Abstract:
In a GPRS or EDGE or UMTS system, a real-time video service is provided by selecting one of a small number, e.g. four, predetermined channel coding rates applicable for video, applying the rate to the video data in the application layer, and transmitting the data over the radio interface to a mobile system together with, for each burst, a header indicating the selected coding rate and a temporary flow indicator. The short header allows video payload capacity to be increased. One of the predetermined channel coding rates is a {fraction (1/1)} or transparent rate.

Description:
CROSS-REFERENCE TOP RELATED APPLICATION  
         [0001]    This application claims priority of European Patent Application No. 00301207.7, which was filed on Feb. 16, 2000.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a digital mobile telecommunications system such as the General Packet Radio Service (GPRS), the Enhanced Data Rate for GSM Evolution (EDGE) and the Universal Mobile Telecommunications System (UMTS). Such systems are being developed to accommodate data users in addition to voice users.  
           [0004]    2. Description of the Related Art  
           [0005]    In such systems, for example GPRS/EDGE, in the link layer of the 7-layer protocol stack, application packets are segmented into fixed size Radio Link Control (RLC) blocks, which also include a header. The RLC blocks are then distributed into bursts which occupy one time slot in the GPRS/EDGE system.  
           [0006]    In the ETSI Tdoc SMG2 WPA 127/99, WPB 003/99, “Two burst based link quality control proposal for EGPRS”, January 1999, the major telecommunications companies have agreed on a scheme for general data. In this scheme, there are several combinations for modulation and coding schemes. The header formats, channel coding and interleaving techniques differ from one scheme to another.  
           [0007]    Video data streams have different characteristics to other data streams. In video, the bit rate is variable. Although a rate controller can be used to stabilize the average bit rate in a certain time interval, the number of bits generated in each frame or picture block varies significantly. Furthermore, the error sensitivities of the parameters and particular bits within those parameters are highly dependent on the bit stream context.  
           [0008]    Also, parameters in the video bit stream have variable length codes. This can cause loss of synchronisation. This means that a single bit error may be equivalent to a burst of error. Use of existing interleaving schemes spreads the errors within the video bit stream, causing more damage.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the invention to provide a method and apparatus for coding video data streams to allow their efficient transmission by a mobile telecommunications system.  
           [0010]    According to the invention in a telecommunications system, a method of supplying a real-time video data service characterized by the steps of defining a plurality of channel coding rates applicable to video data, said plurality including a {fraction (1/1)} coding rate; selecting one of said rates and applying it to the video data; and transmitting the coded video data over a link to a video receiver.  
           [0011]    Also according to the invention, a mobile radio telecommunication system comprising a core network, at least one Support Node, at least one Radio Network Controller and at least one Mobile Station, the system being arranged for supply of a real-time video service to said Mobile Station characterized in that said system is arranged to select one of a plurality of channel coding rates, said plurality including a {fraction (1/1)} rate, to apply said selected rate to a video signal, and to supply the coded signal to said Mobile Station.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will be described by way of example only with reference to the accompanying drawings in which:  
         [0013]    [0013]FIG. 1 is a highly schematic illustration of a mobile telecommunication system;  
         [0014]    [0014]FIG. 2 illustrates the 7 layer protocol stacks;  
         [0015]    [0015]FIG. 3 a  illustrates a downlink header;  
         [0016]    [0016]FIG. 3 b  illustrates an uplink header;  
         [0017]    [0017]FIGS. 4 a  and  4   b  illustrate RLC/MAC format for transparent downlink and uplink modes of coding;  
         [0018]    [0018]FIGS. 4 c  to  4   e  illustrate RLC/MAC format for three other modes of coding for the downlink; and  
         [0019]    [0019]FIG. 5 illustrates interleaving and mapping for the transparent mode.  
