serdes.DFECDR
Decision feedback equalizer (DFE) with clock and data recovery (CDR)
Description
The serdes.DFECDR
System object™ adaptively processes a sample-by-sample input signal or analytically processes
an impulse response vector input signal to remove distortions at post-cursor taps.
The DFE modifies baseband signals to minimize the intersymbol interference (ISI) at the clock sampling times. The DFE samples data at each clock sample time and adjusts the amplitude of the waveform by a correction voltage.
For impulse response processing, the hula-hoop algorithm is used to find the clock sampling locations. The zero-forcing algorithm is then used to determine the N correction factors necessary to have no ISI at the N subsequent sampling locations, where N is the number of DFE taps.
For sample-by-sample processing, the clock recovery is accomplished by a first order phase tracking model. The bang-bang phase detector utilizes the unequalized edge samples and equalized data samples to determine the optimum sampling location. The DFE correction voltage for the N-th tap is adaptively found by finding a voltage that compensates for any correlation between two data samples spaced by N symbol times. This requires a data pattern that is uncorrelated with the channel ISI for correct adaptive behavior.
To equalize the input signal:
Create the
serdes.DFECDR
object and set its properties.Call the object with arguments, as if it were a function.
To learn more about how System objects work, see What Are System Objects?
Creation
Description
returns a DFECDR
object that modifies an input waveform with the DFE and determines the clock sampling
times. The system object estimates the data symbol according to the Bang-Bang CDR
algorithm.dfecdr
= serdes.DFECDR
sets properties using one or more name-value pairs. Enclose each property name in quotes.
Unspecified properties have default values.dfecdr
= serdes.DFECDR(Name
,Value
)
Example: dfecdr = serdes.DFECDR('Mode',1)
returns a DFECDR object
that applies specified DFE tap weights to input waveform.
Properties
Unless otherwise indicated, properties are nontunable, which means you cannot change their
values after calling the object. Objects lock when you call them, and the
release
function unlocks them.
If a property is tunable, you can change its value at any time.
For more information on changing property values, see System Design in MATLAB Using System Objects.
DFE Properties
Mode
— DFE operating mode
2
(default) | 0
| 1
DFE operating mode, specified as 0
, 1
, or
2
. The operating mode you select determines the DFE tap weights
values that the object applies to the input waveform.
Mode Value | DFE Mode | DFE Operation |
---|---|---|
0 | Off | The object bypasses DFE and the input waveform remains unchanged. |
1 | Fixed | The object applies the DFE tap weights specified in
TapWeights to the input waveform. |
2 | Adapt | The object determines the optimum DFE tap weights and applies them to the input waveform. |
Data Types: double
TapWeights
— Initial DFE tap weights
[0 0 0 0]
(default) | row vector
Initial DFE tap weights, specified as a row vector in volts. The length of the vector determines the number of taps. The vector elements signify the tap weights or tap strength. Set the tap weight to zero to initialize the tap.
Data Types: double
MinimumTap
— Minimum value of adapted tap weights
-1
(default) | real scalar | real-valued row vector
Minimum value of the adapted tap weights, specified as a real scalar or a
real-valued row vector in volts. Specify as a scalar to apply to apply the same
minimum value to all the DFE taps, or specify as a vector that has the same length as
the TapWeights
.
Data Types: double
MaximumTap
— Maximum value of adapted tap weights
1
(default) | nonnegative real scalar | nonnegative real-valued row vector
Maximum value of the adapted tap weights, specified as a nonnegative real scalar
or a nonnegative real-valued row vector in volts. Specify as a scalar to apply the
same maximum value to all the DFE taps, or specify as a vector that has the same
length as the TapWeights
.
Data Types: double
EqualizationGain
— Controls for update rate of tap weights
9.6e-5
(default) | positive real scalar
Controls for update rate of tap weights, specified as a unitless nonnegative real scalar. Increase the value of this property for the DFE adaptation to converge faster at the expense of more noise in the DFE tap values.
Data Types: double
EqualizationStep
— DFE adaptive step resolution
1e-6
(default) | nonnegative real scalar | nonnegative real-valued row vector
DFE adaptive step resolution, specified as a nonnegative real scalar or a
nonnegative real-valued row vector in volts. Specify as a scalar to apply the same
value to all the DFE taps, or specify as a vector that has the same length as
TapWeights
.
EqualizationStep
specifies the minimum change in DFE taps
from one time step to the next to mimic hardware limitations. Setting
EqualizationStep
to zero yields DFE tap values without any
resolution limitation.
