DECODING METHOD

Abstract: This invention relates to a decoding method of a telecommunication network, in which method: information frames are received and data contained in them is buffered until the amount of the buffered data corresponds to a predetermined amount; the buffered data is decoded, a parity check is made for the buffered data; and the buffered data is transmitted further in the telecommunication network. In order to improve the connection between the parts of the telecommunication network, the accuracy of the buffered data is resolved by a parity check and an additional check, the additional check taking into account results obtained in connection with the processing of data received earlier in time.

Description: Decoding method This invention relates to a decoding method of a telecommunication network, in which method: information frames are received and data contained in them is buffered until the amount of the buffered data corresponds to a predetermined amount, the buffered data is decoded, a parity check is made for the buffered data, and the buffered data is transmitted further in the telecommunication network. This invention is specifically connected to the GSM mobile telephone system (Groupe Special Mobile) , and more precisely to a method by which the quality of the connection between the mobile, that is, the subscriber unit, and the mobile services switching centre can be improved. In situations in which there is an active traffic channel in a base station but the mobile does not transmit radio signals to the base station on said connection continuously, the base station has no way of detecting from a received signal whether the subscriber unit has transmitted the frame or not. Therefore, the base station constantly receives noise from the radio path, detects and decodes it, and sends speech blocks further in TRAU-frames to a transcoder. The above-mentioned situation occurs in the GSM system at least in connection with a handover when the handover is made between two base stations, or when a subscriber unit uses discontinuous transmission, that is, is in a DTX state. In the DTX -tate the subscriber unit sends one speech frame of " ) ms in length in accordance with the GSM specifica.. ons, after which it has a pause of 460 ms in length before transmitting the next speech frame. According to the GSM specifications, the base station has to be able to detect 99,8% of all defective speech blocks sent via the base station. This detection is based on a parity check made in connection with the decoding of speech blocks . Because in said special situations all the speech blocks received from the radio path by the base station are defective, a defective speech block passes through the base station in compliance with the GSM specifications every tenth second on the average without the decoding of the base station detecting the error. The object of this invention is to eliminate the above-mentioned drawback and to improve the quality of the connection between the subscriber unit and the mobile services switching centre. This object is achieved with the method of the invention that is characterized in that the accuracy of the buffered data is resolved by a parity check and an additional check, the additional check taking into account results obtained in connection with the processing of data received earlier in time. The invention is based on the idea that the number of forwarded defective speech blocks marked as faultless will decrease significantly when an additional check based on information obtained in the processing of previous speech blocks is made in addition to a parity check to ascertain the accuracy of the buffered speech block. Thus, the most important advantage of the method of the invention is that the connection between the subscriber unit and the mobile services switching centre can be significantly improved by very simple procedures. For example, in channel decoding of a GSM base station the utilization of the invention can be carried out as a whole by means of a software change, whereby the hardware costs of the base station will not rise. A first preferred embodiment of the method of the invention is characterized in that in said additional check the number of successive parity checks showing error is checked, and if said number exceeds a predetermined limit value, the buffered data is found to be defective. In this embodiment of the invention the first faultless speech block following a long string of defective speech blocks is rejected. As regards a base station according to the GSM specifications, the probability of a defective speech block being interpreted as faultless diminishes to less than 0,0004%. This means that a defective speech block passes through the base station once in 5000 seconds. A second preferred embodiment of the method of the invention is characterized in that said additional check is based on measurements made in connection with the reception of the information frames, whereby the measurement results of the information frames that have transmitted the buffered data are compared to measurement results of such earlier information frames that are known to have been faultless, and that the buffered data is found to be defective if in the comparison the difference of said measurement results is found to exceed a predetermined limit value. In this embodiment of the invention other available information in addition to the parity check is utilized in identifying defective blocks. For example, the specifications of the GSM system determine that a base station should control the quality of signals received during normal traffic operation, e.g. by calculating the Bit Error Rate BER on the basis of a signal received from the output of a receiver of the base station. In addition to this, the GSM base station carries out for example RSSI (Received Signal Strength Indication) measurements, in which the power level of the received signal is measured, and measurements of the noise ratio of the received signal, i.e. SNR (Signal Noise Ratio) measurements. In this embodiment of the invention existing measurement results, or preferably their average, are utilized so that when the measurement results obtained in connection with the reception of a speech block in the buffer are compared to the corresponding results from faultless speech blocks received earlier;: the accuracy of the speech block in the buffer can be checked. The preferred embodiments of the method of the invention are disclosed in the appended dependent claims 2-10. In the following, the invention will be described in more detail by means of a few preferred embodiments with reference to the accompanying