AN APPARATUS AND METHOD FOR THE ENHANCEMENT OF SPEECH QUALITY IN MOBILE COMMUNICATIONS

Abstract: In conventional TDMA cellular mobile telephone systems, connections between one mobile station and the network conventionally utilise one time slot per TDMA frame and both the codec and the error control coding applied to the voice signal are based on a fixed number of bits. In areas of high interference, the user can request the enhancement of speech quality manually or by registering a permanent request. The network responds by allocating at least one further slot per TDMA frame to the connection. The same voice information is then spread over two slots while the additional capacity is utilised to implement a more powerful error control coding in addition to, or in place of, the regular error control coding. In a further embodiment, the additional capacity is utilised to apply an improved codec and so ameliorate the speech quality above that even of a fixed line.

Description: An apparatus and method for the enhancement of speech quality in mobile communications 1. Field of invention The invention relates to cellular mobile telephone systems using time division multiple access, and specifically to the enhancement of speech quality of a connection in such a cellular mobile telephone system. 2. Background art In cellular mobile telephone systems, poor speech quality is usually due to interference or coverage problems. The perceived quality of a voice connection is difficult to quantify but generally has good correlation with the bit error rate (BER) of a transmission link. High BER generally manifests itself in poor speech quality, while a low BER means that the perceived speech quality is good. Currently, when a mobile user experiences poor speech reception the only action he or she can take is to physically move to a different position and in that way hopefully bypass the interference. Meanwhile the network will continuously try to find a better channel in another cell. However, in areas of high interference, the quality may degrade substantially before a new channel with better quality can be found. During this period of high interference, the end user may encounter so severe a degradation of speech quality that he or she has to terminate the call earlier than intended. In other cases, the BER may be so high that the network is unable to maintain the connection and will cause the call to be dropped. The effects of inferior signal /30383 quality are therefore not just discomforting and irritating for the end user but may also significantly reduce the mean holding times of calls in areas of high interference. Thus operator revenue will likewise suffer from bad speech quality. There are three main reasons for bad speech quality in cellular telephone systems. These are: low signal strength, tight frequency reuse and congestion. Conventionally, bad signal quality resulting from low signal strength has been fought by improving the radio coverage in areas where coverage is sparse. This may include improving the link resources, for example, by installing better antennas with higher gain, e.g. diversity gain, or by increasing the base station output power. A more costly solution is to install an additional base station in the area of low signal strength. The interference resulting from tight frequency reuse is conventionally combated in a number of ways. The most obvious way is to build new sites in the high interference area. However, this is not only costly but also assumes that there are frequencies available for new cells to use. If the spectrum is limited, it may be difficult to find a frequency that can be used in that area but will not lead to interference problems in other areas. Then the only viable solution is to redraft the frequency plan. An alternative remedy utilised in the prior art is to effectively move the cell borders by changing the parameters in the base station controller (BSC). Specifically, the base station controller is configured to cause a mobile station experiencing interference to perform hand-off to another base station early, that is, before the signal strength between the mobile and the new base station has reached an optimum level. While this method mav Dermit the interference to be bypassed, it is very likely that the user will continue experiencing inferior speech quality as a result of the low signal strength. Congestion leads to poor signal quality when a mobile user reaches the limits of a cell but is unable to obtain a free channel in an adjacent cell owing to over-subscription. Since traffic distribution will never be evenly spread across a network, there will always be situations in which traffic peaks will lead to congestion. Conventionally, this is dealt with by the mobile station holding the old channel as long as possible while continuously attempting handover to a better traffic channel. However while no free channel is assigned to the mobile station, the signal quality will eventually become very bad, since the mobile station is essentially connected to the 'wrong 1 base station in terms of signal strength. There then arises the danger that the call will be dropped prematurely. The above described methods for improving quality are necessary and useful, however, they will not always be sufficient. Furthermore it may not be financially viable to attempt to fully exclude areas of high interference in a cellular telephone network. A network that is capable of operating with perfect quality and sufficient capacity to cope with all possible traffic conditions is not cost-effective to design and would require a huge available spectrum and a large number of base stations. Subscribers to a cellular telephone network cannot be expected to pay