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We develop a family of tractable models for UAV mobility. Based on a system of coupled stochastic differential equations, the resulting models exhibit realistic trajectories and flexible covariance structures, making them suitable for both small scale and large scale applications. Closed form solutions for steady state and transient position distributions, as well as position covariance, are derived. These results are subsequently used to obtain distance outage probabilities and pathloss. The analytical results allow us to assess the impact of a variety of system parameters related to device control, navigation and position measurement errors. Finally, using measured UAV flight data, we show an excellent goodness of fit between the proposed model and the data. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel relay selection scheme for orthogonal frequency-division multiplexing systems by combining conventional bulk and per-subcarrier selection schemes and analyze its outage performance over equally spatially correlated channels. Specifically, the combined selection scheme selects only two relays at the first attempt and performs per-subcarrier selection over these two relays. We analyze the asymptotic outage performance of the combined selection scheme in the high signal-to-noise ratio (SNR) region and prove a generalized theorem. This theorem states that the combined selection can achieve an optimal outage probability equivalent to the per-subcarrier selection at a high SNR without using the full set of available relays for selection. This unique property is termed the equivalence principle, and it holds for all correlation conditions. To explore this principle, we consider three examples: decode-and-forward, fixed-gain amplify-and-forward (AF), and variable-gain AF relay systems. Furthermore, two extended applications, i.e., antenna selection and branch selection, are also considered to reveal the feasibility and the expandability of the equivalence principle. Our analysis is verified by Monte Carlo simulations. The proposed combined selection and the proved theorem provide a general and feasible solution to the tradeoff between system complexity and outage performance when relay selection is applied. [ABSTRACT FROM AUTHOR]

Using mutually shift-orthogonal spreading codes, block-spread code-division multiple access (BS-CDMA) systems have been shown to achieve multiuser interference (MUI)-free reception when synchronization between the base station and the subscriber stations is achieved. In practice, when carrier frequency offset (CFO) is present, orthogonality among users is destroyed, and MUI occurs. This paper presents three methods of designing the spreading and despreading codes for uplink BS-CDMA systems to reduce MUI due to CFO. We show through rigorous derivation that all three codes can completely eliminate MUI due to CFO. In particular, by fixing the spreading codes at the transmitter, despreading codes that were obtained by minimizing the interference power or their cross correlation with the spreading codes are shown to yield a performance close to a synchronous system. The advantages and disadvantages of the BS-CDMA systems using the three proposed codes are discussed. [ABSTRACT FROM PUBLISHER]