Introduction
During the last years, the increasing demand arising from residential subscribers for high data rate connections to and within homes spurred an intense research activity, both from academia and industry, towards innovative transmission techniques. This lead, on one side, to the development of transmission systems yielding a more efficient utilization of conventional copper line telephone, such as in the case of multi-tone transmissions for high speed asymmetric digital subscriber line (ADSL). However, alternative access techniques avoiding the bottleneck of wired telephone line (and the relevant fees due to telecom companies) received particular attention, too. Among them, we recall radio access tecniques, either terrestrial in the millimetric wave band (the so-called wireless local loop, WLL) or via direct-to-home (DTH) satellite (e.g. the NETSYSTEM wideband internet access broadcast via Astra satellites). In this panorama chracterized by a flourishing of proposals, power line communication (PLC) systems, i.e. data transmission techniques over conventional electrical wiring, emerged as a possible alternative for providing digital communication services to residential and business users [1-5]. As matter of fact, the possibility of using electrical wirings as carriers for data networks within homes and offices without any new cabling represents a strong point in favor of PLCs. Accordingly to this, the recent years witnessed a tremendous growth of the interest in the development of PLC, often with the strong support of major electrical companies. The outcome of the main studies on this subject is represented by an abundance of PLC-based systems proposed for various kind of applications: industrial, commercial and residential. In particular, the main applications of PLC for residential users are in the following fields:
• residential local area networks (LANs);
• intelligent in-home devices, i.e. embedded networked controllers of a variety of in-home devices including appliances, light switches, security systems, and electric meters;
• ubiquitous and low cost IP connections.
However, the perspective of a widespread diffusion of PLCs within homes and buildings rises serious concerns among radio listeners and broadcasters [6-7], which are extremely worried about possible detrimental interferences caused by PLCs on radio receivers.
This report aims at providing some additional elements for a better evaluation of this controversial issue. In the following, we will review some topics related with digital transmission over power lines. After, we will report some preliminary experimental data concerning the interference at radio frequency generated by PLCs. Finally, we will draw some comments followed by the customary section devoted to the conclusions.
Features of Power Line Communications
Channel Characterization
In PLC, the physical carrier medium for digital data is the conventional power wiring (commonly available in almost any building) that acts therefore as a transmission line, upon proper de-coupling of transmit and receive equipments from the 220 V (or 125 V) power network voltage. It is fairly evident that the characteristics of power wiring are well far from optimal as far as data transmission is concerned. Let us now briefly review the main channel characteristics for powerline communications [4].
Uncertain network topology. The PLC wired channel is time-varying since there can be a lot of plugged in electrical devices that are turned on and off all the time. Furthermore, the computers or appliances that want to communicate with each other may stay in different phases.
Attenuation. There are many factors that cause attenuation, such as the phase coupling losses, the device impedance, and insertion losses due to the appliances that are plugged in the power line. Another major attenuation factor is the very large inductance at the main transformer that attenuates the high frequencies signal normally used for the data communication. Moreover, attenuation in power lines greatly varies with the frequency. Actually, the frequency response exhibits deep attenuation peaks (notches) at some random frequencies. The attenuation is frequency and time dependent and can reach up to 60 dB in the deepest notches.
Noise or interference. The PLC's medium is inherently noisy. The main sources of noise are electric appliances that are plugged into the power line and cause impulses that reveal very harmful for the data signal. For example, the dimming of the lighting system, the starting of an air compressor in the air conditioning system or a refrigerator, a vacuum cleaner or electrical drills cause harmonics similar to the switching power supplies (battery charger).
Line impedance. Experimental measurements reported in the technical literature demonstrated that a typical power line exhibits a low value impedance. For instance, the results reported in Ref. [1] indicate an impedance of about 0.1-100 W in the frequency range100 kHz-1 MHz, while the results reported in Ref. [8] indicate an impedance of about 10 W in the frequency range10 kHz-100 kHz.
