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The development cycle of a modem starts with the channel analysis, proceeding with the .... The Windows application has to build a job out of several functions,.
Power-Line Analysing Tool for Channel Estimation, Channel Emulation and Evaluation of Communication Systems Gerd Bumiller GmbH Unterschlauersbacher-Hauptstr. 10, D-90613 Großhabersdorf Tel.: +49 9105 9960-51, Fax: +49 9105 9960-19 Email: [email protected]

Abstract The power-line analysing tool PLATO developed by the company GmbH is used for channel estimation, channel emulation and evaluation of communication systems in a frequency band between 9 kHz and 30 MHz. The configuration of the PLATO system is a portable PC with Windows 95/98/NT, a special PLATO PCI Board and front-ends for analog adaptation. With the specially developed remote-controlled real-time operating system for the DSP it is easily possible to combine modules to a communication system and test it in real-time. The tests can be done together with channel estimation to get the real environment for the analysis in the simulation or with the channel emulator for laboratory tests.

Introduction Transmission systems for power-line communication have to cope with channels not designed for communication. Especially systems with bitrates of 1 Mbps or more have to use frequencies between 0.5 MHz and 30 MHz were no standards for power-line communication and no ready models for channel and noise exist. Developing an inflexible system on this basis of knowledge is dangerous. The development cycle of a modem starts with the channel analysis, proceeding with the creation of a first channel model. Based on this channel model a communication system will be designed and optimised and verified in simulations. After this the communication system must be implemented and tested in the field. If the system fails and which is likely, the cycle must be started again. The channels, the system failed have to be analysed, a better channel model will be created and a new communication system designed. And after the tests the cycle will continue. As we have seen in other papers, the channel analysis has been started and the first channel models are build. Doing the channel analysis has shown that the equipment which could be bought does not fit the requirements. Based on the experience of Prof. Dostert and Mr. Zimmermann (University of Karlsruhe) a channel analysis system is specified and will be realised in co-operation with our company in the power-line analysing tool PLATO. It is intended to use this channel analysis in the beginning of the second and further development cycles. Optimum analysis of a failure of a communication system in simulations is possible if a good model for the channel in which the communication system fails is available. The power-line analysing tool PLATO is also designed for the test of communication systems and to include the channel analysis into these tests. The costs of a test are dominated by the organisation and installation of the modem and not by the duration of the test. To make the channel analysis and the test of the communication systems together is more accurate, faster and cheaper. The development cycle is closed and the end of one cycle is the beginning of the next. If we have a new communication system, or we want to compare different systems, it is necessary to have a channel emulator for the laboratory tests. Therefore, a special application of the power-line analysing tool PLATO is designed to allow usage as a channel emulator. With the channel emulator, it is possible to make comparable and repeatable laboratory tests of different communication systems in a real environment. This is also interesting for the standardisation or for the system evaluation of the power industry.

Concept of Power-Line Analysing Tool PLATO The power-line analysing tool PLATO is a portable PC with Windows 95/98/NT and PLATO Windows application, a PLATO PCI board and front-ends. The following figure presents the concept of the system:

Portable PC

Various Front-Ends

BNC Connections

Serial-/ ConfigrationPorts PCMCIA-Card for Communication

BNC Dual Port RAM

FPGA 2

SDRAM 128MB

Flash

FPGA 1

Impedance Measurement

ChannelAnalysis

Antenna

DC/DC

Evaluation Communication System

PCI Controller

Dual A/D A/D Dual D/A D/A Synth.

DECT

2x Stereo Input 1x Mono Input 2x Stereo Output 1x Mono Output

Serial- / ConfigurationPorts

PLATO PCI-Board figure 1: Concept power-line analysing tool PLATO The portable PC includes the PLATO PCI board and perhaps a PCMCIA card for cellular communication. With this cellular communication it will be possible to get remote access and control during long term measurements. Even the remote compose of new jobs should be possible. The PLATO PCI board includes a high-performance DSP with 1600 MIPS from Texas Instruments and 128 MB SDRAM, several high-performance high-capacity FPGAs, mono ADC and mono DAC for 64 MSPS with 12-bit resolution, dual ADC and dual DAC for 40 MSPS with 12/10-bit resolution, Dual-Port RAMs and a programmable synthesiser to generate the sampling clock. The DSP is controlled by a the specially developed remote-controlled real-time operating system, which will be presented later in this paper. For the configuration and communication with the front-ends a serial port and special configuration ports are provided. The control unit for a programmable gain control is included in one FPGA of the PCI board and can be linked over the configuration port. The several front-ends are used for the different measurement and communication tasks and the analog adaptation to the distribution line. The front-ends have IDs and data needed for calibration could be stored inside. Even not power-line specific front-ends can be connected. The flexible concept of the PLATO-System with the high-performance DSP and FPGAs meets all requirements. The usage of high-resolution ADCs and DACs allows channel analysis and communication up to 30MHz. In complex applications, even with this high-performance DSP decoding online may not be possible, but then one second of communication with full sampling rate can be stored into the large RAM and decoded offline. Due to the internal timer of the system continues counting, the synchronisation with the partner of communication can be maintained.

