1. World Academy of Science, Engineering and Technology 62 2012
Design and Performance Evaluation
Of Modulation Techniques Suitable
For ADSL modem.
Mr. Ashraf S M Touba Prof. Ivan Bagaric
technologies. Each of the DSL technologies mixes
Abstract — One of the significant problems in ADSL modem different. Asymmetrical digital subscriber line (ADSL) is a
design is the demand for bandwidth for high speed data communication system that transfers both analog and
communications and interactive media transmission which digital information on a copper wire pair. The analog
has been growing explosively. The objective of this paper , is information can be a standard POTS or ISDN signal. The
to achieve the high bit rate in demand of bandwidth which
maximum downstream digital transmission rate (data rate
depends on design and evaluating the Modulation /
Demodulation – Regardless of the including any types of
to the end user) can vary from 1.5 Mbps to 9 Mbps
coding techniques and Filters - and simulated reaching to the downstream and the maximum upstream digital
appropriated technique which will has been considered to transmission rate (from the customer to the network) varies
design requirements of ADSL Up/Down stream specifications from 16 kbps to approximately 800 kbps. The data
using Matlab coding . transmission rate varies depending on distance, line
Keywords — ADSL ,Up /Down stream, Mathlab, Bit rate. distortion and settings from the ADSL service provider.
I. INTRODUCTION II. ADSL TECHNOLOGY.
Digital subscriber line is the transmission of digital A. What does meaning of ADSL Modem?
information, usually on a copper wire pair. Although the The idea behind the Asymmetrically that In ADSL, the
transmitted information is in digital form, the transmission data throughput in the upstream direction, (the direction to
medium is usually an analog carrier signal (or the the service provider) is lower, hence the designation
combination of many an analogue carrier signals) that is of asymmetric service, Figure 1 shown the means of
modulated by the digital information signal. Digital Asymmetrically ;the digital data sent over analogy media
subscriber line (DSL) was first used in the 1960s to that enables faster data transmission over copper telephony
describe the T-1 circuits that were extended to the line than a conventional voice band modem can provide. It
customer premises. Later the same term was used to does this by utilizing frequencies that are not used by a
describe ISDN basic rate interface (BRI) (2B+D, 144 voice telephone call that will being the subscriber line
Kbps) and primary rates interface (PRI) (23B+D, 1.544 same to on the lines. In general; the speed factors depend
Mbps). There are several different digital subscriber line type and thickness of copper wire ,situation of carrying
technologies. Each of these DSL technologies usually has a wire (external noises and interfaces)and depends on the
prefix to indicate the specific variant of DSL technology. distance from the local exchange , maximum range of
Hence, the “x” in xDSL indicates that there are many
ADSL without a repeater is 5.5Km,as a distance decreases
forms of xDSL technology. DSL transmission allows high-
toward the telephone company office the data rate
speed data transmission over existing twisted pair
telephone wires. This has the potential providing high- increases .
speed data services without the burden of installing new In July 2002, the ITU completed G.992.3 and G.992.4,
transmission lines (e.g., for Internet access). DSL service two new standards for ADSL technology collectively
dramatically evolved in the mid 1990s due to the called “ADSL2”. In January 2003, as users of ADSL
availability of new modulation technology and low cost chipsets based on the first generation of ADSL standards
electronic circuits that can do advanced signal processing passed the 30-million mark, G.992.5 officially joined the
(e.g., echo canceling and multiple channel demodulation). ADSL2 family as ADSL2plus, (or ADSL2+ as it is
This has increased the data transmission capability of commonly known).Several other features and
twisted pair copper wire to over 50 Mbps. improvements were also incorporated in the form of new
The data transmission capability of a DSL system varies Annexes. Carriers, service providers,[6] and subscribers
based on the distance of the cable, type of cable used, and have played a key role in the completion of ADSL2,
modulation technology. There are several different DSL having provided valuable feedback from the field that the
ITU in turn incorporated into the standards in the form of
Ashraf Touba PhD student, University of Singidunum ,Belgrade ,Serbia new features and performance improvements.
(E-mail: asmg28@gmail.com).
Ivan Bagaric ,Full professor ,University of Singidunum ,Belgrade,Serbia
(E-mail: ibagaric@singidunum.ac.rs).
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2. World Academy of Science, Engineering and Technology 62 2012
Fig.2. Constellation diagram for MPSK
(Illustrated for M=8).
Fig.1. The Downstream/Up stream of ADSL Modem
Each message point is spaced (2π/M) distance from its
adjacent message point. The receiver block diagram of
B. The Modulation schemes are used in ADSL MPSK is given below:
Modem.
Digital modulation schemes are classified into two large
categories: constant envelope and non-constant envelope.
Under constant envelope class, there are three subclasses:
FSK, PSK, and CPM. Under non-constant envelope class,
there are three subclasses: ASK, QAM, and other non-
constant envelope modulations. In M-ary phase shift
keying systems, the phase of the carrier is allowed to take Fig.3. Receiver Block diagram for MPSK.
on one of the (M) possible values;
Since the in-phase and quadrature components of M-ary
(1) QAM are independent, the Pc(1-Pe)2 where the Pe is the
probability of error of either component. The constellation
Thus (M) possible signals that would be transmitted during for the in-phase (or) quadrature component has geometry
each has signaling interval of similar to (PAM) with corresponding number of amplitude
(T) is: levels.
