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im trying to understand the difference between a guard band, and the cyclic prefix, or at least the relationship between the two.

Prior to now, I was under the interpretation that the cyclic prefix acts as a guard band, which transforms the linear convolution into a circular convolution which makes demodulating the signal and channel equalisation much easier as well as preventing ISI. However, when using the OFDMModulator object in MATLAB, I see an example usage as follows:

mod = comm.OFDMModulator(...
    'FFTLength' ,           64, ...
    'NumGuardBandCarriers', [5;6], ...
    'InsertDCNull',         true, ...
    'PilotInputPort',       true, ...
    'PilotCarrierIndices',  [12;26;40;54], ...
    'CyclicPrefixLength',   16, ...
    'NumSymbols',           1);

So, we can look at the subcarrier utilisation for one symbol by calling showResourceMapping(mod), and we see:

enter image description here

This is sort of what I expected, because we set NumGuardBandCarriers to [5;6] so, 5 subcarriers at the lower end and 6 at the higher end, but there are a few things I dont quite get:

(1) what relevance does the 'CyclicPrefixLength' parameter have on this mapping - when I change this value to larger numbers the graph remains the exact same.

edit: I have an answer to this. It does not affect the actualy subcarrier allocation of course, but when you actually use the ofdm object to modulate some data, then the length of the result depends on how long the CP is (L_fft + L_CP)

(2) why is that the guard band is spread across the lower and upper subcarrier inidices - If the guard band is the cyclic prefix, I was expecting that you just take the last N symbols out of the FFT and prepend them to the Data.

(3) what is the relationship/dependecy between the number of guard band carriers and the cyclic prefix length?

edit: Ok I understand - for reference, there is a difference between a guard band and a guard interval. The guard band has nothing to do with the cyclic prefix. This is purelly allocation of null subcarriers at the edge of the OFDM symbol which provide buffers between consecutive OFDM symbols to protect the integrity of transmitted signals by reducing intersymbol interference. In contrast the guard interval is a synonym for the cyclic prefix, which itself has nothing to do with subcarrier allocation. Instead we simply prepend the result of the IFFT in the modulator with the final N symbols generated in the modulation process.

Perhaps someone else would still like to expand but this is how I understand it.

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I think of it as 2 sets of guard bands, one set in the frequency domain (unused spectrum), and one set in the time domain (OFDM cyclic prefix). One guards against adjacent frequency channel splatter (offset errors plus Doppler). One guards against before/after time domain splatter (synchronization latencies plus multipath interference).

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  • $\begingroup$ That is a good way to think about it thanks. I often find resources talking about the time domain splatter but not the frequency domain splatter, that's where the confusion stems from. $\endgroup$
    – Jonah F
    Oct 14 at 17:04

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