CROSSTALK NOISE AND INTERFERENCE NOISE
IN xDSL TELECOMMUNICATION SYSTEMS
Zoran R. Petrović1, Milan Lj. Janković2 and Miroslav L. Dukić1
1
Faculty of Electrical Engineering, Belgrade, Bul.Revolucije 73
2
Community of Yugoslav PTT, Belgrade, Palmotićeva 2
ABSTRACT - The
basic
requirements
in
modern
telecommunications are related to the usage of the interactive
broadband services, such as the access to Internet with the
wide bandwidth, videoconference etc. One of the possible, and
today very attractive solutions is the application of xDSL
technology in realization of the access networks via standard
copper telephone lines. This article presents an overview of the
operation conditions of the xDSL systems provided that the
dominant are: crosstalk noise and interference noise (impulse
noise and radio noise). The results of the analysis show that in
real work conditions the additional signal processing is
necessary first of all at the level of line coding, in order to
achieve the necessary transmission quality in xDSL systems.
1. INTRODUCTION
The improvement of the quality of everyday life and a
growing volume of surrounding information to be processed
requires much faster access to existing and future services.
Visionaries have spoken of a future where the common
person has instanteneous access to data spread around the
globe. Engaging in a live videoconference, or perhaps
watching a personalized newcast are just two of examples of
many. For this vision to become reality, a global broadband
information infrastructure must be built that provides low-cost
access to the customers and sources of information. What
connects to virtually every home and business in the
industralization world? Phone lines connect more than 700
million sites today. Data rates of several kb/s are possible over
phone lines using dial-up modems. This is enough to spark the
appetite of the Internet surfer but is not nearly enough to satisfy
the desire for immediate information on demand. Similarly,
video and audio applications at dial-up modem data rates leave
users demanding more.
Telecom service users are becoming increasingly interested
in a high-speed access to Internet services and databases
uploaded with various kind of multimedia content. The access
to such sophisticated and resource-challenging services has to
be supported by a comparable advancement in the "last
kilometer" communication technologies.
Broadband copper transmission (xDSL), targeted mainly
for residential users, has received much attention from telecom
operators in the past few years. xDSL is the acronym for the
family of Digital Subscriber Line technologies, where the “x”
represents the various forms of these technologies:
Asymmetric DSL (ADSL), Rate Adaptive DSL (RADSL),
High-bit-rate DSL (HDSL), Symmetric DSL (SDSL), Veryhigh bit rate DSL (VDSL), etc. [1].
xDSL services are dedicated, point-to-point, public
network access technologies that allow multiple forms of data,
voice, and video to be carried over twisted-pair copper wire on
the local loop (“last kilometer”) between a network service
provider’s (NSP’s) central office and the customer site, or on
local loops created either intra-building or intra-campus. xDSL
is expected to have a significant impact by supporting highspeed Internet/intranet access, online services, video-ondemand, TV signal delivery, interactive entertainment, and
voice transmission to enterprise, small office, home office, and,
ultimately, consumer markets. The major advantage of highspeed xDSL services is that they can all be supported on
ordinary copper telephone lines already installed in most
commercial and residential buildings.
There are many types of noises that couple through
imperfect balance (or imperfect/insufficient twisting) into the
phone line. In the last few years various investigations
concerning the use of residential power lines as a high-speed
communication medium. Because of limited bandwidth and a
very high level of time variant signal attenuation and high
levels of noise, communication system must be capable of
fighting the frequency selective and broadband interference.
Very sophisticated digital signal processing and transmission
are very useful in these conditions. This paper provides an
overwiev, of the most common, which are crosstalk noise,
impulse noise and radio noise.
The paper is organized as follows. In Sec.2 xDSL system
reference model is presented. The main characteristics of
impairments for xDSL are summarized in Sec. 3, while the last
section contains concluding remarks
2. xDSL SYSTEM REFERENCE MODEL
Based on an analysis of the generalised telecommunication
network configuration, shown in Fig 1, which is used for the
development of the access network technologies and models,
xDSL system reference model is shown in Fig 2, [2,3].
Customer
Network
Public Network
Access Network
Core
Network
Local
Switching
Local
Distribution
Final
Drop
Basic Interface
Fig.1 - Generalized network configuration.
Customer
Premisis
Equipment
(CPE)
VC
U-C2 U-C
VA
U-R2
T-SM
•
T
B
Splitter
Digital
Broadcast
ATU-C
Broadband
Network
ATU-C
Narrowband
Network
ATU-C
Network
Management
U-R
ATU-C
Loop
T.E.
