ByFaaDoOEngineers.com 1 INTRODUCTION Broadband Powerline communications are sets of equipment, software and management services that when overlaid on the electric grid provides users with communication means over existing “power lines”. The new technology operates in the 1-30 MHz and can deliver data rates up to 200Mbps. The rationale behind providing high bit-rate data services exploiting the power grid resides in the vast infrastructure in place for power distribution, and the penetration of the service could be much higher than any other wire line alternative. In spite of the renewed interest in Power line communications, this technology still faces several technical challenges and regulatory issues: the power line channel is extremely difficult to model; it is a very noisy transmission medium; Power line cables in the 240V secondary distribution systems are often unshielded, thus becoming both sources and targets of electromagnetic interference (EMI); transformers can introduce severe distortion in the absence of bypass couplers. Since the power-line network is not designed for communications purposes, the channel suffers from multipath fading and frequency selectivity. A transfer characteristic model for the low voltage indoor power line based on the transmission line theory is developed. To model the transfer characteristics of power lines, basically there are two essential factors: the model parameters and the modeling algorithms. These two factors determine the reliability and accuracy of the model. From the ways the model parameters are obtained, the modeling technique can be classified into two approaches: the top-down approach and the bottom-up approach. In the top-down approach, the model parameters are retrieved from measurements. This approach requires little computation and is easy to implement. However, since the parameters depend on the measurement results, the model is prone to measurement errors. On the contrary, the bottom-up approach starts from theoretical derivation of model parameters. Although this approach requires more computational efforts comparing to the top-down approach, it however describes clearly the relationship between the network behavior and the model parameters. Moreover, this modeling approach is more versatile and flexible since all the parameters are formulated, making it easy to predict the changes in the transfer function should there be any change in the system configuration. The model described in this project adopts this bottom-up approach. Depending on the modeling algorithms used, the above approaches can be achieved in the time domain or the frequency domain. First frequency domain modeling, using scattering matrix is used to obtain the transfer function of the channel from which the attenuation in the signal strength and the delay or phase distortion at different frequencies is calculated. Scattering matrix gives the relationship of the incident (a) and reflected (b) waves. Broadband Power Line Communication is only interested in the transfer function in the forward direction, which is the ratio of the incident power into the receiver over the power injected by the transmitter. This can readily be expressed by b 2 / a 1 or S 21 in the scattering matrix. Secondly, IFFT (Inverse fast Fourier transform) is used to calculate the impulse response from the channel transfer function to know the multipath environment of the power line channel in time domain modeling and an echo model is developed in Simulink to represent this physical characteristics.
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By FaaDoOEngineers.com
1
INTRODUCTION
Broadband Powerline communications are sets of equipment, software and
management services that when overlaid on the electric grid provides users with
communication means over existing “power lines”. The new technology operates in
the 1-30 MHz and can deliver data rates up to 200Mbps. The rationale behind
providing high bit-rate data services exploiting the power grid resides in the vast
infrastructure in place for power distribution, and the penetration of the service could
be much higher than any other wire line alternative. In spite of the renewed interest in
Power line communications, this technology still faces several technical challenges
and regulatory issues: the power line channel is extremely difficult to model; it is a
very noisy transmission medium; Power line cables in the 240V secondary
distribution systems are often unshielded, thus becoming both sources and targets of
electromagnetic interference (EMI); transformers can introduce severe distortion in
the absence of bypass couplers.
Since the power-line network is not designed for communications purposes, the
channel suffers from multipath fading and frequency selectivity. A transfer
characteristic model for the low voltage indoor power line based on the transmission
line theory is developed. To model the transfer characteristics of power lines,
basically there are two essential factors: the model parameters and the modeling
algorithms. These two factors determine the reliability and accuracy of the model.
From the ways the model parameters are obtained, the modeling technique can be
classified into two approaches: the top-down approach and the bottom-up approach.
In the top-down approach, the model parameters are retrieved from measurements.
This approach requires little computation and is easy to implement. However, since
the parameters depend on the measurement results, the model is prone to
measurement errors. On the contrary, the bottom-up approach starts from theoretical
derivation of model parameters. Although this approach requires more computational
efforts comparing to the top-down approach, it however describes clearly the
relationship between the network behavior and the model parameters. Moreover, this
modeling approach is more versatile and flexible since all the parameters are
formulated, making it easy to predict the changes in the transfer function should there
be any change in the system configuration. The model described in this project adopts
this bottom-up approach. Depending on the modeling algorithms used, the above
approaches can be achieved in the time domain or the frequency domain. First
frequency domain modeling, using scattering matrix is used to obtain the transfer
function of the channel from which the attenuation in the signal strength and the delay
or phase distortion at different frequencies is calculated. Scattering matrix gives the
relationship of the incident (a) and reflected (b) waves. Broadband Power Line
Communication is only interested in the transfer function in the forward direction,
which is the ratio of the incident power into the receiver over the power injected by
the transmitter. This can readily be expressed by b2 / a1 or S21 in the scattering matrix.
