WO2003001826A2

WO2003001826A2 - Method and apparatus for transmitting data

Info

Publication number: WO2003001826A2
Application number: PCT/EP2002/004858
Authority: WO
Inventors: Stephen Hill; Stephen John Barrett; Stephen Aftelak
Original assignee: Motorola Inc
Priority date: 2001-06-22
Filing date: 2002-04-29
Publication date: 2003-01-03
Also published as: GB2376849B; WO2003001826A3; GB0115397D0; GB2376849A

Abstract

This invention relates to a method and apparatus for transmitting data. A source encoder (203) receives data from a data source (201). The source encoder (203) produces a data stream that has a varying data rate. This stream is fed to a data segmenter (205), which packetises the data stream before feeding it to a data compressor (209). The data compressor (207) compresses the data stream so that it can be transmitted over a continuous circuit switched link which has a link data rate which is lower than the maximum data rate of the data stream. The compression is variable such that only data packets having data rates higher than the link data rate are compressed. Such compression is either lossy or loss-free compression depending on the compression level required. The compressed data is transmitter using a conventional transmitter (211) and received by a conventional receiver (213). The invention is applicable to cellular communication systems.

Description

METHOD AND APPARATUS FOR TRANSMITTING DATA

Field of the Invention

This invention relates to a method and apparatus for transmitting data over a shared resource in a communication network.

B ackground of the Invention

In a cellular communication system each of the remote terminals (typically mobile stations) communicates with typically a fixed base station. Communication from the remote terminal to the base station is known as uplink and communication from the base station to the remote terminal is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of remote terminals 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.

As a remote terminal moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the remote terminal and the base station of the first cell, to being between the remote terminal and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.

All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a remote terminal is routed through the fixed network to the destination specific for this call. If the call is between two remote terminals of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other remote terminal currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a remote terminal and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.

A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the remote terminals and the base stations. This spectrum must be shared between all remote terminals simultaneously using the system.

One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of remote terminals can be accommodated in the same wideband spectrum by allocating different codes for different remote terminals. Codes are chosen to minimise the interference caused between remote terminals typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in 'Spread Spectrum CDMA Systems for Wireless Communications', Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858- 5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe. Traditional traffic in mobile cellular communication systems has been circuit switched voice data where a permanent link is set up between the communicating parties. In the future it is envisaged that data communication will increase substantially and typically the requirements for a remote terminal to transmit data will not be continuous but will be at irregular intervals. Consequently, it is generally considered to be inefficient to have a continuous link set up between users. Therefore a significant increase in packet based data traffic is expected, where the transmitting remote terminal seeks to transmit the data in discrete data packets when necessary. An example of a packet based system is General Packet Radio Service (GPRS) introduced to the Global System for Mobile communication (GSM). Further details on data packet systems can be found in

'Understanding data communications: from fundamentals to networking, 2nd ed.', John Wiley publishers, author Gilbert Held, 1997, ISBN 0-471-96820-X.

In a packet based system where a high number of remote terminals may require resources for packet transmissions at unknown and irregular intervals, it is important for optimal utilisation of the limited resource to schedule the order and time for transmission of the individual packets. This becomes even more important when different data packets have different requirements with respect to delay, bit error rate etc. Therefore most packet based systems contain schedulers which control when the individual data packets are transmitted and therefore share the available resource, whether time-slots in a TDMA system or power and codes in a CDMA system. An introduction to schedulers can be found in 'Service discipline for guaranteed performance service in packet-switching networks', Hui Zhang, Proceedings of the IEEE, volume 83, no. 10, October 1995.

The problem of efficient resource utilisation is especially pertinent to bursty data streams where the instantaneous data rate may vary significantly. An example of such a bursty data stream is a video signal where a source encoder typically will have an output data rate that varies significantly for the same quality level dependent on the nature of the picture being encoded. For example, a fast moving image with high level of detail will generate a much higher data rate than a relatively static image with low level of detail. As the nature of the image typically changes during a transmission the data rate may vary by several orders of magnitude.