Data Types: double
Taps2x
— Multiply DFE tap weights by a factor of two
false (default) | true
Multiply DFE tap weights by a factor of two, specified as true or false. Set this property to true to multiply the DFE tap weights by a factor of two.
The output of the slicer in the serdes.DFECDR
System object from the SerDes Toolbox™ is [-0.5 0.5]. But some industry applications require the slicer output
to be [-1 1]. Taps2x
allows you to quickly double the DFE tap
weights to change the slicer reference.
CDR Properties
CDRMode
— Determine CDR order
1st order
(default) | 2nd order
Determine the CDR order to enable phase and frequency tracking.
1st order
— Only tracks the phase.2nd order
— Tracks both the phase and frequency.
Count
— Early or late CDR count threshold to trigger phase update
16
(default) | real positive integer greater than 4
Early or late CDR count threshold to trigger a phase update, specified as a
unitless real positive integer greater than 4. Increasing the value of
Count
provides a more stable output clock phase at the expense
of convergence speed. Because the bit decisions are made at the clock phase output, a
more stable clock phase has a better bit error rate (BER).
Count
also controls the bandwidth of the CDR which is
approximately calculated by using the equation:
Data Types: double
ClockStep
— Clock phase resolution
0.0078
(default) | real scalar
Clock phase resolution, specified as a real scalar in fraction of symbol time.
ClockStep
is the inverse of the number of phase adjustments in
CDR.
Data Types: double
PhaseOffset
— Clock phase offset
0
(default) | real scalar in the range [−0.5, 0.5]
Clock phase offset, specified as a real scalar in the range [−0.5, 0.5] in
fraction of symbol time. PhaseOffset
is used to manually shift
the clock probability distribution function (PDF) for better BER.
Data Types: double
ReferenceOffset
— Reference clock offset impairment
0
(default) | real scalar in the range [−300, 300]
Reference clock offset impairment, specified as a real scalar in the range [−300,
300] in parts per million (ppm). ReferenceOffset
is the deviation
between transmitter oscillator frequency and receiver oscillator frequency.
Data Types: double
Sensitivity
— Sampling latch metastability voltage
0
(default) | real scalar
Sampling latch metastability voltage, specified as a real scalar in volts (V). If
the data sample voltage lies within the region (±Sensitivity
),
there is a 50% probability of bit error.
Data Types: double
PhaseDetector
— Clock phase detector option
BangBang
(default) | BaudRateTypeA
Clock phase detector option used in the clock data recovery. You can choose between bang-bang (Alexander) or baud-rate type-A (Mueller-Muller).
FrequencyStep
— Internal gain for frequency tracking
1/2e11
(default) | nonnegative real scalar
Internal gain for the frequency tracking loop, specified as a nonnegative real scalar.
Data Types: double
FrequencyCount
— Frequency tracking update
16
(default) | nonnegative integer scalar
Once every FrequencyCount
symbols, update the system phase
rotator clock with the frequency estimate.
Data Types: double
Advanced Properties
SymbolTime
— Time of single symbol duration
1e-10
(default) | real scalar
Time of a single symbol duration, specified as a real scalar in seconds (s).
Data Types: double
SampleInterval
— Uniform time step of waveform
6.25e-12
(default) | real scalar
Uniform time step of the waveform, specified as a real scalar in seconds (s).
Data Types: double
Modulation
— Modulation scheme
2
(default) | 3
| 4
Modulation scheme, specified as 2
, 3
or
4
.
Modulation Value | Modulation Scheme |
---|---|
2 | Non-return to zero (NRZ) |
3 | Three-level pulse amplitude modulation (PAM3) |
4 | Four-level pulse amplitude modulation (PAM4) |
Note
IBIS does not support a PAM3 modulation scheme, so you cannot export the System object to IBIS-AMI model if you set the modulation scheme to PAM3.
Data Types: double
WaveType
— Input waveform type
'Sample'
(default) | 'Impulse'
Input waveform type, specified as one of these:
'Sample'
— A sample-by-sample input signal.'Impulse'
— An impulse-response input signal.
Data Types: char
Usage
Syntax
Input Arguments
x
— Input baseband signal
scalar | vector
Input baseband signal, specified as a scalar or vector. If you set the
WaveType
property to 'Sample'
, then the
input signal is a sample-by-sample signal specified as a scalar. If you set the
WaveType
property to 'Impulse'
, the input
signal is an impulse-response vector signal.
