Selected coding and modulation techniques for reliable power line communications.
Already available of power distribution grid offers vast deployments for reliable communication services, regarding transmission rates, services and deployments. PLC far behind (xDSL, CATV), but PLC has a chance to be on the market in the access network. Power line can be found in all building and residences which is not the case for other access network methods. This paper firstly have describes some terms related to the PLC system and secondly reports several modulation and coding techniques by means of MATLAB[TM]/SIMULINK for realization optimum communication via low-voltage power line network grid.
Low-Voltage Distribution Grid
The voltage that connects to our house is 240 volts. The interface between different voltage transmission lines and the distribution system is the electrical substation. Substations use transformers to "step down" voltages from the higher transmission voltages to the lower distribution system voltages. Transformers located along distribution lines further step down the line voltages for household usage. A single low-voltage line consists of four wires-three phases and the neutral. Each building is connected to the network grid through cable-boxes. In typical configurations, there are usually several residences (typically between 5 and 20, in U.S.A. and Canada ) connected to the secondary side of the same transformer as shown in Figure 1.
[FIGURE 1 OMITTED]
PLC Multipath Signal
Power transmission line has a tree-like topology. With branches formed by additional wires tapered from the main path and having:
1. Various lengths.
2. Various terminated loads.
The delay on the path can be found by using the following function:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1)
When we merge signal spreading on all paths together frequency response Expression:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2)
Where A (li, f) is the signal attenuation proportional to length and frequency.
The total signal attenuation on the PLC channel consists of tow parts:
1. Coupling losses (depending on transmitter design)
2. Line losses (very high and can range from 40-100dB/Km). Complex propagation constant:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3)
Depending on the primary cable parameters (R, L, G, C).The frequency response H (f) of the transmission line with length (l) can be expressed as follows (U(x) : is the voltage at distance x): [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (5)
Consider the frequency in MHz R/unit length proportional to [check]f because it is dominated by the skin effect and G/unit length proportional to f. Cables can be regarded as weakly lossy with real valued characteristic impedances and simplified expression for the complex propagation constant [pi].
[gamma] = [k.sub.1] [square root of f + [k.sub.2] f + [jk.sub.3] f (5)
Where k1, k2 and k3 are the parameters summarizing material and geometry properties. Based on these derivations, an approximation formula for the attenuation factor [alpha]:
[alpha] (f) = [a.sub.o] + [a.sub.1] [f.sup.k] (6)
That is able to characterize the attenuation of typical electrical power distribution lines with only three parameters, being easily derived from the measured transfer function . Now the propagation loss LdB is given at the length l and the frequency f as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (7)
Unfortunately, in the case of the PLC, we cannot stay only with the additive white Gaussian noise (AWGN) *. The noise scenario is much more complicated since five general classes of noise can be distinguished in power distribution line channel [4, 5] these five classes are Figure. 2:
1. Colored background noise.
2. Narrow--band noise.
3. Periodic impulse noise asynchronous with the main frequency.
4. Periodic impulse noise synchronous with main frequency.
5. Asynchronous impulse noise.
1, 2 and 3 are considered as background noises because they remain for a period of minutes or sometimes of hours . Usually background noise in a communications system limits the channel capacity. On contrary, the noise types 4 and 5 are timevariant in term of microseconds or millisecond and there impact on useful signals is much stronger and may cause single-bit or burst errors in data transmission . Impulse is the worst noise in PLC environment because it can easily reach several dB over other noise types. Below mathematical expressions describe impulse noise model.
[t.sub.iat] = [t.sub.w] + [t.sub.d] = [t.sub.arr, i + 1] - [t.sub.arr, i] (8)
Where; [t.sub.w] : impulse width. [t.sub.arr] : Arrival time. [t.sub.d] : Impulse distance. [t.sub.iat] : Inter-arrival time.
The train of impulses can be described as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (9)
Where; Ai: impulse amplitude. imp: generalized impulse function.
[A.sub.i]; [t.sub.w] And [t.sub.arr] are random variables whose statistical properties may be investigated by measurements. More information can be found in .