Operating System and Windows Application In the PLATO system different modules like a communication system and channel measurement should work together controlled by the Windows application. It is useful if the different modules employ the same drivers for communications with the PC and over the FPGAs with the ADCs and DACs. A communication system has to cope with a continuos data stream and for all these reasons a real-time operating system on the DSP is needed. Because Windows 95/98/NT is not a real-time operating system, it is not possible to control the jobs on the DSP with the Windows application in real-time. The Windows application has to build a job out of several functions, send it to the DSP and then the job has to run on the DSP independently of the Windows application. In the Windows application it must be possible to watch all results of the functions and to start new jobs which interrupt or are appended to running jobs. Due to all of these requirements, it is clear that the PLATO Windows application and the operation system on the DSP belong together and no standard operating system for the DSP could be used. All functions which could be selected by the jobs are stored in the program space of the DSP. A special definition how to build these functions ensures that it is possible to append the functions to each other. In the parameters of each function is defined which function is to follow. If a job is finished, an idle function provided by the operating system will be called. During the initialisation, the operating system on the DSP communicates with the Windows application. On this way the Windows application knows all available functions, their version numbers and the available space in the data memory. With this information the Windows application is able to manage the data space for the jobs designed by the user. The Windows application is able to read and write data via PCI bus, hostport-interface and DMA channel directly into the memory of the DSP without using resources of the DSP-CPU. On this way the parameters for the functions and the data-sets are written into the memory of the DSP and the results of the functions could be read by the Windows application. Over a mailbox in the PCI bus the DSP is able to send a trigger to the Windows application if a special state of the job is reached. On this trigger the Windows application fetches the results and translates these into the style specified by the user. Direct memory access channels are used to copy the data to or from the ADCs and DACs. Only the control of these DMA-Channels requires resources from the DSP processor, but with the usage of Dual-Port RAM the blocks transferred by the DMA-Channels are large enough. The control of the DMA-Channels supports several modes. In one mode, slots of a time division multiple access frame can be selected and start different functions after copying the data into different workspaces. It is not necessary to use all slots of the TDMA channel. Therefore the DSP-CPU will have more time to work with the data. This design of the operating system provides full flexibility to build the jobs and watch all results of the functions by the Windows application. When the job is transmitted into the memory of the DSP, the DSP runs independently of the Windows application. The requirements of the real-time capabilities of the system can be reached. Due to the structure of the operating system with intensive use of the 5 available DMA-Channels nearly all resources of the DSP-CPU are available for the user functions. The characterisation of the operating system is remote-controlled and running in a real-time environment with special support for communication systems. Of course, the Windows application is the user interface of the power-line analysing tool PLATO. It controls the PCI board, like sampling rate, transmission mode and so on, and controls the front-ends. Jobs for measurement and testing of communication system will be designed, executed, saved and loaded. Functions out of different modules can be combined. Several jobs can be started based on time by a scheduler. The data management for the workspace of the DSP will be done. Data generated externally will be loaded from files and stored into the workspace of the DSP. The results of the DSP will be shown, saved to a file or database, or visualised in a toolbox. The visualisation could be printed or included into other documents. A remote access and control via a cellular communication system may be possible. The power-line analysing tool PLATO will be distributed in three versions. First version only allows to work with ready designed jobs and gets a user interface adapted to these jobs. In the second version it is possible to design jobs with the functions given or modules included. In the last version it is also possible to develop own functions and include them into the system. Independent of the version, the system will be distributed with several modules for communication systems, channel analysis and channel emulation.

Test of Communication Systems The parts of a communication system can be characterised by the OSI layer model. In the power-line analysing tool PLATO, the two lowest layers of the model - the physical layer and the medium access layer – of a system can be tested and analysed. Testing physical layer of a communication system using OFDM (Orthogonal Frequency Division Multiplex) might look like the example given in the following figure.

