GB2380914A

GB2380914A - Data transmission in a cellular communications system

Info

Publication number: GB2380914A
Application number: GB0124677A
Authority: GB
Inventors: Stephen John Barrett; Luis Lopes; Stephen Aftelak
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-10-13
Filing date: 2001-10-13
Publication date: 2003-04-16
Anticipated expiration: 2021-10-13
Also published as: GB2380914B; GB0124677D0

Abstract

A cellular network architecture (12, 13) supports multiple interleaving intervals on the air interface (21) whilst permitting efficient backhaul (14) dimensioning by performing channel coding (15) and bit interleaving (17) operations in a radio network controller, RNC (12). Further improvements are provided by the inclusion of a compressor (25) in the RNC (12) which reduces the number of data bits to be transported over the backhaul link (14). De-compression apparatus (27) located in a node B (13) re-constitutes the original data stream for onward transmission over an air interface (21) to a user equipment (20).

Description

DATA TRANSMISSION IN A CELLULAR COMMUNICATIONS SYSTEM This invention relates to a data transmission in a cellular communication system and has particular applicability to the so-called third generation systems 3G, (UMTSUniversal Mobile Telecommunications System) currently under standardisation.

In a cellular communication system remote terminals (which may or may not be mobile) communicate with a fixed base station. Communication from a terminal to the base station is known as the uplink and communication from the base station to the terminal is known as the downlink. The total coverage area of the system is divided into a number of separate cells, each covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring 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 its final destination.

In UMTS, a radio network controller (RNC) communicates with a number of base stations (termed node B's) which in turn communicate with a number of remote terminals often termed user equipment (UE). The user equipment may be a mobile 'phone, lap top computer, paging device, etc. The user equipment, node B and RNC equate to the mobile station, base station transceiver and base station controller of the global mobile communication system (GSM) or general packet radio system (GPRS).

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 and employed in UMTS, is by a technique known as code division multiple access (CDMA). In a direct sequence 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 narrow-band signal is thus spread and transmitted as wide-band signal. At the receiver the original narrow-band signal is re-generated 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 as 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 wide-band spectrum by allocating different codes for different terminals. Codes are chosen to minimise the interference caused between the mobile terminals typically by choosing orthogonal codes where possible. A further description of CDMA communication systems can be found in"Spread-spectrum CDMA Systems for Wireless Communications", Glisic and Vucetic, Artech House Publishers, 1997, ISBN 0-89006-858-5.

Traditional traffic in mobile cellular communication systems has been circuitswitched voice data but in the future it is envisaged that data communication wdf increasingly comprise data other than voice such as E-Mail and Internet. This will require discrete data packets being transmitted between the remote terminals, base stations and network controllers. GPRS is an example of a packet base system.

Further details on data packet systems can be found in"Understanding Data Communications from Fundamentals to Networking", second edition, John Wiley Publishers, author Gilbert Held, 1997, ISBN 0-471-96820-X.

Figure 1 shows, in schematic block diagram form, a current configuration of a UMTS cellular communications system comprising a RNC 1, node B 2 and user equipment (UE) 3. The RNC 1 and node B 2 are linked via a fixed link termed lub 4 and often referred to as the"backhaul". The backhaul link may be hard wired or may be an optical or microwave link. The communications link between the node B 2 and UE 3 is over an air interface 5.

An RNC typically controls several node Bs and each node B typically serves a multiplicity of user equipments within its coverage area. However, only one node B and user equipment are shown here for the sake of clarity.

The RNC 1 also interfaces with a core network (not shown) which may include a PSTN (public switched telephone network) and is capable of receiving and transmitting data packets therefrom and thereto. Data packets received by the RNC 1 may be destined for any user equipment served by any of the node Bs under the control of the RNC 1.

One of the tasks of an RNC is to ensure that any given data packet is sent to the appropriate node B via the lub backhaul link for onward transmission to the destination UE. Such a packet, when received at the node B 2, is passed through a channel coder 6, interleaver 7 and RF modulation circuit 8 prior to being transmitted over the air on the downlink to the UE 3 as a coded RF signal. The principles of operation of the modules 6,7, 8 are well known to those skilled in the art.

