A Power Line Communication Explained (1984)
Challenges and Technologies
This paper reviews the sources of attenuation, noise and distortion encountered when communicating over AC power wiring. Various technologies which have been used to address these challenges, such as spread spectrum and digital signal processing, are then examined in light of the known channel conditions.
While the idea of sending communication signals on the same pair of wires as are used for power distribution is as old as the telegraph itself, the number of communication devices installed on dedicated wiring far exceeds the number installed on AC mains wiring. The reason for this is not, as one might think, the result of having overlooked the possibility of AC mains communication until recent decades. In the 1920’s at least two patents were issued to the American Telephone and Telegraph Company in the field of “Carrier Transmission Over Power Circuits”. United States Patents numbers 1,607,668 and 1,672,940, filed in 1924 show systems for transmitting and receiving communication signals over three phase AC power wiring. Others have suggested that what was required for power line communication to move into the main stream was a commercialized version of military spread spectrum technology. It has been suggested that this is what was needed in order to overcome the harsh and unpredictable characteristics of the power line environment. Commercial spread spectrum power line communication has been the focus of research and product development at a number of companies since the early 1980’s. After nearly two decades of development, spread spectrum technology has still not delivered on its promise to provide the products required for the proliferation of power line communication. This paper, after reviewing basic power line communication characteristics, examines the advantages and disadvantages of various power line communication technologies from the perspective of extensive research and field experience with each prospective technology. While earlier thoughts that a new technology was needed to overcome communications challenges were correct, it is the intent of this paper to demonstrate that the key technology required is digital signal processing technology (DSP) and that the application of spread spectrum techniques actually decreases reliability in many common situations.
Power Line Characteristics
Evaluation of any communication technology is only relevant in the context of the operating environment. This seemingly obvious point, frequently bypassed in textbook analysis, cannot be overlooked in the field of power line communications. We begin by examining three common assumptions which must be modified in order to be applicable to power line analysis. The majority of engineering texts rely heavily on the principle of superposition. Unfortunately, the conditions required for superposition to be applicable (i.e., linearity and time invariance) are not met for the majority of power line networks. One cause of nonlinearity is when a packet's signal voltage adds to the AC line voltage and causes power supply diodes to turn on and off at the packet carrier frequency. A common example of time variance is when the impedance at a point of a power line network varies with time as appliances on the network are alternately drawing and then not drawing power from the network at twice the AC line frequency. Another area of confusion arises from the common view that wiring capacitance dominates signal propagation effects. This simplified view is rooted in assumptions which do not accurately reflect power wiring environments. While it is true that wire capacitance is dominant for cases where the termination or load impedance is much greater than the characteristic impedance of the wire, power lines are frequently loaded with impedances significantly below the characteristic impedance of the wire. Common examples of...
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