I am trying to understand, possibly in an intuitive way, why would OQAM/FBMC when used with a Phydyas filter lead to a signal that does not need any cyclic prefix just like in any OFDM signal also created by an IFFT If the cyclic prefix is absent how would ISI be avoided?
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$\begingroup$ Hi Ali! Are you aware of why OFDM uses a cyclic prefix instead of just a silent guard interval? $\endgroup$– Marcus MüllerCommented Jun 21, 2020 at 11:54
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$\begingroup$ yes I am I am aware of that $\endgroup$– ali khalilCommented Jun 21, 2020 at 16:15
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$\begingroup$ Wonderful, that makes answering easier. $\endgroup$– Marcus MüllerCommented Jun 21, 2020 at 16:35
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$\begingroup$ what's the difference between OQAM and OQPSK? $\endgroup$– robert bristow-johnsonCommented Jul 16, 2021 at 22:33
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1 Answer
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Since FBMC doesn't depend on the channel acting as circular convolution on the transmit signal like OFDM does, it makes no sense to use a cyclic prefix. (By the way, not all OFDM is cyclic-prefix OFDM, some OFDM-schemes simply use a silent guard interval, but these are the exception.)
To battle ISI without usage of a guard interval, one has to fall back to doing an equalizer per sub-channel in FBMC (something that OFDM reduces to multiplication with the inverse of the complex channel coefficient, but bought at the expense of rate loss due to necessity of the guard interval/cyclic prefix).
However, that equalizer is typically relatively benign in complexity, since the FBMC subchannels are designed to not see very many symbols in delay spread.
That's exactly what the PHYDYAS-style FBMC does: it uses the techniques used in modern single-carrier communications per carrier, and uses OQAM to combat inter-carrier-interference, whilst employing FFT methods for the multicarrier modulation.
answered Jun 21, 2020 at 16:43
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$\begingroup$ I am reading and still trying to understand but can I very roughly conclude that OQAM is there to combat inter-carrier interference while the PHYDYAS filter combats intersymbol interference? If that is the case this would already be a big step in my understanding..that will help me digging further into available literature $\endgroup$ Commented Jun 21, 2020 at 16:54
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$\begingroup$ @alikhalil I'm confused by your terminology. PHYDYAS is a working group, and it uses a per-subchannel equalizer (p. 19ff), exactly as described in my answer. A filter alone can't solve ISI, unless that filter is adaptive to the channel. And in that case, we'd call that filter an equalizer. $\endgroup$ Commented Jun 21, 2020 at 16:55
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$\begingroup$ o.k. as I said this was very helpful and will certainly enable me to dig further. $\endgroup$ Commented Jun 21, 2020 at 17:01
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$\begingroup$ "However, that equalizer is typically relatively benign in complexity, since the FBMC subchannels are designed to not see very many symbols in delay spread" What does this sentence mean? $\endgroup$ Commented Mar 7, 2021 at 1:38
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$\begingroup$ Pretty much what it says: Your equalizer is not very complex, because when you build an FBMC system, you'll be smart, and make the channels not wide enough that the delay spread is larger than a few symbols (at most). $\endgroup$ Commented Mar 7, 2021 at 13:18