System Architecture for Power-Line Communication and Consequences for Modulation and Multiple Access Gerd Bumiller iAd GmbH Unterschlauersbacher-Hauptstr. 10, D-90613 Großhabersdorf, Germany Phone: +49 9105 9960-51, Fax: +49 9105 9960-19, Email:
[email protected]
PaperID: KE1137 Affiliation: T15) System Architectures Keywords: System Architecture, OFDM, TDMA/CDMA, Channel Characteristic
Abstract In this paper the requirements for power-line communication and the medium access and communication method are discussed, and a short overview of the behaviour of the transmission channel is given. Proving the statements given here is beyond the scope of this paper, but it could be the basis of further and hopefully intensive discussion of system architecture.
1.
Introduction
System architecture for power-line communication is very critical and has to be done in an early state of the development. The power-line channel is very hostile, because many concepts work partly, and prove only after completion of development to apply to less than 50 - 90% of the cases, which is insufficient to go to market. Usually funding for a second approach is not available anymore and the company leaves the PLC community, as we have seen several times during the last years.
2.
Requirements for the Power-Line Communication System
On the one hand power-line communication competes with communication via dedicated wires like field buses, telephone lines with voice-band-modems, as well as LAN systems, and on the other hand with radio based systems like DECT, GSM, ISM-band modems or wireless LAN. All these technologies have already been available for a long time, are successfully employed in their respective fields and, due to their production on a large scale, can be
offered for very low prices. However, radio based systems require an infrastructure, such as GSM, which makes operation of the system very expensive. A profitable application for the power-line communication would have to be a system where the wiring would impose the main expenses, while the distribution line already exists and a radio based system would not be feasible or too expensive. These prerequisites for power-line communication systems are met by medium voltage networks for the following reasons. Medium voltage cables are up to several dozens of kilometres long, which makes the later installation of communication wires prohibitively expensive. At the same time the only feasible radio based solution available for such applications is GSM, using the infrastructure of a network provider along with the associated costs. Therefore, at present GSM-based systems are employed only for small numbers of selected communication points only. Power-line communication is the only alternative allowing communication with numerous clients over such distances. Thus makes even quite expensive communication technology can be used profitably in this field. The requirements mentioned above are certainly also met on the mains network between the last transformer and the end customer or within industrial plants; these applications, however, are much more cost intensive. Whether power-line communication will ever be competitive for installation in small networks in family homes or flats, is uncertain.
This analysis shows coverage of distances or large areas by an existing infrastructure – the distribution line network – and a greater number of users is crucial profitable employment of power-line communication. Channel analysis studies [1, 2] have shown that the requirements to a communication system will be hard to meet and, due to the high attenuation of the channel, repeaters and appropriate routing algorithms are necessary, or in some cases even a large number of repeaters in some cases. Since the channel characteristics change over time, often even abruptly, the routing has to be adapted continuously. This can only be provided by automatic routing. Due to the great effort required to transmit data via PLC, provide a data rate lavishly to achieve a sufficient communication even with a poor utilisation, such as e.g. Ethernet, is not feasible. Efficient usage is only possible if the characteristics of data volume, data streams and the corresponding delay requirements can be limited. It will, for instance, be very difficult to efficiently accommodate for voice transmission with the need for a constant data rate and only a minimal delay, a web browser, needing large amounts of data with a high data rate but rarely, and a control application in facility management with a high number of independent small data packets with one system. It is then sometimes better to dispense with certain applications or serve them with a specially developed system in order to provide a well suited system for a broad scale of applications. The last described market is the focus of ; it requires a system with independent data packets of a size in the range of 32 to 256 byte, that can be sent to the clients separately. The clients must be able to respond; therefore, bi-directional communication is necessary. The topology of the distribution line network varies greatly. In low voltage networks often a tree structure, sometimes also with closed rings, can be assumed. It is therefore called a meshed network. The topology of the distribution line network is often not known by the network management system and, thus, cannot be exploited; the system has to assume an unknown random structure. To allow automatic routing in such a system,
each node must provide data on its environment, which will be transmitted as overhead in the system. However, in a ring structure, as often found in medium voltage networks, the topology can be exploited very efficiently data by an appropriate network management system.
3.
