Abstract:
Methods, receivers and transmitters all adapted to assist in the performance of layer 2 frame delineation are provided which are applicable in the context of OFDM, and also applicable to any layer 1 design facilitating high data rate transmission. Advantageously, the invention allows the identification of where a layer 2 frame begins notwithstanding the fact that the previous layer 2 header, which would normally be used to identify the location of the next layer 2 frame, has been received in error. As a consequence, rather than discarding considerable quantities of data after an erroneous header, for example all data until a super-frame boundary, layer 2 frames can be correctly received staring with the next header received in a non-erroneous layer 1 frame.

Description:
FIELD OF THE INVENTION 
     The invention relates to methods and systems for layer 2 frame delineation recovery, for example for delineation recovery for MAC frames transmitted over OFDM (Orthogonal Frequency Division Multiplexing) symbols. 
     BACKGROUND OF THE INVENTION 
     In OFDM based transmission systems, the number of data bits carried by one OFDM symbol (a type of layer 1 frame) tends to be very large compared to that of conventional low speed transmission systems. For example, if one OFDM symbol consists of 1024 sub-carriers with QPSK modulation and rate one half channel encoding, and 80% capacity used for data bits, the number of data bits which would be transmitted for a single OFDM symbol would be about 100 bytes. If channel conditions allow, the number could be even as high as 800 bytes with the 256 QAM modulation scheme and near rate one coding. With these high-speed data transmission schemes, it is important to have an efficient data-packing scheme because even a fraction of the symbol that is not used for transmission could end up being several hundred bytes long. 
     It has become common to have variable length layer 2, or MAC frames, in order to support the bursting nature of data arrivals. It is desirable to have layer 2 frames which are capable of carrying complete upper layer transport units, for example IP packets. Thus, rather than forcing the layer 2 frame to be a fixed multiple of the layer 1 frame length, it has become common to allow the layer 2 frame to have a variable length thereby avoiding the potential wastage which would occur in the event that the layer 2 frame was a fixed size and the upper layer packet does not fit into the layer 2 frame. 
     In a variable length layer 2 frame, the length information is located in the header of the layer 2 frame so the receiver knows the end of the frame as well as the beginning of the next frame. Disadvantageously, when the header is in error, the receiver does not know where the next frame starts and consequently will lose data in subsequent layer 2 frames until a new layer 2 frame boundary is detected. In one effort to deal with this issue, layer 1 frames are grouped in “super-frames” for example consisting of 10 layer 1 OFDM symbols. The beginning of such a super-frame is always used to start the transmission of a new layer 2 frame. Thus if a header is in error and the receiver is unable to determine the next layer 2 frame boundary, the receiver will always know that there will definitely be a layer 2 frame boundary at the start of the next super-frame. Unfortunately, this can result in a substantial loss of data. 
     An example of this is shown in FIG.  1 . In  FIG. 1  a sequence of layer 1 OFDM symbols is indicated by  10 . Two scheduling intervals  11  and  12  are shown, these being equivalent to the above referenced super-frames. A MAC frame  12  starts at the beginning of the first scheduling interval  11 , and similarly a MAC frame  18  starts at the beginning of the second scheduling interval  12 . Scheduling interval  11  also includes MAC frames  14  and  16  transmitted in sequence after MAC frame  12 . MAC frames  12 ,  14  and  16  have respective headers  13 ,  15  and  17 . Similarly, during the second scheduling interval  12 , MAC frames  20 ,  22 ,  24  and  26  follow MAC frame  18 , and the MAC frames  18 ,  20 ,  22 ,  24  and  26  have respective headers  19 ,  21 ,  23 ,  25  and  27 . In the event during reception over a scheduling interval  11 , MAC frame  14  is received with an error in its header  15 , the system is unable to determine the length of MAC frame  14  and as such does not know where the start of the next MAC frame  16  will be. For this reason the entirety of MAC frames  14  and  16  will be lost, and the receiver can only start decoding MAC frames again beginning with MAC frame  18  because it knows that a new scheduling interval  12  begins there because that is the start of a super-fame. A similar loss is shown during scheduling interval  12  in which there is shown to be an error in the header  21  of MAC frame  20  resulting in the loss of the entirety of MAC frames  20 ,  22 ,  24  and  26 . In OFDM based very high data rate transmission systems, these types of loss can result in severe performance degradations since the number of data bits in one symbol is large and each header carries other sensitive control information. 
