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Abstract- in this paper , a design and
simulation for Asymmetric Digital
Subscriber Line (ADSL)/AsymmetricDigital Subscriber Line 2 (ADSL2)
Initialization Process is presented which
can be applied to different telephone
network. The Initialization process is
designed and simulated under the
MATLAB v7 environment. ADSL channelfaces different types of noise, the most
important types are Background and
Crosstalk noises and Intersymbol
interference (ISI) which can be eliminated
by using Time Domain Equalizer (TEQ).Minimum Mean Square Error (MMSE)
algorithm is implemented as TEQ
algorithm. The initialization process where
tested on American National Standard
Institute (ANSI ) define 8 Carrier Serving
Area (CSA) test loops for ADSL service Asa result , 9.02 Mbps and 9.42 Mbps were
achieved over CSA loop-2 for ADSL and
ADSL2 respectively. The TEQ efficiency
for 8 CSA test loop are calculated where it
achieved 93.1% of zero ISI for CSA loop-
3.a Comparison between Near End Cross-
Talk (NEXT) and Far End Cross-Talk
(FEXT) power shows that NEXT havehigher power and narrow band where
NEXT powers were (-45.593) dBm for allloops and FEXT were ranging from (-
76.353 to -68.261) dBm for the 8 loops.
Finally the results show that ADSL2
outperforms ADSL by about 400Kbps
which about 1.1%.
I. IntroductionBroadband or high-speed Internet access is
provided by a series of technologies that
give users the ability to send and receive
data at volumes and speeds far greater than
current Internet access over traditionaltelephone lines. In addition to offering
speed, broadband access provides a
continuous, always on connection (noneed to dial-up) and a two way
capability, that is, the ability to both receive
(download) and transmit (upload) data at
high speeds. Broadband access, along with
the content and services it might enable, has
the potential to transform the Internet: both
what it offers and how it is used. It is likely
that many of the future applications that
will best exploit the technologicalcapabilities of broadband have yet to be
developed [1].
Telephone line based technologies
provide dedicated access to the individual
users. One of the best solutions is DigitalSubscriber Line (DSL) access, which is
targeted for residential users and has
received much attention by many telephone
companies. The architecture of DSLsystems allows telephone companies to useexisting twisted-pair infrastructures, by
which there is no need to lay extra lines for
new services, for their next-generation
broadband access networks.
The most promising of the xDSL
technologies for integrated Internet access
is Asymmetric Digital Subscriber Line(ADSL) by which ADSL is designed to
Design and Simulation of ADSL/ADSL2 Initialization
Process
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interoperate with the telephone, i.e.
allowing voice and high-speed data to be
sent simultaneously over the same line.
Unlike today's computer dialup modems,the customer may use the telephone while
the computer is connected to the Internet
Service Provider (ISP). The achievable data
rate is significantly higher in ADSL by
almost 100 times compared to today's
fastest 56 Kbps modems [2].A recent development is ADSL2 and
ADSL2+. It defines new applications,
services and deployment scenarios.
The first issue it has to deal with is a
wide range of used cables in telephone loop
plant also a varying length of loops from
few hundred of feet to 18000 feet. Themodem should adept well with these
changes [3].
Other users using ADSL or other
communication service running on the same
binder causes noises, these noises is called
crosstalk noises, where background noiserepresents the thermal and environment
effect. Crosstalk noises have the dominant
effect on ADSL system performance.
Other ADSL impartment whenoperating on long loops is dispersive
behavior of the channel, resulting in a wider
received pulse. This causes a time sample
to spread into the neighboring time slots
that causes InterSymbol Interference (ISI).To eliminate this effect ADSL modem
should shorten the channel impulse to
desirable length.
Cioffe [4], described fundamentals of
MCM and how it is analyzed for channels
with ISI and additive Gaussian noise. Inoue
[5] ,was concerned with an improvement topreviously proposed ADSL echo canceller
using modified conjugate gradient for
adaption schema. Arslan [6], studied
different kinds of equalization technique for
discrete multitone transceiver and
developed a time domain equalizers (TEQ)design method to optimize the channel
capacity at the output of the TEQ. Nadhim
[7] studied MCM technique in DSL
systems using the Fourier transform and
then he used the discreet wavelet transform
(DWT) and studied the systemperformance. DALY [8], dealt with the way
of improving the efficiency of multicarrier
communication on the digital subscriber
loop and also examined bit and power
loading algorithms for multitone systems.