Secondly, IFFT (Inverse fast Fourier transform) is used to calculate the impulse
response from the channel transfer function to know the multipath environment of the
power line channel in time domain modeling and an echo model is developed in
Simulink to represent this physical characteristics.
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2
Noise in LV power-line is characterized within two categories: background and
impulsive noise. Many electric appliances frequently cause man-made
electromagnetic noise on power-line channels. Such man-made noise produces an
impulsive distortion on channel causing a burst of noise. A large impulse often causes
the entire transmitted symbol to be corrupted and it can be devastating to the overall
system performance.
The well-known multicarrier technique, orthogonal frequency division multiplexing
(OFDM), is considered as the modulation scheme for Broadband Powerline
communications. By the application of OFDM, the most distinct property of power-
line channel, its frequency selectivity, can be easily coped with. Moreover, OFDM
can perform better than single carrier modulation in the presence of impulsive noise,
because it spreads the effect of impulsive noise over multiple sub carriers. Like other
communications systems, coding can improve the OFDM system performance but
because of the nature of this channel, the achieved improvements are usually very
restricted.
Since power line technology appears to be more mature for the indoor home-
networking scenario than for the outside broadband access one, focus here is on the
development of channel and noise model for indoor power line network and thus
designing of communication system for performance analysis of Broadband PLC
using block coding techniques using software simulation in MATLAB / Simulink. In
addition, network performance analysis of CORINEX Communication, Inc.
Broadband PLC equipments for indoor power line network using measurements of
different network characteristics parameters such as throughput and latency is
performed.
This project is organized as follows. Chapter 2 deals with channel modeling using
transmission line analysis of indoor powerline. Noise modeling is mentioned in
Chapter 3 while Broadband powerline communication system is designed in Chapter
4. Broadband PLC network performance parameters measurements are discussed in
Chapter 5 while conclusion and scope for future work are mentioned at the end.
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3
CHANNEL MODELING
2.1 TRANSMISSION LINE ANALYSIS OF POWER LINE
The electromagnetic theory states that to achieve efficient point-to-point
transmission of power and information, the source energy must be guided. When
power lines are used to transmit high frequency communication signals, they can be
regarded as transmission lines, which guide the transverse electromagnetic (TEM)
waves along them. The cable under study in this project is the typical single-phase
house wirings commonly found in India. The cables are made up of stranded copper
conductors with PVC insulation. The three cables (live, neutral, and earth) are usually
laid inside PVC conduits that are embedded inside the concrete wall. Typically, the
live and neutral cables are used as the PLC transmission channel, which can be
approximated as a close form of the “two-wire transmission line”. According to
Electromagnetic theory, the two-wire transmission line must be a pair of parallel
conducting wires separated by a uniform distance. In the actual installation, the power
cables are simply pulled through the conduit and the separation between them is not
uniform at all. However, the conduit normally has small cross-sectional area and this
limits the variation of the separation between the cables. Hence, the assumption of
uniform separation is reasonable in this case. Based on the above consideration, the
paired power cables are regarded as a distributed parameter network, where voltages
and currents can vary in magnitude and phase over its length. Hence, it can be
described by circuit parameters that are distributed over its length as shown in Fig.2.1
below.
Fig. 2.1 Equivalent circuit of two-wire transmission line
The quantities v (z, t) and v (z +z, t) denote the instantaneous voltages at location z
and z + z, respectively. Similarly, i (z, t) and i (z +z, t) denote the instantaneous
currents at z and z +z, respectively. R defines the resistance per unit length for both
conductors (in / m), L defines the inductance per unit length for both conductors (in
H/m), G is the conductance per unit length (in S/m), and C is the capacitance per unit
length (F/m).
2.2 MODEL PARAMETERS.
Based on the lumped-element circuit of a two wire transmission line as shown above,
model parameters per unit length (m) are:
1.) Resistance „R‟ = ac
ccf /1
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Where „‟= skin depth = and is a function of frequency „f ‟. This effect
causes an increase in the resistance of the cable and it worsens as the current
frequency increases.
2.) Inductance
3.) Conductance
4.) Capacitance
Where a = radius of the copper conductor,
D = distance between conductors,
cpermeability of copper conductor,
dconductivity of the dielectric material,
and dpermittivity of the dielectric material.
