to recover the payload portion. The payload portion is, in accordance with the reception quality reported by the intended receiver, packaged to make efficient use of the transmission re-sources while ensuring a reasonable probability that the intended receiver will be able to recover the payload. The header portion can include indications of the modulation, forward error correction and repetition utilized to package the payload and can indicate the length of the payload.

Description:
[0001]    Further, the characteristics of the data transmission typically are independent of the reception levels experienced at the receiver. Thus, the transmission characteristics are set to a lowest common denominator which is selected to ensure reception under worst case, or near worst case, conditions and is held constant for all transmissions. While this allows for simple system design and operation, it does not make efficient use of bandwidth or other system resources.  
           [0002]    U.S. Pat. No. 5,949,814, also to Odenwalder (“Odenwalder #2”), teaches a system which provides a high data rate supplemental channel for CDMA telecommunications systems. In this scheme, the transmission system includes an in-phase channel set and a quadrature-phase channel set. The in-phase channel set provides a set of orthogonal medium rate control and traffic channels and the quadrature-phase channel set provides the high-rate supplemental channel and an extended set of medium-rate channels that are orthogonal with respect to each other.  
           [0003]    While Odenwalder #2 can increase the downlink data transmission rate, it is not generally suitable for transmitting data to multiple subscriber stations, which have different abilities to receive the transmission. Further, Odenwalder #2 requires certain overhead control communication between the base station and the mobile user in order to commence a high data rate communication therebetween. Such a system is not well suited to systems such as packet-based communication systems where small amounts of data may need to be transferred to users as the necessary overhead can make the communication inefficient relative to the amount of data transferred. Similarly, such a system is not well suited to situations wherein a variety of users need data transmitted to them.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the present invention to provide a novel method, system and apparatus for transmitting data between stations, which obviates or mitigates at least one of the above-identified disadvantages of the prior art.  
           [0005]    According to one aspect of the invention, there is provided a system for transmitting data comprising: a plurality of subscriber stations operable to receive a signal from a base station at a different reception-quality than at least one other subscriber station; and a base station operable to transmit a radio signal to said subscriber stations, the signal including a frame having a fixed duration and comprising at least one block of data, each block including a header packaged by said base station to be recoverable by all of the subscriber stations regardless of their specific reception-qualities, and a payload portion packaged by said base station to be recoverable by at least the intended recipient subscriber station.  
           [0006]    According to another aspect of the invention, there is provided a block for transmission to one of a plurality of subscriber stations each having a reception-quality corresponding to an ability to recover the transmission, the block comprising a payload and a header, the header packaged for recovery regardless of the reception-qualities of the subscriber stations and including information required to recover the payload, the payload being packaged to make efficient use of transmission resources and to enhance reception by an intended receiving subscriber station according to the reception-quality experienced by that subscriber station. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, in which:  
         [0008]    [0008]FIG. 1 is a schematic representation of a network incorporating a data channel in accordance with an embodiment of the invention;  
         [0009]    [0009]FIG. 2 is a schematic representation of the base station shown in FIG. 1;  
         [0010]    [0010]FIG. 3 is a schematic representation of one of the subscriber stations shown in FIG. 1;  
         [0011]    [0011]FIGS. 4 a ,  4   b  and  4   c  are schematic representations of a frame of data blocks for transmission over the network shown in FIG. 1 at three different spreading factors;  
         [0012]    [0012]FIG. 5 is a schematic representation of a block in the frames of FIG. 4 a ; and  
         [0013]    [0013]FIG. 6 is a flowchart of a method of constructing the block of FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring now to FIG. 1, a wireless network system for transmitting data is indicated generally at  20 . System  20  includes a radio base station  24  and a plurality of subscriber stations  28   a ,  28   b  . . .  28   n . In a presently preferred embodiment, radio base station  24  is connected to at least one data telecommunications network (not shown), such as a land line-based switched data network, a packet network, etc., by an appropriate gateway and one or more backhauls (not shown), such as a T1, T3, E1, E3, OC3 or other suitable land line link, or can be a satellite or other radio or microwave channel link or any other link suitable for operation as a backhaul as will occur to those of skill in the art.  
