Coherent Detection Primary Advantage Primary Disadvantage Has better BER for same Eb / N0 compared to non-coherent detection OR Has lower Eb / N0 for same BER compared to non-coherent detection Primary Disadvantage Coherent reference signal must be derived from noisy signal at Rx input Complex & expensive circuitry Noisy reference signal means BER formulas are best case Actual performance is slightly worse Many applications choose non-coherent detection for simplified circuitry and lower cost Sacrifice Eb / N0 for lower cost/reduce complexity circuitry ECE 4710: Lecture #35

Non-Coherent Detection Non-Coherent Rx Circuitry Simple & cheap No carrier recovery for coherent reference Widely used for OOK and BFSK modulation methods Non-coherent OOK Rx is most widely used method in fiber optic communication systems Can’t be used for BPSK, QPSK, OQPSK, QAM  carrier recovery required for measurement of absolute phase of Rx signal BER equations Derivation much more complicated than coherent detection ECE 4710: Lecture #35

Non-Coherent OOK Detection Bandpass Filter : 1) Filter BW = Bp must be large enough to capture most of signal power to preserve signal waveshape (envelope) at output, 2) Eliminate noise PSD outside of signal BW  output noise, n(t), will be bandlimited gaussian noise Envelope Detector : Replaces product detector in coherent Rx  no carrier reference and synchronization circuitry needed Sample & Hold + Threshold Device : Same function as in coherent Rx ECE 4710: Lecture #35

Non-Coherent OOK Detection OOK Bandpass Waveforms Received Signal + Noise Bandpass AWGN represented by qn uniformly distributed random phase ECE 4710: Lecture #35

Non-Coherent OOK Detection BER for equally likely 1’s and 0’s What are conditional PDF’s at output of envelope detector?? For s2 the input to envelope detector is only bandlimited AWGN  “noise only” case Output “noise only” PDF has a Rayleigh distribution Envelope detector output is always positive ECE 4710: Lecture #35

Non-Coherent OOK Detection For s1 the input to envelope detector is sinusoidal bandpass signal + bandlimited AWGN Signal + noise case PDF is Rician s2 is variance of noise PDF at input to envelope detector Io(A)  modified Bessel function (zero order) Io(A) = 1 for A = 0  Rician PDF reduces to Rayleigh when no signal is present  noise only ECE 4710: Lecture #35

Non-Coherent OOK Detection Rician Rayleigh When A / s » 1 Rician PDF  Gaussian PDF ECE 4710: Lecture #35

Non-Coherent OOK BER Bit Error Rate is ECE 4710: Lecture #35 Cannot be solved in closed form Using A / s » 1 an approximate solution can be found or ECE 4710: Lecture #35

Non-Coherent FSK Detection FSK Signal Spectrum “0” “1” Bandpass Filter BW = 2B = 4R  Main lobe + 1st sidelobe  Envelope detector produces waveshape that is  rectangular ECE 4710: Lecture #35

Non-Coherent FSK Detection “1” “0” Dual OOK RF Rx For signal only at Rx input: ECE 4710: Lecture #35

Non-Coherent FSK Detection Assume a “1” is transmitted  s1 Upper channel input is signal energy + bandlimited AWGN Output PDF of envelope detector is Rician Same as OOK with signal “On”  “1” Lower channel input is bandlimited AWGN only Output PDF of envelope detector is Rayleigh Lower channel Rayleigh noise added to upper channel signal + noise Effectively doubles noise power compared to OOK ECE 4710: Lecture #35

Non-Coherent FSK Detection For OOK the average energy per bit is Note that for “1” bit For FSK the average energy per bit is Twice the bit energy compared to OOK “1” “0”  “1” “0” ECE 4710: Lecture #35

Non-Coherent FSK Detection FSK vs. OOK Double the noise power and Double the signal power BER vs. Eb / No performance is the same!! Non-coherent FSK BER is the same as non-coherent OOK BER Note that coherent FSK and coherent OOK also had same BER for matched filter case: ECE 4710: Lecture #35

BER vs. Eb / N0 ECE 4710: Lecture #35 For Pe < 10-3 Non-coherernt OOK & FSK require only 1 dB more than coherent OOK &FSK Coherent OOK & FSK Non-Coherent OOK & FSK BPSK or QPSK 1 dB Non-coherernt OOK & FSK require 4 dB more than coherent BPSK & QPSK ECE 4710: Lecture #35

OOK & FSK Rx Coherent OOK and FSK require carrier recovery & synchronization circuitry Complexity adds significant cost to Rx Only gives 1 dB improvement for BER vs. Eb / N0 Almost all OOK and FSK Rx’s are non-coherent Reduced cost/complexity relative to coherent Rx Only small difference in BER performance Coherent Rx : BPSK/QPSK has significantly better performance (3 or 4 dB) compared to OOK/FSK Use complexity and $$ for better performance ECE 4710: Lecture #35

DPSK Detection Coherent Rx required for BPSK & QPSK Differentially encoded BPSK  DPSK can be detected using partially coherent Rx Partially Coherent? **No carrier recovery circuitry** Use 1-bit period delayed version of incoming Rx signal to provide reference signal for product detector Tx carrier oscillator must be stable from one-bit period to next Delayed carrier input to product detector able to detect phase change (not absolute phase value) Input data must be differentially encoded ECE 4710: Lecture #35

DPSK Detection For optimal detection  bandpass matched filter BER is Much better than OOK or FSK Only slightly worse than BPSK & QPSK (  1 dB) DPSK widely used since BER is almost as good as BPSK but no carrier recovery circuitry is required Effectively a compromise between noncoherent OOK/FSK and coherent BPSK/QPSK ECE 4710: Lecture #35

BER vs. Eb / N0 ECE 4710: Lecture #35 For Pe < 10-3 DPSK requires only 1 dB more than BPSK or QPSK Coherent OOK & FSK Non-Coherent OOK & FSK BPSK or QPSK 1 dB DPSK DPSK has 3 dB better Eb / No compared to non-coherent OOK/FSK ECE 4710: Lecture #35