Background and Motivation

Dynamic channel allocation [1] (DCA) and packet reservation multiple access [2] (PRMA) are techniques which potentially allow large increases in capacity over a fixed channel allocation (FCA) time division multiple access (TDMA) system. Although both DCA and PRMA can offer a significant system capacity improvement, their capacity advantages typically cannot be jointly exploited, since the rapid variation of slot occupancy resulting from the employment of PRMA limits the validity of interference measurements, which are essential for the reliable operation of the DCA algorithm. One alternative to tackle this problem is to have mixed fixed and dynamic frequency re-use patterns, but this has the disadvantage of reducing the number of slots per carrier for the PRMA scheme, thus decreasing its efficiency.

In this paper we propose adaptive modulation as an alternative to DCA. The cells must be frequency planned as in a FCA system using a binary modulation scheme. When adaptive modulation is deployed, the throughput is increased by permitting high level modulation schemes to be used by the mobiles roaming near to the centre of the cell, which therefore will require a lower number of PRMA slots to deliver a fixed number of channel encoded speech bits to the base station (BS). In contrast, mobile stations (MS) near the fringes of the cell will have to use binary modulation in order to cope with the prevailing lower signal-to-noise ratio (SNR) and hence will occupy more PRMA slots for the same number of speech bits. Specifically, our adaptive system uses three modulation schemes, namely binary differential phase shift keying (DPSK) i.e., one bit per symbol at the cell boundary, quaternary differential phase shift keying (DQPSK), i.e., two bits per symbol at medium distances from the BS, and 16 level star quadrature amplitude modulation [3] (16-StQAM), which carries four bits per symbol close to the centre of the cell.