a comparison of 25 gbps nrz & pam-4 modulation used in legacy
TRANSCRIPT
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Abstract
Standards bodies are now examining how to increase the
throughput of high-density backplane links to 25 Gbps. One
method for achieving this is to construct premium backplane
links utilizing advanced materials and connectors. Another
approach is to re-use legacy backplanes by employing PAM-
4 signaling at half of the baud rate. For PAM-4 to offer an
advantage over NRZ, the signal-to-noise ratio (SNR) at the
slicer input, i.e. after equalization, must be ~9.5 dB better than
NRZ to overcome loss of separation between signal levels.
This paper will examine 25 Gbaud NRZ and 12.5 Gbaud PAM-4
signaling across varying levels of channel insertion loss and
crosstalk. Chip parameters such as rise-time and jitter will also
be varied. The paper provides a reference for engineers to use
when considering when it is appropriate to use NRZ signaling
at 25 Gbaud and when it is appropriate to use PAM-4
signaling at 12.5 Gbaud for successful high-density backplane
operation.
Author Biographies
Adam Healey is a Distinguished Engineer at LSI Corporation
where he supports the development and standardization of
high speed serial interface products. Prior to joining Lucent
Microelectronics in 2000, Adam worked for University of
New Hampshire’s InterOperability Lab where he developed
many of the test procedures and systems used to verify
interoperability, performance, and compliance to standards
of 10, 100, and 1000 Mb/s Ethernet products. During his
tenure at Lucent Microelectronics, which later became Agere
Systems and then LSI Corporation, Adam was involved in
wide variety of projects including channel modeling and
equalization strategies for high speed optical and electrical
links, transcoding and error correction coding subsystems,
and transport networking architecture. Adam is a member
of the IEEE and regular contributor to the development
of industry standards through his work in the IEEE 802.3
Ethernet working group and INCITS T11.2 Fibre Channel
Physical Variants task group. Adam was chairman of the
IEEE P802.3ap Task Force chartered to develop the standard
for Ethernet operation over electrical backplanes at speeds
of 1 and 10 Gbps and currently secretary of the IEEE 802.3
Ethernet Working Group. Adam has also previously served
a technical committee chairman and Vice President of
Technology for the Ethernet Alliance. Adam received B.S.
[‘95] and M.S. [‘00] degrees in Electrical Engineering from the
University of New Hampshire.
Chad Morgan earned his degree in Electrical Engineering
from the Pennsylvania State University, University Park, in
1995. For the past 16 years, he has worked in the Circuits
& Design group of TE Connectivity as a signal integrity
engineer, specializing in the analysis & design of high-
speed, high-density components. Currently, he is a Principal
Engineer at TE Connectivity, where he focuses on high-
frequency measurement & characterization of components &
materials, full-wave electromagnetic modeling of high-speed
interconnects, and the simulation of digital systems. Mr.
Morgan is a Distinguished Innovator with numerous patents
at TE Connectivity, and he has presented multiple papers
at trade shows such as DesignCon and the International
Microwave Symposium.
Introduction
The growing demand for instant multimedia access in an
ever-increasing number of digital devices has continued to
push the need for higher aggregate bandwidth in modern
communication hardware. As a result, standards bodies are
now examining how to increase high-density backplane
serial throughput to 25 Gbps per differential pair. As an
example, the Optical Interconnect Forum (OIF) now has
Implementation Agreements defining the criteria for
designing both short- and long-reach 25 Gbps channels [1].
Further, The IEEE 802.3 Ethernet Working Group has begun
discussions on a 100 Gigabit Backplane Ethernet standard
that would consist of 4 lanes, each transporting 25 Gbps of
data.
Currently, there is much debate within standards bodies on
how to achieve acceptable 25 Gbps data transmission across
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
A d a m H e a l e y, L S I C o r p o r a t i o na d a m . h e a l e y @ l s i . c o m
C h a d M o r g a n , T E C o n n e c t i v i t yc h a d . m o r g a n @ t e . c o m
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2
high-density backplane channels of up to 1 meter. One
possible method for achieving this is to construct high-quality
backplane links utilizing low-loss dielectrics, smooth copper,
and low-reflection, low-crosstalk connectors. These channels
still require high-levels of equalization and are more expensive
than legacy backplane systems built for 10 Gbps operation.
Another method that has been suggested is to use PAM-
4 signaling at half of the baud rate to achieve 25 Gbps
transmission across legacy backplanes. The hope is that
successful 12.5 Gbaud PAM-4 operation would allow the use
of cheaper legacy backplane channels. The possible move
to PAM-4 signaling, however, comes with numerous factors
to consider. Engineers attempting to implement PAM-4
would need to use updated simulation algorithms, new
test equipment, newly defined test protocols, and higher-
complexity equalization.
Before considering PAM-4, the first step is to clearly outline
where PAM-4 signaling at 12.5 Gbaud has performance
advantages over NRZ signaling at 25 Gbaud. This paper
will accomplish this by providing NRZ vs. PAM-4 electrical
performance data across a range of different backplane
performance levels.
Backplane performance levels can essentially be categorized
by their insertion loss-to-crosstalk (ICR) ratio. Therefore,
a range of channel insertion loss and crosstalk levels are
included in the paper. Various insertion loss levels are
achieved by studying multiple lengths (1.0 m & 0.75 m),
dielectrics (Improved FR4 as defined by the IEEE P802.3ap
Task Force [2] & Megtron6), and TE Connectivity Z-PACK
TinMan and STRADA Whisper connector systems.. Various
crosstalk levels are achieved by studying multiple connectors
(TE Connectivity Z-PACK TinMan & STRADA Whisper
connectors) and multiple pinouts for each connector. These
pinouts include the manufacturer-recommended pinout, full
near-end crosstalk (NEXT), and full far-end crosstalk (FEXT).