The purpose of interleaving, whereby bits from several code words are mixed up so that bits which are close to one another in a modulated signal are spread over several code words, is to provide some form of error protection. Hence, for example b bits, of a code word are spread onto n bursts. The larger the value of n, the better the error protection, but the longer the transmission delay. All n bursts must be received then before the full b bits comprising a code word can be"re-constituted".

The value of n multiplied by the burst length can be thought of an interleaving interval.

At the UE, the received coded RF signal is demodulated by the RF circuit 9, deinterleaved and decoded by the de-interleaver and decoder modules 10 and 11 respectively.

An plink signal is dealt with in a similar fashion undergoing in the UE 3, channel coding, interleaving and RF modulation. At the node B 2 the received signal is demodulated, de-interleave and channel decoded prior to transmission over the lub 4 to the RNC 1.

A current specification for UMTS supports multiple interleaving intervals on the air interface, for example, 10 milliseconds, 20 milliseconds and 80 milliseconds thereby offering respectively greater degrees of time diversity. However, this can lead to problems with backhaul dimensioning i. e. the bit rate that it needs to support.

Imagine a circuit-switched 144 kilobits per second service requiring an 80 millisecond interleaving interval. In the current implementation, the whole 80 milliseconds worth of data has to be available at the node B before the interleaving and channel coding can be performed. If the part of the delay budget consumed by the backhaul connection is to be kept low, at 10 milliseconds, for example, then the capability to support a burst rate of 144,000 x 80/10 (1.1 megabits per second) will have to be available on the backhaul to carry the packet in question. This can lead to the backhaul having to be greatly over-dimensioned.

In order to permit an efficient backhaul dimensioning together with multiple interleaving intervals, one solution known to the inventors is to set the data transmission rate to 144 kilobits per second with an 80 milliseconds interleaving interval and then to offset the times (e. g. in 10 millisecond intervals) at which the 144,000 x. 08 packets of data belonging to different user equipments are sent on the backhaul. In this way the bit rate required on the backhaul would be smooth.

However, there are some problems with this solution. Firstly, it will be very rare for packets to just happen to arrive from the core network at 10 millisecond intervals or equivalently just happen to be generated by user equipments with equally-spaced timing offsets. This means that for the vast majority of the time some packets will have to be queued (and therefore delayed) waiting for their slot on the backhaul. So the delay could be up to 80 milliseconds in this example. Secondly, node Bs might not be sychronised and since some connections may be carried in soft handover between node B's, this also complicates such an approach. Thirdly, it would also be necessary to take into account these schduling delays on the backhaul in the air interface schduling which is performed at the RNC, thereby further complicating the separation of terrestrial interface design and air interface design. In practice though, not everyone will be carrying 144 kilobits per second with 80 milliseconds interleaving interval. If, for example, there is only one such user and the rest carry speech with a 10 milliseconds interleaving interval, then such schduling of data on the backhaul is not possible because there would be a 1.1 megabits per second blip when the slug of data corresponding to the 144 kilobits per second connection was transported.

Another solution known to the inventors is always to use a 10 millisecond interleaving interval. This simplifies backhaul dimensioning but results in worse air interface capacity ultilisation and less error protection.

According to the present invention and in a first aspect, a method of transmitting data in a cellular communications system includes the steps of; in a network controller, receiving from a core network a digital data stream, channel coding the digital data stream to produce a stream of code words, each code word comprising a plurality of bits,

performing a bit interleaving operation on the stream of code words to produce a succession of data blocks, transmitting over a backhaul link the succession of data blocks to a base station, and in the base station, receiving the succession of blocks and transmitting sequentially each received block to a user equipment over an air interface.

In a second aspect, the invention comprises a method of transmitting data in a cellular communications system, the method including the steps of; in a base station, receiving over an air interface, data blocks from a user equipment wherein the data blocks contain coded and interleaved data bits representing a digital data stream, transmitting the data blocks over a backhaul link to a network controller, and in the network controller, performing a bit de-interleaving operation on the data blocks to produce a stream of code words, and channel de-coding the stream of code words to produce a digital data stream for onward transmission to a core network.

