Abstract:
To mash coverage discontinuities, e.g. of high and low bit rate zones, when transmitting especially multimedia data to a terminal ( 209 ), it is proposed to store the data of a multimedia data stream&#39;s basic flow to a basic cache memory ( 315 ) with higher priority than storing the data of at least one enhancement flow of a multimedia data stream to at least one enhancement cache memory ( 317, 319 ), at least when said terminal ( 209 ) is likely to leave a high bit rate zone. The invention addresses as well a multimedia module ( 205 ), a terminal ( 209 ) and a computer program product adapted to this handling of multimedia streams.

Description:
[0001]     The invention is based on a priority application EP 05 292 230.9 which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a method for transmitting data in a discontinuous coverage radio network to a terminal, said discontinuous coverage radio network comprising at least one high bit rate zone, and said discontinuous coverage radio network and/or said terminal comprising a cache memory. It further relates to a module for transmitting multimedia data in a discontinuous coverage radio network to a terminal, a terminal for use in a discontinuous coverage radio network for receiving multimedia data, as well as a computer program product for transmitting multimedia data in a discontinuous coverage radio network to a terminal.  
         [0003]     In mobile networks, the radio performances are usually not constant. Some kinds of traffic, e.g. real time traffic, have a higher priority compared to other kinds of traffic, for example in case of streaming or data transfer. When a terminal experiences poor radio conditions, the transmission usually stops. A current solution is to reduce the bit rate assigned to the mobile terminal to achieve continuity of service. In case of multimedia data transfer, this is particularly annoying for the user, as the quality of e.g. a video or a piece of music is significantly degraded.  
         [0004]     In order to provide a minimum overall quality to the network, usually call admission control mechanisms are implemented. As a consequence, the capacity of the radio network is dimensioned by worst condition, i.e. service is delivered for a guaranteed percentage of places and time. Thus, the capacity is very low.  
         [0005]     One known approach for trying to tackle this problem is to implement a video coder bit rate adaptation in function of the acknowledgements received by call admission control. In case of H.263 or MPEG-4, the bit rate varies by a factor of 5 between the minimum and the maximum data rate. The adaptation of the transmitted rate in 3G/HSDPA may vary by a factor of 50 at constant radio resources.  
         [0006]     This approach delivers good quality of service, but has several drawbacks: In order to avoid service interruptions, large buffers are used. However, filling these buffers depends on the radio conditions. Under poor radio condition, the buffer is filled only slowly. This results in a very long waiting time for the end-user, before the service starts. Furthermore, there are still situations, where interruptions of the service may occur. And, in cellular networks, the cell border determines the capacity by interferences with other cells and reduced intensity of the radio waves. Thus, at cell borders poor radio conditions can lead to even longer waiting times and eventually interruptions.  
         [0007]     The end-user quality of service is adversely degraded. In terms of quality of experience, e.g. changes in image fluidity are noticeable, in case of videos. Long waiting times before the service starts, and interruptions in the middle of a video or music sequence are quite annoying for the end-user.  
         [0008]     Accordingly, it is an object of the present invention to provide a method for transmitting data in a discontinuous coverage radio network with improved experienced quality for the end-user as well as a multimedia module for transmitting multimedia data, a terminal for receiving multimedia data and computer program product for transmitting multimedia data.  
       SUMMARY OF THE INVENTION  
       [0009]     In a first aspect of the present invention, this object is achieved by a method for transmitting data in a discontinuous coverage radio network to a terminal, said discontinuous coverage radio network comprising at least one high bit rate zone, and said discontinuous coverage radio network and/or said terminal comprising a cache memory, said method comprising the steps of:  
         [0010]     partitioning the cache memory into a basic cache memory and at least one enhancement cache memory;  
         [0011]     filling the basic cache memory with higher speed than the at least one enhancement cache memory, at least when said terminal is likely to leave the high bit rate zone.  