         [0015]    Base station  24  communicates with subscriber stations  28  which, in a present embodiment of the invention, are installed at subscriber premises, as is common in a wireless local loop (WLL) system. The number ‘n’ of subscriber stations serviced by a base station  24  can vary depending upon the amount of radio bandwidth available and/or the configuration and requirements of the subscriber stations  28 .  
         [0016]    A data channel  32  is established between base station  24  and each subscriber station  28  via radio. Data channel  32  carries information to be transferred from base station  24  to respective subscriber stations  28   a ,  28   b  . . .  28   n  as needed. Data channel  32  can be implemented with networks using a variety of multiple access techniques, including TDMA, FDMA, CDMA or hybrid systems such as GSM, etc. In a present embodiment, data transmitted over data channel  32  is transmitted as packets encapsulated within frames, the details of which will be discussed in greater detail below.  
         [0017]    The ability of a subscriber station  28  to properly receive a signal transmitted to it, hereinafter referred to as the “reception quality” of the signal, can depend upon a variety of factors. Measures of reception quality can be determined in different manners according to the multiple access technique employed to transmit the signal. For example, in TDMA or FDMA systems, the received signal strength is the determination most often used. In CDMA systems, the ratio of received bit power to received interference power (often expressed as E s /N o , where E s  is energy per symbol, and N o  is the received interference energy) is a relevant determination. In any event, the reception-quality of channel  32  at each subscriber station  28  can vary depending on a variety of factors, including multipath interference (from the presence of nearby buildings, etc.), radio noise sources (including transmissions by other users or radio noise sources), geographical features, the distance of the subscriber station  28  from base station  24 , the quality of the receiver in the subscriber station  28 , etc. as is well understood by those of skill in the art. With distance, typically a signal attenuates as 1/r N , where r is the distance between the subscriber station  28  and base station  24 , and N&gt;1. In IS-95 CDMA systems, for example, N typically is in the range of 3&lt;N&lt;5.  
         [0018]    As illustrated in FIG. 1, the geographic distribution of subscriber stations  28  with respect to base station  24  need not be symmetric nor will subscriber stations which are physically located close to one another necessarily experience the same or similar reception qualities due to a variety of factors including the geographic environment (the presence or absence of buildings which can reflect or mask signals), the radio environment (the presence or absence of radio noise sources), etc. Thus, in most circumstances subscriber stations  28  served by a base station  24  can have significantly different reception qualities and these reception qualities can change over time.  
         [0019]    In FIG. 1, at one time subscriber stations  28   a  and  28   f  may experience a very good reception quality while subscriber stations  28   b  and  28   g  experience moderate reception quality and subscriber stations  28   c ,  28   d  and  28   e  may experience low reception quality. At a subsequent time, subscriber stations  28   a ,  28   d  and  28   g  can have very good reception, subscriber stations  28   c ,  28   e  and  28   f  may experience moderate reception quality and subscriber station  28   b  may experience low reception quality, etc.  
         [0020]    In the present invention, at appropriate intervals or at predetermined events, each subscriber station  28  will report its present reception-quality to base station  24 . Base station  24  operates to maintain a database of the latest reported reception-qualities and appropriately packages data to be transmitted over data channel  32  to each subscriber station  28 .  
         [0021]    As used herein, the terms “package”, “packaged” and “packaging” refer to the overall arrangement of the transmission of the packaged data for its reception at an intended destination receiver. Packaging of data can include, without limitation, applying different levels of forward error correcting FEC) codes (from no coding to high levels of coding and/or different coding methods), employing various levels of symbol repetition, employing different modulation schemes (4-QAM, 16-QAM, 64-QAM, etc.) and any other techniques or methods for arranging data transmission with a selection of the amount of radio (or other physical layer) resources required, the data rate and probability of transmission errors which are appropriate for the transmission. For example, data can be packaged with rate ¼ FEC coding (each 1 data bit is transmitted in 4 bits of information) and 16-QAM modulation for transmission to a first intended receiver and packaged with rate ½ FEC coding and 64-QAM modulation for transmission to a second intended receiver which has a better reception-quality than the first.  
