Processing Gain Improvement by Means of Replica Subtraction in Code Division Multiple Access Systems

The paper presents a performance evaluation of code division multiple access (CDMA) systems, using signal replica transmission and interference subtraction thereafter in the receiver. Asynchronous direct sequence systems, using two transmission channels, are discussed. The two incoming signals in the receivers are coherently multiplied by IF frequency quadrature components, enabling the elimination of the desired signal in the replica path. Additional low pass filtering, sideband frequency conversion and delay operations occur, prior to the interference subtraction process. System synthesis, parameter design and necessary conditions for adequate CDMA interference reduction are discussed. The results obtained indicate that an improved processing gain may be achieved on account of a 3dB degradation in signal to thermal noise ratio. The time delay in the receiver replica path is a major factor in processing gain determination. The CDMA interference subtraction requirements dictate an appropriate choice of carrier frequencies and strict frequency control in the various transmitters.