PLATO Encoding Pseudo Random Generator

Framer Channel Encoding Interleaver

Modulation Mapper Modulator IFFT Guard-Interval

Transmission Interpolation Mixer Filter Amplifier

Power Line Channel

Sync.

Pseudo Random Generator

Bit Error Rate

Decoding

Demodulation

Deinterleaver

FFT Demodulator

Decoding

Reception Filter Amplifier Mixer Decimation

PLATO figure 2 : example for testing the physical layer of a communication system After generating the data by a pseudo random generator the workflow to build the transmission data is sequential. The basic functions are implemented in the communication module and can be combined to a communication system in the Windows application. The order of the functions can be changed or other functions included. All functions can be attached to each other. The user defines the parameters of the functions and the following function in the Windows application. On this way a lot of different systems can be built by the user. The communication system design can be stored and reloaded. It is possible to use some functions like Fast Fourier Transformation together with other modules. To measure the bit error rate, it is necessary for the receiver to know the data which the transmitter sent. By using a pseudo random generator for generating the data, the receiver only needs a synchronisation with the transmitter. For slow data transmission the internal clock of the system can be used, but for high rate data transmission this is not exact enough. A second way of communication for the synchronisation is not wanted and is not required. The operating system supports TDMA framing and therefore the tests can be done in a twoslot TDMA frame, the first slot only used for synchronisation and the second for communication. The order of the correlation sequence can be very high and, if no synchronisation is possible, then surely communication is not possible either. The power-line analysing tool with the special design of the operating system supports the test of the medium access layer. Because of the large bandwidth and the high performance DSP even CDMA systems can be implemented and tested. The large memory allows to store a lot of interim results, save them into a file and restore this results in the simulation. On this way every function can be verified and failures in the communication analysed. An included channel analysis can also be used in field tests to understand what happens to the system.

Channel Emulator The channel emulator is a special application of the power-line analysing tool PLATO. With the channel emulator it is possible to make comparable and repeatable laboratory tests of different communication systems in a nearly real environment. Therefore the usage of the channel emulator is interesting for standardisation or for the system evaluation of the power industry. It is also possible to optimise a communication system in the laboratory and thus shorten the time of development.

high-performance DSP

PC System

PCI

Interference Signal Generator n(k ⋅ T)

high-performance high-capacity FPGA

Transmitter

62,5 Msamples/s 12 Bit

Transmission Channel

ADC

h(k ⋅T)

62,5 Msamples/s 12 Bit

+

DAC

Receiver

Linear Channel Filter

figure 3 : model for the channel emulator Over a front-end the transmitter of the communication system will be connected to the input of the channel emulator. The front end is used for analog adaptation to the communication system and to emulate the impedance of the distribution line for the transmitter. The high-resolution, high-performance analog-to-digital converter samples the signal. With a sampling rate up to 62.5 MHz the channel emulator can be used for systems which use frequencies below 30 MHz. In a high-capacity FPGA the transmission channel is realised as a linear channel filter and an adder for interference and noise. The high-resolution, high-performance digital-to-analog converter generates the analog output signal. A back-end box will be used for analog adaptation and emulates the impedance of the distribution line for the receiver of the communication system. The channel emulator will be configured by a PC system over the PCI bus. A potable PC, as used for the PLATO field test system, is not necessary. A special Windows application or a HP Vee application can be used as user interface. Including the channel emulator into the laboratory environment for automatically measurement should be possible. The used channel model for the channel emulator is based on the work of M. Zimmermann and K. Dostert, presented on this conference (as ‘A Multi-Path Signal Propagation Model for the Power Line Channel in the High Frequency Range’ [1]). Several sets of parameters will be implemented and can be selected in the user interface. The DSP generates the interference and the noise or noise measured in real environment and stored can be added. By storing the signal of the transceiver, manipulating it with the DSP and adding this signal repeatedly to the new sent signal, the influence of crosstalks can be analysed. If a standard for a power-line communication system is written, it should be possible to test the interoperability of the different communication systems.

Impedance measurement1 The design of appropriate amplifiers and coupling devices requires detailed information about the impedance characteristics of power line networks at different locations (substations, meters, indoor sockets). Therefore one front-end is designed for impedance measurement. This front-end and the system configuration is shown in the following figure.

