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My Question is, Can we use the Rake Receiver with OFDM modulation? As you know, Rake receiver is usually used with DSSS modulation, so can we use it with OFDM?

in other words, Does the Rake Receiver depends on type of modulation or on the channel environment (I mean, If the channel has high multi-path, the Rake Receiver will be better to be used regardless the type of modulation)

Thank you

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  • $\begingroup$ where will you use the Rake receiver with OFDM, before or after FFT operation? $\endgroup$ – AlexTP Jun 7 '18 at 8:11
  • $\begingroup$ @AlexTP ..Thank you so much for your response, It'd be used before the FFT, for example, in multi-path channel, many channel taps will be detected representing the multi-path of channel, so that we can use matched filter for each taps in order to demodulate each tap by a separated demodulator. For example let's say that we are going to used two demodulators, so that, we will take the two highest SNR of two taps and demdulate them .. then go ahead. Or, in other words, Instead of highest SNR, let's say we will take the two lowest BER of taps then demodulate them and go ahead.. Is that possible? $\endgroup$ – Zeyad_Zeyad Jun 7 '18 at 8:57
  • $\begingroup$ I am not sure I understand what you want to do. First, for the last sentence, how can you get BER (bit error rate, right?), or any kind of error rate, at the modulation stage without channel decoding (and error detection mechanism like CRC)? Second, what do you mean matched filter, is it the notion to remove en.wikipedia.org/wiki/Pulse_shaping? Last, how are you able to grab the channel tap in Rake receivers (that use delay and correlators by definition) without using spreading modulations (like in DS-CDMA)? $\endgroup$ – AlexTP Jun 7 '18 at 9:22
  • $\begingroup$ Can I please ask you to correct that "Reciever" and remove the Camel Case wherever it is not required in the next cycle of edit of this question? $\endgroup$ – A_A Jun 7 '18 at 9:28
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    $\begingroup$ :( I'm a bit sad because I really wanted to know what you're talking about. Good luck with it! $\endgroup$ – Marcus Müller Jun 7 '18 at 20:26
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You can basically apply the rake receiver whenever.

However, OFDM was invented especially to make that unnecessary.

The point of a rake receiver is to demodulate the different paths independently and sum up the observed information. It is a time-domain decomposition of the channel impulse response, if you will.

OFDM considers the effects of the multipath channel, but in frequency domain, and converts the complicated time-domain equalization problem (which the Rake, partially, solves) to a frequency-domain problem, where the channel properties are independent between OFDM subcarriers and trivial to correct. It's the frequency-domain decomposition of the channel, if you will.

So, if you do OFDM, and your channel delay spread is within what your OFDM system can deal with, adding a rake in before seems redundant.

Your "pick the stronges rake path" is simply like a bad OFDM synchronizer. So, this is worse than just doing the very standard Schmidl&Cox synchronization for packetized OFDM, or to fixed-lag correlation methods used in streaming OFDM applications.

regarding how can I get the BER, I can do it similarly to this paper journals.sagepub.com/doi/pdf/10.1155/2016/3137014 .. But anyway, don't care of that method, let's consider the method of selecting the strongest signals. It means, "the signal can reach the receiver via multiple distinct pathways. The rake receiver is used to correct this effect, selecting the stronger signals.Each of these multipath components in rake receiver is called finger" so then, we can demodulate the different taps in different demodulators

Yeah, no. An OFDM synchronization would already do that for you. Rake and OFDM solve the same problem: Multipath == frequency selective channels. But OFDM inherently solves the problems of single carrier systems; applying Rake before doing OFDM is IMHO a dead end.

You need to read up on both when to use rake, and when to use OFDM. Recent papers aren't the way here; they are either low-quality (explaining concepts that have been standard for decades), or extremely special in application. Get a good communications textbook!

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