The Power Line as a Medium

The power line medium is widely viewed as the most ideal infrastructure to allow connectivity between everyday devices. Unfortunately, it is also one of the most difficult environments to communicate in. Unpredictable noise levels and sources, along with dynamic attenuation characteristics make most conventional communication techniques ineffective.

These obstacles involved in communicating over the power line are well discussed in various literature such as: "Power Line Communications: Channel Properties and Communication Strategies" , from which some excerpts have been posted later on in this section.

Repetitive Noise

Many of the devices best suited for PLC, cause some of the worst obstacles evident on the power line. Devices such as dimmer switches on lights, and switching power supplies used by many computers, use the 60Hz AC power characteristics to operate properly. As the direction of the current on the power line changes, components inside these devices 'switch' to compensate.

This causes noise, or a 'spike', on the power line, and as these devices 'switch' along with the AC power cycles, repetitive noise in the form of 'spikes' is created. In many cases these noise spikes can be many times greater in amplitude than the average ambient noise, thereby making the effect of repetitive noise a greater obstacle to overcome, in many case, than ambient noise on the power line.

Dynamic Attenuation

A major benefit of the power line as a connectivity infrastructure is the fact that almost all devices in the home are connected to power in some form or another. However, that very fact creates another great obstacle for PLC: Unpredictable, constantly changing attenuation characteristics.

In most electronic communication situations, calculations can be made to determine to what extent the signals will be attenuated during their travel between source and destination. These calculations usually involve simple factors such as distance between the devices and the communication medium used to connect the devices.

Generally speaking, the inherent impedance of the communication medium and the distance that the signal has to travel along that medium will determine how much of the signal will be lost during the trip. However, on the power line, these calculations become unattainable. The impedance of the power line is not a constant, as it is in most communication mediums. The impedance on the power line is affected by the type and number of devices attached to it at one time.

As devices turn on and off, and even during the regular use of many devices, their effect on the impedance of the power line changes. This creates a purely dynamic impedance environment. In general, the physical distance between two devices can usually be used to determine just how far the signals have to travel. This is not the case on the power line.

The power wiring in any house or building is a complex network and in many cases two devices, only meters apart physically, may be hundreds of meters apart when traveling by the power line. Both these factors further increase the complexity of determining the amount of attenuation a signal will realize between two devices.

Excerpts From: "Power Line Communications: Channel Properties and Communication Strategies"

The following excerpt is taken directly from the thesis by L. Selander refered to above.

It provides a more detailed, technical look into using the power line as a communication medium and some of the problems that are faced in doing so.

"Chapter 4 - Measurements of the Characteristics of the Low-Voltage Grid"

Excerpts From: "PLC and Forward Error Correction"

There is a growing interest in the use of PowerLine Carrier (PLC) for data communication using the intrabuilding electric power distribution circuits. Power lines were not designed for data communications and exhibit highly variable levels of impedance, signal attenuation and noise. Many studies are available that describe in some detail the impedance, signal attenuation and noise characteristics of powerline networks. In their study of residential powerline noise sources, Vines et al. identified 4 types:

1- Sources that generate impulse noise in synchronism with the 60 Hz power frequency.

2- Smooth spectrum noise generated by loads not synchronous with the power frequency (Ex : Universal motor in an electric drill)

3- Non synchronous, single-event impulse noise, (Ex : thermostat or lighting switching)

4- Non synchronous periodic noise.

In general, intrabuilding powerline noise can be considered to consist of continuous, relatively low level background noise punctuated by high level noise impulses. Background noise is typically Gaussian, and its effects on communication performance is well understood. In the case of impulse noise, its time-domain characteristics (amplitude, width and interarrival time) is very important to determine the influence on data communication systems. Chan and Donaldson characterized noise impulses on PLC networks. They conclude:

1- Impulse strength is typically more than 10 dB above the background noise level and can exceed 40 dB.
2- Impulse frequency for the dominant impulse train is usually 120 Hz.
3- Impulse width can vary up to a few percent of the impulse period for 120 Hz impulse noise.
4- Because noise as well as wanted signal are subjected to attenuation, noise sources close to the receiver will have the greatest effect on the received noise structure, particularly when the network attenuation is large.
5- Harmful effects of impulse noise on data communications systems can be expected.

Item 4 above bears significant consequences. It says that when a noise source is located close to a receiver, and when the signal is attenuated (across-phase communication, or attenuation caused by the combination of line impedance and the presence of low impedance loads along the communication link) a local noise source could make a receiver exceed its noise tolerance (signal to noise ratio), yielding erroneous received data.

Single-event noise type can be easily overcome, by simply repeating the data packet. However, the situation is more complex with line-synchronized noise type, because this type of noise can introduce one (or more) error 120 times per second.

If the data transmission process lasts more than 1/120 second (or 8.33 mSec), errors will likely occur, and packets will be lost. In many circumstances, it is unlikely that repeating the packet will circumvent the problem. Breaking the data packet into smaller segments lasting less than 8.33 mSec, and transmitting these segments between noise bursts is also impossible, because the transmitter could have a local noise source, different from the noise source at the receiver.

In other words (and remembering item 4 above), in any PLC system, one should assume that noise at the receiver is unknown to the transmitter. Consequently, it could be difficult (if not impractical) for an intelligent transmitter to analyze the powerline noise characteristics and adapt its transmission strategy, because noise at the receiver end is very likely to be different from noise measured at the transmitter. Moreover, such an intelligent transmitter could not broadcast messages to N receivers, each receiver having its own and different local noise characteristics.

Local noise is not the sole source of communication errors.

Sudden impedance variation can also induce similar effects. Different loads (some compact fluorescent lights, personal computer power supplies, switching power supplies) are known to make the impedance of the communication link change abruptly with time, generally synchronously with the powerline frequency. (These loads generally offer a higher impedance level around the zero crossing of the power wave.) For the same reasons as above, impedance variations at the receiver end is almost undetectable at the transmitter location, making adaptive algorithms difficult to implement. The same principles apply to impedance variation: more severe effects are generally observed when the impairment source is close to the receiver, and the impedance between the receiver and the transmitter is high. Once again, repeating a greater than 8.33 mSec packet will likely not overcome line synchronous, impedance variation related error bursts.