In this paper, we propose a novel solution to improving wireless channel quality of wireless local area networks (WLANs) in fast-mobile environments, which uses a media-access-control (MAC) layer approach: adaptive frame-length control and block acknowledgement (ACK). In fast-mobile environments, using short frame lengths can suppress channel estimation error and decrease frame errors. However, it increases the MAC overhead, resulting in decreased throughput. To solve this tradeoff, we combined block ACK, which is specified in IEEE802.11e as an optional function, with adaptive frame-length control. Although adaptive frame-length control considering this tradeoff has previously been investigated, the targets were different from WLANs using orthogonal frequency division multiplexing (OFDM) in fast-mobile environments. The MAC-overhead reduction using block ACK is suitable for our frame-length control because it does not change the frame format in the physical layer. Also, it is a new idea to use block ACK as a solution to improving channel quality in fast-mobile environments. In this paper, we evaluate our method through computer simulations and verify the effectiveness of adaptive frame-length control that can accommodate relative speeds.

In conventional road-vehicle communication systems, user terminals in the vehicles have to directly connect to wireless access points (APs). However, vehicle speeds are so fast that the channel condition between the terminals and the APs constantly changes because of changing path loss and time-varying fading. In this paper, to compensate for such deterioration, we propose to reduce the relative speed between the terminals and the APs by an inter-vehicle packet relay technique. If a terminal can send data via other vehicles running at lower speeds, the relative speed will decrease, which suppresses the dynamic range of path loss and deterioration by fading. We, first, validate our method by a numerical analysis using a statistical path-loss model. The numerical analysis verifies that our method is able to suppress deterioration caused by path loss and time-varying fading. However, in the numerical analysis, geometric propagation of paths is not considered; instantaneous and rapid loss changes are not considered. Therefore, we evaluate our method by computer simulations using a geometric propagation model. In the simulations, phase difference between multiple paths and loss fluctuation within one frame duration affect the performance. From the results of the simulations, we validate our method. Furthermore, we investigate the combination of our method and the selection diversity technique, which can suppress channel fluctuation and may enhance the performance of our method. Moreover, we measure interference in the overlapped zone between two AP areas. From the measurement, we show that our packet relays do not cause a problem in interference between areas.

In road-vehicle communication systems, the transmission rate between user terminals in the vehicle and the access points degrades due to changing path-loss and time-varying fading. In this paper, we used an inter-vehicle packet relay technique to improve channel quality in road-vehicle communication systems. We evaluated this method using numerical analysis to validate our method.

In this paper, a low-cost radio-on-fiber (ROF) system for a 36 GHz band road-vehicle communication system (RVCS) is proposed and demonstrated. Optical components for 10 Gb/s baseband transmission systems, which are becoming lower in cost, are used for the proposed system. The signal is transmitted in the optical link in the form of an intermediate frequency (IF) signal of the 5.8 GHz band. The third subharmonic of a local oscillator wave (LO) is transmitted simultaneously with the IF signal from the central station to the remote stations (RSs). This scheme enables the realization of RSs without costly millimeter-wave synthesizers. In such a configuration, the influence of intermodulation distortion between the IF signal and the LO subharmonic and degradation of the carrier-to-noise ratio (CNR) could possibly be a problem, and so an analysis and experiments were carried. It was clarified that the compression of dynamic range caused by the simultaneous transmission was small. Frequency tripling of the LO degraded the CNR of the LO; however, this effect was compensated for by increasing the optical modulation index (OMI) of the LO subharmonic. Increasing the OMI of the LO subharmonic proved to have no influence on the IM3 characteristics of the RF signal analytically and experimentally. The proposed low-cost system proved to have sufficient characteristics for millimeter-wave RVCS.