for an increase in resources that they have not requested and that, after all, can alleviate a problem which is encountered only in limited locations or during certain periods. In turn, the operators would rightly be reluctant to invest in such resources if the cost cannot be passed on to the end user. It is known that the αualitv of a transmission can be imυroved bv usins error- SUBSTfTUTE SHEET (RULE 26) control coding. The provision of better coding over the air interface between a mobile station and base station will clearly reduce the bit error rate of the radio connection. However, in a TDMA connection between the mobile station and a base station, the number of bits transferred in any one burst is fixed. Error control coding is essentially the calculated use of redundancy. A strong error control code will in general be longer than a weak one. Accordingly in a TDMA connection there is a trade-off between the level of error control, i.e. the number of coding bits, and the amount of traffic information transmitted. Increasing the error-control will reduce the number of information bits that can be transmitted in each burst. However, the transmission of fewer speech bits per burst in turn results in inferior speech quality. SUMMARY OF INVENTION It is accordingly an object of the present invention to provide an arrangement and method for a cellular mobile telephone system based on TDMA that enable the improvement of the perceived quality of a voice connection in areas of high interference. It is a further object of the invention to provide an arrangement and method for a cellular mobile telephone system based on TDMA that enables the improvement of speech quality with essentially no increase in the operating costs borne by an operator. These and further objects are achieved in a cellular telephone communication system based on TDMA by providing means in a mobile station and/or a base station for allocating at least one additional time slot for a particular connection. Specifically, the mobile station may be provided with manually operable means, possibly in the form of a key, with which the end user can operate to request the allocation of one or more additional time slots from the network when the speech quality of a connection is perceived to be inferior. The request can be activated during a call, or the mobile telephone may be configured prior to a call to launch a request when the quality of the connection, i.e. the BER, falls below a predetermined level automatically. Alternatively, the network may be configured to automatically allocate at least one additional time slot to a communication with a specific mobile station when the level of interference is found to exceed a predetermined level. The invention further resides in a mobile station comprising manually operable means for requesting the allocation of at least one additional time slot for a particular connection. A method is further proposed whereby the speech quality of a connection may be improved by allocating at least one additional time slot to a connection. The additional time slot or slots may be used to increase the amount of error control coding transmitted with each fixed amount of user or speech information. In an alternative embodiment, the additional time slot is used to transmit an improved and expanded speech coding scheme to provide enhanced speech quality. The additional time slots are preferably allocated on request from the user. The request may occur during the call or an equivalent function be enabled prior to a call to allow the mobile telephone to issue a request when the quality of the connection warrants the use of an additional slot. The additional time slots may be allocated by the base station equipment when the bit error rate of a connection for an end user reaches a predetermined level and when the end user has registered a request with the network requesting the enhancement of speech quality. In periods of interference, the speech quality of a connection can be improved by increasing the redundancy of a time slot or burst by spreading the user or speech data over two or more time slots instead of one, and utilising the additional capacity for an improved error control code. Furthermore, by utilising the additional time slots to accommodate an improved, and therefore longer, speech coding scheme (codec) the overall perceived quality of speech may be increased. Since the additional time slots can be allocated upon request by the end user, for example during periods of high interference, the additional cost of the connection can be passed onto the user directly by the operator. The end user need not request the improved speech quality if he or she is unwilling to pay, and the operator is not obliged to make costly additional resources generally available. BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the present invention will become apparent from the following description of the preferred embodiments that are given by way of example with reference to the accompanying drawings, in which: Fig. 1 schematically depicts a cellular mobile telephone system, Fig. 2 illustrates the conventional allocation of time slots for a connection, Fig. 3 illustrates the allocation of time slots for a connection according to the present invention, Fig. 4. shows a procedure for boosting speech quality initiated by a user, and Fig. 5 shows a procedure for boosting speech quality based on a registered user request. DETAILED DESCRIPTION OF THE DRAWINGS A link of a typical cellular mobile telephone system represented by a GSM system is illustrated in Fig. 1. The cellular telephone system comprises a network including several base transceiver stations (BTS) 20, generally located in the centre of