Code - Spread CDMA using Low - RateConvolutional

Convolutional Codes P al Frenger, P al Orten, and Tony Ottosson Communication Systems Group, Department of Signals and Systems, Chalmers University of Technology, S-412 96 G oteborg, Sweden. phone: +46 31 772 1758, fax: +46 31 772 1748, email: [email protected] 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 exible and powerful scheme than those proposed up to now. Keywords| CDMA, low-rate convolutional codes, orthogonal codes, superorthogonal codes, rate-compatible, maximum free distance.