Power line channel characteristics
Assessment of commercial viability of a communication technology (or any technology for that matter) is only relevant in the context of its operating environment rather than the theory of communications in general. While the evolution of wireless communication for decades yielded significant characterization of the wireless medium resulting in huge advancements in wireless communications and communication technology in general, there was, in comparison, only small amount of characterization performed on the power line as a ubiquitous communication medium, and its specific challenges only now are becoming better understood.
The typical noise on the power line network is both time and frequency dependent. Some of the key characteristics of the power line environment, especially in the lower frequency region are:
• Impulse and tonal noise
• Significant and variable attenuation and propagation loss
• Often severe interference with time varying noise sources
• Dynamically changing channel due to load and noise variation
As one would expect, there are many sources of noise on the power line network. Some are due to the devices connecting to it and others due to the network itself, which in many cases is old and was never provisioned with communication in mind. Below are a few typical examples.
Activation of many kinds of devices can be a source of impulse noise. However, the most common impulse noise sources are light dimmers. These devices introduce high impulse noise, as they connect the lamp to the AC line part way through each half AC cycle. When the bulb is set to medium brightness impulses of several tens of volts are imposed onto the power lines at twice the AC line frequency.
Another form of noise is “tones”. The most common sources of tonal noise are switching power supplies, which are common in many electronic devices such as personal computers and electronic fluorescent ballasts. Many devices, such as televisions and computer monitors contain other high speed switching systems. The fundamental frequency of these systems is anywhere from 15 kHz to 1 MHz or more. The noise that these devices inject onto the power mains is typically rich in harmonics of the switching frequency. Figure 1
presents the spectrum of a typical DC charger injecting into the power main switching frequency harmonics at 70 kHz (main frequency), 140 kHz, 210 kHz, 280 kHz, etc.
Figure 1 – Typical DC charger spectrum
While typical channel simulations often rely on the principle of superposition, which inherently assumes linearity and time-invariance of the noise sources, neither of these assumptions holds for the power line environment, making theoretical analysis extremely difficult. As an example, the impedance at any point of the power line network varies with time as appliances are added, removed or change their power draw from the network. It is also not uncommon to observe different signal attenuation in different directions along the same path, i.e. signal attenuation from point A to B compared to the signal attenuation from point B to A.
Another challenge presented by the high variety of devices connected to the power line and the variation of load is the variance in line attenuation and its frequency dependence. Loads that present low network impedance at communication frequencies compared to the characteristic impedance of the wiring (e.g. heating elements), cause the wiring inductance, rather than its capacitance, to dominate the propagation effects of the communication signal.
presents an example of a real life power line channel that combines many of the noise sources discussed above.
Figure 2 - Unpredictable noise signature in narrowband channel (10-500KHz)