Effect of Varying Topologies on the Performance of Broadband over Power Line

Vinay Kumar Chandna (Member IEEE) and Mir Zahida Abstract— Power line channel is an alternative to broadband data communication. However, LV power lines present a very harsh environment for high frequency communication signals. The high frequency signal undergoes attenuation due to line loss and impedance mismatch because of branches and loads. Also, the dynamics of system changes the topology of the network very frequently. In this paper, a typical BPL system has been simulated based on echo model of the power line. The effect of varying topologies on the performance of the BPL system has been investigated using different number of branches, load impedances and line lengths. The simulation shows that with the increase in number of branches, the bit error rate increases and poses very little effect on the bit error rate due to variation of load. Key words – Bit Error Rate, BPL, Digital Communication, OFDM.

1. INTRODUCTION The present distribution system is very complex in nature, as it is closed and the possibility of expansion is difficult. In Indian scenario, there is a little scope for the installation of different channels for voice, energy, broadband and various other data services. Broadband over power lines (BPL) is an alternative choice for high-speed data transmission [1]. The key reason for the use of BPL technology is the fact that every home and office is connected to the power grid and contains electric wiring and the data through the same wiring can easily be transmitted to the consumer without additional installations and thus reduces cost considerably. BPL provides transmission of high-speed data over existing electric wiring and has the potential to provide a truly ubiquitous method to access the Internet. The power line offers a very harsh environment to the BPL signal and is very difficult to model for high frequency data. In this paper, an echo-based model has been used for the power line channel. This model takes into account the line attenuation due to heat loss and radiation. Also the effect of branches and loads is accounted for by taking into consideration the reflected and delayed version of the signal at the points of impedance discontinuities of the channel. OFDM is used as the modulation technique as it eliminates the adverse effects of impulsive noise and multipath effects which characterize the power line channel. The inter-symbol interference is eliminated through the use of a cyclic prefix [2]. Vinay Kumar Chandna Asstt. Professor with Electrical Department, Jamia Millia Islamia, Jamia Nagar, New Delhi 110018 (vinaychandna@yahoo.co.in) Mir Zahida M.Tech student with Electrical Department, Jamia Millia Islamia, Jamia Nagar, New Delhi 110018 (zahid1399@yahoo.co.in)

In this paper, a point-to-point BPL system model has been developed. The power line, between the transmitter and the receiver, with different line lengths, different number of branches and different load impedances has been modeled. The performance of the system is studied by calculating the bit error rate (BER) of the system. The BER is used to observe the effect of the varying network topologies on the system performance.

2. PROBLEM FORMATION The topology of the low voltage distribution system (DS) differs from place to place depending on various factors viz. network location, subscriber density, characteristic of load, etc [3]. Thus, it is very much necessary to know the consequences of these varying topologies on the performance of the broadband over power line (BPL) system. In this paper, a typical OFDM BPL system has been simulated in MATLAB / SIMULINK with varying number of branches, different loads and various line lengths and the bit error rate (BER) has been calculated. The multi-path echo- model of the power line is used for simulation purposes. The receiver and transmitter are designed based on the orthogonal frequency division multiplexing (OFDM) technique. The variation of the BER with the variation in the number of branches, the line length and the loads are mapped that gives us an insight into the effects of varying topologies on the performance of the BPL system. For simplicity, in this paper, continuous linear load is considered for simulation. The same may be extended to non-linear and discontinuous loads.

3. BROADBAND OVER POWER LINE TOPOLOGY The topology of the BPL network is decided by the topology of the type of distribution network being used. The paper describes the typical topology of the distribution system where low voltage network is used as the transmission medium (OFDM). The low voltage distribution network is realized by using different type of cables (underground or overhead), transformer units etc. As the topology of the low voltage distribution system differs from place to place depending on several factors viz. network location (residential or commercial), user density (high user density in commercial areas to medium user density in urban residential areas and low user density in rural areas), the network length (distance between transformer unit and user) and the network design (no. of branches etc.), therefore, in this paper a typical BPL system is simulated, as shown in fig.1,using different number of branches and variation of load.