figures, in which Figure 1 shows basic elements of the GSM system, Figure 2 shows a flow diagram that illustrates a first preferred embodiment of the method of the invention, and Figure 3 shows a flow diagram that illustrates a second preferred embodiment of the method of the invention. Figure 1 shows basic elements of the GSM system. A mobile services switching centre MSC takes care of the switching of the incoming and outgoing calls in a corresponding manner as a fixed telephone network exchange. In addition to this, it performs operations characteristic of mobile telecommunication, such as location management of subscribers. Mobile subscriber units MS are connected to the centre MSC by means of base station systems. The base station system comprises a base station controller BSC and base stations BTSl and BTS2. The base station controller BSC is used to control several base stations BTS. The GSM system is digital as a whole and speech transmission also takes place completely digitally. The speech coding and rate adapting functions of the network between the base stations BTS and the mobile services switching centre MSC have been centered on one transcoder unit TRAU (Transcoder/Rate Adaptor Unit) . In the GSM system the TRAU can be located in several alternative places according to the choices made by the manufacturer. When the transcoder is located separate from the base station BTS (as in Figure 1) , data is transmitted between the base station and the transcoder in so called TRAU frames of 320 bit in size, which means that the transmission of one frame takes 20 ms at a transmission speed of 16kbit/s. The GSM system is described in more detail for example in The GSM System for Mobile Communications, by M. Moyle and M-B. Pautet, Palaiseau, France, 1992, ISBN:2-9507190-0-7, for which reason its structure and operation will not be described in greater detail in this context. Figure 2 shows a flow diagram that illustrates a first preferred embodiment of the method of the invention. The method shown in the figure can be preferably applied in connection with channel decoding of a GSM base station, for instance. A radio frequency burst is received in block 1. The data contained in the received burst is fed to a buffer in blocks 2 and 3 in a manner known per se, after which the contents of the buffer are checked. When the amount of data fed to the buffer corresponds to one TRAU block of 320 bits in size (corresponds to data contained in 8 bursts) , a transition is made to block 4 in which the speech block is decoded and after the decoding a parity check is made to check the accuracy of the speech block. There are 3 check bits in one speech block for the parity check. One bit is reserved in the TRAU frame for indicating the accuracy of the frame. If it is found in block 5 that the speech block in the buffer is defective on the basis of the parity check, the above-mentioned check bit is given the value 1, after which a transition is made to block 9 where a TRAU frame is assembled and transmitted further. If it is found in the parity check of block 4 that the data in the buffer is faultless, an additional check is made in block 7. The additional check in question is based on previous speech blocks. In Figure 2 the speech block in the buffer is found defective if the preceding N speech blocks have been found defective, N being preferably between 5-100. If the speech block in the buffer is found to be faultless also in the additional check of block 7, the check bit is given the value 0 in block 8, after which a TRAU frame is assembled and transmitted further in the telecommunication network. On the other hand, if the buffered speech block is defective on the basis of the additional check made in block 7, a transition is made to block 9. When the TRAU frame has been assembled and transmitted further in the telecommunication network, the buffer is emptied, after which the assembling of the next speech block will start from block 1. In other words, in Figure 2 the first faultless speech block following a long string of defective speech blocks is rejected. This means that the first sequence of 20 ms in length is lost, whereby the quality of the call is correspondingly impaired (the transcoder TRAU in Figure 1 ignores, that is, does not forward those TRAU frames which were given 1 as the value of their check bit) . If said impairing is thought to be unreasonable, the case shown in Figure 2 can proceed so that the first speech block is copied from the buffer to a special memory before it is transmitted further. If the next buffered speech block is found to be faultless in the parity check, the speech block in memory is marked faultless, after which it is again transmitted further and a new speech block in the buffer is transferred to memory. Thus, the speech blocks are sent at a delay of 20 ms . The delay is caught up with after some time, for example when the parity check indicates that the speech block in the buffer is defective. Alternatively, the delay can be caught up with by skipping one speech block. Figure 3 shows a flow diagram that illustrates a second embodiment of the method of the invention. The method shown in the figure can be preferably applied in connection with channel decoding of the GSM base station, for instance. Said method makes use of SACCH multiframes (Slow Associated Control Channel) that have a duration of 104 frames, that is, about 480 ms on the traffic channel. During one SACCH multiframe a mobile MS on a traffic channel normally transmits 100 radio bursts, of which 96 contain speech and 4 signalling. During the remaining 4 frames (so called empty frames) , the mobile MS does not transmit anything. When the mobile MS uses discontinuous transmission (DTX) , the mobile may leave some of the bursts untransmitted. Even then it transmits at least 12 bursts during a SACCH multiframe (1 signalling block divided into 4 bursts and 1 block containing noise information divided into 8 bursts) . That is, during one SACCH multiframe the base station BTS receives 88 bursts (=22 speech blocks) that the mobile MS has possibly never transmitted at all. The GSM base station calculates the signal noise ratio (SNR) and the power level RSSI for each received radio burst. For each received signalling and speech block it also calculates the bit error rate (BER) . According to the second embodiment of the invention the base station stores measurement results of such bursts (12 per SACCH multiframe) that the mobile will transmit in any case, even when it uses discontinuous transmission. Thus, the base station holds in memory the measurement data of last 12 reliable bursts, that is: SNR_1...SNR_12, RSSI_1...RSSI_12, and