         [0012]     By managing the cache memory according to the present method, it is made sure that a least the basic part of data to be transmitted, i.e. the data necessary to provide a minimum service, is always sufficiently available, even in cases of disruptions of radio coverage, e.g. due to coverage discontinuities, because at least the cached basic data may still be transmitted by a low bit rate parent coverage network, or because, if caching is done at the terminal level, service delivery to the user can continue even with temporarily no coverage at all. As soon as the terminal is back in a high bit rate zone, full service can be resumed by caching and further transmitting also enhancement data, which is improving the service, but not is not indispensable.  
         [0013]     It will be noted, that it is not only possible to split one cache memory in different parts, one for basic data and at least one for enhancement data, but also to provide separate cache memories.  
         [0014]     In preferred embodiment of the method according to the present invention, multimedia data is transmitted, and the data of a multimedia data stream&#39;s basic flow is stored into the basic cache memory with higher priority than the data of at least one enhancement flow of the multimedia data stream is stored into the at least one enhancement cache memory, at least when said terminal is likely to leave the high bit rate zone.  
         [0015]     In order to improve experienced quality for the end-user the continuity of multimedia service is achieved according to the present invention by relying on caching the basic flow with priority. A multimedia data stream is often split into several flows depending on the relevance of the contained multimedia information: The basic flow contains all necessary data for delivering the multimedia stream with a basic quality. The enhancement flow(s) provide additional information, e.g. higher resolution, that is not really necessary for a basic quality of experience for the end-user. The splitting is possible through various existing.  
         [0016]     Especially, when the terminal receiving the multimedia data stream is approaching the border of a high bit rate zone, i.e. a known discontinuity of the radio network, the basic flow—containing all the absolutely necessary data for experiencing the multimedia without noticeable quality loss—is filled with high priority into the basic cache memory to make the basic flow data available even if the end-user leaves the high bit rate zone. Advantageously, the basic cache memory is always filled with high priority to mask unforeseeable coverage discontinuities or interruptions. Depending on capacity, the at least one enhancement cache memory is filled, too, and the at least one enhancement flow is transmitted to the terminal as well.  
         [0017]     There are two preferred ways of selectively filling the parts of the cache memory or the dedicated cache memories. One preferred way is to fill the basic cache memory and the at least one enhancement cache memory at constant rate, the basic cache memory being filled in priority and the at least one enhancement cache memory being filled, when resources are available. Thereby, optimal use of available resources is made. The other preferred way is to fill the basic cache memory with a higher bit rate than the at least one enhancement cache memory. Thereby, the quality of experienced service is optimized, as a maximum of enhancement flow data is delivered.  
         [0018]     In preferred embodiments of the present invention the method comprises the further steps of:  
         [0019]     estimating the terminal&#39;s travel time until entering again a high bit rate zone;  
         [0020]     filling at least as much data of the basic flow into the basic cache memory as needed for the time outside a high bit rate zone.  
         [0021]     By extrapolating the amount of data needed to bridge the time gap, while the end-user is not in a high bit rate zone, and anticipatorily filling the basic cache memory with at least this amount, it is made sure that the end-user will in any case get the demanded multimedia service without any disturbance in spite of the fact that he temporarily leaves the high bit rate zone. Furthermore, as the amount of basic flow data is more precisely known, eventually resources can be allocated to filling enhancement flow cache memories, too, or to any other tasks, while still having cached enough basic flow data for a good quality of experience.  
         [0022]     It has proven to be advantageous, in case the discontinuous coverage radio network comprises at least two high bit rate zones, to determine the travel time statistically based on the terminal&#39;s direction and speed of movement as well as based on the distance of any neighboring high bit rate zone. If there is only one high bit rate zone, one could use a database with probabilities of for how long an end-user leaves the high bit rate zone correlated with speeds and directions of movements.  
