Title of Invention	A METHOD FOR TRANSMITTING VOICE OVER A RADIO INTERFACE IN A DIGITAL RADIO COMMUNICATIONS SYSTEM HAVING MOBILE STATIONS AND BASE STATIONS AND A BASE STATIONS SYSTEM
Abstract	In the method according to the invention for transmitting voice over a radio interface, at least one signal is transmitted between a mobile stations (MS) and a base station (BS). At least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions of the radio interface is determined from the signal. The characteristic value RXLEV, RXQUAL, ta, C/I) or values are compared with a threshold value (S,S1, S2) and a voice coder (SC1, SC2) is selected in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions.

Title of Invention

A METHOD FOR TRANSMITTING VOICE OVER A RADIO INTERFACE IN A DIGITAL RADIO COMMUNICATIONS SYSTEM HAVING MOBILE STATIONS AND BASE STATIONS AND A BASE STATIONS SYSTEM

Abstract

In the method according to the invention for transmitting voice over a radio interface, at least one signal is transmitted between a mobile stations (MS) and a base station (BS). At least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions of the radio interface is determined from the signal. The characteristic value RXLEV, RXQUAL, ta, C/I) or values are compared with a threshold value (S,S1, S2) and a voice coder (SC1, SC2) is selected in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions.

