[show abstract][hide abstract] ABSTRACT: Adaptive modulation techniques are becoming very important for mobile communication systems due to dynamic characteristics of the medium. Diversity is another technique which further improves the performance of mobile communication system. In this paper the performance of adaptive M-QAM modulation technique has been evaluated in term of BER with diversity. A comparison of different values of the constellation size M has been done by simulation for variable number of paths variable number of paths, L = 1, 2, 3, 4, 5, 6. The performance is compared with the performance of GMSK, DQPSK, and MPSK modulation in the same diversity environment using MRC technique. A detailed performance comparison of modulation techniques shows that the diversity improves the performance of all modulation schemes, especially at lower values of diversity orders. The improvement in the performance is less at higher values of diversity orders. It is noticed that the improvement in BER performance is significant if the number of paths is less than 4, and there is no substantial improvement in BER performance if the number of paths is greater than 6, due to the increase of estimation errors.
Advanced Communication Technology, 2008. ICACT 2008. 10th International Conference on; 03/2008
[show abstract][hide abstract] ABSTRACT: In this paper we derive general expressions for the performance of hierarchical multilevel quadrature amplitude modulation constellations (M-QAM), over flat Rayleigh fading environments with imperfect channel estimation. Cases of diversity reception with identical channels and with dissimilar channels, employing maximal ratio combining (MRC) are considered. Since hierarchical constellations are usually employed to achieve unequal bit error protection, the individual bit error rate (BER) of each bit stream is analyzed and it is shown that poor channel estimation has a more severe impact in the performance of the least protected bit streams.
IEEE Transactions on Communications. 01/2007; 55:1852-1856.
[show abstract][hide abstract] ABSTRACT: The use of generalized logarithmic likelihood ratio (GLLR), as a metric for soft-input decoding, was shown to provide gains over the conventional logarithmic likelihood ratio (LLR). However, computational complexity of the GLLR is greater than in the case of LLR, so the choice of GLLRs must be justified by a notable performance improvement. In this paper, we investigate the judiciousness of the adoption of GLLRs or LLRs for soft decoding analyzing the performance of both metrics in two different scenarios. Metrics expressions are derived and channel estimation errors are considered in analysis and simulation. Numerical results show that GLLRs outperform LLRs but, in general, the achieved improvement is relatively small. Therefore, application of GLLR metrics requires cautious analysis
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