Proven time-domain simulation methods will be used to
complete 25 Gbaud NRZ vs. 12.5 Gbaud PAM-4 comparisons.
Once package and chip parasitics are added to the multiple
channel models, simulations will be completed for a given
driver rise time, jitter level, and equalization scheme. When
all baseline simulations are complete, results will then be
augmented to show the effects of varying rise time and jitter.
Ultimately, the goal of the paper is to provide a detailed and
reliable reference for engineers to use when considering the
appropriate use of 25 Gbaud NRZ signaling and when it is
appropriate to use 12.5 Gbaud PAM-4 signaling for successful
high-density backplane operation.
Description of Channels
Figure 1 shows all parameters for the eight backplane
channels that are studied in this paper. As shown, overall
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Figure 1: Simulated 1.0 m & 0.75 m Backplane Channels Using “Improved FR4” or Megtron6 Dielectrics and ZPACK TinMan or STRADA Whisper Connectors from TE Connectivity.
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3
channel lengths of ~1 m and ~0.75m are implemented by
concatenating daughtercard lengths of 0.127 m (5.0”) with
backplane lengths of either 0.4318 m (17.0”) or 0.762 m
(30.0”). In order to study legacy channels that were designed
for 10 Gbps, some channel permutations are implemented in
“improved FR4” and with Z-PACK TinMan connectors from TE
Connectivity. In order to study high-performance channels,
other channel permutations are implemented in Panasonic
Megtron6 with H-VLP smooth copper and STRADA Whisper
connectors from TE Connectivity. All other parameters shown
in Figure 1 are typical of modern backplane channels, such as
counterbored vias with 0.254 mm (0.010”) remaining stubs.
For each of the eight channel permutations, multiple crosstalk
patterns are studied. For example, crosstalk effects are
examined when the manufacturer-recommended connector
pinout is implemented, as shown in Figures 2 and 3 for
STRADA Whisper and Z-PACK TinMan connectors from TE
Connectivity. Crosstalk effects are also studied when all eight
nearby aggressors are producing near-end crosstalk (all-
NEXT) or when all eight nearby aggressors are producing
far-end crosstalk (all-FEXT). Generally, crosstalk effects
are more prevalent when utilizing PAM-4 modulation, since
full-swing aggressors can produce high-magnitude crosstalk
on 1/3-swing bits. For this reason, it is important to study
crosstalk carefully under multiple conditions.
All channel performance data for the four ~1.0 m channels is
shown in Figure 4, and all channel performance data for the
four ~0.75 m channels is shown in Figure 5 (following two
pages). For insertion loss (IL), return loss (RL), and ICR, limit
lines are included from the IEEE 802.3 specification [3] as
a reference. Although insertion loss deviation (ILD) is not
shown, all four ~1.0 m channels and all four ~0.75 m channels
fall within the ILD bounds specified by IEEE 802.3.
In Figures 4 and 5, note that all eight channels meet the IEEE
10GBASE-KR limits for insertion and return loss. Because
these limit lines are only specified to 6 GHz, they are not
definitive for successful 25 Gbps operation. However, they
do serve as a useful definition for the lowest acceptable
performance of a legacy 10 Gbps channel. When examining
ICR plots, it is clear that all STRADA Whisper connector
channels surpass the fit-ICR limit with at least 10 dB of margin.
The Z-PACK TinMan conector channels, on the other hand,
just fail the fit-ICR limit when using the recommended pinout.
This is intentional, as it was desirable to examine NRZ vs.
PAM-4 performance for a 1 m, “improved FR4” legacy channel
that just violates the 10GBASE-KR fit-ICR specification.
Note that the connectors for the eight channels in Figures
4 and 5 were carefully chosen. Z-PACK TinMan connector
channel performance is similar to numerous other connectors
Figure 2: STRADA Whisper connector & footprint showing crosstalk configurations.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Figure 3: Z-PACK TinMan connector & footprint showing crosstalk configurations.
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4
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
(a) (b)
(c) (d)
(e) (f)
(g) (h)
Figure 4: 1 meter Channel Data - (a) Differential Insertion Loss, (b) Differential Return Loss, (c) Recommended Pinout Crosstalk, (d) Recommended Pinout ICR, (e) All NEXT Crosstalk, (f) All NEXT ICR, (g) All FEXT Crosstalk, & (h) All FEXT ICR
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5
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Figure 5: 0.75 meter Channel Data - (a) Differential Insertion Loss, (b) Differential Return Loss, (c) Recommended Pinout Crosstalk, (d) Recommended Pinout ICR, (e) All NEXT Crosstalk, (f) All NEXT ICR, (g) All FEXT Crosstalk, & (h) All FEXT ICR
(a) (b)
(c) (d)
(e)
(g) (h)
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6
in legacy backplane systems (Max RL = -10 dB to 5 GHz,
Max XTALK = -30 dB to 5 GHz). STRADA Whisper channel
performance, on the other hand, represents industry best-in-
class performance (Max RL = -15 dB to 12.5 GHz, Max XTALK
= -45 dB to 12.5 GHz). A goal of this paper is to study how
connector performance impacts time-domain results.
As a final point, it is interesting to note that lower-loss
dielectrics and shorter systems buy little in the way of
improved ICR at 6 GHz. In simulations including NEXT, only
about 5 dB of improvement is seen in going from 1.0 m to
0.75 m or in going from “Improved FR4” to Megtron6. In
simulations including FEXT, almost no improvement is seen.
Later, the paper will show how this translates to time-domain
performance.
Simulation ConditionsProbability of Symbol Error
Consider a serial link utilizing NRZ modulation where the
distance between symbols is 2d and there is additive white
Gaussian noise with standard deviation . The probability of
a symbol error is given by Equation 1.
