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Broadband system to increase bitrate in train communication networks
Computer Standards & Interfaces 31 (2009) 261–271
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Computer Standards & Interfaces j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c s i
Broadband system to increase bitrate in train communication networks C. Rodríguez-Morcillo a,⁎, S. Alexandres b, J.D. Muñoz b a b
Instituto de Investigación Tecnológica, Universidad Pontificia Comillas, Cl. Santa Cruz de Marcenado 26, 28015 Madrid, Spain Departamento de Electrónica y Automática, Universidad Pontificia Comillas, Cl. Alberto Aguilera 23, 28015 Madrid, Spain
A R T I C L E
I N F O
A B S T R A C T
Article history: Received 18 May 2007 Received in revised form 27 March 2008 Accepted 4 May 2008 Available online 18 May 2008
MVB (Multifunction Vehicle Bus), defined in IEC 61375, has been broadly adopted as the communication standard between embedded control systems on-board modern trains. In this work a new method to take advantage of the full bandwidth of the channel using an OFDM technique is described. With this new method it is possible to share the physical medium between standard MVB traffic and new OFDM traffic. A 90 Mbps theoretical bitrate can be achieved. The results of this work have been validated in a test bench including standard MVB nodes transmitting on a line similar to a real vehicle bus. © 2008 Elsevier B.V. All rights reserved.
Keywords: IEC 61375 MVB OFDM Programmable logic Bandwidth optimization
Contents 1. 2.
Introduction . . . . . . . . . . . . Standards discussion . . . . . . . . 2.1. EIA-709 (LonWorks) . . . . . 2.2. IEC 61375-1 (TCN) . . . . . . 2.3. IEC-61158-3 (Profibus) . . . . 2.4. Industrial Ethernet . . . . . 3. Communication system . . . . . . . 3.1. OFDM review . . . . . . . . 3.2. System description . . . . . 3.3. System configuration . . . . 4. Practical system implementation . . 4.1. Line modelling . . . . . . . 4.2. Prototyping . . . . . . . . . 4.3. Coupling to MVB and filtering 5. Results . . . . . . . . . . . . . . 6. Conclusions . . . . . . . . . . . . Acknowledgments. . . . . . . . . . . . References . . . . . . . . . . . . . . .
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1. Introduction Several factors drive the installation of new communication systems on-board trains: investment by companies and governments facilitates the development of new systems; furthermore, the rapid
⁎ Corresponding author. Tel.: +34 91 5422800; fax: +34 91 5423176. E-mail addresses: [email protected] (C. Rodríguez-Morcillo), [email protected] (S. Alexandres), [email protected] (J.D. Muñoz). 0920-5489/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.csi.2008.05.007
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advances of the technology, the social adoption of the information and communication technologies (ICT) and the demand for new services by users in the railway sector have led to the installation or redesign of new on-board communications systems. In recent decades, railway operators planned to develop a modern and versatile communication system on-board trains. The objective was to allow the interchange of information between different sets of equipment embedded in a train. In addition, it was necessary to unify an interface between devices, so that vehicles developed by different manufactures could interact in the same network.
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Table 1 Comparison of industrial buses Bus
Bitrate [Mbps]
Physical medium
Audio/video
Advantages
Disadvantages
LonWorks (ANSI/EIA-709)
0.078
Twisted pair
No
Low speed. No audio. No video.
TCN (IEC 61375)
1
Twisted pair Optical fibre Twisted pair Optical fibre Twisted pair
Low bitrate audio
Established. It is used by NYCT and others Promoted by Siemens and Adtranz Derivate of LonWorks
AS-5370
1.5 5 1.25
E1 PDH (UIT G.732)
2.048
WorldFIP (EN50170-3)
2.5
Ethernet (IEEE 802.3)
10
Profibus (IEC 61158)
12
Twisted pair Optical fibre Coaxial Twisted pair Twisted pair Optical fibre
Audio
Audio Audio
Established. It is used by NYCT Promoted by Alstom
Video
Difunded
Audio and video
Very fast
Not extensible. No audio. No video. Requires a license Not extendible
Proprietary components Requires couplers. Not deterministic Requires couplers
At the end of 1988, the International Electrotechnical Commission (IEC) and the Union Internationale des Chemins de fer (UIC) began to develop an international standard detailing all requirements for the communication process of all on-board equipment. The major requirements of this development are: • Interoperability at train level to allow communication between railway vehicles of different countries. • Interoperability at vehicle level to allow communication between devices of different manufacturers. • Improvement in the operation and introduction of new services. • Support for configuration and maintenance and a general cost reduction in the life-time of products. After ten years of work, the IEC 61375-1 or Train Communication Network (TCN) was obtained [1]. Different European projects for its validation and standardization have been carried out around TCN. ROSIN (96-99) (Railway Open System Interconnection Network) [2,3], TrainCom (00-04) (Integrated Communication System for Intelligent Train Applications) [4,5] and EuRoMain (02-05) (European Railway Open Maintenance System) [6,7]. Finally, in October 1999, in Kyoto (Japan), TCN achieved the state of International Standard as IEC 61375-
1. Later, the IEEE Vehicular Society included TCN in its IEEE Standard for Communications Protocol Onboard Trains, as P1473-T. In spite of the increasing use of TCN in communications on-board trains, there are other communication standards that perform the same functions, like LonWorks (ANSI/EIA 709) [8] and Profibus (IEC 61158-3) [9]. Table 1 [10] shows others industrial communication fieldbuses used for interconnecting embedded systems on-board trains. In recent years, the limitations of TCN have been detected. This bus was designed as a real time bus for control and supervision purposes. These applications typically require a low bandwidth, so that the bus was designed with a bitrate of 1.5 Mbps. But today, the users and operators of railway systems are demanding new services, like new embedded control systems, digital audio, video-surveillance and user information, with more bandwidth requirements. The solution, a priori, is the installation of new wiring for these new systems. Nevertheless, the installation and maintenance costs grow rapidly. This paper presents an approach using a similar solution to others sharing techniques like PLC or ADSL. The solution shares the physical wire used by TCN with a high frequency channel using an OFDM technique. Bitrate increases to several Megabits per second. The paper is organized in four sections. After this introduction, Section 2 briefly reviews the norm IEC 61375 on Train Communication Network and other standards used in train networks. The following section describes the communication system proposed, introducing the OFDM architecture. Section 4 describes the prototype used for system validation. Finally, the last two sections present results and the conclusions. 2. Standards discussion This section describes several industrial fieldbus standards used to support broadband communication between electronic embedded systems in train vehicles. In addition, Table 1 synthesizes and compares these and others standards. 2.1. EIA-709 (LonWorks) EIA-709 and IEEE 1473-L, known as LonWorks, specifies the physical layer LON (Local Operating Network) and the LonTalk communications protocol. This standard supports control systems by means of a point to point network in a hierarchical structure (masterslave). Interoperability is allowed between different manufactures and good results are obtained. Nevertheless, maintenance and modification are expensive and difficult in these networks. LonWorks is used in multiple industrial applications, for example in Amtrak's Acela Express [11]. The main disadvantages of this network are the low bitrate of some kbps, and the fact that it is a non-deterministic network, which does not allow the system to function in real time.