Analysis of Transmission Channel
The choice of the communication method must take the requirements to be met by the system and the transmission channel into consideration. The transmission channel is characterised by the transfer function between transmitter and receiver and the interference the receiver sees. Since the distribution line network was not designed for communication the wave impedances are not matched and reflections, multipath propagation and frequencyselective channels may occur. As every branch is already a point of reflection, terminations at the end points would bring only little improvement. The terminators draw power from the network and therewith increase the fundamental attenuation and reduce the possible range. Therefore it is much more reasonable to choose a communication method such as OFDM that can use the multipath propagation efficiently. Even though the attenuation along the cable is relatively low, the loss as du to branches can increase the attenuation considerably on a comparably short distance, especially with numerous branches. Since often a directed feeding of transmission power is not possible, the greatest part of energy is already lost at the sender. Due to the, at times, very high attenuation between transmitter and receiver, a dynamic range of the signal exceeds 80 dB requiring an automatic gain control (AGC) at the receiver. The transfer functions are temporarily constant and change abruptly with load changes. Therefore modelling of a power-line channel as fading channel is disadvantageous. A periodic abrupt switching between two transfer functions with the network frequency is absolutely possible with switching power supply unit. In the following figure only one transfer function is assumed.
figure 1: signal to noise over frequency and time The interference at the receiver consists of 3 components. The first is coloured background noise on a low level mostly. Secondly there is impulse noise, that is partly synchronous to the power frequency, covering the entire bandwidth and often have a much higher amplitude than the received signal. The third component are narrow band interferences, that can also be of much higher amplitude than the actually received signal. So there remain single spaces within the frequency / time domain where a positive signal to noise ratio can be found. A prediction where those are is difficult and for non-periodic impulse noise not possible at all. It is easier to limit the influence of such interferences and to retrieve the most data by strong channel encoding. In addition small independent packets from various clients, that can be sent over a variety of routes to reach each unit, have to be transmitted. Due to the limited available bandwidth a bandwidth efficiency of 0.5 to 3 Bit / Hz / s is required.
4.
Medium Access
The requirements include access to the same transfer medium for several clients. Therefore, the various possibilities of channel access shall be discussed here. Theory distinguishes three methods, that can however also be applied in combination with each other. In Frequency Division Multiple Access FDMA each transmitter uses different carrier frequencies, thus the transmission symbol spectra are not overlapping. For many clients this leads to very narrow spectral channels. However, the characteristics of the transmission channel require a broad band spectrum for transmission. That rules FDMA as main access method out. For the realisation of duplex a client has to transmit and receive at the same time. Because of the high attenuation of the channel, the transmit and receive signal have a very large amplitude difference. As a result the receive signal can only be processed with elaborate filters.
In Code Division Multiple Access CDMA, the entire available frequency band and time are considered a unit where all clients send at the same time. The receivers receive the superposition of all signals and filter the signals addressed to them by a correlation of the received signal with a code sequence. In order to successfully calculate the correlation on the receive signal all signals must have an amplitude within the simultaneously available dynamic range of the receiver. That means it is not possible to adjust to the amplitude of one signal with an AGC. The remaining dynamic range is not sufficient for the high attenuation on the transmission channel. If the different signals at the receiver have widely differing amplitudes successive cancellation has to be used for the decoding process [4]. It is questionable whether the necessary quality with the available transmission channel can be achieved. Another widely used method is to regulate the transmit power in a way that all signals reach the receiver with nearly the same amplitude. For packet oriented transmission with sometimes several receivers that poses a severe organisational problem. Moreover the good transmit paths are herewith downgraded to the quality of the worst path. All these disadvantages exclude this access method from use in Power-Line Communication. In Time Division Multiple Access TDMA the entire frequency band at one point is used by only one transmitter. The clients basically share the medium in time slots. The sender can use the entire frequency band for its transmission and the temporary loss of single frequency ranges can be overcome. Duplex is realised with time slots as well. That way a client does not have to transmit and receive at the same time. As there is always only one transmitter active the receivers’ AGCs can adjust to the amplitude of the received signal and achieve a considerably higher dynamic range of amplitudes, that can be received. All these advantages call for use of this access method on power lines. A disadvantage of this access method that needs to be mentioned is that the delay especially for short alerts clearly increases and the full bandwidth cannot be used efficiently for short messages. Data transmission with a multi-carrier method requires fixed block sizes for the data packets. Considered with regard to multi-carrier method and channel encoding these data blocks should be as large as possible. If smaller amounts of data are transmitted in one block the unused remainder is ineffective overhead. To minimise overhead the data packets have to be as small as possible. The optimum compromise depends on the characteristics of the data to be transmitted, and therefore, also on the desired application.
