Constant Amplitude Transmission of Multi-code CDMA Symbols Using Symbol Level Quantization and MPSK Modulation

摘要

Multi-code CDMA system provides high speed and variable rate services with different quality requirements. Since envelope fluctuation of the multi-code signal is large, the system requires highly linear power amplifier. In this paper, we consider a phase modulated multi-code CDMA system where the multi-level symbol produced by adding orthogonal codes is MPSK modulated for transmission. When a large number of channels are used, however, the system becomes sensitive to channel impairments since distance between vectors in the MPSK signal constellation is small. To get around this problem the system places a nonuniform quantizer which reformats the symbols such that number of symbol states is reduced. However, quantizing the symbol level may introduce nonlinear distortion due to destruction of orthogonality between channels and may cause performance degradation. Degree of distortion is dependent on the quantizer design and the number of channels as well as the choice of orthogonal code set. We suggest a way to design a quantizer and to select the optimal set of orthogonal codes. Simulation results show that the multi-code CDMA system with proposed symbol formatting and MPSK modulation has comparable BER performance to the conventional multi-code CDMA system in AWGN environment. The proposed system is power efficient and the receiver structure is very simple. The system is appropriate for low cost high data rate applications.