Output Arguments
y
— Estimated channel output
scalar | vector
Estimated channel output. If the input signal is a sample-by-sample signal specified as a scalar, then the output is also scalar. If the input signal is an impulse response vector signal, the output is also a vector.
TapWeights
— Estimated DFE tap weight values
vector
Estimated DFE tap weight values, returned as a vector.
Phase
— Relative recovered clock phase
units of SymbolTime
in the range [0,1]
Relative recovered clock phase, returned as a units of
SymbolTime
in the range [0,1].
clkAMI
— AMI clock bus
structure
AMI clock bus, returned as a structure.
Field | Description |
---|---|
clockTime | Time taken to sample the data signal. |
clockValidOnRising | Valid clock time value on the rising edge of a signal. |
interior
— Bus containing additional interior CDR signals
structure
Bus containing additional interior CDR signals, returned as a structure.
Field | Description |
---|---|
clockPhase | Relative clock phase in units of SymbolTime in the
range of [0,1]. |
symbolRecovered | Symbol recovered from data signal at
ClockTime . |
voltageSample | Voltage observed from the data signal at
ClockTime . |
PAM4Threshold | The estimated upper eye at PAM4 threshold. |
CDRedgeVoltage | The voltage observed from the data signal at ClockTime —
SymbolTime /2 . |
CDRCounter | The bang-bang CDR internal counter used to trigger samples. |
CDREarlyLateCount | The bang-bang CDR accumulated (or filtered) signal used to trigger updated to the CDR phase. |
PAMSymbolMiddleVoltage | Estimated PAM4 symbol voltage of the inner eye outer envelope to estimate PAM threshold. |
PAMSymbolOuterVoltage | Estimated PAM4 outer envelope voltage to estimate PAM threshold. |
EyeHeightAbsAve | Estimates eye height. |
PAMThreshold | Array of PAM thresholds for each eye in the PAMn modulation. If the modulation scheme is PAMn, the first (n-1) eyes contain the valid thresholds. The rest of the entries are zero-padded. |
PhaseError | Phase detector error. |
FreqEstimate | Frequency estimate for the 2nd order or frequency tracking loop. |
Object Functions
To use an object function, specify the
System object as the first input argument. For
example, to release system resources of a System object named obj
, use
this syntax:
release(obj)
Examples
Impulse Response Processing Using DFECDR
This example shows how to process impulse response of a channel using serdes.DFECDR
System object™.
Use a symbol time of 100
ps. There are 16
samples per symbol. The channel has 14
dB loss.
SymbolTime = 100e-12; SamplesPerSymbol = 16; dbloss = 14; NumberOfDFETaps = 2;
Calculate the sample interval.
dt = SymbolTime/SamplesPerSymbol;
Create the DFECDR object. The object adaptively applies optimum DFE tap weights to input impulse response.
DFE1 = serdes.DFECDR('SymbolTime',SymbolTime,'SampleInterval',dt,... 'Mode',2,'WaveType','Impulse','TapWeights',zeros(NumberOfDFETaps,1));
Create the channel impulse response.
channel = serdes.ChannelLoss('Loss',dbloss,'dt',dt,... 'TargetFrequency',1/SymbolTime/2); impulseIn = channel.impulse;
Process the impulse response with DFE.
[impulseOut,TapWeights] = DFE1(impulseIn);
Convert the impulse response to a pulse, a waveform and an eye diagram for visualization.
ord = 6; dataPattern = prbs(ord,2^ord-1)-0.5; pulseIn = impulse2pulse(impulseIn,SamplesPerSymbol,dt); waveIn = pulse2wave(pulseIn,dataPattern,SamplesPerSymbol); eyeIn = reshape(waveIn,SamplesPerSymbol,[]); pulseOut = impulse2pulse(impulseOut,SamplesPerSymbol,dt); waveOut = pulse2wave(pulseOut,dataPattern,SamplesPerSymbol); eyeOut = reshape(waveOut,SamplesPerSymbol,[]);
Create the time vectors.
t = dt*(0:length(pulseOut)-1)/SymbolTime; teye = t(1:SamplesPerSymbol); t2 = dt*(0:length(waveOut)-1)/SymbolTime;
Plot the resulting waveforms.
figure plot(t,pulseIn,t,pulseOut) legend('Input','Output') title('Pulse Response Comparison') xlabel('SymbolTimes'),ylabel('Voltage') grid on axis([41 55 -0.1 0.4])
figure plot(t2,waveIn,t2,waveOut) legend('Input','Output') title('Waveform Comparison') xlabel('SymbolTimes'),ylabel('Voltage') grid on
figure subplot(211),plot(teye,eyeIn,'b') xlabel('SymbolTimes'),ylabel('Voltage') grid on title('Input Eye Diagram') subplot(212),plot(teye,eyeOut,'b') xlabel('SymbolTimes'),ylabel('Voltage') grid on title('Output Eye Diagram')
Sample-by-Sample Processing Using DFECDR
This example shows how to process impulse response of a channel one sample at a time using serdes.DFECDR
System object™.