Full Text	- 1A- Description Method for transmitting voice over a radio interface in a digital radio communications system with mobile stations and base stations The invention relates to a method and a base station system for transmitting voice over a radio interface in a digital radio communications system with mobile stations and base stations. Such a base station system is part of a digital radio communications system, for example the GSM mobile radio network (Global System for Mobile Communications", as is known from J. Biala, "Mobilfunk und intelligente Netze [Mobile radio and intellignet networks]", VIEWEG Verlag [publisher], Braunschweig/Wiesbaden, 1995, in particular pages 57 to 92. Mobile communications systems permit communications links to be set up to mobile subscribers by transmitting information, in particular voice information, over a radio interface. If a plurality of subscribers are separated on the same carrier frequency of this radio interface by different time slots, the system is a time-division multiplex mobile communications system. The time-division multiplex method is also referred to as TDMA {Time Division Multiple Access) method. In addition, or as an alternative, to time-division multiplexing, other methods for separating the subscribers can also be used on the radio interface. In the GSM mobile radio system, frequency-division multiplexing is offered in addition to time-division multiplexing. A mobile communications system comprises at least one base station system which contains, for example, a base station controller which is connected to a multiplicity of base stations. The base stations each supply a radio - 2 - coverage area and therefore make available radio resources for mobile stations in the radio coverage area of the respective base stations. The radio coverage areas of adjacent base stations overlap here, so that a transfer procedure for a communications link for a mobile station is possible between two base stations. Each base station can only have a limited supply of radio resources here. Such radio resources are formed, for example in the case of the GSM mobile radio system, by channels which are characterized by their frequency and the time slot. In the GSM mobile radio system, the base station controller, for example, performs the functions of a radio-channel switching device within the base station system and controls the base stations. A base station controller carries out, in particular, the functions of managing the radio channels of the base stations and administering and executing transfer procedures (handovers). In digital mobile radio systems, as a rule digital voice coders are used which generate a data rate which is less than the 64 kbit/s data rate of PCM 30 channels. This is necessary as the available radio resources are limited. For a further development of voice coders, half rate coders which use half the data rate for voice transmission were planned for the GSM system and for the D-AMPS mobile radio system. Full rate voice coders and half rate voice coders are thus available. EP 0 472 511 A2 has disclosed a digital mobile radio system which uses two different voice coders. The bit error rate of the transmission between mobile station and base station is measured regularly here. When there is a low bit error rate, the half rate coder is used, and when there is a high bit error rate the full rate coder is used. - 3 - The invention is based on the object of developing the method and the base station system for improved voice transmission.The method of achieving this object will now be described with further advantageous refinements of the present invention. In the method according to the invention for transmitting voice over a radio interface, at least one signal is transmitted between a mobile station and a base station. At least one characteristic value relating to transmission conditions of the radio interface is determined from the signal. The characteristic value or values are each compared with a threshold value. A voice coder is selected in accordance with the comparison result, a voice coder being used with a higher data rate under good transmission conditions than under poor transmission conditions. This selection of a voice coder makes use of the fact that the subjective voice reproduction quality of both voice coders differs as a function of the transmission conditions. When are there are changing conditions due to noise or co-frequency interference the voice coders behave differently. If the voice coders are advantageously a full rate coder and a half rate coder of a GSM mobile radio system, a subscriber prefers the full rate coder under good channel conditions, while the half rate coder is preferred under poor channel conditions. According to the invention, the voice quality for a subscriber of the radio communications system is improved by means of continuous monitoring of the quality of the transmission conditions. -4- According to one advantageous development of the invention, a reception level, a bit error rate and/or a. value which is proportional to the signal transit time between the mobile station and the base station are determined as a characteristic value. When the reception field strength is high, the bit error rate low and signal transit times short it is possible to assume good transmission conditions. The full rate coder should therefore be selected in such a case. In contrast to the solution proposed in EP 0 472 511 A2, the objective is to select the voice coder with the low data rate in the case of poor quality - i.e. a high bit error rate - so that fewer useful data bits are available for the voice information. For the voice coder with the higher date rate, the ratio of guard bits to useful data bits is advantageously smaller than in the case of the voice coder with the low data rate. The larger proportion of-guard bits leads to a higher level of immunity to interference, and to a higher reproduction quality of the voice information in the case of poor channel conditions. In the case of a half rate coder a different source code method is used, which reduces the source code required in comparison to the full rate coder by more than half, so that additional guard bits are introduced in order to increase the protection of the transmission. The measurements relating to the transmission conditions can be carried out both in the base station and in the mobile station and subsequently signaled to the base station. The decision relating to the voice coder to be selected is advantageously taken in the base station system, for example in the base station or the base station controller. According to one further advantageous refinement of the invention at least two threshold values are used - 5 - switching over from a higher to a low data rate taking place with worse transmission quality than switching over from a low to a higher data rate. A hysteresis is thus planned which prevents continuous switching over between the voice coders when there are transmission conditions in the vicinity of the limits. The continuous switching over can additionally be prevented by means of a time switch which is reset after switching over and prevents renewed switching over for an adjustable time. The decision relating to the voice coder to be selected is advantageously taken both at the start of a conversation and during a connection. In addition, this decision is additionally associated with a selection of a