The electrical supply network cabling was not tested to see if it could transport high frequency (HF) signals. One must therefore consider the constraints of this medium to ensure good transmission of HF signals without disturbing nearby devices, nor affecting the frequencies in the 1-30 MHz band, certain frequencies of this band being reserved for the army or for radio aficionados.
All this must be taken into account in order to provide sufficient bandwidth for the end-user.
Thus the problem arises of limiting the power required to transmit data while ensuring a sufficient bandwidth, and limiting the effects of noise and distortion on the line. The solution: a combination of the strongest signal possible and an optimal coupling between the PLC network and the electrical supply network.
There are two coupling methods: parallel capacitative coupling on the electrical network or inductive coupling using a magnetic core. For interior (indoor) installations, the capacitive coupling is the default when one connects PLC equipment to the electrical plug, the problem therefore only arises for outdoor installations which are much more complex to install.
Data modulation technologies
The main challenge of PLCs is "obtaining" a bandwidth with a low transmission levels, where transmission powers are limited on the power line, or a treatment of the most powerful possible signal to overcome the restriction on transmission levels.
In current solutions, two types of modulation are used: OFDM (Orthogonal Frequency Division Multiplexing) and Spread Spectrum modulation.
OFDM : Orthogonal Frequency Division Multiplexing
OFDM transmission technology is based on simultaneous transmission on n frequency bands (between 2 and 30 MHz) with N carriers per band. The signal is shared between the carriers. The working frequencies are chosen according to regulations, others are "spread out" with the use of software. The signal is transmitted at a high enough bandwidth level and then applied onto several frequencies simultaneously. If one of these decreases the signal will be transmitted nonetheless thanks to the simultaneous transmission. The OFDM signal spectrum provides optimal usage of the allocated band due to the orthogonality of the sub-carriers.
NB: This modulation was chosen by the Homeplug committee, thus all Homeplug standard equipment uses OFDM modulation. This modulation is also used in wireless WiFi (802.11a) transmissions.
SS: Spread Spectrum Modulation
The principle behind Spread Spectrum modulation consists in "spreading out" information over a frequency band much broader than the band actually required, with the aim of counteracting the interference signals and distortion caused by propagation: the signal merges with the noise. The signal is encoded separately, one code is assigned to each user which is then decoded when it reaches its destination. Spreading is ensured by applying a pseudo-random signal called a spreading code. The reception of this signal is seen as noise if the receiver does not know the code. As the signal is transmitted at a lower level than the noise, the bandwidth is low. Spread spectrum modulation is thus optimized to counteract noise, reducing the effects caused by noise.
CDMA Code Division Multiple Access modulation is a type of spread spectrum modulation used in some PLC solutions.
When describing the various existing solutions it can be seen that the solutions which use spread spectrum modulation tend to be narrow bandwidth applications, whereas only those solutions using OFDM to date have been seen to increase in bandwidth.
All PLC solutions must include a robust physical layer and also an efficient network layer access protocol. This protocol controls the division of the transmission media between many customers, while the physical layer specifies the modulation, the encoding and the format of the packets.
The access method used by PLC machines is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), which is the same method used in wireless WiFi networks.
Written by FranÃ§oise Cacciaguerra â€“ November 2003