Joint ITU/IEEE Workshop on Ethernet - Emerging
Applications and Technologies
Standardization of advanced
modulation formats
Pete Anslow, Ciena
Associate Rapporteur ITU-T SG15 Q6
Geneva, Switzerland, 22 September 2012
Outline
NRZ specification methodology
Recent updates
Eye mask
Spectral excursion
Advanced modulation formats
Reference receiver approach
Error vector magnitude
Further work
Geneva, Switzerland, 22 September 2012
2
NRZ specification methodology
The first version of CCITT (now ITU-T) G.957 in December
1990 established the basic methodology for specification of
multi-vendor optical interfaces:
1.E-03
Eye mask
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
Extinction ratio
1.E-09
1.E-10
1.E-11
1.E-12
Path penalty
Launched power
Receiver sensitivity
Link loss
Link dispersion
Geneva, Switzerland, 22 September 2012
SMSR
3
Eye mask update
In recent years (starting with IEEE 802.3ba-2010) the eye
mask test has been updated to allow a proportion of “hits”
“Old” mask could fail
on a single sample
100%
Probability of passing test
90%
80%
New
Proposed
70%
Old
Existing
60%
50%
40%
30%
20%
10%
0%
3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Transmitter bandwidth
“New” ratio mask
allows a proportion
of hits and changes
from all fail to all
pass more abruptly
Geneva, Switzerland, 22 September 2012
4
Spectral excursion
When ITU-T G.698.1 introduced a Mux/Demux into the link
“spectral excursion” was added to the methodology to ensure
that the signal would pass through the filters.
-15 dB points of Tx spectrum must remain within -2 dB points of link filter function
Nominal central frequency
minus maximum spectral
excursion
Nominal
central
frequency
Nominal central frequency
plus maximum spectral
excursion
Nominal central frequency minus
maximum spectral excursion
Nominal
central
frequency
Nominal central frequency plus
maximum spectral excursion
-5
-10 Log(Loss) (dB)
Power relative to peak (dB)
0
-10
-15
-20
Maximum
ripple
2 dB
max
-25
-30
0
Offset From Nominal Central Frequency (GHz)
Geneva, Switzerland, 22 September 2012
0
Offset From Nominal Central Frequency (GHz)
5
Advanced modulation formats
Moving from 10 Gbit/s to 40 Gbit/s and 100Gbit/s , NRZ is no
longer the dominant line-side modulation format.
Formats currently being considered by ITU-T SG15 Q6 at 40G are:
(P) DPSK
(Partial) Differential Phase Shift Key
RZ-DQPSK Return-to-Zero Differential Quadrature Phase Shift Key
DP-QPSK
Dual-Polarization Quadrature Phase Shift Key
ODB
Optical Duobinary
The format currently being considered by ITU-T SG15 Q6 at 100G is:
DP-QPSK
Dual-Polarization Quadrature Phase Shift Key
With the exception of ODB, these are all formats that encode the data
using the phase of the optical signal rather than its amplitude.
Geneva, Switzerland, 22 September 2012
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New methodology needed
Eye mask and extinction ratio are of no use in determining the
quality of the transmitted signal for any of the phase
modulated formats.
DPSK
RZ-DQPSK
DP-QPSK
ODB
Spectral excursion limit adjusted to -5 dB points of Tx
spectrum within -2 dB points of link filter function (red) still
too conservative
8
Good
combination
fails test
7
OSNR penalty (dB)
6
5
Good
combination
passes test
4
Bad
combination
fails test
3
2
1
0
Bad
combination
passes test
-1
-30
-20
-10
0
10
20
30
Frequency offset (GHz)
Geneva, Switzerland, 22 September 2012
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Reference receiver approach
One approach being pursued for (P) DPSK is to define a
reference phase-to-intensity demodulator which is used to
turn the (P) DPSK transmitted signal into an amplitude
modulated signal which then has to meet a “demodulated eye
mask” and a “demodulated extinction ratio”
(P) DPSK signal
NRZ signal
Delay line
interferometer
(P) DPSK
optical
transmitter
Geneva, Switzerland, 22 September 2012
optical
reference
receiver
8
I/Q plane representation
To define a reference demodulator for a more complex
modulation format such as DP-QPSK (or DP-16QAM in the
future) does not seem like the best way forward.
0
Quadrature
The alternative that is being developed is to represent the
magnitude and phase of the optical signal in the centre of
each symbol on an I/Q (in-phase/quadrature) plane –
commonly called a constellation.
0
Geneva, Switzerland, 22 September 2012
In-phase
9
Error vector magnitude
The pass/fail metric being developed is based on the error
vector magnitude (EVM) definition in IEEE Std 802.11 (WiFi)
with an allowed hit ratio
2
0
1
QDC
Remove
offsets
1
Q
Capture
points
2
-1
-1
-2
-2
-2
-1
0
I
1
2
-2
2
Apply
limits
2
0
-1
-2
-2
0
IDC / Imag
Geneva, Switzerland, 22 September 2012
2
1
QDC / Qmag
QDC / Qmag
0
IDC
2
1
Normalise
amplitude
0
0
-1
-2
-2
0
IDC / Imag
2
10
Commercial EVM measurements
Measurements using two commercially available test sets and
the unmodified EVM algorithm from IEEE 802.11
Vendor 1 Mean EVM Max 0.439 std dev 0.0377
Vendor 2 Mean EVM Max 0.417 std dev 0.0364
Vendor 1
Geneva, Switzerland, 22 September 2012
Vendor 2
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Further work
EVM definition
Work is ongoing to identify the key elements in the capture
process that need to be defined (e.g. the phase tracking
algorithm) and the complimentary parameters that need to be
specified (e.g. I/Q imbalance)
Spectral excursion
More measurements of the OSNR penalty vs offset between
the signal and the end-to-end filter function of the link are
needed in order that a less conservative spectral excursion
criterion can be developed.
Once the methods of defining the quality of a transmitter and
ensuring a signal can traverse a link with acceptable penalty
have been defined, the work to define multi-vendor
interoperable phase modulated applications can move forward.
Geneva, Switzerland, 22 September 2012
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Thank you