ATU-R
T.E.
T.E.
POTS-C
PSTN
Access
Node
POTS-R
Phone(s)
T.E.
Premises
Distribution
Network
Fig.2 - xDSL system reference model.
Legend
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
ATU-C: xDSL Transmission Unit at the network end.
The ATU-C may be integrated within an Access Node.
ATU-R: xDSL Transmission Unit at the customer
premises end. The ATU-R may be
integrated within
an SM.
Access Node: Concentration point for Broadband and
Narrowband data. The Access Node may be located at a
local exchange or a remote site. Also, a remote Access
Node may subtend from a central access node.
B: Auxiliary data input (such as a satellite feed) to
Service Module (such as a Set Top Box).
Broadcast: Broadband data input in simplex mode
(typically broadcast video).
Broadband Network: Switching system for data rates
above 2 Mb/s.
Loop: Twisted-pair copper telephone line. Loops my
differ in distance, diameter, age, and transmssion
characteristics depending on network.
Narrowband Network: Switching system for data rates at
or below 2 Mb/s.
POTS: Plain Old Telephone Service.
POTS-C: Interface between PSTN and POTS splitter at
network end.
POTS-R: Interface between phones and POTS splitter at
premises end
PDN: Premises Distribution Network: System for
connecting ATU-R to Service Modules. May be point-topoint or multipoint; may be passive wiring or an active
network. Multipoint may be a bus or star.
PSTN: Public Switched Telephone Network.
SM: Service Module: Performs terminal adaptation
functions. Examples are Set Top Boxes, PC interfaces, or
LAN router.
Splitter: Filters which separate high frequency (xDSL)
and low frequency (POTS) signals at network end and
premises end. The splitter may be integrated into the
ATU, physically separated from the ATU, or divided
between high pass and low pass, with the low pass
function physically separated from the ATU.
The
provision of POTS splitters and POTS-related functions is
optional.
T-SM: Interface between ATU-R and Premises
Distribution Network. May be same as T when network is
point-to-point passive wiring. An ATU-R may have more
than one type of T-SM interface implemented (e.g., a
T1/E1 connection and an Ethernet connection). The TSM interface may be integrated within a Service Module.
•
•
•
•
•
T: Interface between Premises Distribution Network and
Service Modules. May be same as T-SM when network is
point-to-point passive wiring. Note that T interface may
disappear at the physical level when ATU-R is integrated
within a Service Module
U-C: Interface between Loop and POTS Splitter on the
network side. Defining both ends of the Loop interface
separately arises because of the asymmetry of the signals
on the line.
U-C2: Interface between POTS splitter and ATU-C.
U-R: Interface between Loop and POTS Splitter on the
premises side.
U-R2: Interface between POTS splitter and ATU-R.
VA: Logical interface between ATU-C and Access Node.
As this interface will often be within circuits on a
common board, the xDSL Forum does not consider
physical VA interfaces. The V interface may contain
STM, ATM, or both transfer modes. In the primitive case
of point-to-point connection between a switch port and an
ATU-C (that is, a case without concentration or
multiplexing), then the VA and VC interfaces become
identical (alternatively, the VA interface disappears).
VC: Interface between Access Node and network. May
have multiple physical connections (as shown) although
may also carry all signals across a single physical
connection. A digital carrier facility (e.g., a SONET or
SDH extension) may be interposed at the VC interface
when the access node and ATU-Cs are located at a remote
site. Interface to the PSTN may be a universal tip-ring
interface or a multiplexed telephony interface such as
specified in ITU-T Recommendation G.964, or ETSI 300
324. The broadband segment of the VC interface may be
STM switching, ATM switching, or private line type
connections.
3. IMPAIRMENTS FOR xDSL
Subscriber loops have been under study for many years and
different models exist to describe important parameters such
as:
• Cable type (wire diameter, isolation material);
• Cable length;
• Loop structure (load coils, bridged taps);
• Noise sources (crosstalk, impulse noise, radio
frequency interference).
For analogue voice frequency signals normally the
attenuation based on the wire diameter determines the length of
the subscriber loop. Load coils are used in same cases to
extend the range. For digital signals with bandwidths
exceeding voice frequencies normally attenuation, crosstalk
and phase delay limit subscriber loop lengths. In addition
impulse noise can influence the range.
