Analysis of ADSL2´s 4D-TCM performance

Asymmetric digital subscriber line (ADSL) has been gaining popularity as a high speed transmission technology through the copper twisted pair telephone lines. High performance is achieved by using discrete multi-tone (DMT) modulation. DMT divides the channel into a number of independent sub-channels so that more bits are transmitted over sub-channels with higher signal-to-noise ratios. Performance can be further improved by combining DMT with trellis-coded modulation (TCM). In this paper we analyze the performance of a four-dimensional-TCM encoder as it is used in ADSL/ADSL2 modems. First, assuming all the sub-channels are transmitting the same number of bits b, we theoretically evaluate the TCM coding gain for different values of b. Then, we consider the case where sub-channels may transmit different number of bits as in an ADSL transmission. Simulation results are presented to validate our analysis