     Another existing method of trying to deal with this issue is to include a predetermined bit pattern as a frame start and frame end indication. Such start and end indicators may require more bits than the original header information, and are also subject to error. 
     SUMMARY OF THE INVENTION 
     The invention provides methods, receivers and transmitters all adapted to assist in the performance of layer 2 frame delineation. The invention is applicable in the context of OFDM, but is also applicable to any layer 1 design facilitating high data rate transmission. Advantageously, the invention allows the identification of where a layer 2 frame begins notwithstanding the fact that the previous layer 2 header, which would normally be used to identify the location of the next layer 2 frame, has been received in error. This means that rather than discarding a lot of data after an erroneous header, for example all data until a super-frame boundary, layer 2 frames can be correctly received starting with the next header received in a non-erroneous layer 1 frame. 
     The method implemented at a transmitter, provided by one broad aspect of the invention involves prior to transmitting layer 1 frames, adding layer 2 boundary information to each layer 1 frame, the layer 2 boundary information indicating whether there is a layer 2 header within the layer 1 frame and indicating where in the layer 1 frame the layer 2 header begins. The layer 2 boundary information can consist of frame delineation bits added to each layer 1 frame. 
     In one embodiment, the transmitter is required to align the start of each layer 2 frame with one of a number of predetermined layer 2 start positions within layer 1 frames. The boundary information is then used to indicate which one of the possible start positions is the start position of each frame. 
     In some embodiments, the boundary information is transmitted on dedicated OFDM sub-carriers or some other dedicated channel. In another embodiment, the boundary information is transmitted in punctured symbol locations within the layer 1 frames. 
     Another broad aspect provides a transmitter adapted to perform the above described transmit frame delineation functionality. More particularly, this provides layer 1 functionality adapted to, prior to transmitting layer 1 frames, add layer 2 boundary information indicating whether there is a layer 2 header within the layer 1 frame, and to indicate where in the layer 1 frame the layer 2 header begins. 
     According to another broad aspect, the invention provides the functionality executed by a receiver of frames transmitted using the above noted transmit functionality. The receiver receives a sequence of layer 1 frames. For each layer 1 frame received, the boundary information is extracted from the layer 1 frame indicating whether there is a layer 2 header within the layer 1 frame, and indicating where in the layer 1 frame the layer 2 header begins. 
     A determination is made of whether the layer 1 frame is received in error. If so, the layer 1 frames are not passed up to layer 2 until a layer 1 frame is received without error with boundary information indicating a layer 2 header is located within the layer 1 frame. 
     On an ongoing basis, layer 2 headers are examined to determine the location of the next layer 2 header. When the delineation procedure is required due to an error in the header, the receiver looks for a layer 2 header in a location identified by the boundary information. 
     Another broad aspect provides a receiver adapted to implement methods such as described above. Further aspects provide a computer usable medium containing instructions for causing processing hardware to execute the above described methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described with reference to the attached drawings in which: 
         FIG. 1  is an example of layer 2 frames being transmitted over layer 1 frames in super-frames, illustrating how multiple layer 2 frames may be lost in conventional systems due to an error in a single header; 
         FIG. 2  shows the layer 1 and layer 2 frames of  FIG. 1  to which a frame delineation recovery scheme according to the invention has been applied; 
         FIG. 3  is a block diagram of a receiver and transmitter adapted to perform layer 2 frame delineation according to an embodiment of the invention; 
         FIG. 4  is a flowchart of functionality executed by the frame delineation recovery block of  FIG. 3 ; 
         FIG. 5  is a flowchart of layer 2 header processing; 
         FIG. 6  shows an example format for the frame delineation bits; and 
         FIG. 7  is a detailed example of frames and OFDM symbols which feature the frame delineation bits in accordance with the example at FIG.  6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 2 , shown is the same sequence of layer 1 OFDM symbols  10  and MAC frames discussed previously with respect to FIG.  1 . In this example, a frame delineation recovery method has been implemented as will be described in detail below such that notwithstanding the error in the header of  15  of packet  14 , the location of the header  17  of the subsequent frame  16  can be determined, and since that OFDM symbol was not received in error that frame can be received thereby reducing the amount of lost data to consist solely of AC frame  14 . Applying the same method to the subsequent scheduling interval  12  results in only MAC frame  20  being lost instead of MAC frames  20 ,  22 ,  24  and  26  as previously was the case in FIG.  1 . 
