An optimal signal design for synchronous CDMA with multidimensional signaling and QoS constraints

We consider the symbol-synchronous code-division multiple-access (CDMA) channel in which each user transmits one multidimensional symbol per signaling interval in the presence of additive white Gaussian noise (AWGN). The users are assigned quality-of-service (QoS) thresholds that are to be guaranteed by the system. These thresholds are rates at which arbitrarily reliable transmission is possible. For a fixed processing gain, which is proportional to the required bandwidth, we determine the minimum required sum-power (i.e., the sum of the users received powers) over all sets of signature sequences and power-control policies such that the QoS requirements are met. Moreover, our derivation also provides a design which meets this minimum sum-power with successive decoding in a multiuser decision-feedback receiver structure so that single-user coding is sufficient.