transmission channel of the radio interface and/or a transfer procedure (handover). The method according to the invention can thus easily be implemented in existing mobile radio networks. Characteristic values which can be obtained particularly easily from existing mobile radio systems are the reception level and the bit error rate (which are specified as scaled values RXLEV, RXQUAL), which as a rule are already present and can be included in the comparison. The threshold values are advantageously predefined by an operations and maintenance center and can be adapted from base station to base station and/or in accordance with the instantaneous capacity utilization of the mobile radio system. When there is a high level of utilization of individual radio cells it may be necessary to prefer the half rate coder generally. The invention will be explained in more detail below by means of exemplary embodiments and with reference to accompanying drawings, in which: FIG 1 shows a mobile communications system, - 6 - FIG 2 shows a block circuit diagram of components of a base station system, FIG 3 shows a flowchart of the method according to the invention with the reception level as a characteristic value, FIG 4 shows a flowchart of the method according to the invention with a variable for the bit error rate as a characteristic value, FIG 5 shows a flowchart of the method according to the invention with the reception level and a variable for the bit error rate as characteristic values, FIG 6 shows a flowchart of the method according to the invention in combination with a channel change, and FIG 7 shows a flowchart of the method according to the invention in combination with a handover operation. The mobile communications system which is illustrated by way of example in FIG 1 is a known GSM mobile radio system that is composed of one or more mobile switching centers MSC which are networked to one another and form the access to a fixed network PSTN. Furthermore, these mobile switching centers MSC are connected in each case to at least one base station controller BSC via a transcoder unit TRAU. Each base station controller BSC in turn permits a connection to at least one base station BS. Such a base station BS is a radio station which can set up a message connection to mobile stations MS via a radio interface. Each base station BS implements a radio coverage area. According to Fig. 1, the base stations implement a radio - 7 - coverage area, represented in each case by way of example as a hexagon for the sake of simplification, these radio coverage areas covering, as cells Z, the entirety of the radio network. Overlaps are provided between the cells Z so that a mobile station MS in the overlap region can set up a connection to at least two base stations BS. A base station controller BSC, a transcoder unit TRAU and a number of base stations BS form a base station system BSS for which, according to FIG 1, an operations and a maintenance center OMC performs functions for fault monitoring, for network management and for configuration of the individual radio coverage areas. Details on the configuration of a base station system BSS can be found in the Siemens AG product documentation "System Netztibersicht [System network overview]" GSM (TED-NET), A30808-X3232-Y104-1-18, 1995, in particular pages 65 to 67. The base station BS, the base station controller BSC and the transcoder unit TRAU, which are outlined by way of example in FIG 2, form components of the base station system BSS. A base station controller BSC may be integrated as a separate unit or together with a base station BS or other components of the communications network. The transcoder unit TRAU [lacuna] The base station controller BSC has links to a plurality of base stations BS, from which measured values relating to the transmission conditions from or to mobile stations MS arrive. Such measurement variables, which may possibly also only be obtained after internal calculations, are, for example, the reception level RXLEV, a scaled variable relating to the bit error rate RXQUAL, a timing advance ta or a signal-to-noise ratio C/I. These values can also be determined by a mobile station MS. - 8 - A further input variable of the base station controller BSC is the threshold value S which is transmitted as a single value, or in the form of sub-variables S1, S2, for each base station BS by an operations and maintenance center OMC. The base station controller BSC contains a memory device SP in which the characteristic values RXLEV, RXQUAL and, if appropriate, to and C/I, as well as the threshold values S, S1, S2, are stored. In addition to further components which are not significant for the invention, the base station controller BSC contains a control device SE for comparing the characteristic value RXLEV, RXQUAL, ta, C/I with the threshold values S, S1, S2. During these comparisons, a hysteresis is provided using positive and negative offset values with respect to the decisions illustrated in Figs 3 to 7 is provided. An exemplary base station BS which is connected to the base station controller BSC contains a transceiver device TRX and a signal evaluation device SA. The base station BS receives useful information and signaling information from mobile stations MS via an antenna A. The characteristic values RXLEV, RXQUAL are signaled by the mobile station MS, while the data relating to the signal transit time in the form of the timing advance ta and 'data relating to the signal-to-noise ratio C/I are acquired in the base station BS itself from the reception signals. However, it is also possible to use values which are determined only in the mobile station MS or in the base station BS or alternative combinations for the decision process. The characteristic values RXLEV, RXQUAL, to and C/I are signaled to the base station controller BSC by the base station BS. - 9 - A transcoding unit TRAU which is located between base station controller BSC and mobile switching center MSC contains two voice coders SC1, SC2 with different data rates. The voice information which has been received at the network end from the mobile switching center MSC, for example over a PCM link, a communications link, at 64 kbit/s is voice-coded using one of the two voice coders SC1, SC2 and transmitted onto the base station controller BSC at 11.4 kbit/s (SC1) or 22.8 kbit/s (SC2). The data rates arise as a result of convolution coding of the source code and the addition of guard bits. In a full rate coder, for the voice transmission one time slot is used for the voice information per frame of the TDMA system. With a half rate coder, it is only one time slot in every second frame, as a result of which twice the capacity can be supported without changing the radio interface in the channel structure. The selection of a voice coder SC1, SC2 is carried out on the instruction of the control device SE of the base station controller BSC. Decoders or voice coders (not illustrated) of the mobile station MS are to be provided appropriately. The devices SE, SP, SC1, SC2, SA which are designed to carry out the method according to the invention can, according to an alternative embodiment of the invention, also be arranged in other components of the base station system BSS. In particular, the transcoding unit TRAU can be integrated both in the mobile switching centre MSC and in the base station controller BSC. The method according to the invention and the function of the described modules