Time Domain Equalization for ofdm Systems in Time Varying Channels
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Date
2011-07
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Addis Ababa University
Abstract
Orthogonal Frequency Division Multiplexing (OFDM) is an emerging multi-carrier
modulation scheme, which has been adopted for several wireless standards such as IEEE
802.11a and HiperLAN2. OFDM as a transmission technique has been known to have a lot of
strengths compared to any other transmission technique due to its high spectral efficiency,
robustness to the channel fading, immunity to impulse interference and ability to handle very
strong echoes. The efficacy of OFDM implementation in many areas such as DAB (Digital
audio Broadcasting), DVB (Digital Video Broadcasting) and Wireless LAN has gained its
popularity.
A well-known problem of OFDM is its sensitivity to frequency offset between the
transmitted and received carrier frequencies. This frequency offset introduces inter-carrier
interference (ICI) in the OFDM symbol. This thesis investigates time domain equalization
technique by using well designed windows for combating the effects of ICI entitled
equalization with time domain windowing. This method is compared with other methods like
existing frequency domain correlative coding and self cancellation methods in terms of bit
error rate performance, carrier to interference ratio and bandwidth efficiency. Effects of
different orders of windowing on the carrier to interference power ratio are investigated for
various normalized frequency offset values. Through simulations, it is shown that the Time
domain windowing technique is effective in mitigating the effects of ICI. It shows a better
performance in terms of BER and CIR compared to the existing frequency domain correlative
coding and self cancellation techniques. The designed window of leading coefficient and
correlation order value of 1 gives an optimum design based on maximizing CIR.
Over10 dB performance gain has been obtained with employment of windowing compared to
the standard OFDM system without any equalization techniques employed at the normalized
frequency offset of ɛ = 0.15 and BER of 10-3. The time domain windowing scheme shows
better tolerance to frequency offset by considerable reduction of the sensitivity to frequency
error