Reduction of mixed cochannel interference in microcellular shared time-division duplexing (STDD) systems

Abstract

Shared time-division duplexing (STDD) is an attractive candidate for high-quality low-delay multiple access in microcellular systems. STDD provides a considerable increase in capacity over conventional time-division systems with conventional speech activity detection (SAD). By sharing time slots in both directions of transmission, large statistical multiplexing gains are obtained, even with a moderate number of users per carrier. We show by means of theory that STDD is superior to conventional time-division multiple access (TDMA) as well as TDMA/TDD/SAD in terms of capacity. We furthermore demonstrate how a type of mixed cochannel interference particular to STDD can be reduced or almost eliminated by means of directional antennas at the base stations for both reception and transmission, combined with appropriate organization of the time burst. The concept of partially shared TDD (PSTDD) is also introduced, and the tradeoff between capacity gains by means of statistical multiplexing and packets dropped due to interference is studied by means of analysis. To illustrate the introduced concepts and methodology of evaluation, a number of numerical results are given for a frame length of 2 ms. Statistical multiplexing gains of the order of 100% over TDMA are obtained with 40 speech slots per frame and a packet dropping rate of 0.1%.