CHANNEL CODING IN A CELLULAR RADIO SYSTEM

Abstract: The invention relates to a data transfer method and a cellular radio system which comprises at least one transceiver (100, 200) which receives a signal packed into frames, means (202) for channel coding the signal received by the transceiver prior to transmitting the signal which consists of bits, and a vocoder (300) for source coding the signal. The means (202) form and add check bits to the signal to be transmitted, the check bits being used in detecting errors caused by transmitting the signal. The means (202) add the check bits in the channel coding to reduce the number of bits in the frames received by the transceiver.

Description: CHANNEL CODING IN A CELLULAR RADIO SYSTEM FIELD OF THE INVENTION The present invention generally relates to communication systems. More specifically, the invention relates to a digital cellular radio system in which check bits are added to a signal. The invention relates to a data transfer method employed in a cellular radio system comprising at least one transceiver which receives a signal packed into frames and which channel-codes the signal prior to transmission, said signal consisting of bits, said method comprising the steps of source-coding the signal and adding check bits thereto, the check bits being utilized in detecting errors caused by transmitting the signal. BACKGROUND OF THE INVENTION The standard TIA/EIA IS-95 defines channel coding carried out for the signal in a base station of the cellular radio system. The standard defines the channel coding employed in data transfer on an Air interface between a base station and a subscriber terminal of a cellular radio system. The standard defines the channel coding to be carried out in a channel coder of the base station. In the channel coder defined by the standard, the signal is CRC block-encoded (CRC = Cyclic Redundancy Check), convolutional-coded and interleaved. The interleaving is followed by converting the signal into an RF signal and transmitting it on the radio path to the subscriber terminal. The standard TIA/EIA IS-96 determines a variable-rate speech encoder employed in the cellular radio network. As the variable-rate encoder, for example a CELP vocoder (CELP = Code Excited Linear Predictive) has been used. The standard defines PCB (PCB = Parity Check Bits) and CRC calculation for several different data rates, such as 1200 kbit/s, 2400 kbit/s, 4800 kbit/s and 9600 kbit/s. In addition, the standard defines signal interleaving for all the above data rates. The PCB calculation has in the standard been defined to be carried out in the vocoder in connection with the source coding. In prior art, the vocoder is in the TRAU unit (TRAU = Transcoder/Rate Adaptor Unit), located apart from the base station. The TRAU unit is located, for example, at the base station controller or the mobile services switching centre. The TRAU unit is used in signal source coding, adapting the data rate to be suitable for the transfer network, e.g. the PSTN network (PSTN = Public Switched Telephone Network), and for transmitting TRAU frames to the base station. The TRAU unit codes the 64 kbit s signal originating from the PSTN network to a 16 kbit signal, whereby the user data rates are at the aforementioned levels. The TRAU unit serves as the source coder, reducing the bandwidth on the Air interface between the base station and the subscriber terminal. The prior art TRAU frame is e.g. 192 bits long before and after the convolutional coding. The convolutional coding is carried out in the channel coder. It takes 20 ms to transmit a frame. The frame comprises e.g. 172 information bits, 12 CRC bits and 8 tail bits. In the aforementioned case, the data rate is 9600 kbit/s. The frame comprises one MM bit (MM = Mixed Mode Bit). The remaining part of said frame, i.e. 171 bits, is allocated for traffic. In prior art, the calculation of PCB bits, the coding and decoding of PCB bits is carried out in the vocoder i.e. the TRAU unit. The calculation, coding and decoding of the bits requires bit buffering, which causes delay. The CRC block encoding is performed at the base station in connection with channel coding. In calculating CRC block encoding and bit interleaving, additional buffering is required, resulting in additional delay. In addition, the prior art solution takes up a lot of transmission path capacity. CHARACTERISTICS OF THE INVENTION It is therefore an object of the present invention to implement a method by means of which the transmission delay and capacity taken up by the signal may both be reduced. This object is achieved by a data transfer method according to the invention, characterized in that the check bits are added in the channel coding to reduce the number of bits in the frames received by the transceiver. The invention further relates to a cellular radio system comprising at least one transceiver which receives a signal packed into frames, means for channel coding the signal received by the transceiver prior to transmitting the signal, which consists of bits, and a vocoder for source-coding the signal, in which the means form and add check bits to the signal to be transmitted, the check bits being utilized in detecting errors caused by transmitting the signal. The object is achieved with a cellular radio system according to the invention, which is characterized in that the means add the check bits to the signal in the channel coding in order to reduce the number of bits in the frames received by the transceiver. The solution according to the invention provides significant advantages. In the solution, check bits are generated to be added to the signal, and these check bits are utilized in detecting errors resulting from transmitting the signal. The check bits are added in connection with channel coding and not source coding. When the check bits are added only at the channel coding, it is possible to reduce the number of bits in the frames received by the transceiver. In practice, reducing