a cell, base station controllers (BSC) 30, each of which typically controls a plurality of base transceiver stations 20, and mobile services switching centres (MSC) 40, which are responsible for setting up, managing and clearing connections as well as routing calls to the proper cell. MSC's also provide interfaces to the fixed telephone system (PSTN or ISDN) as well as provisions for charging and accounting services. Mobile telephones or mobile stations (MS) 10 moving from cell to cell communicate over an air interface with one, generally the closest, base transceiver station 20. In the system depicted in Fig. 1, the mobile station 10 is provided with a control key or keys, schematically illustrated by a rectangle 11. The control key or keys 11 serve to allow the user to request an enhancement of the speech quality from the network, as will be described in more detail below. In the air interface between a mobile station 10 and a base transceiver station 20, user traffic is transferred by means of a combination of frequency division multiplexing and time division multiple access techniques. Up to eight mobile stations 10 are assigned to a communications channel of fixed bandwidth of, typically, 200 kHz. The user traffic between each mobile station 10 and the network is divided into fixed lengths bursts or time slots. These bursts or time slots are multiplexed into 8-slot TDMA frames which are sent across the channel bv modulatine- the RF carrier. The frames may further be multiplexed together for sending in a fixed time period. Fig. 2 shows the operation of a voice connection to and from a mobile station 10. The allocation of time slots is illustrated for one carrier frequency channel utilised for the downlink. Eight time slots, numbered from 0 to 7 in the figure, make up a TDMA frame and generally one time slot in every eight is allocated to user traffic from a specific mobile station 10. This is illustrated by the shaded areas. The user traffic transmitted in each time slot comprises digitised voice information as well as synchronisation and control bits. The conversion of an analogue voice signal to a digital signal is performed using a speech coding scheme (codec) conventionally designed for a fixed number of bits. The resulting digital signal is then coded to render the signal less sensitive to transmission errors. Error control coding introduces redundancy into the digitised speech signal. Generally, an increased redundancy, i.e. an increased number of error control bits, indicates a more robust connection. However, since the number of bits that can be transmitted in any one burst is limited, a trade-off occurs between the number of error control bits and the voice information bits transmitted per time slot to obtain acceptable speech quality at low interference levels and sufficient error control power to maintain this speech quality at higher interference levels. When a mobile user passes through areas of very high interference (low C I rates), the resulting bit error rate (BER) may exceed that which can be corrected by the error control coding utilised for one time slot and the perceived voice quality will deteriorate. In accordance with the invention, the quality of a voice connection is enhanced by utilising a more powerful error correction scheme and therefore increasing the redundancy in the error control coding of the digitised voice signal. However, rather than reduce the number of digitised voice bits transmitted in an allocated slot, which in effect would result in a deterioration of the speech quality at low interference levels, the same amount of voice information is transmitted spread over two time slots. The additional capacity provided by this second time slot is then available to substantially increase the error control coding redundancy. This increased allocation of time slots per connection is illustrated in Fig. 3. Here two consecutive time slots per frame are allocated to a single mobile station 10. If the BER is particularly high, more than one supplementary time slot per frame could be allocated to a single connection. The consecutive arrangement of slots allocated to a single mobile station 10 is given by way of example only. The slots may be also distributed in another manner through the TDMA frame. The improved error control coding may be implemented by replacing the normally used coding with a superior scheme for those time slots allocated to a connection. A further option would be to pre-code the digitised speech, for example on an application level, and subsequently subject the pre-coded signal to the normal error control coding process. This latter procedure has the advantage that the existing functions, which in some cases may be imposed by a standard, can be retained. Since the enhancement of speech quality as described above naturally brings with it an increased allocation of resources to a single connection, it is preferred that this function is provided in accordance with a request from the user. Accordingly, the mobile station 10 is equipped with one or more control keys 11, which may be dedicated function keys, or be a specified combination of keys already present on the keypad. When the user experiences inferior speech quality during a connection he may request the enhancement of the speech quality by using a control key 11. Once the period of interference has passed, the user may request that the connection be returned to the single time slot, either with the same or a separate dedicated function key or a specified command entered via the keypad. The request issued by the user may result in one additional time slot being allocated, in which case several requests must be issued to obtain a connection wherein several time slots