Chapter 7 Error Probabilities for Noncoherent Systems Noncoherent
Chapter 7 Error Probabilities for Noncoherent Systems Ø Noncoherent Detection of OOK Ø Noncoherent Detection of FSK Ø Differential Phase Shift Keying Huseyin Bilgekul EEE 461 Communication Systems II Department of Electrical and Electronic Engineering Eastern Mediterranean University EEE 461 1
Noncoherent Detection • Phase is unknown • Use filter and techniques such as envelope detection or feedback to demodulate signal H(f) Envelope Detector Sampler Threshold detector EEE 461 2
Noncoherent Detection EEE 461 3
On-off Keying EEE 461 4
On-off Keying EEE 461 5
On-off Keying EEE 461 6
On-off Keying EEE 461 7
On-off Keying Pdf if 0 is sent Rayleigh distributed noise only case Pdf if 1 is sent Rician distributed Threshold • Optimum threshold value occurs where densities intersect • Threshold depends on bit SNR, problem in fading channels EEE 461 8
On-off Keying • • BER depends on the bandwidth of the bandpass filter used. Pe becomes smaller as Bp is decreased. Minimum bandwidth allowed with no ISI is Bp=2 B=R=1/T Noncoherent OOK is worse than coherent OOK. EEE 461 9
On-off Keying EEE 461 10
Noncoherent FSK Detector • Filters matched to RF pulses for 0 and 1 • Decide 0 if vu < v. L Decide 1 if v. L > vu H 1(f) Env. Det Sampler vu Comparator H 0(f) Env. Det Sampler v. L EEE 461 11
Noncoherent FSK Detector • Noise outputs of the upper and lower receiver channels is similar. • When signal is present in the lower channel, the upper channel is only noise which is Rayleigh distributed. The lower channel is signal plus noise and it is Rician distributed. EEE 461 12
Noncoherent FSK Detector • The probability of error is thus obtained as: • OOK and FSK are equivalent on Eb/No basis. • Noncoherent FSK requires at most 1 d. B more Eb/No coherent FSK. • Noncoherent FSK is much easier to build. EEE 461 13
Noncoherent FSK Detector EEE 461 14
Differential Phase Shift Keying • Phase shift keyed signals can not be detected noncoherently. • Using the delayed version of the signal as phase reference partial coherence can be obtained. EEE 461 15
Differential Phase Shift Keying • For additive white Gaussian noise the BER performance can be obtained and approximated as: • Suboptimum detection (Band pass filter) is similar to that of OOk and FSK. • Optimum demodulation of DPSK requires 1 d. B more Eb/No BPSK. EEE 461 16
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