Performance Analysis of Direct-Sequence Code-Division Multiple-Access Communications with Asymmetric Quadrature Phase-Shift-Keying Modulation

This article considers a quaternary direct-sequence code-division multiple-access (DS-CDMA) communication system with asymmetric quadrature phase-shiftkeying (AQPSK) modulation for unequal error protection (UEP) capability. Both time synchronous and asynchronous cases are investigated. An expression for the probability distribution of the multiple-access interference is derived. The exact bit-error performance and the approximate performance using a Gaussian approximation and random signature sequences are evaluated by extending the techniques used for uniform quadrature phase-shift-keying (QPSK) and binary phase-shiftkeying (BPSK) DS-CDMA systems. Finally, a general system model with unequal user power and the near–far problem is considered and analyzed. The results show that, for a system with UEP capability, the less protected data bits are more sensitive to the near–far effect that occurs in a multiple-access environment than are the more protected bits.