A DSP implemented dual 9600/7200 bps TCM modem for mobile communications over FM voice radios

This paper describes a dual 9600/7200 bit per second (bps) modem designed by the authors and implemented on a DSP based hardware platform. Designed for use in mobile fading environments using inexpensive FM voice grade radios, the modem employs both 16 state, 16-QAM TCM (trellis coded modulation) and 16-state, 8-PSK TCM for a net data throughput of 7200 bps and 4800 bps, respectively (less some overhead for the radio attack time, synchronization and the multiple access contention protocol). The dual modem design implements a form of adaptive error control where the lower data rate 8-PSK TCM modem provides coverage where the 16-QAM TCM modem cannot. In addition to a description of the modem design and the challenges faced, this paper provides some insight into several of the key techniques used in the DSP implementation as well as theoretical and simulated bit error rate curves for the modem