Hello David
Spread spectrum as a technique is a method of increasing the resilience of a wireless signal. Such a technique spreads data signals across a wider range of frequencies (channel width) than strictly necessary. This approach makes the signal less susceptible to interference making it more resilient.
The concept itself is about how data is “spread” over a channel’s bandwidth, not the entire electromagnetic spectrum. So if you have a 40 MHz channel width, a spread spectrum technique is used to place the data on the wireless signal in a robust manner.
Now it’s interesting that the OCG states that OFDM is a spread spectrum technology, because strictly speaking it isn’t, although it does spread data across multiple frequencies.
I talk more about OFDM in this post. But as a summary, let me just say that OFDM splits the data into many low-rate streams and transmits them in parallel over many closely spaced subcarriers. The subcarriers are orthogonal to each other (no interference between them), which makes OFDM very spectrally efficient.
Real spread spectrum, in contrast, such as DSSS, intentionally spreads a signal over a much wider bandwidth than needed, often using a pseudo-random code (like in DSSS) or hopping between frequencies (like in FHSS, which is another such method).
Even so, I still suggest that for the purposes of the exam, stick with what the OCG says. I just hope my explanation has helped to decipher the information a bit more successfully.
Now QAM is a modulation technique. This is the method with which data is actually encoded onto each subcarrier, and this is used specifically with OFDM. DSSS also uses modulation techniques, but not QAM. It typically uses simpler techniques. I think at this point you’re probably beyond the information you need for certification, but it is interesting to further explore these concepts.
I hope this has been helpful!
Laz