MTCM versus TCM codes in a fading environment

It is well known that conventional trellis-coded modulation (TCM) codes are optimally designed in an additive white Gaussian noise (AWGN) environment. Multiple TCM (MTCM) codes were proposed as a solution to the poor performance of conventional TCM codes over fading channels. However, it is observed that the performance of MTCM codes may be inferior to that of conventional TCM codes under certain situations. In this article, we attempt to provide a clear and quantitative description of these conditions. In particular, a method is presented to easily determine if a particular MTCM code will perform better than its equivalent conventional TCM code by simply examining the number of code states, the code rate and the code design (connectivity)