Consider a quaternary signaling scheme in which the
(2) received signal is defined by x(t) =ai+w(t);0≤ t ≤T Where
the amplitude ai = ± a/2 ± 3a/2 and w(t)is the sample
function of a white Gaussian noise process of zero mean
E =energy per symbol and usually M is taken as power of 2
and power spectral density N0/2. The signal space,
i.e. M =2k .
representing the signal component of x(t) has two
Recognizing that each Sk(t) may be written in terms of a
characteristic features. It is one-dimensional in that the
sinusoid and co-sinusoid , which are orthogonal and then
signal component is scaled of time function;
suitably scaling to fulfill the conditions of
(3)
(8)
The appropriate form for the orthogonal waveforms
φi(t) and φj(t) is given by : There are four message points,
(9)
(4)
(5)
The signal space diagram is as depicted below:
The coordinates of the message points can be calculated by
(6)
(7)
A constellation diagram is a representation of a signal
modulated by a digital modulation scheme. It displays the
signal as a two-dimensional scatter diagram in the complex Fig.4 Signal Space diagram for M -ary QAM.
plane at symbol sampling instants thus will given as :
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3. World Academy of Science, Engineering and Technology 62 2012
C. Spectrum (Bandwidth) Requirements. meet the (ITU G.992.4) as standard for splitter less
The signal bandwidth for the communications channel ADSL2 with data rate mandatory capability reduced to
needed depends on the symbol rate, not on the bit rate. 1.536 Mb/s downstream , 512Kb/s Up stream. That
An example of how symbol rate influences spectrum modem considered the 512-QAM Modulation in Upstream
requirements can be seen in eight-state Phase Shift Keying and 1024-QAM downstream in AWGN channel without
(8PSK). It is a variation of PSK. The phase of the signal any types of filters, Error correction code and
can take any of eight values at any symbol time. Since 23= decode…etc.
8, there are three bits per symbol. This means the symbol
rate is one third of the bit rate. This is relatively easy to
decode. Bandwidth efficiency describes how efficiently the
allocated bandwidth is utilized or the ability of a
modulation scheme to accommodate data, within a limited
bandwidth, as shown in ( table 1) below the theoretical
bandwidth efficiency limits for the main modulation types;
as we have more number of bits per symbol means the
modulation will increasing in the order and the following
bandwidth efficiency of power spectrum density has
Fig.6. Modem design of M-QAM
reduced powerfully as shown in (Figure 5)
Table 1. Bandwidth efficiency corresponding to
the symbol rate .
Modulation Theoretical Bandwidth
Format Efficiency limits
BPSK 1 bit/sec/Hz
QPSK 2 bit/sec/Hz
8 PSK 3 bit/sec/Hz
16 PSK 4 bit/sec/Hz
32 PSK 5 bit/sec/Hz
Fig.7(a) Time domain waveforms of 16QAM
Fig.5 Power Spectrum of Selected M-ary Modulation
Schemes.
III. DESIGN AND IMPLEMENTATION
Clearly have been identified that to producing an ADSL
modem which has a efficient bandwidth meeting a higher
bit data rate should design the proper digital modulation
.In this paper that examined each type of M-ray Mod/Dem
practically .The different types of M-ray Modem start by
BPSK to 256 QAM ; that has been designed separately in
terms of Modem design as shown in (Figure .6)
,Time/Frequency domain waveforms as shown in Fig.7(b)Frequency domain waveforms of
(Figure.7), Bit error rate (BER) in theoretically and 16QAM
practically as shown in (Figure .8) the bit error rate for M-
QAM starting with BPSK to 1024 QAM ; theoretical and
simulation. The proper modem which has been chosen to
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4. World Academy of Science, Engineering and Technology 62 2012
The modem has been designed and the parameters
calculated to introducing the simulated frequency spectrum
which measured from null to null approach as upstream
part of 512 QAM ,Downstream of 1024QAM The
practical frequency spectrum shown in (Figure .10) is
clearly demonstrated the availability of high order
modulation as feature to have high data rate in amount of
channel bandwidth.
IV. CONCLUSION
Fig.8 The theoretical and practical BER of M-QAM
The needs of the broadband come to involves of digital
The consideration parameters and result of simulation of modulation techniques, which most have been worked out
specifically ADSL2 Modem shown in (table.2). the range to get acquainted which best modem could be considered
of down/up streams will have bandwidth channels (960 to leads to have the appropriate bandwidth efficiency as
kHz , 116 kHz) respectively; ITU G.992.4 ADSL2 increasing of order of M-ary. The aim of these paper to
Modem theoretically spectrum shown in (Figure.9). introduce the main process of efficient design that could
uses as base which easily implemented with any filters
Table .2 the design parameters and results. ,error correction parts to enhancing data rate and less
complexity .
ADSL Data Channel Ratio of M-ray
Parameters Rate(Rb) Bandwidth(B (2Rb/2) technique REFERENCES
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Fig .10 ITU G.992.4 ADSL2, simulated spectrum Conference March 25-29, 2007 TUNISIA.
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