[FIGURE 2 OMITTED]
Today PLC standard exists only for system used for simple control, and switching purposes. It deals with the systems occupying a frequency range up to 525 kHz, standard limits for unwanted radiation generated by restricting the maximum signal amplitude in the power line grid. In Europe, there is a standard CENELEC EN-500651. This standard provides a frequency spectrum from 9 to 140 kHz for PLC. The band allocations are illustrated in Figure 3.
[FIGURE 3 OMITTED]
Power Line Communication Model Proposed
It is important that the selected model be identical with the real environment with all its features. Since an adequate model has not been standardized yet, the question of its choice is still remaining open. Our model of the PLC environment is shown in Figure 4 and consists of the following parts with particular coding and modulation techniques:
1. The Transmission Part
2. Transmission Channel.
3. The Receiving Part.
[FIGURE 4 OMITTED]
Noise Simulation Model
According to frequency domain, time domain analysis and summarized results obtained in Reference , we propose a model for generation impulse noise and background noise, which are useful for simulation of power line modem. Impulse noise is usually characterized as a random pulse waveform whose amplitude is much higher than the background noise. Three important properties of impulsive noise are: amplitude, width and inter-arrival time distributions. These properties have been studied in Reference . Using MATLAB/Simulink a subsystem has been simulated as shown in Figure 5 and 6 below.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Modulation Techniques for PLC System Design
PLC channel characteristics impose important factors in the choice of the proper modulation techniques for realization optimum PLC system design, these factors are: noise and impulse disturbances; a time variant frequency-selective nature of the channel and limitations with regard to the EMC. These factors will be discussed with details in the next sections. Experience from wireless systems or x Digital Subscriber Line(x DSL) technologies preferring the second technology, but this solution is not ideal because of the obligatory of the initial estimation of the phase and the stability of the updating of parameters during a session.
Three different types of modulation schemes can be used. The following discussion describes the advantages and the drawbacks of each of these methods in the PLC transmission environment.
Singal Carrier Modulation (SCM)
In SCM information modulated in amplitude, phase or frequency changes of the carrier frequency fo. Wideband signal of this type has its frequency spectrum placed around fo. He SCM modulation is cheap and easy to implement. Because of the strong inter-symbol interference ISI due to Notches in the transfer function of the channel powerful detection and equalization techniques can be used to eliminate this but less complex linear equalizers cannot be used . Instead of this the Decision Feedback Equalizer (DFE) .this solution is a goof solution it also minimizes the colored noise, but Impulse noise can cause error propagation. To avoid this; the impulse noise detector could be used to stop an adaptation of the equalizer at that moment .
The necessity of addition complex techniques will reduce the simplicity; therefore the SCM is not promising modulation technique for the use in the PLC systems.
Spread Spectrum Modulation (SSM)
The basic idea of spread spectrum (SS) system is the use of cod sequence that spread narrowband signals over wider bandwidth . This technique is suitable for PLC system because:
1. Low power density spectrum
2. Limitation of radiation
3. Robustness to narrowband interferences
4. Robustness to selective attenuations
But we need a large bandwidth which may be limit the maximum data rate. Therefore there are three methods of how the variable-rate Code-Division Multiple Access (CDMA) can be implemented by:
3. Variable spreading
The spread spectrum technique is quite an interesting solution for the PLC .
Multi-Carrier Modulation (MCM)
The Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme is suitable for frequency-selective channels because its ability to cope with this feature by dividing the available bandwidth into N equally spaced narrow band subchannels . Each narrowband subcarrier can be modulated using various modulation formats; BPSK, QPSK and QAM (or differential equivalents). A substitution advantage of OFDM is its adaptability, since it is possible to choose the optimum modulation scheme individually for each subchannel. It is also possible to fade out the signal on some frequencies because of very bad conditions for transmission or regulatory restrictions .
Because of the high spectral efficiency, robustness against channel distortion, high flexibility and adaptability, it is expected that the OFDM will become the most favorable modulation scheme in all PLC system design.
Coding Techniques for the PLC System Design
In the field of coding techniques, there is no general channel model that has been standardized . Thus a lot of different coding schemes look suitable for the PLC depending on the choice of the channel model. In the following we will mention jus some of them .