SA, SE are explained below with reference, to Figures 3 to 7. According to Fig. 3, the signal evaluation device SA determines the reception level RXLEV as a characteristic value for the transmission conditions to - 10 - a mobile station MS. The determination of the reception level RXLEV is carried out repeatedly, for example at cyclical intervals. The reception level RXLEV is compared with the threshold value S in the control device SE. If the reception level RXLEV is greater than the threshold value S, the full rate coder SC1 is selected for the voice transmission. If the particular reception level RXLEV is smaller than the threshold value S, the half rate coder SC2 is selected. A time switch is provided for the repetition of the decision so that the comparison is repeated with new measured values after a certain waiting time. According to FIG 4, as an alternative to the reception level RXLEV, a quality parameter, the bit error rate (BER), expressed by the scaled value RXQUAL, is used for the comparison with the threshold value S. At a high bit error rate (large RXUAL, poor transmission conditions) greater than the threshold value S, the half rate coder SC2 is selected, and if the bit error rate is smaller the full rate coder SC1 is selected. According to a third exemplary embodiment according to Fig. 5, the reception level RXLEV and the bit error rate are included in the comparison with two threshold values S1, S2. The half rate coder SC2 is selected if the reception level RXLEV is smaller than the first threshold value S1 and RXQUAL is greater than the second threshold value S2 (the bit error rate large). The selection of a voice coder SC1, SC2 is carried out according to further exemplary embodiments, Figs 6 and 7, in combination with decisions relating to a channel change or a transfer procedure (handover). In the method according to Fig. 6, a decision relating to the channel change is taken after the evaluation of GR 96 P 264 8 -11- the characteristic values RXLEV, RXQUAL, ta, C/I. If a channel change is not possible or if no channels with better transmission conditions are available, the comparisons according to Fig. 5 are carried out and a voice coder SC1, SC2 is selected. In the method according to Fig. 7, a decision relating to a handover is taken first and the voice coder SC1, SC2 is redefined only if no suitable adjacent cells for the connection to be transferred to can be determined. The sequence of the voice coder change, the channel change or the transfer procedure is to take place as quickly and imperceptibly as possible. This is ensured largely by procedures of the radio interface management in the base station controller BSC. Network-end measurements of the adjacent base stations (BS) (measurement reporting/radio link control) and the timing advances ta for the respective base stations BS can be used as selection criteria for a channel or an adjacent radio coverage area. A list of preferred channels or base stations BS which specifies respectively available radio resources and specific adjacent relationships between the channels and the radio coverage areas can be taken into account here. A particular advantage of the method according to the invention is that when there are limited radio resources, for example if only one transceiver device TRX is present in the base station (and there are thus few alternatives to a qualitatively poor radio channel) or if there are no suitable handover candidates among the adjacent cells, it is nevertheless possible to improve the voice quality. Likewise, simple implementation in existing mobile communications networks, for example in the GSM mobile radio network, is possible without having to perform wide-ranging circuit modifications. -12-WE CLAIM: 1. A method for transmitting voice over a radio interface in a digital radio communications system having mobile stations (MS) and base stations (BS), comprising the steps of : - transmission of at least one signal between 3 mobile station (MS) and a base station CBS); - determination of at least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions of the radio interface from the signal; comparison of the characteristic value (RXLEV, RXQUAL, ta, C/I) with at least one threshold value (S, S1, S2); - selection of a voice coder (SC1, SC2) in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions. 2. The method as claimed in claim 1, wherein the determination step comprises determination of a reception level, a bit error rate and/or a value which is proportional to the signal transit time (ta) between the mobile station (MS) and the base station CBS) as a characteristic value (RXLEV, RXQUAL,ta, C/I). 3. The method as claimed in claim 1 or 2, wherein the determination step comprises using at least two threshold values, - 13 - switching over from a higher to a low data rate taking place with worse transmission quality than switching over from a low to a higher data rate. 4. The method as claimed in one of the preceding claims, wherein the selection of a transmission channel of the radio interface additionally takes place in accordance with the comparison result. 5. The method as claimed in one of the preceding claims, wherein the ratio of guard bits to useful data bits is lower for the voice coder (SC1) with higher data rate than in the case of the voice coder (SC2) with low data rate. 6. A base station system (BSS) for a digital radio communications system with mobile stations (MS) and base stations (BS), said system comprising : - at least one transceiver device (TRX) for transmitting voice over a radio interface; at least two voice coders (SC1, SC2) with different data rates ; - a signal evaluation device (SA) for determining at least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions of the radio interface; - 14- a storage device (SP) for storing the characteristic value (RXLEV, RXG3UAX, ta, C/I) and at least one threshold value (S, S1, S2); and a control device (SE) for comparing the characteristic value (RXLEV, RXQUAL, ta, C/I) with the threshold value (S, S1,S2) and for selecting a voice coder (SC1, SC2) in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions. 7. The base station system (BSS) as claimed in claim 6, wherein a first voice coder (SC1) being designed for full rate voice coding, and a second voice coder (SC2) being designed for half rate voice coding. 8. The base station system (BSS) as claimed in claim 6 or 7, wherein the characteristic value (RXLEV, RXQUAL) is related to a reception level and to a bit error rate. 9. The base station system (BSS) as claimed in claim 6, 7, or 8, wherein the characteristic value (ta) is related to a value which is proportional to the signal transit time between a mobile station (MS) and the base station (BS). - 15 - 10. The base station system (BSS) as claimed in one of claims 6 to 9, wherein at least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions is signaled by a mobile station (MS) to said base station. In the method according to the invention for transmitting voice over a radio interface, at least one signal is transmitted between a mobile stations (MS) and a base station (BS). At least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions of the radio interface is determined from the signal. The characteristic value RXLEV, RXQUAL, ta, C/I) or values are compared with a threshold value (S,S1, S2) and a voice coder (SC1, SC2) is selected in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions.

