Abstract:
Data blocks to be received each comprise a header and a data field including respectively first and second codes for error detection. The decoding of a signal segment received having first and second portions corresponding respectively to the header and to the data field of a block gave rise to a verification of the first code in the first portion. If the first code is correct, a check is made to verify whether the second portion of the decoded signal has a second correct code. A data block is considered to be detected either in the presence of first and second correct codes, or in the presence only of a first correct code when a data likelihood criterion, evaluated during the decoding of the second portion of the signal, is fulfilled.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to digital communication techniques, and more particularly to the reception of data blocks into which are inserted error detection codes.  
         [0002]     It is very usual to make provision for such codes in transmitted data block formats. They are generally codes appended to the data of the block by applying a systematic cyclic coding. One speaks of parity checking or CRC (“cyclic redundancy checksum”) codes.  
         [0003]     The invention is more particularly aimed at systems where provision is made for at least two codes of this kind within a data block, to check the integrity of distinct parts of the block: a header and one or more data fields.  
         [0004]     Often, the information contained in the header of a block is more important than that contained in the data fields. The header may thus specify information useful for the reception of the remainder of the data of the block or for the proper operation of the receiver (identification of the recipient of the block, format of the data, modulation, error correcting coding applied, information useful for the link layer protocols, etc). To further protect this information, a coding scheme that is more robust to errors is applied to the header and the former is furnished with a CRC distinct from that used for the data field.  
         [0005]     Such a distinction is for example envisaged for the transmission of data in the cellular radio communication system of EGPRS type (“Enhanced General Packet Radio Service”) standardized by the ETSI (“European Telecommunications Standards Institute”). This system makes it possible to transmit, between a base station and a mobile station, modulated-signal blocks that may form the subject of several modulations and of several transmission error correcting codings. The modulation and the coding are selected block by block by choosing a Modulation and Coding Scheme MCS from among nine schemes MCS-1 to MCS-9 specified in the ETSI EN 300 909 European standard, Digital cellular telecommunications system (Phase 2+); Channel coding (GSM 05.03, version 8.5.1, Release 1999) published by the ETSI in November 2000. Each of these schemes offers a level of data protection that is more or less robust and correspondingly a bigger or smaller information bit rate.  
         [0006]     The header of an EGPRS radio block comprises an 8-bit CRC, while its data field comprises a 12-bit CRC.  
         [0007]     The choice of the modulation and coding scheme applied to a block results from a link adaptation algorithm whose input variables comprise, for each mobile station, parameters representing the quality of reception observed by the receiver (the mobile station in the downlink direction or the base station in the uplink direction). Mediocre quality (high bit error rate) attests to difficult propagating conditions, and requires an increase in the protection against errors for the mobile concerned and hence a reduction in the useful bit rate.  
         [0008]     The presence of a block originating from a given mobile station or destined for the latter is decided by the receiver when the CRC of the header is correct and when the content of the header designates this mobile station. This decision makes it possible to update the relevant quality estimators.  
         [0009]     The size of the CRC of the header (8 bits) gives rise to a false detection probability of {fraction (1/256)}, i.e. around once every 5 seconds. When a false detection such as this occurs, the link adaptation relating to the mobile station designated in the header is disturbed. In particular, if no block were transmitted (noise alone), the quality indicator for the mobile station in question degrades, so that it undergoes a decrease in useful bit rate, although it perhaps benefits from excellent radio conditions.  
         [0010]     The link adaptation techniques are not the only ones that may be disturbed through the false detection of data blocks. For example, such is the case also for automatic block repetition (ARQ) techniques with incremental redundancy (IR). In these techniques, the data of a block badly decoded by the receiver are preserved by the latter and a repeat thereof is requested. The data preserved then enrich those obtained following the repeat so as to increase the probability of detection. It is important to prevent a block from being detected in error in the presence of noise alone since this may introduce confusion into the controlling of the sending of blocks.  
         [0011]     These drawbacks may be attenuated by increasing the size of the CRC of the header. However, this is to the detriment of the transmission bit rate, and is not possible in a system whose formats are already standardized.  
