A study of Signal Detection for Road-to-Vehicle Communications in ITS

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1 A study of Signal Detection for Road-to-Vehicle Communications in ITS MASUO UMEMOTO Yokosuka ITS Research Center Telecommunication Advancement Organization of Japan Hikarino-oka 3-2-1, Yokosuka, Kanagawa JAPAN Abstract: A current DSRC system with 5.8 GHz for Electronic Toll Collection (ETC) is briefly discussed. The d=1 modulation code and direct demodulation system using the low IF signal are proposed for DSRC systems in the next generation. Furthermore, a simulation result of direct demodulation using Viterbi detection is presented. Key-Words: Modulation code, Viterbi detection, QPSK, Low IF, Vehicle communications, ITS 1 Introduction Recently, Intelligent Transport Systems (ITS) has become the focus of global attention. Various info-communication services in the ITS must take an important role in safe and comfortable driving, and they are being researched and developed all over the world. The various info-communication services are provided with mobile communications, Dedicated Short-Range Communications (DSRC), and broadcasting. A variety of DSRC uses, such as automatic toll collection, traffic information services, emergency dispatch services, and so on, is attractive. However, new services require new on-board equipment. As a result, cars will be possibly filled up with many communication terminals. Therefore, a multimode wireless communication terminal[1][2] using software radio technology has been proposed and developed. On the other hand, DSRC systems have to be designed to provide a short range, wireless link to transfer information between vehicles and roadside systems, or in an inter-vehicle communication network. In this paper, a DSRC system using a 5.8 GHz band is studied in order to encourage the continued development of DSRC services between vehicles and roadside systems. A current DSRC system for Electronic Toll Collection (ETC)[3] is briefly described in Section 2. In the next generation, DSRC systems require a higher transfer rate of several Mbps and a larger communication zone of around 0 meters. Therefore, digital modulation system and its related circuit [4] are being studied. A QPSK (quadrature phase shift keying) system is expected as a key candidate. Modulation code for the QPSK and a new receiving system are discussed in Section 3. A d=1 code is proposed, where the parameter d controls the minimum transition period of the modulation code. The d=1 code offers longer transition length than that of an original data sequence. The code with a longer transition length maybe immune to multipath propagation channel. Furthermore, a receiving system using with a low IF conversion technique is discussed. Finally, in Section 4, we give conclusions. 2 DSRC Systems 2.1 Current ETC system An outline of the standard for Electronic Toll Collection (ETC) system [5] in Japan is listed in Table1. In five years, the ETC system will be introduced at about 730 tollgates out of the total 13 tollgates of national expressways.

2 A TDMA system having 8 (maximum) time slots is adopted. Therefore, a data transfer rate per user becomes actually around 0 kbps, and communication time is also short because of a small communication zone. To encourage the continued development of DSRC services, we have to study techniques for higher data rate and larger communication zone. antenna, Doppler frequency shift to the receiving signal of 5.8 GHz in the vehicle is calculated and shown in Figure 2. Item Table 1. ARIB STD-T55 Specifications Carrier Frequency Transmission Power Modulation system Total transfer data rate Communication zone size Multiple access Access control ISM band (5.8GHz) Roadside station: Max. 3 mw Mobile station: Max. mw ASK(Split phase code) 24 kbps Max. 30 meters TDMA-FDD system Active system Fig. 1 Free space loss of 5.8GHz signal 2.2 DSRC channel of a large communication zone To consider roughly path loss in a communication zone of 0 meters, free space loss L f is estimated, which loss is given as follows: L f = ( 4πd λ )2 (1) where λ is a wave length, and d is distance from a roadside base station to a vehicle. Fig. 1 shows the loss. To enlarge the DSRC communication zone from 30 meters to 0 meters, around db loss must be compensated. In a case of the communication zone of 0 meters, for example, when a vehicle moves on a road lane which is 3 meters apart from a roadside Fig. 2 Doppler frequency shift effect at several speeds Fig. 3 shows a relationship between the vehicle and the roadside antenna in a plane figure.