Thus, the backhaul link between the RNC and the base station/node B carries data which is already channel coded and interleaved.

Placing the channel coding and interleaving functions in the RNC rather than base station/node B advantageously allows a backhaul bit rate reduction whilst still permitting variable interleaving intervals.

In the example, of a 80 millisecond interleaving interval and a 144 kilobits per second service, the node B can now transmit 10 milliseconds, say, data blocks to the UE as soon as they arrive from the RNC without having to wait for a full 80 milliseconds worth of data to arrive. Hence the node B can be"drip fed"in a just-intime manner i. e. 10 milliseconds at a time. Therefore the backhaul delay is minimised to just 10 milliseconds permitting a backhaul requirement of no more than 144 kilobits per second (multiplied by the channel coding rate).

If the channel coder used to perform the channel coding step in the RNC is a 1/3 rate convolutional coder (as will be well known to those skilled in the art), then the lub/backhaul link carries 3 times the source data rate (from the core network) to be

transmitted. However, this is a significant improvement than the worst case of 8 times the source data rate as in the example described above with reference to Figure 1.

In order to gain further improvement, the invention also provides in a third aspect, data compression apparatus comprising a compressor adapted to receive from a coder a sequence of code words comprising a plurality of systematic bits and redundant bits, and further adapted to transmit a first part of the sequence to receiving apparatus, and from a second part of the sequence, extract the systematic bits comprising the second part and transmit the extracted systematic bits to the receiving apparatus.

The compression apparatus may include a bit interleaver for performing a bit interleaving operation on the sequence of code words prior to them being fed to the compressor.

In one embodiment, the compression apparatus includes the receiving apparatus and said receiving apparatus consists of a decompresser adapted to reconstitute the second part of the sequence on receipt of the extracted systematic bits.

In a preferred embodiment, a channel coder is connected to the input of a bit interleaver whose output is, in turn, connected to the compresser. The channel coder, bit interleaver and compresser are all located in a network controller. The decompresser is located in a base station or node B and receives the first part of the sequence of code words and the extracted systematic bits over a backhaul link.

The decompresser is further adapted to transmit the first part of the sequence and the reconstituted second part of the sequence to a user equipment over an air interface.

The decompresser may be adapted to extract the systematic bits comprising the received first part of the sequence of code words and may include a coder for reconstituting both first and second parts of said sequence of code words.

The decompresser may also include a bit interleaver for performing a bit interleaving operation on a reconstituted sequence of code words.

The preferred embodiment provides a further improvement in backhaul efficiency as the redundant bits included in part of a code word stream are not sent to the base station but reconstituted in the base station (given that the channel coding and interleaving operations of the network controller are known to the base station) for onward transmission to the user equipment.

According to a further aspect of the invention there is provided a method of transmitting data including the steps of; receiving a digital data stream, coding the digital data stream to produce a stream of code words, each code word comprising a plurality of systematic bits and redundant bits, performing a bit interleaving operation on the stream of code words to produce a succession of data blocks each comprising a plurality of interleaved systematic bits and redundant bits, transmitting a first part of the succession of datablocks to receiving apparatus, from a remaining second part of the succession of data blocks, extracting the systematic bits comprising the remaining second part of the succession of data blocks and transmitting the extracted systematic bits to the receiving apparatus, at the receiving apparatus receiving the transmitted first part of the succession of datablocks and transmitting same to a data terminal, receiving the extracted systematic bits comprising the second part of the succession of data blocks, extracting the systematic bits from the first part of the succession of data blocks received and performing a coding and bit interleaving operation on the systematic bits extracted from the first part of the succession of blocks and from the extracted systematic bits comprising the second part of the succession of blocks, thereby reconstituting said first and second parts of said succession of data blocks, and transmitting said re-constituted second part of the succession of data blocks to the data terminal.

Some embodiments of the invention will now be described by way of example only, with reference to the drawings of which; Figure 1 is a schematic block diagram of a known configuration of a third generation cellular communications system, Figure 2 is a schematic block diagram of a configuration of a third generation cellular communications system in accordance with the invention, And Figure 3 is a schematic block diagram of a further configuration of a third generation cellular communications system in accordance with the invention.