In Equation 1, the variable Q is also referred to as the signal-
to-noise ratio (SNR) and it is defined to be . NRZ
modulation may also be considered to be 2-level pulse
amplitude modulation (PAM-2). The expression for the
probability of a symbol error can be generalized to PAM-L,
where L is the number of amplitude levels, as shown in
Equation 2.
The factor of (L - 1) is associated with the fact that the inner
symbols in the PAM-L constellation are more prone to error. In
addition, the argument of the complementary error function is
reduced by a factor of 1/ (L - 1) to account for the reduction in
the distance between symbols when the peak amplitude d is
held constant. The probability of error for various values of L
is shown in Figure 6.
For increasing L, the SNR required to achieve a target
probability for symbol error also increases. Comparing NRZ
modulation and PAM-4, one can see that the SNR must be
9.6 dB larger for PAM-4 to achieve the same symbol error
probability as NRZ. However, since each PAM-L symbol can
convey log2(L) bits of information, the symbol rate is reduced
accordingly.
It is clear that for PAM-L to have an advantage over PAM-2,
the reduction in symbol rate must yield an improvement in
SNR that overcomes the increased SNR requirement for the
same symbol error ratio. SNR improvement may be realized in
a variety of ways and is not limited to the effective reduction
in the channel insertion loss.
Transmitter Model
The transmitter model shown in Figure 7 includes pre-driver
and driver stages for independent control of rise and fall
times and output return loss. The pre-driver consists of a
voltage source vs(t) that drives the low pass filter formed by
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Equation 1
Equation 2
10 15 20 25 30 35 4010-20
10-15
10-10
10-5
100
Prob
abilit
y of
erro
r
SNR, dB
L = 2
L = 3
L = 4
L = 5
L = 6
L = 7
L = 8
Figure 6: Symbol error probability as a function of SNR
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Rpd and Cpd. The values of Rpd and Cpd are chosen to set the
pre-driver rise and fall times. The filter output voltage vi(t)
then controls a voltage source which represents the driver.
The driver includes the on-die termination, represented in a
simplified form by Rpd and Cpd, and is connected to a channel
that consists of the backplane channel of interest plus the
transmitter and receiver device packages, and the receiver
on-die termination.
The device package model is selected to be representative of
a large package that might be used for a high channel count
device such as a switch. The insertion loss and return loss for
the selected model are shown in Figure 8. For simplicity, the
same device package model is used for both the transmitter
and the receiver.
Given the package model, the single-ended on-die
termination resistance is set to 50 Ω and the parasitic
capacitance is tuned to yield the desired return loss
performance. A single-ended on-die termination capacitance
of 0.25 pF was found to just touch the transmitter differential
output return loss mask defined by the OIF CEI-25G-LR
implementation agreement [1]. For simplicity, the same on-die
termination is used for both the transmitter and receiver.
Figure 9 shows the impact of the choice of package model
and on-die termination. It is clear that the insertion loss of
the terminated channel is measurably larger than the channel
in isolation. Furthermore, additional insertion loss deviation
artifacts are visible reflecting the interaction between than
channel, device packages, and termination impedances.
Note that voltage source vs(t) incorporates a finite impulse
response filter with three symbol-spaced taps that
implements de-emphasis. The delay of this filter is one unit
interval which implies that there is one pre-cursor tap and one
post-cursor tap. The voltage source also incorporates voltage
scaling to set the driver output amplitude as well as phase
modulation of the clock for the generation of jitter. Both
deterministic (sinusoidal) and random jitter components are
defined for the transmitter.
The transmitter parameters, other than symbol rate and
modulation format, are kept constant for the both the PAM-2
and PAM-4 simulation cases. Specifically, the same device
packages and on-die termination networks are used, the peak
differential output voltage is constant, and the jitter is fixed in
absolute time (ps). The premise is that the design techniques
that would be employed to realize the PAM-2 solution could
also be leveraged by the PAM-4 solution. This is reflected in
the near-end eye diagrams shown in Figure 10. Sensitivity to
variation in rise-time and jitter will also be studied later in this
paper.
Receiver Model
Two receiver architectures will be considered in this study.
The first architecture reflects a “conventional” approach which
is heavily reliant on analog signal processing.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
ChannelRd
Cd Ct Rt
Rpd
Cpdvs(t)
vi(t) vo(t)
vi(t)
Figure 7: High-level view of the transmitter and channel model
Figure 8: Package differential insertion loss and return loss for Pkg35mm_T21mm115ohmLoXtalk_BGALoXtalk.s8p’model from
‘www.ieee802.org/3/ba/public/tools/PkgModels40GHz.zip’
Figure 8: Package differential insertion loss and return loss for Pkg35mm_T21mm115ohmLoXtalk_BGALoXtalk.s8p’model from ‘www.ieee802.org/3/ba/public/tools/
PkgModels40GHz.zip’
Figure 9: Example of driver/receiver package and driver capacitance effect on channel performance
(a) (b)
Figure 9: Example of driver/receiver package and driver capacitance effect on channel performance
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The first architecture reflects a “conventional” approach which
is heavily reliant on analog signal processing. The analog front
end (AFE) for this architecture consists of a programmable
gain amplifier (PGA), continuous time equalizer (CTE), and
analog circuitry required for the timing recovery and high-
speed decision feedback equalizer implementation. Digital
circuitry is used where possible, especially in adaptation loops
and management functions. This approach will be associated
with PAM-2 or NRZ modulation.
The second architecture features a digital signal processing
(DSP) based receiver. The analog front end includes a PGA
and CTE as before, and an analog-to-digital converter (ADC)
that renders the analog signal at the AFE output into a series
of digital words for subsequent post-processing. The DFE is
implemented in the digital domain and may be supplemented
by other structures that are readily realized in DSP. This
approach will be associated with PAM-4 modulation.