Fig. 1. TCN buses architecture.
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Fig. 2. Digital system block diagram.
2.2. IEC 61375-1 (TCN) Standard IEC 61375 allows communication between devices installed in the same or in different vehicles in a train. The main digital services covered are traction, control and supervision devices (doors, lights, air conditioning and so on), and central and auxiliary management devices. Communication between on-board systems happens at two level architecture. The standard defines two hierarchical buses: WTB (Wire Train Bus) connects the equipment in different vehicles at 1 Mbps in the upper level; and MVB (Multifunction Vehicle Bus) connects embedded systems inside a vehicle at 1.5 Mbps in the lower level (see Fig. 1). Both buses use a master-slave protocol with baseband Manchester encoding. At the moment, MVB is being introduced in a large number of European train networks. In some cases, the MVB bus is used to interconnect all the vehicles of a train if its length is short (as in underground trains). However, WTB has not had the same acceptance, using in some cases E1, T1 or Ethernet networks to interconnect different vehicles of the train. One disadvantage of this standard is the low bitrate. In addition, the standard lacks clarity in some important points, for example, the objective of the bus redundancy (point 3.3 of the IEC-61375). 2.3. IEC-61158-3 (Profibus) This is one of the most used architectures in automation and control in the US and Europe. The standard was created in 1989 in Germany by a group of fieldbus manufacturers. It allows a bitrate between 9.6 kbps and 12 Mbps and a maximum of 126 devices connected on the same bus. The standard also supports audio and low bitrate video in real time communication. The standard is focused on the application and link layers. Many applications of Profibus are being used in Europe, for example, the remote supervision system in the Warsaw (Poland) underground system developed by Kontron. The network is working in a deterministic and real time scheme that makes possible the traffic supervision of all underground networks. 2.4. Industrial Ethernet Industrial Ethernet (IE) is the name given to IEEE-802.3 and 802.11 networks for automation and control systems. The objective of IE is to
add a functionality to open or proprietary protocols, such as Modbus, Profibus, CANopen, DeviceNet or Fieldbus. The idea is to increase the speed, the distance and the performance, in addition to replacing the classic master-slave to peer to peer architecture with better interoperability. However, this technology presents several difficulties in industrial communications migration. For example, the protocols TCP/IP and the control algorithms are not designed for deterministic and real time processes in industrial environments. Nowadays the implementation of industrial Ethernet is focused on the upper layer of OSI (link and applications layers). To conclude, in this section several standards used in communications on-board trains have been analyzed. Most of them have a low bitrate that are not able to transmit new video systems, and their expansion is difficult and expensive. In the next section, the modification of the MVB (IEC 61375) to increase the bus bitrate by optimizing the bandwidth is presented. 3. Communication system In order to take advantage of the available bandwidth of the MVB bus, the digital communication system proposed allows a broadband communication over a twisted pair transmission line. Like others
Table 2 OFDM parameters Parameter
Value
Distance between carriers Number of carriers Bandwidth used Time between consecutive samples Sample frequency of the transmitted signal Frame duration Symbol duration Samples per frame Samples per symbol Samples at FFT/IFFT Symbols per frame Pilots per frame Bits per QAM symbol Data bits transmitted per frame Data bits transmitted per symbol Bitrate Data sample frequency Carrier frequency
200 kHz 14 2.8 MHz 19.53 ns 51.2 MHz 499.92 μs 6.17 μs 25,596 316 256 81 2 4 4424 56 8 Mbps 2 MHz 20 MHz
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Fig. 3. Maximum work frequency vs. wireline length.
modern broadband systems (PLC, 802.11, xDSL), the system proposed uses OFDM to increase the bitrate and to optimize the bandwidth. In this case, the bandwidth available is split between TCN and the OFDM system. In the following sections, the OFDM technique and the system developed are described.
Each set of subcarriers, which are modulated in Quadrature Amplitude Modulation (QAM), generates an OFDM symbol. Supposing it can be expressed like Eq. (1),
3.1. OFDM review
sðt Þ ¼ ∑ di d e
0 Ns −1
B j2π d @Fc −
i−
1 Ns 2C Ad ðt−ts Þ
TSmb
; ts V t V ts þ TSmb
ð1Þ
i¼0
sðt Þ ¼ 0; t b ts 1 t N ts þ TSmb The performance principle of OFDM consists in splitting the digital information bit stream into small blocks and then transmitting these blocks simultaneously over a number of subcarriers.
Where the symbol begins in t = ts, s(t) is the OFDM symbol as a time function, di are the complex QAM symbols of each subcarrier, Ns is the
Fig. 4. Frequency response of a 30 m transmission line on-board train.
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Fig. 5. Line amplifier scheme.
number of symbol subcarriers, TSmb is the OFDM symbol duration, and Fc is the carrier frequency of the OFDM signal. The real and imaginary parts in the last expression correspond with the in-phase and quadrature components of the OFDM signal. The final OFDM signal is modulated by multiplying by a cosine and a sine of the desired carrier frequency (Fc). Finally, the OFDM frame is made by linking several symbols and separating each frame by a silence. The interferences between symbols are minimized by introducing a cyclic extension between each symbol. One advantage is that today this technique could be fully implemented in a programmable logic device. This has been a clear advantage for system prototyping. More details and information about OFDM can be seen in [12]. 3.2. System description Fig. 2 shows a block diagram of the prototype. In this diagram, the upper and bottom row blocks correspond to transmitter and receiver units, respectively. Noise and channel blocks represent the transmission line. Both transmitter and receiver blocks are fully digital systems and are divided into three sections, as shown at Fig. 2. From left to right, the first section represents the front-end, the next section is the process on frequency domain, and the right section is the process on time domain. Next a brief description of the system is given. Firstly, different input sources are time multiplexed on the transmitter block and a data bit stream is generated. This stream is split into groups of equal size and then each group is processed through a QAM modulator, obtaining the real and imaginary parts (a QAM symbol) of each one. After that, QAM symbols are distributed over OFDM subcarriers. Next, this information is transformed to time domain by an IFFT process, and the core of the OFDM symbol is obtained. Then, each OFDM symbol is built by extending the result of the IFFT with cyclic extensions. An OFDM frame is composed by the linking of OFDM symbols (windowing) and pilot symbols (which are well known symbols). These pilots are inserted as start delimiter at the beginning of the frame. This frame is then modulated over a carrier. Finally, the D/A conversion and line-driving processes are carried out. On the other hand, the receiver front-end is composed of an A/D converter and a selective filter bank. Once the start delimiter of the frame has been detected, the receiver demodulates the input signal to base band. Then, a synchronization process is carried out in order to obtain the OFDM symbols of the frame. After each symbol is obtained, an FFT process is carried out in order to decompose each symbol into its subcarriers.
