STDD, an approach to low delay, high quality wireless speech communications

Previously, we (see Wong, Sundberg and Seshadri, GLOBECOM ´93, p.1649-1653, 1993) proposed a low-delay multiple access scheme, called shared time-division duplexing (STDD). This scheme allows both the up-link and downlink traffic to share a common channel, thereby achieving high statistical multiplexing gain even with a low population of simultaneous conversations. Here, we focus on transmitting speech using this multiple access scheme by a suitable choice of speech coder, channel coder and modulation. In particular, we consider low delay, high quality speech coding and digital modulation systems based on adaptive DPCM, with QDPSK or pseudo-analog transmission (skewed DPSK). The choice of the alternative systems depends on required end-to-end delay, recovered speech quality and bandwidth efficiency. Typically, with a total capacity of 1 MBaud, 2 ms frame and 8 kBaud speech coding rate, low delay STDD is able to support 48 pairs of users compared to 38, 35 and 16 for TDMA with speech activity detection, basic TDMA and PRMA respectively. This corresponds to respective gains of 26%, 37% and 200%