Both architectures set the transmitter de-emphasis, CTE
transfer function, sampling point, and the coefficients of the
receiver’s equalizer to minimize the mean-squared error (or
maximize the SNR) at the decision point.
Continuous Time Equalizer
Both architectures under consideration utilize a CTE but the
design parameters vary as a function of the symbol rate.
The variation is due to proper placements of the peaking
frequency as well as constraining the bandwidth to avoid
integrating excess noise. The template for the continuous time
equalizer transfer function is given in Equation 3.
The parasitic poles pp are chosen so the insertion loss of the
filter is 2 dB at the fundamental frequency, i.e. half of the
symbol rate, when the gain value k is zero. The value of p1
is fixed at 12.9 π Grad/s to foster consistent mid-band gain
between the two filters. The set of gain values k is defined so
that the peaking increases in 1 dB increments up a maximum
of 10 dB. The resulting transfer functions are illustrated in
Figure 11.
Electronics noise is modeled as additive white Gaussian noise
with power spectral density N0/2 referred to the input of the
CTE. This means that the CTE will shape this noise according
to its k setting.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Figure 10: Comparison of eye diagrams from PAM-2 (NRZ) and PAM-4 transmitters. (a) PAM-2 (NRZ) near-end eye (b) PAM-4 near-end eye
Equation 3
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Analog Equalizer
Decision feedback equalizers (DFE) with a relatively large
number of taps have been successfully implemented with
largely analog signal processing techniques [4]. Apart from
the challenges of closing the critical timing path, one of the
factors that influence the performance of the receiver is latch
metastability. Metastability occurs when the input signal
is not large enough for the latch to resolve a discernable
logic level at its output. The method chosen for modeling
latch metastability is the simple but conservative overdrive
model in which the signal is required to exceed the decision
threshold by a certain amount, otherwise, an error occurs.
The number of taps for each equalizer as well as the minimum
latch overdrive is set according to Table 1.
DSP-Based Equalizer
Recent deployments of 10 Gbps serial links using digital
signal processing (DSP) technology have been enabled
by enhancements to analog-to-digital converter (ADC)
performance and the scaling of CMOS technologies.
DSP-based receivers have advantages over their analog
counterparts in several areas. Equalizer structures that are
challenging to realize in the analog domain have relatively
straight-forward digital counterparts. A simple example is the
feed-forward equalizer (FFE). While this structure is realizable
in the analog domain, the delay line is subject to variation due
to process, voltage, and temperature and does not readily
scale with data rate. Analog multiplication must be carefully
implemented to achieve sufficient linearity and resolution.
Finally, the bandwidth limitations and noise accumulation of
these cascaded components has an adverse effect on the
SNR. All of these elements are trivial operations in the digital
domain, and the digital architecture readily scales with speed
(within the bounds of the digital clock rate).
This simple example highlights essential benefits of the
DSP-based architecture. The first is a lower sensitivity to
power, voltage, and temperature variation. While this is still
a consideration for the ADC, once the signal is rendered as
a sequence of digital values, the processing is consistent
regardless of the corner case. Not only is the processing more
consistent, it is also more predictable. Cycle-accurate models
are readily generated that are not reliant on analog models
that must account for variation of a number of parameters
and at times may not be very precise. Furthermore, correct
implementation of the signal processing path may be checked
with robust digital verification techniques. Finally, a DSP-
based implementation is inherently portable to other process
nodes without the need for extensive analog re-work.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Figure 11: Comparison of continuous time equalizers for PAM-2 (NRZ) and PAM-4 receivers. (a) PAM-2 (NRZ) continuous time equalizer (b) PAM-4 continuous time equalizer
10-1
100
101
102
-10
-5
0
5
10
15
Mag
nitu
de,
dB
Frequency, GHz
10-1
100
101
102
-10
-5
0
5
10
15
Mag
nitu
de, d
B
Frequency, GHz
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Porting into smaller process geometries holds the promise of
a faster digital clock rate, a smaller receiver, lower power, or
some combination thereof.
However, DSP-based receivers present their own set of
challenges. The most obvious challenge is the upper limits
on the digital clock frequency for a given technology node.
Well known techniques such as parallelism pipelining and
parallelism can circumvent these problems in many cases.
However, if we consider the feedback structure of the DFE,
the iteration bound presents a challenge for high-speed
operation that cannot be addressed by pipelining alone.
For this particular example, look-ahead techniques [5] can
be employed to relax the iteration bound but this particular
architecture scales exponentially for an increasing number of
DFE taps. For a PAM-4 receiver based on DFE, the need to
resolve 4 levels and propagate 2-bit decisions (each symbol
represent 2 bits) translates to an implementation that requires
twice the complexity of the corresponding NRZ receiver with
half as many taps. One approach to the scaling problem is
to shift more emphasis to the FFE and limit the number of
DFE taps. Other novel architectures could be considered that
achieve performance comparable to DFE with superior scaling
properties [6].
In addition, a performance limiting factor for the DSP-based
receiver is the quantization noise introduced by the ADC. The
resolution of the ADC is defined by its effective number of
bits (ENOB). This quantity is less than the actual number of
bits (ANOB) in the ADC output word, as the ENOB includes
the non-idealities in the conversion process. It is also affected
by the scale of the input signal relative to the ADC full-scale
range. Since the ADC quantization step is relative to the
full scale range, signals smaller than the full scale range see
effectively more quantization noise while larger signals are
clipped introducing non-linearity. It is the responsibility of the
PGA and automatic gain control (AGC) loop to balance these
trade-offs.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Table 1: Default simulation parameters
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11
For the purpose of this simulation study the DSP-based
equalizer is assumed to include both a FFE and a DFE. The
PGA is configured so that ADC clips the input signal with a
relative frequency no greater than 1E−6. The number of taps
for each equalizer as well as the ENOB of the ADC are set
according to Table 1.
