Multiplex communications – Communication over free space – Signaling for performing battery saving
Patent
1996-10-22
1999-04-20
Hsu, Alpus H.
Multiplex communications
Communication over free space
Signaling for performing battery saving
370342, 370470, 370510, 34082544, 371 671, 375200, 375368, 455 383, H04B 7216, H04J 1302
Patent
active
058963740
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a code division multiple access (CDMA) in mobile communications systems, and more particularly to a variable rate transmission method, a transmitter and receiver using the same method, which can realize apparent variable rate transmission by transmitting data contained in frames of a fixed length at a constant transmission rate.
BACKGROUND ART
In CDMA mobile communications, the output data (transmitted data) from a voice codec undergoes primary modulation, followed by secondary modulation using a pseudorandom sequence (spreading code sequence), to be spectrum-spread to a wideband signal and transmitted. The bit rate of the spreading code sequence is called a chip rate, and is tens to hundreds of times faster than the transmission rate. Generally, the primary modulation uses binary or quadrature phase shift keying, and the secondary modulation uses binary phase shift keying.
Considering voice communications, the information amount of a transmitted voice signal is not constant, but varies from time to time. Accordingly, the transmission rate can be changed by dividing the transmitted data into frames of a fixed duration, and by transmitting data of a variable bit length in each frame, thereby achieving effective transmission of information in each frame period. This can obviate useless transmission, thereby saving power consumption of the transmitter.
The following method is taken to transmit data of a different transmission rate in the CDMA system. First, data whose transmission rate is lower than the frame transmission rate is transmitted using a part of transmission frames (see, for example, R. Padovani, "Reverse link performance of IS-95 based cellular systems", IEEE Personal Communications, vol.1, pp.28-34, 3rd Quarter 1994). On the other hand, data whose transmission rate is higher than the frame transmission rate is divided into a plurality of transmission channels, and the divided data are spread using different spreading codes to be transmitted.
It is necessary in this method, however, to provide a receiving side with the transmission rate information. Alternatively, when the transmission rate information is not provided, it is necessary to predetermine the values the transmission rate can take, to carry out the error detection of the received data for all the transmission rates, and to output the received data with the transmission rate at which the error is not detected as the right data.
In this case, if an error occurs during the transmission of the transmission rate information, the effective length in the received frames cannot be decided, which makes it difficult to correctly recover the transmitted data at the receiving side even if no error has occurred during the data transmission.
Thus, it is difficult for the conventional data transmission method to vary the transmission rate during the communications to achieve a variable rate transmission. In addition, when the data is transmitted at a rate considerably lower than a maximum transmission rate, burst transmission occurs because of blanks in the frames, during which data transmission is not carried out. Such burst-like transmission presents a problem in that it causes an EMI (Electromagnetic Interference).
FEC (Forward Error Correction) of the transmitted data is generally used to improve the transmission quality in the case where many errors can occur during transmission as in a mobile communications environment. In this case, the transmitting side transmits transmitted data (including the transmission rate information) which has undergone error correcting encoding, and the receiving side carries out the error correcting decoding followed by extraction of the transmission rate information to decide the effective data length in each received frame. Accordingly, the transmission rate information cannot be obtained until the end of the error correcting decoding. Thus, the decoding is performed before deciding the data length to be decoded, and hence the error correction cannot fully achie
REFERENCES:
patent: 5509020 (1996-04-01), Iwakiri et al.
patent: 5511073 (1996-04-01), Padovani et al.
Adachi Fumiyuki
Higashi Akihiro
Ohno Koji
Okumura Yukihiko
Hsu Alpus H.
NTT Mobile Communications Network Inc.
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