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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) L7/ 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 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 neglectedin the wideband AQAM scheme. In this chapter the impact of co-channel interference on the wideband AQAM schemeis 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 phenomenonin the DFE. 7.1 Impact of Error Propagation Error propagation is a phenomenon that occurs, whenever an erroneous decisionis fed back into the feedback filter of the DFE. When a wrong decision is fed back, the feedbackfilter 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. Intuitively, the effects of this error will last throughout the memory span of the feedback filter. This causes an error propagation throughout the feedback filter, until the memory of the feedback filter is cleared of any erroneous feedback inputs. The performance of the fixed modulation modes of our AQAM scheme in conjunction 257 258 CHAPTER 7. PRACTICAL CONSIDERATIONS OF WIDEBAND AQAM Transmission Burst type: DFE Parameters: No. of feedforward taps, N f No. of feedback taps,Nb Decision Feedback Recursive Kalman Channel Estimator Parameters: Measurement error covariance Matrix, R ( k ) = g1 System error covariance Matrix,Q ( k ) 6 (see Equation 3.47) Initial Channel Estimate Vector h(o) Number of RKCE taps Channel Parameters: Typical Urban Rayleigh-faded Weights Normalized Doppler Frequency: 3.25 Non-Spread Speech Burst of Figure 4.13. 35 7 Past Decision 0

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