Near-orthogonal coding for spread spectrum and error correction

Since the spreading operation and error correction must share the bandwidth available in a CDMA system, it is appropriate to approach these problems jointly. Several papers have addressed this problem with promising results. Hui (1984) has shown that under certain assumptions, the system performs better when more bandwidth is devoted to error correction. Giallorenzi (see PhD thesis, Univ. of Virginia, Charlottesville, VA, 1994) shows that combining error correction decoding and multiuser detection significantly improves system performance. We extend this research by considering not only simultaneous despreading and decoding, but also the simultaneous encoding and spreading. We consider a coded asynchronous CDMA system over an AWGN channel with constant information rate, R_I. Each user´s transmission rate is R_tx=R_I·Q·N where 1/Q is the convolutional code rate and N is the spreading factor. The receiver matches to each signature sequence and performs maximum likelihood sequence detection