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Description
Optical PAM-4 modulation is becoming increasingly pervasive for short-range links up to
several kilometers, as evidenced by the decision of the IEEE 802.3bs Task Force to adopt it for its emerging
400 GbE standard. The move from NRZ
signaling to higher-order formats offers increased capacity at the same symbol rate, circumventing the
formidable challenge of having to significantly improve the bandwidth of today's components; 100G and 400G
connectivity is possible with multi-lane optical PAM 4, squeezing more Gbits/s through commercially-available
CMOS nodes and commodity optics such as DMLs and EMLs.
The shift to a new format poses renewed challenges to technology providers, who must now ensure that their
components and systems will deliver on the required performance. As such, IEEE 802.3bs is defining new
standardized measurements, one of which is the Transmitter and Dispersion Eye Closure for Quaternary PAM
(TDECQ). In this example, we show how the measurement can be performed with the TDEC_Meter module in
VPItransmissionMaker Optical Systems.
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Typical Results
The simulation setup is shown in
:
A 53.125 Gbit/s EML-based PAM-4 transmitter modulates the required Short Stress Pattern Random-Quaternary (SSPRQ)
sequence onto an O-band optical carrier, and launches the signal over 10 km of SSMF. Direct-detection is employed
at the receiver, and after equalization with the reference 5-tap symbol-spaced FFE
(),
the signal is analyzed by the TDEC_Meter. The module automatically determines the average power levels for the
lowest and highest symbols, as well as the thresholds required for the TDECQ calculation
().
The calculation requires taking signal level histograms at the left (0.45 U.I.) and right (0.55 U.I.) sides of
the optimum sampling point of the eye diagram
().
The "left" and "right" CDFs and PDFs of each sub-eye threshold can be optionally plotted by
the TDEC_meter
(). Also
visualized in the PDF plots of Figure 5 are Gaussian distributions centered on each sub-eye threshold:
These represent the amount of Gaussian noise that needs to be added to the PAM-4 signal, in order to reach
the target Symbol Error Rate (SER) of 4.8*10-3. A higher-quality, more open eye diagram implies that more
noise needs to be added to reach the target SER, which results in a better (i.e. lower) TDECQ value. The
measurement is designed to penalize PAM-4 signals with non-equal eye openings. In the example shown, a
TDECQ value of 2.66 dB is obtained, meeting the maximum specification of 3.4 dB for 200GBASE-LR4 [1].
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Further Information
Keywords: PAM-4, TDECQ, EML
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
[1] IEEE P802.3bs™/D3.3 Draft Standard for Ethernet Amendment 10: Media Access Control Parameters, Physical Layers and Management Parameters for 200 Gb/s and 400 Gb/s Operation