the number of bits means that transmission path capacity may be saved for other uses. The check bits are generated and added to a signal advantageously when the frame rate of the signal is substantially at the maximum. In an order determined by source coding, the transceiver serving as the channel coder receives a signal packed into the frame. Channel coding is carried out at the same rate as source coding, resulting in that signal buffering is not required to the same extent as before. The reduced buffering requirements enable reducing signal delay. When receiving a signal containing check bits, the signal is first CRC block-decoded. After the CRC block decoding, the signal is PCB decoded, and then it is transmitted to the vocoder. DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in closer detail with reference to the examples in the attached drawings, in which Figure 1 shows a cellular radio system to which the method of the invention is applicable, Figure 2a illustrates a first preferred location for the TRAU unit, Figure 2b illustrates a second preferred location for the TRAU unit, Figure 3 shows a schematic diagram for a base station of the cellular radio system, Figure 4a shows a prior art TRAU frame, and Figure 4b shows a TRAU frame formed according to the solution of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The solution is applicable to spread spectrum systems, particularly to CDMA systems employing frequency hopping. In the following, the invention will be described based on the aforementioned facts without, however, restricting thereto. Figure 1 shows a cellular radio system comprising subscriber terminals 100, base stations 200, a TRAU unit 300, a base station controller 400, and a mobile services switching centre 500. In the solution according to the invention, the base station 200 is connected to the subscriber terminal 100 via a signal 250 transmitted on the radio path. The connection between the base station 200 and the subscriber terminal is referred to as an Air interface. In addition, the base station 200 communicates with the base station controller 400. The connection between the base station 200 and the base station controller 400 is referred to as an Abis interface. The base station controller 400 further communicates with the mobile services switching centre 500. The connection between the base station con- trailer 400 and the mobile services switching centre 500 is referred to as an A interface. The operating frequency of the cellular radio system illustrated by the figure may be e.g. 800 MHz. The cellular radio system may also be represented by e.g. the PCS system (PCS = Personal Communication System), operating in the 1.9 GHz frequency range. The TRAU unit 300 comprised by the cellular ra- dio system is located between the base station 200 and the base station controller 400. In the arrangement according to the figure, the mobile services switching centre 500, the base station controller 400 and the TRAU unit 300 are connected via a digital transmission link 450, by utilizing e.g. PCM technique. The TRAU unit 300 and the base station are connected with practically a similar transmission link 350. The base station 200 comprises an antenna 201 , and the subscriber terminal comprises an antenna 101. The antennas 101 , 201 function as transmitting and receiving antennas. The base station 200 according to the figure transmits, using the antenna 201, a signal to the subscriber terminal 100, which receives the signal by the antenna 101. In the solution according to the invention, the subscriber terminals 100 are e.g. mobile phones. The base station 200 establishes a connection to the subscriber terminal 100 by means of the signal 250 transmitted on the radio path. The mobile services switching centre 500 receives a signal from e.g. the PSTN network, the signal comprising PCM channels which usually have the data rate of 64 kbit/s. In Figure 2a, the TRAU unit 300 is located at the base station controller 400. The TRAU 300 according to the figure serves as a transcoder. The TRAU unit 300 codes and decodes data contained by the signal, said data being for example coded speech. In addition, the TRAU unit adapts different bit rates, such as the 64 kbit/s data rate used in the PCM signal, into a data rate of 16 kbit/s. In Figure 2a, the TRAU unit 300 and the base station 200 transmit to each other data packets which are arranged as TRAU frames. According to the solution of the figure, the mobile services switching centre 500 supplies a signal to the TRAU unit 300 via the base station controller 400. Said signal forms a channel whose rate is 64 kbit/s. The TRAU unit 300 transcodes the signal into a rate of 16 kbit/s, and transmits the transcoded signal further to the base station 200. In Figure 2b, the TRAU unit 300 is located at the mobile services switching centre 500. In Figure 2b, the transcoder codes the signal provided by the mobile services switching centre 500 so that after the transcoding the channel rate is 16 kbit/s. According to the solution of the figure, the signal 450 received by the base station controller 400 has already been transcoded, and therefore the signal has the rate of 16 kbit/s. The TRAU unit 300 codes a signal formed e.g. from speech. The TRAU unit 300 varies, as needed, the coding rate employed in the signal encoding. In practice, the speech coding is carried out by a variable-rate speech encoder, i.e. a vocoder. The vocoder may be implemented by e.g. DSP technique. The TRAU unit 300 codes a PCM signal supplied by e.g. the PSTN network so that the signal rate will be lower. During the signal encoding, the data rate of the signal decreases to e.g. 9.6 kbit/s. The TRAU unit 300 operates in the case described above as a source coder. It is assumed in the solution ac- cording to the invention that the TRAU unit 300 serves as a remote transcoder, which means that the TRAU unit 300 is located separate from the base station 200. The TRAU unit 300 decodes the 