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Hyeong-Seok YuHyeong-Seok YuVada Lab.Vada Lab.
[email protected]@vada1.skku.ac.kr
Baseband Pulse TransmissionBaseband Pulse TransmissionCorrelative-Level CodingCorrelative-Level Coding
Baseband M-ary PAM TransmissionBaseband M-ary PAM TransmissionTapped-Delay-Line EqualizationTapped-Delay-Line Equalization
Eye PatternEye Pattern
2
Correlative-Level CodingCorrelative-Level Coding
Correlative-level coding (partial response signaling) adding ISI to the transmitted signal in a controlled
manner Since ISI introduced into the transmitted signal is
known, its effect can be interpreted at the receiver A practical method of achieving the theoretical
maximum signaling rate of 2W symbol per second in a bandwidth of W Hertz
Using realizable and perturbation-tolerant filters
3
Correlative-Level CodingCorrelative-Level Coding
Duobinary Signaling Duobinary Signaling Dou : doubling of the transmission capacity of a straight binary
system
Binary input sequence {bk} : uncorrelated binary symbol 1, 0
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01k
kk
if symbol b isa
if symbol b is
+= −
1−+= kkk aac
4
Correlative-Level CodingCorrelative-Level Coding
Duobinary Signaling Duobinary Signaling Ideal Nyquist channel of bandwidth
W=1/2Tb
)exp()cos()(2
)exp()]exp())[exp((
)]2exp(1)[()(
bbNyquist
bbbNyquist
bNyquistI
fTjfTfH
fTjfTjfTjfH
fTjfHfH
πππππ
π
−=
−−+=
−+=
≤
=otherwise
TffH b
Nyquist
2/1||
,0
,1)(
2cos( )exp( ), | | 1/ 2( )
0,b b b
I
fT j fT f TH f
otherwise
π π− ≤=
)(
)/sin(
/)(
]/)(sin[
/
)/sin()(
2
tTt
TtT
TTt
TTt
Tt
Ttth
b
bb
bb
bb
b
bI
−=
−−+=
ππ
ππ
ππ
5
Correlative-Level CodingCorrelative-Level Coding
Duobinary Signaling Duobinary Signaling The tails of hI(t) decay as 1/|t|2, which is a faster rate of decay
than 1/|t| encountered in the ideal Nyquist channel. Let represent the estimate of the original pulse ak as
conceived by the receiver at time t=kTb
Decision feedback : technique of using a stored estimate of the previous symbol
Propagate : drawback, once error are made, they tend to propagate through the output
Precoding : practical means of avoiding the error propagation phenomenon before the duobinary coding
^
ka
^
1
^
−−= kkk aca
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Correlative-Level CodingCorrelative-Level Coding
Duobinary Signaling Duobinary Signaling
{dk} is applied to a pulse-amplitude modulator, producing a corresponding two-level sequence of short pulse {ak}, where +1 or –1 as before
11 1
0k k
k
symbol if either symbol b or d isd
symbol otherwise−
=
1−+= kkk aac
1−⊕= kkk dbd
10
02k
kk
if data symbol b isc
if data symbol b is
= ±
7
Correlative-Level CodingCorrelative-Level Coding
Duobinary Signaling Duobinary Signaling |ck|=1 : random guess in favor of symbol 1 or 0
0,1||
1,1||
isbsymbolsaycIf
isbsymbolsaycIf
kk
kk
><
8
Correlative-Level CodingCorrelative-Level Coding
Modified Duobinary Signaling Modified Duobinary Signaling Nonzero at the origin : undesirable Subtracting amplitude-modulated pulses spaced 2Tb second
1−+= kkk aac( ) ( )[1 exp( 4 )]
2 ( )sin(2 )exp( 2 )
IV Nyquist b
Nyquist b b
H f H f j fT
jH f fT j fT
ππ π
= − −
= −
2 sin(2 )exp( 2 ), | | 1/ 2( )
0,b b b
IV
j fT j fT f TH f
elsewhere
π π− ≤=
2
sin( / ) sin[ ( 2 ) / ]( )
/ ( 2 ) /
2 sin( / )
(2 )
b b bIV
b b b
b b
b
t T t T Th t
t T t T T
T t T
t T t
π ππ π
ππ
−= −−
=−
9
Correlative-Level CodingCorrelative-Level Coding
Modified Duobinary SignalingModified Duobinary Signaling