3 Vehicle θ 0 meters Road side antenna 3 meters Ts=3Tc. The IF signal at a condition of Ts=Tc is shown in (b). Fig. 3 Relationship between a vehicle and a roadside antenna 1 symbol period Doppler frequency shift effect in the large communication zone must be a study issue. 3 Signal detection in a low IF system 3.1 Low IF signal with QPSK modulation Recently, high speed analog to digital converters (ADC) of several hundreds mega sample per second have been available. If such an ADC is used, direct demodulation of an IF signal can be done, while the IF frequency is low or almost the same as a data rate frequency. A signal processing diagram of the direct demodulation is shown in Figure 4. BPF LNA Local Oscillator Low frequency IF The RF received signal is amplified and frequency down converted into an IF signal. The IF signal is directly quantized through the ADC. Two examples of a low frequency IF signal waveform with the QPSK modulation are shown in Figure 5 (a), (b). In (a), a single symbol period (Ts ) equals three times as the cycle period of the IF signal (Tc), or x IF BPF ADC DEMO Fig. 4 A scheme for direct demodulation of a low frequency IF signal Fig 5. (a) The IF signal waveform with QPSK modulation at a condition of Ts=3Tc 1 symbol period time Fig. 5 (b) The IF signal waveform with QPSK modulation at a condition of Ts=Tc 3.2 Direct demodulation In the QPSK modulation signal, 2 bits are transmitted in a single modulation symbol. The phase of the carrier takes one of 4 eqaully spaced values, such as 0, π/2, π and 3π/2, which each value of phase corresponds to a unique pair of message bits. The phase of the carrier, or the phase of the IF signal is detected through at least two sampling data in a single symbol period. This means that direct demodulation of the IF signal is done through a sampling process by the ADC. Target values of the QPSK modulation signal are known in advance, the message bits are,

4 therefore, detected with the minimum squared error. 3.3 Modulation code If we employ a channel code such as a convolution code, the frequency bandwidth of the modulation signal may become wider, and complicate decoding processing may be required. In this paper, d=1 code is studied, where the parameter d controls the minimum transition period of the modulation code. Note that a random bit sequence is characterized by d=0 code. Table 2 shows a coding table of a typical d=1 code, which was used in magnetic recording channel. Table 2 coding table of d=1code Data Code Data Code The logical ones in the code sequence indicate the positions of a transition 1-1 or -1 1 of the corresponding modulation code sequence. The code sequence 0... would be converted to the modulation code sequence; The minimum transition length Tmin of the modulation code sequence is given by Tmin=4/3Tb, (2) where Tb is the bit period of original data. Pseudorandom data ( d=0 code) Fig. 6 Power spectrum of pseudorandom data and that of the corresponding modulation code sequence by using d=1 code. Fig. 6 shows the power spectrum of pseudorandom data and that of the corresponding modulation code sequence by using d=1 code. Although two bits data are converted into three bits code according the coding table, the frequency bandwidth of modulation code sequence is almost the same as that of data sequence. 3.4 Viterbi detection When applying the d=1 constrain code to modulation code, Viterbi detection is used in direct demodulation processing. Fig. 7 shows a trellis diagram for the modulation code. Modulation code Sequence by using d=1 code Fig. 7 Trellis diagram for the d=1 modulation code

5 In the direct modulation process, the low IF signal at a condition of Ts=Tc is employed, and then then the low IF signal is sampled at a rate of four times in a symbol period. Fig. 8 shows a simulation result of byte error rate by the direct demodulation It is assumed that the low IF signal suffers from additive white Gaussian noise. Simulation results of pseudorandom data are also shown in Figure 8. The low IF signal of this data is also sampled at a rate of four times in a symbol period. Furthermore, detection based on the minimum squared error is used. Byte error rate Modulation code by using d=1 code Pseudorandom data (d=0 code) Radio System by Parameter Controlled and Telecommunication Toolbox Embedded Digital Signal Processing Chipset, ACTS mobile communications summit 98, pp.5-120, Greek, June [2]M. Umemoto and N. Yazawa, Development of Multimode Wireless Communication Terminals in ITS, Submitting to the World Congress on ITS, Turin, Italy, Nov. 20. [3] Electronic Toll Collection System (ARIB STD- T55 Version 1.1 in Japanese), Feb [4]Yoshida et al, DC offset Canceller in a Direct Conversion Receiver for QPSK Signal Reception, PIMRC 98, pp , Sept [5]Y. Fujimori, Outline of ETC system introduction plan and the standards draft, 6 th World Congress on ITS, Toronto, Nov SNR Fig. 8 Byte error rate of direct demodulation An improvement of signal to noise ratio (SNR) is ensured. 4 Conclusions The d=1 modulation code and direct demodulation system using the low IF signal were proposed for DSRC systems in the next generation. The effect of the d=1 code in a multipath condition will be a further study issue. References [1]H. Harada and M. Fujise, Multimode Software

Project: IEEE P Working Group for Wireless Personal Area Networks N

Project: IEEE P Working Group for Wireless Personal Area Networks N Project: IEEE P802.15 Working Group for Wireless Personal Area Networks N (WPANs( WPANs) Title: [Introduction of vertically connected wireless system] Date Submitted: [ 14 JAN, 2004] Source: [Ami Kanazawa