16 kbit/s signal received from the base station 200 back to a 64 kbit/s signal. Accordingly, the TRAU unit 300 operates as a speech encoder i.e. vocoder. The base station 200 serves as a channel coder in the solution of the figure. The vocoder and channel coder transmit and receive data packets i.e. frames at intervals of e.g 20 ms. Channel coding minimizes the problems caused by the signal becoming weaker, such as errors in the signal. In the channel coding, extra bits are added to the signal. These extra bits contain, from the user's point of view, less essential information i.e. redundancy. Further, channel coding corrects errors. Channel coding minimizes signal errors, which result particularly from the signal going through the Air interface. Figure 3 shows a schematic block diagram of the base station 200. The base station 200 comprises means 202 for coding and decoding the PCB bits of the signal. In addition, the means 202 have been arranged to CRC block- encode and CRC block-decode the signal. In addition, the means 202 com- prised by the base station 200 have been arranged to convolutional-code and convolutional-decode the signal. Further, the means 202 comprised by the base station 200 have been arranged to interleave and deinterleave the signal. The subscriber terminal 100 comprises substantially the same parts as the base sta- tion 200. According to the solution of the figure, the channel coding is carried out by means of block encoding and convolutional coding. Besides the base station 200, the channel coder may also be located at e.g. the subscriber terminal 100. The base station 200 further comprises an antenna 201. The means 202 convert the interleaved signal to an RF signal and transmit the RF signal via the antenna 201 to the subscriber terminal 100. Like the base station 200, the subscriber terminal 100 also comprises the means 202. By utilizing block encoding, it is possible to detect errors in the signal. The means 202 CRC block-encode the signal and inserts the encoded signal into the blocks. In the block encoding, the means 202 add extra bits to the signal; for example, two bits may be added. Prior to carrying out the block encoding, the means 202 calculate parity for the signal to be block-encoded. To the end of each block, parity or e.g. checksum is inserted. Errors in the signal may be detected by using the parity or checksum. Upon receiving a signal containing check bits, a block and frame check corresponding to the check bits is decoded from the check bits at the transceiver. In the block decoding, e.g. 161 actual data bits and e.g. 12 check bits are received. After this, with the aid of the check bits, information on the correctness of the actual signal may be obtained. Further, the means 202 convolutional code the signal. In the convolu- tional coding, one bit generates a plurality of bits, for example two bits. By convolutional coding, all the errors that can be detected by block encoding can be corrected. The bit to be convolutional-coded does not alone influence the bits to be generated, but they are also affected by the state of the bit preceding the bit to be convolutional-coded. The means 202 PCB and CRC code the signal 350 received from the TRAU unit 300. Additionally, the means 202 PCB and CRC decode the signal transmitted by the subscriber terminal 100 to the base station 200. According to the solution of the invention, the PCB and CRC coding are performed in the base station 200 serving as a channel coder and not in the TRAU unit 300 serving as a vocoder. The aforementioned location for the PCB and CRC coding and decoding enables saving capacity of a signal 350 propagating on the transmission path. Additionally, the delay of the signal using the transmission path will be smaller due to the TRAU 300 transcoding and transmitting the bits it has encoded to the base station in the same order as the bits arrive in the TRAU unit 300. In addition, the channel coding is carried out at the frame rate em- ployed in the source coding, making it possible to reduce the transmission path delay. Figure 4a illustrates a prior art TRAU frame propagating between the base station 200, serving as a channel coder, and the TRAU unit 300, serving as a source coder. The frame comprises 172 bits out of which 171 are allocated for traffic. The frame comprises an MM bit (MM = Mixed Mode Bit), used to indicate the frame type. The data rate of the frame in the figure is 9.6 kbit/s. Figure 4b illustrates a TRAU frame formed by the method according to the invention, the TRAU frame propagating between the channel coder and the source coder. The frame comprises only 161 bits out of which 160 are allo- cated for traffic. The frame also comprises an MM bit to indicate the frame type. In this case, the data rate is as low as 8.05 kbit/s. As the solution of the invention requires 11 bits less than the prior art solution, the capacity saved can be used for other purposes. The capacity saved may be used e.g. in connection with traffic concentrators of the Frame Relay type between the vocoder and the base station. In the solution of the invention, the number of bits in the frame of the signal 350 is reduced by carrying out the PCB and CRC coding and decoding at the base station 200. In addition, the PCB and CRC coding and the PCB and CRC decoding may be carried out at the subscriber terminal 100. Moving the PCB and CRC calculation from the TRAU unit 300 reduces signal buffering, thus making the signal delay smaller. At the same time data transfer between the base station 200 and the TRAU unit 300 may be reduced whereby the capacity saved may be used for other purposes. Adding the PCB and CRC bits is advantageously carried out when the TRAU frame rate is substantially at the maximum. Although the invention is above described with reference to the example in the attached drawings, it is obvious that the invention is not restricted thereto but may be varied in many ways within the inventive idea of the attached claims.

