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I am new to signal processing and am currently doing research into the subject. I am learning about Orthogonal Frequency Division Multiplexing and have a question. Most definitions of OFDM mention its modulation scheme can be adapted to suit channel conditions, an example being changing to a more robust scheme if heavy noise is present.

My question is if the adaptation is applied to every sub-carrier or can each scheme be applied on a per sub-carrier basis? Will every sub-carrier be forced to switch to QPSK for example or can ones that are not affected keep using a high data rate scheme?

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Either of your suggested approaches would work (adapting modulation/coding scheme on a per-subcarrier basis or changing all subcarriers at once). There's no theoretical constraint that prevents you from doing that; it's simply a question of whether the added efficiency of tuning individual subcarriers is worth the increased system complexity.

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  • $\begingroup$ Thanks, I'm new to this and a lot of literature mentioned both approaches, didn't want to misunderstand it. $\endgroup$ – Valten1992 May 1 '14 at 14:47
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Theoretically, both. However, considering the fact that OFDM is a wideband modulation scheme in general, the Tx signal would face very different kinds of channel response across spread out subcarriers. So it makes much more sense to individually control the modulation such that a higher-order modulation symbol is sent on subcarriers with good channels while a lower-order on subcarriers suffering with bad channels.

Think about it: if adaptation treats the whole band as one block of spectrum, (1) how will you define this channel as good or bad within such a wide and diverse spectrum (with so many good and bad subchannels)? (2) how will this adaptation be different then as compared to a single carrier modulation?

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