The duration of a time slot with TDMA determined by block size and the available bandwidth. If the time slot is too short, an impulse noise has too much influence on the transmission block and destruction of the block cannot be prevented. The bandwidth, block size and duration of the transmission depend on several factors. To avoid severe restriction by the transfer by the access method, it is useful to combine FDMA and TDMA, whereas FDMA will have as few channels as possible.
5.
Communication Method
Due to the many possible transmit paths and the timevarying channel pulse response it is not possible to measure the channels at the beginning and provide a channel estimation for data transmission. A channel estimation at the start of every transfer is very inefficient for small data packets and a transmission of the estimation results to the sender is not reasonable. So the sender has no information on the transfer channel. The following things can be said about the send signal: • •
•
•
Narrow band transmission methods can disappear in a notch of the frequency-selective channel. In Frequency Hopping many narrow frequency bands exist parallel. Are these used at the same time by several clients the receiver will have a similar problem with the dynamic range as in CDMA. If they are not used simultaneously, the desired bandwidth efficiency cannot be achieved. For Direct Sequence Spread Spectrum very precise synchronisation and channel estimation are needed which increases the effort spent in channel estimation and that way drastically reduces the efficiency for small data packets. Moreover, it is difficult to achieve the desired bandwidth efficiency. Multiple Carrier methods are recommendable for frequency-selective channels and also reach the desired bandwidth efficiency. The variety of these methods makes a closer examination necessary.
Multiple Carrier are realised efficiently in digital signal processing using fast Fourier transformation. These methods are often called OFDM – orthogonal frequency division multiplex – or DMT – discrete multitone. Here a signal that has been modulated with the information is transformed by inverse Fourier transformation in the transmitter and is sent preceded by a cyclic prefix of itself as guard interval. If the transmission uses a linear channel, the receiver receives
the send signal convolved with the channel impulse response. If the channel impulse response remains unchanged during transmission and the relevant part of the impulse response is shorter than the length of the guard interval, the convolution can be considered cyclic [4]. After applying fast Fourier transformation in the receiver, the sub channels, consisting of a complex symbol, are multiplied with weighting factors. There are no interferences between these individual channels. Since the guard interval is not used for transmitting data it should be short compared to the entire symbol. The Fourier transformation in the receiver changes the characteristics of the interferences. Impulse noise in the time domain before the Fourier transformation is distributed equally over all channels within the frequency domain. If the impulse energy is limited with respect to the received signal before the Fourier transformation and the duration of the pulse is short compared to the length of the symbol, the equally distributed noise can easily be handled without further measures. The following measurement shows transmission symbols of ’s transmission method, that are disturbed by immensely higher pulses.
The difference between the individual multi-carrier methods is the modulation of the information on the signal before the inverse Fourier transformation and the demodulation or decoding, respectively. According to requirements and application all possibilities of the digital data transmission can be employed with either coherent or incoherent demodulation. The differences between each of these methods are accordingly big. In coherent demodulation the information is mapped on symbols and modulated directly to sub-channels. For the demodulation the weighting factor resulting from the transfer function needs to be known and channel estimation is necessary. If the application is a point-topoint connection like with ADSL on phone lines [4], the transfer function can be estimated at the beginning and that information be transmitted to the sender. The sender adapts its mapping of information to the quality of each sub-channel and the receiver corrects the weighting factors and can easily decode. Slow changes in the transfer function can be tracked without necessitating a complete channel estimation. This is disadvantageous for Power-line communication due to the absolutely necessary estimation of the channel for coherent demodulation.
Complex Input Signal RED - Real GREEN - Imaginary
Amplitude [mV]
300 200 100 0 -100
20
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80
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-200 -300
Time [ms]
Figure 2: signal disturbed by impulse noise Signal of the equivalent complex baseband channel measured at the receiver, filtered with the transmission bandwidth. Transmission burst with preamble and OFDM-signal between 57 and 72 ms. Yet a transmission with a word error rate < 10-3 is possible. If the characteristics of the impulse noise in the mains network is known, the minimum duration of a transmission signal can be determined. On the other hand narrow band noise is concentrated on few channels by Fourier transformation. Since this noise usually lasts for a longer period of time, the decoding can take this into consideration if additional redundancy due to channel encoding is present in the signal [5].
