Research On An Improved Scheme And Sequence Application Of Multi-carrier CDMA System

High-rate data transmission becomes indispensable for nowadays and future mobile communication system. However, when the data is being transferred in mobile channel, there will be inter-symbol interference (ISI) because of delay spreading. Multi-carrier CDMA is one of effective techniques to mitigate the ISI. It is an integrated modulation method of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) technologies. In a quasi-synchronous code-division multiple-access (QS-CDMA) system, loosely synchronous (LS) code can eliminate inter symbol interference (ISI) and multiple access interference (MAI) by using zero correlation zone (ZCZ). Generalized LS (GLS) code is a general description of LS code. Adjacent cell interference (ACI) can be minimized by applying GLS in QS-CDMA system.First of all, an improved Multi-carrier CDMA system is proposed in this thesis. Compared with conventional MC-CDMA system, the main difference is that this scheme performs the spreading in the time and frequency domain simultaneously. This system can utilize the transmission bandwidth more efficiently and achieve frequency diversity. Furthermore, this system lowers the data rate of each carrier branch, so it is easier to make spreading sequence synchronized than the conventional system. Then the performance of the system is analyzed in a Rayleigh fading channel in this thesis.Next, based on the Gaussian approximation method, the bit error rate of multiple cellular QS-CDMA system using a GLS code family and scrambled LS codes is estimated and their simulations and performance comparison are presented. It is shown that the GLS code improve the system performance greatly, much better than the LS code.Finally, generalized loosely synchronous (GLS) codes are applied to the MC-DS-CDMA system and the bit error rate of the new system is obtained. The system overcomes MAI of OFDM when there is frequency offset because of the ZCZ characteristic of GLS code. The simulation result shows that the system using GLS code performs better than the MC-DS-CDMA using Walsh sequence, and has high frequency utilizing rate.