In this thesis, we have focused on channel estimation techniques for SISO-OFDM and MIMO-OFDM systems. We have aimed to design spectrally efficient and lower com- plexity system to improve the channel estimation performance.
We have studied pilot-based and Expectation-Maximization (EM)-based channel estimation techniques and have compared their performances in terms of BER, MSE and iteration numbers used in the EM algorithm. We have shown that the EM-based channel estimation algorithm has better performance compared to pilot-based channel estimation for SISO-OFDM systems. We have also compared these algorithms for STBC-OFDM and V-BLAST structured OFDM and have shown that the EM algorithm either improve the channel estimation performance or increase the bit error rate performance.
We have studied two different methods which have different number of pilot sym- bols and different pilot arrangement in time-frequency axes. We have utilized the EM algorithm to increase channel estimation performance for both methods. We have pro- posed a lower complexity and spectrally efficient pilot arrangement and compared it with existing pilot arrangement method. We have also used these methods for STBC-OFDM and V-BLAST OFDM. We have shown that the method which has lower number of pilots has the same performance for high SNR values. We have concluded that, the proposed technique is suitable to use for high SNR values in SISO-OFDM, STBC-OFDM and V-BLAST OFDM systems, since it is spectrally efficient and has lower computational complexity.
We have proposed an iterative channel estimation technique, which establishes a link between decision block and channel estimation block, with lower complexity. We have compared this technique with the EM-based channel estimation technique and shown that they have the same performance.
Finally, we have studied joint channel estimation and signal detection technique for V-BLAST-OFDM systems. We have updated the worst estimated channels using the EM algorithm and we have improved the channel estimation performance. Thus, we have increased the BER performance of the system slightly.
As a future work, these iterative channel estimation techniques for SISO-OFDM and MIMO-OFDM will be extended to multiuser communication systems.
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