2.3 MODELING THE INDOOR POWERLINE
Here the indoor power cables are approximated to be a two-wire transmission line
with solid core conductor for the ease of implementation using software simulation as
shown in Fig.2.2. The dielectric material, between the cable conductors, is
inhomogeneous in both space (due to the round shape of the cable conductor) and
contents (mixture of insulation and air). But since the cables are of close proximity to
each other, the thickness of the insulation„t‟ is comparable with that of the air space
between the conductors. In this model, the dielectric is assumed to be just a mixed
content material and the effects of the inhomogeneous in space are neglected to keep
the model tractable.
Fig. 2.2 Approximate model of the power line
Here, distance between the two conductors (Live and Neutral) „D‟= 2t + 2t + 2a
where t = thickness of insulation = 0.7 mm
a = radius of copper conductor = 0.63 mm
Therefore, D = 4.06 mm
Also, Conductivity of copper C = 5.8 x 107 S/m
)2/cosh(
)2/cosh(
)2/cosh(
aDaC
aDaG
aDaL
d
d
c
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Relative permittivity of dielectric [PVC (4) & air (1)] r = 0.8
Conductivity of dielectric d = 1 x 10-5
S/m
The length of transmission line is taken to be „S‟=5 m with shunt stub
terminated in an open circuit as shown in fig.2.3 considering the fact that, indoor
power lines are radial N-branched network as shown in fig.2.4 below.
Fig. 2.3 Configuration of simulated network
Fig. 2.4 A simplified indoor power line channel
In the above figure, port 1 is the transmitter from where the signal is sent, and
port 2 is the receiver where the signal strength is measured.
2.4 TRANSFER FUNCTION MODELING
There are three main types of attenuation for a wave propagating in the forward
direction. The first one is the line attenuation, which is caused by the heat loss and
radiations along the power line. This line attenuation is always present and it depends
on the length of the wave path and the frequency of the wave. The second type of
attenuation is caused by reflections arising from the points of impedance
discontinuities on the propagation channel. The reflected wave from the unmatched
points will interfere with the original incident wave. This kind of interferences may be
constructive or destructive, giving rise to attenuation if it is destructive. The last type
of attenuation is caused by the delayed version of the forward propagating wave
falling out of phase with the main incident forward wave, giving rise to destructive
interference and hence overall signal attenuation. Thus, frequency-domain modeling
approach using scattering matrix technique is used to account for all these reflected
By FaaDoOEngineers.com
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and delayed paths in the power network. Scattering matrix gives the relationship of
the incident (a) and reflected (b) waves as shown in the fig.2.5 below.
Fig. 2.5 Scattering parameters
Scattering parameters or S-parameters are properties used to describe the electrical
behavior of linear electrical networks when undergoing various steady state stimuli by
small signals.They are members of a family of similar parameters used in electronics
engineering, other examples being: Y-parameters, Z-parameters, H-parameters, T-
parameters or ABCD-parameters.They differ from these, in the sense that S-
parameters do not use open or short circuit conditions to characterize a linear
electrical network; instead matched and unmatched loads are used. Moreover, the
quantities are measured in terms of power. Although applicable at any frequency, S-
parameters are mostly used for networks operating at radio frequency (RF) and
microwave frequencies. S-parameters change with the measurement frequency so this
must be included for any S-parameter measurements stated, in addition to the
characteristic impedance or system impedance. S-parameters are readily represented
in matrix form and obey the rules of matrix algebra. The S-parameter matrix
describing an N-port network will be square of dimension 'N' and will therefore
contain N2 elements. At the test frequency each element or S-parameter is represented
by a unitless complex number, thus representing magnitude and angle, or amplitude
and phase. For all ports the reflected power waves may be defined in terms of the S-
parameter matrix and the incident power waves by the following matrix equation:
[b] = [S] [a]
where S is an N x N matrix the elements of which can be indexed using conventional
matrix (mathematics) notation. The phase part of an S-parameter is the spatial phase
at the test frequency, not the temporal (time-related) phase.
The S-parameter matrix for the 2-port network is probably the most common and it
serves as the basic building block for generating the higher order matrices for larger
networks. In this case the relationship between the reflected, incident power waves
and the S-parameter matrix is given by:
2
1
2221
1211
2
1
a
a
SS
SS
b
b
Expanding the matrices into equations gives:
and 2221212
2121111
aSaSb
aSaSb
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Each equation gives the relationship between the reflected and incident power waves
at each of the network ports, 1 and 2, in terms of the network's individual S-
parameters, S11 , S12, S21 and S22 . If one considers an incident power wave at port 1
(a1) there may result from it waves exiting from either port 1 itself (b1) or port 2 (b2).