per frame are allocated. Alternatively, the number of time slots required for the connection (up to a maximum of 8) may be specified in a single request. Fig. 3 illustrates the procedure for this user initiated boost of speech quality. In step 1, the mobile station 10 notifies the network of the request to boost speech quality. This request is handled by the MSC 40 which issues an assignment request to the BSC 30 if the network can allow the modification. After this there will follow a resource upgrading or downgrading procedure, during which the number of time slots allocated will be altered and the appropriate error coding scheme adapted to the increased capacity designated. Subsequently, the BSC 30 notifies the MSC 40 of the completion of this procedure in step 3, and in step 4 the modification is acknowledged by the MSC 40 to the MS 10. Since a correlation exists between the perceived speech quality and the bit error rate of a connection, the mobile telephone 10 may be provided with certain functions enabling it to monitor the line and indicate to the user when the connection would benefit from the use of an additional time slot. In the same way, the mobile station 10 could prompt the user to order the release of an additional time slot when the connection is good enough to provide acceptable speech quality with the standard error control coding provided in one time slot per frame. In a further embodiment this latter arrangement is extended. Specifically, the functionality of the manually operable control keys 11 is incorporated in the phone itself by the user changing the settings of the telephone 10 prior to a call. When this function is enabled, the mobile telephone itself automatically requests an additional time slot when the BER becomes too high and subsequently releases an additional time slot when the BER drops again. The enabling of this function could be achieved using a dedicated control key 11 or by lodging the request with a specified key configuration through the normal key pad of the telephone. In a further embodiment, the user may not be required to manually request the boosting of speech quality but may instead register a permanent request with the network. The network continually monitors the signal level and quality of the channels so that any increase in the carrier to interference ratio (C/T) will be detected. If a request from the user is registered, the quality of a connection can be boosted automatically by the network by allocating additional time slots and utilising a more powerful error control coding. Such a request may be stored in a register in the network, for example together with identifying information about the user and other supplemental services provided to the user in the home location register (HLR). As for the manually requested procedure the network may reduce the number of time slots allocated when the BER of the connection returns to an acceptable, or at least less noticeable, level. This procedure is illustrated in Fig. 4. In this procedure it is assumed that the register containing the recorded user request has been interrogated and the BSC 30 is aware that the user requires the automatic enhancement of speech quality. In step 1 of this procedure the BSC 30 determines that the BER of a connection is such that the user would benefit from the allocation of an additional time slot, or the removal of an added time slot. In step 2, the BSC 30 informs the BTS 20 which additional slots are to be allocated or released. This command is acknowledged in step 3. The MS 10 is then notified of the change in configuration in step 4, and this command is acknowledged in step 5. Steps 2 and 3 are performed as many times as needed depending on the BER of the connection and the strength of the utilised error control scheme. It will be understood that a combination of the two procedures illustrated in Figs. 4 and 5 may also be implemented, whereby a manual request will be processed only when this function is registered by the network for the user. By utilising the additional time slots allocated to utilise an improved error control coding, the speech quality can be brought up to the ordinary speech quality of a connection that is subjected to little or negligible interference. In a further embodiment, the additional time slot or slots allocated on request to a user are used to accommodate an improved speech coding scheme (codec). The resulting connection then has speech quality that, depending on the codec utilised, can be far superior to that of even a fixed line. For practical reasons it may be preferable to limit the number of time slots utilised to implement an improved codec. On request by the user as described above, additional time slots could then be allocated to improve the error control of a connection. It will be understood that the enhanced speech quality attained through the allocation of supplementary time slots and utilising an improved error control coding or codec may be provided in only the downlink connection or in both the downlink and uplink connections. In some cases the user may request that the speech quality in an uplink connection be boosted when the network detects a deterioration. Since the boost in speech quality is initiated by the user, either by a request during a connection or by a previously registered function request, the cost of allocating the additional resources may be passed on directly to the user. The user in turn will only have to pay for those periods of increased time slots which he requested. It may also be of interest for certain emergency services to have permanent access to this form of speech quality enhancement. It will be understood that the invention may also be applicable to data traffic.

