Power Line Communication (PLC) refers to provisioning of communication services over the electricity power grid. The main characteristics of PLC involves sending high frequencies using a PLC Carrier unit over power lines and injecting the frequencies (or picking them up) via a coupler linked directly to the power grid. It is general to categorize the PLC systems as Broadband PLC and Narrowband PLC. Broad band PLC systems cater to the need of high data rate applications such as providing internet access over the wall socket and home entertainment. Narrowband PLC systems are meant to provide low data rate applications such as power system communication in a substation environment, building/home automation, automatic meter reading, real time pricing and energy management. It is surprising to note that in the last 7 years, India could provide broadband connectivity to less than 2.5 million subscribers as against its target of 20 million in 2010. The broadband subscriber market is expected to increase to 30 million by 2013. Broadband services are the current requirement of rural development. Where broadband access has lagged notably in rural areas, PLC opens up a new communication infrastructure. Nearly 70% of the population lives in rural areas in India, where the issues of information, education and communication become paramount for development of the same. At this juncture it is relevant to mention that gap between target and achievement to accomplish broadband penetration in India will go over 30 million by 2013. Adequate infrastructure for Internet information services in the rural areas need to be made available to rural folk to connect them with urban and semi urban population as knowledge and education help develop stronger economies and more social equalities. SCADA (also called tele control) in substations has one or more computers with appropriate application software (Master Stations) connected by a communication system (wire, radio, power line carrier or fibre optics), to a number of remote terminal units (RTUs) which are referred to as intelligent electronic devices (IEDs), placed at various locations to collect data and to perform intelligent autonomous (local) control of electrical systems and report results back to the remote master(s). Use of an appropriate data highway which loops through the substation is thus necessary.
The design of PLC modem (modulator demodulator) to facilitate the power line as a medium of communication is a challenging task. Power lines constitute a rather hostile medium for data transmission. Varying impedance, considerable noise and high attenuation are the source of degradation of the signals on power line. Moreover, typical power lines are unshielded, high frequency current flowing in power lines turns power lines into antennas and electromagnetic radiation will propagate out of the wires. International Amateur Radio Union (IARU) are complaining because the claim that communication through the power lines, particularly those operating in the range 1.6 megahertz to 30 megahertz, challenge the fundamental purpose of EMC (Electromagnetic Compatibility) standards. Optimum signal to noise ratio (SNR) or in other words, carrier that requires much less power becomes the significant performance parameter in this case. We need to not only utilize existing technologies to build the MODEMs but also create novel ones.
Efficient MODEM design mainly concerns with efficient modulation scheme that suits the power line channel. Efficiency of the modulator clearly is the transmission power which in turn radiates less. With the availability of fast processing signal processors, the claim is that PLC is now mature and ready to be standardized worldwide within an international body of standardization. The technological consortiums are working together to achieve this within the IEEE, and is also working on the issues of interoperability and coexistence of different PLC devices on a common electrical network. In this paper two of the signal processing schemes of PLC, modulation and coding are discussed. In Section II of this paper the use of orthogonal frequency division modulation (OFDM) for a power line channel is acknowledged. In Section III details of different schemes to combat noise effects are given. In Section IV the simulated results for data transmission over fading channel, modelled as power line channel are discussed. The concluding remarks in the Section V provide the knowledge of very recent technical advances that need to be tested for appropriate physical implementation.
MODULATION SCHEME FOR PLC
An easy way to comply with the conference paper formatting requirements is to use this document as a template and simply type your text into it. Electric power companies such as Cinergy, Consolidated Edison and Progress Energy reported encouraging results to suppress noise effects, with Intellon largely contributing to the advances to solve noise problems. INT 5130 chipset, the first to offer, lead other networking companies to announce products to meet standards set by an industrial alliance called Home plug. Intellons chip uses orthogonal frequency division modulation to combat multipath channel effects that are inherent of power line channel so that the data suffers less loss.
A. Definition of OFDM:
Orthogonal frequency division multiplexing is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A sub carrier carrying a portion of the user information is transmitted in each band. Each sub carrier is orthogonal (independent of each other) with every other sub carrier, differentiating OFDM from the commonly used frequency division multiplexing (FDM).