         [0012]     An object of the present invention is to minimise the risks of false detection of such blocks having several codes for error detection.  
       SUMMARY OF THE INVENTION  
       [0013]     The invention thus proposes a method for receiving data blocks each comprising a header and a data field including respectively first and second codes for error detection, comprising the steps of: 
        decoding a first portion of a signal received;     ignoring the signal received if the decoded signal of the first portion has a first incorrect code; and     if the decoded signal of the first portion has a first correct code, determining a block header in accordance with the first portion and for at least one value of this block header: 
            decoding a second portion of the signal received;     if the decoded signal of the second portion has a second correct code, validating the block header and a corresponding data field determined in accordance with the second portion; and     if the decoded signal of the second portion has a second incorrect code, validating the block header if a likelihood parameter evaluated during the decoding of the second portion fulfils a validation criterion and rejecting the block header if the likelihood parameter does not fulfil the validation criterion.    
               
 
         [0020]     Fortuitous detection of a block header in the presence of noise only will generally be prevented by virtue of the likelihood criterion applied. This criterion will not be fulfilled in the presence of noise alone whereas the reception under poor conditions of a relevant block (first code correct but second code incorrect) will generally give rise to a greater likelihood than that observed in the presence of noise only. The criterion may thus consist in comparing the likelihood parameter with a suitably chosen, fixed or adaptive, threshold.  
         [0021]     Another aspect of the invention concerns a device for receiving data blocks comprising processing means devised for implementing a method as defined hereinabove. The invention also proposes a cellular radio communication base station as well as a mobile station, including such a device for receiving data blocks.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a diagram of a GPRS type network to which the invention may be applied.  
         [0023]      FIG. 2  is an exemplary schematic diagram of a unit for controlling packets of such a network.  
         [0024]      FIGS. 3 and 4  are flow charts of procedures for receiving blocks according to two embodiments of the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]     The invention is described below in its non-limiting application to EGPRS type networks. These networks are developed for the transmission of data in packet mode in cellular networks of GSM (“Global System for Mobile communications”) type.  
         [0026]     The EGPRS network illustrated in  FIG. 1  is built on a GSM infrastructure, and conventionally divided into a network core, also called Network and Switching Subsystem or NSS, and a radio-access network also called Base Station Subsystem or BSS.  
         [0027]     For the packet service, the switches of the NSS are called GPRS Support Nodes or GSNs. A distinction is made between the SGSNs (Serving GSNs)  5  which are linked to the BSS through an interface called Gb, and the GGSNs (Gateway GSNs, not represented) which serve as a gateway with external packet transmission networks, such as the Internet, for example.  
         [0028]     A general description of the radio interface, called Um, between the mobile stations (MS)  10   a - 10   b - 10   c  and the base stations (BTS)  20   a - 20   b  of the BSS is provided in the technical specification ETSI TS 101 350, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2” (GSM 03.64, version 8.5.0, Release 1999), published by ETSI (European Telecommunications Standards Institute) in August 2000.  
         [0029]     Each base station  20   a - 20   b  is supervised by a base station controller or BSC  21  through an interface called Abis. In order to manage the transmission of GPRS packets, the BSS further comprises an entity  22  called packet control unit or PCU. The locating of the PCU within the BSS is not standardized. In the example represented in  FIG. 1 , the PCU  22  is situated between the BSC  21 , with which it communicates via an interface called Agprs, and the NSS, with which it communicates via the interface Gb.  
         [0030]      FIG. 2  illustrates a possible structure of a PCU  22  situated between an SGSN  5  and a BSC  21 , as in the example of  FIG. 1 . The reference  40  designates the Gb interface controller for the link with the SGSN  5 . The data manipulated by the PCU  22  between the Gb interface controller  40  and the Agprs interface controller  42  transit via a buffer memory  41  where packet queues are recorded.  