The function of pilot symbols inserted by the transmitter is to allow the measurement of the attenuation and the phase angle introduced by the channel during the signal propagation in each subcarrier. These measurements are realized by Channel Estimation block. Then a correction process with these measurements is performed to minimize the channel effects in the symbols of the frame. Finally, the corrected QAM symbols are decoded and demultiplexed to different outputs. 3.3. System configuration The OFDM technique has a set of parameters that configure the system. These parameters are linked to each other to maintain the OFDM carriers orthogonally so that the system operation is optimal. Table 2 contains the most representative parameters of the OFDM system and the values used in the test bench for our prototype. 4. Practical system implementation Different implementations for the communication system were carried out in laboratory conditions. In a first case, the transmission line was modelled and simulated to verify the bandwidth available as well as the system reliability. Once a Matlab line model had been analyzed and tested, a laboratory prototype of the full communication system was implemented on FPGA platform. This platform allowed a complete debugging and optimizing process for the system performance. Finally a MVB bus that includes several MVB devices and the communications prototype was set up. Both systems were tested, sharing the same transmission line successfully. The following sections explain the tasks performed.
Table 3 Summary of the resources used by the system Absolute values
Family Device Total ALUTs Total registers Total pins Total memory bits DSP block 9-bit elements Total PLLs
Stratix II EP2S60F1020C4 – 10,870 – – – –
Resources Used
Total
Used (%)
– – 12,494 – 82 652,840 232 5
– – 48,352 – 719 2,544,192 288 12
– – 26% – 11% 26% 81% 42%
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Table 4 Resources used by system's blocks Block
ALUTs
ALMs
LC combinationals
LC registers
Memory bits
DSP 9 × 9
DSP 18 × 18
Data in/out Transmitter Multiplexer QAM modulator Pilot insertion IFFT Cyclic extension Windowing Modulator Others (Tx)
273 4482 23 6 53 3340 200 208 436 216
161 2766 17 6 32 2009 146 155 261 140
88 2906 21 6 51 2029 118 123 361 197
245 3814 9 0 29 3146 193 200 237 0
288 497,248 0 96 64 34,560 13,312 28,096 421,120 0
0 68 0 0 0 24 8 8 28 0
0 0 0 0 0 0 0 0 0 0
0 18 0 0 0 12 0 0 6 0
0 4 0 0 0 0 1 1 2 0
Receiver Filter Silence detector Demodulator Synchronization FFT Channel estimation Channel correction QAM demodulator Demultiplexer Others (Rx)
7739 1160 52 115 50 5509 221 88 224 89 231
4887 648 28 67 34 3591 121 50 138 57 153
4434 591 46 63 48 2987 205 86 207 25 176
6811 1160 42 109 28 5061 187 56 9 85 74
155,304 65,520 0 11,520 24,576 46,680 2528 0 0 384 4096
164 0 0 12 0 96 40 16 0 0 0
0 0 0 0 0 0 0 0 0 0 0
10 0 0 6 0 0 4 0 0 0 0
18 0 0 0 0 12 4 2 0 0 0
4.1. Line modelling The line normally used in embedded systems on railways vehicles is a shielded twisted pair (STP) with special characteristic for out-
DSP elements
DSP 36 × 36
door and hostile environments. We used an STP from Raychem (99TM1121-20) for the research. The model obtained was deeply analyzed in order to verify frequency response and bandwidth performance.
Fig. 6. Filter and coupling scheme used in TCN.
Fig. 7. Filter frequency response for TCN signal.
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Fig. 8. TCN signal frequency spectrum.
The mathematical model obtained was checked and compliant with line transmission classical theory [13,14]. Several tests were made to obtain the physical parameters, such as attenuation as well characteristic impedance, using analyzers and measurement instrumentation (HP4194A–HP41941A). Also, the maximum operation frequency as a function of line transmission length with a 30 dB attenuation was calculated (see Fig. 3). In consequence a trade off between line length and bandwidth operation exists. Given that typical vehicle length is about 30 m, the maximum bandwidth for this bus length is 30 MHz (see Fig. 3). To
justify this conclusion, the frequency response of this wire, shown in Fig. 4, was measured. 4.2. Prototyping After analyzing the transmission line and determining the main functional characteristics of the system, the OFDM parameters were selected in order to develop a laboratory prototype. The platform is built using a Stratix II EP2S60 DSP board. This board has a Stratix II EP2S60F1020C4 FPGA from Altera, two A/D converters (AD9433BSQ)
Fig. 9. Above, the MVB master frame, and below, same master frame with OFDM signal. Transmitter end point.
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Fig. 10. PSD of the transmitted signals.
from Analog Devices and two D/A converters (DAC904) from Texas Instruments. In this board the digital processing described in Section 3.2 (the transmitter and receiver blocks from Fig. 2) is implemented using Quartus II software. The interface with the bus is composed of a line driver amplifier (OPA2677) from Texas Instruments, with a 4 V/V gain, and a BALUN transformer (T2.5-6T-X65) from Mini-Circuit (see diagram at Fig. 5). A summary of results is shown in Tables 3 and 4.
4.3. Coupling to MVB and filtering The MVB standard defines three physical mediums for the communication process: • Electrical Short Distance (ESD): Uses a RS485 transmission system without galvanic isolation. It supports up to 32 devices and 20 m cable length. It is used inside racks.
Fig. 11. OFDM and MVB signals at receiver end.
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Fig. 12. Power Spectral Density signals at receiver.
• Electrical Middle Distance (EMD): Uses a RS485 transmission system with galvanic isolation using transformers. It supports up to 32 devices and 200 m cable length. It is used to communicate devices inside a vehicle. • Optical Glass Fibre (OGF): Star topology. It supports 2 km line length and it is used only in critical vehicles like a locomotive unit to avoid electromagnetic interference. Given that the proposed communication system will be used in a vehicle (bus larger than 20 m), the medium interface of the devices must be EMD.
On the other hand, two different couplings to the bus are possible, capacitive or inductive, to avoid the short circuiting of the transmission line. We selected a capacitive coupling, because the energy losses are lower than with inductive coupling [15]. Moreover, MVB devices use capacitive coupling. In this way all coupler devices in the bus are similar. The next step was the bandwidth analysis. MVB presents a wide spectrum and it is necessary to reduce it with a passive low pass filter in order not to disturb the OFDM system. Fig. 6 presents the MVB filtering and coupling circuit. In this circuit, the isolation transformer and the filter's capacitor form an LC network with a 2.7 MHz cut-off frequency. The filter frequency response is shown in Fig. 7. The objective of this filter
Fig. 13. QAM constellation at OFDM receiver.