Digital Transmission Techniques for PLCs
The transmission techniques adopted for PLC must be capable of enhancing signal robustness with respect to distortions and interferences experienced during the propagation in power lines.
The most popular approach, addressed to as 'spread-spectrum', consists in artificially expanding the spectral content of the signal before transmitting it [9]. The spectral spreading is accomplished by multiplicating the useful data stream with a pseudo-random binary sequence at higher symbol rate. As a consequence, the resulting signal sent on the power line yields a bandwidth occupancy several tens of times wider than the original one. The receiver reconstructs the original data stream 'squeezing' the spread-spectrum signal by means of a correlation with a local replica of the pseudo-noise sequence used in the transmitter. This 'de-spreading' operation shrinks also those parts of the signal spectrum heavyly affected by deep notches of the frequency response of the channel, thus minimizing distortions.
An alternative approach for digital transmissions over highly frequency-selective channels consists in splitting the high speed data stream into several low rate fluxes and transmitting each on them by using a different carrier frequency, denoted 'sub-carrier' [10]. This technique, also known as 'multi-carrier transmission', reveals robust against frequency-selective distortions because a notch in the frequency response affects only one sub-carrier which is bearing a low data rate stream.
Standards for PLCs
While there is not a single ubiquitous standard across all industries, three de-facto standards have emerged for home networking: X-10, EIA-709, and CEBus. All three of these technologies support PLC for providing services to existing homes (EIA-709 supports additional media including unshielded twisted-pair, too) and all three of these standards have been widely adopted by multiple vendors and effectively set the stage for explosive growth in home products ready for networked access. Among the three technologies, EIA-709 in particular has found world-wide acceptance in both commercial and home environmental control, security, and lighting applications.
X-10
The PLC technology called X-10, developed by X-10 Corporation [11], uses existing AC wiring to send high-frequency command signals from remote locations. The signal travels over AC wires by taking advantage of the physical characteristics of AC current. AC current alternates voltage between 120 volts positive and 120 negative 60 times per second. The resulting 60 Hz power curve reveals a point in every cycle where the voltage is zero: the so-called zero crossing point. The signal itself is a 5 volt, 120 kHz pulse at the zero crossing. PLC transmitters and receivers can send and receive only during a 200 millisecond window. In this window the voltage on the line does not exceed 6 volts. Information is sent as a coded digital sequence that contains information for identifying the receiver, and for commanding its operation (on, off, dim, brighten). The original version of the system supported low bit rate only (60 kb/s), but extension to higher speed is currently investigated.
EIA-709
EIA-709 originally proposed by Echelon LonWorks [12] and subsequently standardized by the Electronic Industries Association (EIA), defines a communication protocol for networked control systems in a home, supporting both simple on/off devices and complex devices. Physical communication occours over power lines inside and outside of homes over 120V AC to 240V AC wiring. The power line channel occupies the bandwidth from the 125kHz to 140 kHz and communicates at 10 kb/s by using spread-spectrum technology. The standard supports both two and three phase electrical configurations and calls out a narrow-band power line signaling technology that meets regulatory requirements for North America and the European Union.
CEBus
The Electronic Industries Association (EIA) defined a standard communication protocol for PLC, named CEBus (Consumer Electronics Bus) [13], also EIA IS-60, which was further developed by CEBus Industry Council. The CEBus standard is an open standard that provides separate physical layer specification documents for communication on power lines and other media. Intellon [14] is a private company producing products that conform to the CEBus standard. The Intellon technology is oriented toward providing control capabilities to home networks and consists of two fundamental components -- a transceiver implementing spread spectrum technology and a micro controller to run the protocol. Data packets are transmitted by the transceiver at about 10 kb/s, employing spread-spectrum technology. Each packet contains the necessary sender and receiver addresses. Intellon offers products ranging from chip sets to board solutions, depending on the level of integration the manufacturer wants to perform on their own.