Simulation Parameters
Unless otherwise specified, all simulations are performed with
the parameters summarized in Table 1.
Note that the a common symbol error ratio target, i.e.
probability of symbol error, yields a common bit error ratio
target under the assumption that the PAM-4 symbols are
generated from a Gray code and that detection errors map
the symbol of interest only to adjacent symbols.
Simulation Results
The results of the simulations are presented in terms of SNR
margin which is defined to be the difference between the SNR
at the decision point and the SNR required to achieve the
target probability of symbol error. In general, SNR margins
greater than or equal to zero imply the probability of symbol
error is less than the target (good) and SNR margins less than
zero imply the probability of symbol error may exceed the
target (bad).
It should be noted that a negative SNR margin does not
assure that the target probability of symbol error was not
achieved. The relationship between SNR and error probability,
as discussed earlier in this paper, assumes that the noise
term represents the standard deviation of an unbounded
Gaussian amplitude distribution. In practice, components
of the noise term, such as inter-symbol interference (ISI)
and crosstalk, are in fact bounded i.e. the likelihood of that
component exceeding some maximum amplitude is zero.
Since the relationship between SNR and the probability of a
symbol error does not take this into account, the SNR margin
metric can be viewed as conservative.
That said, SNR margin is readily derived from simulation data
and provides a single-value as a figure of merit that may
easily be compared across a large number of simulation cases.
Since margin is reported, the higher SNR values required to
maintain a constant symbol error probability with increasing
L is built into the calculation and no longer needs to be
explicitly considered. Furthermore, the ability of Forward
Error Correction (FEC) to improve the performance of the link
may be readily evaluated in terms of SNR.
For the purpose of these simulations, the SNR at the decision
point is computed as the square of the mean of the outer-
most symbol divided by the sum of the variances of the
individual error terms (due to ISI, crosstalk, jitter-induced
amplitude error, etc.). This quantity is reported in units of
decibels.
The first set of simulation results investigates the relative
impact of each of the link impairments. Referring to Figure 12,
0.75 m and 1 m channels are examined using the PAM-2 and
PAM-4 transmitter and receiver reference models. Channels
are described by an index where 1 and 2 correspond to the
STRADA Whisper connector and 3 and 4 correspond to the
Z-PACK TinMan connector. In addition, 1 and 3 represent
channels built with Megtron 6 while 2 and 4 represent
channels built with materials satisfying the definition of
“improved FR4” used by the IEEE P802.3ap Backplane
Ethernet Task Force. Manufacturer recommended pinouts are
used for these channels.
The graphs illustrate the cumulative reduction of SNR margin
as impairments are added. The “ISI only” values represent the
SNR margin with only residual ISI considered. The “+FEXT”
values are the SNR margin considering residual ISI and all far-
end crosstalk aggressors. The “+NEXT” values include residual
ISI, far-end crosstalk aggressors, and all near-end crosstalk
aggressors. The final set of values, labeled “ENOB” represent
the SNR margin with all impairments considered. Note that,
for the PAM-2 cases, there is no ADC in the receiver reference
model and therefore the SNR degradation due to quantization
error is zero.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
© 2012 Tyco Electronics Corporation. All rights reserved. | STRADA Whisper, TE Connectivity, the TE connectivity (logo), Z-PACK and Z-PACK TinMan are trademarks. Megtron is a trademark of Panasonic Corporation.Other logos, product and/or company names might be trademarks of their respective owners.
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12
Next, the sensitivity of SNR margin to variations in link
parameters is explored. First, crosstalk is manipulated by
specifying pinouts that differ from the manufacturer’s
recommendations. Two such cases are considered. The first is
the “All FEXT” case where the victim receiver is surrounded
by other receivers (co-propagating or far-end aggressors).
The second is that “All NEXT” case where the victim receiver
is surrounded by transmitters (counter-propagating or near-
end aggressors). The SNR margin for these cases is compared
to the values obtained for the manufacturer recommended
pinouts. These results are summarized in Figure 14. In Figure
14, the largest variation with pinout appears for the Z-PACK
TinMan connector channels for which the SNR margin was
negative even for the recommended pinout. For the STRADA
Whisper connector channels, the variation is considerably
smaller, and this reflects that fact that for these low-noise
channels, crosstalk is not a dominant impairment
The comparison of the total SNR margin for the PAM-2 and