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Fig. 14. Bit Error Rate measured.
is to reduce the MVB signal energy so that, at the OFDM band, it is irrelevant. In Fig. 8, the Power Spectral Density (PSD) of the MVB signal is shown. As we can see, from 12 MHz the energy is lower than −100 dB. In the OFDM system it is necessary to filter at the receiver to eliminate the MVB signal. This is done at the FPGA with a band-pass 40th order FIR filter with cut-off frequencies at 17 and 23 MHz. 5. Results This section presents some relevant results obtained in the test bench described in Section 4.2.
Fig. 9 shows a measurement of the transmitting signal at the transmitter end point, when both systems are working simultaneously. Note how clearly the OFDM signal is added to the MVB signal generated. The PSD of the signals is shown in Fig. 10. Note that MVB energy is spread until 12 MHz and OFDM is distributed around 20 MHz, which shows that the filters are working properly. Fig. 11 represents both signals at receiver end point. Note the attenuation introduced by the channel. In the OFDM receiver, after AD conversion, the signal is filtered to eliminate the MVB spectrum and the noise induced during the propagation. The PSD of the signal before and after the filter is shown in Fig. 12. Finally, Fig. 13 shows the QAM
Fig. 15. System bitrate as function of bandwidth and modulation.
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constellation of the signal received, where some wrong points due to the noise of the line can be observed. To evaluate the robustness of the system as a function of the Signal-to-Noise Ratio (SNR) and the QAM used, the system had been simulated varying these parameters and measuring the Bit Error Ratio (BER). The results are shown in Fig. 14. Note that if a low index modulation is selected, the system becomes more robust. However the bitrate is reduced. Obviously it is necessary to reach a trade off between bitrate and noise robustness. For example, in our test bench, a value of 16-QAM with a bandwidth of 2.8 MHz was selected, which resulted in an 8 Mbps bitrate with very low BER. Finally, to appreciate the system's capacity, Fig. 15 shows the bitrate as a function of the bandwidth used and the QAM index selected. From this figure it is easy to see that using the full bandwidth available in a MVB bus (15 MHz), and a QAM index of 256, it is possible to reach a bitrate of 90 Mbps. Obviously, if the system is noisy the BER will be high with this configuration. 6. Conclusions In recent years, bandwidth requirements due to the installation of new embedded system on-board trains are growing rapidly. After one decade, the standard IEC 61375 is much extended. However, new broadband requirements require new solutions. We propose a new communications system based on OFDM, which increases the bitrate of the transmission line by several Mbps. It shares the line with actual IEC 61375 equipment, which optimizes the use of the bandwidth available in the line. The advantage of this solution is that it improves performance, so covering new system requirements, and does not require the installation of new wiring, thus reducing installation and maintenance costs. The results obtained from the test bench are 8 Mbps with 16-QAM in a bandwidth of 2.8 MHz with a 30 m cable. Nevertheless, using all the bandwidth available in the transmission line it is possible to reach 90 Mbps. The flexibility of the OFDM technique makes it possible to distribute the bandwidth between different channels; for example, to have a bidirectional communication. Finally, it is necessary to emphasize that, to permit communications through the train, repeaters need to be installed on each vehicle. Acknowledgments The authors thank the scholarship provided by the ICAI Engineers' Association for the research, and the companies Altera, SEPSA, Renfe and VAC for the resources made available for the study. This work has also been partially supported by the Ministry of Science and Technology and by the research project DPI2004-06940-C02-02 of the Spanish Plan Nacional de I+D+i. References [1] IEC, Standard 61375-1 for Electric Railway Equipment — Train bus Part 1: Train Communication Network, Oct. 1999. [2] P. Umiliacchi, C. Pentimalli, A. Piazza, ROSIN — TR 1045 — Summary of the Specification and Development Work, Public deliverable, 1997. [3] ROSIN: Railway Open System Interconnection Network, Rosin Consortium, 1996– 2000 available online at http://www.labs.it/rosin/default.htm. [4] E. Renner, Integrated Communication System for Intelligent Train Applications, Public Deliverable, 2001. [5] TrainCom: Integrated communication System for Intelligent Train Applications, TrainCom Consortium, 2001–2004 available online at http://www.traincom.org.
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[6] P. Umiliacchi, R. Shingler, EuRoMain: European Railway Open Maintenance System. Final Report, Public Deliverable, 2006. [7] EuRoMain: European Railway Open Maintenance System, EuRoMain Consortium, 2002 available online at http://www.euromain.org. [8] Echelon Corporation, LonWorks, 2002, available online at http://www.smarthomeforum.com/lonworks.asp?ID=17. [9] IEC 61158, PROFIBUS Standard — DP Specification, available online at http://www. profibus.com. [10] G. Neff, Tunneling Routers for Trainline Networks, Presented in LonWorld2000 conference, 2000, pp. 1–14, Orlando, FL (USA). [11] F. Nardelli, “LonWorks™ On Amtrak's Acela Express,” Presented in APTA Rail Transit Conference, Baltimore (United States), 2002. [12] R.v. Nee, R. Prasad, OFDM for Wireless Multimedia Communications, Artech House Publishers, London, England, 2000. [13] R.W.P. King, H.R. Mimno, A.H. Wing, Transmission Lines, Antennas and Wave Guides, McGraw-Hill Book Company, Inc, Cambridge, England, 1945. [14] P.C. Magnusson, G.C. Alexander, V.K. Tripathi, Transmission Lines and Wave Propagation, 3rd edCRC Press, Corvallis, Oregon, 1992. [15] G. Berterreix and M. Bonet, Transmisión de datos por la red eléctrica (PLC) en banda angosta, Ph. D Thesis, Electrical Engineering Department., Comahue National University, 2006.
C. Rodríguez-Morcillo. He received the MS and PhD degree in Electrical Engineering from the Universidad Pontificia Comillas of Madrid, Spain, in 2001 and 2007, respectively. Since 2001, he has been with the Institute for Research in Technology, Comillas Pontifical University of Madrid. Currently, he is a researcher of the institute. His major current research interests include hardware design, embedded communications systems and digital systems design.
S. Alexandres. He received the MS and PhD degrees in Telecommunications from the Polytechnical University of Madrid, Spain, in 1985 and 1991. From 1991 to 1998, he was an associate professor of the Telecommunications School, UPM, Madrid. Since 1998 he has been with the Department of Electronics and Automatics for the School of Engineering and member of the research staff of the Institute of Research, Universidad Pontificia Comillas of Madrid. At present he is a professor and responsible for the electronics and automatics research group. His major current research interests include embedded communications systems, digital architectures and hardware design.
J.D. Muñoz. He received the MS and PhD degrees in Electrical Engineering from the Universidad Pontificia Comillas of Madrid, Spain, in 1991 and 2002. Since 1991 he has been with the Department of Electronics and Automatic Control and member of the research staff of the Institute of Research, both of the Universidad Pontificia Comillas de Madrid. His main research interests include digital systems design, computer architecture, motor drives control and design of embedded systems for automatic control applications.
