Which method provides better noise immunity?
I know you can transmit more data via psk, but what about which method is less susceptible to noise?
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1What do you mean about being able to "transfer more data" using PSK? Also, you'll need to better-define the schemes that you're comparing; there are many flavors of each that will have subtle differences in performance. In terms of noise susceptibility, you need to define a quantitative metric that has a level playing field between the two, such as measuring the systems' bit-error rate versus the signal to noise ratio $\frac{E_b}{N_0}$. Another more complicated analysis problem would be to calculate the modulation-constrained channel capacity for the two. – Jason R Sep 20 '12 at 22:47
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1You can transfer more bits in a given bandwidth with PSK or that is what I gather from what I have read. My only criteria is if you send bit 01010101 with a FSK Mark of 100hz and space of 200hz and send the same stream with psk on 100hz which has the best probability of being decoded by the receiver. Those frequencies I picked are arbitrary. – SamFisher83 Sep 20 '12 at 23:02
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As you say in your comment, PSK signals are more bandwidth efficient, but FSK signals have good noise rejection for their data rates. It is thus difficult to compare them on even terms.
The traditional way of measuring noise immunity is measuring/calculating the probability of a bit error given the bit energy vs. noise energy (Eb/No). This is a good way of making everything fair
The following plot uses this paper for the FSK analysis, and this page for the BPSK analysis.
As you can see, BPSK wins quite handily. I won't attempt to prove it, but QPSK has the exact same BER curve as BPSK. You can intuitively see that if you think about how the real and imaginary portions of the symbol are orthogonal to each other, so you are essentially just sending two orthogonal BPSK signals at the same time. The BER curves of the higher PSK types (e.g. 8-PSK), though, do get worse because the symbols are closer together, so more energy is needed to distinguish them in the presence of noise.
So why does anyone use FSK? Mostly because it's pretty simple to implement, particularly the non-coherent receivers.