     
    
     DETAILED DESCRIPTION  
       [0020]    In FIG. 1 in the GPRS system  10 , a Mobile Station (MS)  12  having a wired connection R to Terminal Equipment (TE)  14  such as a laptop computer, a modem, a video camera etc., is connected through a radio interface Um to a Radio Network Controller (RNC)  16 . The RNC  16  is connected through a radio interface Iu-ps to an Enhanced Gateway GPRS Support Node (E-GGSN)  17  and through the E-GGSN to an Enhanced Serving GPRS Support Node (E-SGSN)  18 . The E-GGSN connects through interfaces Gi to a Medium GateWay  19  and then the Public Switched Telephone Network (PSTN)  20 , and also to a Multimedia Internet Protocol (IP) network  21 .  
         [0021]    RNC  16  transmits signals to the cell within which a Mobile Station  20  is located. Communications between the RNC  16  and the MS  20  are controlled by a 7 layer protocol stack, FIG. 2. The lowest layer  22  comprises the GSM Radio Frequency layer, also known as the physical layer. The second lowest layer  24  is the Radio Link Control/Medium Access Control RLC/MAC layer, above which lies the Logical Link layer (LLC)  26 . Higher layers  28  are not relevant to the present invention.  
         [0022]    The system of FIG. 1 and the protocol stack of FIG. 2 are standard arrangements, however the areas which are altered to allow implementation of the invention are shown shaded.  
         [0023]    [0023]FIG. 3 a  illustrates an RLC/MAC header for use with video data streams according to the invention. The header comprises three fields, an Uplink State Flag (USF)  30 ; a Temporary Flow Identity (TFI)  32 ; and Coding Schemes (CS)  34 .  
         [0024]    The USF is 3 bits in length and the TFI is 7 bits in length. Both are defined in ETSI, EN301 349, GSM 4.60 Version 6.3.0, 1999-04.  
         [0025]    Comparison of the header according to the invention with prior art standard headers will show that it is substantially shorter in length. The final block indication or sequence counter has been removed. This functionality is achieved by using the End Of Sequence (EOS) code in the video bit stream.  
         [0026]    In the uplink from the MS  12  to the BTS  18 , there is no need for USF  30 , and the uplink header is illustrated in FIG. 3 b;  it comprises a TFI field  32 ′ and a CS field  34 ′.  
         [0027]    The CS field  34  specifies the channel coding rate for the video payload. Four options are provided, i.e.,  
                                                   CS   CODING RATE                           00   1/1           01   2/3           10   1/2           11   1/3                      
 
         [0028]    When CS is 00, no channel coding is performed, and this will be referred to as the transparent mode. In this mode, the video data is not protected by channel coding schemes at all. This mode can be used when propagation conditions are good.  
         [0029]    The four modes corresponding to the four coding rates will now be described in more detail.  
         [0030]    In the transparent mode RLC/MAC format illustrated in FIG. 4 a,  there are eight stealing bits  38 , which are used to indicate that the payload includes video data. In the USF  30 , a (36, 3) Block Code is applied and there are 36 bits in the corresponding block  40 . The TFI  32  and CS  34  are combined in the header block H; in addition to the nine header bits, there are three tailing bits; a (⅓, 4) convolutional coding is applied, and the header block  42  has 36 bits. The video payload VP has 1312 bits in its block  44 . The total block length is 1392 bits.  
         [0031]    [0031]FIG. 4 b  shows the RLC/MAC format for transparent mode uplink. There are eight stealing bits  50 . The header H  46  has 9 bits for TFI and CS plus 3 tailing bits. A (⅓, 4) convolutional coding is applied and the block  52  has 36 bits. The video payload VP  48  has 1312 bits, carried over to block  54 . An additional block  56  called a time diversity (TD) block has 36 bits to give a total block length of 1392 bits. The 36 bits in block  56  allow the application of time diversity, and repetition of the header  46 , indicated by the dotted line TD. The repetition can reduce errors, unless both blocks  52 ,  56  are damaged when error correction cannot be made.  
         [0032]    [0032]FIGS. 4 c,    4   d  and  4   e  show only the RLC/MAC downlink formats for the other three CS codes. The same coding schemes are used for the corresponding uplinks.  