B. How OFDM works:
In practice, the OFDM bundles data over narrowband carriers transmitted in parallel at different frequencies. Spectral efficiency is achieved by using these parallel sub channels that are as closely spaced as possible in frequency without overlapping/interfering, by being orthogonal. Typically, ISI (Intersymbol Interference) comes from multipath delays. By its very nature OFDM defeats multipath distortion. OFDM combats it by transmitting bits in parallel, with each bit being transmitted rather slowly. For example, suppose to transmit 1,000,000 bits per second and to transmit them one at a time, each takes one microsecond to send. Any delay spread longer than one microsecond would cause delayed reflections from multipath to overlap the direct signal for the next bit, thus causing ISI. If instead 1000 bits are transmitted in parallel at a time, on 1000 separate OFDM sub channels, the transmission is 1000 times slower; that is, one millisecond to send them. A multipath delay spread of 1 microsecond would only overlap 1/1000th of the transmission interval for any given bit, thus avoiding the ISI.
C. Why OFDM is applicable to Power Line channel:
Power line is characterized as a multipath channel and hence the disturbances present in the line that carries high frequency signal are equivalent to multipath disturbances. Choice of OFDM as modulation scheme helps to combat the noise effects on the line.
D. OFDM Implementation using DFT (Discrete Fourier Transform):
OFDM can be seen as either a modulation technique or a frequency multiplexing technique. In an OFDM system, data is carried on narrowband sub carriers modulated with different possible techniques, such as binary phase shift keying (BFSK) or quadrature amplitude modulation (QAM). Each sub channel is modulated with a separate symbol and then all the sub channels are frequency-multiplexed, using orthogonal frequencies to increase the channel efficiency. Data is transformed into time domain using IFFT at the transmitter and transformed back to frequency-domain using FFT at the receiver. The total number of sub-carriers is equivalent to the number of points of the IFFT/FFT.
FORWARD ERROR CORRECTION CODING
A. Random and Burst errors:
Random errors are a common phenomenon in memory less channel. Single randomly distributed bit errors occur whose occurrence is independent from bit to bit. Burst errors are associated with memory channels in which noise is not independent from one transmitted symbol to the other; therefore errors tend to occur in clusters. Error control coding schemes with random error correction capacity alone do not greatly improve the system performance of such channels. Only a low transmitting power density will be possible for future broadband PLC, due to strict emission regulations for electromagnetic (EM) compatibility. Burst error correcting codes such as BCH or RS (Reed Solomon) codes find application in such cases to combat the impulse/burst noise effects that last for certain duration. Durations for which the impulse effects stay on line can be analyzed and suitable error correction schemes to be applied.
B. Interleaving and Concatenation:
Thus in general, transmission errors have a strong time/frequency correlation. Interleaving the channel coded data provides diversity in the time domain. Interleaving breaks the time correlation and enables the decoder to eliminate or reduce local fading throughout the band. Interleaving depth should be large enough to break long straight errors. Concatenation is another means of improving the error correcting capacity of the system.
C Recent Advances in channel coding for Power Line:
A. Interleaving and Concatenation principles are utilized to form a revolutionary turbo codes that claim to approach Shannon limit. Extensive research on Convolutional Turbo Codes (CTC), Block Turbo codes (BTC) and means of implementing them with fast processors is being conducted. Space time codes are other revolutionary approaches that utilize diversity in fading channel to mitigate the channel multipath noise effects. As power line channel is a multipath channel, the efficiency of the approach will be coding gain at reduced decoding complexity.
SIMULATION RESULTS FOR THE PERFORMANCE OF OFDM.
MATLAB 7.0 is used for the simulation of the results. The text data written in ‚input.txt file is applied as input to the OFDM transmitter block. The power line channel is modelled by adding random multipath noise to the OFDM data. Text data being transmitted using phase shift keying (PSK) modulation is compared with OFDM transmission. System performance in the form of probability versus SNR plots are found and are shown in Figure 1. Figure 2 shows the level of the IFFT (Inverse Fast Fourier Transform) output of the transmitter that corresponds to text data of consideration here. FFT of size 128 with 32 carrier points is used for the simulation studies. Figure 3 shows the received data which are transmitter strengths distorted with added multipath noise. OFDM receiver shows reduced errors with increase in SNR. There is a coding gain of approximately 2 to 2.5 dBs (decibels) for orthogonal frequency modulation as compared to simple phase shift keying modulation.
CONCLUSION and SCOPE OF FUTURE WORK
Coding gain of approximately 2 to 2.5 dBs provided with OFDM over simple PSK modulation acknowledges the performance of OFDM. We have to always remember that EMC constraints put a limit on the carrier power and efforts to have high coding gain are to be approached. This can be made possible with coded OFDM (COFDM) that uses efficient burst error correction. OFDM and COFDM implementations for real time data text, voice image and video are to be studied using presently available processors. Turbo BCH and Turbo Convolutional codes that use less complex decoding and modulation with desired high bit rate signal constellations are to be developed, proper choice of which results in an efficient MODEM technology.