However if, according to the definition of S-parameters, port 2 is terminated in a load
identical to the system impedance (Z0) then, by the maximum power transfer theorem,
b2 will be totally absorbed making a2 equal to zero. Therefore
1
1
1
111
V
V
a
bS and
1
2
1
221
V
V
a
bS
Similarly, if port 1 is terminated in the system impedance then a1 becomes zero,giving
2
1
2
112
V
V
a
bS and
2
2
2
222
V
V
a
bS
Each 2-port S-parameter has the following generic descriptions:
S11 is the input port voltage reflection coefficient
S12 is the reverse voltage gain
S21 is the forward voltage gain
S22 is the output port voltage reflection coefficient.
Here, S21 gives the Network Transfer Function.
2.5 DETERMINATION OF TRANSFER FUNCTION
In order to find out the degree of signal degradation in the power line channel
between two access point, software simulation is done using MATLAB programming
to obtain the transfer function whose magnitude (dB) Vs frequency plot gives the
attenuation in the signal strength and angle (radian) Vs frequency plot gives the phase
distortion or delay. The MATLAB program is as given below.
Impulse response plot after running the program is as obtained below. In the impulse
response, the multiple propagation paths can be seen.
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10
0 1 2 3 4 5 6 7
x 10-6
-1
0
1
2
3
4
5
6
7
Time (microseconds)
outp
ut
'y(t
)'
0 1 2 3 4 5 6 7
x 10-6
0
2
4
6
8
Time (microseconds)
input
'u(t
)'
Fig. 2.9 Impulse response plot at output and input, the multiple propagations paths can
be seen
The dominant paths of the impulse response are sufficiently covered by the simple
N=6 path model from which the attenuations and delays are calculated to develop a
six path echo model in SIMULINK as shown below. Impulsive noise is added in the
echo channel model which is explained in the later chapters.
Path Number Attenuation ‘Cn’ Delay ‘ n’
1 0.875 0
2 0.1775 0.315e-6
3 0.07 0.579e-6
4 0.0525 1e-6
5 0.0375 1.3e-6
6 0.0375 1.8e-6
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Fig. 2.10 Echo model representing the multi-path channel model of Broadband PLC
developed in Simulink
1
Out1
imp
Signal From
Workspace
Product5
Product4
Product3
Product2
Product1
Product
z-N
Delay5
z-N
Delay4
z-N
Delay3
z-N
Delay2
z-N
Delay1
z-N
Delay
0.037
Constant5
0.037
Constant4
0.052
Constant3
0.07
Constant2
0.177
Constant1
0.875
Constant
Add1
In1
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NOISE MODELING
3.1 NOISE DESCRIPTION
Since the power-line network is not designed for communications purposes, the
channel exhibits an unfavorable frequency selective transfer function. Furthermore,
this channel is distorted by impulsive noise and by severe narrowband interference.
Unlike many other communication channels, power-line channel does not represent
an additive white Gaussian noise (AWGN) environment. Noise in LV power-line is
characterized within two categories: background and impulsive noise. Many electric
appliances frequently cause man-made electromagnetic noise on power-line channels.
Such man-made noise produces an impulsive distortion on channel causing a burst of
noise. A large impulse often causes the entire transmitted symbol to be corrupted and
it can be devastating to the overall system performance. Background noise usually
consists of coloured background noise and narrowband noise. Here only coloured
background noise in residential environment is considered.
3.1.1 COLOURED BACKGROUND NOISE
Coloured background noise power spectral density (psd) is relatively lower and
decrease with frequency. This type of noise is mainly caused by a superposition of
noise sources of lower intensity. Contrary to the white noise, which is a random noise
having a continuous and uniform spectral density that is substantially independent of
the frequency over the specified frequency range; the coloured background noise
shows strong dependency on the considered frequency. The parameters of this noise
vary over time in terms of minutes and hours.
For the model of the coloured background noise psd, the measurements have shown
that a first-order exponential function is more adequate, as formulated by equation
given below;
1/
1.ff
OCBN eNNN
with No the constant noise density, N1 and f 1 are the parameters of the exponential
function, and the unit of psd is dBV/Hz1/2
. The psd of coloured background noise in
residential environment according to [2] is given by following equation:
6,3/][.3535)( MHzf
BN efN for residential environments
Matlab program for plot of coloured background noise is as given below:
%Equation for coloured background noise for resi. environment function y = cbnpsd(f) y = -35 + 35*exp (-(f/6)); %coloured background noise power spectral density