“Power line communications (PLC) is a global technology with worldwide interest in its development. In its simplest terms, PLC modulates communication signals over existing power lines. This enables devices to be networked without introducing any new wires or cables. This capability is extremely attractive across a diverse range of applications that can leverage greater intelligence and efficiency through networking. ” – Texas Instruments. Introduction: Let me start with a question and provide you a relevant answer to this: Why Power Line Communication became important?
Power line Communication is currently emerging technology that is consequently attracting many researchers. Digital communications over power lines is a very old idea that was introduced back at the early 1920s. Many patents were filed in this area. Since then, all the utility companies around the world are successfully using this technology for metering, control and maintenance. The liberalizations of telecommunications and the deregulation of electric utilities also added new dimensions to the potential application of the electricity infrastructure.
Recently, there is being a vast growth and a rising interest in exploiting the power grid to provide broadband Internet access to residential customers, offices and industries. The most astonishing aspect of this idea is the presence of infrastructure that is in place for power distribution, and the rise of multimedia services over it could be much more valued than any existing wired alternative. For example, if we try to rebuild the whole transmission system using optical fiber instead of cabling it would cost a great deal and would be irrelevant, whereas providing access to the Internet y using the existing cable lines is more beneficial. As electrical power is indispensible so is the access to the Internet. PLC modems used are normally plugged into an electrical outlet, and the combination of these two networks is quite interesting and makes it a good option. Looking into the statistics of other broadband options such as xDSL and cable modems have only reached 10 percent of US. Even though 60 percent of households are connected to the Internet, there is a tremendous opportunity for power line communications to bridge this gap.
On the other hand, there is also a growing development in the area of reusing in-building power line cables to provide a broadband LAN within the home or office. The major advantage offered by power-line-based home networks, is the availability of an existing infrastructure of wires and wall outlets, so new cable installation is not required. Analyzing the above discussion we can come to know that Power Line Communication is unimaginably important and sustainable. MIMO-OFDM based Broadband PLC: Initially the Power lines were not designed for communication purpose.
But researchers began to exploit the power lines and used it for communication purposes. Digital transmission is the core of any communications system, including Power Line Communications (PLC) systems. When a new communication channel or medium is investigated by researchers they are many considerations that we need take look into among the known modulation and coding techniques. Unusual channels such as the PLC channel or medium may require adaptation of the known techniques or inspire research into developing new modulation and coding techniques with better performance.
There might be more than one technique that suits the PLC channel. For different applications, different channel conditions should be taken into considerations. A unique condition may outperform the others and robustness under different conditions is of prime importance as the PLC channel is not so easy to predict. Fair amount of research is going on in modulation and coding for the two classes of PLC systems, namely narrowband and broadband PLC systems. Lately, research on MIMO-OFDM based broadband power line communications is progressing rapidly and outperforming the other techniques.
As we know smart grid is the future of the electric power line network, which allows the exchange of information among electric power providers, consumers and electric industries. As BPLC allows high-speed data transmission over existing electric power lines, these channel experiences severe channel distortions due to multipath fading and impulse noise. The Middleton class A model for impulse noise and the Zimmermann frequency model for power line multipath fading is used to overcome them. Middleton class A model, whose pdf is defined as,
Where, ?2 = ?2G + ?2I (?2G is the Gaussian noise variance and ?2I is the pure impulse noise variance), ? = ?2G/?2I and A is the impulse index. And now the Zimmerman frequency PLC channel mode, whose transfer function at jth antenna path is expressed as, Where, L is the number of fading paths. ?0, ?1 and u are the power line cable parameters, and |gj,l |?1 is the weighting factor of the jth antenna and lth fading path. dj,l / vp is equivalent to the corresponding path delay ?j,l (where dj,l represents its length) as follows: Where, ?r is non-insulation dielectric constant of the cable and c0 is the speed of light.