precoding
2
21 1
0
k k k
k k
d b d
symbol if either symbol b or d is
symbol otherwise
−
−
= ⊕
=
10
Correlative-Level CodingCorrelative-Level Coding
Modified Duobinary SignalingModified Duobinary Signaling
|ck|=1 : random guess in favor of symbol 1 or 0
| | 1, 1
| | 1, 0k k
k k
If c say symbol b is
If c say symbol b is
><
11
Correlative-Level CodingCorrelative-Level Coding
Generalized form of correlative-level codingGeneralized form of correlative-level coding |ck|=1 : random guess in favor of symbol 1 or 0
Type of Type of classclass
NN ww00 w w11 w w22 w w33 w w44 commentscomments
II 22 1 11 1 DuobinaryDuobinary
IIII 33 1 2 11 2 1
IIIIII 33 2 1 2 1 ––11
IVIV 33 1 0 1 0 ––11 ModifiedModified
VV 55 -1 0 2 0 -1-1 0 2 0 -1
∑−
−=
1
sin)(N
n bn n
T
tcwth
12
Baseband M-ary PAM Trans.Baseband M-ary PAM Trans.
Produce one of M possible amplitude level
T : symbol duration 1/T: signaling rate, symbol per
second, bauds Equal to log2M bit per second
Tb : bit duration of equivalent binary PAM :
To realize the same average probability of symbol error, transmitted power must be increased by a factor of M2/log2M compared to binary PAM
MTT b 2log=
13
Tapped-delay-line equalization Tapped-delay-line equalization
Approach to high speed transmission Combination of two basic signal-processing operation Discrete PAM Linear modulation scheme
The number of detectable amplitude levels is often limited by ISI
Residual distortion for ISI : limiting factor on data rate of the system
14
Tapped-delay-line equalization Tapped-delay-line equalization
Equalization : to compensate for the residual distortion Equalizer : filter
A device well-suited for the design of a linear equalizer is the tapped-delay-line filter
Total number of taps is chosen to be (2N+1)
∑−=
−=N
Nkk kTtwth )()( δ
15
Tapped-delay-line equalization Tapped-delay-line equalization
P(t) is equal to the convolution of c(t) and h(t)
nT=t sampling time, discrete convolution sum
∑∑
∑
−=−=
−=
−=−∗=
−∗=∗=
N
Nkk
N
Nkk
N
Nkk
kTtcwkTttcw
kTtwtcthtctp
)()()(
)()()()()(
δ
δ
∑−=
−=N
Nkk TkncwnTp ))(()(
16
Tapped-delay-line equalization Tapped-delay-line equalization
Nyquist criterion for distortionless transmission, with T used in place of Tb, normalized condition p(0)=1
Zero-forcing equalizer Optimum in the sense that it minimizes the peak distortion(ISI) – worst
case Simple implementation The longer equalizer, the more the ideal condition for distortionless
transmission
±±±==
=
≠=
=Nn
n
n
nnTp
.....,,2,1
0
,0
,1
0,0
0,1)(
17
Adaptive Equalizer Adaptive Equalizer
The channel is usually time varying Difference in the transmission characteristics of the individual links that
may be switched together Differences in the number of links in a connection
Adaptive equalization Adjust itself by operating on the the input signal
Training sequence Precall equalization Channel changes little during an average data call
Prechannel equalization Require the feedback channel
Postchannel equalization synchronous
Tap spacing is the same as the symbol duration of transmitted signal
18
Adaptive EqualizerAdaptive Equalizer
Adaptation may be achieved By observing the error b/w desired pulse shape and actual pulse
shape Using this error to estimate the direction in which the tap-weight
should be changed Mean-square error criterion
More general in application Less sensitive to timing perturbations
: desired response, : error signal, : actual response Mean-square error is defined by cost fuction
