| Due to the high spectrum efficiency, ease of implementation (via FFT), and lowreceiver complexity (single-tape equalization), etc., orthogonal frequency multiplexingtechnique (OFDM) has been adopted and is currently considered by many applicationstandards, including digital audio and video broadcasting (DAB, DVB) in Europe,high-speed ADSL modems over twisted pairs in the United States, local area mobilewireless broad-band standards IEEE802.11a, MMAC, HIPERLAN/2, and the fourthgeneration mobile system. In the case of low carrier frequency and low terminal movingspeed, the channels can be regarded as time-invariant or slow-varying (the channel'sparameters are time-invariant or change a little during an OFDM symbol period), overwhich the intersymbol interference (ISI) and intercarrier interference (IC) can beeliminated by employing adequate cyclic prefix (greater than channel length), andtherefore the distortion of received signals can compensated by just a single-tap equalizerin the frequency domain. However, future wireless applications are expected to operate at high transmitfrequencies, at high levels of mobility, and at high capacities, resulting in a fading that isboth time- and frequency- selective. In such cases, the channel variation over an OFDMsymbol destroys the orthogonality among subcarriers, resulting in inter-carrier interference(ICI) and performance degradation. The ICI cancellation operation is indispensable in thereceiver to recover the orthogonalities among subcarriers and therefore improve the systemperformance. The topics of this paper are listed as follows: the simulation, analysis, andestimation of time-varying channels; analysis and cancellation of ICI in OFDM systemscaused by channel variations; and equalization of OFDM systems in time-varying channels.The author's research and main contributions are listed as follows. 1. Channel's characteristics greatly influence the performance of wirelesscommunication systems. Wireless channels are usually both time- and frequency-selective,and it is essential to investigate channels' characteristics and their simulation methods forthe communication system design and performance analysis. Based on the time delay line IVmodel (TDL), we proposed a time varying channel simulation method, which exploitsmulti-rate filtering and interpolation. The method has the advantages of low computationalcomplexity and convenience to implement, which lays a foundation for the followingchapters, including ICI cancellation, equalization, and optimization of OFDM systems intime-varying channels. 2. We provide an ICI analysis in in both time- and frequency- domain, while existingliteratures analyze the ICI effects exclusively in frequency domain, and obtain thefollowing important results: (1) The received signal is composed of two components, i.e.,desired ICI free term and ICI term; (2) the desired ICI free term is determined by theaverage channel impulse response, and ICI term is determined by the channel's fluctuationfrom the average during an OFDM interval. 3. Based on the above ICI analysis, we propose a time-domain ICI mitigation method,whose complexity is linear in the OFDM symbol length. Theoretical analysis andsimulation results indicate that the proposed method can effectively mitigate ICI. 4. Channels' rapid variations lead to the loss of orthogonality among subcarriers,resulting in inter-carrier interference (ICI) in orthogonal frequency division multiplexing(OFDM) systems. In this case, traditional channel estimation methods become inapplicable.Choi, etc., have proposed an algorithm for fast-varying channel estimation, but it requires ahigh computational complexity and needs the channel statistical knowledge, which can notbe obtained accurately in the practice. LINARTZ, etc., put forward that in the practice,Channel varies almost linearly during an OFDM block when the normalized Dopplerfrequency shift is...
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