Communication Techniques On The Power Line

Many of the previous attempts at developing a reliable, robust PLC technology resulted in failures caused by overcomplicated designs. The problems faced when trying to communicate in a harsh environment, such as the power line, can be very daunting. These problems often cause designers to attempt overly complex methods, which result in being unreliable, difficult to work with, and extremely costly.

Communication Techniques

There are many common techniques used for digital communications, each with different strengths and weakness. Obviously, not all would be suitable for the harsh power line environment, so a select few that are commonly used in PLC are explained in further detail below:

Frequency Shift Keying (FSK)

Using FSK, the digital data is modulated with a simple carrier wave to produce a resultant signal that contains two different frequencies. In this method, frequency A represents a digital 0, while frequency B represents a digital 1.

The receiver in an FSK system must use hardware, such as a Phase Locked Loop (PLL), in order to 'lock' on to the signal so that the filtered signal that is received can give an accurate representation of the original signal. However, as is the case with the Costas loops used in many PSK systems, the unpredictable noise present on the power line makes it very hard for the PLL to maintain a stable 'lock', causing the incoming signal to become corrupted and difficult to receive accurately.

Phase Shift Keying (PSK)


Binary-Phase Shift Keying

A widely accepted and flexible technique called Binary-Phase Shift Keying (BPSK) can be used effectively in a PLC solution. BPSK is a method of data encoding that uses phase angle measurements of the incoming signal to determine the data being received. This is accomplished by modulating the data being sent with a simple carrier wave. The receiving hardware measures the phase angle of the incoming signal and uses the phase information to determine a 1 or 0.

One problem often encountered when using BPSK is the need for the receiver to be able to 'sync' to the incoming signal. In conventional BPSK, a specific phase angle or range of phase angles represents a digital 1 or 0. If the receiver drifts out of 'sync' with the incoming signal, the measured phase angles will fall out of the accepted ranges and the data will be lost. Therefore, this type of system requires the hardware to be able to continually readjust its sample timing in order to maintain its 'sync'. Most standard BPSK systems accomplish this continual syncing using hardware such as a Costas Loop. This hardware is usually quite costly and proves to be ineffective as the level of ambient noise in the system increases.

To overcome the unique communication obstacles within the power line as discussed earlier, some non-standard modifications are required.

Differential Binary Phase Shift Keying

Differential BPSK uses conventional BPSK methods, but instead of representing digital data with a specific phase angle value, digital data is defined as a specific value of difference in phase angles between the current set of samples and the previous set of samples.

This change in technique reduces the need for the hardware to 'sync' to the incoming signal. Since the data is represented by differences in measured phase angle, the only phase relationship that matters is that between the two most recent measurements. If the two measurements have a phase difference close to 0 degrees, the data is considered a 0. If the measurements differ by close to 180 degrees, the data is considered to be a 1; or vice versa.

By reducing the need for the hardware to 'sync' to the incoming signal, not only is there a cost savings in hardware, but the system is also able to handle higher levels of ambient noise without losing the incoming data.

Error Detection/Correction

Error detection and correction is key to any reliable communication system. In the harsh environment of the power line this is even more important, as the likelihood of errors in data during transmit from source to destination is much more likely. Any errors generated in a data packet need to be recognized and filtered out by the system.

Standard techniques such as CRC16 are widely available and used, but are not always effective enough in a PLC environment. Error detection/correction techniques that are specialized for the specific problems involved in effective PLC communication have been developed by various companies and individuals. These type of techniques have been implemented in the technology used in Synaptech's solutions.

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 presents an in-depth examination of some of the possible communication methods suitable for the unique power line environment.

"Chapter 6 - A Modulation Method for the Power-Line Communication Channel"

Excerpts From: "PLC and Forward Error Correction"

When analyzing the different PLC technologies available on the market today, one notices two fundamental and distinct design approaches.

Certain well-known PLC technologies assume that the modulation and demodulation technique they implement is sufficient to guarantee the minimal threshold of reliability that the consumer demands. Such technologies would have us believe that they possess an ultra-sturdy Magic Bullet of sorts that will lay waste to the obstacles on its path. Such technologies give little importance to communication error management and do not implement Forward Error Correction (FEC).

The second, and in our opinion, more prudent and more realistic approach assumes that a PL communication channel is an extremely difficult medium and that communication errors will occur. Technologies conceived from that point of view actively manage communication errors and implement, for example, an FEC technique.

Most Powerline Carrier systems and technologies offer some kind of error detection technique, (like Cyclic Redundancy Check or CRC) so that erroneous packets are not interpreted. To further enhance the possibility of reliable communication in hostile environments requires appropriate signaling schemes and error control strategies. It is well known that Forward Error Correction (FEC) techniques can significantly improve communication reliability.

Simply stated, FEC adds redundant information to the original message, allowing the receiver to retrieve the message even if it contains erroneous bits. Facing line synchronous error bursts, (and particularly when item 4 above is involved) typical non-FEC systems are often unable to deliver any usable data. They only detect that the message has one or more errors, but are unable to correct the erroneous bit(s) in the received packet, and reject the data packet. On intrabuilding Powerline networks, Donaldson et al observed typical FEC coding gains of 15 dB at 10 –3 decoded Bit Error Rate (BER). (Our own experiments also show spectacular results.) This is a very significant improvement of noise tolerance, (Signal-to-noise- ratio) as well as other impairments tolerance like sudden impedance variation. "Stated differently, FEC coding typically enhances decoded Bit Error Rate values three order of magnitude" relative to systems not implementing any Forward Error Correction techniques.

FEC is a well known and relatively simple technique that significantly increases communication reliability in noisy channels like Powerline. But FEC is not totally "free"! It involves adding redundant data , meaning that more bits per message must be carried out. In other words, it decreases the throughput (number of effective data bits delivered per second) in noiseless environments, where FEC would not be mandatory. But powerline is not such an environment. In noisy networks, FEC can even increase the net throughput over non-FEC systems, by reducing the need for packet repeating. In severe situations, (particularly in presence of line synchronous impairments, causing error bursts) FEC enabled products can often deliver data where other non-FEC products simply crash.