A low-delay multimode speech terminal

Abstract

The intelligent, adaptively reconfigurable wireless systems of the near future require programmable source codecs in order to optimally configure the transceiver to adapt to time-variant channel and traffic conditions. Hence we developed a programmable 8-16 kbits/s low-delay speech codec, which is compatible with the G728 16 kbits/s ITU codec at its top rate and offers a graceful trade-off between the speech quality and bit rate in the 8-16 kbits/s range. The issues of robustness against channel errors strongly influenced the algorithmic design of the 8-16 kbits/s speech codec, and hence special attention is devoted to these issues. Source-matched Bose-Chaudhuri-Hocquenghem (BCH) codecs combined with unequal protection pilot-assisted 4- and 16-level quadrature amplitude modulation (4-QAM, 16-QAM) are employed in order to transmit both the 8 and the 16 kbits/s coded speech bits at a signalling rate of 10.4 kBd. In a bandwidth of 1728 kHz, which is used by the Digital European Cordless Telephone (DECT) system 55 duplex or 110 simplex time slots can be created. Good toll quality speech is delivered in an equivalent bandwidth of 15.71 kHz, if the channel signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) are in excess of about 18 and 26 dB for the lower and higher speech quality 4-QAM and 16-QAM modes, respectively.