Performance Of The OFDM-CDMA System In The Downlink Channel

With the development of the high ratio wireless digital communication system, the wireless digital communication system is more sensitive to the frequency selective fading of the radio channel. However, the OFDM scheme can combat the frequency selectivity of the radio channel, has high frequency spectrum efficiency and can be transmitted easily. So the OFDM scheme is paid much attention to recently.So far, the OFDM scheme was successfully applied in the DAB system, the DVB-T system, IEEE 802.11 a/g standard, HIPERLAN2 standard and the ADSL system (DMT modem). On the other hand, the OFDM scheme is one of the important schemes in the Beyond 3G/4G.The performance of a MC-CDMA system (a important type of OFDM-CDMA combined systems) in the downlink channel is investigated in this paper. Our creative investigations mainly include:1. The performance of a cellular MC-CDMA system in the downlink channel. In this subject, the co-channel interference on the downlink channel of the MC-CDMA cellular system is investigated and the analytic formula of the co-channel interference is induced. Then, a new scheme of reducing co-channel interference is proposed. Based on the proposed scheme, the downlink channel capacity of a MC-CDMA cellular system with co-channel interference is obtained by the Monte Carlo simulations. It is proven by the simulations that the capacity of a MC-CDMA cellular system using the proposed scheme can be increased 15%.2. Fast frame synchronization in an OFDM system. In this subject, a novel frame synchronization method based on maximal length shift-register sequences (m-sequences) for OFDM systems is proposed. An m-sequences is added directly to OFDM signals in the time domain. By utilizing the autocorrelation characteristics of m-sequences, fast frame synchronization can be obtained over slowly fading radio channels. In addition, if we combine the m-sequences based method with the guard interval based method, further improvement can be obtained over fading channels. Moreover, because the power of the m-sequences is low enough, the addition of the m-sequences does not influence on the capacity of the system.Channel estimation of an OFDM/MC-CDMA system. Effects caused by carrier frequency offset over successive OFDM symbols are investigated in this subject. Through theoretical analyses, we proved that the carrier frequency offset lead to the phase rotation changing linearly over3. successive OFDM symbols. Based on this conclusion, a novel linear phase interpolation method is proposed to combat the problem when block-type pilot arrangement is used in a MC-CDMA system. Theoretical analyses and computer simulations show that the proposed scheme effectively reduces the negative impact of carrier frequency offset on system performance.4. Narrow-band interference rejection in a MC-CDMA system using the transform domain adaptive filter. Single-tone interference can be rejected by the conventional threshold exciser. However, because of the "leakage" of an OFDM system, the single-tone interference may be transformed to narrow-band interference. The narrow-band interference is charactered by dynamic amplitude and multi-tone. The performance of the threshold exciser under narrow-band interference decreases badly. So, the transform domain adaptive filter is used in the MC-CDMA system to reject narrow-band interference on the AWGN channel. From the simulation results, the performance of the MC-CDMA systems using the transform domain adaptive filter is much better than that of the systems using the threshold exciser.5. Reduction of PAPR for an OFDM system. High PAPR is the main problem of an OFDM system. In this subject, a solution is proposed to reduce the PAPR of an OFDM system.