License-free Communication Research Articles

Experimental studies on the effects of fog and haze on free‐space optical link at 405 nm

AbstractFree‐space optics (FSO) is a perfect line of sight communication which is gaining a lot of popularity in the current research as it offers many advantages such as license free communication, high bandwidth, high speed, low bit error rate, and so on. Like the other communication technologies, FSO also suffers from adverse effects due to the climatic changes in the atmosphere. Some of the prominent effects that degrade the strength of the signal in free space are the fog, snow, haze, and rain. In this paper, the effects of fog and haze on the FSO link were investigated with the support of both simulated and experimental results. An experimental setup consisting of a single‐channel FSO system enclosed in a glass cabinet is used for generating the experimental results, and the simulation is carried out in OptiSystem using the Kim, Kruse, and Alnaboulsi prediction models. Results demonstrate that the simulated and experimental results follow the same pattern that as the intensity fog or the intensity of haze increases the received voltage levels decline.

Incoherent DOA Estimation in Uniform Antenna Arrays with Inordinate Spacing Using a Subband Hopping Approach

Given a certain transmission frequency, the Shannon spatial sampling limit defines an upper bound for the antenna element spacing. Beyond this bound, the exceeded ambiguity avoids correct estimation of the signal parameters (i.e., array manifold crossing). In this survey, the problem of simultaneous signal and direction-of-arrival (DOA) estimation of broadband sources is addressed when the element spacing of a uniform array antenna (uniform linear array (ULA)) is inordinate. It is illustrated that one can resolve the aliasing ambiguity by utilizing the inherent frequency diversity of the broadband sources. An algorithm based on maximum likelihood estimation (MLE) has been developed to estimate the transmitted data signal and the DOA of each source. Through confirmatory simulation, it is shown that the performance gain of the proposed setup is potentially significant, specifically under a low signal-to-noise ratio (SNR) and when the transmitters are closely spaced. This relaxes the stringent maximum element-spacing constraint of ULAs pertinent to the upper-bound frequency of transmission and suggests that the element spacing—which in practical applications results in detrimental element coupling—can be conveniently increased, in particular under wide transmission spectrum and low SNR (e.g., license-free communication). A method similar to the frequency hopping approach (i.e., subband hopping) is utilized for the problem of source associations and identification. In the sequel, a suboptimal subspace-based algorithm is proposed and its performance is investigated. An approximate expression for the estimation error has also been developed to gauge the behavior of the proposed setup.