Since ADSL channel does not change
during data transmission and only minornoises level changes occurs, all problems
are handled during ADSL modem start-up
process, which is known as ADSL
initialization process. In this work
ADSL/ADSL2 initialization process in
simulated under MATLAB version 7environment.
II. ADSL/ADSL2 InitializationThe initialization process allows the ATU-
C and ATU-R to establish their
communications. The process allows the
two modems to identify themselves to eachother, determine line conditions available tosupport communication, exchange
parameters that define the request
connection, allocate resources, and begin
normal communication. The process is
divided into four phases as described in
ANSI standard:Activation and Acknowledgment: TheATU-R begins the initialization process by
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transmitting the appropriate tones to the
ATU-C. When this segment of initialization
is complete, the ATU-R and ATU-C have
negotiated the timing method used betweenthem and have determined which device is
the master. At the end of this procedure, the
ATU-R and ATU-C are in state capable of
analyzing the line condition. In this paper,
the ATU-C and ATU-R are assumed to
have negotiated the timing method andperfectly handshaking and all activation and
acknowledgment process is done perfectly.
Transceiver Training: During this process,
the ATU-R and the ATU-C send signals
that allow their partner to determine line
conditions and adjust the equalization of
their transceiver. Transceiver training alsodetermines if ADSL is operating in FDM or
Echo cancellation mode.Channel Analysis: The modem exchange
information on the upstream and
downstream bearer channel required for the
connection the latency paths they will beplaced in, and the bandwidths for each
channel requested. Information about
specific features supported or requested is
also exchanged. The modems then performtest that determine the loop quality and
SNR for each specific 4 kHz DMT tone.
Exchange: Having gathered the
information about quality of the connection
and the requested configuration, themodems configure themselves and
exchange information about their
configuration, the specific bandwidth
allocated to the requested bearer channels is
assigned, the specific DMT tones and the
amount of the data encoded in each tone are
determined and assigned. The connection istested in both directions after which each
modem notifies its peer that it is ready to
enter normal communications. In this
thesis, this phase is represented by bit
loading process.
The ADSL test loop modelingprocess simulate the ANSI CSA loops as
ADSL modem test loop, noise modeling
process simulate AWGN, NEXT and FEXT
noises for ADSL service. Channel analysis
process measure ADSL modem
performance on test loops with theexistence of noises by calculating signal to
noise ratio (SNR) assuming free ISI
channel. Since the used test loop is not free
ISI, therefore ISI elimination process is
required. Channel equalization process
eliminate channel ISI by using Time
Domain Equalizer (TEQ) and measuremodem performance on test loop after
equalization by calculating equalizer output
signal to noise ratio (SNR). Bit loading
processes calculates modem performance in
terms of bit rate and calculate modem bit
loading table using equalizer output (SNR).
A. DSL channelThe transmission characteristics of DSL
loops determine the performance of DSLsystems. DSL loops are based on existing
analog telephone subscriber loops which
were originally developed for voice
communication. A subscriber loop consistsof twisted pair cables that connect a localcentral office to customer premises. Two
subscribers are connected to each other
through central offices.
In order to facilitate the development
of broadband communication on the DSL,
the ANSI and ESTI standardization bodies
specified test loops which would hopefully
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be representative of a large cross-section of
the DSL links encountered in practice.
The ANSI introduced the Carrier
Serving Area (CSA) engineering guidelinesin the early 1980s to shorten subscriber
loop length, which reduces loop
deployment cost and supports all future
digital services. A carrier serving area
(CSA) is a plant administration area
subsection of the main loop plant. [4].These generic lines are referred to as ADSL
CSA test-loops [9].
Figure 1. Transmission Line
Schematic.
The ANSI CSA test loop is modeled and
used as test loops as is shown in Fig 1. Theload impedance shown in Fig. 1 isconsidered to be a real constant (100 inANSI loops). The eight standard CSA loops
are used as test channels in the
ADSL/ADSL2 initialization simulation.
