A trellis-coded 28.8 kbps telephone line MODEM algorithm is implemented in real time using an ADSP-21020 DSP chip. The MODEM algorithm conforms to the ITU-T V.34 standard. We implement it in a half-duplex mode in this study. The important building blocks of the MODEM algorithm includes adaptive speed and bandwidth, shell mapping, precoding, 4-dimensional TCM, nonlinear decoding, and pre-emphasis.
We obtain performance of the V.34 MODEM algorithm implemented in this study. Compared to the V.32 MODEM algorithm, we obtain the following improvements : 0.3 to 0.5 dB coding gain from 4-dimensional TCM, about 1 dB shaping gain from shell mapping, and 10 dB gain in mean square error from precoding.