We can use space-time code (STC) coding technique because the spatial effect in the form of three different phases of the power line. It is possible to design space-time block codes using 4-pulse amplitude modulation (4-PAM) as a modulation scheme with rate 1 and diversity
Other coding schemes are based on the assumption that the multipath signal propagation model can be used to describe the PLC channel. Because of the independent random variables the line is mismatched. Therefore, the PLC channel can be roughly modeled as the Rayleigh fading channel .
Since the impulse noise is present in PLC channel, we can expect a strong burst of errors during transmission. Therefore, the coding schemes have to be integrated with the interleaving technique that disperses errors and allows code capabilities designed to control independent errors to be fully exploited.
Finally, interleaving may result in significant performance degradation if there are residual errors after decoding . At this time, integrated solutions are also known. One of them is the bit-interleaved coded modulation (BICM) [11, 12, and 13], where its performance in combination with the Grey mapping of bits onto signals comes very close to the optimum for the AWGN channel.
Simulation of Modulation and Coding Techniques
A designed system has been simulated using Simulink. Particular points have been considered in this system that the power line environment consists of a group of signals which have been merged or mixed together to realize the approximate media of power line grid. As shown in figures 8, 9 and 10 the spectacular result obtained that the error rate calculation is always zero
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
The combination of M-FSK with permutation coding gives rise to a constant modulator output envelope and includes frequency and time diversity. Frequency and time diversity can be expected to give robustness against narrow band noise and impulse noise .
The Simulink model presents a simplified OFDM transmission system, which introduces the application of the IFFT/FFT elements in the process of data modulation/demodulation according to the IEEE802.11a standard. In the same way as the other simulink models, the system was divided into three basic sections Figure 11:
* Receiver, and
* Measurement tools
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
This paper analysis the basic feature of the transmission environment of power line network (LV) and possibilities for modeling and simulation of information signal transmission in this environment by mean of PLC technology. Together with the PLC transmission environment, possible modulation and coding techniques are introduced in order to create a complete picture of the PLC transmission path.
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(1) Adnan S. Obeed, (2) Nitin M. Kulkarni and (3) Arvind D. Shaligram
(1&2) Dept. of Electronic Science, Fergusson College, FC Road, Pune, Maharashtra, India.
(3) Department of Electronic Science, University of Pune, Ganeshkhind Road, Pune, Maharashtra, India E-mail: email@example.com, firstname.lastname@example.org
Adnan S. Obeed got a diploma on Electrical engineering from Nancy II, France 1988; in 1999 he received his B.E in electrical engineering from University of technology, Department of Electrical engineering, Baghdad, Iraq. He worked as an aircraft electrical engineer for 15 years. He received M.Sc. in Electronic Science from Department of Electronic Science University of technology Al-Rashid College, Baghdad, Iraq. He was a lecturer from 2004-2005 in technical College, Musiab, Ministry of higher Education and Scientific Research Baghdad, Iraq. Now he is doing a Ph.D. research work under Indian scholarship (ICCR) since 2006 in Power Line Communication technology in University of Pune Department of Electronic Science.
Dr. Nitin M. Kulkarni received M.Sc. and Ph.D. degree from Department of Electronic Science, University of Pune. Since 1986, he has been with Department of Electronic Science, Fergusson College Pune. Presently, he is a Reader in Electronic Science. His research interests are intelligent instrumentation and sensor network. He is a member of SERNET and Instrumentation society of India.
Dr. Arvind D. Shaligram, presently Head of the Department of Electronic Science at University of Pune, has a research experience of more than 25 years. After completing Ph.D. degree from University of Pune, he joined the University department as faculty member and continues to works till date. He has guided 18 Ph.D students and 12 M.Phil students so far. His major research interests are in the fields of intelligent systems, Optoelectronic sensors and systems, biomedical instrumentation and sensor networks and their applications.
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|Author:||Obeed, Adnan S.; Kulkarni, Nitin M.; Shaligram, Arvind D.|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Sep 1, 2009|
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