     Referring now to  FIG. 3 , details of an example transmitter and receiver adapted to implement the frame delineation recovery will be described. Shown is transmitter functionality generally indicated by  40  connected through a wireless channel  44  to receiver functionality generally indicated by  42 . Layer 2 functionality is generally indicated by  46  for the transmitter  40  and  70  for the receiver  42 . The transmit layer 2 functionality  46  generates layer 2 data  48  and layer 2 boundary information  50 . These are passed to respective coding/modulation blocks  52 ,  54 . The boundary information in this embodiment will be implemented as frame delineation (FD) bits. A detailed example of the FD bits is presented below. The FD bits identify if, and where, a new MAC frame begins within the layer 1 OFDM symbol. In some embodiments, the coding and modulation applied to the frame delineation bits by coding/modulation block  54  is stronger than that applied to the data by coding/modulation block  52  such that there is a higher probability that at the receiver  42  the frame delineation bits are detected correctly. Data output by the two coding/modulation blocks  52 ,  54  is passed to an OFDM transmitter  56  which puts the modulated and encoded data and FD bits onto OFDM sub-carriers and transmits OFDM symbols over the channel  44 . In some embodiments, the frame delineation bits are transmitted on a separate sub-carrier or sub-carriers to the data information. 
     The receiving functionality  42  includes an OFDM receiver  58  which receives the layer 1 OFDM symbols and passes the signals from the sub-carrier used for the frame delineation bits to a first decoding/demodulation block  60 , and passes the signals from the remaining sub-carriers used for data to a second decoding/demodulation block  62 . The decoding/demodulation block  60  performs decoding and demodulation on the sub-carrier containing the frame delineation bits and passes these frame delineation bits to a MAC frame boundary detection block  64 . This block processes the boundary information such as frame delineation bits to determine whether or not a new MAC frame begins in that layer 1 OFDM symbol, and if so where in the symbol the new MAC frame begins. This information  69  is passed to a MAC frame delineation recovery block  68 . The decoding and demodulation block  62  performs decoding and demodulation on the remaining sub-carriers and passes the data thus produced to an error detection block  66  which determines whether the symbol was received in error for example by performing a CRC check. The error detection block  66  produces an output  67  to the MAC frame delineation recovery block  68  which indicates whether or not the OFDM symbol was received in error. The MAC frame delineation recovery block  68  uses the information received from the MAC frame boundary detection block  64  and the error detection block  66  to decide either to discard the layer 1 OFDM symbol all together, or to pass it up to layer 2 functionality  70 . It is noted that the error detection function can be considered a function performed between layer 1 and layer 2, a function performed at layer 1 or a function performed at layer 2. This simply depends upon where the function is implemented. 
     The functionality implemented by the MAC frame delineation recovery block  68  will now be described with reference to FIG.  4 . 
