Cancellation techniques of co-channel interference in asynchronous spread spectrum multiple access systems

A spread spectrum multiple access (SSMA) communication scheme employing asynchronous code-multiplexing technique has been used in both mobile and satellite communication systems. One shortcoming of the SSMA system is the co-channel interference due to the correlations among the pseudo noise (PN) sequences being used by multiple channels. Such co-channel interference in the reception limits the number of channels that can share one common frequency band. Furthermore, there is a distant-dependent problem of different received-powers from different transmitters. This paper describes a technique for cancelling the co-channel interference in asynchronous SSMA systems. This technique does not treat the co-channel interference as thermal noise. It considers the co-channel interference as components that can be recreated at each receiver. If the correlations among the PN sequences of all the active channels become known, the co-channel interference can be cancelled using the linear algebraic approach. As a result, a larger number of channels can share the same frequency band more efficiently. Further, if initial synchronization can be implemented then the distant-dependent problem can be alleviated.