Channel Estimation,Interference Cancellation And Signal Detection Technologies For OFDM And MIMO Communication Systems

OFDM (Orthogonal Frequency Division Multiplexing) is currently a hot research topic in the field of bandwidth wireless communication. As a multicarrier parallel transmission technique, OFDM can effectively combat the multipath fading channel and convert the frequency selective channel into flat channel in frequency domain. It is a potential candidate for 4G.Meanwhile, Multiple input multiple output (MIMO) transmit multiple data streams in parallel subchannels in spatial, thus greatly increasing the system throughput without additional bandwidth and power requirements. MIMO is also a key technology for the future communication systems.In order to realize the no-error high-data wireless transmission, many key technologies need to be studied. In this thesis, we focus on the channel estimation, interference cancellation and signal detection techniques for OFDM and MIMO systems.For the channel estimation in OFDM systems, we analyzed the baseband mathematics models of OFDM transmission in multipath slow and fast fading channel. For slow fading channels, we propose a channel estimation algorithm saving bandwidth. For fast fading channels using Jake's model, we derived a minimum mean-square error (MMSE)-based channel estimation algorithm.One of the major problems in OFDM systems is the high sensitivity to frequency-offset errors. We investigated the intercarrier interference (ICI) problem due to the channel time-selectivity. We derived the signal to interference power ratio (SIR) and proposed two methods for ICI cancellation. Through computer simulation, we confirmed that the proposed algorithms get a big performance gain compared to the conventional OFDM system.Using a cyclic prefix (CP) as the guard interval is a simple way to combat the intersymbol interference (ISI), however also reduces the transmission efficiency of the system. Because choosing CP a priori is common, for some channels with a very long delay spread, it is possible that the length of CP is beyond the channel length. We analyzed the interference due to the insufficient CP for OFDM systems and derived the SIR. In order to cancel interference, we have proposed two filter designs with different criteria. Simulation results show the performance improvement with the interference cancellation filter.Time domain equalizer (TEQ) is another effective method against the decrease of the transmission efficiency. We have taken an overview of three classical TEQ designs. Since the high complexity of the existed methods, we proposed a modified method and showed its advantages through simulation.Signal detection for MIMO systems is a hot research topic. The performance of the conventional detection algorithm, ordered successive interference cancellation (OSIC), is not good enough due to the error propagation. We proposed two modified OSIC algorithms and compared the proposed methods and the conventional algorithm.For the interference cancellation problem of MIMO-OFDM systems over time-varying channels, we presented a model for MIMO-OFDM systems with ISI and ICI. Then, an interference analysis has been presented. To mitigate ISI and ICI, in the minimum mean-square error (MMSE) sense, an optimal equalizer is developed. We proposed the interference cancellation techniques for mitigating interference since the complexity of the optimum equalizer is too huge. In this system, channel estimation becomes critical, and we proposed a scheme for estimating channel parameters. The system is evaluated through simulations.