BER_1...BER_2. A radio frequency burst is received in block 11 of Figure 3. The data of the received burst is input to the buffer in blocks 12 and 13 in a manner known per se, after which the contents of the buffer are checked. In block 12 the power level RSSI and the noise ratio SNR of the signal are measured for each burst in a manner known as such. In block 12 the bit error ratio BER is also measured for each received signalling or speech block. The above-mentioned measurement results are also buffered. When the amount of data input to the buffer corresponds to one TRAU frame that is 320 bits in size (corresponds to data contained in 8 bursts) , a transition is made to block 14 in which the speech block is decoded and a parity check is made to check its accuracy. There are 3 check bits in one speech block for the parity check. One bit is reserved in the TRAU frame for indicating the accuracy of the frame. If it is found in block 15 that the speech block in the buffer is defective on the basis of the parity check, a transition is made to block 20, in which the check bit is given the value 1, after which a TRAU frame is assembled and transmitted further. On the other hand, if it is found in the parity check that the speech block in the buffer is faultless, an additional check is made in block 17 after this. The additional check in question is based on the measurements made in block 12. Because the TRAU frame in the buffer is assembled from eight bursts, to each of which are made the measurements mentioned above, the average of the buffered measurement results is calculated first. A buffered speech block may then have three reference values, that is, the average BER, RSSI and SNR values. These reference values are next compared to the average of the measurement results of the reliable bursts in the memory of the base station. If there are several measurement results in use, as in the example of Figure 3, the bit error rate BER is preferably given most emphasis. If the difference between the reference values and measurement results exceeds a predetermined limit value, it is found in block 18 that the buffered speech block is defective, whereby a transition is made to block 20 in which the check bit is given the value 1, a TRAU frame is assembled and transmitted. On the other hand, if it is found in block 18 that the buffered block is faultless, a transition is made to block 19 in which the check bit is given the value 0, a TRAU frame is assembled and transmitted further. After transmitting the TRAU frame the buffer is emptied", after which a transition is made to block 11, where the assembling of the next speech block is started. It is to be understood that the above description and the figures related thereto are only meant to illustrate the present invention. Thus, the method of the invention can also be applied in other connections than in connection with the GSM system, although the invention is explained above by way of example specifically by means of the GSM system. Therefore, the embodiments of the method of the invention can vary within the scope of the attached claims.

Claims: Claims : 1. A decoding method of a telecommunication network, in which method: information frames are received and data contained in them is buffered until the amount of the buffered data corresponds to a predetermined amount, the buffered data is decoded, a parity check is made for the buffered data, and the buffered data is transmitted further in the telecommunication network, c h a r a c t e r i z e d in that the accuracy of the buffered data is resolved by a parity check and an additional check, the additional check taking into account results obtained in connection with the processing of data received earlier in time. 2. A method according to the claim 1, c h a r- a c t e r i z e d in that said telecommunication network is a telecommunication network between a base station (BTSl, BTS2) and a mobile services switching centre (MSC) of a cellular radio system, preferably the GSM system, whereby the data buffered in connection with channel decoding is transmitted further as TRAU blocks. 3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that a radio unit (MS) that transmitted the information frames being received uses discontinuous transmission. 4. A method according to any one of claims 1 to 3, c h a r a c t e r i z e d in that in said additional check the number of successive parity checks showing error is checked, and if said number exceeds a predetermined limit value (N) , the buffered data is found to be defective. 5. A method according to claim 4, c h a r a c- t e r i z e d in that said predetermined limit value (N) is between 5 - 100. 6. A method according to claim 5, c h a r a c- t e r i z e d in that the data found to be defective in the additional check is marked defective and copied into a memory device before it is transmitted further in the telecommunication network, and if the next parity check indicates that the new buffered data is faultless, the data stored in the memory device is also considered faultless, whereby the data stored in the memory device is marked faultless, and the data stored in the memory device is then transmitted further in the telecommunication network. 7. A method according to any one of claims 1 to 3, c h a r a c t e r i z e d in that said additional check is based on measurements made in connection with the reception of the information frames, whereby the measurement results of the information frames that have transmitted the buffered data are compared to measurement results of such earlier information frames that are known to have been faultless, and that the buffered data is found to be defective if in the comparison the difference of said measurement results is found to exceed a predetermined limit value. 8. A method according to claim 7, c h a r a c¬ t e r i z e d in that the comparison of the measurement results is made by comparing the averages of the measurement results. 9. A method according to claim 7 or 8, c h a r a c t e r i z e d in that in connection with the reception of the information frames one or some of the following quantities is measured: the bit error rate (BER) , the power level of the received signal (RSSI) and/or the noise ratio of the received signal (SNR) , the comparison being based on one or some of the above- mentioned quantities. 10. A method according to any one of claims 7 to 9, c h a r a c t e r i z e d in that the information frames to be received are SACCH frames of the GSM system.

Inventors: KOIVU VESA, KOIVU, VESA
Applicants: KOIVU VESA, NOKIA TELECOMMUNICATIONS OY, KOIVU, VESA
Application year: 1995
Main classification: H03M13
IPC codes: H03M13, H04L1