         [0031]     Between the PCU  22  and the BTS  20   a - 20   b , the information is carried by 320-bit frames of TRAU (Transcoder/Rate Adaptor Unit) type. These TRAU frames are formatted and processed by a module  44  and transmitted by way of synchronous interface circuits  45  which form MIC subchannels at 16 kbit/s with the BTSs  20   a - 20   b . Several 16-kbit/s (sub)channels can be multiplexed in time on the Agprs interface and separated by the BSC  21  for routing to the BTSs.  
         [0032]     A module  46  of the Agprs interface controller  42  implements the radio protocols of layer  2  of the SOI model, that is to say the RLC/MAC (Radio Link Control/Medium Access Control) protocols described in the European Standard ETSI EN 301 349, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol” (GSM 04.60, version 8.3.1, Release 1999), published by ETSI in October 2000.  
         [0033]     The RLC sublayer forms the interface with the upper-layer protocol, called LLC (Logical Link Control). It carries out the segmentation and the reassembling of LLC protocol data units (LLC-PDU). The MAC sublayer manages the multiplexing of the blocks pertaining to the various active Temporary Block Flows (TBF) on the physical channels available, by arbitrating between the various mobile users through a scheduling mechanism.  
         [0034]     The format of the RLC/MAC blocks used in an EGPRS system is described in the aforesaid European Standard ETSI EN 301 349. Nine modulation and coding schemes, called MCS-1 to MCS-9, are provided. They correspond to schemes for protection against transmission errors. The scheme used for a given block, as well as any puncturing scheme that may be applied, are indicated in a field CPS (“Coding and Puncturing Scheme indicator”) of the EGPRS RLC/MAC header. Each RLC block contains a number of bytes depending on the modulation and coding scheme adopted for this block. A block may be transmitted every 20 ms per physical channel.  
         [0035]     The EGPRS RLC/MAC header forms the subject of a channel coding distinct from that of the data of the block. The level of protection of this header against transmission errors is higher than that of the data so as to ensure better robustness of the signalling information. The header comprises in particular the following elements: 
        the transport flow indicator (TFI) which identifies the TBF to which the block pertains and hence the MS with is the receiver thereof (in the downlink direction) or the sender thereof (in the uplink direction);     the CPS;     information useful to the ARQ mechanism implemented in the RLC layer;     an 8-bit error detection code, herein below denoted CRC1.        
 
         [0040]     In addition to this header, the RLC/MAC block comprises a data field including a 12-bit CRC, hereafter denoted CRC2, for detecting badly received data.  
         [0041]     The technical specification TS 100 911, version 8.13.0, “Digital cellular telecommunications system (Phase 2+); Radio Subsystem Link Control” (3GPP TS 05.08 Release 1999), published in February 2002 by the 3GPP (“Third Generation Partnership Project”) organisation describes the radio measurements to be made by the BTSs and the MSs on signals received. These measurements group together levels and indicators of quality of the signals. Among the latter, the EGPRS system provides for the calculation by an MS, over a downlink and for each of the two possible modulations (GMSK and 8-PSK), of indicators related to the bit error probability (BEP) over a radio block. Found among them are in particular the MEAN_BEP which is a mean of the bit error probability over a radio block, and the CV_BEP which is a coefficient of variation of the bit error probability over a radio block, i.e.  
       CV_BEP   =           variance   ⁢           ⁢     (   BEP   )       MEAN_BEP       .         
 
 The BEP degrades abruptly when a block is detected in error by the MS although nothing was transmitted by the BTS. 
 
         [0043]     In the uplink direction, similar measurements may be made by the BTS in relation to each MS.  
         [0044]     The MS for which the indicators related to the BEP are to be updated following detection of a block is that which corresponds to the TFI included in the header of the block.  
         [0045]     The measurements made by the MS (for example the MS  10   a  of  FIG. 1 ) are uploaded to the PCU  22  in a PACKET DOWNLINK ACK/NACK message, as specified in paragraph 11.2.6a of the aforesaid Standard EN 301 349. The measurements performed by the BTS (for example the BTS  20   a  of  FIG. 1 ) are, for their part, transmitted to the PCU  22  in a TRAU frame.  
