Superposition Coding with Unequal Error Protection for the Uplink of Overloaded DS-CDMA System

The overloaded CDMA schemes exploited in direct sequence CDMA (DS-CDMA) systems are mainly to accommodate a greater number of users than the available spreading factor N. In this paper, a superposition coding CDMA (SPC-CDMA) with unequal error protection (UEP) is proposed as one of the overloaded CDMA schemes for the next generation mobile communication systems. It exploits the available power control in most base stations to adapt the transmitted power of active users in the uplink channel. In this scheme, the active users are divided into G groups and each group consists of K users. The K users share the same spreading sequence and are distinguished by different received power levels. At the receiver side, the system first performs despreading for group detection followed by multiuser receiver to estimate the K user signals in each group. It is shown through simulations that better performance are achieved compared to the conventional DS-CDMA and existing overloaded collaborative spreading CDMA (CS-CDMA) schemes, in additive white Gaussian noise (AWGN) and fading channels. Hence, the proposed scheme maximizes the system capacity K-fold compared to conventional DS-CDMA system without requiring extra spreading codes, with average signal to noise ratio (SNR) cost of only 1dB and 2 dB over AWGN and fading channels respectively at BER of 10−3. On the other hand, for the same N, K and power constraints, SPC-CDMA scheme achieves twofold increase in data rate with 0.7 and 4 dB gains over AWGN and fading channels respectively, compared with overloaded CS-CDMA scheme in the same system capacity. In addition, the proposed scheme can also attain different levels of UEP for different users’ requirements by adjusting their fractions of transmitted powers.