Contents lists available at ScienceDirect
Computer Standards & Interfaces j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c s i
Broadband system to increase bitrate in train communication networks C. Rodríguez-Morcillo a,⁎, S. Alexandres b, J.D. Muñoz b a b
Instituto de Investigación Tecnológica, Universidad Pontificia Comillas, Cl. Santa Cruz de Marcenado 26, 28015 Madrid, Spain Departamento de Electrónica y Automática, Universidad Pontificia Comillas, Cl. Alberto Aguilera 23, 28015 Madrid, Spain
A R T I C L E
I N F O
A B S T R A C T
Article history: Received 18 May 2007 Received in revised form 27 March 2008 Accepted 4 May 2008 Available online 18 May 2008
MVB (Multifunction Vehicle Bus), defined in IEC 61375, has been broadly adopted as the communication standard between embedded control systems on-board modern trains. In this work a new method to take advantage of the full bandwidth of the channel using an OFDM technique is described. With this new method it is possible to share the physical medium between standard MVB traffic and new OFDM traffic. A 90 Mbps theoretical bitrate can be achieved. The results of this work have been validated in a test bench including standard MVB nodes transmitting on a line similar to a real vehicle bus. © 2008 Elsevier B.V. All rights reserved.
Keywords: IEC 61375 MVB OFDM Programmable logic Bandwidth optimization
Contents 1. 2.
Introduction . . . . . . . . . . . . Standards discussion . . . . . . . . 2.1. EIA-709 (LonWorks) . . . . . 2.2. IEC 61375-1 (TCN) . . . . . . 2.3. IEC-61158-3 (Profibus) . . . . 2.4. Industrial Ethernet . . . . . 3. Communication system . . . . . . . 3.1. OFDM review . . . . . . . . 3.2. System description . . . . . 3.3. System configuration . . . . 4. Practical system implementation . . 4.1. Line modelling . . . . . . . 4.2. Prototyping . . . . . . . . . 4.3. Coupling to MVB and filtering 5. Results . . . . . . . . . . . . . . 6. Conclusions . . . . . . . . . . . . Acknowledgments. . . . . . . . . . . . References . . . . . . . . . . . . . . .
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1. Introduction Several factors drive the installation of new communication systems on-board trains: investment by companies and governments facilitates the development of new systems; furthermore, the rapid
⁎ Corresponding author. Tel.: +34 91 5422800; fax: +34 91 5423176. E-mail addresses: [email protected] (C. Rodríguez-Morcillo), [email protected] (S. Alexandres), [email protected] (J.D. Muñoz). 0920-5489/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.csi.2008.05.007
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advances of the technology, the social adoption of the information and communication technologies (ICT) and the demand for new services by users in the railway sector have led to the installation or redesign of new on-board communications systems. In recent decades, railway operators planned to develop a modern and versatile communication system on-board trains. The objective was to allow the interchange of information between different sets of equipment embedded in a train. In addition, it was necessary to unify an interface between devices, so that vehicles developed by different manufactures could interact in the same network.
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Table 1 Comparison of industrial buses Bus
Bitrate [Mbps]
Physical medium
Audio/video
Advantages
Disadvantages
LonWorks (ANSI/EIA-709)
0.078
Twisted pair
No
Low speed. No audio. No video.
TCN (IEC 61375)
1
Twisted pair Optical fibre Twisted pair Optical fibre Twisted pair
Low bitrate audio
Established. It is used by NYCT and others Promoted by Siemens and Adtranz Derivate of LonWorks
AS-5370
1.5 5 1.25
E1 PDH (UIT G.732)
2.048
WorldFIP (EN50170-3)
2.5
Ethernet (IEEE 802.3)
10
Profibus (IEC 61158)
12
Twisted pair Optical fibre Coaxial Twisted pair Twisted pair Optical fibre
Audio
Audio Audio
Established. It is used by NYCT Promoted by Alstom
Video
Difunded
Audio and video
Very fast
Not extensible. No audio. No video. Requires a license Not extendible
Proprietary components Requires couplers. Not deterministic Requires couplers
At the end of 1988, the International Electrotechnical Commission (IEC) and the Union Internationale des Chemins de fer (UIC) began to develop an international standard detailing all requirements for the communication process of all on-board equipment. The major requirements of this development are: • Interoperability at train level to allow communication between railway vehicles of different countries. • Interoperability at vehicle level to allow communication between devices of different manufacturers. • Improvement in the operation and introduction of new services. • Support for configuration and maintenance and a general cost reduction in the life-time of products. After ten years of work, the IEC 61375-1 or Train Communication Network (TCN) was obtained [1]. Different European projects for its validation and standardization have been carried out around TCN. ROSIN (96-99) (Railway Open System Interconnection Network) [2,3], TrainCom (00-04) (Integrated Communication System for Intelligent Train Applications) [4,5] and EuRoMain (02-05) (European Railway Open Maintenance System) [6,7]. Finally, in October 1999, in Kyoto (Japan), TCN achieved the state of International Standard as IEC 61375-
1. Later, the IEEE Vehicular Society included TCN in its IEEE Standard for Communications Protocol Onboard Trains, as P1473-T. In spite of the increasing use of TCN in communications on-board trains, there are other communication standards that perform the same functions, like LonWorks (ANSI/EIA 709) [8] and Profibus (IEC 61158-3) [9]. Table 1 [10] shows others industrial communication fieldbuses used for interconnecting embedded systems on-board trains. In recent years, the limitations of TCN have been detected. This bus was designed as a real time bus for control and supervision purposes. These applications typically require a low bandwidth, so that the bus was designed with a bitrate of 1.5 Mbps. But today, the users and operators of railway systems are demanding new services, like new embedded control systems, digital audio, video-surveillance and user information, with more bandwidth requirements. The solution, a priori, is the installation of new wiring for these new systems. Nevertheless, the installation and maintenance costs grow rapidly. This paper presents an approach using a similar solution to others sharing techniques like PLC or ADSL. The solution shares the physical wire used by TCN with a high frequency channel using an OFDM technique. Bitrate increases to several Megabits per second. The paper is organized in four sections. After this introduction, Section 2 briefly reviews the norm IEC 61375 on Train Communication Network and other standards used in train networks. The following section describes the communication system proposed, introducing the OFDM architecture. Section 4 describes the prototype used for system validation. Finally, the last two sections present results and the conclusions. 2. Standards discussion This section describes several industrial fieldbus standards used to support broadband communication between electronic embedded systems in train vehicles. In addition, Table 1 synthesizes and compares these and others standards. 2.1. EIA-709 (LonWorks) EIA-709 and IEEE 1473-L, known as LonWorks, specifies the physical layer LON (Local Operating Network) and the LonTalk communications protocol. This standard supports control systems by means of a point to point network in a hierarchical structure (masterslave). Interoperability is allowed between different manufactures and good results are obtained. Nevertheless, maintenance and modification are expensive and difficult in these networks. LonWorks is used in multiple industrial applications, for example in Amtrak's Acela Express [11]. The main disadvantages of this network are the low bitrate of some kbps, and the fact that it is a non-deterministic network, which does not allow the system to function in real time.
