Power line communications stands for the use of power supply grid for communication purpose. Power line network has very extensive infrastructure in nearly each building. Because of that fact the use of this network for transmission of data in addition to power supply has gained a lot of attention. Since power line was devised for transmission of power at 50-60 Hz and at most 400 Hz, the use this medium for data transmission, at high frequencies, presents some technically challenging problems. Besides large attenuation, power line is one of the most electrically contaminated environments, which makes communication extremely difficult. Further more the restrictions imposed on the use of various frequency bands in the power line spectrum limit the achievable data rates.
Power lines connect the power generation station to a variety of customers dispersed over a wide region. Power transmission is done using varying voltage levels and power line cables. Power line cable characteristics and the number of crossovers play an important role in determining the kind of communication technology that needs to be used. Based on the voltage levels at which they transfer power lines can be categorized as follows
1. High-tension lines: These connect electricity generation stations to distribution stations. The voltage levels on these lines is typically in the order of hundreds of kilovolts and they run over distances of the order of tens of kilometers.
2. Medium-tension lines: These connect the distribution stations to pole mounted transformers. The voltage levels are of the order of a few kilo volts and they run over distances of the order of a few kilometers.
3. Low-tension lines: These connect pole-mounted transformers to individual households. The voltage levels on these lines are of the order of a few hundred volts and these run over distances of the order of a few hundred meters.
High-tension lines represent excellent carriers for RF energy as we only find open wire equipment with very few crossovers. A transmission power of about 10 watts is often sufficient to overcome distances of more than 500 kilometers. Around the year 1922 the first carrier frequency system (CFS) began to operate on high-tension lines in the frequency range of 15-1500 KHz. During the past and even nowadays the main purpose of CFS was to maintain the operability of the power supply. While in former times speech transmission was dominated, today we have more and more digital data communications due to the rapid progress of overall automation. Through the application of modern digital modulation and coding schemes, a significant enhancement of bandwidth efficiency could be achieved for CFS.
Medium- and low-tension lines are characterized by large number of cross connections and different conductor types (e.g. open wire and cable). Long distance RF signal propagation is extremely bad in this environment because of high attenuation and impedance matching problems. Around the year 1930 ripple carrier signaling (RCS) began to operate on these lines. These used frequency range below 3 KHz down to 125 Hz with amplitude shift keying (ASK) modulation technique. The data rates achieved by RCS was of the order of a few bits per second. Load management and automatic reconfiguration of power distribution networks were among the most important tasks performed by RCS.
We see that the use of power line communications in the past was mainly for use by the Utility Corporations (UCs) in maintaining the seamless power supply [Juj 98]. The UCs generally regarded the power distribution wiring as a “natural” medium for their communication needs, as all-important stations are connected. Recently, data communications over low-tension lines has gained a lot of attention [Bro99, Kai98]. This is fuel by the explosive growth of Internet along with advances in digital signal processing, error correction coding and electronic hardware. These helped in achieving medium to high data rates over low-tension power lines. Digital devices using low-tension power lines can be categorized based on the bandwidth they use. They are
Low bandwidth digital devices
These devices use carrier frequencies in the range 0-500 KHz and are primarily used for building automation. Different kinds of buildings may be upgraded into “smart homes” by using their power wires for communications. It should be noted that additional wiring for communication purposes is cost effective only in buildings under construction, whereas retrofitting is normally ruled out. A “smart house” can be defined as a building equipped with numerous sensors and actuators, where e.g. heating, air-conditioning, illumination can be automatically and remotely controlled and supervised. Furthermore safety systems such as burglar or fire alarms may be included [Dos97, Bra97].
Frequencies used by these devices is restricted by the limitations imposed by the regulatory agencies [Bro99]. These regulations are developed to ensure harmonious coexistence of various electromagnetic devices in the same environment. The frequency restrictions imposed in two of the main markets, North America and Europe, are shown in figure 1. Federal Communications Commission (FCC) and European Committee for Electro technical Standardization (CENELEC) govern regulator rules in North America and Europe respectively.
In North America frequency band from 0 to 500 KHz can be used for power line communications. However the regulatory rules in Europe are more stringent. The spectrum is divided into five bands based on the regulations. They are
1. Frequency Band from 3 – 9 KHz: The use of this frequency band is limited to energy provides; however, with their approval it may also be used by other parties inside consumer’s premises.
2. Frequency Band from 9 – 95 KHz: The use of this frequency band is limited to the energy providers and their concession-holders. This frequency band is often referred as the “A-Band”
3. Frequency Band from 95 – 125 KHz: The use of this frequency band is limited to the energy provider’s costumers; no access protocol is defined for this frequency band. This frequency band is often referred as the “B-Band”
4. Frequency Band from 125 – 140 KHz: The use of this frequency band is limited to the energy providers customers; in order to make simultaneous operation of several systems within this frequency band possible, a carrier sense multiple access protocol using center frequency of 132.5 KHz was defined. This frequency band is often referred to as the “C-Band”
5. Frequency Band from 140 – 148.5 KHz: The use of this frequency band is limited to the energy provider’s customers; no access-protocol is defined for this frequency band. This frequency band is often referred to as the “D-Band”.
Thus in Europe power line communications is restricted to operate in the frequency range from 95 – 148.5 KHz. Apart from band allocation, regulatory bodies also impose limits on the radiations that are emitted by these devices. These reflect as restrictions on the transmitted power in each of these frequency bands.
Various protocols have been developed for use by low bandwidth digital devices for communication on power line. Each of these protocols different in the modulation technique, channel access mechanism and the frequency band they use. Various products based on these protocols are available in the market and are mainly used for home automation purposes. A brief overview of these protocols is presented here.
High bandwidth digital devices
High-speed data communication over low-tension power lines has recently gained lot of attention. This is fueled by the unparalleled growth of the Internet, which has created accelerating demand for digital telecommunications. High bandwidth digital devices are designed to exploit this market. More specifically, these devices use the existing power line infrastructure within the apartment, office or school building for providing a local area network (LAN) to interconnect various digital devices. It has to be noted that the existing infrastructure for communications like telephone line, Cable TV has very few outlets inside the buildings. By use of gateways between these and Power line LANs a variety of services can be offered to customers. Some of the applications include high-speed Internet access, multimedia, smart appliances/remote control, home automation and security; data back up, telecommunications, entertainment and IP-telephony.
High bandwidth digital devices for communication on power line use the frequency band between 1 MHz and 30 MHz. In contrast to low bandwidth digital devices, no regulatory standards have been developed for this region of the spectrum. Devices using this unlicensed band need to be compliant with the radiation emission limits imposed by the regulatory bodies. It should be noted that internationally agreed, distress, broadcast, citizen band and amateur radio frequencies also occupy this portion of the spectrum. Hence, the technologies being developed for high-speed digital communication over power line should have the ability to mask certain frequency bands for future compatibility. In the section that follows gives a brief overview of power line channel characteristics in the frequency band between 1 MHz and 30 MHz.