Constellation-constrained capacity for trellis code multiple access systems

Trellis code multiple access (TCMA) is a narrow-band multiple access scheme. There is no bandwidth expansion, so users are using the same bandwidth as one single user. The load (the number of bits per channel use) of the system is therefore much higher than the load in, for example, conventional CDMA systems. It is shown here that the maximum number of users in a TCMA system is usually modest. This fact is based on constellation-constrained capacity, i.e., the maximum capacity of a system with a specific constellation of the transmitted symbols. The minimum required signal-to-noise ratio for reliable transmission is also given for different code rates.