In incoherent modulation the information is not modulated directly to sub channels, but in such a way that the receiver can determine the information from the difference of two successive symbols. Since in multicarrier methods two dimensions are used, time and frequency, the successive symbol can also be defined in both dimensions. In the time domain sub-channels of the same frequency of temporary successive OFDM-symbols are considered successive symbols. This is good for a broadcasting system like DAB, where one sender transmits many symbols to all receivers. However, the requirement here is to send small data packets independently of each other and thus it would be a rather inappropriate choice. In the frequency domain successive symbols are subchannels within the same OFDM symbol on adjacent frequencies. The frequency spacing between the two channels has to be so small that the weighting factors of these sub-channels differ only slightly. During demodulation the weighting factors are automatically cancelled by division of the symbols. It must not be ignored however that, due to the frequency-selective transfer function, the quality of transmission on the subchannels may differ widely. In order to compensate this, an appropriate channel encoding is required. There is no dependency between the OFDM symbols in this definition and also no channel estimation is necessary.
Therefore, it is possible that a data packet consist of only one symbol, which excellently meets the requirements. The transmission method applied by uses transmission symbols for transmitting one data packet that consist of a preamble for setting the AGC and synchronisation, and an OFDM symbol with guard interval, that is modulated incoherently over subchannels on adjacent frequencies. Since transmission symbols are independent of each other channel encoding must also use only one transmission symbol, which makes the use of a convolutional code with a viterbidecoder favourable at such limited data lengths. That is also what is used by .
[4]
R. Fischer, “Mehrkanal- und Mehrträgerverfahren für die schnelle digitale Übertragung im Ortsanschlußleitungsnetz,” Shaker Verlag Aachen, dissertation, 1997.
[5]
D. Galda, H. Rohling, “Narrow band Interference Reduction in OFDM-based Power Line Communication Systems,” in ISPLC 2001, Malmö, Sweden, pp. 345-351, 2001.
[6]
M. Deinzer, M. Stöger, “Integrated PLC-modem based on OFDM,” GmbH, Großhabersdorf, technical paper for ISPLC, 1999.
[7]
G. Bumiller, “Power-Line Analysing Tool for Channel Estimation, Channel Emulation and Evaluation of Communication Systems,” GmbH, Großhabersdorf, technical paper for ISPLC, 1999.
[8]
M. Sebeck, G. Bumiller, “A Network Management System for Power-Line Communications and its Verification by Simulation,” GmbH, Großhabersdorf, technical paper for ISPLC, 2000.
[9]
G. Bumiller, Markus Sebeck, “Complete PowerLine Narrow Band System for Urban-Wide Communication,” GmbH, Großhabersdorf, technical paper for ISPLC, 2001.
[10]
G. Bumiller, M. Deinzer, “Narrow Band PowerLine Chipset for Telecommunication and Internet Application,” GmbH, Großhabersdorf, technical paper for ISPLC, 2001.
[11]
G. Bumiller “Network Management System for Telecommunication and Internet Application,” GmbH, Großhabersdorf, technical paper for ISPLC, 2001.
[12]
M.Sebeck, G. Bumiller, “Power-Line Analysing Tool for Channel Estimation, Channel Emulation and Noise Characterisation,” GmbH, Großhabersdorf, technical paper for ISPLC, 2001.
OFDM symbol
Preambel
Guard
data block t send symbol
Figure 3: structure of a send symbol
6.
Conclusion
The company has been developing power-line modems for 10 years and the author has been working on system and communication designs of several PLC systems for more than 5 years. A lot of papers were presented by us on the ISPLCs [6-12] of the last years. In this paper the author summarised his experiences on the power-line channel and encourages the system architecture. Hopefully, discussions.
this
paper
will
initiate
intensive
7.
References
[1]
O. Hooijen, “Aspects of Residential Power Line Communications,” Shaker Verlag Aachen, dissertation, 1998.
[2]
M. Zimmermann, “Energieverteilnetze als Zugangsmedium für Telekommunikationsdienste,” Shaker Verlag Aachen, dissertation, 2000.
[3]
S. Müller-Weinfurtner, “OFDM for Wireless Communications: Nyquist Windowing, PeakPower Reduction, and Synchronization,” Shaker Veralg Aachen, dissertation, 2000.
