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[SOLVED] Wideband systems in Multiple Access

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iVenky

iVenky

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I just couldn't get this sentence that I take from Rappaport book on Wireless communication in the chapter on "Multiple Access"-

"In wideband systems, the transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel. Thus, multipath
fading does not greatly vary the received signal power within a wideband channel, and frequency selective fades occur in only a small fraction of the signal bandwidth at any instance of time"

But the truth is if the bandwidth is larger than the coherence bandwidth then the multipath fading is prominent.

---------- Post added at 21:15 ---------- Previous post was at 19:41 ----------

I think it is somewhat related to frequency diversity but I still need some clear explanation of this.

Much awaiting for your replies.
 

Not an expert on this but I did read Rappaport. My thought is that, with flat fading, the entire signal bandwidth would drop out at times. But with frequency-selective fading, and especially in the case of a wideband signal, the portion of the bandwidth that might fade at a given point in time is not large, and since the majority of your bandwidth is unaffected by the fading, you'll probably still be able to decode the signal. So all he is noting is that, actually, wideband signals are robust in frequency selective channels.
 
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    iVenky

    iVenky

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I thing I am getting it. Yep it's somewhat similar to frequency diversity. If one frequency attenuates due to fading other one may not (though we would have multipath delay) and with the help of that frequency it is indeed possible to decode that signal.

Am I coming to a right conclusion?

Thanks by the way.
 

I agree that there is an analogy with frequency diversity. Also, if your signal is fading in only a small portion of its bandwidth, those can be restored by using an equalization algorithm to compensate for the fading in that bandwidth. It would be good to get some validation from other members of the forum though.
 

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