In this thesis work, we implement the 2400 bits/s DQPSK modem using a TMS320C31 DSP for telephone voiceband channels which conforms to the ITU-T V.26ter specification. This modem algorithm is operated in a full-duplex mode with echo cancellation. In order to transmit low rate digital voice under 2400 bits/s, we modify the handshake sequence of the V.26ter modem for fast start-up.
The building blocks of the modem algorithm include the square timing recovery, the T/2-spaced equalizer, and the data-driven echo canceller. The equalizer adopts the LMS algorithm to compensate channel distortion and the echo canceller is implemented in a two real-valued structure. We test the convergence of the echo canceller with various voiceband channels. By modifying the recommended handshake sequence of V.26ter, we can reduce the start-up time about 50% without performance degradation.
The bit error rate(BER) performance of the V.26ter modem implemented in this work is compared with that of the ITU-T V.22bis modem which uses frequency division multiplexing(FDM) for full-duplex transmission. The performance of the v.26ter modem is 7 to 8 dB superior to that of the V.22bis modem.