A splitter is used to separate the voice
signal from ADSL signal, hence a highpass
filter with low transition frequency isneeded. For the purposes of modling, a fifth
order Chebyshev high-pass filter with cut-off frequency of 4.8 kHz was used to
simulate splitter effect.
Figure 2. Configuration of the Eight
Standard CSA loops. Number Represent
Length/Thickness in Feet/Gauge. Vertical
Lines Represent Bridge-Taps.
B. Telephone Line ImpairmentsSubscriber loops which connect the
customer premises to a Central switching
Office (CO), were developed and deployed
for voice transmission. The term loop refers
to the twisted copper pair telephone line
from a CO to the customer. This termoriginates from current flow through a
looped circuit from the CO on one wire and
returning on another wire [10].
All signals sent over conventional
pair-cable telephone lines are subject to lineattenuation, dispersion and electrical noise.
Line attenuation and some forms of in-band
noise both increase with frequency [11].
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Intersymbol interference (ISI) is an
unavoidable consequence of both wired and
wireless communication systems. ISI
causes spreading of received signal.As transmissions occur more frequently
(that is, the symbol rate is increased, which
corresponds to decreasing symbol time T
for each successive), then a given channel
shape exhibits increasing overlap of
transmissions and thus more ISI.Equalization methods adaptively configure
the receiver to mitigate ISI [12].
Multicarrier Modulation (MCM)
systems use Cyclic Prefix (CP) to separates
the symbols in time in order to decrease ISI.
As it is well known, the signal going
through the line is linearly convolved withthe impulse response of the line. If the
impulse response is shorter than the
duration of the cyclic prefix, each symbol
can be processed separately, and there is no
ISI [11].
Another impairment of data transmission is
white noise. One special kind of noise is
Additive White Gaussian Noise (AWGN)
which represents background noise asalready specified by its name. It has a flat
spectrum and a Gaussian amplitude
probability. Normally it is characterized by
its power spectral density.
Since AWGN have constant PSD( /2), it is easly built by generating arandom signal process and muliplying it bythe AWGN power, where AWGN power is
the AWGN PSD (Watt/Hz) multiplied by
the used bandwidth.
For simulation, different levels ofAWGN have been used, all moving in a
range between (-170 to -140 dBm/Hz)
single sided power spectral density [13].
For ADSL, however, -140 dBm/Hz seems
to be the most used value for AWGN [14].
The performance of DSL transceivers
can be impaired by interference from. Thetwisting of the wire pairs reduces this
inductive coupling (also referred as
crosstalk), but some signal leakage remains.
Crosstalk is most pronounced at the
segment of cable near the interfering
transmitters. The crosstalk resulting fromother transmission systems in the same
cable (and especially the same binder group
with the cable) is a primary factor limiting
the bit rates and loop reach achievable by
DSLs. There are two types of crosstalk
[12].
Near End Cross-Talk (NEXT) is amajor impairment for systems that share the
same frequency band for upstream and
downstream transmission. NEXT noise is
seen by the receiver located at the same end
of the cable bender of the transmitter that is
the noise source.Far End Cross-Talk (FEXT) is the
noise detected by the receiver located at the
far end of the cable from the transmitter
that is the noise source. FEXT is less severethan NEXT because the FEXT noise is
attenuated by traversing the full length of
the cable [12].
In order to evaluate a NEXT or a
FEXT crosstalk signals, the PSD functionof the interfering service has to be known.
The PSD of downstream and upstream for
ADSL will be discussed in this subsection.
This paper will focus on Standard
T1.413 by ANSI as a reference for the
ADSL service. The ADSL upstream
according to ANSI can be given by
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the is the same for both thedownload and upload stream except for the
differences between the and coefficient. The NEXT forADSL service can be calculated using
where U is the number of disturbers in thecable, is the couplingcoefficient for 49 NEXT disturbers and is the frequency in Hz. The FEXT for
ADSL service can be calculated using
Where
is the channel gain (frequency
response), 8 x 10-20
is the coupling
coefficient for 49 FEXT disturbers and isthe coupling path distance.Generating a crosstalk signal is
conveniently accomplished by defining a
filter transfer function in terms of thedesired power spectral density (PSD). If
the input to the filter is a white noise
process with unit variance, then the PSD is
equal to the magnitude squared of the filter
transfer function. The filter transferfunction is Where is the PSD in Watts/Hz.The total Noise PSD will be
Since there are different PSD origins, each
PSD is can be simulated independently and
the resulted signal is summed to generate
the total noise signal.
