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Adaptive thu phát không dây P6

Mobile communications channelstypically exhibit time-variant channel quality fluctuations [ 131 and hence conventional fixed-mode modems suffer from bursts of transmission errors, even if the system was designed to provide a high link margin. As argued throughout this monograph, an efficient approach of mitigating these detrimental effects is to adaptively adjust the transmission format based on the near-instantaneous channel quality information perceived by the receiver, which is fed back to the transmitter with the aid of a feedback channel [ 151 | 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 Vy_ Adaptive Modulation Mode Switching Optimization B.J. Choi L. Hanzo 6.1 Introduction Mobile communications channels typically exhibit time-variant channel quality fluctuations 13 and hence conventional fixed-mode modems suffer from bursts of transmission errors even if the system was designed to provide a high link margin. As argued throughout this monograph an efficient approach of mitigating these detrimental effects is to adaptively adjust the transmission format based on the near-instantaneous channel quality information perceived by the receiver which is fed back to the transmitter with the aid of a feedback channel 15 . This scheme requires a reliable feedback link from the receiver to the transmitter and the channel quality variation should be sufficiently slow for the transmitter to be able to adapt. Hayes 15 proposed transmission power adaptation while Cavers 9 suggested invoking a variable symbol duration scheme in response to the perceived channel quality at the expense of a variable bandwidth requirement. Since a variable-power scheme increases both the average transmitted power requirements and the level of co-channel interference 17 imposed on other users of the system instead variable-rate Adaptive Quadrature Amplitude Modulation AQAM was proposed by Steele and Webb as an alternative employing various star-QAM constellations 16 17 . With the advent of Pilot Symbol Assisted Modulation PS AM 18-20 Otsuki et al. 21 employed square constellations instead of star constellations in the context of AQAM as a practical fading counter measure. Analyzing the channel capacity of Rayleigh fading channels 22-24 Goldsmith et al. showed that variable-power variable-rate adaptive schemes are optimum approaching the capacity of the channel and characterized the throughput performance of .