PAM-4 reference models is given in Figure 13. From these
results, it is evident that positive SNR margin is achieved
for the STRADA Whisper connector channels while there is
significant negative margin for the Z-PACK TinMan connector
channels. Referring the Figure 12, the PAM-2 reference model
operating over the Z-PACK TinMan connector channels
shows negative SNR margin even for the ISI only indicating
that there is significant residual ISI in these cases. The PAM-
4 reference model yields small positive margins in the “ISI
only” case with the exception of the 1 m Z-PACK TinMan
connector channel. Reflecting on the transfer functions for
the Z-PACK TinMan connector channels shown in Figures 4
and 5, significant insertion loss deviation is evident and the
residual ISI may be a consequence of reflected energy that is
out of the reach of the DFE. In addition, higher crosstalk levels
in the Z-PACK TinMan connector channels are the next largest
contributor to margin degradation.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
1 2 3 4ISI only 7.2 6.0 -2.8 -4.0+ FEXT 6.7 5.5 -3.6 -4.5+ NEXT 6.7 5.5 -4.1 -5.3+ AWGN 6.0 3.3 -4.2 -5.6+ UJ 4.4 2.2 -4.4 -5.7+ ENOB 4.4 2.2 -4.4 -5.7
-8.0-6.0-4.0-2.00.02.04.06.08.0
SNR
mar
gin,
dB
1 2 3 4ISI only 7.6 8.9 1.0 0.4+ FEXT 7.0 8.1 -0.6 -0.9+ NEXT 7.0 8.1 -0.9 -1.3+ AWGN 6.4 6.5 -1.0 -1.6+ UJ 5.6 5.4 -1.2 -1.8+ ENOB 3.4 2.8 -1.9 -2.6
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
SNR
mar
gin,
dB
1 2 3 4ISI only 5.9 0.6 -3.9 -6.7+ FEXT 5.5 0.5 -4.5 -6.9+ NEXT 5.4 0.5 -5.2 -7.8+ AWGN 3.6 -1.9 -5.4 -8.4+ UJ 2.5 -2.2 -5.5 -8.4+ ENOB 2.5 -2.2 -5.5 -8.4
-10.0-8.0-6.0-4.0-2.00.02.04.06.08.0
SNR
mar
gin,
dB
1 2 3 4ISI only 7.9 8.3 0.6 0.1+ FEXT 7.2 7.5 -2.4 -2.5+ NEXT 7.2 7.5 -2.4 -2.5+ AWGN 6.0 4.6 -2.6 -3.1+ UJ 5.0 3.6 -2.8 -3.3+ ENOB 2.7 1.1 -3.4 -4.3
-6.0-4.0-2.00.02.04.06.08.0
10.0
SNR
mar
gin,
dB
Figure 12: SNR margin reduction as a function of various impairments. Channel index [1, 2] = STRADA Whisper, [3, 4] = ZPACK TinMan, [1, 3] = Megtron 6, [2, 4] = “Improved FR4”. Recommended pinouts. (a) PAM-2, 0.75 m channels (b) PAM-4, 0.75 m channels (c) PAM-2, 1 m channels (d) PAM-4, 1 m channels
© 2012 Tyco Electronics Corporation. All rights reserved. | STRADA Whisper, TE Connectivity, the TE connectivity (logo), Z-PACK and Z-PACK TinMan are trademarks. Megtron is a trademark of Panasonic Corporation.Other logos, product and/or company names might be trademarks of their respective owners.
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13
output rise-time. As reported in Table 1, the default value
for this parameter is approximately 18.6 ps (20 to 80%) as
measured at the package pin. The pre-driver rise-time is
varied in order to manipulate the rise-time observed at the
The scope of the analysis is then narrowed to the two 1 m
STRADA Whisper connector channels where transmitter and
receiver parameters are varied to investigate their impact on
SNR margin. The first parameter considered is the transmitter
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Figure 13: Comparison of PAM-2 (NRZ) and PAM-4 SNR margin. Channel index [1, 2] = STRADA Whisper connector, [3, 4] = Z-PACK TinMan connector, [1, 3] = Megtron 6, [2, 4] = “Improved FR4”. Recommended pinouts. (a) 0.75 m channels (b) 1 m channels
1 2 3 4PAM-2 4.4 2.2 -4.4 -5.7PAM-4 3.4 2.8 -1.9 -2.6
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
SNR
mar
gin,
dB
Channel index
1 2 3 4PAM-2 2.5 -2.2 -5.5 -8.4PAM-4 2.7 1.1 -2.3 -3.8
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
SNR
mar
gin,
dB
Channel index
1 2 3 4FEXT/NEXT 4.4 2.2 -4.4 -5.7All FEXT 4.4 2.2 -5.1 -5.7All NEXT 4.7 2.3 -4.7 -6.5
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
SNR
mar
gin,
dB
Channel index
1 2 3 4FEXT/NEXT 3.4 2.8 -1.9 -2.6All FEXT 3.4 2.8 -3.2 -3.5All NEXT 3.6 2.9 -1.9 -3.0
-4.0-3.0-2.0-1.00.01.02.03.04.05.0
SNR
mar
gin,
dB
Channel index
1 2 3 4FEXT/NEXT 2.5 -2.2 -5.5 -8.4All FEXT 2.5 -2.2 -5.6 -7.6All NEXT 2.5 -2.3 -7.3 -10.3
-12.0-10.0
-8.0-6.0-4.0-2.00.02.04.0
SNR
mar
gin,
dB
Channel index
1 2 3 4FEXT/NEXT 2.7 1.1 -2.3 -3.8All FEXT 2.7 1.1 -3.4 -4.3All NEXT 2.8 1.1 -3.1 -5.5
-6.0-5.0-4.0-3.0-2.0-1.00.01.02.03.04.0
SNR
mar
gin,
dB
Channel index
Figure 14: Comparison SNR margin for various crosstalk configurations. Channel index [1, 2] = STRADA Whisper connector, [3, 4] = Z-PACK TinMan connector, [1, 3] = Megtron 6, [2, 4] = “Improved FR4”. “NEXT/FEXT” corresponds recommended pinouts. (a) PAM-2, 0.75 m channels (b) PAM-4, 0.75 m channels (c) PAM-2, 1 m channels (d) PAM-4, 1 m channels
© 2012 Tyco Electronics Corporation. All rights reserved. | STRADA Whisper, TE Connectivity, the TE connectivity (logo), Z-PACK and Z-PACK TinMan are trademarks. Megtron is a trademark of Panasonic Corporation.Other logos, product and/or company names might be trademarks of their respective owners.
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14
pin without changing other aspects of the transmitter such
as return loss performance. Three additional values, again
referenced to the package pin, are considered: 16.4, 20.8, and
24.3 ps. The results are summarized in Figure 15.