         [0033]    In FIG. 4 c  for CS field equals 01 and coding rate ⅔, again there are 8 stealing bits in block  60 . A (36, 3) Block Code is applied to the 3 bit USF, and there are 36 bits in block  62 . The header H of 9 bits again has an additional 3 tailing bits; a (⅓, 4) convolutional coding is applied and block  64  has 36 bits. The video payload (VP) has 432 bits plus 6 tailing bits. A (⅓, 7) convolutional coding is applied with puncturing 2 bits, giving 1312 bits in block  66 . In FIG. 4 d  for CS field equals 10 and coding rate ½, blocks  70 ,  72 , and  74  are similar to block  60 ,  62  and  64  in FIG. 4 c.  The video payload VP is 650 bits plus 6 tailing bits. A (⅓, 7) convolutional coding is applied with puncturing 656 bits giving a 1312 bit block  76 .  
         [0034]    In FIG. 4 e  for CS field equal to 11 and coding rate ⅓, blocks  80 ,  82 ,  84  are similar to blocks  60 ,  62 ,  64 . The video payload VP is 868 bits and there are 6 tailing bits. A (⅓, 7) convolutional coding is applied with puncturing 1310 bits, giving a 1312 block  86 .  
         [0035]    The bit rates for the video data are as follows.  
                                                       VIDEO BIT RATE           CODING RATE   (kbps)                           1/1   65.6           2/3   43.4           1/2   32.5           1/3   21.6                      
 
         [0036]    The transparent mode allows a higher video bit rate to be transmitted over a telecommunications system than has previously been possible.  
         [0037]    The selected one of the four CS is applied to the video data stream by a video encoder in TE  14  in FIG. 1. The video encoder implements its own error protection schemes, which are naturally suitable for video. In effect, the RLC/MAC layer  24  in FIG. 2 is transparent, and channel coding is performed in the physical layer  22 . The channel coding overhead will continue to exist in all parts of the network. However, since the main bandwidth limitation is in the air interface, it is preferable to optimize the efficiency between the MS  12  and RNC  16 . The selection of the coding rate is thus made by the video encoder in accordance with either video quality of service or channel conditions. For example, if the channel is very bad, a more powerful channel coding scheme is selected.  
         [0038]    From FIGS. 4 c  to 4e it will be clear that all the bits of the RLC block, which have been subjected to channel coding, have also been interleaved and mapped into four bursts as conventionally applied in the EDGE system. However, the payload in the transparent mode, FIGS. 4 a  and  4   b,  is not interleaved at all.  
         [0039]    One option is to disable the standard EDGE interleaver when sending video data by the transparent mode. The EDGE interleaver is located in the physical layer in the MS  12  in FIG. 1. However this may be impractical or impossible for existing mobiles.  
         [0040]    An alternative inventive method is proposed. The payload is first de-interleaved to cancel out the subsequent interleaving process in the EDGE interleaver. The arrangement is illustrated in FIG. 5. The stealing bits of block  38  and the header block  42  are interleaved to each of four bursts  90 ,  92 ,  94 ,  96  as shown by the full lines. However, the video payload  44  is divided into four parts A B C D and one part is supplied to each burst, as shown by the dotted lines. Thus part A is supplied to burst  90 , part B to burst  92  etc; i.e. the parts in A B C D are mapped to the bursts  90 ,  92 ,  94 ,  96  in order.  
         [0041]    The advantages of the invention are that the RLC block header is made much smaller than in conventional arrangements so that the video payload capacity is increased. More importantly, when the transparent mode is used and channel protection is performed in the application layer, the same level of protection provided by non-selective ⅓ convolutional coding can be achieved at rates lower than ½. The extra capacity made available can be used to improve error resilience or video quality or both.  
         [0042]    While the invention has been described in respect to a mobile telecommunications system, it will also find application in the fixed packet switch telecommunications network when real-time video services are supplied.