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?