Multicarrier Cdma System, The Residual Frequency Offset Estimation And Compensation Methods

For the 3rd generation wireless communications, direct sequence code division multiple access (DS-CDMA) techniques have received a great deal of attention owe to the strong anti-interference capability, the high-capacity, high-security, and so on. In a high data rate application, the serious effect of inter-code interference (ICI) and the inter-symbol interference (ISI) would bring on the obvious degression of system performance. As a multi-carrier transmission technique, the orthogonal frequency division multiple (OFDM) technique could transmit the high speed data by dividing it into parallel low-speed data, which can reduce the effect of ICI and ISI.The multi-carreir CDMA (MC-CDMA) system is the combination of CDMA and multicarrier modulation systems, being focused on in the communication field in recent years. Multicarrer CDMA has many advantages: higher efficiency of frequency spectrum, high speed data transmission, capability of narrow band interference rejection and inter-symbol interference rejection capability, and so on. It has been one of the standby schemes for beyond 3rd Generation wireless communication, sometimes called it the 4th Generation. But MC-CDMA system still has some problems. Compared with the single-carrier modulation, the MC-CDMA system is more sensitive to frequency offset which is the same with OFDM, even a small residual frequency offset will cause significant degradation of the system performance. Hence, it is quite important to pay more attention to the research on the appropriate method to estimate the amount of residual frequency offset and compensate it for the MC-CDMA system.Papers about RFO estimation are almost based on the OFDM systems, which do not probably adapt to the MC-CDMA system. So the main work of this paper is to find and improve one method which could be suitable and estimate the RFO effectively for the MC-CDMA systems.On the base of analysis on the basic theory of MC-CDMA, this paper researches on MC-CDMA systems, analyses the effect of the RFO to the MC-CDMA systems from theories and simulation results. Almost current papers consider that it is the non-synchronization between the transmitter and receiver oscillations bring about frequency offset. But in the mobile environment, the Doppler Effects exist, which also contribute to the RFO. Based on this theory, this paper defines a formula of RFO which is contributed by these two ways. After that this paper improve a estimation method which utilizes inserting pilot subcarriers before transmitting, estimates the amount of RFO by extracting the pilot subcarriers extracted in frequency domain then compensates for it in time domain at the receiver. At last, the simulation results show that the improved method's estimation performance is better than the original one's as the constellation modulation is more complex.