Research On Channel Characteristics Of MIMO-OFDM Systems

Nowadays, mobile communications is one of the most vigorous industries inthe world. While the fast growing mobile communication technologies providethe users continuously with new services and applications, the boosting users'demands have also facilitated new mobile technology innovations. Future mobileservices and applications will transfer gradually from dominating voice servicesto diversified data services. The supporting technology will change subsequentlyfrom circuit switching to packet switching. The features of future mobilecommunications technology will be: to provide better quality services tocustomers with lower costs on the basis of more efficient wireless networks andaccess technologies.To respond to ITU's initiation, many countries in the worldhave commenced their researches on the next generation mobile systemsMoreover, consensus of the objectives of future mobile communication systemhas already been reached, which is: to aggregate various access systems, such ascellular system, fixed wireless access system, nomadic system, wireless regionalarea network (WRAN) and integrate with the full IP network to provide userswith the maximum wireless transmission capacity of 100 Mbps and 1 Gbpsrespectively in the high-speed and low-speed environments;to realize theseamless integration of cellular network, wireless regional area network (WRAN),radio and television broadcasting, and satellite communication systems. All theseefforts aim to energize people to communicate with anybody at anytime,anywhere with any communication mode.In conventional communication systems, one antenna at the transmitter (Tx)and one at the receiver (Rx)—a so called SISO (Single Input Single Output)antenna system, creates a bottleneck in terms of capacity. Whatever themodulation scheme used, the coding strategy employed, the radio channel willalways set the limit for the telecom design. The investigation of Telatar hadshown that the introduction of MIMO technology could obtain much morecapacity than SISO. The benefit of using the MIMO technology lies in thecreation of orthogonal information channels, the combination of Tx and Rxdiversity and the increase in the antenna gain. The MIMO-OFDM system isMIMO system modulated by OFDM, its main advantage lies in transformingbroadband MIMO channel into a great deal of narrowband MIMO channel byusing DSP technology, it could avoid many operations of using space-timeequalization arithmetic, so the design of receiver become simple.It is thought that MIMO-OFDM technology will be the most likelytechnology selected by the next generation mobile telecommunication systems, soits very important to research the characteristics of channel for MIMO-OFDMsystems. Owing to the restrict of power,space diversification,temporal changingand variety of frequency, its difficult to apply the radio channel, however, if wecan conquer these restrictions and let data transport through the badlyenvironment, the return will be huge, so its very meaningful to research thewireless channel of MIMO-OFDM systems.Its mainly research the characteristics of channel of MIMO-OFDM systemsin this thesis:1.Research MIMO radio channel with frequency,temporal,space selectivefading. Discussing signal model, basic structure, and propogation scene ofwideband MIMO systems, introducing SVD and EVD methods, deducing thesignal model of MIMO-OFDM systems.At last, based on study channel modelingtheory (correlation model, ray-tracing model and the scattering model) andsimulation method, proposed the simulation results of correlation model.2.Investigate the channel capacity of MIMO systems.Firstly, analysischannel capacity of MIMO systems with transmit power is distributed averagely,simulation results show that the capacity is enlarged by using multiple antennas.Secondly,analysis how the channel capacity was affected by parameters ofsystems and environments,simulation results indicate the relationships ofchannel capacity with antenna's relativity and Rician factor.Thirdly, analysischannel capacity of MIMO systems with transmit power is distributed adaptively,simulation results show that the capacity is larger than capacity of MIMOsystems with transmit power is distributed averagely.3.Research the channel estimation methods based on LMMSE algorithm ofMIMO-OFDM sytems, a two-pass algorithm is proposed, which jointly estimatesboth the channel power delay profile (PDP) and the significant channel response.Simulations show that Two-Pass algorithm performs approximately the same asconventional LMMSE algorithm in number of pilots is less than the channelresponse time and requires no PDP knowledge in advance.4.Explore the channel estimation methods based on LS algorithm ofMIMO-OFDM sytems.To reduce PAR and enhance system's performance,introducing a special training sequence whose IFFT is a chirp sequence withconstant amplitude,and inserting zeros in every odd sub-carrier index.Simulationsshow that the method increase the performance of the channel estimation forMIMO-OFDM systems in the presence of ISI.5.Based on former investigation, proposed the direction of research in thefuture: To adapt the temporal variety channel,should introduce channel trackingmethod;Besides the parameters involved in this thesis,are there any other factorsimpact the increasing of channel capacity?If there are,how they affect the channelcapacity;How to acquire channel state information exactly and feed back it totransmitter quickly?Is there any method to answer this question;Explore the blindand semi-blind channel estimation techniques,consider how to reduse theircompute complexity,i.e.how to balance the spectrum efficiency and computeefficiency.