Selection of an optimal modulation scheme for digital communications over low voltage power lines

There is an ever increasing demand for environmental management of buildings, office automation, security monitoring or remote control of customer appliances and remote meter reading. The AC low voltage power line would be an ideal communications medium for digital data. Several systems for power line communications (PLC) were presented. The reliability of PLC is strongly influenced by time-variant and frequency-selective attenuation and interference on the power line channel. Various experiments were made to evaluate different PLC systems and set up a channel model. This paper gives a short summary of typical transfer characteristics and interference to be observed at power line networks. It is shown that at transmission over a single phase of the power supply system the maximum phase jitter of the time-variant transfer function is in most cases below 90°. In the second part important modulation schemes are evaluated for applicability to power line communications. This evaluation considers chirp modulation, direct sequencing phase shift keying modulation (PN/PSK) and hopping techniques with spectral redundancy given by carriers with M different frequencies. The selection of the optimal modulation technique considers the European standard EN 50065, which restricts both the frequency range and the maximum signal amplitude. This paper deals with modulation schemes, using M frequencies, namely classic frequency hopping modulation (MFH) and the multifrequency,phase modulation (MFH/PSK and MF/PSK). Significant advantages of M frequency phase modulation in comparison with classic frequency hopping are pointed out