This paper deals with the performance of the multilevel PSK spread spectrum （SS）systems with trellis coded modulation（TCM） for indoor communications. A tight upper bound of the performance

which is verified by computer simulation

is derived.The moment method based on the maximum entropy is exploited to extimate the probability density function for the multiple access interference and multipath interference. The construction criteria of TCM in indoor channel is discussed briefly. The effect of the phase error on the system performance is studied.The numerical and simulation results show that the performance of the trellis coded SS system is improved considerably over that of the conventional uncoded SS system.