Claims: CLAIMS 1. A data transfer method employed in a cellular radio system comprising at least one transceiver (100, 200) which receives a signal packed into frames and which channel-codes the signal prior to transmission, said signal consisting of bits, said method comprising the steps of source-coding the signal and adding check bits thereto, the check bits being utilized in detecting errors caused by transmitting the signal, characterized in that the check bits are added in the channel coding to reduce the number of bits in the frames received by the transceiver (100, 200). 2. A method as claimed in claim ^characterized in that the check bits are added to the signal just prior to channel coding the signal. 3. A method as claimed in claim 1, characterized in that the source coding is carried out prior to adding the check bits, and wherein the source-coded signal is transmitted at different data rates, as needed, forward to be channel-coded. 4. A method as claimed in claim 3, characterized in that the check bits are added to the signal in connection with the channel coding when the frame rate of the signal is substantially at the maximum. 5. A method as claimed in claim 3, characterized in that the check bits are added when the frame rate is substantially 9.6 kbit/s. 6. A method as claimed in claim 3, characterized in that the signal packed into a frame is received in an order established at the source coding and at the frame rate employed in the source coding. 7. A method as claimed in claim ^ characterized in that the source coding is carried out apart from the transceiver (100, 200) serving as a channel coder. 8. A method as claimed in claim 1, characterized in that upon receiving a signal containing check bits, a block check corresponding to the check bits is decoded from the check bits at the transceiver (100, 200). 9. A method as claimed in claim ^characterized in that the method is used in a cellular radio system comprising a vocoder (300) to which the transceiver transmits a signal, the method comprising CRC block decoding and PCB decoding the signal containing check bits. 10. A method as claimed in claim 1, characterized in that CRC and PCB bits are used as the check bits. 11. A method as claimed in claim ^characterized in that the transceiver used is a subscriber terminal and/or a base station. 12. A cellular radio system comprising at least one transceiver (100, 200) which receives a signal packed into frames, means (202) for channel cod- ing the signal received by the transceiver prior to transmitting the signal, the signal consisting of bits, and a vocoder (300) for source coding the signal, in which the means (202) form and add check bits to the signal to be transmitted, the check bits being utilized in detecting errors caused by transmitting the signal, characterized in that the means (202) add the check bits to the signal in the channel coding to reduce the number of bits in the frames received by the transceiver (100, 200). 13. A radio system as claimed in claim 12, characterized in that the means (202) add check bits to the signal just prior to channel coding the signal. 14. A radio system as claimed in claim 12, characterized in that the vocoder (300) transmits, at different frame rates, the signal it has source-decoded to be channel-coded, the source coding being performed, if required, prior to adding the check bits. 15. A radio system as claimed in claim 14, characterized in that when the frame rate is substantially at the maximum, the means (202) add the check bits to the signal. 16. A radio system as claimed in claim 14, characterized in that the means (202) add the check bits to the signal when the frame rate is substantially 9.6 kbit/s. 17. A radio system as claimed in claim 14, characterized in that the transceiver (100, 200) receives the signal packet into a frame in an order source-coded by the vocoder (300) and at the frame rate employed in the source coding. 18. A radio system as claimed in claim 12, characterized in that the vocoder (300) is placed separate from the transceiver (100, 200) serving as the channel coder. 19. A radio system as claimed in claim 12, characterized in that, upon receiving a signal containing the check bits, the means (202) are arranged to decode a block check from the check bits. 20. A radio system as claimed in claim 12, characterized in that, upon receiving a signal containing check bits, the means (202) CRC block- decode and PCB decode the signal. 21. A radio system as claimed in claim 12, characterized in that the check bits added by the means (202) are CRC and PCB bits. 22. A radio system as claimed in claim 12, characterized in that the transceiver is a subscriber terminal (100) and/or a base station (200).

Inventors: SIIRA MIKKO, SIIRA, MIKKO
Applicants: NOKIA TELECOMMUNICATIONS OY, SIIRA MIKKO, SIIRA, MIKKO
Application year: 1998
Main classification: H03M13
IPC codes: H04L1