SE515700C2 - Device, mobile phone and method of improving call quality in a time-shared mobile telephone network - Google Patents

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

20 25 30 A .__._ l_ __. Low signal strength has been radially and radically enhanced to improve coverage in areas with poor coverage. This may include improving link resources, for example by installing better antennas with higher gain, such as diversity gain, or by increasing the base station's output power.A more expensive solution is to install an additional base station in the area with low signal strength.

Interference caused by the small distance between the frequencies is traditionally combated in a number of ways. The obvious way is to build new stations in the area with high interference. However, this is not only expensive but also presupposes that there are available frequencies that can be used in new cells. If the spectrum is limited, it can be difficult to find a frequency that can be used in the area without causing interference problems in other areas. The only possible solution is to redo the frequency plan.

An alternative measure used in known solutions is to change the parameters of the Base Station Controller (BSC) and thereby surface the boundaries of the cell. The control means is configured to cause a mobile terminal which experiences interference to switch to another base station early, ie. before the signal strength between the mobile phone and the new base station has reached an optimal level. Although this method can make it possible to avoid interference, it is very likely that the user will continue to experience poor speech quality due to the low signal strength.

Spamming leads to poor signal quality when a mobile user reaches the boundaries of a cell but cannot get a free channel in an adjacent cell due to over-establishment.

Since the traffic will never be evenly distributed over a network, there will always be situations in which traffic peaks will lead to Spärming. Traditionally, this is handled by the mobile terminal holding the old channel for as long as possible, while it continuously tries to make a handover to a better traction channel. Since no free channel is assigned to the mobile terminal, it will be possible. v «. i u. .. ....;: ..__ - _ _ _ _ _ _, ¿z 'I; ;. .. .. .. - - '' ..; . . 515 0 Û .l 'I .. I' 1 2 .: z.. . . I 1,,. .. .. .. - - riaikvaliteteii - ter h- 'gen is connected to the "wrong" base station with respect to signal strength. The risk then arises that the call will be disconnected too soon.

The quality-enhancing procedures described above are necessary and useful, but are not always sufficient. Furthermore, it may be economically disadvantageous to try to completely avoid areas of high interference in a cellular telephone network. A network that can operate with perfect quality and sufficient capacity to be able to handle 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 which, after all, solve a problem that only occurs in limited areas or during certain periods. The operators will in turn be reluctant to invest in such resources if the cost cannot be borne by the end user.

It is known that the transmission quality can be improved by using error handling coding. Better coding over the air interface between a mobile terminal and a base station will significantly reduce the bit error rate of the radio link. In a TDMA connection between the mobile terminal and a base station, a fixed number of bits are transmitted in each burst. Error handling coding basically involves the calculated use of redundancy. A strong error handling code is generally longer than a weak one. Therefore, in a TDMA connection, there is a balance between the level of error handling, ie. the number of coding bits and the amount of traffic information. Increasing error handling reduces the number of pieces of information that can be sent in each burst, which in turn results in poor speech quality.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an apparatus and method for a cellular mobile telephone system based on TDMA, 10 15 20 25 30 -: v | - s, .- ..f --- -; _ ", _". . . . f. - z. ~ -.- * -> a. = f. u »-u» I "1,2. .nr. ay uf.. v! gg _,. vu '' '"' "'... <:' --- '- II ll ^'" 'r * “^” 'L' ^ '“experienced the quality of a voice connection in areas S61 P F- i! and F- OC? OC! C) H E? v-i CF 'm: .'21 v-r ß-, _l O !! <1 CL (I) II with high interference.

It is a further object of the invention to provide an apparatus and method for a cellular mobile telephone system based on TDMA which enables improvement of speech quality substantially without increasing the operating costs which fall on an operator.

These and further objects are achieved in a cellular telephone communication system based on TDMA by providing means in a mobile terminal and / or a base station for assigning at least one additional time slot to a particular connection. In particular, the mobile terminal may be provided with manually operable means, e.g. a button, by means of which the end user can request the assignment of one or more additional time slots from the network when the speech quality of a connection is perceived as too poor. The request can be activated during a call or the mobile phone can be configured before a call to send a request when the quality of the connection, ie. bit error rate, will be lower than a predetermined level. Alternatively, the network may be configured to automatically allocate at least one additional time slot for communication with a particular mobile terminal when the interference level is found to exceed a predetermined level.

The invention further relates to a mobile terminal comprising manually actuable means for requesting the allocation of at least one additional time slot for a certain connection.

