Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Patent
1996-06-20
1999-01-19
Vo, Don N.
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
375262, 704201, 704501, H04B 166
Patent
active
058621784
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/ Fl 95/00390 filed Jul. 5, 1995.
FIELD OF THE INVENTION
The invention relates to a method for speech transmission in a mobile communications system, the method comprising compressing a speech signal to a small number of speech coding bits by a speech coding method, and channel encoding the speech coding bits.
BACKGROUND OF THE INVENTION
In telecommunications systems transmitting digital speech, a speech signal is usually subjected to two coding operations: speech coding and channel coding.
Speech coding comprises speech encoding performed in the transmitter by a speech encoder and speech decoding performed in the receiver by a speech decoder. The speech encoder in the transmitter compresses a speech signal so that the number of bits used for representing the speech signal per a unit of time is reduced, whereby less transmission capacity is required for transmitting the speech signal. The speech decoder in the receiver performs a reverse operation and synthesizes the speech signal from the bits generated by the speech encoder. However, the speech synthesized in the receiver is not identical with the original speech compressed by the speech encoder; the original speech has changed more or less as a result of the speech coding. In general, the more the speech is compressed in the speech coding, the more its quality deteriorates during the coding. In the pan-European GSM mobile communication system (Global System for Mobile Communication), for example, the speech encoder of a full-rate traffic channel compresses a speech signal to a transmission rate of 13 kbit/s. The speech synthesized by the corresponding speech decoder is clearly of a poorer quality than the speech transmitted by, for instance, a public switched telephone network (PSTN).
Thus, when a speech coding method is selected, a compromise must be made between the quality offered by the method and the transmission capacity required by it. Another factor to be considered in the selection is the complexity of the implementation of the speech coding method: the quality of speech can usually be improved without increasing the transmission rate if higher requirements for the method as regards calculation capacity and thereby also higher costs of the implementation are allowed. On account of the continuous development of speech coding methods and the implementation techniques, more and more advanced methods are available for speech transmission in the existing telecommunications systems. After the development of the method employed in the GSM, speech coding technology has advanced to such an extent that, as compared with the above-mentioned 13 kbit/s speech coding method, a higher quality of speech can now be achieved at a much lower transmission rate, e.g. 8 kbit/s.
Channel coding comprises channel encoding performed in the transmitter by a channel encoder, and channel decoding performed in the receiver by a channel decoder. The purpose of channel coding is to protect speech coding bits to be transmitted against errors occurring in the transmission channel. Channel coding can either be used for merely detecting whether the transmission has caused any errors in the speech coding bits without any possibility of correcting them, or it may be capable of correcting errors caused by the transmission, provided that the number of errors does not exceed a given maximum, which is dependent on the channel coding method.
The selection of the channel coding method employed depends on the quality of the transmission channel. In fixed transmission networks the probability of errors is often very low, wherefore not much channel coding is required, whereas in wireless networks such as mobile telephone networks the probability of errors in the transmission channels is often very high, and the channel coding method employed has a significant effect on the resulting quality of speech. Mobile telephone networks usually employ both error-detecting and error-correcting channel coding methods concurrently.
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REFERENCES:
patent: 5115469 (1992-05-01), Taniguchi et al.
patent: 5271089 (1993-12-01), Ozawa
patent: 5511096 (1996-04-01), Huang et al.
patent: 5657333 (1997-08-01), Ikekawa
patent: 5684893 (1997-11-01), Shikakura
Haavisto Petri
Jarvinen Kari
Vainio Janne
Nokia Telecommunications Oy
Vo Don N.
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