Fig. 1. TCN buses architecture.
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Fig. 2. Digital system block diagram.
2.2. IEC 61375-1 (TCN) Standard IEC 61375 allows communication between devices installed in the same or in different vehicles in a train. The main digital services covered are traction, control and supervision devices (doors, lights, air conditioning and so on), and central and auxiliary management devices. Communication between on-board systems happens at two level architecture. The standard defines two hierarchical buses: WTB (Wire Train Bus) connects the equipment in different vehicles at 1 Mbps in the upper level; and MVB (Multifunction Vehicle Bus) connects embedded systems inside a vehicle at 1.5 Mbps in the lower level (see Fig. 1). Both buses use a master-slave protocol with baseband Manchester encoding. At the moment, MVB is being introduced in a large number of European train networks. In some cases, the MVB bus is used to interconnect all the vehicles of a train if its length is short (as in underground trains). However, WTB has not had the same acceptance, using in some cases E1, T1 or Ethernet networks to interconnect different vehicles of the train. One disadvantage of this standard is the low bitrate. In addition, the standard lacks clarity in some important points, for example, the objective of the bus redundancy (point 3.3 of the IEC-61375). 2.3. IEC-61158-3 (Profibus) This is one of the most used architectures in automation and control in the US and Europe. The standard was created in 1989 in Germany by a group of fieldbus manufacturers. It allows a bitrate between 9.6 kbps and 12 Mbps and a maximum of 126 devices connected on the same bus. The standard also supports audio and low bitrate video in real time communication. The standard is focused on the application and link layers. Many applications of Profibus are being used in Europe, for example, the remote supervision system in the Warsaw (Poland) underground system developed by Kontron. The network is working in a deterministic and real time scheme that makes possible the traffic supervision of all underground networks. 2.4. Industrial Ethernet Industrial Ethernet (IE) is the name given to IEEE-802.3 and 802.11 networks for automation and control systems. The objective of IE is to
add a functionality to open or proprietary protocols, such as Modbus, Profibus, CANopen, DeviceNet or Fieldbus. The idea is to increase the speed, the distance and the performance, in addition to replacing the classic master-slave to peer to peer architecture with better interoperability. However, this technology presents several difficulties in industrial communications migration. For example, the protocols TCP/IP and the control algorithms are not designed for deterministic and real time processes in industrial environments. Nowadays the implementation of industrial Ethernet is focused on the upper layer of OSI (link and applications layers). To conclude, in this section several standards used in communications on-board trains have been analyzed. Most of them have a low bitrate that are not able to transmit new video systems, and their expansion is difficult and expensive. In the next section, the modification of the MVB (IEC 61375) to increase the bus bitrate by optimizing the bandwidth is presented. 3. Communication system In order to take advantage of the available bandwidth of the MVB bus, the digital communication system proposed allows a broadband communication over a twisted pair transmission line. Like others
Table 2 OFDM parameters Parameter
Value
Distance between carriers Number of carriers Bandwidth used Time between consecutive samples Sample frequency of the transmitted signal Frame duration Symbol duration Samples per frame Samples per symbol Samples at FFT/IFFT Symbols per frame Pilots per frame Bits per QAM symbol Data bits transmitted per frame Data bits transmitted per symbol Bitrate Data sample frequency Carrier frequency
200 kHz 14 2.8 MHz 19.53 ns 51.2 MHz 499.92 μs 6.17 μs 25,596 316 256 81 2 4 4424 56 8 Mbps 2 MHz 20 MHz
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Fig. 3. Maximum work frequency vs. wireline length.
modern broadband systems (PLC, 802.11, xDSL), the system proposed uses OFDM to increase the bitrate and to optimize the bandwidth. In this case, the bandwidth available is split between TCN and the OFDM system. In the following sections, the OFDM technique and the system developed are described.
Each set of subcarriers, which are modulated in Quadrature Amplitude Modulation (QAM), generates an OFDM symbol. Supposing it can be expressed like Eq. (1),
3.1. OFDM review
sðt Þ ¼ ∑ di d e
0 Ns −1
B j2π d @Fc −
i−
1 Ns 2C Ad ðt−ts Þ
TSmb
; ts V t V ts þ TSmb
ð1Þ
i¼0
sðt Þ ¼ 0; t b ts 1 t N ts þ TSmb The performance principle of OFDM consists in splitting the digital information bit stream into small blocks and then transmitting these blocks simultaneously over a number of subcarriers.
Where the symbol begins in t = ts, s(t) is the OFDM symbol as a time function, di are the complex QAM symbols of each subcarrier, Ns is the
Fig. 4. Frequency response of a 30 m transmission line on-board train.
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Fig. 5. Line amplifier scheme.
number of symbol subcarriers, TSmb is the OFDM symbol duration, and Fc is the carrier frequency of the OFDM signal. The real and imaginary parts in the last expression correspond with the in-phase and quadrature components of the OFDM signal. The final OFDM signal is modulated by multiplying by a cosine and a sine of the desired carrier frequency (Fc). Finally, the OFDM frame is made by linking several symbols and separating each frame by a silence. The interferences between symbols are minimized by introducing a cyclic extension between each symbol. One advantage is that today this technique could be fully implemented in a programmable logic device. This has been a clear advantage for system prototyping. More details and information about OFDM can be seen in [12]. 3.2. System description Fig. 2 shows a block diagram of the prototype. In this diagram, the upper and bottom row blocks correspond to transmitter and receiver units, respectively. Noise and channel blocks represent the transmission line. Both transmitter and receiver blocks are fully digital systems and are divided into three sections, as shown at Fig. 2. From left to right, the first section represents the front-end, the next section is the process on frequency domain, and the right section is the process on time domain. Next a brief description of the system is given. Firstly, different input sources are time multiplexed on the transmitter block and a data bit stream is generated. This stream is split into groups of equal size and then each group is processed through a QAM modulator, obtaining the real and imaginary parts (a QAM symbol) of each one. After that, QAM symbols are distributed over OFDM subcarriers. Next, this information is transformed to time domain by an IFFT process, and the core of the OFDM symbol is obtained. Then, each OFDM symbol is built by extending the result of the IFFT with cyclic extensions. An OFDM frame is composed by the linking of OFDM symbols (windowing) and pilot symbols (which are well known symbols). These pilots are inserted as start delimiter at the beginning of the frame. This frame is then modulated over a carrier. Finally, the D/A conversion and line-driving processes are carried out. On the other hand, the receiver front-end is composed of an A/D converter and a selective filter bank. Once the start delimiter of the frame has been detected, the receiver demodulates the input signal to base band. Then, a synchronization process is carried out in order to obtain the OFDM symbols of the frame. After each symbol is obtained, an FFT process is carried out in order to decompose each symbol into its subcarriers.