The impairments that high-speed digital services must deal
with on the copper loop are well understood. All xDSL
schemes increase the normal, modest analog voice passband,
sometimes to about 400kHz or so and as far as 1.1MHz. The
impairments at these frequencies fall into two main categories:
physical and electrical. Physical impairments arise from the
local loop being engineered and optimized for analog voice.
Service providers added load coils to extend the reach of voice
passband but at some time choke off higher frequencies. In
many cases, especially outside of the United States, mixed
diameters were used instead of load coils for same purpose,
although mixed diameters are still common in the United
States as well. Bridged taps connecting to laterals made it
easier to add and delete customers, but resulted in unterminated
branches that reflect signals and cause interference at the
higher xDSL frequencies. Splices between different wires bend
and flex in the wind, causing "micro-outages" of short
duration. And splices are not always done properly among
pairs within the same bundle. Although each pair is colorcoded, various factors make it possible to splice wires from
separate pairs together. Such split pairs are seldom fatal for
voice, but a real problem for digital services. Short but
untwisted drop cables to the premises had minimal effect on
overall voice quality, but are a concern in xDSL environments.
Dampness is always a problem, whether caused by rain (aerial
cables at risk) or ground water (buried cable at risk). Finally,
the use of proprietary line extenders and digital loop carrier
(DLC) pose additional problems for xDSL mainly due to their
operation only in the voice passband.
There are significant differences between the United States
copper access infrastructure and the rest of the world. In the
United States, heroic measures are taken to provide "universal
service" to anyone who wants it, resulting in many extremely
long loops and creative methods which conspire to defeat easy
conversion to xDSL. Outside of the US, remote areas are more
often served by public phones on shorter loops, and there are
fewer local exchanges per capita. Load coils are rarer on access
lines outside US, and bridged taps are more often the result of
installation oversights than of systematic planning.
Electrical loop impairments are also called interferers or
disturbers. At xDSL frequencies, a major concern is radio
frequency interference (RFI). Many amplitude modulation
(AM) radio stations broadcast in the same range as xDSL
methods operate. Aerial wires are much more at risk,
especially in areas, where AM stations are particularly dense.
Even amateur radio can be a concern for xDSL schemes that
operate on aerial cable above 1 MHz or so.
3.1 NOISE
Examples of intrinsic noise impairments are thermal noise,
echoes and reflections, attenuation and crosstalk. There are
also other components that reside in the cable infrastructure
that can impair the operation of xDSL systems. These include
surge protectors, radio frequency interference (RFI) filters,
and, in some networks, bridged taps and loading coils. Another
intrinsic impairment is the condition of the cable infrastructure,
which exhibits faults such as split pairs, bunched pairs, leakage
to ground, low insulation resistance, battery or earth contacts,
and high-resistance joints. All these impairments reduce DSL
performance.
Examples of extrinsic impairments are impulsive noise
originating from lightning strikes, electric fences, power lines,
machinery, arc welders, switches, fluorescent lighting, and so
on. There is also radio interference from broadcasting and
radio transmitters.
The noise sources mentioned above can alternatively be
classified as capacity or performance limiting. Capacity
limiting noise is usually slowly changing, such as thermal
noise and crosstalk. These noise levels are often predictable
and relatively easy to take into account when the telco creates
deployment-planning rules.
Performance limiting noise, such as impulses and RFI, is
intermittent in nature. It is geographically variable and
unpredictable, and therefore is usually accounted for in
planning rules by using a safety margin. xDSL systems seek to
use additional signal processing, such as error correction with
interleaving and adaptive line codes, to mitigate such sources
of noise.
3.1.1 IMPULSE NOISE
Impulse noise is nonstationary crosstalk from temporary
electromagnetic events in the vicinity of phone lines. Examples
of impulse generators are as diverse as the opening of a
refrigerator door (the motor turns on/off), control voltages to
elevators (phone lines in apartment buildings often run through
elevator shafts), and ringing of phones on lines sharing the
same binder. Each of these effects is temporary and results in
injection of noise into the phone line through the same basic
mechanism as RF noise ingress, but typically at much lower
frequencies.
Differential (metallic) induced voltages are typically a few
millivolts, but can be as high as 100mV. Such voltages may
sound small, but the severe attenuation of high frequencies on
twisted pair means that an impulse can appear enormous to a
receiver in comparison to received xDSL signal levels. The
common-mode voltages caused by impulses can be 10s of
volts in amplitude. Typical impulses last 10s to 100s of
microseconds but can span time intervals as long as 3ms.