Typically, each OFDM subcarrier has flat (constant) frequency channel characteristics due to its narrow bandwidth, such that the frequency selective fading transfer function can be translated (digitized) and is approximated as follows: In this paper they designed a MIMO-OFDM system that contains I(transmit antennas) & J(receive antennas). In the OFDM transmitter, the kth subcarrier modulation signal, S(k) experiences the following inverse fast Fourier transform (IFFT): Where, s(n) is the nth (= 0, 1, …, N-1) time sample and N is the number of subcarriers.
For BPLC the transmitting and receiving antennas are limited unlike MIMO in wireless systems. For a 3-phase 4-wire the number transmitting and receiving antennas is 2 and 1 for single-phase 2-wire. MIMO-OFDM can be used both indoor and outdoor but SISO-OFDM is mostly used only indoors. The sheath covering the cables acts as the ground. 3- Phase 4- Wire Power Line Interior Structure A space frequency encoder (SF) is used to reduce error probability caused by the interference in MIMO channel. The S1 & S2 are two SF encoder vectors that are formed by arranging the same sub-carrier signal samples and S2 is circular shifted version of S1.
S1 and S2 are converted to time sample vectors by IFFT i. e. , to s1 & s2. Now they are transmitted through the antenna to the receiver. The transmitting process occurs at the transmitter section where modulator and signal encoder is present and the receiver section consists of linear combiner and detector. The cyclic prefix (CP) is added to the OFDM modulated sample vectors (s1 & s2) before transmission to prevent ISI (inter-symbol interference) due to multipath delay. The signal that is recovered by the fading is sent to SF decoder for decoding process i. e. FFT, inverse circular-shifted process, MRC process and then by removing the CP which was added before to recover the data stream. Here they considered MRC with and without cross talk. 2?2 MIMO-OFDM PLC system block diagram is given below: Issues: EMC Issues (EMC –Electromagnetic compatibility) The ability of a device or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances in the form of interferences to any other system in that environment, even to itself. Below diagram will help us understand the characteristics of EMC.
Disturbances over PLC: Noise Classification: * Colored background noise * Narrowband noise * Periodic impulsive noise, asynchronous to the main frequency * Periodic impulsive noise, noise, synchronous to the main frequency * Asynchronous impulsive noise Mainly colored background noise and Asynchronous impulsive noise is taken into consideration for channel modeling as it has greater impact on the communication in power lines at low frequencies. Colored background noise has Quasi-Static behavior and can be modeled by where, | s| b| c| Max| -94| -80| -0. 4| Min| -124| -100| -0. 6| Average| -105| -90| -0. 5|
Fig below shows us the 3-D graph between background noise spectrum, frequency and Time. Random Impulsive Noise is caused by frequency bursts generated by electrical devices connected to the powerline. We can measure noise using the setup, which is shown below: Cross talks (other disturbances): There are other disturbances like Cross talks, which can affect the communication over Power lines that have to be considered. Cross talk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. There are two types: . NEXT (Near End Cross talk): Interference between two pairs in a cable measured at the same end of the cable as the interfering transmitter NEXT Formation 2. FEXT (Far End Cross talk): Interference between two pairs of a cable measured at the other end of the cable with respect to the interfering transmitter. FEXT Formation Standardizations: PLC Standardization bodies: Importance of Regulations and standards is equally important when compared to the technological developments. One should know about the governing bodies, which implement standards to control the usage of spectrum and other parameters.
If one violates the standards and doesn’t know about these bodies can never achieve what one wants to achieve. International Bodies: IEC committee Regional Bodies: TC 77, CISPR, Product Committees, ITU, ISO, Asia and Pacific Organizations, CENELEC. National Bodies (In USA): RA, FCC, Reg TP Conclusion: Power line communication is the future and many researches are going on extensively. Interesting feature is MIMO-OFDM technology and its implementation and modeling that will improve the broadband power line communication and at present which is considered as the future of Power line communication.