Performance of Adaptive Modulation over Frequency Selective Fading Channel in VANET Environment

Abstract

The Doppler effect can cause both time and frequency selective fading in VANET because of the node mobility and time-varying channel. The significant effect is that the Doppler spread become much longer than the symbol period. Consequently, the orthogonality among subcarriers on OFDM system will no longer hold and the resulting intercarrier interference (ICI) will decrease system performance, e.g. bit error rate (BER). In this paper, we propose adaptive modulation schemes to maintain BER with three thresholds tested. Unlike the typical scheme that transmits data with serial adaptive modulation, the proposed scheme transmits its data based on the change in the gain channel that is instantaneously attenuated on each subcarrier. The proposed adaptive modulation performance is compared to non-adaptive modulation. The simulation results show that adaptive modulation requires a lower SNR of 18 dB to achieve BER 10-3 compared with QPSK and 16-QAM modulation. In addition, we also observed the increase in system capacity comparison with Doppler variation. Adaptive modulation experienced the largest increase in system capacity that is 59% compared to non-adaptive modulation (BPSK) by applying the lowest frequency doppler normalized with $f_{d}=0.000128$.