A method is further described in which the speech quality of a connection can be improved by assigning at least one additional time slot to a connection. The additional time slot or slots can be used to increase the amount of error handling code sent with each fixed amount of user or speech information. In an alternative embodiment, the additional time slot is used to transmit l0_ 15 20: i fi 30 H me: 515 700 5 4.1. _ ... The additional time slots are preferably allocated at the request of the user. Requests can be made during the call or a corresponding function can be activated before a call to have the mobile phone send out a request when the quality of the connection justifies the use of an additional time slot. The additional time slots can be assigned by the base station equipment when the bit error rate of a connection to an end user reaches a predetermined level and when the end user has registered a request with the network for improved speech quality.

During periods of interference, the speech quality of a connection can be improved by increasing the redundancy of a time slot or burst by spreading user or speech data over two or fl of your time slots instead of one and using the additional capacity for improved error handling coding. Furthermore, the overall perceived speech quality can be improved by using the additional time slots to transmit an improved and therefore longer speech coding scheme (codec). Since the additional time slots can be allocated at the request of the end user e.g. during periods of high interference, the additional cost of the connection may be charged to the user directly by the operator. The end user does not need to request improved speech quality if he or she is not willing to pay and the operator does not need to make costly additional resources publicly available.

Brief Description of the Drawings Further objects and advantages of the present invention will become apparent from the following description of preferred embodiments, by way of example only, with reference to the accompanying drawings, in which Figure 1 schematically shows a cellular mobile telephone system; Figure 2 shows conventional allocation of time slots to a connection Figure 3 shows the allocation of time slots to a connection according to the present invention, 10 15 20 Btw 30 515 700§§ @ TQ§ fl¿ g¿ u '.- 6 Figure 4 shows a method for improving the speech quality at the request of the user, and Figure 5 shows a method for improving the speech quality based on a registered user request.

Detailed Description of the Drawings A link in a typical cellular mobile telephone system, represented by a GSM system, is shown in Figure 1. The cellular telephone system comprises a network comprising your base transceiver station (BTS) 20, generally located in the middle of a cell. cell, base station controllers (BSCs) 30, each of which typically controls a number of base stations 20, and mobile services switching centers (MSCs) 40 which are responsible for connecting, managing and. disconnect connections and also to route calls to the correct cell. The exchanges also provide interfaces to the fixed telephone system (P STN or ISDN) and solutions for billing and accounting services. Mobile telephones or mobile terminals 10 which move between cells communicate over an air interface with one, usually the nearest, base station 20.

In the system shown in Figure 1, the mobile terminal 10 is equipped with a control button or buttons, which are shown schernatically as a rectangle 11. The control button or buttons ll enable the user to request an improvement in the speech quality from the network, as will be the case. described in more detail below.

In the air interface between a mobile terminal 10 and a base station 20, user traffic is transmitted by a combination of frequency sharing and time sharing technology.

Up to eight mobile terminals 10 are assigned to a fixed bandwidth communication channel of typically 200 kHz.

The user traffic between each mobile terminal 10 and the network is divided into bursts of fixed length, or time slots. These bursts or time slots are multiplexed into TDMA-10 15 20 25 30 g 515 1 - '.n »- 7 slots with 8 slots, which are separated over the karial by nodulating the carrier. The frames can further be multiplexed together to be transmitted for a certain period of time.

Figure 2 shows the function of a voice connection to and from a mobile terminal 10. The assignment of time slots is shown for a carrier frequency channel used for the downlink. Eight time slots, numbered from 0 to 7 in the figure, form a TDMA frame and, in general, a time slot in each group of eight is assigned to user traffic from a particular mobile terminal 10. This is illustrated by the shaded areas.

The user traffic that is transmitted in each time slot includes digitized voice information together with synchronization and control bits. The conversion of an analog speech signal into a digital signal is performed using a speech coding scheme (codec) which is conventionally designed for a fixed number of bits. The resulting digital signal is then encoded to make the signal less susceptible to transmission errors.

Error handling coding introduces redundancy in the digitized speech signal. Increased redundancy, ie. a higher number of error handling bits generally indicates that the connection is more robust. However, since the number of bits that can be transmitted in a single burst is limited, a compromise is made between the number of error handling bits and the speech information bits transmitted for each time slot so that acceptable speech quality is achieved at low interference levels and sufficient error handling crane is achieved to maintain this interval quality. .

