Code-Spread CDMA using Low-Rate Convolutional Codes

Abstract

In CDMA systems maximum theoretical performance can be obtained by spreading with low-rate error control codes. Previously, orthogonal and superorthogonal codes have been proposed for this purpose. We present in this paper a family of rate-compatible lowrate convolutional codes with maximum free distance. The performance of these codes for spectrum spreading in a CDMA system is evaluated and shown to outperform that of orthogonal and superorthogonal codes as well as conventional coding/spreading schemes. We also show that our low-rate codes will give simple encoder and decoder implementation. With these codes any 1=n rate can be obtained regardless of the constraint length, resulting in a more flexible and powerful scheme than those proposed up to now. Keywords--- CDMA, low-rate convolutional codes, orthogonal codes, superorthogonal codes, rate-compatible, maximum free distance. I. Introduction S PREAD spectrum systems have been used for decades as a way of achieving robustness agai..