C. Equalization for DiscreteMultitone Modulation
With DMT, the problem of fully equalizing
a channel is converted into partitioning the
channel into small subchannels which is
more efficient to implement in high-speed
transmission. However, this does not imply
that equalization is not required in a DMT
system. The spectra of each inverse FFT(IFFT) modulated subchannel is a sampled
sinc function which is not bandlimited.
Demodulation is still possible due to
the orthogonality between the sinc
functions. An ISI channel, destroysorthogonality between subchannels so that
they cannot be separated at the receiver.
One way to reduce ISI with a shorter
cyclic prefix is to use an equalizer. Sincethe length of a DMT symbol is longer thana symbol in single carrier modulation,
equalization is simpler. Also, noise
enhancement by the equalizer is not an
issue because the equalizer does not affect
the SNR in each subchannel, which are the
primary parameters to determine the
performance of a DMT system.
The ADSL standard uses a guardperiod, time-domain equalization, and
frequency-domain equalization. The Time-
Domain Equalizer (TEQ) shortens the
channel to a length of a predetermined but
short guard period. The TEQ can be
implemented as an FIR filter whose filter
coefficients are trained during initialization.
This combination has been standardized
and is implemented in practical systems [4].
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Minimum Mean-Squared Error Impulse
Response Shortening Algorithm
Chow and Cioffi [15] are the first to apply
channel shortening equalization tomulticarrier modulation. They use the
MMSE design method to shorten a given
channel to the length of the cyclic prefix.
The idea behind the MMSE TEQ
design method may be explained by Fig. 3.
The structure consists of an FIR equalizerin cascade with the channel and a parallel
branch that consists of a delay and an FIR
filter with a target impulse response (TIR).
The goal in the MMSE design of the vector
of TEQ taps (w) is to minimize the mean
square of the error between the output of
the equalizer and the output of the TIR.Assume that the error is zero for any given
input signal. That means the impulse
responses of both branches are equal. In
other words, the equalized channel impulse
response (upper branch) would be equal to
a delayed version of the TIR. Setting thenumber of taps of the TIR to a desired
length forces the equalizer channel impulse
response to have the same length.
Figure 3. Block diagram of Minimum
Mean-Squared Error (MMSE) Equalizer[6].
Fig 4. shows a TIR and Shortened
Impulse Response (SIR). The MMSE
design method formulates the square of the
difference between the TIR and SIR as the
error and minimizes it. The method
minimizes the difference between the TIRand SIR both inside and outside the target
window.
In fact, the difference between the
TIR and SIR inside the target window does
not cause ISI. Both the TIR and SIR inside
the target window have higher amplitudes,which mean that difference inside the target
window might contribute more to the MSE
than the difference outside.
Since the MMSE method in general
cannot force the error to become exactly
zero, some residual ISI will remain. To
maximize channel capacity, the residual ISIshould be placed in frequency bands with
high channel noise.
Figure 4. Target Impulse Response (TIR)
and Shortened Impulse Response (SIR)[6].
This ensures that the residual ISI
would be small compared to the noise andthe effect on the SNR would be negligible
[4]. The MMSE design method does not
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have a mechanism to shape the residual ISI
in frequency. MMSE algorithm is
implemented in chapter five and used to
eliminate ISI.
Table 1. Performance Comparison
between Selected TEQ algorithems [16].