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Fig.1. Typical simulated BPL system

4. POWER LINE CHANNEL MODELING The power line has to be modeled for high speed data communication taking into consideration various factors like the input impedance, the signal attenuation due to line losses, noise, the effect of branches and loads etc. The channel model used in this paper is presented in [4]. In this model, the two intrinsic line parameters – characteristic impedance (Z0) and propagation constant (γ) are derived first. Then the transfer function for high frequency BPL system is obtained using an echo model based on the top down approach thus maintaining the accuracy of the conventional bottom up approach while simplifying the modeling algorithm. The propagation of signals over power lines introduces an attenuation, which increases with the length of the line and the frequency. The line attenuation is caused by the heat loss and radiation on the power line and is a function of characteristic impedance Z0 and propagation constant γ. The two intrinsic line parameters Z0 and γ dominate the wave behavior along the power line. The two parameters are expressed as:

γ = √ ((R + jωL) (G + jωC)) (1)

Z0 = √ ((R + jωL) / (G + jωC)) (2)

Where, R, L, G and C are the resistance, inductance, conductance and capacitance per unit length of the power line conductor respectively. In order to derive γ and Z0, the four primary line constants have to be derived first. R=1 / π (√ π f µc / σc) (3) G = π σ / (cosh-1 (D / 2a)) (4) L = µ / π cosh-1 (D / 2a) (5) C = π ε / cosh-1 (D / 2a) (6)

Where, a: radius of the conductor. D: distance between conductors. f: wave frequency µc: permeability of conducting material σc: conductivity of conducting material σ: conductivity of dielectric material between conductors ε: permeability of dielectric material between conductors. From (1), after approximation, γ can be written as

γ = ½ [R/Z0 +GZ0] + jω√LC (7) The attenuation constant i.e. the real part of γ is:

α = ½ [R/Z0 +GZ0] (8)

After substituting the values of R, G and Z0, attenuation constant is approximated as, α = k1√f + k2 f (9) Where k1and k2 are constants depending on the material and dimensional characteristics of the conductors. In addition to the frequency dependent attenuation that characterizes the power line channel, deep narrowband notches occur in the transfer function which may spread over the whole frequency range. These notches are caused by multiple reflections at impedance discontinuities. An echo model of the channel as shown in fig.2 describes this behavior. [4-8].

3τ hLτ2τ

∑

0g 1g 2g 3ghLg

1τ

Fig.2. Multipath echo model of power line

The echo model can be described by a discreet time impulse response h (t) as

H (t) = ∑ gi. δ ( t - τi ) (10) Taking into account the line attenuation due to heat loss and radiation and taking the Fast Fourier Transform, the channel response is given as

H (f) = ∑ gi. e-α (f) l. e-2 π f τi (11)

Where, gi is the weighting factor representing the product of the reflection and transmission factors along the path i, τi is the path delay introduced by path i and is the ratio of the path length li and phase velocity νp. τi = li / νp (12)

5. RECEIVER AND TRANSMITTER MODEL Due to a number of distributed branches of various lengths and varying connected load impedances, power line channel is characterized by a multi-path fading environment. The delayed waves due to loads and branches interfere with the direct waves causing inter symbol interference (ISI) that degrades network performance. The effect of multi path is reduced by orthogonal frequency division multiplexing (OFDM) as it is based on parallel broadband data transmission. The OFDM signals are generated by means of inverse fast Fourier transform as shown in Fig.3. [9-11].

MultiportSelector

SelectRows

MatrixConcatenation

1

IFFT

IFFTcomplex (0,0)

complex (0,0)* ones (81,1)

complex (1,0)

Add CyclicPrefix

U YS

-1

complex (0,0)* ones (81,1)

-1

Fig3. Generation of OFDM signal using IFFT.

The high speed data being transmitted is first coded. The coded data is modulated using M-QAM technique. The modulated data is fed into the IFFT circuit generating an OFDM signal. The signal is then fed into a guard time insertion circuit in order to reduce ISI. At the receiver, all the steps carried out at the transmitter are reversed. The guard time is first removed. The signal is then passed through the fast Fourier transform circuit in order to change the signal back to frequency domain. Finally the demodulation and decoding steps are carried out and the high-speed data bits are recovered. Fig.4. represents the configuration of OFDM transmission system.

6. SIMULATION RESULTS The simulation of the BPL system is carried out in MATLAB/SIMULINK. The performance of the BPL system is studied for different number of branches, different loads and varying line lengths. The results are shown in fig. 5, fig. 6 and fig.7 respectively. From the simulation results the effect of varying topologies on the performance of the BPL system are inferred as:

Fig.4. Transmitter and receiver model of an OFDM system.