Claims: Claims: 1. A cellular telephone communication system comprising at least one mobile station 10 adapted to communicate with a cellular telephone network (20, 30, 40) over a time division multiple access connection, characterised in that at least one of said mobile station and said network comprises means (11) for initiating the use of at least one additional time slot for a connection. 2. System as claimed in claim 1, characterised in that said means includes manually operable means (11) comprised in said mobile station (10) for requesting the allocation of at least one additional time slot. 3. System as claimed in claim 1 or 2, characterised in that said manually operable means (11) comprised in said mobile station (10) are adapted to be enabled prior to the establishment of a connection and to automatically request the allocation of at least one additional time slot when the signal quality falls below a predetermined level. 4. A system as claimed in claim 1, characterised in that said network (20, 30, 40) comprises means for registering a request for enhanced speech quality and means for responding to said request by allocating at least one additional time slot for a connection. 5. A system as claimed in claim 4, characterised in that said network (20, 30, 40) further comprises means for monitoring the signal quality of a connection, said means for responding to said request being adapted to allocate at least one additional time slot for a connection when the signal quality falls below a predetermined level. 6. A mobile telephone for use in a TDMA cellular telephone communication system, characterised by comprising manually operable means (11) for requesting the allocation of an additional time slot for a connection. 7. A mobile telephone as claimed in claim 6, characterised in that said manually operable means are adapted to enable a function of said mobile telephone (10) to request one of the allocation and the removal of an additional time slot for a connection when the signal quality of the connection reaches a predetermined level. 8. A mobile telephone as claimed in claim 6, characterised by comprising manually operable means for requesting the removal of at least one additional time slot for a connection. 9. A method for enhancing speech quality between a mobile station (10) and a time division multiple access cellular telephone communication network, characterised by utilising at least one additional time slot to increase the amount of transmitted information representing a fixed amount of traffic information. 10. A method as claimed in claim 9, characterised by utilising the at least one additional time slot to increase the amount of error coding information transmitted to reduce the bit error rate (BER) of a connection. 11. A method as claimed in claim 9 or 10, characterised by utilising the additional time slot to increase the number of voice coding information (codec) bits to enhance the speech quality of a connection. 12. A method as claimed in any one of claims 9 to 11, characterised by allocating at least one additional time slot to a connection upon a request by the end user. 13. A method as claimed in any one of claims 9 to 12 characterised by allocating at least one additional time slot according to a registered request by the end user. 14. A method as claimed in claim 13, characterised by allocating at least one additional time slot when the bit error rate of a connection exceeds a predetermined level. AMENDED CLAIMS [received by the International Bureau on 10 April 2000 (10.04.00); original claims 1 - 14 replaced by new claims 1 - 13 (3pages)] 1. A node (30, 40) for use in a cellular telephone network according to a Time Division multiple Access (TDMA) standard, arranged to communicate with at least one mobile terminal (10), characterized in that it is arranged to receive a request from the mobile terminal (10) to allocate to a connection occupying at least one time slot for transmitting a certain amount of pay load per time unit, at least a further time slot when the quality goes below a certain level and use this further time slot to increase the amount of information transmitted, representing said payload for the connection. 2. A node (30, 40) according to claim 1, arranged to receive and register said request before the connection is set up, to supervise the quality of the connection and automatically allocate said at least one further time slot when the quality goes below a certain level. 3. A node (30, 40) according to claim 1, arranged to receive said request while the connection is set up and allocate said at least one further time slot when the request is received. 4. A mobile telephone for use in a cellular TDMA network, characterized in that it comprises means (11) for, while a connection occupying at least one channel, for transmitting a certain amount of payload per time unit is connected, requesting the allocation of a further channel to the connection to increase the amount of information transmitted representing said payload. 5. A mobile telephone according to claim 4, characterized in that said means (11) for requesting the allocation of a further channel is arranged for manual operation. 6. A mobile telephone according to claim 4 or 5, characterized in that it comprises means for supervising the bit error rate and, when the bit error rate exceeds a certain threshold value, inform a user of the mobile telephone that the quality of the connection could be improved by the allocation of a further channel. 7. A mobile telephone according to any one of the claims 4-6, characterized in that it comprises means for supervising the bit error rate and, when the bit error rate exceeds a certain threshold value, automatically sending a request for the allocation of a further channel to a unit in the mobile telecommunications network. 8. A method of improving the connection quality between a mobile terminal (10) and a TDMA cellular network, of a connection occupying at least one channel for transmitting a certain amount of payload per time unit, characterized by the steps of transmitting a request for the allocation of at least one further channel to the connection from the terminal (10) to a unit (30, 40) in the network; and using at the least one further channel in the connection for increasing the amount of information transmitted concerning the payload. 9. A method according to claim 8 wherein the further channel is used to transmit error coding information concerning the payload. 10. A method according to claim 8 or 9, wherein the further channel is used to increase the number of voice coding information bits. 11. A method according to any one of the claims 8-10, further comprising the steps of supervising the connection quality, by the terminal (10), sending the request for allocation of at least one further channel automatically from the terminal when the connection quality goes below a certain threshold value. 12. A method according to any one of the claims 7-9, wherein the user initiates the request for allocation of at least one channel in dependence of the perceived quality. 13. A method according to claim 12, wherein the terminal supervises the connection quality, and informs the user that the quality could be improved, when this is the case.

Inventors: CERWALL CARL PATRIK, CERWALL, CARL, PATRIK
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: H04Q7
IPC codes: H04J3, H04L1, H04W88