In Figure 2, an RNC 12 communicates with a node B/base station 13 over a (lub) backhaul link 14. The RNC 12 includes a channel coder 15 and decoder 16 and an interleaver 17 and de-interleaver 18. The node B 13 includes a base band and RF module 19. The node B communicates with a UE 20 over a radio link 21. The UE 20 includes a base band and RF module 22, an interleaver-de-interleaver module 23 and a channel coder-decoder module 24.

In accordance with the invention, the coding and interleaving (and decoding and deinterleaving) functions have been moved from the node B to the RNC (cf. Figure 1).

In operation, assume that a data packet has been received at the RNC 12 from a core network (not shown). The packet is then channel coded (by the channel coder 15) and then bit-interleaved (by the interleaver 17) with an interleaving interval of 80 milliseconds say. The resulting bit-stream is then transmitted over the backhaul link 14 in 10 milliseconds blocks to the node B 13. So the backhaul delay is 10 milliseconds.

As soon as the node B 13 receives a 10 milliseconds block, it transmits it over the air interface 21 to the UE 20 (once the necessary modulation has been performed by the RF module 19).

When all 80 milliseconds worth of data has been received at the UE 20, it is demodulated, de-interleave and decoded in the usual manner by the modules 22, 23 and 24 respectively.

For the case of uplink transmissions, a data packet is channel coded and bitinterleaved, then modulated in the UE 20 by the modules 24,23 and 22 respectively in the usual manner, and transmitted over the air interface 21 to the node B 13. The packet received at the node B 13 is then demodulated and transmitted by the node B 13 via the backhaul link 14 to the D interleaver 18 and decoder 16 of the RNC 12.

The RNC 12, node B 13 and UE 20 of Figure 3 comprise the same components as shown in Figure 2 with the following additions. The RNC 12 further includes a compression module 25 and the channel coder 15 is configured as a 1/3 rate forward error correction coder. The node B 13 is further provided with a decompression module 27.

In operation, assume that a 800 bit data packet arrives at the RNC from the core network and is channel coded by the channel coder 15 which produces a 2,400 bit stream of systematic and redundant bits. This bit stream is then interleaved by the interleaver 17 with an interval of 80 milliseconds say, and passed on the compression module 25.

In the compression module 25, the interleaved bit stream is segmented into 8 frames comprising equal numbers of bits. In this example, the first five frames are transmitted over the backhaul link 14 to the node B 13. As the node B 13 receives these 5 frames, it sends them directly over the air interface 21 to the UE 20. So at the end of the fifth frame, 500 systematic bits and 1,000 redundant bits have been sent.

In the compression module 25 in the RNC, the remaining 300 systematic bits from

Ih the 6th, 7th and 8th frames are extracted and transmitted in a sixth frame to the node B 13 over the backhaullink 14.

The decompression module 27 in the node B 13 receives the sixth frame comprising the extracted systematic bits and also retrieves the systematic bits from the first five frames. It is able to perform this retrieval operation because the channel coder and interleaver operations of the RNC are known to the node B or are deterministic and therefore the decompression module 27 in the node B 13 knows the position (though not the value) of the systematic bits as they are transmitted in each frame.

The decompression module 27 is further configured to include a channel coder and a bit interleaver which operate in the same fashion as the channel coder 15 and interleaver 17 of the RNC 12.

Thus, the decompression module 27 re-generates the 8 frames of the coded and interleaved bit stream previously produced in the RNC 12, the node B 13 then transmits frames 6,7 and 8 over the air to the UE 20.

Once the UE 20 has received all 8 frames, it can perform a de-interleaving and decoding operation in the known manner.

As the RNC 12 does not need to transmit anything on the 7th and 8th frames, this results in a 25% saving on the backhaullink 14. In this example, the data rate when

transmitting over the backhaul, is 3 x 100 bits/frame. If there are many calls on this system, this released bandwidth could be exploited via statistical multiplexing or alternatively, it could be used for carrying RNC-node B signalling.

In another embodiment, the RNC 12 transmits the first four frames of redundant and systematic bits. Then the 5th frame needs to include 400 systematic bits. The 6th, 7'" and 8'"frames are not sent, thereby giving a further saving on the backhaullink.