One method for allocating resource to a bursty service is by providing resource allocation sufficient to allow for the maximum data rate. However, this would clearly be inefficient as for most of the time only a fraction of the allocated resource would be used.

Another method is to packetise the data stream and allocate part of a shared resource to the service. The packetising may result in varying data packet sizes and/or varying intervals between transmission of packets. By using a common resource for a high number of bursty traffic services statistical multiplexing results in an improved efficiency in the utilisation of the resource. However, in this case the quality of service (for example in terms of dropped packets due to interference from other services) will depend on other services sharing the resource. This complicates providing a guaranteed quality of service because this is dependent upon the statistics of all the packet streams. These statistics may be difficult to obtain, and the admission control decision is computationally complex to implement.

A need therefore exists for a system for transmitting data in which one or more of the abovementioned disadvantages may be alleviated.

Summary of the Invention

Accordingly there is provided a method for transmitting data over a shared resource in a communication network comprising the steps of : receiving a data stream having a varying data rate; provisioning from the shared resource, a circuit switched data link having a link data rate below the maximum data rate of the data stream; compressing the data stream to a compressed data stream having a compressed data rate not higher than the link data rate using variable data compression; and transmitting the compressed data stream over the circuit switched data link.

Preferably the variable data compression is such that a minimum compression factor is used and the data stream and compressed data stream comprise a plurality of data packets. According to a feature of the invention the variable data compression comprises both lossy and non-lossy compression dependent on the compression level required to compress the data stream.

According to a second aspect of the invention there is provided, an apparatus for transmitting data over a shared resource in a communication network comprising: means for receiving a data stream having a varying data rate; means for provisioning from the shared resource, a circuit switched data link having a link data rate below the maximum data rate of the data stream; means for compressing the data stream to a compressed data stream having a compressed data rate not higher than the link data rate using variable data compression; and means for transmitting the compressed data stream over the circuit switched data link.

Thus the advantages of the invention includes the provision of efficient data communication while allowing the resource for transmitting data to be independent of other data services being supported. It also includes the advantage of data compression having a low complexity and computational resource requirement as it enables compression to be as low as possible to provide for data communication at the link data rate.

Brief Description of the Drawings

An embodiment of the present invention is described below, by way of example only, with reference to the Drawings, in which: FIG. 1 is an illustration of a cellular communication system according to prior art;

FIG. 2 illustrates a block diagram of an apparatus for transmitting data in accordance with an embodiment of the invention;

FIG. 3 illustrates a packetised data stream having a varying data rate; and

FIG. 4 illustrates how the data packets of FIG. 3 are affected by the data compressor in accordance with an embodiment of the invention.

Detailed Description of a Preferred Embodiment

Fig. 2 illustrates a block diagram of an apparatus for transmitting data 200 in accordance with an embodiment of the invention.

A data source 201 generates data to be transmitted. A specific example, is a video data source, which records a picture and digitises these. The digitised signal is fed to a source encoder 203 which generates a digital video signal based on the digitised signal from the data source.

The output of the source encoder 203 is connected to a data segmenter 205 which receives a data stream (the digital video signal) having a varying data rate from the source encoder 203 and separates this data stream into discrete data packets. The data segementer 205 is connected to a data compressor 207. The data compressor 207 is also connected to a resource provisioner 209 for provisioning a circuit switched data link from a shared resource, such as a shared common channel on the air interface. Alternatively, the total air interface resource may be seen as the shared resource from which a circuit switched data link is provisioned. The circuit switched data link is provisioned such that the link data rate is below the maximum data rate of the data stream from the data segmenter 205. The data compressor 207 compresses the data stream to a compressed data stream having a compressed data rate not higher than the link data rate. This compression is done by use of variable data compression.

The data compressor 207 is connected to a transmitter 211 which transmits the compressed data stream over the circuit switched data link provisioned by the resource provisioner 209.

The transmitted compressed data stream is received by a receiver 213, which is connected to a de-compressor 215. The de-compressor 215 decompresses the received data output from the receiver in a process that reverses the function of the compressor 207. The data stream output by the de-compressor 215 is then processed in a conventional fashion.

