POWER LINE COMMUNICATIONS
This invention relates to an apparatus and method for use in extracting communications signals from power lines, and 5 for applying communications signals to power lines.
BACKGROUND OF THE INVENTION
It is known to transport telecommunications signals over an electricity distribution or power transmission network. Patent application WO 94/09572 Al (Norweb) describes such a network. Delivering a telecommunications service in this manner is attractive as it overcomes one of the greatest costs in providing a new telecommunications network i.e. installing cabling to each subscriber. Existing electricity distribution cabling is used to carry the telecommunications signals.
FIG. 1 shows an example of an electricity distribution network which is adapted to carry telecommunications signals. Mains electricity enters the network from an 11 kV 20 transmission line 105 and is transformed by substation 100 into a 415V supply which is delivered over cable 120 to subscribers SI, S2, S3. Abase station BS couples telecommunications signals VB, such as voice and data signals, at injection point 110 onto distribution cable 120. The tele- 25 communications signals propagate over the cable on radio frequency carriers to transceiver units TRX at subscriber premises SI, S2, S3. In the upstream direction, telecommunications signals are transmitted from the subscriber transceiver units towards the base station. 30
One of the problems with using electricity distribution cables to transport RF signals results from the structure of the cable. The inner section of the cable comprises a group of phase lines, each line carrying one of the three supply phases. At radio frequencies (RF) the capacitance which 35 exists between these separate lines causes signals which originate on one line to leak or couple onto neighbouring lines. The process of coupling between phase lines introduces a phase shift. Therefore, after propagating along the lines, the components of a communications signal on each 40 line will no longer be in phase with each other but will be of different phase and amplitude. This causes problems with receiving equipment.
A further problem with using power lines to transport telecommunications signals is that of interference from 45 electrical equipment connected to the lines. Thermal noise on the lines is generally at a much lower level than other sources of interference such as broadband interference. Broadband interference occurs periodically, occupying the same frequencies as the RF carriers used to carry telecom- 50 munications signals, and occurring at levels which are high enough to corrupt the wanted signals. Interference signals propagate along the network along with the wanted RF signals, and this reduces the signal to interference ratio at the base station receiver. Some of the sources of interference are 55 domestic appliances, particularly appliances having a motor, businesses with machinery, street lighting and other street furniture. In addition, the emissions of external sources such as radio transmitters can be picked up by the network. External sources are a particular problem where part of the 60 distribution network uses overhead lines.
European Patent Application EP 0 063 296 (Westinghouse) describes a communication system for a three-phase power distribution network in which a remote device has three receiver front ends and a demodulator for 65 independently reconstructing three received signals, one from each line.
U.S. Pat. No. 4,918,422 describes an apparatus for extracting inbound information generated line-to-line in a multi-phase electric distribution system. Signalling is achieved by current pulses.
JP 02-177,731 describes a receiver for a distribution line which eliminates 3-phase noise.
SUMMARY OF THE INVENTION
The present invention seeks to address the above problems.
According to a first aspect of the present invention, there is provided an apparatus for extracting communications signals carried upon RF carriers from power lines, which lines suffer from leakage of signals between them, the apparatus comprising:
means for receiving an RF signal from each of at least two of the lines, which received signals include at least one component of a wanted communications signal; means for variably weighting the received signals so as to increase the ratio of wanted signal to interference plus noise S/(l+N), which interference occupies the same frequencies as the RF carriers. This has the advantage that the components of the wanted communications signal which reach the apparatus via direct and coupled paths are optimally combined to compensate for the phase shift occurring in the coupling process. It also has the advantage that when a wanted communications signal is present on a first line in combination with unwanted interference, the interference on a second line can be used to cancel or minimise the interference on the first line.
The communications signals can be voice, data or broadband.
Preferably the apparatus further comprises: a measuring unit, coupled to the output of the weighting means, which is operable to measure a quantity of the weighted signal which is indicative of the ratio of wanted signal to interference plus noise; and a control means, coupled to the measuring unit, for controlling the weighting means whereby to increase the value of the measured quantity. This allows the apparatus to adapt to changing conditions on the lines, such as during a period when interference occurs on the lines.
Preferably the weighting means weights the received signals in phase and sums the weighted signals. The weighting means may also scale the received signals in amplitude.
Preferably the control means performs an iterative technique in which perturbations are applied to the weighting means. This establishes an optimum combination of the received signals with minimal disruption.
The measuring unit can measure bit error rate of the weighted signal. Alternatively the measuring unit can: measure at a predetermined period during the weighted signal when the wanted signal is known to be absent; measure at a predetermined period during the weighted signal when a carrier of the wanted signal is present, but the modulating signal is known to be absent; or measure during a period when a test signal is known to be present in the signal.
Advantageously the weighting means is operable to weight the received signals in a different manner during each of a plurality of time periods. These time periods can correspond to time slots on a time-shared carrier frequency, such as the carriers used in the Digital European Cordless Telephone (DECT) protocol. This has the advantage of minimising the amount of equipment required, by sharing it among a plurality of subscribers.
Advantageously the weighting means stores settings for use with weighting signals from a particular subscriber who