Loop
CSA
Bit Rate as a percentageof Match Filter Bound
(MFB)
Rb(MFB)(Mbps)
GeneralMMSE
MSSNR MinISI
1 92% 62% 98% 8.45
2 90% 75% 97% 9.68
3 91% 82% 99% 8.11
4 92% 61% 98% 8.05
5 85% 72% 98% 8.53
6 93% 80% 99% 7.77
7 77% 74.2% 96.0% 7.75
8 56% 71% 99% 6.90
A comparison of selected TEQ algorithmscan be shown in Table 1 as percentage to
Match Filter Bound (MFB) which
represents perfect equalization, the number
of TEQ taps is 17. The Minimum Mean
Squared Error (MMSE) is the mostcommonly used in commercial ADSL
modems. MMSE design methods are
relatively easier to implement with adaptive
algorithms and are efficient in the sense ofcomputational complexity [4]. MMSEalgorithm is implemented in chapter five
and used to shorten test loop impulse
response.
D. Bit LoadingThe process of assigning information and
energy to each of the subchannels is calledbit loading in multichannel transmission
[12]. As was described before, the encoder
takes the data bit stream and encodes it into
N QAM constellation points for each
subchannel. This encoding is doneaccording to the bit loading table which
defines the number of bits carried by each
tone.
The ADSL system has the following rules
for bit loading as given in ANSI T1.413:
Only integer number of bits isallowed, if the resulted is notinteger, it is rounded to the least
integer.
The minimum number of bits thatcan be carried on any sub channel is
2 (4-QAM), so any subchannel that
cannot carry 2 bits is turned off.
The maximum number of bits thateach subchannel can carry is 15 bits,
if the resulted is greater than 15bits, it is replaced by 15 bits
In ADSL2 system, the same rule can be
applied except for the following, asdescribed in ITU G.992.3:
The minimum number of bits thatcan be carried on any sub channel is
1bit (2-QAM), rather than 2 bits for
ADSL, any subchannel that cannot
carry 1 bit is turned off.
E. Channel AnalysisThis section will discuss the process of
calculating ADSL signal to noise ratio
(SNR) which is needed to determine the
channel capacity.
During channel analysis processATU-C send signal know as Medly. This
signal consists of binary random data
transmitted to ATU-R. All subchannels are
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used and each subchannel is modulated
with 4-QAM signals at maximum allowable
power.
On the other side, the ATU-R willreceive the Medly signal but after the signal
was filtered by the channel. The ATU-R
will need also to estimate channel noises.
The received Medly signal power and its
calculated noise are used to calculate each
subchannel SNR value.Fig. 5 shows the ADSL downstream
mask. Using ADSL PSD mask, the total
power can be calculated, as stated
previously only ADSL downstream ATU-C
to ATU-R will be simulated. The used PSD
mask that is the downstream PSD over
POST with overlap spectrum so thebandwidth that will be used for
transmission is from 25.875 KHz to 1104
KHz.
Figure 5. ADSL Downstream PSD Mask.
The maximum transmitted power
PSD is -36.5 dBm/Hz but a margin of 3.5dBm is used as the filter pass band ripple as
was described in ANSI T1.413[17]. Theresulted maximum power will be -40
dBm/Hz. Hence the total power is given by:
Also
where
is the transmitted signal power
spectrum and represent subchannelpower.Using the generated noise and theknown channel impulse response, the SNR
for each of the DMT subchannels can be
calculated. Hence
where
is the SNR for the
subchannel,
is the channel gain
(frequency response) for the subchannel(which can be calculated by taking thesquare of the absolute value of the Fourier
transform of the channel impulse response), is the noise PSD for the subchannel, assuming free ISI channel. The
SNR for a channel without ISI is referred to
as
, where MFB stand for Match
Filter Bound. The average SNR for DMT
channel can be calculated using.
Where term is called the average SNRor geometric SNR (.
The Time domain equalizer (TEQ)
algorithm is not perfect and do not shorten
the channel to (v+1). The residual will
cause ISI and that will affect on the systemperformance. To calculate SNR that include
the residual ISI the following equation can
be used :
where is the equalizer output SNR, is the equalizer gain (frequencyresponse) and
is the ISI gain of the
residual ISI.
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The represents the equalizeroutput SNR which is used to calculate
channel capacity. In order to calculate the
efficiency of the equalizer, the total bit ratefor equalized channel and Match Filter
Bound channel using the geometric SNR of
the equalizer output and thegeometric SNR of the free ISI channel are used:
To calculate the number of bits that each
subchannel can carry, the default bit-loading algorithm is used to calculate thebit loading table as explained in subsection
D where :
Here, is the number of bits that can becarried on the subchannel and is themodulation gap.