         [0046]     The PCU  22  implements a link adaptation algorithm to select a coding and modulation scheme for each radio link, on the basis of the measurements that have been uploaded to it. It avails itself for example of tables that give a correspondence between a pair (MEAN_BEP, CV_BEP) and one of the schemes MCS-1 to MCS-9. It is therefore important that this algorithm be executed on relevant input parameters, in particular the MEAN_BEP which is the most meaningful.  
         [0047]      FIG. 3  shows the procedure that the BTS can apply on reception of a radio signal segment liable to correspond to an RLC/MAC data block if an MS has sent such a block in the period considered. Given the translation (mapping) of the RLC/MAC blocks onto the physical layer, this signal segment is recovered in four radio bursts over a physical channel. After deinterleaving of the signal segment, the latter comprises a header portion in which the header of any block that may have been sent is located and a data portion in which the remainder of the data of any such block is located.  
         [0048]     In step  50 , the BTS proceeds to the channel decoding of the header portion in accordance with the code applied by the MS (cf. aforesaid ETSI EN 300 909 Standard). It then verifies whether the CRC1 recalculated agrees with that appearing in the decoded header (test  51 ). If the CRC1 is incorrect, the signal segment is ignored (step  52 ), the BTS considering itself to have picked up merely noise. If the CRC1 is correct, the BTS identifies the MS with the aid of the TFI found in the decoded header (step  53 ) then it decodes the data portion of the signal segment (step  54 ) in accordance with the code indicated in the CPS field of the decoded header. It then verifies whether the CRC2 recalculated agrees with that appearing in the decoded data field (test  55 ).  
         [0049]     If the CRC2 is correct, the RLC/MAC block is declared received without error (step  56 ). Otherwise, an additional test  57  is performed to evaluate a likelihood criterion.  
         [0050]     The decoding  54  being carried out with the aid of a Viterbi trellis decoder adapted to the convolutional code applied by the MS, this decoder is capable of providing, together with the values of the estimated symbols, a value of a metric that has been optimized in the trellis (see G. D. Forney, “The Viterbi Algorithm”, Proc. IEEE, Vol. 61, No. 3, March 1973, pp 268-278). This metric is indicative of the likelihood of the decoded symbols. Test  57  consists in comparing it with a predetermined threshold.  
         [0051]     If this threshold is not reached, the BTS ignores the signal segment, considering itself to have fortuitously observed a correct value of the CRC1 in the presence of noise. If the metric of the Viterbi decoder exceeds the threshold of test  57 , the BTS considers itself on the other hand to have actually received a block originating from the MS associated with the TFI, but that it received it under poor conditions (false CRC2). It then declares the block received, but with an error (step  58 ) so that this block will not be acknowledged. After step  56  or  58 , the BTS proceeds to the updating  59  of the BEP estimator for the MS associated with the TFI so that the MEAN-BEP and CV_BEP indicators are despatched to the PCU at the appropriate time. On the other hand, this updating  59  does not take place after step  52 , in particular when the likelihood criterion is not satisfied in test  57 .  
         [0052]     The probability of false detection of a block is small when the threshold of step  57  is high. Furthermore, the latter should not be taken too high so as to avoid rejecting blocks that have actually been received. On a metric scale normalized to 1, a threshold of 0.7 decreases the probability of false detection by 10% and a threshold of 0.82 decreases this probability by 99%. A preferred value of the threshold at 0.78 decreases the probability by 90%.  
         [0053]     The processing applied by the MSs for the reception of downlink blocks is illustrated by  FIG. 4 . It is very similar to that of  FIG. 3 . The difference is that when the CRC1 is correct, the MS simply verifies in step  63  whether the TFI is indeed allocated to it. The signal segment is ignored if it is another MS which is pertinent. If the TFI is indeed pertinent to the MS, the latter decodes the data portion of the signal segment and continues as indicated with reference to  FIG. 3 .  
         [0054]     In an application of the invention to an ARQ mechanism with incremental redundancy, the step  56  gives rise to an acknowledgement of the block, whilst the step  58  gives rise to a storing of the decoded symbols of the block and to a repetition request. The blocks eliminated by the test  57  will not disturb the ARQ mechanism.