         [0022]    [0022]FIG. 2 shows an example of base station  24  in greater detail. Base station  24  comprises an antenna  40 , or antennas, for receiving and transmitting radio-communications over communication channel  32 . In turn, antenna  40  is connected to a radio  44  and a modem  48 . Modem  48  is connected to a microprocessor-router assembly  52  such as a SPARC processor system manufactured by SUN Microsystems. It will be understood that assembly  52  can include multiple microprocessors, as desired and/or that the router can be provided as a separate unit, if desired. The router within microprocessor-router assembly  52  is connected to a backhaul  56  in any suitable manner, which in turn connects base station  24  to a data network (not shown).  
         [0023]    Referring now to FIG. 3, an example of a subscriber station  28  is shown in greater detail. Subscriber station  28  comprises an antenna  60 , or antennas, for receiving and transmitting radio-communications over communication channel  32 . In turn, antenna  60  is connected to a radio  64  and a modem  68 , which in turn is connected to a microprocessor-assembly  72 .  
         [0024]    Microprocessor-assembly  72  can include, for example, a StrongARM processor manufactured by Intel, that performs a variety of functions, including implementing A/D-D/A conversion, filters, encoders, decoders, data compressors, de-compressors and/or packet disassembly. As seen in FIG. 3, microprocessor-assembly  72  interconnects modem  68  and a data port  76 , for connecting subscriber station  28  to a data client device, such as a personal computer, personal digital assistant or the like which is operable to use data received over communication channel  32 . Accordingly, microprocessor-assembly  72  is operable to process data between data port  76  and modem  68 .  
         [0025]    Referring now to FIGS. 4 a  through  4   c , a frame for transmission over channel  32  is indicated generally at  100 . In a presently preferred embodiment of the invention, data is transmitted over channel  32  in frames  100  which require ten milliseconds of transmission time, although longer or shorter transmission times for frame  100  can be selected if desired.  
         [0026]    As understood by those of skill in the art, frame  100  can be measured in terms of a duration of time. In turn, that duration can carry a given number of symbols for transmission. In turn, those symbols can represent data, the actual amount of data being represented by a symbol depending on how the data is packaged into a symbol. In a CDMA embodiment, symbols can be packaged using a combination of the CDMA spreading factor, modulation, repetition and encoding. Thus, it will be appreciated that, while the duration of frame  100  remains constant, the effective amount of data transmitted within a frame will depend on the packaging of the data. The application of these concepts to the present invention will be discussed in greater detail below.  
         [0027]    In the present invention, a frame  100  is configured to transmit a number of data blocks B 1  through B i , where each block B i  carries a fixed number of traffic symbols and thus the number of blocks in a frame  100  depends upon the CDMA spreading factor, chip rate and the transmission duration of the frame. In a present embodiment of the invention, a CDMA system with a chip rate of three-million, eight-hundred and forty thousand chips per second (3.84 Mcps) is employed and a block B i  with one-thousand two-hundred traffic symbols is employed.  
         [0028]    [0028]FIG. 4 a  shows frame  100  employed with a CDMA spreading factor of four, so that eight blocks (B 1  through B 8 ) are included in frame  100  and frame  100  thus includes nine-thousand, six-hundred traffic symbols. In FIG. 4 b , a CDMA spreading factor of eight is used, so frame  100  includes four blocks (B 1  through B 4 ) and four-thousand, eight-hundred traffic symbols and in FIG. 4 c , a CDMA spreading factor of  16  is employed, so frame  100  includes two blocks (B 1  and B 2 ) for two-thousand, four-hundred traffic symbols. The present inventor has determined that, by maintaining the number of traffic symbols in blocks B constant and the frame duration constant, undesired complexity at modem  68  can be avoided, although it is contemplated that frame structures with different numbers of traffic symbols can be employed, if desired.  
