RAKE receiver

[gndpoints]

Hi, I'm wondering if somebody can help explain clearly how RAKE receiver actually works. I know that it consists of several correlators each tuned to the delay of a single multipath component which is longer than a single chip period (thus resolvable) and then optimally combine them. What confuses me is that even if ISI is not a problem, how does the receiver handles inter-chip interference (since a multipath component arrives more than one chip later)?

Thanks

 

[johnwiss]

Basically the RAKE will do what you say and the intra-chip components will not be resolvable. A standard equalizer may be used to "flatten" the channel effects caused by the residual ISI invisible to the RAKE.

There will be a large gain for most practical systems with just the RAKE and multipath robust modulation (e.g., bpsk with spreading). In the spread systems, the residual ISI will be helped by the despreading operation which will follow the RAKE receiver. In fact the RAKE will suffer if these close-together multipath components are included in the combiner. For example, if you implement 2 sample/chip combining choose the fingers to be at least a chip away form each other at the closest and if you get two large correlation peaks closer than that just chose one--not both.