Low complexity, low delay speech coding for indoor wireless communications

The paper presents a low complexity, low delay variable rate speech coding method suitable for low power (CDMA or TDMA) indoor wireless communications. The indoor wireless communications channel is characterized by relatively deep fades resulting in bursty errors. A robust speech codec is required to combat this bursty nature. Low power operation is essential, which negates the applicability of LD-CELP (G.728) for indoor wireless applications, Historically, ADPCM (G.721) has been proposed, which lacks robustness to random and bursty errors and has a relatively high bit rate (32 kb/s). To circumvent the need for relatively complex echo-cancellation, low end to end delays are required which implies that interleaving is not applicable. In the paper, a low complexity, low delay, variable rate CELP algorithm is proposed and techniques to improve the robustness to bursty errors are investigated. The new technique is compared to ADPCM on the basis of complexity and performance (MOS) over measured indoor wireless channels at 1.7 GHz with various combinations of space, frequency and code diversity