         [0029]    Each block B i  has the structure shown in FIG. 5, including a header  104  and payload  108 . It is intended that header  104  be receivable by all subscriber stations  28  in system  20  that have at least a predetermined minimum reception quality. Accordingly, header  104  is packaged in a robust manner to increase the probability that subscriber stations  28  will be able to receive it (i.e.—the frame error rate, or FER, for subscriber stations to receive and understand header  104  is less than a level selected by the operator of system  20 ). In a present embodiment of the invention, header  104  comprises ten header information bits which are ultimately packaged into one-hundred and twenty traffic symbols by: coding the information bits for forward error correction (FEC) to yield thirty coded bits (a rate ⅓ FEC code); using a repetition factor of eight to repeat the resulting bits for eight repetitions to obtain two-hundred and forty bits; and then modulating those bits using QPSK modulation to yield the one-hundred and twenty traffic symbols of header  104 . While this packaging is presently preferred for header  104 , it is contemplated that a wide range of other packagings can be employed for header  104 , as will be apparent to those of skill in the art.  
         [0030]    Of the ten header information bits of header  104 , five bits are presently employed to represent a Length value and the remaining five bits to represent a Block Format.  
         [0031]    In the present invention, while header  104  is packaged to be receivable by all subscriber stations  28 , payload  108  is packaged to provide efficient use of radio channel  32  when transmitting information to an intended recipient subscriber station  28 . Accordingly, the modulation, FEC coding; symbol repetitions, etc. of payload  108  will be varied from block B to block B, depending upon the intended recipient subscriber station  28  and its reception quality.  
         [0032]    In a present embodiment of the invention, a symbol repetition factor of four, three, two or one can be employed; modulation schemes of 64-QAM; 16-QAM; 4-QAM can be employed; and eight different FEC puncturing masks can be employed (to obtain code rates from ⅓ to ⅘). Further, a length multiplier is required to be available to the receiver so that it can correctly interpret the contents of payload  108  and in a present embodiment of the invention, multiplier values of eight, sixteen, thirty-two, sixty-four and one-hundred and twenty-eight can be employed. Thus, the particular modulation scheme can be represented with two bits of information (to select from four possible modulations); the symbol repetition factor with two bits (to select from four possible repetition rates); the FEC puncture mask with three bits (to select from eight possible puncture masks), the length multiplier with three bits (to select from five possible multiplier values). However, as will be apparent to those of skill in the art, many combinations of these parameters are redundant, contradictory or are unlikely to be useful in system  20 . For example, transmissions at 64-QAM modulation with no symbol repetition and low levels of FEC coding are unlikely to be required in system  20 .  
         [0033]    Accordingly, to reduce the overhead (header  104 ) required to transmit the payload  108 , thirty-two selected combinations, which are deemed most useful, of the modulation, FEC puncture mask, length multiplier and symbol repetition factors are selected and these combinations are defined as entries in a look up table, known to base station  24  and subscriber stations  24  and the entries of which can be accessed by five bits of information which comprise the Block Format. The actual combinations of factors selected for inclusion in the look up table are not particularly limited and it is contemplated that they will be selected by the manufacturer of base stations  24  and subscriber stations  28  in view of the expected range of operating conditions of a system  20 .  
         [0034]    The remaining five information bits of header  104  represent a Length parameter which, represents the value to be multiplied by the length multiplier from the Block Format, to determine the number of information bits in the payload  108 , as this number is necessary for a receiver to know before attempting to interpret payload  108 . Essentially, the Length and length multiplier parameters are employed to determine if payload  108  is less than full with valid bits (which can occur depending upon the FEC coding, modulation, and repetition levels used to transmit and the amount of data to be transmitted). As blocks B always have the same number of traffic symbols, pad symbols are added to payload  108  to fill it, if necessary and, to save computational complexity, these pad bits are added after FEC coding, repetition and interleaving has been performed on the payload symbols (as described below). Accordingly, information as to the actual length of payload  108  is required by the receiver to allow for de-interleaving, FEC de-coding, etc. to be performed correctly on the payload  108 .  