Further aspects, variations and details of embodiments of the invention are described in the following.

Depending on the source, the data source and source encoding will often result in a data stream, which has a significantly varying data rate. For example, a data source for a speech may consist in a microphone, an analog amplifier and a digital to analog converter, which samples the amplified microphone signal at a fixed resolution and at fixed intervals. This generates a fixed data rate signal, which is however a very inefficient representation of the signal. The signal is therefore fed to the source encoder which encodes the microphone signal as a digital speech signal. This encoding is based on knowledge of speech signals, as is well known in the art, and therefore provides a highly efficient signal containing sufficient information to regenerate a speech signal at the receiver. However, as the information of speech signals varies during speech some voice encoders have a variable output rate. For example, during pauses in the speech there is virtually no important information to convey and therefore the source encoder will reduce its variation to a minimum data rate. Other examples of variable source encoding include video signals where the data rate depends on the level of detail and changes in the picture. In the current embodiment, the output of the source encoder is fed to a data segmenter, which segments the data into discrete packets. In other embodiments the source encoder directly outputs data in discrete packets. FIG. 3 illustrates a packetised data stream having a varying data rate. The data stream represents the output of the data segmenter, which is fed to the data compressor, hi this example, each data packet has a fixed duration but the data size and thereby the data rate of each packet varies. Thus packet 301 has a higher data rate than packet 303. The maximum data rate of the packetised data stream is given by data packet 307. The minimum data rate is zero as for some time intervals no data packets are generated (e.g. corresponding to the silence periods for a speech signal).

Transmission of a varying data rate data stream such as shown in FIG. 3 is problematic. One option is to allocate a circuit switched link between transmitter and receiver. However, in order to ensure that the data stream can be communicated over this link conventional approaches require that a link is provisioned which has a data capacity equal or larger than the maximum data rate. It is clear that for the majority of the time this data link will not be fully utilised and therefore that a significant loss in capacity is incurred.

Alternatively, systems exist which uses a fully packetised approach where packets are sent on a common resource shared with other packet systems. Due to statistical multiplexing between the packet sources, a more efficient resource utilisation is obtained. However, the quality of service and indeed whether a given data packet can be communicated is dependent on other users and therefore it is complicated to guarantee a given quality of service.

In accordance with the described embodiment of the invention, the resource provisioner provisions a circuit switched data link. However, contrary to conventional approaches this provisioning is such that the data capacity of the link is below the maximum data rate of the varying data stream. Instead the resource provisioner allocates a lower resource level so that the link data rate is below the maximum data rate of the data stream.

The exact data rate of the provisioned link can depend on many factors such as the quality required, the cost of the resource, the available bandwidth, the complexity requirements for compressing etc. It may dynamically be determined or may be predetermined for a given service, hi the preferred embodiment, a given service has a predetermined link data rate dependent on the service determined to give a reasonable quality level, for example if a speech signal is to be transmitted the link data rate is set to 4 kbps and if a video signal is transmitted it is set to 384 kbps. Hence, in this embodiment, the resource provisioner is informed or detects that a speech signal is to be transmitted and consequently it provisions a 4 kbps circuit switched link.

It will be clear that the provision of the circuit switched link can be done according to any known method or system for resource allocation without detracting from the current invention.

When a circuit switched data link has been provisioned, the varying data rate data stream is transmitted over this link.

For this purpose the data compressor uses variable data rate compression to generate a compressed data stream having a compressed data rate not higher than the link data rate using variable data compression.

In accordance with the embodiment, the variable compression is in response to the link data rate and the current rate of the varying data rate. Preferably this is such that a minimum of available compression is used which will result in a compressed data rate below the link data rate.

