Code-spread CDMA using maximum free distance low-rate convolutional codes

In code-division multiple-access (CDMA) systems, maximum total throughput assuming a matched filter receiver can be obtained by spreading,vith low-rate error control codes. Previously, orthogonal, hi-orthogonal and super-orthogonal codes have been proposed for this purpose. We present in this paper a family of rate-compatible low-rate 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 super orthogonal codes as well as conventionally coded and spread systems, We also show that the proposed low-rate codes will give simple encoder and decoder implementations. With these codes, any rate 1/n, n less than or equal to 512, are obtained for constraint lengths up to Il, resulting in a more flexible and powerful scheme than those previously proposed.