Numerous studies of impulses have resulted in both
analytical models for impulses based a statistical analysis of
over 105 impulses by various groups. However, others insist
that impulses defy analysis and prefer to just store
representative worst-case waveforms. The area of impulse
modeling thus remains controversial, probably because the
causes of impulses are so diverse that any distillation for
engineering and measurement purposes necessarily has some
bias. The most widely used analytical model is the COOK
pulse [4]. Cook recovered over 105 impulses and via computer
analyzed some 89000 of them on many different phone lines as
a basis for this model of the next subsection. The ADSL
standard, however, uses two measured impulses instead of the
Cook pulse and another empirical formula from Bellcore to
relate test results to performance.
3.1.2 RADIO NOISE
Radio noise is remnant of wireless transmission signals on
phone lines, particularly AM radio broadcasts and amateur
(HAM) operator transmissions. Radio frequency (RF) signals
impinge on twisted-pair phone lines, especially aerial lines.
Phone lines, being made of copper, make relatively good
antennae with electromagnetic waves incident on them leading
to an induced charge flux with respect to earth ground. The
common mode voltage for a twisted pair is for either of the two
wires with respect to the ground - usually these two voltages
are about the same because of the similarity of the two wires in
a twisted pair. Well-balanced phone lines thus should see a
significant reduction in differential RF signals on the pair with
respect to common-mode signals. However, balance decreases
with increasing frequency, and so at frequencies of xDSLs
from 560kHz to 30MHz, xDSL systems can overlap radio
bands and will receive some level of RF noise along with the
differential xDSL signals on the same phone lines, [5]. This
type of xDSL noise is known as RF ingress.
3.2 CROSSTALK
Crosstalk causes by the far the largest contribution to
capacity limiting noise for xDSL systems, so it is worth
examining in a little more detail here. There are two very
different types of crosstalk in multipair access network cables,
near-end crosstalk (NEXT) and far-end crosstalk (FEXT), as
shown in Fig.3.
3.3 POWER DISTRIBUTION NETWORK NOISE
During the last years the market of the Local Area Network
(LAN), and also Wide Area Network (WAN), exploded. The
main goal of these communications systems is to enable a
high-speed (up to 10Mbits/s) communication services without
the necessity of additional cabling and additional interface
(wall socket). Also, the cessation of the telecom monopoly for
the national PTT’s on January 1, 1998 and deregulation in
energy market initiated significant changes concerning fixed
network telecommunications. Currently alternative fast
communication links over so-called ‘last mile’ are of major
interest to establish real competition. The electrical distribution
power grid could turn out to be an ideal candidate to cope with
the existing standard communication links based on copper
wiring.
There are three standards from three different parts of the
world, Europe, Japan and USA.
In Europe residential power line communications is
allowed in frequency band 3-148.5kHz. This band is divided
into 5 sub-bands, one of which, the so-called A sub-band (995kHz) is to be used by electricity suppliers and their licenses.
Fig.3 - Illustration of crosstalk, [5].
NEXT is interference that appears on another pair at the
same end of the cable as the source of interference. Its level is
substantially independent of the length of the cable. FEXT, on
the other hand, is interference that appears on another pair at
the opposite or far end of the cable to the source of the
interference. Its level is attenuated at least as much as the
signal itself if both have traveled the same distance.
NEXT affects any systems which transmit in both
directions at once (e.g., echo-canceling systems), and where it
occurs it invariably dominates over FEXT. NEXT can in
principle be eliminated by not transmitting in both directions in
the same time, separating the two directions of transmission
into either nonoverlapping intervals in time or nonoverlapping
frequency bands. This is how xDSL systems attempt to avoid
self-NEXT by using frequency or time-division duplexing.
Fig.4 shows the crosstalk spectra of upstream ADSL,
HDSL and ISDN.
Fig.4 - xDSL downstream crosstalk psd ( power spectral
density), [5].
In Japan the available frequency band is limited from 10450kHz by the regulation of the Ministry of Posts and
Telecommunications (Final Report of the Study Group for
Power Line Communications, July 1986).
FCC’s (Federal Communications Commission) rule No. 15
allows communications over power line, in USA, out of AM
band (535-1705kHz). In the past the mostly used band is 50500kHz.