The results indicate that the solution based on PAM-4
modulation is relatively insensitive to increases of rise-time on
this order while the PAM-2 solution loses 0.7 to 0.9 dB of SNR
margin. This may be explained by the fact the unit interval
for the PAM-4 solution is twice that of the PAM-2 solution. In
addition to this, the penalty resulting from increasing rise-time
is muted by the equalizer which attempts to compensate for
the apparent reduction in bandwidth.
The next parameter to be considered is jitter. For this
experiment, jitter is added to the default values given in Table
1 as receiver deterministic jitter (default value was 0). This
jitter is sinusoidal and of sufficiently high frequency to not
be tracked by the clock and data recovery unit. In this way,
the tolerance of the system to additional jitter, or in another
manner of speaking, the horizontal margin can be evaluated.
Peak-to-peak receiver deterministic jitter amplitudes of 2, 4,
and 6 ps are considered. Figure 16a shows the SNR margin as
a function of the added jitter expressed in absolute time units.
Considering cases where the SNR margin is greater than zero,
the PAM-4 solution suffers an approximately 1 dB reduction in
margin from 0 to 6 ps where the PAM-2 solution sees a 2 dB
reduction in margin. However, considering Figure 16b, where
the added jitter is normalized to the unit interval, one can see
that the margin for the PAM-4 solution is decreasing at an
accelerated rate. This seems to agree with the conventional
wisdom that PAM-4 has reduced jitter tolerance due to the
additional data dependent jitter caused by the unconstrained
transitions between levels (refer to Figure 10b). Thus, while
PAM-4 appears to suffer a larger penalty per unit interval
of jitter, the unit interval is twice that of the PAM-2 solution
offering the same throughput. It follows that if the low jitter
design practices that would be used to realize the PAM-2
solution could be applied to the PAM-4 solution, a net benefit
could result.
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
SNR
mar
gin,
dB
Added sinusoidal jitter, ps peak-to-peak
PAM-2, Megtron 6PAM-2, "Imp. FR4"PAM-4, Megtron 6PAM-4, "Imp. FR4"
Forward Error Correction (FEC)
Both the NRZ and PAM-4 solutions fail to achieve positive
SNR margin for the Z-PACK TinMan connector channels.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
15.0 17.0 19.0 21.0 23.0 25.0
SNR
mar
gin,
dB
Transmitter output rise time (20 to 80%), ps
PAM-2, Megtron 6PAM-2, "Imp. FR4"PAM-4, Megtron 6PAM-4, "Imp. FR4"
Figure 15: Degradation in SNR margin due to increasing rise and fall times for 1 m STRADA Whisper connector channels (manufacturer recommended pinouts).
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
0.000 0.050 0.100 0.150 0.200
SNR
mar
gin,
dB
Added sinusoidal jitter, UI peak-to-peak
PAM-2, Megtron 6PAM-2, "Imp. FR4"PAM-4, Megtron 6PAM-4, "Imp. FR4"
Figure 16: Degradation in SNR margin due to added sinusoidal jitter for 1 m STRADA Whisper channels (manufacturer recommended pinouts). (a) Jiiter in ps
(b) Jitter in unit intervals
© 2012 Tyco Electronics Corporation. All rights reserved. | STRADA Whisper, TE Connectivity, the TE connectivity (logo), Z-PACK and Z-PACK TinMan are trademarks. Megtron is a trademark of Panasonic Corporation.Other logos, product and/or company names might be trademarks of their respective owners.
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15
Further, additional SNR margin for the STRADA Whisper
connector channels may be of interest for operation at lower
symbol error probabilities. The symbol error probability for
either receiver could be improved with the application of
FEC. FEC operates by adding redundancy in the form of
parity check information to the outgoing data which is used
by the receiver to identify and correct errors. This effect may
be represented by an effective coding gain in decibels which
may then be added to SNR margin computed in this paper to
estimate the improvement.
The selection of an error correcting code must consider
trade-offs between coding gain, over-clocking to maintain
consistent throughput with the overhead of the code, and
added latency. Since the DFE is a staple equalizer for these
applications, the performance of the code in the presence of
burst errors must be carefully considered. Burst errors may
be observed at the output of the DFE, especially under stress
conditions, since a decision error leads to a higher propensity
to make mistakes detecting subsequent symbols.
However, the application of a Reed-Solomon code, with pre-
coding, has been estimated to provide coding gain as high
as 5.4 dB for a PAM-4 system under these conditions [7].
Considering the most challenging channel considered in this
study, the 1 m Z-PACK TinMan “improved FR4” connector
channel, an initial SNR margin of −3.8 could theoretically be
improved to a SNR margin of 1.6 dB. More detailed analysis
would be required to quantify the exact improvement and
this is beyond scope of this paper. Naturally, FEC could
also be applied to NRZ modulated links with comparable
improvement. However, for the same 1 m Z-PACK TinMan
“improved FR4” connector channel, the SNR margin was −8.4
dB and may not be easily salvaged even with the use of FEC.
Observations and Conclusions
This paper has examined 25 Gbps NRZ vs. PAM-4 signaling
across multiple backplane channels with varying degrees of
insertion loss and crosstalk. The goal of this work was not
only to quantify sources of SNR margin degradation for both
types of modulation, but ultimately to determine if and when
premium channels and/or PAM-4 signaling are required for
successful 25 Gbps operation.
When examining sources of SNR margin degradation in time-
domain simulations, several trends become clear. First, it is
apparent that channel reflections (i.e. connector reflections)
can be far more detrimental to system performance than
channel loss (i.e. channel length and dielectric loss). This is
partially true because channel loss can be more effectively
compensated by equalization. While lossier channels do
require more equalization, which then increases AWGN
amplification, it is still clear that the primary source of SNR
margin degradation is due to reflections, when they are
present. This effect can be seen by comparing Z-PACK
TinMan connector channel performance (noticeable
reflections) to STRADA Whisper connector channel
performance (minimal reflections).