     The functionality begins with the receipt and decoding of a layer 1 OFDM symbol (step  4 - 1 ). The error detection block  66  produces the output  67  indicating whether or not the symbol was received in error. If the symbol was not in error (No path, step  4 - 2 ) then in step  4 - 3  the symbol data is passed up to layer 2 functionality  70 . If on the other hand, the error detection block does indicate that the symbol was received in error (Yes path, step  4 - 2 ) then at step  4 - 4  if the erroneous symbol does not contain a header (no path, step  4 - 4 , known from previously correctly decoded header) then the process returns to step  4 - 1  and the receipt and decoding of the next OFDM symbol. On the other hand, the frame delineation recovery step, generally indicated by step  4 - 5  is executed in the event the erroneous symbol did in fact contain a header. (yes path, step  4 - 4 ) Step  4 - 5  is a loop which is now repeated until a symbol is received which is not in error and which contains a MAC header. This begins with the receipt and decoding of a layer 1 OFDM symbol at step  4 - 6 . In step  4 - 7 , if that symbol was received in error (Yes path) then that is the end of that iteration of the loop and the loop continues back at step  4 - 6  with the receipt of another symbol. On the other hand, if the symbol was not received in error (No path) then the MAC frame boundary information received from the MAC frame boundary detection block  64  is examined to determine whether or not the symbol contains a MAC frame header. In the event there is no MAC frame header, (No path, step  4 - 8 ) then further information is still being received for a MAC frame for which the header was received in error. As such, there is no purpose for this data and that iteration of the loop ends and the process continues with the receipt of another layer 1 OFDM symbol at step  4 - 6 . On the other hand, if the symbol does contain a MAC frame header (Yes path, step  4 - 8 ) then the MAC frame header position determined from the frame boundary information received from the MAC frame boundary detection lock  64  is passed up to layer 2  70  at step  4 - 9 . Following this, the symbol data is also passed up to layer 2 at step  4 - 3 . Whenever an error occurs in the symbol the entire symbol may be discarded. It is also possible that portions of the erroneous symbol may be of use and these can be passed up to layer 2. 
     On the transmit side  40  MAC frame boundary information  50  can be generated at any appropriate point in the processing. This can be done at layer 2 or layer 1 for example. 
     On the receive side  70  a modified layer 2 frame reception method is required. Referring now to  FIG. 5 , a flowchart of layer 2 frame header processing is shown. The details of processing of the data portion of the layer 2 processing are not shown as these can be implemented in any conventional manner. The method starts with layer 2 correctly receiving a layer 2 header in a known location (step  5 - 1 ) either because of a super-frame mechanism, because layer 1 indicates where it is using boundary information, or through some other start-up procedure. That header is decoded to determine the location of the header for the next layer 2 frame by examining the length field (step  5 - 2 ). Now, some time later, layer 2 will look in the expected location for the next header. If that header is correctly received (Yes path, step  5 - 3 ), then that iteration of header processing ends, and steps  5 - 2 ,  5 - 3  are repeated until a header is not found in the expected location (No path, step  5 - 3 ) due to layer 1 symbol in error. Then layer 2 must wait until it receives boundary information from layer 1 indicating the location of a layer 2 header (step  5 - 4 ). When such boundary information is received, the process flow returns to step  5 - 2  which again determines an expected location for the next layer 2 header. 
     An example implementation of the frame delineation bits will now be described with reference to FIG.  6 . In this example it is assumed that there are four bits included in each OFDM symbol which indicate firstly whether there is a MAC frame boundary within the symbol and secondly where that MAC frame boundary occurs. Referring now to  FIG. 6 , the first bit  80  indicates whether the symbol contains a MAC frame boundary in accordance with the legend of Table  81 . In this example, zero indicates that there is no new frame while a one indicates that there is a new frame. The following three bits  82  are used to indicate the start position of the new frame. These bits indicate to a resolution of ⅛ of an OFDM symbol the position of the frame running from 000 indicating that the frame starts right at the beginning of the symbol through to 111 indicating that the frame starts ⅞ths of the way through the symbol. This assumes that the MAC layer is constrained to start new frames only at positions which are ⅛th multiples of OFDM symbols in length. 