Today in Europe started investigations in frequency band
200kHz-20MHz, [6].
However, at the higher frequencies that are to be used for
communications a very high level of time-variant signal
attenuation and high levels of non-Gaussian noise characterize
the low voltage network.
The noise sources at the residence, listed in order of
strength are:
• Appliances at the residence;
• Appliances at a neighbor’s residence connected to the
same distribution transformer;
• Background noise entering the residential circuit from
the primary side of distribution transformer and
perhaps from electromagnetic radiation.
The noise can be modeled as a summation of four noise
types:
• Background noise: always present, and its spectral
density is a decreasing function of the frequency.
• Impulse noise: due to all kinds of switching
operations noise bursts can be expected on the
channel. The very powerful noise bursts normally
taking no longer then 0.1ms and they having
approximately a Poison distribution.
• Noise synchronous to the power system frequency
(i.e. 50Hz, 60Hz). This kind of noise is mainly caused
by silicon-controlled rectifiers, causing noise having
large peeks in the power spectral density at multiplies
of the power system frequency. Levels are not exceeds
–45dBW per harmonics.
•
Noise at frequencies unrelated to the power system
frequency. This type of noise is predominantly caused
by horizontal retrace pulses in the video signal of TVsets and computer monitors with levels up to –45dBW
per harmonics.
The second very important channel characteristic is
attenuation. Measurements are show that attenuation on
residential power line exceeds 20dB.
The attenuation on the channel could be divided into two
parts:
• Coupling losses, i.e. signal power lost due to the
interaction of the capacitive network coupling circuits
and the (often very low) channel impedance. Coupling
losses were found to be very high (up to 40dB), highly
frequency dependant (variations of 30dB within the A
sub-band are unusual) and different for each set of
transmitter/receiver locations.
• Line losses, i.e. signal power lost due to the leakage of
signal into undesired directions. If increase linearly (in
dB) with the distance between transmitter and receiver
and amounts on average 80dB/km, with the worst-case
values of the over 100dB/km.
4. CONCLUSIONS
xDSL technology encompasses a wide class of point-topoint digital access transmission methods over voice-grade
single pair copper wires. The enabling technologies are highly
sophisticated digital signaling processor (DSP) techniques and
advanced modulation methods, such as:
• Discrete Multi-Tone (DMT) – a multicarrier
transmission technique that uses a Fast Fourier
Transform (FFT) and Inverse FFT to allocate the
transmitted bits among many narrowband QAM
modulated tones depending on the transport capacity
of each tone.
• Carrierless Amplitude and Phase (CAP) – line code
similar to QAM,
which extend the potential data transmission rates into the
broadband range. For any of the modulation schemes the xDSL
design parameters can be traded off for a given level of
performance. Both CAP and DMT xDSL modems use line
coding to reduce the transmitted symbol rate to confine the
spectrum in the lower frequency, which corresponds to lower
signal loss. For shorter loops with less loss, the upper spectral
range of the data transmission channel can be extended to send
the symbols faster, which increases throughput.
xDSL implementation can also be divided into two broad
categories: baseband or passband. Baseband systems use a
simple line coding technique to transport data in the 0-100kHz
band. For this reason, baseband systems cannot coexist with
analog telephony on the same pair. Digital passband systems
generate two or more channels well above the baseband by
amplitude and phase modulation plus filtering, leaving the
baseband free to support legacy voice/modem service. In spite
of their relatively simple coding scheme, implementing
baseband systems is complex because of the need for hybrid
circuitry to couple the transmitter and receiver to a single
twisted pair such that one does not interfere with the other.
Furthermore, echo cancellers must be used to detect the
presence of echoes due to line imperfections.
Recently, a series of technological improvements and
standardization initiatives have been launched to remove many
of the practical barriers to rapid adoption of xDSL technology
in the customer arena. The common threads running through
all these new xDSL proposals are low speeds and splitterless
implementations.
Low speed xDSL - Low speed versions of xDSL limit the
downstream speed to 1.5-2.0Mb/s and upstream rates to 100200kb/s or so. The major change from previous
implementations is a big decrease in the high-frequency
spectra. Either CAP or DMT modulation may be used.
Lowering the line rate has several benefits for manufacturers
and service providers. Most obviously, line span range is
extended by operating in a lower-loss region. Thus, it becomes
easier for service providers to offer uniform predictable service
over their range of loop lengths. Lower top-end frequencies
also theoretically mean less risk of interference with other
traffic on the cable. Some low-speed DMT implementations
could choose to separate upstream and downstream channels,
eliminating the need for echo cancellation.
