Code allocation schemes for the downlink of a TD-CDMA system

CDMA cellular system capacity is very limited by multiple access interference (MAI). Since MAI is very sensitive to the spreading code properties of other active users, the code allocation of all active users should require careful planning in order to attain the best system performance. This paper deals with the code allocation schemes in the downlink of a TD-CDMA system. The focus is to compare various code allocation scenarios in order to maximize the system performance determined by the bit-error-rate at the output of a conventional RAKE receiver. Although TD-CDMA system performance has already been studied, neither an explicit strategy for assigning codes between users nor the effects of the code allocation schemes on the system performance were presented. Different code allocation schemes have been investigated. It is shown that the system performance is very sensitive to the allocated codes. A theoretical study is carried out and a new strategy maximizing the system performance via code assignment is proposed. Simulation results are presented to illustrate the importance of code allocation.