We propose time-resolved EVM for characterization of 16-QAM transmitters. By designing a mask test, different impairments can be separated and quantified. The impact from quadrature error and timing skew are investigated experimentally

Andrekson, Peter

Eliasson, Henrik

Fatadin, I.

Johannisson, Pontus

Lundström, Carl

Chalmers Publication Library

Chalmers Publication Library
Mask testing of 28 gbaud 16-QAM transmitters using time-resolved error vector
magnitude
This document has been downloaded from Chalmers Publication Library (CPL). It is the author´s
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Signal Processing in Photonic Communications, SPPCom 2015; Omni Parker
HouseBoston; United States; 27 June 2015 through 1 July 2015
Citation for the published paper:
Lundström, C. ; Eliasson, H. ; Fatadin, I. et al. (2015) "Mask testing of 28 gbaud 16-QAM
transmitters using time-resolved error vector magnitude". Signal Processing in Photonic
Communications, SPPCom 2015; Omni Parker HouseBoston; United States; 27 June 2015
through 1 July 2015 pp. SpS4D.3.
http://dx.doi.org/10.1364/SPPCOM.2015.SpS4D.3
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SpS4D.3.pdf Advanced Photonics © 2015 OSA
Mask Testing of 28 Gbaud 16-QAM Transmitters
using Time-Resolved Error Vector Magnitude
Carl Lundstro¨m1, Henrik Eliasson1, Irshaad Fatadin2, Pontus Johannisson1, and Peter A. Andrekson1
(1) Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden
(2) National Physical Laboratory, Teddington, Middlesex, TW11 0LW, U.K.
carl.lundstrom@chalmers.se
Abstract: We propose time-resolved EVM for characterization of 16-QAM transmitters. By de-
signing a mask test, different impairments can be separated and quantified. The impact from quadra-
ture error and timing skew are investigated experimentally.
OCIS codes: (060.1660) Coherent communications; (060.4080) Modulation; (060.2330) Fiber optics communications
1. Introduction
The new generation of coherent fiber-optic transmission systems utilizes modulation formats such as quaternary phase
shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM), implying that ways of characterizing and
testing transmitters for such formats need to be developed. The root mean square error vector magnitude, EVMRMS,
has been established as a way to quantify performance [1, 2]. However, a good evaluation method should account for
all possible transmitter errors including incorrect modulator biases, quadrature errors, timing skews etc., and ideally
also give information about the origin of the problem and available margins. It should correlate well with the system
performance as measured in terms of the bit error rate (BER). An approach based on mask testing of time-resolved
EVM, denoted EVMTR, has been proposed to help address these issues [3]. An EVMTR plot is formed by including
all samples of the signal and plotting the EVM versus t modulo Ts, where Ts is the symbol duration. Thus, an EVMTR
plot shows the deviation from the correct constellation point over many superimposed symbol slots.
Traditional mask testing for on-off-keying eye diagrams uses the fact that different transmitter impairments affect
the eye in different ways and eventually cause mask hits. The same principle can be applied using EVMTR, and this
approach has been experimentally shown to correlate better than EVMRMS to the BER for a polarization-multiplexed
(PM) QPSK signal [4]. However, this approach needs to be extended also to larger QAM constellations. In this paper,
we apply mask testing based on EVMTR to a 16-QAM signal. A transmitter for this format can be implemented in
different ways with numerous different potential impairments such as incorrect modulator bias levels, quadrature error
(QE) and timing skew, and various gain/amplitude imbalances to name a few. Constructing a mask test method is thus
a complex problem. In this initial study we have investigated two common impairments, QE and timing skew that can
be considered typical. Additionally, we consider both a parabolic and a polygonal mask shape.
2. Experimental setup
The experimental setup is shown in Fig. 1. To generate a 16-QAM signal at 28 Gbaud, a dual-polarization IQ modulator
was driven with four binary drive signals to obtain two phase-stable “sub-QPSK” signals in orthogonal polarizations.
The timing skew and QEs could be individually adjusted using the delays and phase biases shown in Fig. 1. Using a