However, this results in a higher instantaneous data rate on the 5th frame.

Alternatively, the 100 additional systematic bits are spread out over the first 4 frames, although the average rate is then somewhat greater than 3 x 100 bits/frame.

The node B13 extracts the first four frames worth of coded and interleaved data and transmits these, frame by frame as they are received, over the air to the UE20. The additional systematic bits sent in the first four frames are also extracted and stored and then from these and the 300 systematic bits received in the 5th frame, the full 8 frames of the coded and interleaved bit stream are re-generated. Then the node B13 transmits the 5th, 6th, 7th and 8th frames worth of data over the air to the UE20.

Claims

CLAIMS 1. A method of transmitting data in a cellular communications system including the steps of; in a network controller, receiving from a core network a digital data stream, channel coding the digital data stream to produce a stream of code words, each code word comprising a plurality of bits, performing a bit interleaving operation on the stream of code words to produce a succession of data blocks, transmitting over a backhaul link the succession of data blocks to a base station, and in the base station, receiving the succession of blocks and transmitting sequentially each received block to a user equipment over an air interface.
2. A method of transmitting data in a cellular communications system, the method including the steps of; in a base station, receiving over an air interface, data blocks from a user equipment wherein the data blocks contain coded and interleaved data bits representing a digital data stream, transmitting the data blocks over a backhaul link to a network controller, and in the network controller, performing a bit de-interleaving operation on the data blocks to produce a stream of code words, and channel de-coding the stream of code words to produce a digital data stream for onward transmission to a core network.
3. Data compression apparatus comprising a compressor adapted to receive from a coder a sequence of code words comprising a plurality of systematic bits and redundant bits, and further adapted to transmit a first part of the sequence to receiving apparatus, and from a second part of the sequence, extract the systematic bits comprising the second part and transmit the extracted systematic bits to the receiving apparatus.

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4. Data compression apparatus as claimed in Claim 3 and further including a bit interleaver for performing a bit interleaving operation on the sequence of code words prior to them being fed to the compressor.
5. Data compression apparatus according to Claim 3 or Claim 4 including the receiving apparatus and wherein said receiving apparatus consists of a decompresser adapted to reconstitute the second part of the sequence on receipt of the extracted systematic bits.
6. Data compression apparatus as claimed in Claim 5 wherein the decompresser is further adapted to transmit the first part of the sequence and the reconstituted second part of the sequence to a user equipment over an air interface.
7. Data compression apparatus as claimed in Claim 5 or Claim 6 wherein the decompressor is adapted to extract the systematic bits comprising the received first part of the sequence of code words and includes a coder for reconstituting both first and second parts of said sequence of code words.
8. Data compression apparatus as claimed in Claim 7 wherein the decompresser includes a bit interleaver for performing a bit interleaving operation on a reconstituted sequence of code words.
9. A method of transmitting data including the steps of; receiving a digital data stream, coding the digital data stream to produce a stream of code words, each code word comprising a plurality of systematic bits and redundant bits, performing a bit interleaving operation on the stream of code words to produce a succession of data blocks each comprising a plurality of interleaved systematic bits and redundant bits, transmitting a first part of the succession of datablocks to receiving apparatus, from a remaining second part of the succession of data blocks, extracting the systematic bits comprising the remaining second part of the succession of data blocks and transmitting the extracted systematic bits to the receiving apparatus,

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at the receiving apparatus receiving the transmitted first part of the succession of data blocks and transmitting same to a data terminal, receiving the extracted systematic bits comprising the second part of the succession of data blocks, extracting the systematic bits from the first part of the succession of data blocks received and performing a coding and bit interleaving operation on the systematic bits extracted from the first part of the succession of blocks and from the extracted systematic bits comprising the second part of the succession of blocks, thereby re-constituting said first and second parts of said succession of data blocks, and transmitting said re-constituted second part of the succession of data blocks to the data terminal.
10. A method of transmitting data substantially as hereinbefore described with reference to Figures 2 and 3.
11. Data compression apparatus substantially as hereinbefore described with reference to Figure 3.