When a mobile user passes through areas with very high interference (low C / I ratios), the resulting bit error rate may exceed what can be corrected by the error handling coding used for a time slot and the perceived speech quality will deteriorate.

In accordance with the invention, the quality of a speech connection is improved by using a more powerful error handling scheme and thereby increasing the redundancy in the error handling coding of the digitized speech signal. Rather than reducing the number of digitizers in a assigned degraded speech quality at low interference levels, the same amount of speech information is transmitted, spread over two time slots. The additional capacity achieved through this second time slot is then available to significantly increase the redundancy in the error handling coding. This extended allocation of time slots per connection is shown in Figure 3.

Two consecutive time slots in each frame are assigned here to a single mobile terminal 10. If the bit error rate is particularly high, more than one additional time slot for each frame can be assigned to a single connection. The arrangement with successive time slots assigned to a single mobile terminal 10 is only intended as an example. The time slots can also be distributed in another way in the TDMA frame.

The improved error handling coding can be realized by replacing the commonly used coding with a superior schedule for the time slots assigned to a connection. Another possibility would be to pre-encode the digitized speech, e.g. at the application level and there fi er expose the pre-coded signal to the usual error handling coding process. This latter procedure has the advantage that existing functions, which in some cases may be dictated by a standard, can be maintained.

Since the improvement of speech quality as described above naturally leads to an increased allocation of resources to a single connection, it is preferable that this function is provided at the request of the user. The mobile terminal 10 is therefore equipped with one or more control keys 11 which may be specially assigned function keys or a special combination of keys already present on the keypad. When the user experiences substandard speech quality during a connection, he can request improvement of the speech quality by using a control button ll. When the interference period is over, the user can request that the connection return to only one time slot, either with the same or another specially assigned function key or a special command that is keyed on the keypad. Requests sent by the user may result in an additional time slot being allocated. In this case, the request must be made 10 15 20 v '25 30 515 700: fax ff t., .I 9. Alternatively, the number of time slots required for the connection (up to a maximum of 8) can be specified in a request.

Figure 3 shows the procedure for this improvement of speech quality at the request of the user. In step 1, the mobile terminal 10 informs the network of requests to improve the voice quality. This request is handled by the mobile network exchange 40 which sends an assignment request to the base station controller 30 if the network can allow the change. Thereafter, a resource upgrade or downgrade takes place, in which the number of allocated time slots is changed and the appropriate error coding scheme for the increased capacity is activated. Then, in step 3, the base station controller 30 informs the mobile network exchange 40 that the procedure has been completed, and in step 4 the change of the mobile network exchange 40 to the mobile terminal 10 is confirmed.

Because there is a connection between the perceived speech quality and the bit error rate of a connection, the mobile terminal 10 may be equipped with certain functions that allow it to monitor the line and indicate to the user whether the connection could be improved if an additional time slot was used. Similarly, the mobile terminal 10 may prompt the user to request that an additional time slot be released when the connection is good enough to provide acceptable speech quality with the standard error handling coding provided with one time slot per frame.

In a wider embodiment, the latter arrangement is extended. Specifically, the functions of the manually controllable control buttons ll are built into the phone itself by the user changing the settings in the phone 10 before a call. When this function is active, the mobile phone itself automatically requests an additional time slot when the bit error rate becomes too high and removes an additional time slot when the bit error rate drops again. Activation of this function could be achieved by using a specially assigned control key ll or by sending the request by pressing a specified key configuration on the regular keypad of the telephone. 10 15 20 ° fi 515 700 n. .- 10 In a wider embodiment, the user does not need to manually request the improvement of speech quality but can instead register a permanent request in the network. The network continuously monitors the signal level and the quality of the channels so that any increase in carrier to interference (C / I) ratio can be detected. If a request from the user is registered, the quality of a connection can be improved automatically by the network by allocating tid your time slots and using a more powerful error handling coding. Such a request can be stored in a register in the network, e.g. together with information about the user's identity and other additional services provided to the user in the Home Location Register (CPR). As with the manual request procedure, the network can reduce the number of time slots allocated when the bit error rate of the connection returns to an acceptable, or at least less noticeable, level.

