> The advantage of this system ? > each subcarrier can change it's phase (and amplitude) much faster > than a carrier of limited bandwidth could do, but during "symbol > integration time" there is no carrier overlap, the orthogonality > remains perfect. >
Yes, that is certainly the forte of OFDM, and indeed most OFDM systems use amplitude and phase simultaneously to pack more symbols into the time/frequency space. fred harris, aka Mr. DSP, once told me that "a really good signal fills up the channel and looks like white noise", or words to that effect. Some have mentioned that other phase modulated systems are orthogonal, "so why not call them OFDM also?" but generally speaking, they are called DPSK or DQPSK, or whatever, to be more descriptive... even though they may be "orthogonal". The OFDM flavors with many amplitude levels in the symbols that are used in UHF/microwave are probably not so good for HF work close to the noise floor. But a dynamically variable OFDM signal with variable amplitude levels that changes according to SN could take advantage of when propagation is good. It could revert to a single amplitude level, (making it -PSK), to wring every dB out when the SN is worse. I foresee and recommend this variable OFDM symbol approach as the best next step in fast HF modems for hams. Bonnie VR2/KQ6XA
