A quick question, is there any way to do channel estimation on a (G)FSK receiver. I'm trying to build a DASH7 receiver.
I found that there is a block for a OFDM channel estimator, but i can't find it for FSK.
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Sign up to join this communityA quick question, is there any way to do channel estimation on a (G)FSK receiver. I'm trying to build a DASH7 receiver.
I found that there is a block for a OFDM channel estimator, but i can't find it for FSK.
In FSK, you don't care about the frequency-varying phase properties of your channel – they simply don't matter to your signal, so a (relative) amplitude estimate for all mark frequencies would technically suffice.
Most FSK receivers don't even do that – it's only helpful to know that "mark frequency A gets through stronger than frequency B" if you use that to adjust your decision boundaries (e.g. for an MAP decision, you'd need that knowledge).
I don't have access to any DASH7 phy layer documentation, but if it's really derived from RFID tag technology, then you can be 100% certain the thing works without channel state information. Complexity wouldn't allow that. That doesn't mean you can't make it work better at further distances – but it does imply that if you're well within the specified ranges and environments, getting a channel estimate will not help you significantly much.
Wikipedia says "range up to 2 km, less than 200 kHz bandwidth", and that basically means your second observable multipath component would have to have a 1.5 km longer way to travel than the first one you receive – that is physically practically irrelevant, as that delayed version of the signal will be so much weaker than the quicker path that it can't lead to significant frequency-selectivity.
So the answer to your question is "you don't do that at all".
The rest of this answer, hence, is of merely academic interest:
Now, if your channel is actually strongly frequency-selective, so that knowing the relative amplitude becomes desirable, you can also be sure that the transition between different frequencies (which is Gauss shaped, transmitter-side) isn't nice anymore – and that's bad for your Gauss-expecting receiver.
Now, the only thing that could be FSK-specifically done here is some form of nonlinear emphasis on some values – that really doesn't sound very tempting, because estimating the necessary distortion to counteract the nonlinear memory-affected effect of a frequency selective channel on the FM demodulation of a frequency transition sounds pretty hard. And whenever possible, we try to compensate systems where they are linear – that gives us at least some guarantees about invertibility/well-posedness of the inverse problem.
So, don't try to deal with your FSK in any special way. Instead, just do what works for any transmission type:
The first step – transmission of something you know – is in many systems done via a decision feedback equalizer, where "something you know" is what you received and decided for, but that means block-wise operation in an FSK case, since your system is not linear in modulation pulses, but has memory.