Design and Field Test of a 256-QAM DIV Modem

The increasing demand for digital transmission facilities has led to the development of modems which allow the transmission of T1 (1.544 Mbit/s) data streams over the existing analog FM and AM/SSB links. The DTS model 2001 data in voice (DIV) modem uses 256-QAM modulation to transmit a T1 data stream in a single supergroup. This paper describes the technical requirements and design of the modem. A number of particularly interesting aspects of the design are discussed in more detail. These items are 1) the extensive use of digital signal processing in the implementation; 2) the techniques used in the demodulator control loops; 3) the use of a low-rate error correction codec and interleaver for random and burst error correction; and 4) the use of signaling constellation customization as a design tool.