Really basic question about OFDM 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?
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.