Other PLC Systems
In today's market, there are a number of companies that currently develop powerline communication products and aim for 1-10 Mb/s data rates. Adaptive Networks [15], like Intellon and Echelon, offers power line chip sets based on spread spectrum technology. However, it offers both low- and high-speed chip sets, with data rates of 19.2 kb/s and 100 kb/s respectively, and future versions will transmit at 10 Mb/s. Interlogis [16] proposes another PLCsystem based on FSK modulation transmitting up to 350 kb/s, with future expansions up to 1 Mb/s. Enikia Inc. [17] uses a proprietary technology capable of supporting a bit rate of 10 Mb/s. Swiss company Ascom [18], developed a PLC system operating at 3 Mb/s. Other PLC systems are developed by Domosys Corp. [19]. Further information on PLC systems and standards can be found in [20-25].
An Experiment
The Experimental Setup
In order to assess the possible interference caused by PLC on the reception of shortwave broadcasts, a simple experiment has been carried out during the afternoon of March 2nd, 2001 at the Faculty of Engineering of the University of Pisa, in Italy. The multiband radio receiver used in the experiment was a Sangean ATS 818 ACS wich includes also a tape cassette recorder. This particular model was selected because of its recording capability and because it is a commercial-grade receiver with average performance as far as selectivity and sensitivity are concerned. The receiver, that was battery operated, was placed on a desk in the middle of a classroom and a square loop made of standard 2.5 mm2 electric wire was placed on the floor around the desk. The loop was meant to simulate a domestic power line carrying digital signals and therefore it was closed on a 50 W resistor and fed with a voltage waveform v(t) provided by a Wavetek generator. Concerning the signal, in order to simulate a digital transmission, we generated a square wave at frequency f0 with peak-to-peak (pp) voltage of 2 V, measured in loaded conditions. The classroom is located inside the main building of the Faculty of Engineering and during the experiment the only other electric device active inside was a network analyzer placed 6 metres far apart from the receiver. Shortwave reception trials described hereafter were carried out by using only the receiver's telescopic antenna fully extracted. The experimental setup is sketched in Fig. 1.
Measurements
The first reception trial was performed with the Wavetek generator turned off (i.e. without simulated PLC interference). The BBC-WS on 9410 kHz revealed to be the strongest and the best quality signal available under such circumstances. Actually, despite non optimal operating conditions, reception was very good, as testified by the tape recording enclosed to this report. The Wavetek receiver was then turned on in order to simulate the effect of a PLC signal. The frequency of the square wave was set approximately to f0 =1882 kHz, so as to produce a 5th harmonics at 9410 kHz, i.e. right superimposed on the radio carrier of the BBC's signal. Actually, the frequency f0 was kept sweeping a few kilohertz across the radiofrequency channel band centered around 9410 kHz. Despite its strong signal level, the BBC-WS turned out to be badly affected by the interference caused by the loop. This is again testified by the tape recording enclosed to this report.
Comments
According to the results of the experiment above, we can draw some comments about the possible effects of PLC on shortwave reception. Let us first start with some reasons of concern.
1) The voltage used in the experiment (2 V pp) is comparable with that of practical systems [8], and it is even lower than that delivered by some commercial devices for PLCs. For instance, the power amplifier decribed in Ref. [26] can deliver up to 6 V pp.
2) The load impedance (50 W) is comparable to that of real power lines (1-100 W).
3) The signal of BBC-WS was probably the strongest available during the experiment at that location, and the detrimental effect of PLC interference will have even worst effects on weaker signals.
4) The receiver was battery operated, thus not directly connected to the power line. In the case of receive operations with power line supply, a further amount of PLC inerference is likely to be injected into the receiver through the power supply cable.
5) The interference generator was operating at about 2 MHz, but many PLC systems, currently under development status, foresee much higher rates, up to 30 Mb/s (that roughly corresponds to a 30MHz signal bandwidth), and this could cause a very high interference level.
We acknowledge also that the experiment did not fully reproduce the operating conditions of a real PLC system, as is apparent from the following considerations.
