Techniques to Enhance Spectral Efficiency of OFDM Wireless Systems

In recent years Orthogonal Frequency Division Multiplexing (OFDM) based technologies are in wide use for wireless communication systems. This is because OFDM elegantly overcomes the adverse effects of frequency selective fading channels and offers high spectral efficiency. Investigation of techniques to further enhance the spectral efficiency of OFDM based wireless systems is the prime objective of this thesis. As a first step, a comparison is made between OFDM and Multi Carrier Spread Spectrum (MC-SS) scheme. OFDM is found to offer relatively better performance than MC-SS under channel estimation and synchronization errors. This is the motivation to select OFDM for further investigation and performance enhancement in this thesis. Though OFDM has many advantages, yet it is severely affected by Inter Carrier Interference (ICI), which is caused by residual phase error, carrier frequency offset and Doppler frequency spread. To track the residual phase error, pilot sub carriers are embedded between the data sub carriers. To reduce the pilot overhead, it is proposed in this thesis to load data bits on pilot sub carriers without degrading system performance. It is found that up to 15% increase in spectral efficiency can be obtained by using this technique. To mitigate the impact of ICI, due to Doppler frequency spread, a novel technique of using adaptive sub carrier bandwidth is proposed in this work. This technique enhances the spectral efficiency in the range of 10% to 30% over OFDM systems which use fixed sub carrier bandwidth. OFDM systems need a large Guard Interval (GI) to overcome the effect of Inter v

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