Abstract:
A system and method for broadband communications over power lines has a low-voltage transformer for the house connection unit and/or the electricity distribution inside the house. The system method allow for several parallel asynchronous data communications in different sub-channels with individual transmit power in each sub-channel. Sub-channel separation uses pass-band filters with high stop-band attenuation. High data rate in each sub-channel is achieved through the use of discrete wavelet multi-tone modulation. Coarse synchronization in each sub-channel and the optimization of the coefficients of the time-domain equalizer are carried out using a training sequence.

Description:
REFERENCE DATA 
     This application is a continuation of international PCT application PCT/EP2004/050485 (WO04091113) filed on Apr. 7, 2004 under priority of European application EP03100939.2 of Apr. 8, 2003, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and a method for broadband data communications over a power line distribution network comprising the area between the low-voltage transformer station and the house connection unit and/or the electricity distribution within the house. 
     BACKGROUND ART AND ASSESSMENT THEREOF 
     Prior art systems for data communication over electric power distribution lines, for example between a low-voltage transformer station and a house connection unit and/or within a house, commonly use at least one master station and slave modems which have to register to the master station. The same network architecture is also used for broadband data communications over the electricity distribution inside the house. 
     Such systems are based either on time division multiple access (TDMA) and/or use orthogonal frequency division multiplexing (OFDM) techniques. Although OFDM techniques allow for higher data rates than time division multiple access techniques, they have a major drawback in that they have poor stop-band attenuation. 
     Disadvantages of the master-slave approach are that the transmit level of the master station must be high enough to allow to reach the most distant slave modem and that the communication bandwidth must be shared between several slave modems. 
     Main disadvantages of the prior art systems for data communication over electric power distribution lines are:
         a high transmit level needed to reach the most distant slave modem, resulting in corresponding electromagnetic radiation emissions,   complex random access schemes required to control the permission to transmit of the slave modems,   the master station represents a single point of failure, and   the need for time synchronization between different master stations to avoid interferences between the ongoing simultaneous power line communications, if several power line master-slave systems are used at the same time.       

     These drawbacks are main barriers to the broad deployment of power line communication. 
     SUMMARY OF THE INVENTION 
     An aim of the present invention is to provide a system and a method for data communication over power lines allowing to achieve high data communication rates. 
     Another aim of the present invention is to provide a system and a method for data communication over power lines allowing that several power line modem to power line modem data transmissions happen simultaneously and asynchronously over the power distribution network. 
     Another aim of the present invention is to provide a system and a method for data communication over power lines allowing the use of different transmit power levels of the power line modems. 
     Still another aim of the present invention is to provide a system and a method for data communication over power lines not requiring any synchronization between different power line communications happening in parallel. 
     These aims are achieved with a system and a method having the characteristics of the respective independent claim, variant embodiments being given by the dependent claims. 
     These aims are achieved in particular with a method for data communication from a plurality of senders to a plurality of receivers being connected over a single electric power network having a determined data transmission channel bandwidth, comprising the step of simultaneously asynchronously transmitting data over a plurality of peer-to-peer transmission channels established between the senders and the receivers, as well as with a system for data communication comprising a plurality of communication devices for transmitting and/or receiving data over an electric power network having a determined data transmission channel bandwidth, the communication devices each comprising a transceiver system designed for asynchronously transmitting data over a plurality of peer-to-peer transmission channels established between them. 
     According to a preferred embodiment of the inventive data communication method, the power line channel bandwidth is divided into n sub-channels of the same or of a different bandwidth, n being an integer greater than two. According to a preferred embodiment of the present invention, the n sub-channels are for example separated by using digital filters having high stop-band attenuation. 
     The inventive power line modem preferably includes means for detecting communication activity, for example in a pre-selected subset of the n sub-channels, in order to identify one free sub-channel for transmitting the data to be sent over the physical channel. The data is modulated using for example discrete cosine modulated filter banks or discrete wavelet modulation. The receiver performs symbol synchronization and time-domain equalization of the sub-channel impulse response, and carries out the inverse function to the one employed by the sender, in order to recover the data using for example discrete cosine modulated filter banks or discrete wavelet filter bank. 
