Multi user interference evaluation for a one-code multi carrier spread spectrum CDMA system with imperfect time and frequency synchronization

Abstract

Multi-carrier spread-spectrum (MC-SS) modulation is a new kind of spread-spectrum modulation. The spreading code is designed in the frequency domain. Hence, the system is dual to a direct-sequence spread-system (DS-SS) where the spreading code is given in the time domain. Due to this dualism, first receiver designs for MC-SS used a frequency domain RAKE receiver. Further investigations focused on the properties of the time signal. It could be proved, that the time autocorrelation function for MC-SS signals is sufficiently good to permit the use of a RAKE receiver in the time domain. In this paper, we propose a MC-SS CDMA system with only one spreading code. Orthogonalization between the users could be achieved by adding a frequency offset to the main carrier frequency. The offset should be equal to the carrier spacing. In the case of ideal time and frequency synchronisation the multiple access interference (MAI) is apparent from the autocorrelation function of the code. Without perfect time and frequency synchronisation, a different kind of description of the code properties is required. We show that the evaluation of the ambiguity function of the MC-SS time signal is a way to describe the code properties in the time-frequency plane. This function could be used to estimate the strength and the form of MAI. We give equations for evaluation of the ambiguity function of a desired MC-SS code and demonstrate a way for MAI estimation. Finally, we give numerical results.