An interference immune modulation and demodulation technique for the downlink of spread spectrum communication systems

In this paper, a novel method for modulation and demodulation of user information bit in spread spectrum code division multiple access (CDMA) systems is proposed. Synchronous CDMA poses intrinsic protection against co-channel interference due to orthogonal spreading codes used. However, in the presence of multipath, the signals lose their orthogonality property, leading to an increased cross correlation. In this paper we show that the performance of the system will be close to single user system if we assign two codes to each user and these two codes are used for modulation and demodulation. The well-known maximum length sequence codes are good candidates for the present modulation and demodulation technique. The limiting factor to the system capacity is the maximum number of codes and the capacity is half of that number.