polarization controller and a linear polarizer (at an angle between the two sub-QPSK signals), these were combined
with a 6 dB power ratio into a single-polarization 16-QAM signal. A tap prior to the polarizer allowed studying the
sub-QPSK signals individually. Directly after the transmitter, the signal was split, sending a part into an optical modu-
lation analyzer (OMA, EXFO PSO-200), which measures the signal using equivalent-time sampling and characterizes
it in terms of e.g. EVMTR or EVMRMS. We note that the OMA shows the actual signal without applying any DSP. The
other part of the signal was passed through a noise-loading setup (using a variable optical attenuator and an EDFA)
and received with a coherent receiver based on a 100 Gsample/s real-time sampling oscilloscope with 33 GHz analog
bandwidth. The linewidths of both signal and LO lasers were ∼100 kHz. After sampling, the data was processed in
offline DSP. The DSP included IQ angle imbalance compensation based on Gram-Schmidt orthogonalization, resam-
pling to two samples per symbol, decision-directed least-mean square equalization, and frequency offset estimation
and carrier phase estimation based on the blind phase search method [5]. Finally, the BER was evaluated by direct
error counting.
SpS4D.3.pdf Advanced Photonics © 2015 OSA
Binary Pattern 
Generator 28 Gbaud
Signal
Laser DP IQ-modulator
Transmitter
Polarization
Controller
LP
OMA90 %
10 % VOA EDFA
90o
Hybrid
Receiver
LO
Laser 33 GHz
100 GSample
Oscilloscope
X
Y
X´
Y´
XI XQ YI YQ
Super Delay Sub Delay
Super QE Bias Sub QE Bias
Fig. 1. The experimental setup showing how a PM-QPSK signal is converted into a single-
polarization 16-QAM. PC: Polarization controller. LP: Linear polarizer. OMA: Optical modulation
analyzer. VOA: Variable optical attenuator.
3. Experimental results and discussion
To quantify the effects of the transmitter impairments, QE and timing skew were introduced separately into the two
sub-QPSK signals, which we, refer to as the large and small sub-QPSK, respectively. The OSNR penalty at a BER
of 10−3 was measured for various levels of each impairment. An EVMTR plot was also saved for mask testing. The
OSNR penalty is shown in Fig. 2, with 1 dB penalty marked in the figures. As expected, induced QE on the large
sub-QPSK yields a larger penalty than induced QE on the small sub-QPSK. The QE that yields about 1 dB OSNR
penalty is 5 degrees and 17.5 degrees, respectively. Timing skew, however, yields similar, and relatively low penalties
for the values tested, with a skew of about 25 percent of the symbol slot required in both cases to incur an OSNR
penalty of more than 1 dB.
Fig. 3 shows EVMTR as measured directly after the transmitter with induced impairments giving approximately a
1 dB OSNR penalty. The EVMTR for the ideal case without induced impairment is also shown. Previous works [3, 4]
have not included any discussion on mask design, and only suggested potential mask shapes. A suitable mask should
be equidistant from the EVMTR-rails for an ideal signal, and show similar performance for all potential transmitter
impairments. At the same time it is desirable that it is not too complex to define. The two impairments considered here,
QE and skew, qualitatively affect the EVMTR in two different ways, QE tends to push the bottom rail up towards higher
EVM values, and skew tends to push the side rails toward the center of the symbol slot. Here, we have investigated
a simple mask construction, namely a parabola with an offset vertex above the zero EVM line, giving only two free
parameters. By optimizing the parabola, with respect to the variance of the number of mask hits for the different
impairments studied the best mask choice can be found. We additionally require the mask to give at least three times
as many hits for any impairment giving 1 dB penalty than for the ideal case and also do not consider samples in
the top of the sampling window (normalized EVM > 0.7). Another possible mask design is a polygon. A polygon
with six vertices (two isoceles trapezoids) that closely approximated the previously found parabola gave slightly more
consistent performance among the different impairments. This is attributed to the polygonal mask being slightly more
0 5 10 15 20 25
0
1
2
3
4
5
Quadrature error (degrees)
O
SN
R 
pe
na
lty
 a
t B
ER
=1
0−
3  
(dB
) (a)
 