FIG. 4 illustrates how the data packets of FIG. 3 are affected by the data compressor in accordance with an embodiment of the invention. In FIG. 4 the dashed line 401 illustrates the link data rate (or circuit bandwidth) of the provisioned circuit switched link. Data packets 309, 311 and 315 all have a data rate, which is below the link data rate. They can therefore be transmitted using the circuit switched link without any requirement for compression and they are therefore left unaffected by the data compressor. Data packets 303 and 305 have a data rate, which is higher than the link data rate, and therefore the variable data compression is applied to these packets.

hi the preferred embodiment, the data compressor comprises means for operating a plurality of different compression algorithms having different compression factors. Typically, compression algorithms which provides higher compression factors will be more complex than for lower compression factors and therefore require higher computational resources. Data compression can either be lossy or loss-free. For loss free compression no information is lost and the original data stream can be recovered. For lossy data compression some information is lost and consequently the quality of the underlying signal (e.g. speech or video signal) may be degraded. Typically, the higher the compression factor the higher the degradation in quality. In the preferred embodiment the data compressor comprises functionality for running both lossy and loss-free compression algorithms with different compression factors.

The data rate of the data packets 303 and 305 is of such magnitude that the compression factor required to reduce the data rate to that of the link data rate can be achieved by a loss-free data compression algorithm. The data compressor therefore selects an appropriate loss-free compression algorithm and applies it to the data packets (note different compression algorithms and factors are used for the two data packets in the given example). Consequently, the compressed data packets 303 and 305 can now be transmitted over the circuit switched link.

The data rate of data packets 301 , 307 and 313 exceed the link data rate by a significant margin. In this example, it is not feasible to use a loss-free compression algorithm and instead a lossy compression algorithm is chosen to achieve the required compression factor. In one embodiment an appropriate lossy compression algorithm is chosen and applied to the whole data packet. However, in the preferred embodiment a combination of two or more compression algorithms are used such that part of the data packet is compressed using a lossy data compression algorithm and part of the data is compressed using a loss-free data compression algorithm, h the example of FIG. 4, both loss-free and lossy data compression is used.

It will be clear that any known variable data rate compression technique, including any compression or combination of compression algorithms, can be used without detracting form the current invention. This includes both source compression algorithms where the nature of the underlying signal is used in the data compression (typical of lossy data compression algorithms such as the well known MPEG compression techniques), as well as source independent algorithms which compresses the data without consideration of the underlying signal (typical of loss-free compression algorithms).

Following the compression, all data packets have a compressed data rate below or equal to the link data rate and can therefore be transmitted using the circuit switched link. This is done in the transmitter, which is well known in the art. Similarly the receiver is a conventional receiver and the de-compressor reverses the function of the compressor. In some embodiments the de-compressor can automatically determine the compression scheme used by the data compressor from the received compressed data stream but in the preferred embodiment the data compressor embeds signalling information in a data packet header. This information contains information on which compression algorithm(s) have been used on which data. The receiver decodes this information before de-compression and uses the information to select the appropriate de-compression algorithm(s).

In the preferred embodiment, the output to the data compressor is packetised and is transmitted in packets over the air interface. The data compressor compresses these packets or segments individually. The compression is thus performed on a per data packet basis such that each packet is compressed independently of other data packets. In some embodiments, a plurality of data packets from the data segmenter is combined into a single data packet on the air interface and in this case the compression may be done individually for each input data packet to the data compressor or may be done individually for each air interface data packet only. In the preferred embodiment the decompression performed at at the receiver can then be performed individually and independently for each received data packet resulting in low complexity receivers as well as a high immunity to errors as transmission errors will only affect small units of data (i.e. a single data packet).

The above description has focussed on an embodiment having functionality divided into separate functional units. However, it will be apparent that many variations of distributing and combining the described functionality is possible. For example the data source, source encoder, data segmenter and data compressor may be implemented as a single functional entity and the intermediate signals described in the above embodiment may not be present. For example the data source and source encoder may not be separable but be a single functional unit. As a specific example of this, the sampling of a signal may be controlled according to the content of the underlying signal e.g. by reducing the sample rate of a speech signal during quiet periods or reducing the sampling resolution during high signal levels. In other embodiments the source encoder comprises functionality for directly generating a signal having a data rate below the link data rate of the switched circuit link. In this case the data compression is an integral and inseparable part of the source encoding process.

The preferred embodiment is especially applicable to a cellular mobile communication system such as GSM or UMTS. The apparatus and method may be embedded in the User Equipment such as a mobile phone, user terminal or subscriber unit, or may equally well be used in the fixed network and specifically in a base station. It is thus equally applicable to uplink and downlink.

