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Adaptive thu phát không dây P7

Practical Considerationsof Wideband AQAM In deriving the upper bound performance of wideband AQAM portrayed in Figure 4.21(b), various assumptions were made and stated in Section 4.3.1. However, in order to provide a more accurate comparison between AQAM and constituent fixed modulation modes, those its assumptions must be justified and their effects have to be investigated. Specifically, perfect, i.e. error-free feedback was assumed for the DFE, while in practice erroneous decision can be fed back, which results in error propagation | Adaptive Wireless Tranceivers L. Hanzo C.H. Wong M.S. Yee Copyright 2002 John Wiley Sons Ltd ISBNs 0-470-84689-5 Hardback 0-470-84776-X Electronic Chapter Practical Considerations of Wideband AQAM In deriving the upper bound performance of wideband AQAM portrayed in Figure 4.21 b various assumptions were made and stated in Section 4.3.1. However in order to provide a more accurate comparison between AQAM and its constituent fixed modulation modes those assumptions must be justified and their effects have to be investigated. Specifically perfect i.e. error-free feedback was assumed for the DFE while in practice erroneous decision can be fed back which results in error propagation. Consequently the impact of error propagation is studied in the context of both fixed and adaptive QAM schemes. Furthermore as stated in Section 4.3.1 perfect modulation mode selection was assumed whereby the output SNR of the DFE was estimated perfectly prior to transmission. However in stipulating this assumption the delay incurred between channel quality estimation and the actual utilization of the estimate was neglected in the wideband AQAM scheme. In this chapter the impact of co-channel interference on the wideband AQAM scheme is also investigated. In this respect interference compensation techniques are invoked in order to reduce the degradation resulting from the co-channel interference. Let us now commence our investigations by studying the error propagation phenomenon in the DFE. 7.1 Impact of Error Propagation Error propagation is a phenomenon that occurs whenever an erroneous decision is fed back into the feedback filter of the DFE. When a wrong decision is fed back the feedback filter produces an output estimate which is erroneous. The incorrect estimate precipitates further errors at the output of the equalizer. This leads to another erroneous decision being fed back into the feedback filter. Consequently this recursive phenomenon degrades the BER performance of the DFE. .