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Modulo-lattice coding for non-orthogonal access

In this paper, we study the transmission over an overloaded downlink wireless communication system, using non-orthogonal multiple access, i.e., one in which the base station transmits to multiple users on the same carriers. The base station as well as the user equipments are assumed to be equipped with single-antennas. First, we show that if the system is to be designed so as to maximize the sum-throughput, without any fairness constraint, then orthogonal transmission is optimal; and we derive the associated power split among the carriers. Second, we show that if some fairness constraint is targeted, e.g., weighted sum-throughput or some desired constraint on the reliability at the users, then non-orthogonal schemes generally outperform orthogonal schemes. In this case, we derive the corresponding optimal power allocation policy among the users and carriers. Next, focusing on reducing receiver's complexity in non-orthogonal schemes, we design a practical efficient lattice-based coding scheme for non-orthogonal access in which the entire coding complexity is placed on the transmitter side with the important advantage that the user equipment devices only realize simple single-decoder operations, i.e., no successive-interference cancellation. We describe the encoding and decoding modulo-lattice operations, along with the appropriate system parameters; and illustrate our results through some simulation results.