Is it valid to spread an OFDM waveform using direct sequence spread spectrum (DSSS)? Both operations are linear so it seems like this should work.
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You can do a lot of things. Some are sensible!
Remember why you're using OFDM (frequency-domain equalization) and remember that the temporal structure of the OFDM symbol (as the IDFT of the data symbol vector usually with a cyclic prefix) is what makes that possible. Applying DSSS afterwards completely breaks that. So, you only add complexity, you lose all benefits of OFDM doing this.
However, spreading data before putting it into an OFDM transmitter, and despreading it after an OFDM receiver, can be done, and preserves the temporal properties of the OFDM symbol, and hence the advantages.
The advantage of that would be the same as for other DSSS systems: a resilience to narrowband interferers or short bursty interferers, depending on the length of the spreading relative to the OFDM symbol length. However, in terms of Eb/N0, a bipodal DSSS with a spreading factor of N is equivalent to a repetition code of the same length – and hence, it would make more sense to use a good rate 1/N channel code, together with an interleaver of at least that length, to convert your info bits to N times as much code bits prior to symbol mapping, instead of spreading symbols with a N-sequence.
answered Mar 26, 2022 at 14:05
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$\begingroup$ If the purpose of spreading in this context is to bring the signal below the noise floor, then spreading before ofdm wouldn’t achieve that, correct? $\endgroup$ Commented Mar 27, 2022 at 11:41
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1$\begingroup$ Just the same as for DSSS: it doesn't do it by itself. A DSSS receiver can accumulate energy coherently to lift a signal out of noise, but if I just multiply my transmit symbols with a DSSS sequence, they don't lose power. It's that I can then scale them down and still receive them that allows me to do that. You're right, however, if you're trying to push an OFDM system below noise floor, that's going to be a complicated and potentially badly behaving receiver, as synchronization becomes hard, but is very important for OFDM. (See the ICI problem!) $\endgroup$ Commented Mar 27, 2022 at 11:56
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1$\begingroup$ A general remark: if you want to ask something related to "I want to build an OFDM system that operates below noise floor", then say that in your next question! Don't abstract your questions too much from the problem you're solving – you might not know yet what is important for that problem, and what can be abstracted away. Rather be over-specific and get an answer that can be generalized, than being under-specific and not get an answer. $\endgroup$ Commented Mar 27, 2022 at 11:57
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