Splitterless xDSL - A splitterless xDSL modem allows
direct attachment to the household wiring plant just like an
analog modem. A high-pass filter is incorporated on the DSL
modem to reduce the DSL low-frequency energy and shield
the modem from the voice frequency band. Transmitter power
is also reduced to allow meeting the objectives for voice
frequency band interference. However, the reduced power
level imposes limitations on the upstream speed, since span
length and bit rate are directly traded off against each other. As
a result, splitterless xDSL systems tend to have upstream rates
in the 100-200kb/s range for spans in the 6000m range.
Because splitterless xDSL modems lack a low-pass filter, they
must provide better equalization than before to react to line
conditions. High-speed splitterless ADSL is feasible and is
being readied for the market by several vendors.
G. Lite - G. Lite is a low-speed splitterless ADSL solution
being developed in the International Telecommunication
Union - Telecommunication Standardization Sector (ITU-T). It
is intended to support speeds of 96kb/s to possibly 256kb/s
downstream, with 32-128 kb/s upstream. Data rates as high as
1.5-2.0 Mb/s of downstream bandwidth, and more modest rates
upstream, are also envisioned. G.Lite also has the support of
the Universal ADSL Working Group (AWG) a congregation
of software and hardware computer vendors, network element
vendors and operating companies interested in the quick
development of a universal ADSL standard for high-speed
Internet access. Efforts like G. Lite are highly significant
because they attempt to eliminate many practical installation
problems, and, coupled with lower speeds, remove some of the
risks of introducing new services into an already complex
mixture carried by the feeder network.
REFERENCES
[1] Z. Petrović, ‘Digitalne pretplatničke petlje’, uvodno
predavanje, TELFOR99, Beograd, novembar 1999.
[2] ITU-T, ‘Study group 15-Report R51’, COM 15-R51E,
Geneva, September 1999.
[3] ADSL Forum, ‘ADSL Forum System Reference Model’,
TR-001, May 1996.
[4] J. Cook et al., ‘The Noise and Crosstalk Environment for
ADSL and VDSL Systems’, IEEE Communication
Magazine, Vol.37, No.5, May 1999.
[5] S. Thomas, J. Cioffi, P. Silverman, ‘Understanding
Digital Subscriber Line Technology’, Prentice Hall PTR,
Upper Saddle River, NJ 07458, 1999.
[6] K.M. Dostert, ‘Power Lines as High Speed Data
Transmission Channels-Modeling the Physical Limits’,
Proc. of IEEE ISSSTA 1998, Sun City, S. Africa,
pp.585-589
[7] G. Marubauashi and S. Tachikawa, ‘Spread Spectrum
Transmission on Residential Power Line’, Proc. of IEEE
ISSSTA 1996, Mainz, Germany, pp. 1082-1086
[8] M. Ferro et al., ‘VDSL Technology and its relationships
with Electromagnetic Compatibility’, Proc. of ISSLS
1998, Venice, Italy, March 1998.
[9] M. Janković, Z. Petrović, M. Dukić, ‘A Techno-Economic
Study of the Broadband Access Network Implementation
Models’, Proc of DRCN2000, Munich, Germany, April
2000.
SADRŽAJ: Osnovni
zahtevi
u
savremenim
telekomunikacijama odnose se na korišćenje interaktivnih
širokopojasnih servisa, kao što su pristup Internetu velikim
protokom, video konferencija i sl. Jedno od mogućih i danas
vrlo atraktivnih, rešenja je primena xDSL tehnologije u
realizaciji mreža za pristup preko standardnih telefonskih
kanala. U ovom radu je izložena analiza uslova rada xDSL
sistema u uslovima dominantnog prisustva šuma preslušavanja
i šuma interferencije; impulsni šum i radio šum. Rezultati
analize ukazuju da je u realnim uslovima rada, neophodno
dodatno procesiranje signala, pre svega, na nivou linijskog
kodiranja, da bi se u xDSL sistemu ostvario potreban kvalitet
prenosa.
ŠUM PRESLUŠAVANJA I ŠUM INTERFERENCIJE U
xDSL TELEKOMUNIKACIONIM SISTEMIMA
Zoran Petrović, Milan Janković, Miroslav Dukić