         [0023]     In a second aspect of the present invention, this object is achieved by a module for transmitting multimedia data in a discontinuous coverage radio network to a terminal, said discontinuous coverage radio network comprising at least one high bit rate zone, with  
         [0024]     a basic cache memory and at least one enhancement cache memory;  
         [0025]     means for storing the data of a multimedia data stream&#39;s basic flow to the basic cache memory with higher priority than storing the data of at least one enhancement flow of a multimedia data stream to the at least one enhancement cache memory, at least when said terminal is likely to leave the high bit rate zone.  
         [0026]     Preferred embodiments of the module according to the present invention comprise means for estimating the terminal&#39;s travel time until entering again a high bit rate zone, when the terminal is likely to leave the high bit rate zone.  
         [0027]     In a third aspect of the present invention, this object is achieved by a terminal for use in a discontinuous coverage radio network for receiving multimedia data, said discontinuous coverage radio network comprising at least one high bit rate zone, with  
         [0028]     a basic cache memory and at least one enhancement cache memory;  
         [0029]     means for storing the data of a multimedia data stream&#39;s basic flow to the basic cache memory with higher priority than storing the data of at least one enhancement flow of a multimedia data stream to the at least one enhancement cache memory, at least when said terminal is likely to leave the high bit rate zone.  
         [0030]     In a last aspect of the present invention, this object is achieved by a computer program product for transmitting multimedia data in a discontinuous coverage radio network to a terminal, said discontinuous coverage radio network comprising at least one high bit rate zone, said computer program product comprising program means for performing the step of:  
         [0031]     storing the data of a multimedia data stream&#39;s basic flow to a basic cache memory with higher priority than storing the data of at least one enhancement flow of a multimedia data stream to the at least one enhancement cache memory, at least when said terminal is likely to leave the high bit rate zone.  
         [0032]     Preferred embodiments of the computer program product according to the present invention comprise means for estimating the terminal&#39;s travel time until entering again a high bit rate zone, when the terminal is likely to leave the high bit rate zone. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     A detailed description of the invention is provided below. Said description is provided by way of a non-limiting example to be read with reference to the attached drawings in which:  
         [0034]      FIG. 1  is a flowchart explaining a preferred embodiment of the method according to the present invention;  
         [0035]     FIGS.  2  to  5  show the transmission of video data in a discontinuous coverage radio network with pico cells for various positions of a user terminal;  
         [0036]      FIG. 6  shows schematically a first embodiment of a multimedia module according to the present invention cooperating with a terminal;  
         [0037]      FIG. 7  shows schematically a multimedia module according to the present invention cooperating with an embodiment of a terminal according to the present invention;  
         [0038]      FIGS. 8 and 9  show different possibilities for managing the data transmission speed. 
     
    
       [0039]     The present invention will be described more in detail with reference to the example of an end-user asking for video streaming service in a heterogeneous network with a discontinuous radio coverage, specifically a low bit rate parent network with pico cells as high bit rate zones. It is to be understood, that the present invention ma be applied to transmission of any kind of data in any kind of discontinuous coverage radio network.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0040]     In  FIG. 1 , step  101  the end-user is in a pico cell and asks for video streaming service. Of course, the end user could as well be in a hot spot or any other high bit rate zone. At the beginning of the video streaming service delivery, the end-user is staying in the pico cell, and the multimedia network controlling the delivery sends the video stream separated into a basic flow and a first and second enhancement flow (step  103 ). Then, the end-user moves near to the pico cell border (step  105 ). The multimedia network reacts by extrapolating the most probable data volume of basic flow data needed for good quality of experience till reentering a pico cell (step  107 ). For extrapolation, the multimedia network determines the travel time statistically from the distances to the surrounding pico cells and the estimation of the end-user&#39;s velocity, i.e. his speed and direction.  
         [0041]     The multimedia network will then fill the basic cache memory with high priority (step  109 ), such that the basic cache memory contains the whole basic quality video stream corresponding to the travel of the end-user before reaching the next pico cell. It will start sending the basic flow data from the cache memory with high priority as soon as the end-user leaves the pico cell, instead of sending all flows. Consequently, for the end-user, the service does not seem to be interrupted. The video stream can still be playing during his travel in the parent coverage network (step  115 ).  
