0
$\begingroup$

Is it possible to create a set of complex orthogonal sinusoids using FFT or IFFT In other words a kind of "Multiple Frequency Shift Keying" using multiple sinusoids that are created with a fourier transform ( FFT or IFFT) and where one frequency is sent at a time?

$\endgroup$
0
$\begingroup$

Yes, basically this is OFDM with an on/off constellation for each subcarrier instead of a QAM constellation as often used. So you would map your data to FFT values (each FFT bin is either 1 or 0), then take the IFFT to create the transmit waveform, with the cyclic prefix etc as in standard OFDM implementations.

$\endgroup$
  • $\begingroup$ I would think of this more as an OFDM - OOK approach (On/Off keying) rather than frequency shift keying since we typically think of each frequency bin in the FFT as its own sub-carrier. I guess you could instead call the frequency between two bins as a subcarrier and use 2 bins for the modulation as a true FSK signal. The result is the same it is just a matter of how you want to map the data and how you want to conceptualize what you are doing. Either of these approaches would be unusual so likely more a point of fun discussion. $\endgroup$ – Dan Boschen Oct 21 '18 at 13:03
  • $\begingroup$ I don't see the advantage of using an OOK approach vs the closest alternative of BPSK for transmitting one bit/symbol; we would choose BPSK (or QPSK) over a typical QAM mapping when consideration is more toward power efficiency vs spectral efficiency. $\endgroup$ – Dan Boschen Oct 21 '18 at 13:05
  • $\begingroup$ I would want to think that sending one sinusoid at a time has certain advantages especially in a situation where data rate is very low and where orthogonality is needed so that a receiver can distinguish between one sinusoid and another one since every sinusoid is carrying a different kind of information (without being modulated, but by means of a look up table known for the transmitter and the receiver) so when the receiver FFT's the incoming signal (it has been IFFT'd at the transmit) only the sinusoid that was sent will pop up $\endgroup$ – Hatem Tawfik Oct 21 '18 at 17:29
  • $\begingroup$ I am not sure I completely follow, do you think that if we sent ALL the sub-carriers at the same time the receiver could not distinguish them? They are orthogonal and therefore we can transmit them simultaneously and the receiver can distinguish each. Further when we decide to use multiple options for the symbol in the subcarrier, we still are only transmitting one of those symbols at a time- within that subcarrier. So first thing to be clear on, we can send ALL the subcarriers at the same time- that is no issue in a linear channel. $\endgroup$ – Dan Boschen Oct 21 '18 at 17:50
  • $\begingroup$ So for a "BPSK" subcarrier, you decide to either send +1 or -1 corresponding to bit 1 or bit 0. So if I wanted to send the sequence 10110, the FFT coefficients would be 1 -1 1 1 -1. What specifically are you proposing to send instead in this case? Five separate FFT blocks one at a time? What would they be? $\endgroup$ – Dan Boschen Oct 21 '18 at 17:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.