1. 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

2. 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

3. 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

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

5. 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

6. 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

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

8. 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

9. 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

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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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