Least-Mean-Square AlgorithmLeast-Mean-Square Algorithm
na ne ny
2nE eε =
19
Adaptive EqualizerAdaptive Equalizer
Ensemble-averaged cross-correlation
Optimality condition for minimum mean-square error
Least-Mean-Square AlgorithmLeast-Mean-Square Algorithm
[ ]2 2 2 2 ( )n nn n n n k ex
k k k
e yE e E e E e x R k
w w w
ε−
∂ ∂∂ = = − = − = − ∂ ∂ ∂
[ ]( )ex n n kR k E e x −=
0 0, 1,....,k
for k Nw
ε∂ = = ± ±∂
20
Adaptive EqualizerAdaptive Equalizer
Mean-square error is a second-order and a parabolic function of tap weights as a multidimentional bowl-shaped surface
Adaptive process is a successive adjustments of tap-weight seeking the bottom of the bowl(minimum value )
Steepest descent algorithm The successive adjustments to the tap-weight in direction opposite to
the vector of gradient ) Recursive formular (µ : step size parameter)
Least-Mean-Square AlgorithmLeast-Mean-Square Algorithm
minε
/ kwε∂ ∂
1( 1) ( ) , 0, 1,....,
2
( ) ( ), 0, 1,....,
k kk
k ex
w n w n k Nw
w n R k k N
εµ
µ
∂+ = − = ± ±∂
= − = ± ±
21
Adaptive EqualizerAdaptive Equalizer
Least-Mean-Square Algorithm Steepest-descent algorithm is not available in an unknown environment Approximation to the steepest descent algorithm using instantaneous
estimate
Least-Mean-Square AlgorithmLeast-Mean-Square Algorithm
( )
( 1) ( )ex n n k
k k n n k
R k e x
w n w n e xµ−
−
=+ = +
)
) )
LMS is a feedback system In the case of small µ,
roughly similar to steepest descent algorithm
22
Adaptive EqualizerAdaptive Equalizer
Training mode Known sequence is transmitted and synchorunized version is generated
in the receiver Use the training sequence, so called pseudo-noise(PN) sequence
Decision-directed mode After training sequence is completed Track relatively slow variation in channel characteristic
Large µ : fast tracking, excess mean square error
Operation of the equalizerOperation of the equalizer
23
Adaptive EqualizerAdaptive Equalizer
Analog CCD, Tap-weight is stored in digital memory, analog sample and
multiplication Symbol rate is too high
Digital Sample is quantized and stored in shift register Tap weight is stored in shift register, digital multiplication
Programmable digital Microprocessor Flexibility Same H/W may be time shared
Implementation ApproachesImplementation Approaches
24
Adaptive EqualizerAdaptive Equalizer
Baseband channel impulse response : {hn}, input : {xn}
Using data decisions made on the basis of precursor to take care of the postcursors The decision would obviously have to be correct
Decision-Feed back equalizationDecision-Feed back equalization
00 0
n k n kk
n k n k k n kk k
y h x
h x h x h x
−
− −< >
=
= + +
∑∑ ∑
25
Adaptive EqualizerAdaptive Equalizer
Feedforward section : tapped-delay-line equalizer
Feedback section : the decision is made on previously detected symbols of the input sequence Nonlinear feedback loop by
decision device
Decision-Feed back equalizationDecision-Feed back equalization
(1)
(2)
nn
n
wc
w
=
)
)n
nn
xv
a
=
)
Tn n n ne a c v= −
(1) (1)1 1 1
(2) (2)1 1 1
n n n n
n n n n
w w e x
w w e a
µµ
+ +
+ +
= −
= −
) )
) ) )
26
Eye PatternEye Pattern
Experimental tool for such an evaluation in an insightful manner Synchronized superposition of all the signal of interest viewed within a
particular signaling interval Eye opening : interior region of the eye pattern
In the case of an M-ary system, the eye pattern contains (M-1) eye opening, where M is the number of discreteamplitude levels
27
Eye PatternEye Pattern