Are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Sequence (DSSS) modulations? I am confused because I have read that these are also used e.g. with Gaussian Frequency-Shift Keying (GFSK) modulation? Can modulated signal be modulated again?
FHSS and DSSS alter the frequency and amplitude of a signal and so are technically modulations but they are not like other modulation schemes because they take place at a different point in the transmission chain, so you can have FHSS with M-QAM but you can't have M-QAM with BPSK for example.
When you transmit data using a M-QAM scheme, you take your bits, convert your bits into symbols and then use your modulation scheme to map those symbols to a waveform and then that waveform then is passed on for transmission.So in other words, in M-QAM for example, you are modulating the carrier signal with the symbol data.With FHSS and DSSS you are modulating the carrier signal but you are modulating it by using a unique user specific code or some other code / pattern so the two modulations are very different.