2. POWER LINE COMMUNICATIONS

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

1. Low bandwidth digital devices

2. High bandwidth digital devices