         [0035]    [0035]FIG. 6 shows a flowchart of the process of constructing a block B for transmission. As shown, the ten information bits of header information are first FEC encoded at  200  to yield thirty encoded bits for a rate ⅓ FEC code. In the present embodiment of the invention, a second order Reed-Muller coder is employed, although other suitable coders will also occur to those of skill in the art, which also performs a symbol repetition of order eight to obtain two-hundred and forty encoded bits. Next, the encoded bits are mapped to appropriate symbols for transmission at  204  and, in the present embodiment of the invention, QPSK modulation is employed so that the two-hundred and forty encoded bits are mapped to one-hundred and twenty traffic symbols for transmission.  
         [0036]    While processing of the payload bits can be performed after processing of the header bits has been completed, in a presently preferred embodiment of the invention, the payload bits are processed in parallel with the processing of the header bits to reduce processing latency.  
         [0037]    As shown in the Figure, a cyclical redundancy check (CRC) value is first calculated for the payload information bits at  208  and this value is included, with the payload information bits, as part of the bits to be transmitted. In a present embodiment of the invention, this CRC value is determined from the systematic code generated by a g CRC16 (D) function which produces a sixteen bit CRC code, although other suitable CRC functions will be apparent to those of skill in the art.  
         [0038]    Next, the information bits and the CRC bits are FEC encoded at  212  and, in a present embodiment of the invention, this is accomplished with a Turbo coder with subsequent puncturing of the code. As mentioned above, the degree to which the resulting code is punctured is selected according to the reception quality of the intended recipient of the block B which is being constructed. At  216 , the resulting bits are interleaved using a Relative Prime Interleaver in a present embodiment of the invention.  
         [0039]    After this coding and interleaving, the bits are mapped to symbols at  224 , according to the selected M-ary modulation scheme, where M can be four, sixteen or sixty-four (i.e. 4-QAM, 16-QAM or 64-QAM). Again, the modulation scheme employed is selected according to the reception quality of the intended recipient of the block B being constructed. If the number of bits to be mapped is not divisible by log 2 (M), then symbol rate pad bits are added at  220  to fill the available bit space before the symbol mapping at  224 .  
         [0040]    Next, symbol repetition is performed at  228  at the desired repetition rate, if any. In a present embodiment of the invention, repetition is performed on a symbol by symbol basis, e.g.—given a sequence of bits s 1 , s 2 , s 3 , s 4  and repetition rate of two, the resulting sequence will be s 1 , s 1 , s 2 , s 2 , s 3 , s 3 , s 4 , s 4 .  
         [0041]    At this point, if the number of symbols to be transmitted are less than the number of traffic symbols available for payload  108 , in this specific embodiment of the invention one thousand and eighty traffic symbols, then DTx padding symbols are appended to the channel symbols at  232 . Finally, the channels symbols and the appended DTx padding symbols, if any, are interleaved using a Relative Prime Interleaver at  236  and the resulting traffic symbols are placed in block B at  240 , after the header bits (which are not interleaved, i.e.—header bits always appear at the beginning of block B). The resulting block B can then be processed by the physical channel processes for transmission.  
         [0042]    In operation, each subscriber station  28  reports its reception quality to base station  24 . In an embodiment of the present invention, a subscriber station  24  reports to base station  40  the signal to noise ratio and/or the frame error rate at which it receives frames  100  of channel  32 . This reporting can be performed at an appropriate interval selected by the operator of system  20 , as a trade-off exists between the frequency of the reporting, the relevancy/accuracy of the last reported information and the use of the transmission resources between subscriber station  28  and base station  24  for reporting this information.  
         [0043]    Reception of a block B at a subscriber station follows an inverse set of operations, as will be apparent to those of skill in the art. It should be noted that de-interleaving of traffic symbols can be performed in parallel with the decoding of the header bits, to reduce overall latency at the receiver.  