         [0042]     If there are enough resources available, the multimedia network not only fills the basic cache memory, but also the cache memory corresponding to the first enhancement flow (step  111 ). If, there are still enough resources, even the cache memory corresponding to the second enhancement flow is filled (step  113 ). The filling is done at constant rate. Depending on actual radio capacity and available resources, data from the first or even the second enhancement flow may be sent eventually to the end-user.  
         [0043]     As soon as the end-user enters the next pico cell he will again receive the full video stream through pico cell transmission (step  117 ).  
         [0044]     As a result of the present invention, the user terminal has a significantly higher availability and a better service quality, the network resources are used preferably in good radio conditions, and consequently the overall capacity is higher: The invention has been tested in 3G/HSDPA networks with 256 kps flows. A standard network of this type cannot handle more than 2 of such flows, and with only mediocre availability. By utilizing the caching technique according to the present invention, the same network can handle around 30 of such flow with excellent availability.  
         [0045]     The general concept of masking coverage discontinuities is illustrated in FIGS.  2  to  5 . The discontinuous coverage radio network is made of a parent coverage radio network  201  with low bit rate and high bit rate pico cells  207   a,b,c . The end-user in pico cell  207   a  asks for video streaming service through his terminal  209  ( FIG. 2 ). A high bit rate transmission connection is established via pico cell  209 , and multimedia module  205  in the parent coverage network  201  to video server  203 . The video stream may be received from the video server  203  as a basic flow containing the data necessary for a given experience quality of the video stream and one, two, three or more additional enhancement flows. It is also possible to separate the data stream into basic and enhancement flows at the network level, e.g. in the multimedia module  205 .  
         [0046]     While watching the video, the end-user moves around in the pico cell with a velocity {right arrow over (v)} 1 , i.e. with a certain speed and into a certain direction. By doing so, he comes near to the border of pico cell  207   a  ( FIG. 3 ). The multimedia network  205  determines the expected travel time, if the end-user continues to travel with the actual velocity {right arrow over (v)} 2  and depending on the distances D 1 , D 2  to the surrounding pico cells  207   b,c . To improve the accuracy statistical data concerning possible changes of velocity are also taken into account. Based on the resulting travel time, the multimedia network  205  estimates the needed amount of basic flow data needed to bridge the travel time outside a pico cell  207   a,b,c  without loss of experience for the end-user and fills the basic flow memory cache with at least this amount of data. If there are available resources left, the multimedia network fills also the at least one enhancement cache memory.  
         [0047]     It is also possible to continuously fill the basic cache memory and the at least one any enhancement cache memory, but with different bit rates, the basic cache memory being filled faster than the enhancement cache memory. There is always a provision of data available, at least in the basic cache memory, in case of deterioration or interruption of the transmission, not only when the end-user leaves temporarily a pico cell zone, as shown in  FIG. 4 .  
         [0048]     The low bit rate transmission through the parent coverage network  201  as shown in  FIG. 4  may either slowly continue to fill the basic cache memory, if it is located in the terminal  209 , or send the content of the basic cache memory, if it is located on the network side, for example in the multimedia module  205 . It is also possible to provide a basic cache memory in both terminal  209  and network  201  and filling the content of the network basic cache memory into the terminal basic cache memory. This applies mutatis mutandis for any enhancement cache memory.  
         [0049]     As soon as the end-user enters the next pico cell  207   c , the transmission is again handled with a high bit rate ( FIG. 5 ).  
         [0050]      FIG. 6  shows more in detail an embodiment of a multimedia module  205  according to the present invention. It receives the video data stream from the video server over the connection  301  with its receiver  311  that transmits it to the filler  314 . The video data stream is split up in a basic flow and one or more enhancement flows that are selectively filled as previously explained into dedicated parts of a cache memory, in the present example into basic cache memory  315 , first enhancement cache memory  317  and second enhancement cache memory  319 , wherein basic cache memory  315 , first enhancement cache memory  317  and second enhancement cache memory  319  are partitions of one cache memory. The amount of basic flow data to be filled with priority into the basic cache memory  315  is determined by the estimator  329  estimating the travel time to the next pico cell. The sender  321  then sends the data of the various flows with appropriate priority over connection  305  to terminal  209 .  
