An Experimental Setup for In-Circuit Optimization of Broadband Automotive Power-Line Communications Petrus A. Janse van Rensburg†, Hendrik C. Ferreira‡, and Abraham J. Snyders‡ †
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Department of Electrical Engineering Walter Sisulu University P.Bag 1421, East London, 5200, South Africa Phone: +27-82-200-6207, Fax +27-43-702-9226 E-mail:
[email protected]
Abstract — In this paper, an experimental setup for broadband automotive power-line communications is presented. Modular prototyping equipment is used to construct both a transmitting and receiving node on the wiring harness of a motorcar. Initial characterizing measurements are used to choose typical point-to-point communication paths. Some benchmarking results at 1 Mbps (4-FSK) and 5 Mbps (2-FSK) are also presented. This experimental setup is intended to study the suitability of various modulation techniques and frequency bands. Gathered data is also intended to help trade off performance against complexity/cost.1
Department of Electrical and Electronic Engineering University of Johannesburg P.O.Box 524, Auckland Park, 2006, South Africa Phone +27-11-489-2463, Fax +27-11-489-2357 E-mail: {hcf,ajs}@ing.rau.ac.za
Fig. 1(c) shows that architecturally, power-line communications is the ultimate solution for automotive electronics, as no additional communication network or cabling is necessary. No simpler architecture exists, because power will always have to be delivered to every single load. Furthermore, faultfinding and maintenance of the power / communication network is simplified to such an extent that even a circuit diagram becomes obsolete [1].
Keywords – power-line communications, modulation techniques, measurements, automotive electronics. I. INTRODUCTION
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[1], it was argued that power-line communications holds tremendous potential for the simplification of automotive wiring harnesses. In Fig. 1, the traditional wiring harness topology is compared to a more modern multiplexed topology as well as the ultimate ideal, a power-line communication topology. The scope of application is limited to two corners of the vehicle for simplicity. N
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Fig. 1(a) emphasizes that all active components require battery power to be sourced via a control point (typically some switch close to the ignition key). The number of power cables required, is equal to the number of components requiring control. Multiplexing, an intermediate solution, reduces the thickness and weight of the wiring harness drastically between the ignition and the multiplexed nodes. See in Fig. 1(b). Unfortunately, every single node of the communication bus still needs to be supplied with power. Also, a complicated communication system is introduced. This increase in manufacturing and assembly cost, together with semiconductor expenses, typically cancels the cost savings obtained by the lighter power cabling system. To summarize, it can be said that multiplexing facilitates a lighter, more sophisticated electrical wiring system at the expense of increased complexity and cost. 1 This work was supported in part by the S.A. National Research Foundation under Grant No. 2053408.
0-7803-8844-5/05/$20.00 c 2005 IEEE.
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(c) Fig. 1. a) Traditional cabling system versus b) multiplexed cabling system. Solid lines indicate power cables while the dashed lines represent a communication bus. Black dots (nodes) represent the interface between power and communication circuitry – this is where multiplexing takes place. c) Power-line communication system. Black dots represent transmitter / receiver nodes.
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II. COUPLING As the motorcar power line carries dc, coupling can be accomplished simply by using a series capacitor. An ideal capacitor would block the dc power voltage, and pass high frequencies perfectly for a wide range of terminating impedance values. Consider a coupling capacitor C terminating into a resistor R, constituting a high-pass filter. The -3dB (half-power) cutoff point fLF would be dependent on component values as expressed by (1): 1 (1) f LF 2SRC The suitability of a certain coupling capacitor depends on the impedance of the load into which it terminates. When transmitting data into the wiring harness, the (varying) equivalent impedance of all connected parallel loads dominates. When receiving data from the wiring harness though, the receiver input impedance stays constant.
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Ignition Wiper motor Alternator Battery
The voltage transfer function for all possible cable routes between the above points were measured in both directions, and measurements repeated with battery disconnected. This resulted in 24 voltage transfer function graphs, of which two are shown in Fig. 2. +20
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Power-line communications thus provides a network architecture in which only one power wire network is necessary, independent of the number of electrical / electronic devices used. However, it must be emphasized that the number of transmitter / receiver nodes is directly proportional to the number of devices, and these would have to be cheap, mass-produced, integrated modules.
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As an initial choice, two 1µF metal-film capacitors were placed in series with each terminal. (The symmetry of such a setup improves EMI). Thus the total series capacitance was 0.5 µF. When measuring data received from the power line, the terminating impedance is 1 Mȍ, resulting in a -3 dB cutoff point of 0.318 Hz. When transmitting a signal into the power line though, the access impedance can be very low. An equivalent impedance of say 0.1 ȍ yields a cutoff frequency of 3.18 MHz. Therefore only frequencies above 3 MHz will be injected efficiently.
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Another pitfall to be avoided, is the frequency response of the coupling capacitor itself. This capacitor should operate predictably over the frequency range of interest, and therefore its self-resonant point should be much higher than the upper limit of the communication frequencies. As an initial check, the transfer function of the coupling/measuring setup was determined by short-circuiting the output and input terminals (each with a 1µF series capacitor) of the HP3577B Network Analyzer. This transfer function was flat (as expected above 0.318 Hz) and showed no visible deviation below 50 MHz. III. INITIAL MEASUREMENTS In order to investigate the viability of certain frequency bands, extensive transfer function measurements were done bi-directionally over several different cable paths. Based on the type of load together with accessibility of terminals, the most appropriate measuring points were chosen as:
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Fig. 2. Measured amplitude response of some cable paths: (a) ignition to wiper motor and (b) battery to alternator. Take note of the promising transfer function shape between 10 MHz and 30 MHz in Fig. 2(a), however some cable paths exhibited more erratic transfer functions between 10 MHz and 30 MHz as is shown in Fig. 2(b).