Full Text

- 1A-
Description
Method for transmitting voice over a radio interface in a digital radio communications system with mobile stations and base stations
The invention relates to a method and a base station system for transmitting voice over a radio interface in a digital radio communications system with mobile stations and base stations.
Such a base station system is part of a digital radio communications system, for example the GSM mobile radio network (Global System for Mobile Communications", as is known from J. Biala, "Mobilfunk und intelligente Netze [Mobile radio and intellignet networks]", VIEWEG Verlag [publisher], Braunschweig/Wiesbaden, 1995, in particular pages 57 to 92.
Mobile communications systems permit communications links to be set up to mobile subscribers by transmitting information, in particular voice information, over a radio interface. If a plurality of subscribers are separated on the same carrier frequency of this radio interface by different time slots, the system is a time-division multiplex mobile communications system. The time-division multiplex method is also referred to as TDMA {Time Division Multiple Access) method. In addition, or as an alternative, to time-division multiplexing, other methods for separating the subscribers can also be used on the radio interface. In the GSM mobile radio system, frequency-division multiplexing is offered in addition to time-division multiplexing.
A mobile communications system comprises at least one base station system which contains, for example, a base station controller which is connected to a multiplicity of base stations. The base stations each supply a radio

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coverage area and therefore make available radio resources for mobile stations in the radio coverage area of the respective base stations. The radio coverage areas of adjacent base stations overlap here, so that a transfer procedure for a communications link for a mobile station is possible between two base stations. Each base station can only have a limited supply of radio resources here. Such radio resources are formed, for example in the case of the GSM mobile radio system, by channels which are characterized by their frequency and the time slot.
In the GSM mobile radio system, the base station controller, for example, performs the functions of a radio-channel switching device within the base station system and controls the base stations. A base station controller carries out, in particular, the functions of managing the radio channels of the base stations and administering and executing transfer procedures (handovers).
In digital mobile radio systems, as a rule digital voice coders are used which generate a data rate which is less than the 64 kbit/s data rate of PCM 30 channels. This is necessary as the available radio resources are limited. For a further development of voice coders, half rate coders which use half the data rate for voice transmission were planned for the GSM system and for the D-AMPS mobile radio system. Full rate voice coders and half rate voice coders are thus available.
EP 0 472 511 A2 has disclosed a digital mobile radio
system which uses two different voice coders. The bit
error rate of the transmission between mobile station
and base station is measured regularly here. When there
is a low bit error rate, the half rate coder is used,
and when there is a high bit error rate the full rate coder is used.

- 3 -
The invention is based on the object of developing the
method and the base station system for improved voice
transmission.The method of achieving this object will
now be described with further advantageous refinements of the present invention.
In the method according to the invention for transmitting voice over a radio interface, at least one signal is transmitted between a mobile station and a base station. At least one characteristic value relating to transmission conditions of the radio interface is determined from the signal. The characteristic value or values are each compared with a threshold value. A voice coder is selected in accordance with the comparison result, a voice coder being used with a higher data rate under good transmission conditions than under poor transmission conditions.
This selection of a voice coder makes use of the fact that the subjective voice reproduction quality of both voice coders differs as a function of the transmission conditions. When are there are changing conditions due to noise or co-frequency interference the voice coders behave differently.
If the voice coders are advantageously a full rate coder and a half rate coder of a GSM mobile radio system, a subscriber prefers the full rate coder under good channel conditions, while the half rate coder is preferred under poor channel conditions. According to the invention, the voice quality for a subscriber of the radio communications system is improved by means of continuous monitoring of the quality of the transmission conditions.

-4-
According to one advantageous development of the
invention, a reception level, a bit error rate and/or a.
value which is proportional to the signal transit time
between the mobile station and the base station are
determined as a characteristic value. When the
reception field strength is high, the bit error rate
low and signal transit times short it is possible to
assume good transmission conditions. The full rate
coder should therefore be selected in such a case.
In contrast to the solution proposed in EP 0 472 511 A2, the objective is to select the voice coder with the low data rate in the case of poor quality - i.e. a high bit error rate - so that fewer useful data bits are available for the voice information. For the voice coder with the higher date rate, the ratio of guard bits to useful data bits is advantageously smaller than in the case of the voice coder with the low data rate. The larger proportion of-guard bits leads to a higher level of immunity to interference, and to a higher reproduction quality of the voice information in the case of poor channel conditions. In the case of a half rate coder a different source code method is used, which reduces the source code required in comparison to the full rate coder by more than half, so that additional guard bits are introduced in order to increase the protection of the transmission.
The measurements relating to the transmission conditions can be carried out both in the base station and in the mobile station and subsequently signaled to the base station. The decision relating to the voice coder to be selected is advantageously taken in the base station system, for example in the base station or the base station controller.
According to one further advantageous refinement of the invention at least two threshold values are used

