Power line communication front-ends based on ADSL technology

The practical implementation of communication over power lines (PLC) using an Asymmetric Digital Subscriber Line (ADSL) front-end is discussed. Both PLC and ADSL modems are based on the same principle of using their respective transmission medium in a frequency range they were never originally designed for. One property both technologies share is the large power that is required when transmitting signals onto the communication medium. The frequency ranges are also comparable. Therefore, ADSL front-ends are evaluated as a power line interface. This paper presents the front-end topology and discusses the signal-to noise measurements that were performed to determine, using Shannon’s theorem, the theoretical data throughput. 1. POWER QUANTIFICATION PROBLEM In some communication applications, the use of the copper conductors of the power distribution as a communication channel might be tempting. Basically, almost all houses and industry buildings are coupled to the power grid, and power outlets are available in virtually all rooms in a building. Electrical energy metering applications may for instance benefit from a reliable data transport via the power cable. In a deregulated electricity market, there will be a necessity to broadcast, at predefined times, the price table of the electrical power for the upcoming period. In the other direction, the customer consumption data are sent back at regular times to the power system operator and retailer. Also information on the quality of the power delivered is important. The communication channel for such applications does not have to be very time critical, but a sufficient reliability level is desired. ‘Traditional’ systems, e.g. wireless, are a candidate, but may be ‘too good’ and may suffer from a bandwidth shortage, making them expensive to use. Telecommunication through a power line is not a new idea – it has even been tested for internet applications – but suffers from substantial technical problems [1], [2], [3]: • Low impedance of the gird, requiring a high transmitting power; • High attenuation of the medium, only a limited distance can be covered; • Impedance varies substantially twice every period of the fundamental of the power system voltage, due to the many non-linear loads (diode rectifiers, saturated inductor cores, etc.); • Impedance changes significantly over a longer period due to the switching on and off of the loads and reswitching of the medium; • Extremely high interference with different characteristics due to a variety of noisy loads; • Multiphase layout of the power system with different topologies (wye or delta connected); • Parallel paths may be available; • Availability or not of an eventually earthed neutral. Many of the difficulties encountered when implementing a Power Line Communication (PLC) system are similar to problems that had to be overcome in ADSL (Asymmetric Digital Subscriber Line) communication systems [5]. Therefore, concepts of the ADSL technology can be adopted to suit PLC requirements. 2. ADSL VS PLC ADSL is a technology enabling high-speed data access over an existing telephone network. The frequencies used (up to 1.1 MHz), are out of the ‘voice band’ and allow downstream data rates up to 10 Mbps depending on the loop conditions, with a smaller upstream data rate. The transmission method applied in ADSL is discrete multitone (DMT). The available transmission bandwidth is then divided in subchannels or ‘tones’. Depending on the signal-to-noise-ratio (SNR) detected at the receiver end in each subchannel, a different amount of bits and transmission energy is assigned to each tone [5]. PLC also intends to use an existing channel out of the band for which it was originally intended. The traditional ‘power bandwidth’ consists of the fundamental power system frequency and its (odd) harmonics, of which the amplitude normally decays quickly towards higher frequencies, where a significant amount of noise is encountered. The frequency bands allowed for this type of communication are, for Europe, set in [6]. This standard distinguishes four bands in the range up to 148.5 kHz, with V 425 0-7803-7448-7/02/$17.00 ©2002 IEEE different access protocols (Fig. 1). Higher frequencies are not allowed, in order to avoid interference with long wave (LW) radio broadcasting. In the US, however, these radios do not exist and the band up to 0.5 MHz is released for PLC. Fig. 1. PLC communication bands as defined in the European standard. The fact that a communication channel has to be established in an electromagnetic environment not intended for the chosen frequencies, makes many problems common to both technologies. First of all, a sort of insulation has to be established against the signals in the traditional frequency band for which the systems were designed. The voltage levels of these signals can be dangerously high in case of power systems. This system also has a high ‘short-circuit power’, requiring high insulation levels and fast protection against overvoltages and overcurrents. Therefore, the differences may not be underestimated. The noise characteristics and constantly altering impedance make the power line environment very challenging for communication and generally yield lower data rates. 3. COUPLING CIRCUIT The insulation and filtering requirements present both in PLC and ADSL lead to adapted coupling circuits. The main principles used are capacitive and inductive coupling. Such circuits have to offer a bi-directional and flat pass band for the higher communication frequency. 3.1. Capacitive coupling A possible capacitive coupling circuit is given in Fig. 2 [4]. It is bi-directional in the frequency range up to 0.5 MHz. The power system frequency is damped by the LC-filter at the grid terminals at a rate of 40 dB/decade, reducing the 230 V/50 Hz voltage to a mere 60 mV. From 10 kHz on, almost no distortion is present. The zener diodes block voltage spikes and possible resonances in the circuit. 10μF 12μH 3.9 mH 0,68 μF V 1 R 1 D 1 D 2 L 1 C 2 L 2 C 1