DSP-System Reflection Coefficients

Amplitude/Phase Estimation

Transmitted Signal

Reflected Signal

Directional Coupler

figure 4: system configuration for impedance measurement Due to the two-channel ADC it is possible to characterise the impedance using a directional coupler. Transmitting continuous-wave signals at different frequencies and estimating the amplitude and phase of the transmitted and the reflected signal component allows to determine the reflection coefficients at the transmitting point. The impedance of the network versus frequency can be calculated from the reflection coefficients by the DSP. In the laboratory the complex calibration factors of the front-end can be determined and stored into the front-end. After adaptation of the front-end or a reboot of the DSP-Board a communication will be started for initialisation over the serial port. In this initialisation the complex calibration factors will be transmitted to the DSP.

Channel Analysis1 The architecture of the PLATO system offers the signal acquisition and processing capabilities for basic instrumentation tasks as well as for advanced channel analysis applications. The various analog front-ends allow to perform all necessary measurements for complete characterisation of power line channels. A database included in the Windows application allows an intelligent management of the measurement data. Basic instrumentation features comprise the functions of a Digital Storage Oscilloscope (DSO) with spectrum analysis capabilities. The programmable pre-amplifier, the high-resolution analog-to-digital converter (ADC) and the on-board memory facilitates the acquisition of signals up to 30 MHz with a length and dynamic range not offered by standard instruments. The digital signal processor (DSP) performs all the signal processing tasks, e.g. the FFT, for the spectrum analysis functions. Specially built triggers and filters select the interesting sections of the measurement data and reduce the stream of data. The PC acts as control-terminal, storage and display media. Besides online signal processing by the DSP, off-line processing of the data can be performed by the PC using provided functions of the PLATO software. Moreover the data can be exported for the processing with numerical computation packages like MATLAB™.

1

Written in co-operation with M.Zimmermann, University of Karlsruhe, Germany

In addition to standard instrumentation features described above, advanced instrumentation tasks can be performed, such as statistical noise analysis, high-resolution channel estimation and long term measurements. The statistical analysis and modelling of noise in a communication channel like power-line networks requires numerous measurements. Especially the characterisation of spikes on the distribution line is a task calling for intelligent signal pre-processing capabilities. Asynchronous spikes on the distribution line may sometimes be a rather rare event. The intervals between the impulses are considerably longer than the width of the impulses. Continuous sampling and online detection of impulses by intelligent signal pre-processing considers only the relevant part of the signal. Such characterisation of impulsive noise delivers better results compared to sampling and saving of fixed length blocks with off-line signal processing. Channel estimation is a further important task when characterising a communication channel. High-resolution channel estimation can be performed by the Plato system using optimised periodic broadband pseudo noise signals. Correlation analysis in the receiving DSP allows to determine the complex transfer function and the impulse response respectively. All instrumentation tasks described above can be performed as periodic long term measurements. Tests of communication systems can be included in this periodic measurements. For this measurements it is necessary to have triggers for the interesting sections and filters for reducing the datastream. The long term measurements are a very important feature for the characterisation of time-variant channels like the power-line channel.

Conclusion In this paper we presented the system design, the operating system and the applications of the power-line analysing tool PLATO. With this tool, the development cycle of channel estimation, channel modelling, optimisation of the communication system and the field tests is shortened. If the field test of the communication system fails, a new development cycle beginning with channel estimation can easily be started. For the company GmbH the power-line analysing tool PLATO is the key to develop a powerful communication system with megabit solution on the power-line. The design flow for implementation of this communication system is given and tested by the implementation of the integrated PLC-Modem in the CENELEC-Band presented on this conference in the paper ‘Integrated PLC-Modem based on OFDM’ [2]. The development cycle for the megabit solution started with a modified version of the CENELEC-Band communication system, but up to now a lot of new ideas are being implemented and will be tested. Further, the results of research at the University of Erlangen-Nürnberg, partly presented on this conference in the paper ‘Comparison and Optimisation of Differential Encoded Transmission on Fading Channels’ [3], will be implemented and tested. GmbH for developing a high-quality power-line With these partners and tools, the basis of the company communication system is good and even the evolution of the communication system should be guaranteed.

References [1]

A Multi-Path Signal Propagation Model for the Power Line Channel in the High Frequency Range, M. Zimmermann and K. Dostert, University of Karlsruhe, Germany, Paper presented on ISPLC’99

[2]

Integrated PLC-Modem based on OFDM, M. Deinzer and M. Stöger, iAd GmbH, Germany, Paper presented on ISPLC’99

[3]

Comparison and Optimisation of Differential Encoded Transmission on Fading Channels, L. Lampe and R. Fischer, University Erlangen-Nürnberg, Germany