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switching over from a higher to a low data rate taking
place with worse transmission quality than switching
over from a low to a higher data rate. A hysteresis is
thus planned which prevents continuous switching over
between the voice coders when there are transmission
conditions in the vicinity of the limits. The
continuous switching over can additionally be prevented
by means of a time switch which is reset after
switching over and prevents renewed switching over for
an adjustable time.
The decision relating to the voice coder to be selected is advantageously taken both at the start of a conversation and during a connection. In addition, this decision is additionally associated with a selection of a transmission channel of the radio interface and/or a transfer procedure (handover). The method according to the invention can thus easily be implemented in existing mobile radio networks. Characteristic values which can be obtained particularly easily from existing mobile radio systems are the reception level and the bit error rate (which are specified as scaled values RXLEV, RXQUAL), which as a rule are already present and can be included in the comparison.
The threshold values are advantageously predefined by an operations and maintenance center and can be adapted from base station to base station and/or in accordance with the instantaneous capacity utilization of the mobile radio system. When there is a high level of utilization of individual radio cells it may be necessary to prefer the half rate coder generally.
The invention will be explained in more detail below by
means of exemplary embodiments and with reference to
accompanying drawings, in which: FIG 1 shows a mobile communications system,

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FIG 2 shows a block circuit diagram of components of a base station system,
FIG 3 shows a flowchart of the method according to the invention with the reception level as a characteristic value,
FIG 4 shows a flowchart of the method according to
the invention with a variable for the bit error
rate as a characteristic value,
FIG 5 shows a flowchart of the method according to
the invention with the reception level and a
variable for the bit error rate as
characteristic values,
FIG 6 shows a flowchart of the method according to the invention in combination with a channel change, and
FIG 7 shows a flowchart of the method according to the invention in combination with a handover operation.
The mobile communications system which is illustrated by way of example in FIG 1 is a known GSM mobile radio system that is composed of one or more mobile switching centers MSC which are networked to one another and form the access to a fixed network PSTN. Furthermore, these mobile switching centers MSC are connected in each case to at least one base station controller BSC via a transcoder unit TRAU. Each base station controller BSC in turn permits a connection to at least one base station BS. Such a base station BS is a radio station which can set up a message connection to mobile stations MS via a radio interface.
Each base station BS implements a radio coverage area. According to Fig. 1, the base stations implement a radio

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coverage area, represented in each case by way of
example as a hexagon for the sake of simplification,
these radio coverage areas covering, as cells Z, the
entirety of the radio network. Overlaps are provided
between the cells Z so that a mobile station MS in the
overlap region can set up a connection to at least two
base stations BS. A base station controller BSC, a
transcoder unit TRAU and a number of base stations BS
form a base station system BSS for which, according to
FIG 1, an operations and a maintenance center OMC
performs functions for fault monitoring, for network
management and for configuration of the individual
radio coverage areas.
Details on the configuration of a base station system BSS can be found in the Siemens AG product documentation "System Netztibersicht [System network overview]" GSM (TED-NET), A30808-X3232-Y104-1-18, 1995, in particular pages 65 to 67.
The base station BS, the base station controller BSC and the transcoder unit TRAU, which are outlined by way of example in FIG 2, form components of the base station system BSS. A base station controller BSC may be integrated as a separate unit or together with a base station BS or other components of the communications network. The transcoder unit TRAU [lacuna]
The base station controller BSC has links to a plurality of base stations BS, from which measured values relating to the transmission conditions from or to mobile stations MS arrive. Such measurement variables, which may possibly also only be obtained after internal calculations, are, for example, the reception level RXLEV, a scaled variable relating to the bit error rate RXQUAL, a timing advance ta or a signal-to-noise ratio C/I. These values can also be determined by a mobile station MS.

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A further input variable of the base station controller
BSC is the threshold value S which is transmitted as a
single value, or in the form of sub-variables S1, S2,
for each base station BS by an operations and
maintenance center OMC.
The base station controller BSC contains a memory device SP in which the characteristic values RXLEV, RXQUAL and, if appropriate, to and C/I, as well as the
threshold values S, S1, S2, are stored. In addition to
further components which are not significant for the
invention, the base station controller BSC contains a
control device SE for comparing the characteristic
value RXLEV, RXQUAL, ta, C/I with the threshold values
S, S1, S2. During these comparisons, a hysteresis is
provided using positive and negative offset values with
respect to the decisions illustrated in Figs 3 to 7 is
provided.
An exemplary base station BS which is connected to the base station controller BSC contains a transceiver device TRX and a signal evaluation device SA. The base station BS receives useful information and signaling information from mobile stations MS via an antenna A. The characteristic values RXLEV, RXQUAL are signaled by the mobile station MS, while the data relating to the signal transit time in the form of the timing advance ta and 'data relating to the signal-to-noise ratio C/I are acquired in the base station BS itself from the reception signals. However, it is also possible to use values which are determined only in the mobile station MS or in the base station BS or alternative combinations for the decision process.
The characteristic values RXLEV, RXQUAL, to and C/I are signaled to the base station controller BSC by the base station BS.

