On the Design and Optimization of OFDM Systems
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In this thesis, some selected aspects of OFDM systems have been investigated. We focus on the modulation and demodulation techniques, such as efficient equalization, robustness to carrier frequency offset (CFO) and CFO estimation. An OFDM/OQAM system with pulseshaping can achieve higher spec- trum and power e±ciency than conventional OFDM/QAM systems with guard interval, while it needs more complicated equalizer to counteract the multipath effects. We derive some theoretical expressions that are useful for selecting appropriate equalizer length. This allows an efficient equalization with a complexity only slightly higher than that of OFDM/QAM systems with guard interval. OFDM systems are much more sensitive to CFO than single carrier systems. The robustness to CFO of OFDM systems can be improved by using appropriate pulseshaping. The optimal pulseshapes are found by variational calculus or numerical optimization methods. We find that even when using these optimal pulses, the effects caused by uncompensated CFO can not be completely eliminated. Traditionally, the CFO is estimated before demodulation. For an OFDM system with many subchannels, each subchannel can be viewed as flat-fading, thus the problem of CFO estimation can be simplified based on subchannel signals. We propose several blind CFO estimation methods for OFDM/OQAM systems based on subchannel signals. Numerical simulations are performed to illustrate the performance of the suggested CFO estimation methods and validate the theoretical analysis. Simulation results show that better performance can be achieved based on estimation of subchannel signals than that based on channel signal before demodulation.