The second most dominant source of SNR margin degradation
occurs when significant connector crosstalk is present. In
channels using Z-PACK TinMan connectors, significant SNR
margin degradation from crosstalk can be seen with both
NRZ and PAM-4 modulation. On the other hand, in channels
using STRADA Whisper connectors, there is far less connector
crosstalk. In these channels SNR margin degradation from
crosstalk is almost invisible with NRZ modulation and is only
small with PAM-4 modulation. It is worth noting that SNR
margin degradation can change noticeably with Z-PACK
TinMan connector channels when varying the pinout and
number of aggressors. STRADA Whisper connector crosstalk
levels are so low, that crosstalk degradation does not seem to
be highly sensitive to the connector pinout assignment.
System SNR margin can also vary according to changes in
driver rise-time and system jitter. In the case of rise-time, it
turns out that SNR margin is not very sensitive. In fact, when
looking at driver output rise-times between 16.4 ps and 24.3
ps (20-80%), PAM-4 SNR margin is virtually constant and NRZ
SNR margin only changes by less than 1 dB. On the other
hand, SNR margin is more sensitive to the amount of system
jitter. When adding up to 6 ps of peak-to-peak sinusoidal
jitter at the receiver, SNR margin can be degraded by up to
2 dB. It is worth noting that PAM-4 signals tend to degrade
more rapidly than NRZ signals with increased jitter, but they
also have twice the unit interval width. Therefore, though
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
© 2012 Tyco Electronics Corporation. All rights reserved. | STRADA Whisper, TE Connectivity, the TE connectivity (logo), Z-PACK and Z-PACK TinMan are trademarks. Megtron is a trademark of Panasonic Corporation.Other logos, product and/or company names might be trademarks of their respective owners.
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16
decreasing more rapidly, PAM-4 SNR margin is not degraded
as much as NRZ SNR margin when induced jitter reaches 6 ps
peak-to-peak.
Ultimately, there are two major questions that are paramount
in comparing 25 Gbps NRZ vs. PAM-4 signaling across
modern backplane channels. First, what type of signaling
gives better SNR margin? Second, will PAM-4 signaling
allow the use of legacy channels with “improved FR4” and
connectors such as Z-PACK TinMan connector?
To answer the first question, the PAM-4 solution offered
superior performance in the majority of cases. There are
multiple reasons for this. First, channel ICR is better at PAM-
4’s 6.25 GHz fundamental frequency than it is at NRZ’s 12.5
GHz fundamental frequency. Second, the PAM-4 solution
employed advanced equalization in a DSP-based architecture
that was enabled by the lower symbol rate. Finally, several
impairments such as jitter and noise were held fixed between
the two cases and therefore their impact on the system
operation at the lower symbol rate was muted.
To answer the second question, PAM-4 modulation and
advanced equalization by themselves are not sufficient to
enable operation over all 10GBASE-KR compliant channels.
Simulation results show that 12.5 Gbaud PAM-4 achieves
positive SNR margin for 0.75 m or 1.0 m STRADA Whisper
channels using either Megtron 6 or “improved FR4” materials.
However, in the absence of premium connectors such as
STRADA Whisper, additional measures, such as Forward Error
Correction, would be required to achieve robust operation.
To state things differently, simulations in this paper show that
there are only two scenarios where positive SNR margin was
achieved. When running 25 Gbaud NRZ signaling, one must
use premium dielectrics (Megtron 6) and premium connectors
(STRADA Whisper) for successful operation. When running
12.5 Gbaud PAM-4 signaling, one can use either dielectric
(Megtron 6 or “improved FR4”), but one must use premium
connectors (STRADA Whisper) for successful operation.
Forward Error Correction may be investigated as a means to
support Z-PACK TinMan channels but this is beyond the scope
of this paper.
References
[1] Optical Internetworking Forum, “OIF-CEI-03.0 - Common
Electrical I/O (CEI) - Electrical and Jitter Interoperability
agreements for 6G+ bps, 11G+ bps, 25G+ bps I/O,” September
2011.
[2] J. Goergen, “IEEE802.3ap FR-4 Materials Review: Past
Review and Future Recommendations,” IEEE 802.3 100 Gb/s
Backplane and Copper Cable Study Group interim meeting,
May 2011.
[3] IEEE Std 802.3™-2008, “Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) access method and Physical
Layer specifications,” Section 5, Annex 69B, December 2008.
[4] S. Quan, F. Zhong, W. Liu, P. Aziz et al, “A 1.0625-to-
14.025 Gb/s Multimedia Transceiver with Full-rate Source-
Series-Terminated Transmit Driver and Floating Tap Decision
Feedback Equalizer in 40nm CMOS”, Digest of Technical
Papers, IEEE Intl. Solid States Circuits Conf., pp. 348-349, Feb,
2011.
[5] K. Parhi, “Design of Multiplexer-Loop-Based Decision
Feedback Equalizers,” IEEE Trans. VLSI Sys., vol. 13, no. 4, Apr.
2005.
[6] A. Pola et al., “A New Low Complexity Iterative
Equalization Architecture for High-Speed Receivers on Highly
Dispersive Channels: Decision Feedforward Equalizer (DFFE),”
Proceedings ISCAS 2011, May 2011.
[7] S. Bhoja, W. Bliss, C. Chen, et al., “Precoding proposal for
PAM4 modulation,” IEEE P802.3bj™ 100 Gb/s Backplane and
Copper Task Force interim meeting, September 2011.
A Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
© 2012 Tyco Electronics Corporation. All rights reserved. | STRADA Whisper, TE Connectivity, the TE connectivity (logo), Z-PACK and Z-PACK TinMan are trademarks. Megtron is a trademark of Panasonic Corporation.Other logos, product and/or company names might be trademarks of their respective owners.