Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Patent
1997-07-23
1999-09-14
Chin, Wellington
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
455432, 455536, 455550, 370328, 370329, 370336, 370345, H04Q 718, H04Q 720, H04Q 722
Patent
active
059536667
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a digital mobile communication system in which mobile stations and the fixed network comprise speech coders, the speech coders of the fixed network being located remote from the base stations in transcoders, and especially to controlling the transcoders in a mobile to mobile call.
BACKGROUND OF THE INVENTION
In recent years, digital mobile communication systems for fully digital speech and data transmission have been introduced, resulting in a uniform quality of speech. As far as the mobile communication network is concerned, the most limited resource is the radio path between the mobile stations and the base stations. To reduce the bandwidth requirements of a radio connection on the radio path, speech transmission utilizes speech encoding providing a lower transmission rate, for example 16 or 8 kbit/s instead of the 64 kbit/s, transmission rate typically used in telephone networks. For speech encoding, both the mobile station and the fixed network end must have a speech encoder and a speech decoder. On the network side, the speech coding functions may be located in several alternative places, for example at the base station or in the mobile services switching center. The speech encoder and decoder are often located remote from the base station in so called remote transcoder units. In the latter case, speech encoding parameters are sent between the base station and the transcoder unit in specific frames.
In each mobile originating or mobile terminating speech call, a remote transcoder is connected to the speech connection on the network side The remote transcoder decodes the mobile originating speech signal (uplink direction), and encodes the mobile terminating speech signal (downlink direction). This type of an arrangement causes no problems as long as only one of the speech participants is a mobile station and the other, for example, a subscriber of a public switched telephone network (PSTN).
If the call takes place between two mobile stations (Mobile to Mobile Call, MMC), the operation of the mobile communication network involves a remote transcoder on the connection between the calling mobile station and the mobile services switching center, and, correspondingly, a second remote transcoder between the called mobile subscriber and the (same or another) mobile services switching center. These remote transcoders are then interconnected by the mobile services switching center (centers) as a result of normal call switching. In other words, for each MMC call there are two transcoder units in a series connection, and speech encoding and decoding are performed twice for the call. This is known as tandem coding. Tandem coding presents a problem in mobile communication networks as it degrades the speech quality due to the extra speech encoding and decoding. So far, tandem coding has not been a major problem because relatively few calls are MMC calls. However, as the number of mobile stations increase, the number of MMC calls will also be higher and higher.
A DISCLOSURE OF THE INVENTION
It is an object of the present invention to prevent tandem coding and consequently to improve speech quality in mobile to mobile calls.
This object is achieved by a digital mobile communication system comprising mobile stations and first speech coders in said mobile stations, a radio network including base stations and second speech coders, said second speech coders being remote from the base stations in transcoders which communicate with the base stations by transmitting frames containing control information and speech coding information, and in which mobile communication system the uplink direction is the traffic direction from the base station to the transcoder and the downlink direction is the reverse direction. The mobile communication network according to the invention is characterized in that station contain a speech coding disabled indicator in a mobile to mobile call, disabled indicator in an uplink frame received from the base station, for not carrying out th
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Chin Wellington
Ferguson Keith
Nokia Telecommunications Oy
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