Several of the cable paths showed a promising transfer function shape between 10 MHz and 30 MHz as is shown in Fig. 2(a). This confirms the results in [2]. However, some cable paths exhibited more erratic amplitude response between 10 MHz and 30 MHz as can be seen in Fig. 2(b). Most cable paths were reasonably symmetrical with respect to direction. Another observation was that disconnection of the battery did not have a predictable influence on the transfer function.
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The above observations led to the conclusion that the 10MHz to 30-MHz section is a promising frequency band, but that the communication strategy for each cable path would have to be optimized for effective data transfer at high data rates. IV. EXPERIMENTAL SETUP
In order to facilitate testing and BER measurements, a 2047-bit periodic pseudo-random bit sequence is generated internally on all digital boards. Some modules also have data acquisition functionality, allowing internal signals and other measurements to be captured and displayed on a softwarebased oscilloscope (see Fig 5).
The experimental setup consists of various interchangeable modular prototyping boards that are commercially available [3]. Either a serial cable or IP network interface connects the hardware to a PC, which allows adjustment of various parameters. Fig. 3 shows the transmitter setup comprising of a digital modulator feeding into in a 0-MHz to 80-MHz analog IF modulator.
Fig. 5. Software-based oscilloscope window showing internally captured signals.
V. OPTIMIZATION OF COMMUNICATION PARAMETERS In [4], a direct sequence spread spectrum technique was implemented for automotive power-line communications, using BPSK at a carrier frequency of 4 MHz. However, only 15.75 kbps was transmitted.
Fig. 3. Transmitter setup comprising of a digital modulator feeding into in a 0-MHz to 80-MHz analog IF modulator.
The receiver circuit shown in Fig. 4, performs the required inverse operations i.e. analog demodulation followed by digital demodulation. The final output is cascaded into a BER measurement module.
For this setup though, various modulation strategies are possible. FSK derivatives such as CPFSK, MSK, GFSK and GMSK are options to be investigated. Also, by using different digital modulator and demodulator boards, BPSK and QPSK are possible combined with spreading codes such as Gold sequences, maximal length sequences, Barker codes, and GPS C/A codes. As an initial benchmarking exercise, a 5-Mbps 2FSK scheme was implemented at a center frequency of 20 MHz. The data transfer rate was confirmed (167 megabits of data transferred in 30 seconds) and the spectrum of the output was also verified with an IFR 2397 3-GHz spectrum analyzer (see Fig. 6).
Fig. 4. Receiver comprising of an analog IF demodulator feeding into a digital demodulator. The last module measures bit error rates.
Fig. 6. Spectrum of the 20-MHz modulated 2-FSK signal.
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The performance of this scheme, connected back-to-back through a 24-dB attenuator, is summarized in Fig. 7.
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For low CNR ratios, the two schemes show good correlation, but the output power could not be made large enough for proper comparison at higher CNR ratios. Theoretically, a maximum output level of -12 dBm can be sourced by the modulator, depending on impedance matching etc. However, only -52 dBm was received at the demodulator input, with a noise floor level of -60 dBm. This observation does not correspond with the voltage transfer function shown in Fig. 2(a), which predicted losses closer to 20 dB. Possible factors that warrant investigation, include: impedance matching, contact resistance, parasitic impedances of measurement cables, coupling circuitry, as well as power levels when measuring voltage transfer functions.
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VI. CONCLUSION
CNR (dB) Fig. 7. Performance of the 2-FSK scheme connected back-to-back through a 24-dB attenuator. Carrier-to-noise ratios were measured at the receiver input.
A slightly more robust 4-FSK scheme at a lower 1-Mbps data rate was chosen for the first test on the automotive wiring harness. The cable path between the ignition and wiper motor was chosen for the first attempt. Back-to-back results are compared with in-circuit results in Fig. 8.
Initial benchmarking results at 1 Mbps (4-FSK) and 5 Mbps (2-FSK) were presented showing the negative impact of attenuation on a 20-MHz modulation (center) frequency. This experimental setup can now be used to determine optimum modulation schemes and frequency bands. Furthermore, gathered data will also be used to trade off performance against complexity and cost.
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Although automotive power-line communications has been proposed and implemented to a limited degree, successful broadband communication at high data rates has not been achieved. In this paper, an experimental setup for the optimization of broadband automotive power-line communications is presented. Modular prototyping equipment was used to construct both a transmitting and receiving node on the wiring harness of a motorcar. Initial characterizing measurements were used to choose typical point-to-point communication paths.
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REFERENCES
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CNR (dB) Fig. 8. Comparative performance between back-to-back (Ÿ) and in-circuit (Ƈ) 1-Mbps, 4-FSK schemes. Carrier-to-noise ratios were measured at the receiver input.
[1] P. A. Janse van Rensburg, H. C. Ferreira, “Automotive powerline communications: favourable topology for future automotive electronic trends,” Proc. 7th Int. Symp. Power-Line Comm., 2003, pp. 103-108. [2] A. Schiffer, “Statistical channel and noise modeling of vehicular dc-lines for data communication,” Proc. 51st IEEE Veh. Tech. Conf., May 2000, pp. 158-162. [3] www.comblock.com [4] F. Nouvel, G. el Zein, J. Citerne, “Code division multiple access for an automotive area network over power lines,” Proc. 44th IEEE Veh. Tech. Conf., 1994, pp. 525-529.
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