Use a symbol time of 100
ps, with 8
samples per symbol. The channel loss is 14
dB. Select 12
-th order pseudorandom binary sequence (PRBS), and simulate the first 20000
symbols.
SymbolTime = 100e-12; SamplesPerSymbol = 8; dbloss = 14; NumberOfDFETaps = 2; prbsOrder = 12; M = 20000;
Calculate the sample interval.
dt = SymbolTime/SamplesPerSymbol;
Create the DFECDR System object. Process the channel one sample at a time by setting the input waveforms to 'sample'
type. The object adaptively applies the optimum DFE tap weights to input waveform.
DFE2 = serdes.DFECDR('SymbolTime',SymbolTime,'SampleInterval',dt,... 'Mode',2,'WaveType','Sample','TapWeights',zeros(NumberOfDFETaps,1),... 'EqualizationStep',0,'EqualizationGain',1e-3);
Create the channel impulse response.
channel = serdes.ChannelLoss('Loss',dbloss,'dt',dt,... 'TargetFrequency',1/SymbolTime/2);
Initialize the PRBS generator.
[dataBit,prbsSeed]=prbs(prbsOrder,1);
Generate the sample-by-sample eye diagram.
%Loop through one symbol at a time. inSymbol = zeros(SamplesPerSymbol,1); outWave = zeros(SamplesPerSymbol*M,1); dfeTapWeightHistory = nan(M,NumberOfDFETaps); for ii = 1:M %Get new symbol [dataBit,prbsSeed]=prbs(prbsOrder,1,prbsSeed); inSymbol(1:SamplesPerSymbol) = dataBit-0.5; %Convolve input waveform with channel y = channel(inSymbol); %Process one sample at a time through the DFE for jj = 1:SamplesPerSymbol [outWave((ii-1)*SamplesPerSymbol+jj),TapWeights] = DFE2(y(jj)); end %Save DFE taps dfeTapWeightHistory(ii,:) = TapWeights; end
Plot the DFE adaptation history.
figure plot(dfeTapWeightHistory) grid on legend('TapWeights(1)','TapWeights(2)') xlabel('Symbols') ylabel('Voltage') title('DFE Taps')
You can observe from the plot that the DFE adaptation is approximately complete after the first 10000
symbols, so these can be truncated from the array. Then plot the eye diagram by applying the reshape function to the array of symbols.
foldedEye = reshape(outWave(10000*SamplesPerSymbol+1:M*SamplesPerSymbol),SamplesPerSymbol,[]);
t = dt*(0:SamplesPerSymbol-1);
figure,plot(t,foldedEye,'b');
More About
Adapt Operating Mode
The Init subsystem calls to the serdes.DFECDR
. The
serdes.DFECDR
finds the optimum DFE tap values for the best eye height
opening for statistical analysis. During time domain simulation, DFE uses the adapted values
as the starting point and applies them to the input waveform. For more information about the
Init subsystem, see Statistical Analysis in SerDes Systems
Choose Phase Detector Model in Clock Recovery
You can select which phase detector model the serdes.DFECDR
in clock
recovery: bang-bang or baud-rate type A. The default phase detector model used is
bang-bang.
You can use a DFECDR with baud-rate type-A phase detector model in the SerDes Designer app and then export the model to Simulink®. In this case, automatically adds the Rx_Decision_Time parameter to define the clock position.
If you want to change the phase detector model in Simulink, you need to manually add the Rx_Decision_Time parameter.
Note
You must click the Refresh Init button in the Rx Init subsystem after modifying the Rx_Decision_Time parameter.
Extended Capabilities
C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.
Usage notes and limitations:
IBIS-AMI codegen is not supported in MAC.
Version History
Introduced in R2019a
See Also
DFECDR | CTLE | CDR | serdes.CTLE
| serdes.CDR
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