Radiation mitigation for power line communications using time reversal
Résumé
Power Line Communication (PLC) has emerged in recent years as an attractive communication technique. The main advantage of this technology is its ability to benefit from the existing electrical network infrastructure for the transmission of electromagnetic signals. However, electrical wires were not initially designed to propagate communication signals at frequencies above 1 kHz. As a consequence, the communication channel between the transmitter (Tx) and the receiver (Rx) is a difficult channel, generating attenuation and multiple propagation paths. This chapter focuses on one of the main limitations related to the PLC technology, namely, the generation of unintentional radiated signal. This phenomenon is mainly due to the unbalanced nature of the electrical network. The variation of the impedances of the loads connected to the network as well as the unequal length of the live and neutral wires (due to single-phase switches) converts the differential PLC signal into common-mode current flowing through the network. Consequently, the copper wires used for transmitting the useful signal act as an antenna, and part of the transmitted power is radiated. This not only results in stronger signal attenuation at the Rx but also leads to electromagnetic compatibility issues, as the radiated signal may interfere with other existing services, such as amateur radio or short-wave broadcasting. The presented approach tries to simultaneously reach two complementary goals by means of digital signal processing. The first intention is to focus the transmitted signal at the Rx location. The power gain linked to this energy focalisation allows in turn relaxing the required power level at the Tx, hence generating less electromagnetic interference. A second target is to reduce the level of energy dissipated at any location except the intended Rx. In particular, it is desirable to minimise the level of radiated power from the electrical wires. These two benefits already appear as features of a known technique in the field of wireless transmission: time reversal. This chapter presents an experimental analysis of time reversal as a means to mitigate radiation effects for wired signal transmission.