         [0051]     The multimedia module  205  shown in  FIG. 7  has a receiver  311  for receiving the video data stream and a separator  313  for separating the video data stream into a basic flow and 2 enhancement flows. These are then sent to the terminal  209  according to the present invention via connection  305 . A cache memory  323  for the basic flow and two cache memories  325 ,  327  for the enhancement flows are provided in terminal  209 . The controlling of the selective filling of the cache memories  323 ,  325 ,  327  is done by the filler  331 . As mentioned before, it is also possible to operate a multimedia network with cache memory together with a terminal with cache memory. Furthermore, it is possible to work with only one enhancement cache memory or three, four or more enhancement cache memories.  
         [0052]     The filling of the basic and enhancement parts of a cache memory can for example be handled according to two examples illustrated in  FIGS. 8 and 9 , wherein the multimedia server send data separated into a basic flow and, for example, two enhancement flows. The possibility shown in  FIG. 8  uses a standard RTP server, which delivers data at constant rate to the radio network. The filling of the different parts of the cache memory is done (curve A), and the delivery starts (curve B), when the data level at least in the basic cache memory is sufficient i.e. after waiting time T w . The filling and the delivery are done at least with the same speed a, to ensure that the cache memory is always at least partially filled to mask any discontinuities in transmission.  
         [0053]     According to  FIG. 9 , the basic cache memory part is filled at twice the normal speed, the first enhancement cache memory part at 130% of the normal speed, and the second enhancement cache memory part at normal speed. Thus, the filling (curve C) speed b is higher than delivery (curve D) speed g. this reduces the waiting time T w ′=(g/b)*T w . The cache memory and their filling and delivering can be regulated by RTSP usage, if the cache memory is near saturation.  
         [0054]     The configuration of the present examples of  FIGS. 8 and 9  is in either case such that concerning service delivery the basic flow may be received from both the slow parent coverage network and the fast network, e.g. high bit rate cells, with a preference for reception from the fast network, the slow network being used, when the cache memory is almost empty. Thus, in classical cellular networks, caching facilitates the service delivery in adverse radio conditions and also permits to make use of the extra capacity available in good radio conditions.  
         [0055]     It will be noted, that the cache management as part of the present invention, including the estimation of a terminals travel time, may be implemented as computer program product.  
         [0056]     Although having described several preferred embodiments of the invention, those skilled in the art would appreciate that various changes, alterations, and substitutions can be made without departing from the spirit and concepts of the present invention. The invention is, therefore, claimed in any of its forms or modifications with the proper scope of the appended claims. For example various combinations of the features of the following dependent claims could be made with the features of the independent claim without departing from the scope of the present invention. Furthermore, any reference numerals in the claims shall not be construed as limiting scope.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
         
           
               101 - 117  step  
               201  discontinuous coverage radio network  
               203  video server  
               205  multimedia network  
               207   a,b,c  pico cell  
               209  user terminal  
              D 1 , D 2  distance  
              {right arrow over (v)} 1 , {right arrow over (v)} 2 , {right arrow over (v)} 3 , {right arrow over (v)} 4  velocity  
               301  connection to video server  
               305  connection to user terminal  
               311  receiver  
               313  separator  
               314  filler  
               315  cache memory  
               317  cache memory  
               319  cache memory  
               321  sender  
               322  sender  
               323  cache memory  
               325  cache memory  
               327  cache memory  
               329  estimator  
               331  filler  
              T w , T w ′ waiting time  
              A, C data volume in cache memory  
              B, D data volume out of cache memory  
              a, b, g, d transmission speed