Tham khảo tài liệu 'adaptive wireless tranceivers p13', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Adaptive Wireless Tranceivers L. Hanzo . Wong . Yee Copyright 2002 John Wiley Sons Ltd ISBNs 0-470-84689-5 Hardback 0-470-84776-X Electronic Chapter 13 Adaptive Multicarrier Modulation T. Keller and L. Hanzo1 Introduction High data rate communications are limited not only by noise but especially with increasing symbol rates often more significantly by the Inter Symbol Interference ISI due to the memory of the dispersive wireless communications channel 317 . Explicitly this channel memory is caused by the dispersive channel impulse response CIR due to the different-length propagation paths between the transmitting and the receiving antennae. This dispersion effect could theoretically be measured by transmitting an infinitely short impulse and receiving the CIR itself. On this basis several measures of the effective duration of the impulse response can be calculated one being the delay spread. The multipath propagation of the channel manifests itself by different echos of possibly different transmitted symbols overlapping at the receiver which leads to error rate degradation. This effect occurs not only in wireless communications but also over all types of electrical and optical wave-guides although for these media the relative time differences are comparatively small mostly due to multi-mode transmission or incorrect electrical or optical termination at interfaces. In wireless communications systems the duration and the shape of the CIR depend heavily on the propagation environment of the communications system in question. While indoor wireless networks typically exhibit only short relative delays outdoor networks like the Global System of Mobile communications GSM 13 can face delay spreads in the order of 15 zs. As a general rule the effects of ISI on the transmission error statistics are negligible as long as the delay spread is significantly shorter than the duration of one transmitted symbol. This implies that the symbol rate of communications systems
