This study proposes a new asynchronous Multi-Rate Multiple-Access Differential Chaos Shift Keying (MRMA-DCSK) scheme, ensuring significant data rates for all users. This scheme assigns a unique chaos sequence with a different length to each user. During the first data transmission period, each user transmits the chaos sequence as the reference signal, followed by multiple data bits by sharing the same reference signal in subsequent periods. The proposed scheme affects the bit-error-rate (BER) performance with the number of users, data rate related parameters (L), and length of chaos signals. The simulation results are verified by the derived analysis and show that the proposed scheme achieves higher data rates (from 1/2 to L/L+1) than a conventional scheme while enhancing bit-error-rate (BER) performance.

In this study, aiming at clarifying the characteristics of air-to-ground radio wave propagation in mountainous areas, a transmission experiment was performed between a drone equipped with a transmitter and three receivers set up on the ground using a 920MHz band wireless system at Uchigatani forest, which is located in Yamato-cho, Gujo-shi, Gifu Prefecture. In the experiment, we simultaneously measured the received signal strength indicator (RSSI) and the drone's latitude, longitude, and height from the ground. Then, we verified whether the measured data has the line-of-sight between the transmitter and receivers using a geographic information system and analyzed characteristics of the RSSI, packet loss rate, and fading concerning the height from the ground and distance between the transmitter and receivers. The results showed that increasing the drone's altitude to 90m or more makes the link more stable and that the fading distribution in mountainous terrains is different from in other terrains.

In this paper, a wireless communication network that uses unmanned aerial vehicles (UAVs) in the sky to transmit information between ground users is considered. We highlight a delay-tolerant network, where information is relayed in a store-and-forward fashion by establishing two types of intermittent communication links: between a UAV and a user (UAV-to-user) and between UAVs (UAV-to-UAV). Thus, a flight algorithm that controls the movement of the UAVs is crucial in achieving rapid information transmission. Our study proposes new flight algorithms that simultaneously consider the two types of communication links. In UAV-to-UAV links, the direct information transmission between two UAVs and the indirect transmission through other UAVs are considered separately. The movement of the UAVs is controlled by solving an optimization problem at certain time intervals, with a variable consideration ratio of the two types of links. In addition, we investigate not only the case where all UAVs move cooperatively but also the case where each UAV moves autonomously. Simulation results show that the proposed algorithms are effective. Moreover, they indicate the existence of an optimal consideration ratio of the two types of communication and demonstrate that our approach enables the control of frequencies of establishing the communication links. We conclude that increasing the frequency of indirect communication between UAVs improves network performance.

Radio frequency on free-space optical (RoFSO) technology is regarded as a new universal platform for enabling seamless convergence of fiber and FSO communication networks, thus extending broadband connectivity to underserved areas. In this paper, we investigate the performance to characterize the transmission of code division multiple access (CDMA) based wireless signals over RoFSO system using aperture averaging (AA) technique under strong turbulence conditions. An analytical model including a modified carrier-to-noise-plus- interference ratio (CNIR) form and a novel closed-form expression for the bit-error rate (BER) is derived. Unlike earlier work, our model takes into consideration the effect of using the AA technique modeled by the gamma-gamma distribution, the optical noises, the intermodulation distortion term due to the laser diode non-linearity and the multiple interference access. By investigating the impact of AA on our model in the strong turbulence regime, we show that there is a design trade-off between the receiver lens aperture and the number of users to achieve a required CNIR ensuring a substantial scintillation fade reduction. The presented work can be used as baseline for the design and performance evaluation of the RoFSO system's ability to transmit different broadband wireless services signals over turbulent FSO links in real scenarios.

This paper considers an emergency communication system controlling multiple unmanned aerial vehicles (UAVs) in the sky over a large-scale disaster-affected area. This system is based on delay-tolerant networking, and information from ground users is relayed by the UAVs through wireless transmission and the movement of UAVs in a store-and-forward manner. Each UAV moves autonomously according to a predetermined flight method, which uses the positions of other UAVs through communication. In this paper, we propose a new method for UAV flight considering the non-uniformity of user distributions. The method is based on the Voronoi cell using the predicted locations of other UAVs. We evaluate the performance of the proposed method through computer simulations with a non-uniform user distribution generated by a general cluster point process. The simulation results demonstrate the effectiveness of the proposed method.