6) The wire loop surrounding the receiver was rather tight (2.5 x 2.5 m). In the practice, it the receiver may be placed farther from power wiring, thus experiencing a lower interference.
7) In the practice, power wires run into the walls and this also reduces the interference.
8) It is unlikely that the wirings completely surround the receiver.
9) The periodic square wave at frequency f0 that was used as the test signal has strong power concentration on the harmonics at 3 f0, 5 f0 etc. In the practice, the digital signal that will be transmitted into the power line will have a different spectral content. This issue needs further investigation.
The goal of this simple trial was not to definitely demonstrate that PLC systems actually do (or, hopefully, do not) interfere with radio receivers, in any condition. As matter of fact, the large number of parameters involved, renders this kind of investigation a tremendously prohibitive task. More simply, our goal was to demonstrate that, under some circumstances, electromagnetic coupling between transmissions over electric wirings and a shortwave receiver may occour and cause serious problems for radio listening.
Conclusions ?
As the question mark in the title suggests, the issue of interference from PLCs on radio receivers can not be easily solved and the jury is still out for a final verdict. However, a few preliminary experimental results raise some serious concerns about the possible disturbances induced by PLCs on radio receivers. As matter of fact, it is highly possible that, at least under some unfavourable conditions, a strong interference from PLC can badly affect radio signals, especially the weakest ones. Further investigation on the subject is mandatory indeed in order to provide resolutive answers to this dilemma and to preserve radio communications from possible PLCs' letal electromagnetic pollution.
References
[1] D. Redford, "Spread Spectrum Data Leap through AC Power Wiring", IEEE Spectrum, November 1996, pp. 48-53.
[2] Amitava Dutta-Roy, " Networks for Homes ", IEEE Spectrum, December 1999, Vol. 36, No. 12, available on the web: http://www.spectrum.ieee.org/pubs/spectrum/9912/hnet.html
[3] M. Propp, "The Use of Existing Electrical Powerlines for High Speed Communications to the Home", available on the web: http://ksgwww.harvard.edu/iip/doeconf/propp.html
[4] Aura Ganz,"Technology Developments for Quality Multimedia Delivery for Residences: Coupling of the broadband and home network technologies", available on the web: http://www4.nationalacademies.org/cpsma/cstb.nsf/ 44bf87db309563a0852566f2006d63bb/d4a118651b44c3128525693e0053b588/$FILE/ganz.pdf
[5] A.Ganz, A.Phonphoem, N.Llopis, K.Wongthavarawat, Amherst, and Z.Ganz, " Converged Voice, Video, and Data Wired-Wireless LANs Testbed", MILCOM 1999, Atlantic City, NJ, November 1999.
[6] www.rnw.nl/realradio/html/se35_01_05.htm
[7] www.rnw.nl/realradio/html/medianews.html
[8] P.K. Van Der Gracht, R.W. Donaldson, "Communication Using Pseudonoise Modulation on Electric Power Distribution Circuits", IEEE Transactions on Communications, Vol. 33, No. 9, September 1985, pp. 964-974.
[9] R.C. Dixon, "Spread Spectrum Systems with Commercial Applications", Wiley Interscience, New York 1994.
[10] J.A.C.Bingham , "Multicarrier Modulation for Data Transmission : An Idea Whose Time Has Come", IEEE Commun. Mag., Vol. 28, No. 5, pp.5--14, May 1990.
[15] http://www.adaptivenetworks.com
[20] http://hometoys.com/htinews/apr98/articles/wacks/wacks.htm
[22] www.utc.org/pltf/mainpl.htm
[23] www.exp-math.uni-essen.de/~vinck/plc/
[24] www.eecg.toronto.edu/~tooraj/powerline/refrence.html
[25] www.eecg.toronto.edu/~tooraj/powerline/faq.html
[26] INTELLON technical data sheet of SSC P11 PL Media Interface IC, available on the web: http://www.intellon.com/docs/datasht/24000624.pdf