The function of pilot symbols inserted by the transmitter is to allow the measurement of the attenuation and the phase angle introduced by the channel during the signal propagation in each subcarrier. These measurements are realized by Channel Estimation block. Then a correction process with these measurements is performed to minimize the channel effects in the symbols of the frame. Finally, the corrected QAM symbols are decoded and demultiplexed to different outputs. 3.3. System configuration The OFDM technique has a set of parameters that configure the system. These parameters are linked to each other to maintain the OFDM carriers orthogonally so that the system operation is optimal. Table 2 contains the most representative parameters of the OFDM system and the values used in the test bench for our prototype. 4. Practical system implementation Different implementations for the communication system were carried out in laboratory conditions. In a first case, the transmission line was modelled and simulated to verify the bandwidth available as well as the system reliability. Once a Matlab line model had been analyzed and tested, a laboratory prototype of the full communication system was implemented on FPGA platform. This platform allowed a complete debugging and optimizing process for the system performance. Finally a MVB bus that includes several MVB devices and the communications prototype was set up. Both systems were tested, sharing the same transmission line successfully. The following sections explain the tasks performed.
Table 3 Summary of the resources used by the system Absolute values
Family Device Total ALUTs Total registers Total pins Total memory bits DSP block 9-bit elements Total PLLs
Stratix II EP2S60F1020C4 – 10,870 – – – –
Resources Used
Total
Used (%)
– – 12,494 – 82 652,840 232 5
– – 48,352 – 719 2,544,192 288 12
– – 26% – 11% 26% 81% 42%
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Table 4 Resources used by system's blocks Block
ALUTs
ALMs
LC combinationals
LC registers
Memory bits
DSP 9 × 9
DSP 18 × 18
Data in/out Transmitter Multiplexer QAM modulator Pilot insertion IFFT Cyclic extension Windowing Modulator Others (Tx)
273 4482 23 6 53 3340 200 208 436 216
161 2766 17 6 32 2009 146 155 261 140
88 2906 21 6 51 2029 118 123 361 197
245 3814 9 0 29 3146 193 200 237 0
288 497,248 0 96 64 34,560 13,312 28,096 421,120 0
0 68 0 0 0 24 8 8 28 0
0 0 0 0 0 0 0 0 0 0
0 18 0 0 0 12 0 0 6 0
0 4 0 0 0 0 1 1 2 0
Receiver Filter Silence detector Demodulator Synchronization FFT Channel estimation Channel correction QAM demodulator Demultiplexer Others (Rx)
7739 1160 52 115 50 5509 221 88 224 89 231
4887 648 28 67 34 3591 121 50 138 57 153
4434 591 46 63 48 2987 205 86 207 25 176
6811 1160 42 109 28 5061 187 56 9 85 74
155,304 65,520 0 11,520 24,576 46,680 2528 0 0 384 4096
164 0 0 12 0 96 40 16 0 0 0
0 0 0 0 0 0 0 0 0 0 0
10 0 0 6 0 0 4 0 0 0 0
18 0 0 0 0 12 4 2 0 0 0
4.1. Line modelling The line normally used in embedded systems on railways vehicles is a shielded twisted pair (STP) with special characteristic for out-
DSP elements
DSP 36 × 36
door and hostile environments. We used an STP from Raychem (99TM1121-20) for the research. The model obtained was deeply analyzed in order to verify frequency response and bandwidth performance.
Fig. 6. Filter and coupling scheme used in TCN.
Fig. 7. Filter frequency response for TCN signal.
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Fig. 8. TCN signal frequency spectrum.
The mathematical model obtained was checked and compliant with line transmission classical theory [13,14]. Several tests were made to obtain the physical parameters, such as attenuation as well characteristic impedance, using analyzers and measurement instrumentation (HP4194A–HP41941A). Also, the maximum operation frequency as a function of line transmission length with a 30 dB attenuation was calculated (see Fig. 3). In consequence a trade off between line length and bandwidth operation exists. Given that typical vehicle length is about 30 m, the maximum bandwidth for this bus length is 30 MHz (see Fig. 3). To
justify this conclusion, the frequency response of this wire, shown in Fig. 4, was measured. 4.2. Prototyping After analyzing the transmission line and determining the main functional characteristics of the system, the OFDM parameters were selected in order to develop a laboratory prototype. The platform is built using a Stratix II EP2S60 DSP board. This board has a Stratix II EP2S60F1020C4 FPGA from Altera, two A/D converters (AD9433BSQ)
Fig. 9. Above, the MVB master frame, and below, same master frame with OFDM signal. Transmitter end point.
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Fig. 10. PSD of the transmitted signals.
from Analog Devices and two D/A converters (DAC904) from Texas Instruments. In this board the digital processing described in Section 3.2 (the transmitter and receiver blocks from Fig. 2) is implemented using Quartus II software. The interface with the bus is composed of a line driver amplifier (OPA2677) from Texas Instruments, with a 4 V/V gain, and a BALUN transformer (T2.5-6T-X65) from Mini-Circuit (see diagram at Fig. 5). A summary of results is shown in Tables 3 and 4.
4.3. Coupling to MVB and filtering The MVB standard defines three physical mediums for the communication process: • Electrical Short Distance (ESD): Uses a RS485 transmission system without galvanic isolation. It supports up to 32 devices and 20 m cable length. It is used inside racks.
Fig. 11. OFDM and MVB signals at receiver end.
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Fig. 12. Power Spectral Density signals at receiver.
• Electrical Middle Distance (EMD): Uses a RS485 transmission system with galvanic isolation using transformers. It supports up to 32 devices and 200 m cable length. It is used to communicate devices inside a vehicle. • Optical Glass Fibre (OGF): Star topology. It supports 2 km line length and it is used only in critical vehicles like a locomotive unit to avoid electromagnetic interference. Given that the proposed communication system will be used in a vehicle (bus larger than 20 m), the medium interface of the devices must be EMD.
On the other hand, two different couplings to the bus are possible, capacitive or inductive, to avoid the short circuiting of the transmission line. We selected a capacitive coupling, because the energy losses are lower than with inductive coupling [15]. Moreover, MVB devices use capacitive coupling. In this way all coupler devices in the bus are similar. The next step was the bandwidth analysis. MVB presents a wide spectrum and it is necessary to reduce it with a passive low pass filter in order not to disturb the OFDM system. Fig. 6 presents the MVB filtering and coupling circuit. In this circuit, the isolation transformer and the filter's capacitor form an LC network with a 2.7 MHz cut-off frequency. The filter frequency response is shown in Fig. 7. The objective of this filter
Fig. 13. QAM constellation at OFDM receiver.