 
0 10 20 30
0
0.5
1
1.5
2
(b)
Timing skew (% of symbol duration)
O
SN
R 
pe
na
lty
 a
t B
ER
=1
0−
3  
(dB
)
 
 
Small sub−QPSK
Large sub−QPSK
Fig. 2. OSNR penalties at BER = 10−3 for varying amounts of (a) QE and (b) timing skew on the
two sub-QPSKs, respectively.
SpS4D.3.pdf Advanced Photonics © 2015 OSA
Fig. 3. Time-resolved EVM as a function of time within the symbol slot. The impact of QE and skew
on the large and small sub-QPSK signals, respectively, is quantified in terms of the mask hit ratio
(HR). The OSNR penalty was close to 1 dB in all cases except the ideal case, which is included as
a reference. Two mask shapes, parabolic and polygonal, are shown.
Measured OSNR penalty at BER=10-3 (dB)
-1 0 1 2 3 4
M
as
k h
it r
at
io
0
0.005
0.01
0.015
Skew, small sub-QPSK
Skew, large sub-QPSK
QE, small sub-QPSK
QE, large sub-QPSK
Fig. 4. The number of mask hits for the different impairments versus OSNR penalty.
narrow in its lower part, reducing the number of mask hits for timing skew on the small sub-QPSK. Fig. 3 includes
both of the proposed mask shapes and indicate the hit ratios.
In Fig. 4 the number of mask hits (polygon-shaped mask) is plotted versus the measured OSNR penalty for the
different impairments. The mask hits follow a similar curve, for all impairments though there is a noticeable measure-
ment uncertainty due to the noise of the signal. Clearly, it is possible to find a choice of mask that yields a similar
number of mask hits for all impairments, while the number of hits for the ideal case is significantly lower.
4. Conclusion
We have proposed and used time-resolved EVM measurements for 16-QAM by studying the impact of quadrature
error and timing skew in a dual-parallel IQ modulator 16-QAM transmitter in terms of OSNR penalty, EVMRMS, and
EVMTR-mask hits. It was found that with a suitable choice of mask the number of mask hits can correlate well for
the impairments studied. Further investigations will include more impairments. The performance is very sensitive to
the mask choice and this needs to be carefully studied. Mask tests similar to the proposed ones are candidates for
standardized transmitter testing.
This work was supported by EMRP IND51 “MORSE”. The EMRP is jointly funded by the EMRP participat-
ing countries within Euramet and the European Union. EXFO is acknowledged for the loan of the OMA.
References
[1] R. A. Shafik, M. S. Rahman, and A. R. Islam, “On the extended relationships among EVM, BER and SNR as performance metrics,” in “International Conference
on Electrical and Computer Engineering (ICECE),” (2006), pp. 408–411.
[2] R. Schmogrow, B. Nebendahl, M. Winter, A. Josten, D. Hillerkuss, S. Koenig, J. Meyer, M. Dreschmann, M. Huebner, C. Koos, J. Becker, W. Freude, and J. Leuthold,
“Error vector magnitude as a performance measure for advanced modulation formats,” IEEE Photon. Technol. Lett. 24, 61–63 (2012).
[3] H. Sunnerud, M. Sko¨ld, M. Westlund, and P. A. Andrekson, “Characterization of complex optical modulation formats at 100 Gb/s and beyond by coherent optical
sampling,” J. Lightw. Technol. 30, 3747–3759 (2012).
[4] H. Eliasson, P. Johannisson, H. Sunnerud, M. Westlund, M. Karlsson, and P. A. Andrekson, “Transmitter mask testing for 28 GBaud PM-QPSK,” in “European
Conference on Optical Communication (ECOC),” (2013), p. Tu.3.C.2.
[5] T. Pfau, S. Hoffmann, and R. Noe´, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightw.
Technol. 27, 989–999 (2009).


Mask testing of 28 gbaud 16-QAM transmitters using time-resolved error vector magnitude

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Pontus Johannisson

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