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My question is:

Which type of diversity technique is used by the rake receivers?

In Bahai, et. al. "Multi-Carrier Digital Communications Theory and Applications of OFDM," Second Edition, 2004, on page 27, it is said that RAKE, OFDM, and Equalization, are all using frequency diversity, but here and there on the internet and papers I see it is said that rake is using time diversity!

Thank you!

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3 Answers 3

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Personally, I think it's more natural to describe the RAKE receiver as a time-diversity system.

However, in a wireless system multipath is only resolvable if the bandwidth is large, so RAKE receivers are only useful for wideband signals. So, in a sense, the RAKE receiver depends on the fact that the channel exhibits frequency-selectivity, which can also be described as a form of frequency diversity.

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  • $\begingroup$ I aggree wth @MBaz, because RAKE receiver combines the multipath components received from different time instants. $\endgroup$ Jan 25, 2023 at 10:29
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If it's the rake receiver scheme that I'm familiar with, it's a spread-spectrum signal using a pseudo-random code, and finding multiple correlation peaks in the received signal (which, if you plot it out as delay vs. signal strength looks like a rake -- it took me a long time to figure out that it's not an acronym; it's a metaphor).

So to the extent that one can firmly bin it into either time- or frequency-diversity, I'd put it into time-diversity.

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I just received this email from one of the greatest in wireless communications: Dr. Fuyun Ling. He said: "I think the confusion point is what is the definition of the (optimal) frequency or time diversity. I don't think there is such a rigorous definition. If we define the optimal frequency/time diversities as a receiver combining the signal components containing in all available frequencies/time paths, then an optimal receiver must achieve (optimal) frequency diversity. At the same time, it also achieves (optimal) time diversity. An ideal rake receiver is a matched filter. Thus, it is an optimal receiver. We can think it achieves (optimal) time diversity as well as (optimal) frequency diversity."

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