Table I: Variation of BER with increasing number

of branches for different line lengths

Line length

(meters)

Bit Error Rate (BER)

Number of Branches (N)

1 3 5 7 9 12

100 .129 .369 .44 .47 .48 .49

200 .134 .372 .443 .473 .487 .493

300 .149 .371 .443 .474 .488 .494

400 .177 .372 .444 .474 .488 .494

500 .204 .378 .444 .475 .489 .494

Table II: Variation of BER with variation in the branch terminal loads

Load (ohms)

Bit Error Rate (BER)

Number of Branches (N)

1 3 5

100 .042 .168 .208

200 .061 .224 .353

400 .091 .309 .398

600 .1 .334 .407

800 .106 .338 .410

1000 .108 .353 .423

1200 .116 .354 .424

1500 .116 .360 .432

OFD

M M

odul

ated

Sig

nal

QA

M –

Map

ed S

igna

l

Effect of the no. of branches: From fig.5. it is observed that with the increasing number of branches, there is an increase in the BER. This is due to multiple reflections of the input OFDM signal at these branches. These reflected waves interfere with the direct wave. In case the reflection is destructive attenuation takes place. Thus it is seen that with the increasing number of branches the BER increases. This requires either the increase in the signal power (which would result in an increase in the signal to noise ratio, SNR) or the decrease in the distance between consecutive repeaters in order to keep BER within specified limits. The signal power can be increased only up to a certain limit beyond which interference with the nearby communication lines would exceed the permitted limits. Therefore, with the increasing no. of branches, after increasing the signal power to the permitted limit, the distance between consecutive repeaters needs to be reduced so as to decrease the BER.

0 2 4 6 8 10 12

10-0.7

10-0.6

10-0.5

10-0.4

Number of branches (N)

BE

R

Fig.5. Performance of BPL system (BER) with different no. of branches (N).

Effect of varying line lengths: With the increase in the line length it is observed, from fig.6. that the BER keeps on increasing.

100 150 200 250 300 350 400 450 5000.1

0.15

0.2

0.25

Line length (m)

BE

R

Fig.6. Performance of BPL system with different line lengths.

Thus after a distance of about 500 - 700m the signal would be almost completely lost due to attenuation. Therefore, repeaters have to be installed at distances of about 500 - 700m so as to preserve the signal. These repeaters divide the BPL network into several segments, the lengths of which can be overcome by the applied BPL system. The repeater receives the transmission signal, amplifies it and injects it into the network.

Effect of varying loads: With increasing loads, the BER rate increases. However, as observed from fig.7. the effect of changing loads is more on networks with lesser number of branches. With increasing number of branches, the effect of load on the network goes in decreasing. This is due to the fact that with the increase in the number of branches, the impedance of the network decreases because of a large number of impedances in parallel. More the number of branches, lesser is the network impedance. Therefore, the effect of loads remains hidden in such low impedance networks.

0 500 1000 150010

-2

10-1

100

Load (Ohms)

BE

R

N = 1N = 3N = 5

Fig.7.Performance of BPL system with different load impedances

7. CONCLUSION The effect of the varying number of branches, load impedances and the line lengths, between the transmitter and the receiver, on the performance of the BPL system has been studied. The attenuation due to heat loss and radiation, on the power line, causes increase in bit error ratio (BER) with the increase in line length. This requires an increase in the signal power in order to keep the BER within limits. As the signal power cannot be increased beyond a certain level, because of interference with nearby communication lines, repeaters have to be installed at suitable distances along the line. The increase in the number of branches results in an increase in BER. With increasing number of branches over a particular line length, after increasing the signal power to the allowable limits, the distance between consecutive repeaters has to be reduced to keep BER within acceptable limits. The effect of load on the system performance is also studied and found that there is not much change in the BER due to linear continuous variation of load particularly in networks containing large number of branches.

Number of Branches

Bit

Erro

r Rat

e B

it Er

ror R

ate

Line Length (m)

Load in ohms

Bit

Erro

r Rat

e

8. REFRENCES [1] J. Anatory, M.M> Kissaka and N.H. Mvungi., “Trends in

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9. BIOGRAPHIES V. K. Chandna (M 08) graduated from Nagpur university in 1994 in electronics & power, completed his M.E. in Power System from Walchand college of Engg. Sangli, Maharashtra in the year 1997 and Ph.D. in electrical engineering, from Delhi College of Engineering, Delhi in the year 2008. His employment experience includes Walchand college of Engg., Sangli Nagpur(MAH), Raj Kumar Goel institute of technology, Ghaziabad (UP), Maharaja Agarsen Institute of Technology, Delhi and presently working as Asstt. Professor in Electrical Engg. Department, Jamia Milia Islamia, New Delhi. He has more than 10 papers in International Journal / Conferences of repute. His area of interest is Application of soft computing techniques to power system, SCADA, operation, design and control. Mir Zahida graduated from National institute of Technology Srinagar in electrical engineering in the year 2006. She is presently pursuing her M.Tech in Electric Power System Management from Jamia Milia Islamia. Her area of interest is application of soft computing to power system and digital communication.