     Turning now to  FIG. 7 , a specific example in which the frame delineation bit format described with respect to  FIG. 6  has been employed will now be described. In this example there are MAC frames, MAC frame A  90 , MAC frame B  91 , and MAC frame C  92  which are being transmitted over layer 1 symbols three of which are indicated as symbol one  94 , symbol two  96  and symbol three  98 . It can be seen that symbol one will contain data from MAC frame A  90  entirely, and that there is no MAC frame boundary. As such, the first bit in the frame delineation bits  95  will be a zero indicating no new frame, and the content of the remaining three bits is not relevant. Symbol two  96  on the other hand includes data from MAC frame A  90  and MAC frame B  91  and as such does include a MAC frame boundary. To indicate this the frame delineation bits  97  include a first bit which is a one indicating a new frame. Furthermore, the remaining three bits are set to “101” in this example indicating that the new MAC frame starts at a point ⅝ths of the way through the layer 1 symbol. Similarly, symbol three  98  includes data from both MAC frame B  91  and MAC frame C  92  and as such the frame delineation bits  99  include a first frame delineation bit set to one indicating that there is a new frame. The remaining three bits in this case are set to “100” indicating that the new frame begins at a point ½ of the way through the layer 1 symbol. In this example, if the header of MAC frame A  90  is received in error, OFDM symbol one is discarded, the data for MAC frame A in symbol two is discarded but it is possible to determine from the frame delineation bits in symbol two where the beginning of MAC frame B  91  is and the portion of symbol two  96  relevant to MAC frame B  91  can be extracted. The remainder of frame B is recovered from symbol three  98 . What this means is that MAC frame B does not need to be retransmitted, but rather only MAC frame A  90  needs to be retransmitted. In the above example, three frame delineation bits are used to allow frame delineation to a resolution of ⅛th of an OFDM symbol boundary. Of course it is to be understood that a higher or lower granularity could be achieved by using more or fewer frame delineation bits for this purpose. For example, if the granularity level is M=2 n , then there are M points in the OFDM symbol where a MAC frame can begin. This is a constraint which is added to the MAC layer, namely that layer 2 frames can only start at one of a set of predetermined start positions. As indicated previously, the frame delineation bits may be encoded and modulated separately from the data traffic bits to provide more protection from channel errors. For example, the traffic data may be encoded with rate 5/6 code and modulated with 64QAM, and the frame delineation bits may be encoded with a 1/2 rate code and modulated with QPSK. A small subset of sub-carriers of OFDM symbol can be reserved for the transmission of the frame delineation bits. This reservation of sub-carriers needs to be predefined and known both at the receiver and transmitter so that the receiver can decode and demodulate them separately. 
     In a variable length MAC frame, the header includes information on the length of the MAC frame. The receiver extracts the length information and decides when the next frame starts. The header should be protected by an error detection code so that the length information may not be used incorrectly in case of an error in the header. When a header error is detected, the receiver looks at the frame delineation bits to find the beginning boundary of the next MAC frame. Because frame delineation bits are protected better than traffic bits, they can be correctly decoded even when the traffic bits including the frame header are in error. Therefore the probability that the next MAC frame boundary is lost will be reduced. 
     In the above described embodiments, it has been assumed that the layer 1 frame is an OFDM symbol. More generally, the invention can be applied to other layer 1 protocols which do not use OFDM symbols but support high data rates. Furthermore, the invention has been described with reference to the MAC layer 2 protocol. More generally, the invention is applicable to layer 2 frames which have frame boundaries which are not aligned with the layer 1 boundaries. It is noted that the method provided by the invention to provide layer 2 frame delineation may be used in combination with the super-frame method which enforces the layer 2 frames to be aligned with layer 1 frames periodically. Alternatively, the new frame delineation method can be used on its own without the need for periodic resyncing to the layer 1 frame. 
     In the above described embodiments, the boundary information is transmitted using dedicated OFDM subcarriers. More generally, the boundary information can be transmitted using dedicated channels of any suitable type. Alternatively, the boundary information can be transmitted in punctured symbol locations of layer 1 frames. 
     Embodiments of the invention can be implemented using hardware, software, or any suitable combination of hardware and software. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.