This procedure is shown in Figure 4. In this procedure, it is assumed that the register containing the registered request from the user has been examined and that the base station controller 30 knows that the user should have automatic improvement of the speech quality. In step 1 of the method, the base station controller 30 determines that the bit error rate of a connection is such that the user could benefit from being assigned an additional time slot, or that an additional time slot assigned is removed. In step 2, the controller 30 informs the base station 20 of which additional time slots are to be allocated or removed. This command is confirmed in step 3. The mobile terminal 10 is then informed of the configuration change in step 4 and this command is confirmed in step 5.

Steps 2 and 3 are performed as many times as needed, depending on the bit error rate of the connection and the strength of the error handling scheme used.

It is easily understood that a combination of the two methods shown in Figures 4 and 5 can also be realized, whereby a manual request is only processed when this function is registered by the network for the user. 10 15 20 25 515 700 m; ll By using the additional time slots allocated to use an improved error handling coding, the speech quality can be brought up to normal speech quality for a connection that experiences little or negligible interference. In a further embodiment, the additional time slot or time slots allocated on request to a user are used to include an improved speech coding scheme (codec). The resulting connection then has a speech quality which, depending on which codec is used, can also exceed the quality of a fixed connection.

For practical reasons, it may be advantageous to limit the number of time slots used to implement an enhanced codec. At the request of the user, as described above, additional time slots can then be allocated to improve the error handling of a connection.

It will be appreciated that the improved speech quality achieved through the allocation of additional time slots and the use of an improved error handling coding or codec can be provided only in the downlink connection or in both the downlink and uplink connections. In some cases, the user may request that the speech quality of an uplink connection be improved when the network detects a deterioration.

Since the improvement in speech quality is initiated by the user, either through a request during a connection or through a previously registered request, the cost of allocating the extra resources can be passed directly on to the user. The user, in turn, will only have to pay for periods of extended time slots that he requests.

It may also be interesting for some services such as alarm centers to be able to permanently use this type of improvement in speech quality.

It will be appreciated that the invention may also be applied to data traffic.

Claims (13)

10 15 20 25 30 515 700 12 Patent claims A device (30, 4,) for a .. interception in a cellular telephony meter according to a time division, Time Division Multiple Access - TDMA, standard, arranged to communicate with at least one mobile terminal (10), characterized in that is arranged to receive a request from the mobile terminal (10) that a connection intended to transmit a certain amount of payload per unit of time be increased by an additional channel when the quality drops below a certain level, and to use this additional channel to increase the amount of transmitted information representing said payload for the connection. Unit (30, 40) according to claim 1, arranged to receive and register requests before the connection is connected, to monitor the quality of the connection and to automatically extend the connection when the quality drops below a certain level. Unit (30, 40) according to claim 1, arranged to receive the request while the connection is connected and to extend the connection when the request is received. Mobile telephone for use in a time division, TDMA, mobile telephone network, characterized in that it comprises means (1 l) for requesting, while a connection intended to transmit a certain amount of payload per time unit is connected, the assignment of an additional channel to the connection to increase the amount of transmitted information representing said payload. Mobile telephone according to Claim 4, characterized in that the means (1 l) for requesting the assignment of a further channel is arranged for manual action. Mobile telephone according to claim 4 or 5, characterized in that it comprises means for monitoring the bit error rate and, when the bit error rate exceeds a certain threshold value. inform a mobile phone user that the call quality could be improved by adding an additional channel. 10 15 20 25 30 515 700 13 A mobile telephone according to any one of claims 4-6, characterized in that it comprises means for monitoring the bit error rate and, when the bit error rate exceeds a certain rnelzøl host value mvfnrna ielzf clzinlza Pn 1142 ". .ioiu .Jiuvnu v .. v., mobile phone network device. Method for improving call quality between a mobile terminal (10) and a time-sharing mobile telephone network, in a connection intended to transmit a certain amount of payload per unit of time, characterized in that a request for assignment of at least one additional channel to the connection is sent from the terminal ( 10) to a unit (30, 40) in the network, at least one additional channel is used in the connection to increase the amount of information transmitted regarding the payload. The method of claim 8, wherein the additional channel is used to transmit the payload error coding information. A method according to claim 8 or 9, wherein the additional channel is used to increase the number of voice coding bits, voice coding information. A method according to any one of claims 8-10, wherein the terminal (10) monitors the call quality and the request for assignment of at least one additional channel is sent automatically from the terminal when the call quality is below a certain threshold value. A method according to any one of claims 8-9, wherein the user initiates the request for assignment of at least one channel depending on perceived call quality. The method of claim 12, wherein the terminal monitors call quality and. when this is the case, the user informs that the call quality could be improved.