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Fig. 14. Bit Error Rate measured.
is to reduce the MVB signal energy so that, at the OFDM band, it is irrelevant. In Fig. 8, the Power Spectral Density (PSD) of the MVB signal is shown. As we can see, from 12 MHz the energy is lower than −100 dB. In the OFDM system it is necessary to filter at the receiver to eliminate the MVB signal. This is done at the FPGA with a band-pass 40th order FIR filter with cut-off frequencies at 17 and 23 MHz. 5. Results This section presents some relevant results obtained in the test bench described in Section 4.2.
Fig. 9 shows a measurement of the transmitting signal at the transmitter end point, when both systems are working simultaneously. Note how clearly the OFDM signal is added to the MVB signal generated. The PSD of the signals is shown in Fig. 10. Note that MVB energy is spread until 12 MHz and OFDM is distributed around 20 MHz, which shows that the filters are working properly. Fig. 11 represents both signals at receiver end point. Note the attenuation introduced by the channel. In the OFDM receiver, after AD conversion, the signal is filtered to eliminate the MVB spectrum and the noise induced during the propagation. The PSD of the signal before and after the filter is shown in Fig. 12. Finally, Fig. 13 shows the QAM
Fig. 15. System bitrate as function of bandwidth and modulation.
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constellation of the signal received, where some wrong points due to the noise of the line can be observed. To evaluate the robustness of the system as a function of the Signal-to-Noise Ratio (SNR) and the QAM used, the system had been simulated varying these parameters and measuring the Bit Error Ratio (BER). The results are shown in Fig. 14. Note that if a low index modulation is selected, the system becomes more robust. However the bitrate is reduced. Obviously it is necessary to reach a trade off between bitrate and noise robustness. For example, in our test bench, a value of 16-QAM with a bandwidth of 2.8 MHz was selected, which resulted in an 8 Mbps bitrate with very low BER. Finally, to appreciate the system's capacity, Fig. 15 shows the bitrate as a function of the bandwidth used and the QAM index selected. From this figure it is easy to see that using the full bandwidth available in a MVB bus (15 MHz), and a QAM index of 256, it is possible to reach a bitrate of 90 Mbps. Obviously, if the system is noisy the BER will be high with this configuration. 6. Conclusions In recent years, bandwidth requirements due to the installation of new embedded system on-board trains are growing rapidly. After one decade, the standard IEC 61375 is much extended. However, new broadband requirements require new solutions. We propose a new communications system based on OFDM, which increases the bitrate of the transmission line by several Mbps. It shares the line with actual IEC 61375 equipment, which optimizes the use of the bandwidth available in the line. The advantage of this solution is that it improves performance, so covering new system requirements, and does not require the installation of new wiring, thus reducing installation and maintenance costs. The results obtained from the test bench are 8 Mbps with 16-QAM in a bandwidth of 2.8 MHz with a 30 m cable. Nevertheless, using all the bandwidth available in the transmission line it is possible to reach 90 Mbps. The flexibility of the OFDM technique makes it possible to distribute the bandwidth between different channels; for example, to have a bidirectional communication. Finally, it is necessary to emphasize that, to permit communications through the train, repeaters need to be installed on each vehicle. Acknowledgments The authors thank the scholarship provided by the ICAI Engineers' Association for the research, and the companies Altera, SEPSA, Renfe and VAC for the resources made available for the study. This work has also been partially supported by the Ministry of Science and Technology and by the research project DPI2004-06940-C02-02 of the Spanish Plan Nacional de I+D+i. References [1] IEC, Standard 61375-1 for Electric Railway Equipment — Train bus Part 1: Train Communication Network, Oct. 1999. [2] P. Umiliacchi, C. Pentimalli, A. Piazza, ROSIN — TR 1045 — Summary of the Specification and Development Work, Public deliverable, 1997. [3] ROSIN: Railway Open System Interconnection Network, Rosin Consortium, 1996– 2000 available online at http://www.labs.it/rosin/default.htm. [4] E. Renner, Integrated Communication System for Intelligent Train Applications, Public Deliverable, 2001. [5] TrainCom: Integrated communication System for Intelligent Train Applications, TrainCom Consortium, 2001–2004 available online at http://www.traincom.org.
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[6] P. Umiliacchi, R. Shingler, EuRoMain: European Railway Open Maintenance System. Final Report, Public Deliverable, 2006. [7] EuRoMain: European Railway Open Maintenance System, EuRoMain Consortium, 2002 available online at http://www.euromain.org. [8] Echelon Corporation, LonWorks, 2002, available online at http://www.smarthomeforum.com/lonworks.asp?ID=17. [9] IEC 61158, PROFIBUS Standard — DP Specification, available online at http://www. profibus.com. [10] G. Neff, Tunneling Routers for Trainline Networks, Presented in LonWorld2000 conference, 2000, pp. 1–14, Orlando, FL (USA). [11] F. Nardelli, “LonWorks™ On Amtrak's Acela Express,” Presented in APTA Rail Transit Conference, Baltimore (United States), 2002. [12] R.v. Nee, R. Prasad, OFDM for Wireless Multimedia Communications, Artech House Publishers, London, England, 2000. [13] R.W.P. King, H.R. Mimno, A.H. Wing, Transmission Lines, Antennas and Wave Guides, McGraw-Hill Book Company, Inc, Cambridge, England, 1945. [14] P.C. Magnusson, G.C. Alexander, V.K. Tripathi, Transmission Lines and Wave Propagation, 3rd edCRC Press, Corvallis, Oregon, 1992. [15] G. Berterreix and M. Bonet, Transmisión de datos por la red eléctrica (PLC) en banda angosta, Ph. D Thesis, Electrical Engineering Department., Comahue National University, 2006.
C. Rodríguez-Morcillo. He received the MS and PhD degree in Electrical Engineering from the Universidad Pontificia Comillas of Madrid, Spain, in 2001 and 2007, respectively. Since 2001, he has been with the Institute for Research in Technology, Comillas Pontifical University of Madrid. Currently, he is a researcher of the institute. His major current research interests include hardware design, embedded communications systems and digital systems design.
S. Alexandres. He received the MS and PhD degrees in Telecommunications from the Polytechnical University of Madrid, Spain, in 1985 and 1991. From 1991 to 1998, he was an associate professor of the Telecommunications School, UPM, Madrid. Since 1998 he has been with the Department of Electronics and Automatics for the School of Engineering and member of the research staff of the Institute of Research, Universidad Pontificia Comillas of Madrid. At present he is a professor and responsible for the electronics and automatics research group. His major current research interests include embedded communications systems, digital architectures and hardware design.
J.D. Muñoz. He received the MS and PhD degrees in Electrical Engineering from the Universidad Pontificia Comillas of Madrid, Spain, in 1991 and 2002. Since 1991 he has been with the Department of Electronics and Automatic Control and member of the research staff of the Institute of Research, both of the Universidad Pontificia Comillas de Madrid. His main research interests include digital systems design, computer architecture, motor drives control and design of embedded systems for automatic control applications.