For DSL application the targeted bit
error rate is , the modulationgap for QAM ( ) is 9.8 dB as definedin ANSI, and thus, the for a DSL system
is
where is the DSL margin, for standardADSL system the margin is 6 dB, is thecoding gain for the used error correcting
system, the code gain for ADSL system
with RS code is 4.2 dB and for ADSL2where Trills code is used the code gain is
5.5 dB as defined in section 4.5. The total
bit rate () can be calculated by using: where is the total number of transmittedbits in one frame and is the frame period.
III. Simulation of ADSL/ADSL2Process
The CSA loops modeled with impulseresponses consist of 512 samples and
sampled at a rate of 2.208 MHz using
Linemod[16], the ADSL channel noises, the
MMSE TEQ algorithm were all modeled
using MATLAB then ADSL channelanalysis is calculated finally the bit-loading
table is built, The MATLAB code were
assembled to create a program that
simulate the initialization process for
ADSL/ADSL2 modems, as stated before
the program simulate the downstream
ATU-C to ATU-R only.
As shown in Fig. 6, the program
consists mainly from 6 MATLAB file
codes, that simulate the ADSL/ADSL2
initialization process. Each file represent a
process that was explained in previoussubsections.
Table 2 shows the used parametersfor ADSL/ADSL2 initialization process
simulation.Table 2. ADSL/ADSL2 Test Parameters.
Parameters
Name
Value
Input Power 20 dBm
Number of
Equalizer Tap
19 tap
Margin 6 dB
Code Gain 4.2 dB for ADSL
5.5 dB for ADSL2
AWGN Power -140 dBm/Hz
NEXT Users 10
FEXT Users 10
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A GUI (graphical user interface) was built
to ease the use of the initialization program,IV. Simulation Results and
Discussion
Figure 6. ADSL/ADSL2 Initialization Program Structure.
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In this subsection the result for the ADSL
initialization on CSA test loop-1 is
presented. ADSL2 have similar results
except for bit loading table since ADSL2modem has higher code gain.
A. Channel Modeling ResultsADSL CSA loop-1 was simulated as shown
in figure (5.19) and filtered with high pass
filter to simulate POST splitter effect on the
channel effect. The impulse response of the
channel is much greater than 33 (v+1)
which is the maximum allowable value forADSL. For this reason, the Time Domain
Equalizer (TEQ) is required.
B.Noise Modeling ResultsUsing Eqs (1-5), the noise signals AWGN,
NEXT, FEXT were modeled. Fig. 7 shows
AWGN, NEXT and FEXT power spectrum.The Next Power is -45.593 dBm and FEXT
power is -71.475 dBm. Table 3 shows theNEXT and FEXT power for all loops.
It is noticed that NEXT power did
not change with the change of loops. This is
because NEXT is channel independent,
while FEXT is channel and couple length
dependent.
Since equalizer effect is not only on
the channel and signal but it also affect the
received noise and its residual ISI cause
noise, it called ISI noise.
C.Channel Equalization ResultsBy implementing MSSE an equalizer with
19 tap using MMSE algorithm was used to
shorten the CSA loop-1. The equalizer
shortens most of the channel impulse
response to the required length (v+1), the
remaining impulses will cause ISI as shown
in Fig 8. Table 4 shows the equalizer
efficiency for all CSA loops.
Table 3. CSA Loops NEXT and FEXT
Power.
CSA
Loop
NEXT Power
(dBm)
FEXT Power
(dBm)
10
Users
24
Users
10
Users
24
Users
Loop-1 -45.593 -43.28 -71.475 -69.143
Loop-2 -45.593 -43.28 -72.924 -70.5
Loop-3 -45.593 -43.28 -76.353 -74.157
Loop-4 -45.593 -43.28 -72.17 -69.86
Loop-5 -45.593 -43.28 -72.58 -70.373
Loop-6 -45.593 -43.28 -71.7 -69.497
Loop-7 -45.593 -43.28 -68.261 -65.981
Loop-8 -45.593 -43.28 -68.901 -66.60
Figure 7. AWGN, FEXT and NEXT Power
Spectrum.
