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Adaptive thu phát không dây P12

As argued throughout the previous chapters the book, mobile propagation channels exhibit of time-variant propagation properties [ 131. Although apart from simple cordless telephone schemes most mobile radio systems employ power control for mitigating the effects of received power fluctuations, rapid channel quality fluctuations cannot be compensated by practical, finite reaction-time power control schemes. | 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 12 Burst-by-Burst Adaptive Multiuser Detection CDMA E. L. Kuan and L. Hanzo1 12.1 Motivation As argued throughout the previous chapters of the book mobile propagation channels exhibit time-variant propagation properties 13 . Although apart from simple cordless telephone schemes most mobile radio systems employ power control for mitigating the effects of received power fluctuations rapid channel quality fluctuations cannot be compensated by practical finite reaction-time power control schemes. Furthermore the ubiquitous phenomenon of signal dispersion due to the multiplicity of scattering and reflecting objects cannot be mitigated by power control. Similarly other performance limiting factors such as adjacent- and co-channel intereference as well as multi-user interference vary as a function of time. The ultimate channel quality metric is constituted by the bit error rate experienced irrespective of the specific impairment encountered. The channel quality variations are typically higher near the fringes of the propagation cell or upon moving from an indoor scenario to an outdoor cell due to the high standard deviation of the shadow- and fast-fading 13 encountered even in conjunction with agile power control. Furthermore the bit errors typically occur in bursts due to the time-variant channel quality fluctuations and hence it is plausible that a fixed transceiver mode cannot achieve a high flexibility in such environments. The design of powerful and flexible transceivers has to be based on finding the best compromise amongst a number of contradicting design factors. Some of these contradicting factors are low power consumption high robustness against transmission errors amongst various channel conditions high spectral efficiency low-delay for the sake of supporting interactive real-time multimedia services .