7 Capella Court
Nepean, ON, Canada
K2E 7X1+1 (613) 224-4700
www.optiwave.com© 2009 Optiwave Systems, Inc.

OptiSystem applications:
Optical coherent DSP overview

Universal DSP (1)OptiSystem’s Universal DSP component performs digital domain impairment compensation to aid inrecovering the incoming transmission signal after coherent detection. It supports the following higher
order modulation formats: BPSK, QPSK, 8PSK, 16PSK 8QAM, 16QAM, 32QAM, 64QAM, 128QAM, 256QAM
Note: For QAM modulation formats; square, star, and circular constellation formats are supported.

The Universal DSP component includes 13 functions and algorithms starting with a preprocessingstage (3 functions) followed by the signal recovery stage (10 functions and algorithms):Preprocessing stage (Samples/Symbol = (4 or 8) x Samples per bit)

Add Noise to signal



DC Blocking and Normalization

Main algorithms stage:

Filter (Samples/Symbol = (4 or 8) x Samples per bit)Resampling (Samples/Symbol = 2)

Quadrature Imbalance (QI) Compensation (Samples/Symbol = 2)Chromatic Dispersion (CD) Compensation (Samples/Symbol = 2)

Nonlinear (NL) Compensation (Samples/Symbol = 2)

Timing Recovery (Samples/Symbol = 2)

Adaptive Equalizer - AE (Samples/Symbol = 2)

Down-sampling (Samples/Symbol = 1)

Frequency Offset Estimation - FOE (Samples/Symbol = 1)

Carrier Phase Estimation - CPE (Samples/Symbol = 1)

Note: Any of these stages canbe disabled or enabled (viacheck box). If desired, acustom algorithm stage canbe added between any stage(C++, MATLAB, Python)2

Universal DSP (2)Digital filtering (time or freq.) can be used to compensate forchromatic dispersion. Nonlinear compensation is also
performed using a digital back propagation (BP) method

Carrier phaseestimation: The blindphase search (BPS)
algorithm is used torecover and remove theremaining phasemismatch between thelocal oscillator and thesignal.

Frequency offset estimation: Used todetermine f based on a feedforward
spectral estimation method

A filter can be applied to help in
removing out of band noise

QI compensation is used to mitigate amplitude
and phase imbalances within the in-phase (I) andquadrature (Q) signals.

The adaptive equalizer is used to compensate forresidual chromatic dispersion, polarization mode
dispersion (PMD) and to reduce inter-symbolinterference. For dual-polarization systems, the butterflystructure is used for polarization demultiplexing.

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7 Capella Court
Nepean, ON, Canada
K2E 7X1+1 (613) 224-4700
www.optiwave.com© 2009 Optiwave Systems, Inc.

QPSK back-to-back analysis

QPSK constellation (before DSP)
This is the IQ constellation before
pre-processingThe total number of samples is equal
to the binary sequence length(65536) x Number of samples per bit(4): 262144

The receiver eye is shown for the I-X
channel (after DSP 1, beforeresampling)

QPSK B2B

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QPSK constellation (after resampling)QPSK B2B

Prior to re-sampling the following stages
are performed:
Add noise to signal: Any noise source(noise bin) that falls within the bandwidth of
the transmission channel will be converted
into a signal and added to the opticalsampled signal.

DC Blocking: DC Blocking is applied tooffset any imperfectly biased voltages in themodulators.

Normalization: The received signal isnormalized to the appropriate m-QAM or mPSK gridFilter stage: Filtering can be applied toremove out of band noiseThe input sampled signal is re-sampled at
a rate of 2 samples/symbol. Interpolation isused to adapt the sampled signal
waveform to the new sampling rate (linear,cubic, or step)The 1st and N/2+1 sampled signals areused for re-sampling (where N = Samples
per symbol).

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QPSK constellation (after AE)
The adaptive equalizer is used tocompensate for residual chromatic
dispersion, polarization mode
dispersion (PMD) and to reduce intersymbol interference. For dualpolarization systems, the butterfly
structure is used for polarization
demultiplexing.

The two-stage constant modulus-radiusdirected (CMA-RD) algorithm is used

Down-sampling to 1 sample/symbol is
performed after AE

QPSK B2B

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QPSK constellation (after FOE)
The mixing with the local oscillator
introduces a frequency and phaseoffset, leading to a rotating
constellation diagram.

A spectral based, feedforward
method is used to estimate thefrequency offset (by maximizing theperiodogram of the power order ofthe received signal: 4 for
QPSK/16QAM & 8 for 8PSK/64QAM)QPSK B2B

8QPSK constellation (after CPE)
Carrier phase estimation: The blindphase search (BPS) algorithm isused to recover and remove theremaining phase mismatch between
the local oscillator and the signal.

Several test phases are applied andthe estimated phase is selected fromthe minimum distance sum obtained
between the rotated symbols and thetest phases

QPSK B2B

97 Capella Court
Nepean, ON, CanadaK2E 7X1+1 (613) 224-4700
www.optiwave.com© 2009 Optiwave Systems, Inc.

QPSK transmission example

QPSK constellation (before DSP)
This is the IQ constellation before
pre-processingThe total number of samples is equal
to the binary sequence length(65536) x Number of samples per bit(4): 262144

QPSK 160 km

11QPSK constellation (after resampling) QPSK 160 km
Prior to re-sampling the following stages
are performed:
Add noise to signal: Any noise source(noise bin) that falls within the bandwidth of
the transmission channel will be converted
into a signal and added to the opticalsampled signal.

DC Blocking: DC Blocking is applied tooffset any imperfectly biased voltages in themodulators.

Normalization: The received signal isnormalized to the appropriate m-QAM or mPSK gridFilter stage: Filtering can be applied toremove out of band noiseThe input sampled signal is re-sampled ata rate of 2 samples/symbol. Interpolation isused to adapt the sampled signal
waveform to the new sampling rate (linear,cubic, or step).

The 1st (Value = a) and N/2+1 (Value = b)
sampled signals are used for re-sampling(where N = Samples per symbol).

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QPSK constellation (after CD)
Chromatic dispersion is a static,polarization-independent, phenomenon.
Digital filtering can be used to compensate
for chromatic dispersion resulting frompropagation over fiber. The dispersion
compensating filter can be implemented ineither the frequency domain or time
domainNonlinear compensation (not enabledhere) is performed using a digital backpropagation (BP) method. The received
signal can be digitally propagated throughan inverse fiber model to compensate forCD and fiber nonlinearity.

QPSK 160 km

13QPSK constellation (after AE)
The adaptive equalizer is used tocompensate for residual chromatic
dispersion, polarization mode
dispersion (PMD) and to reduce intersymbol interference. For dualpolarization systems, the butterfly
structure is used for polarization
demultiplexing.

The two-stage constant modulus-radiusdirected (CMA-RD) algorithm is used

QPSK 160 km

14QPSK constellation (after FOE)
The mixing with the local oscillator
introduces a frequency and phaseoffset, leading to a rotating
constellation diagram.

Downsampling toQPSK 160 km15QPSK constellation (after CPE)QPSK 160 km

Carrier phase estimation: The blindphase search (BPS) algorithm isused to recover and remove theremaining phase mismatch between
the local oscillator and the signal.

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BER analysis (OSNR)
The project “112Gbps Coherent DP-QPSK_B2B with DSP.osd” has been setup to perform BER
waterfall curves based on OSNR settings. Simulations can be initiated from the Scripts tab.

Set OSNR component

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