Exact BER Performance for Symbol-Asynchronous Two-User Non-Orthogonal Multiple Access

Two-user non-orthogonal multiple access systems with quadrature phase-shift keying modulation are studied. A time asynchronous scheme for successive interference cancellation receivers that maximally suppresses the inter-user interference compared to any other time asynchronous schemes is proposed. Exact bit error rate (BER) solutions for the two users operating in the additive white Gaussian noise channel using the proposed scheme are derived. A simpler closed-form BER approximation for the weaker user is also presented. Compared to the symbol-synchronous scheme, numerical results show the proposed asynchronous scheme reduces the BER for the stronger signal by a factor of one-half.