     The present invention thus allows a plurality of power line modem to power line modem data communications to happen in parallel and the transmit power can be differently determined for each sub-channel communication, leading to optimally reduced interferences between the sub-channels. According to the invention, there is no need for synchronization between parallel data communications as they occur on separated sub-channels. There is no more single point of failure either, since there is no need for a master station and/or for a complex access mechanism. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention can be obtained when the following detailed description of embodiments of the invention is considered in conjunction with the following drawings, in which 
         FIG. 1  is a conceptual block diagram of a transceiver system in accordance with a preferred embodiment of the present invention. 
         FIG. 2  illustrates a partitioning of the power line channel bandwidth into sub-channels according to a preferred embodiment of the present invention. 
         FIG. 3   a  illustrates three simultaneous asynchronous data transmissions happening in parallel, using different sub-channels of the power line channel bandwidth in accordance with a preferred embodiment of the present invention. 
         FIG. 3   b  diagrammatically represents the three data transmissions of  FIG. 3   a  happening in parallel over the electric power distribution network. 
         FIG. 4  is a block diagram of the sender of  FIG. 1 . 
         FIG. 5  is a block diagram of the receiver of  FIG. 1 . 
         FIG. 6   a  shows the frequency response of a discrete cosine modulated bank of a bandwidth of 1 MHz in accordance with an embodiment of the present invention, 
         FIG. 6   b  shows the frequency response of an analog band-pass filter of 4 MHz employed in the receiver in accordance with another embodiment of the present invention, and 
         FIG. 7  shows 24 sub-carriers that are employed within the normalized frequency range of 0.70 to 0.125 of  FIG. 6   a  according to a preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of a transceiver system  10  implemented in a power line modem in accordance with a preferred embodiment of the present invention. The transceiver system preferably includes a sender  11  based for example on a discrete cosine modulated filter bank or on a wavelet packet modulated filter bank and a receiver  13  also based for instance on a discrete cosine modulated filter bank or on a wavelet packet modulated filter bank. A data communication activity detector  12  is connected to the receiver  13 . The sender  11  and the receiver  13  are connected to the hybrid circuit  14  over which the transceiver system  10  is connected to the electric power distribution network  16 . 
     According to a preferred embodiment of the invention, the transceiver system implemented in the inventive power line modem comprises a sender  11  and a receiver  13 . It is therefore able to both transmit and receive data, possibly at the same time and on different sub-channels. The one skilled in the art will however recognize that it is possible, within the frame of the invention, to build communication devices such as modems able only either to send or to receive data. The transceiver system implemented in such devices then comprise respectively a sender  11  and no receiver  13  or a receiver  13  and no sender  11 . 
       FIG. 2  shows how the bandwidth of the power line communication network is divided into n sub-channels of different bandwidth in accordance with an embodiment of the present invention. The bandwidth of the sub-channels are for example 4 MHz, 2 MHz, 1 MHz or 0.5 MHz. 
     According to a preferred embodiment of the invention, the receiver  13  of the power line modem who wants to transmit selects by the activity detector  12  the different pre-selected sub-channels one after the other and monitors if data communication activity is present on the sub-channel by measuring the signal energy in that sub-channel. The activity detector  12  identifies in this manner a sub-channel that is free to be employed for transmission and communicates this information to the sender  11 . If more than one sub-channel is available, the transceiver system preferably selects the best sub-channel basing on one or more predefined criteria such as for example the sub-channels&#39; bandwidth, frequency range, attenuation characteristics, noise, etc. 
     The selected free power line sub-channel is then used for transmitting the data over the electric power distribution network, such as for example sub-channel  302  is used for data transmission between power line modem B and power line modem  5 , as illustrated in  FIG. 3   a . A peer-to-peer data transmission channel is thus established between these two modems.  FIG. 3   a  and  FIG. 3   b  further illustrates how three communications happen simultaneously through three parallel peer-to-peer transmission channels, each using a different sub-channel of the power line channel bandwidth. These parallel communications are totally independent from each other and can thus be performed asynchronously. The transceiver system of every power line modem A, B, C, R, S and T is preferably implemented according to  FIG. 1 . 
     The inventive data communication method thus allows the generation of a meshed data communication network over an electric power distribution network, where each communication device can establish a peer-to-peer communication with any other device of the network. The transmission power for each peer-to-peer communication is preferably adapted to the characteristics of the transmission line between the two devices. In order to avoid disturbances of the network&#39;s environment, the transmission power must however be kept within certain limits. Thanks to the meshed architecture of the inventive network, one or more communication devices can for example be used as repeaters or relays between two communication devices situated far apart from each other. One or more communication devices of the inventive network can also function as relays or gateways to other networks such as the internet, for example. 
