Transmit Equalization for Asynchronous NOMA Downlink

In this paper, we propose an asynchronous non-orthogonal multiple access (ANOMA) downlink scheme with transmit equalization in frequency-selective channels. In the proposed ANOMA scheme, an intentional symbol timing offset between multiplexed symbols is introduced to reduce inter-user interference (lUI). In addition, finite impulse response filters are employed at the transmitter to reduce inter-symbol interference, thereby simplifying the receiver configuration. We present a filter design method based on signal-to-interference-plus-noise ratio maximization. We show that the IUI of the proposed ANOMA scheme is smaller than that of a conventional synchronous NOMA. Simulation results show that the proposed ANOMA scheme achieves a higher achievable rate region than a conventional synchronous NOMA. Moreover, we demonstrate that the proposed ANOMA scheme achieves a lower peak-to-average-power ratio and greater resilience for the carrier frequency offset than orthogonal frequency-division multiplexing based NOMA.