I. Abstract -We propose to apply DSRC using OFDM to roadside-vehicle communication system as a means to overcome frequency selective fading, shadowing and high-speed hand-over difficulties. We optimized the transmission rate of a subcarrier which is a transmission parameter of the OFDM system. Our goal is to achieve BER of less than under the conditions of transmission rate 2.048 Mbps, carrier frequency 5.8GHz, vehicle speed 180km/h, simulcasting of several roadside transmitters, maximum delay of less than 1 p e c and utilizing forward error correction (FEC). We developed the transmitter and receiver of DQPSK-OFDM system in 5.8GHz band and measured the BER performance to show that OFDM system was applicable to roadside-vehicle communication system. IntroductionThe OFDM system is adopted in European DAB and domestic next generation digital broadcasting because of its robustness against frequency selective fading caused by delayed waves on propagation paths. This modulation is a multicarrier modulation scheme, where data symbols are transmitted in parallel on multiple subcarriers. This technique narrows the subcarrier bandwidth and achieves tolerance against fading. This modulation utilizes the orthogonality relation of frequency with frequency spacing of the subcarrier and the transmission rate of a subcarrier equally, and arranges the carrier by minimum frequency spacing. The spectrum efficiency is higher as a result. The guard interval reduces IS1 and IC1 to utilize this orthogonality relation for the delayed wave in the guard interval. In this paper, we apply the OFDM system to roadsidevehicle communication system, and optimize the transmission rate of a subcarrier and introduce an estimation method of BER performance. We also measured BER performance with a transmitter and receiver we developed in 5.8GHz band.fading, time selective fading, spectrum efficiency) while optimizing the transmission speed of the subcarrier in the OFDM system. First, to study frequency selective fading, we supposed two paths with some delay between them which induce notches by 1 / z [Hz] spacing in the frequency domain. These notches decrease BER performance. It is known that it is necessary to make the transmission rate of a subcarrier less than I / ( 2x7 ) [Hz] to reduce this effect. As to time selective fading, the random FM noise caused by the receiver movement induces interference between subcarriers. It is important to widen the frequency spacing more than a doppler spread between each subcarrier. Because the frequency spacing of the subcarriers and the transmission rate is equal, the transmission rate R [bps] of a subcarrier must satisfy the following relation in order to reduce the effect of the above-mentioned two kinds of fading simultaneously. f d < < R < < ' / 2 m (1) fd:maximum doppler frequency Next, the guard interval duration tg [sec] of the OFDM system must be more than the delay time of the delayed wave to reduce IS1 and IC1 effects.However, it consequently decreases the effective transmission rate compared with the...

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