         [0044]    As mentioned above, header  104  is always packaged into block B in a robust manner to provide a relatively high level of confidence of recovery by all subscriber stations  28   a ,  28   b  . . .  28   n  when frame  100  is transmitted over channel  32 . Such robust packaging is intended to allow every subscriber station  28  served by base station  24  to recover header  104 . Every subscriber station  28  attempts to decode every block B that it receives, even though the payload  108  may be packaged such that a receiving subscriber station  28  will not normally be able to recover it. In such a case, the CRC code which was included in payload  108  at  208  will be incorrect and the subscriber station  28  will discard the block B. If that block B was intended for the subscriber station, a higher level of the protocol stack employed in system  20  will be responsible for retransmitting the data of that payload  108  to the subscriber station  28  in a subsequent block B.  
         [0045]    The payload  108  of a block B can be any type of data received at base station  24 . For example, payload  108  can be one or more TCP/IP packets, or part of a segmented packet, where it is desired to transmit IP packets to a subscriber station  28 . Payloads  108  can be specifically addressed to a particular subscriber stations  28   a ,  28   b  . . . or  28   n , each of which has its own unique address and/or one or more broadcast addresses can be defined for subscriber stations with similar reception qualities. Alternatively, broadcast packets can be packaged for the worst reception quality expected for all of the intended receivers. Data in payload  108  can be combined or segmented, as needed, to fit the size restrictions on the payload in a block B.  
         [0046]    As data is received by base station  24  for transmission to one or more subscriber stations  28 , the data is buffered until a sufficient amount of data is received to fill a frame  100  or until a predefined maximum wait time is exceeded. As will now be apparent to those of skill in the art, the amount of data which is sufficient to fill a frame  100  is dependent upon the Block Format selected to construct each block B i  in a frame  100 . It is contemplated that different blocks B i  within a frame  100  will often have different Block Formats as they are intended for different receivers. Thus, the determination of the receipt of a sufficient amount of data is made assuming the best (i.e. most data rate efficient) encoding and modulation operations, or when the predefined maximum wait time has expired from the receipt of the earliest data, this latter parameter being employed to ensure that a frame  100  is assembled and transmitted before a preselected maximum latency period is exceeded. Any received data which cannot be placed into the assembled frame  100 , due to the Block Format being less data rate efficient, is buffered and assembled in due course into the next frame  100  to be assembled.  
         [0047]    When a sufficient amount of data is received to fill frame  100 , including any data which was buffered from the previous frame  100 , the reception quality last reported by each intended receiver is used to select an appropriate Block Format for each block B which are then assembled and inserted into frame  100 .  
         [0048]    The now-assembled frame  100  is transmitted over channel  32  to subscriber stations  28   a ,  28   b  . . .  28   n . The transmission can occur in the usual manner, using known techniques.  
         [0049]    It is contemplated that system  20  can include more than one channel  32  if desired. In such a case, each channel  32  can have the same spreading factor, or different spreading factors can be employed for different channels  32 . For example, one channel  32  can have a spreading factor of four, to enhance, for a given transmission power level, the likelihood of reception at subscriber stations with poor reception qualities while other channels  32  can have spreading factors of eight, sixteen, etc. to provide efficient data transmissions to subscriber stations with better reception qualities.  
         [0050]    It is to be understood by those of skill in the art that modifications can be made to the above-described method without departing from the present invention. For example, different numbers of header bits, different frame durations, different chip rates, etc. can be employed.  
         [0051]    While the embodiments discussed herein are directed to multiple-access schemes conducted over wireless physical links and using CDMA as a multiple access technique, it will be understood that the present invention can be applied to a variety of physical links, such as over twisted-pair or coaxial links, and a variety of multiple-access schemes such as TDMA, FDMA, OFDM or CDMA.  
         [0052]    The present invention provides a novel data channel in a network having at least one base station and a plurality of subscriber stations. The data channel can be composed of a plurality of frames having a number of data blocks, each having the same number of traffic symbols. The headers of each block are robustly packaged in any appropriate manner, to ensure and/or assist all receivers to recover the header information with a high probability of success (low probability of Frame Error) and the payload of the block is packaged in a manner which is efficient for the intended receiver, as determined from that receiver&#39;s reported reception quality.  
         [0053]    The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.