The invention therefore provides a number of advantages including: Providing a method of guaranteeing a quality of service, which is independent of other users.

Reducing complexity and processing resource of compression algorithms necessary to transmit date. This furthermore reduces power consumption and therefore extends battery life, which is a key parameter for mobile terminals.

Facilitating resource allocation in communication system having services with varying data rates - i.e. for bursty services.

Claims

1. A method for transmitting data over a shared resource in a communication network comprising the steps of : receiving a data stream having a varying data rate; provisioning from the shared resource, a circuit switched data link having a link data rate below the maximum data rate of the data stream; compressing the data stream to a compressed data stream having a compressed data rate not higher than the link data rate using variable data compression; and transmitting the compressed data stream over the circuit switched data link.

2. A method as claimed in claim 1 wherein the variable data compression is such that a minimum available compression factor is used.

3. A method as claimed in any previous claim wherein the data stream comprises a plurality of data packets.

4. A method as claimed in claim 3 wherein each of the plurality of data packets of the data stream is individually compressed.

5. A method as claimed in any previous claim wherein the compressed data stream comprises a plurality of data packets.

6. A method as claimed in any previous claim wherein the variable data compression is a lossy compression.

7. A method as claimed in any previous claim wherein the variable data compression is a non-lossy compression.

8. A method as claimed in any previous claim wherein the variable data compression comprises both lossy and non-lossy compression dependent on the compression level required to compress the data stream.

9. A method as claimed in any previous claim wherein the variable data compression comprises source compression.

10. A method as claimed in any previous claim wherein the variable data compression comprises data compression independent of the information content of the data stream.

11. A method as claimed in any previous claim wherein the data stream is transmitted using discontinuous transmission.

12. A method as claimed in any previous claim further comprising the step of segmenting the data stream prior to the step of compressing the data stream.

13. A method as claimed in any previous claim wherein communication network is a cellular mobile communication system.

14. A method as claimed in claim 13 wherein the shared resource is an air interface resource.

15. An apparatus for transmitting data over a shared resource in a communication network comprising: means for receiving a data stream having a varying data rate; means for provisioning from the shared resource, a circuit switched data link having a link data rate below the maximum data rate of the data stream; means for compressing the data stream to a compressed data stream having a compressed data rate not higher than the link data rate using variable data compression; and means for transmitting the compressed data stream over the circuit switched data link.

16. An apparatus as claimed in claim 15 wherein the means for compressing the data stream comprises means for selecting a minimum available compression factor.

17. An apparatus as claimed in any previous claim 15 to 16 wherein the data stream comprises a plurality of data packets.

18. An apparatus as claimed in any previous claim 15 to 17 wherein the means for compressing is operable to individually compressed each of the plurality of data packets of the data stream.

19. An apparatus as claimed in any previous claim 15 to 18 wherein the compressed data stream comprises a plurality of data packets.

20. An apparatus as claimed in any previous claim 15 to 19 wherein the variable data compression is a lossy compression.

21. An apparatus as claimed in any previous claim 15 to 20 wherein the variable data compression is a non-lossy compression.

22. An apparatus as claimed in any previous claim 15 to 21 wherein the means for compressing comprises means for selecting between lossy and non-lossy compression dependent on the compression level required to compress the data stream.

23. An apparatus as claimed in any previous claim 15 to 22 wherein the means for compressing comprises source compression means.

24. An apparatus as claimed in any previous claim 15 to 23 wherein the variable data compression is independent of the information content of the data stream.

25. An apparatus as claimed in any previous claim 15 to 24 wherein the data stream is transmitted using discontinuous transmission.

26. An apparatus as claimed in any previous claim 15 to 25 further comprising means for segmenting the data stream and wherein the means for compressing is operable to compressing the segmented data.

27. An apparatus as claimed in any previous claim 15 to 26 wherein the shared resource is an air interface resource.

28. A cellular mobile communication system comprising an apparatus as claimed in any of the previous claims 15 to 27.