     As shown in  FIG. 4  illustrating the block diagram of the sender  11  in more details, the data  15  to be transmitted is first interleaved in an interleaver  401  and converted from serial to parallel in a converter  402  and then encoded using a constellation encoder  403 . 
     The parallel output of the constellation encoder  403  is lead to a discrete cosine modulated filter bank or to a wavelet packet modulated filter bank  404 . The bandwidth of the filter bank  404  is for example of 1 MHz and the cosine modulated filter bank or the wavelet packet modulated filter bank preferably has for instance  24  or  64  carriers, each with high stop-band attenuation. 
     The serial output of the filter bank  404  is digitally up-shifted in frequency to the selected free sub-channel&#39;s frequency by a modulator  405  including a frequency generator  407 . The output of modulator  405  is given to a digital-to-analog converter  406  to be transmitted over the selected sub-channel of the electric power distribution network. 
     As shown in  FIG. 5 , the received signal is preferably first band-pass filtered using a band-pass filter  501 , then amplified using a low noise amplifier  502  and up-shifted using a modulator  503  to a chosen Intermediate Frequency (IF). The signal is then amplified again using an automatic gain control  505 , band-pass filtered by the band-pass filter  506 , before being fed to an analog-to-digital converter  507  to be digitalized and possibly over-sampled. 
     According to a preferred embodiment of the invention, coarse synchronization with the emitting modem&#39;s sender is achieved in a synchronization unit  508  employing matched filtering techniques using training symbols known to the receiver  13 . The training symbols are preferably sent by the sender at least once for each newly established peer-to-peer data transmission. The beginning of the sent training symbol is detected by the synchronization unit  508  which will then initiate the synchronization procedure. These training symbols are also used to determine the coefficients of the time-domain recursive equalizer  509 . Fine synchronization together with a compensation of the frequency offset between sender sampling clock and sampling clock of the receiver is carried out based on pilot symbols. 
     To recover the data sent, the output of the time-domain equalizer  509  is fed to a filter bank  510  consisting for example of a discrete cosine modulated filter bank or of a wavelet packet modulated filter bank. The parallel output of the filter bank  510  is fed to a constellation decoder  511 . The parallel output of the constellation decoder  511  is in turn fed to a parallel to serial converter  512  the output of which is fed to a de-interleaving unit  513 . The output of the de-interleaving unit  513  is the estimate  17  of the sent data. 
       FIG. 6   a  shows as an example the frequency response of a discrete cosine modulated filter bank  510  of a bandwidth of 1 MHz. The horizontal axis shows the normalized frequency [×2π rad/s] while the vertical axis shows the amplitude in dB. One can see that the energy of the modulated data signal is confined in a very narrow frequency range and that it is very strongly attenuated outside that range. Thanks to that particular feature, such signals using different frequency ranges can be transmitted on a single transmission line without generating significant cross-talk between each others. Different data transmissions can thus be asynchronously initiated in parallel on neighboring sub-channels without risks of mutual perturbations. 
     On the receiver&#39;s side, in order to retrieve the sent information, the received signal is filtered with a band-pass filter having a frequency response similar to that illustrated in  FIG. 6   b . In  FIG. 6   b , the horizontal axis shows the frequency in MHz while the vertical axis shows the amplitude in dB. By centering the filter&#39;s frequency response on the desired sub-channel, only the signal sent on that sub-channel is received. 
     According to a preferred embodiment of the invention, the transceiver system  10  comprises a sender  11  modulating the data to be transmitted with a discrete cosine modulated filter bank or with a wavelet packet modulated filter bank  404 . The transceiver system  10  is thus a multi-carrier transceiver system and the transmitted data is modulated over a plurality, for example 24, sub-carriers within the frequency bandwidth available in the selected sub-channel ( FIG. 7 ). In  FIG. 7 , the horizontal axis shows the normalized frequency while the vertical axis shows the amplitude in dB. Preferably, the transceiver system  10  is configured such that the level of the transmitting power and the number of encoded data bits, or data rate, can be chosen different for each sub-carrier, depending on predetermined or measured transmission characteristics in the particular sub-carrier frequency band. The transmission characteristics can for example depend on the measured signal-to-noise ratio, the available bandwidth, the attenuation, etc. The transceiver system  10  thus preferably includes a not represented device and/or a memory storage area for determining and/or storing these sub-carrier specific characteristics.