- 9 -
A transcoding unit TRAU which is located between base station controller BSC and mobile switching center MSC contains two voice coders SC1, SC2 with different data rates. The voice information which has been received at the network end from the mobile switching center MSC, for example over a PCM link, a communications link, at 64 kbit/s is voice-coded using one of the two voice coders SC1, SC2 and transmitted onto the base station controller BSC at 11.4 kbit/s (SC1) or 22.8 kbit/s (SC2). The data rates arise as a result of convolution coding of the source code and the addition of guard bits. In a full rate coder, for the voice transmission one time slot is used for the voice information per frame of the TDMA system. With a half rate coder, it is only one time slot in every second frame, as a result of which twice the capacity can be supported without changing the radio interface in the channel structure. The selection of a voice coder SC1, SC2 is carried out on the instruction of the control device SE of the base station controller BSC. Decoders or voice coders (not illustrated) of the mobile station MS are to be provided appropriately.
The devices SE, SP, SC1, SC2, SA which are designed to carry out the method according to the invention can, according to an alternative embodiment of the invention, also be arranged in other components of the base station system BSS. In particular, the transcoding unit TRAU can be integrated both in the mobile switching centre MSC and in the base station controller BSC.
The method according to the invention and the function of the described modules SA, SE are explained below with reference, to Figures 3 to 7.
According to Fig. 3, the signal evaluation device SA
determines the reception level RXLEV as a characteristic value for the transmission conditions to

- 10 -
a mobile station MS. The determination of the reception level RXLEV is carried out repeatedly, for example at cyclical intervals.
The reception level RXLEV is compared with the threshold value S in the control device SE. If the reception level RXLEV is greater than the threshold value S, the full rate coder SC1 is selected for the voice transmission. If the particular reception level RXLEV is smaller than the threshold value S, the half rate coder SC2 is selected. A time switch is provided for the repetition of the decision so that the comparison is repeated with new measured values after a certain waiting time.
According to FIG 4, as an alternative to the reception level RXLEV, a quality parameter, the bit error rate (BER), expressed by the scaled value RXQUAL, is used for the comparison with the threshold value S. At a high bit error rate (large RXUAL, poor transmission conditions) greater than the threshold value S, the half rate coder SC2 is selected, and if the bit error rate is smaller the full rate coder SC1 is selected.
According to a third exemplary embodiment according to Fig. 5, the reception level RXLEV and the bit error rate are included in the comparison with two threshold values S1, S2. The half rate coder SC2 is selected if the reception level RXLEV is smaller than the first threshold value S1 and RXQUAL is greater than the second threshold value S2 (the bit error rate large).
The selection of a voice coder SC1, SC2 is carried out according to further exemplary embodiments, Figs 6 and 7, in combination with decisions relating to a channel change or a transfer procedure (handover).
In the method according to Fig. 6, a decision relating to the channel change is taken after the evaluation of

GR 96 P 264 8 -11-
the characteristic values RXLEV, RXQUAL, ta, C/I. If a channel change is not possible or if no channels with better transmission conditions are available, the comparisons according to Fig. 5 are carried out and a voice coder SC1, SC2 is selected. In the method according to Fig. 7, a decision relating to a handover is taken first and the voice coder SC1, SC2 is redefined only if no suitable adjacent cells for the connection to be transferred to can be determined.
The sequence of the voice coder change, the channel change or the transfer procedure is to take place as quickly and imperceptibly as possible. This is ensured largely by procedures of the radio interface management in the base station controller BSC.
Network-end measurements of the adjacent base stations (BS) (measurement reporting/radio link control) and the timing advances ta for the respective base stations BS can be used as selection criteria for a channel or an adjacent radio coverage area. A list of preferred channels or base stations BS which specifies respectively available radio resources and specific adjacent relationships between the channels and the radio coverage areas can be taken into account here.
A particular advantage of the method according to the invention is that when there are limited radio resources, for example if only one transceiver device TRX is present in the base station (and there are thus few alternatives to a qualitatively poor radio channel) or if there are no suitable handover candidates among the adjacent cells, it is nevertheless possible to improve the voice quality. Likewise, simple implementation in existing mobile communications networks, for example in the GSM mobile radio network, is possible without having to perform wide-ranging circuit modifications.