It is noticed that the equalizers efficiency is
less than that given in section II, since the
equalizer efficiency depends on the
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transmitted power, AWGN power, NEXT
users and FEXT users. Changing of these
parameters will change the equalizers
efficiency.
Figure 8. Comparison between Channel
and Equalized Channel Impulse Response.
Table 4. CSA Loops Equalizer Efficiency.
CSA Loop
Name
Equalizer Efficiency
( ) (%)Loop-1 81.2
Loop-2 89.8
Loop-3 93.1
Loop-4 80.0
Loop-5 85.1
Loop-6 87.1
Loop-7 70.8
Loop-8 85.1
D.Channel Analysis ResultsThe used transmitted signal spectrum
during the SNR calculation is shown in Fig
9. The average transmitted is 20 dBm
which is the maximum allowabletransmitted power.
The signal to noise ration SNR can
be calculated using Eq. 9 and Eq. 11
for and respectively.
Using Eq. 10, the Geometric MFB
(Match Filter Bound) SNR () forCSA loop-1 is 40.182 dB, Geometric
Equalizer SNR ( ) is 32.58 dB andthe Equalizer Efficiency is 81.2 %. Table 5shows the achieved equalizer geometric
SNR ( ) and Match Filter boundSNR ( ) for all CSA loops.
Figure 9. Transmitted Signal Power
Spectrum.
Table 5 CSA Loops Calculated and .CSA
Loop
Geometric
SNR
( )(dB)
Match
Geometric
SNR
(
)
(dB)Loop-1 32.58 40.123
Loop-2 39.57 44.1
Loop-3 36.5 39.21
Loop-4 31.36 39.2
Loop-5 34.39 40.41
Loop-6 33.31 38.24
Loop-7 26.96 38.08
Loop-8 29.8 35.01
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The loops equalizer output SNR
depends on the equalizer efficiency and
more efficient equalization algorithm canachieve better SNR.
.
E.Bit Loading Table ResultsThe bit loading table is calculated by using
Eq. 13 and applied the rules of
ADSL/ADSL2. Using Eq. 15 the total bit
rate for CSA Loop-1 is 6.62 Mbps. Table 6
shows a comparison between ADSL andADSL2 Total Bit Rate.
Table 6 Comparison between ADSL and
ADSL2 Total Bit Rate over CSA Loops.
CSALoops
ADSLTotal Bit Rate
Rb (Mbps)
ADSL2Total Bit Rate
Rb (Mbps)
Loop-1 6.620 7.08
Loop-2 9.02 9.42Loop-3 7.94 8.36
Loop-4 6.2 6.68
Loop-5 7.22 7.68
Loop-6 6.86 7.32
Loop-7 4.6 5.09
Loop-8 5.66 6.07
It is noticed that ADSL2 achieves
better bit rate than ADSL in about 400Kbps which is about 1.1% because it havehigher code gain and more flexible bit
loading rules.
V. ConclusionModling standard loops has important role
in expecting the performance of the systems
before real system installation, it provide
information about potential system
performance, Time Domain Equalizer
(TEQ) can shorten the impulse response todesirable system impulse response length,
for the MMSE algorithm as TEQ algorithm,
TEQ can achieve up to 93% of zero ISI
channel. Also Time Domain Equalizer
(TEQ) increase noise level especially
higher frequencies which cause BER, usingadaptive TEQ can mitigate noise
enhancement, Noise effects on bit rate,
where higher AWGN level, more FEXTand NEXT interfered users causes higher
noise power which leads to reduce bit rate.
NEXT is the dominant channel impartment
compared to AWGN and FEXT, whereNEXT have higher power and narrow
bandwidth, Bit loading algorithm determine
the number of bits that can be carried on
each subchannel, using Default bit loading
algorithm ensure equal probability of error
on all subchannels, where it is fast andsimple and can be done offline, ADSL2
achieve higher bit rate than ADSL by more
than 400 Kbps on CSA loops also ADSL2
have higher code gain and more flexible bitloading role.
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