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On page 137, chapter 5 in the book titled 5G Physical Layer: Principles, Models and Technology, the author says:

An important attribute of the FBMC waveforms is that they typically do not employ any CP or GI, unlike the OFDM-based waveforms. The prototype filters in the FBMC waveforms have long decaying tails which can make them robust to the ISI caused by frequency-selective channels

Question: Why would filter long decaying tails make a signal robust to ISI caused by frequency selectivity?

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long decaying tails which can make them robust

Why would filter long decaying tails make a signal robust

can make them robust, not makes them robust!

You need a long filter length for narrow bandwidth and steep transition width.

So, with a long filter, you can make very narrow subcarrier channels.

Those being narrow, they don't "see" multipath, they only see a flat channel.

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  • $\begingroup$ is the "ISI caused by frequency-selective channels" interference between subcarriers ? $\endgroup$ – AlexTP Jun 30 at 13:10
  • $\begingroup$ no, that'd be ICI. ISI happens between consecutive symbols on the same subcarrier. $\endgroup$ – Marcus Müller Jun 30 at 14:05
  • $\begingroup$ but flat fading is also what each subcarrier sees in OFDM as well, so what is the difference? $\endgroup$ – ali khalil Jun 30 at 14:13
  • $\begingroup$ That one is OFDM and the other is FBMC. You're reading a book on it! OFDM has a rectangular window shape, and doesn't have a long-tailed filter, exactly as it says in the text you're quoting. $\endgroup$ – Marcus Müller Jun 30 at 14:30
  • $\begingroup$ You said that flat fading is the reason of not seeing multipath in FBMC , but flat fading applies also to OFDM for each subcarrier. Your argument is about flat fading $\endgroup$ – ali khalil Jun 30 at 14:53

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