CHAPTER
6
Conclusions and Perspectives
6.1 Conclusions
In this thesis, we propose a low-complexity turbo equalizer structure for multiple-antenna systems. Through the iterative detection, the simple equalizer is capable of removing inter-symbol interference and co-channel interference. The cross-canceling filters in the feedback part remove the co-channel interference effectively. The simulations of the systems with two, three and four transmit antennas show that the equalizer can successfully recover the transmitted symbols in spite of the increase in the antenna number, which implies the increase of co-channel interference,. As long as the a priori information from the previous iteration is not too erroneous, the cross-canceling filters can remove co-channel interference well.
When two receive antennas are adapted instead of a single antenna, the extra cross-filters in the feedforward part can boost the performance of the equalizer. The simulations show that the performance improvement caused by increasing the number receive antennas are dramatic at the first iteration. With the lower error rate at the first iteration, the convergence to the perfect feedback solution becomes faster, that is, less iterations are required. In addition, two receive antennas lower the error rate of the perfect feedback case. The use of interleavers brings the function of
Chapter 6 Conclusions and Perspectives
de-correlation to the data sequence and results in the performance gain. In a turbo system, this de-correlation is especially important. Through simulations, we show the relationship between the interleaver size and the required number of iterations, which provides a trade-off to the system designer. As a conclusion, the turbo equalization we propose is capable of combating frequency-selective channels with the appropriate interleaver size.
6.2 Perspective
From the fundamental view point, the equalizer we proposed is to recover each transmitted sequences of each transmit antenna. When working with one sequence, the other sequences become co-channel interference. The simulations shows that the equalizer can still recover the sequence successfully even when there are four transmit antennas. This idea can be extended to all kinds of co-channel interference including multi-user interference. In addition, the proposed receiving scheme can incorporate with other advanced techniques into the turbo system. With more blocks that generate soft information, the turbo system can be more effective and have faster convergence.
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