Ece 559 Wireless Communications Technologies Lecture 19 & 20: Spread Spectrum Communication Systems
نویسندگان
چکیده
1 Macrodiversity against Shadowing So far, the diversity techniques that are discussed are used to mitigate the eeects of small scale fading. These techniques are often called microscopic diversity. Similar diversity techniques exist in literature with the objective to combat shadow fading, which is known as macroscopic diversity. With macroscopic diversity, signals that are received from a mobile station (MS) by two or more geographically separated base stations are combined together. The most common combining method is selective combining. Best results could be obtained if the path loss from each base station to the received MS is uncorrelated. Let s k be the average received signal strength from the k th base station. By average received signal we assume that Rayleigh fading eeect is averaged out. Thus, s k are independent lognormal RV's with mean m s (dB) and standard deviation (dB). Let log 10 x = s k , then p(x) = 1 p expf ?1 2 2 (log 10 x ? m s) 2 g x > 0 Suppose there are M base stations, the probability that the average signal s is below some threshold is Note that the above expression assumes that selection diversity is used. Fig.1 plots the probability that the normalized received signal power is smaller than the threshold, which is the ordinate in the graph. It is clearly seen that P 4 out < P 3 out < P 2 out < P 1 out .
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