-12-WE CLAIM:
1. A method for transmitting voice over a radio interface in a digital radio communications system having mobile stations (MS) and base stations (BS), comprising the steps of :
- transmission of at least one signal between 3 mobile station
(MS) and a base station CBS);
- determination of at least one characteristic value (RXLEV,
RXQUAL, ta, C/I) relating to the transmission conditions of the
radio interface from the signal;
comparison of the characteristic value (RXLEV, RXQUAL, ta, C/I) with at least one threshold value (S, S1, S2);
- selection of a voice coder (SC1, SC2) in accordance with
the comparison result, a voice coder (SC1) being used with a
higher data rate under good transmission conditions than under
poor transmission conditions.
2. The method as claimed in claim 1, wherein the determination
step comprises determination of a reception level, a bit error
rate and/or a value which is proportional to the signal transit
time (ta) between the mobile station (MS) and the base station
CBS) as a characteristic value (RXLEV, RXQUAL,ta, C/I).
3. The method as claimed in claim 1 or 2, wherein the
determination step comprises using at least two threshold values,

- 13 -
switching over from a higher to a low data rate taking place with worse transmission quality than switching over from a low to a higher data rate.
4. The method as claimed in one of the preceding claims,
wherein the selection of a transmission channel of the radio
interface additionally takes place in accordance with the
comparison result.
5. The method as claimed in one of the preceding claims,
wherein the ratio of guard bits to useful data bits is lower for
the voice coder (SC1) with higher data rate than in the case of
the voice coder (SC2) with low data rate.
6. A base station system (BSS) for a digital radio communications system with mobile stations (MS) and base stations (BS), said system comprising :
- at least one transceiver device (TRX) for transmitting
voice over a radio interface;
at least two voice coders (SC1, SC2) with different data rates ;
- a signal evaluation device (SA) for determining at least
one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the
transmission conditions of the radio interface;

- 14-
a storage device (SP) for storing the characteristic value (RXLEV, RXG3UAX, ta, C/I) and at least one threshold value (S, S1, S2); and
a control device (SE) for comparing the characteristic value (RXLEV, RXQUAL, ta, C/I) with the threshold value (S, S1,S2) and for selecting a voice coder (SC1, SC2) in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions.
7. The base station system (BSS) as claimed in claim 6,
wherein a first voice coder (SC1) being designed for full rate
voice coding, and a second voice coder (SC2) being designed for
half rate voice coding.
8. The base station system (BSS) as claimed in claim 6 or 7, wherein the characteristic value (RXLEV, RXQUAL) is related to a reception level and to a bit error rate.
9. The base station system (BSS) as claimed in claim 6, 7, or 8, wherein the characteristic value (ta) is related to a value which is proportional to the signal transit time between a mobile station (MS) and the base station (BS).

- 15 -
10. The base station system (BSS) as claimed in one of claims 6 to 9, wherein at least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions is signaled by a mobile station (MS) to said base station.

In the method according to the invention for transmitting voice over a radio interface, at least one signal is transmitted between a mobile stations (MS) and a base station (BS). At least one characteristic value (RXLEV, RXQUAL, ta, C/I) relating to the transmission conditions of the radio interface is determined from the signal. The characteristic value RXLEV, RXQUAL, ta, C/I) or values are compared with a threshold value (S,S1, S2) and a voice coder (SC1, SC2) is selected in accordance with the comparison result, a voice coder (SC1) being used with a higher data rate under good transmission conditions than under poor transmission conditions.

Documents:

02391-cal-1997-abstract.pdf

02391-cal-1997-claims.pdf

02391-cal-1997-correspondence.pdf

02391-cal-1997-description (complete).pdf

02391-cal-1997-drawings.pdf

02391-cal-1997-form-1.pdf

02391-cal-1997-form-2.pdf

02391-cal-1997-form-3.pdf

02391-cal-1997-form-5.pdf

02391-cal-1997-gpa.pdf

02391-cal-1997-priority document others.pdf

02391-cal-1997-priority document.pdf

2391-CAL-1997-(11-04-2012)-CORRESPONDENCE.pdf

2391-CAL-1997-(15-10-2012)-CORRESPONDENCE.pdf

2391-CAL-1997-FORM-27.pdf

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Patent Number

194090

Indian Patent Application Number

2391/CAL/1997

PG Journal Number

30/2009

Publication Date

24-Jul-2009

Grant Date

06-May-2005

Date of Filing

17-Dec-1997

Name of Patentee

SIEMENS AKTIENGESELLSCHAFT

Applicant Address

WITTELSBACHERPLATZ 2, 80333 MUNCHEN

Inventors:

#	Inventor's Name	Inventor's Address
1	MICHAEL FAERBER	WINIBALDSTR. 22, 82515 WOLFRATSHAUSEN

PCT International Classification Number

H04B 7/26

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	19653122.5	1996-12-19	Germany