Vol 61, No 4 (2018)

Optical wireless communication is regarded as the next-generation high-speed technology. It has demonstrated its capability to deliver data faster than any other state-of-the-art wireless communication technique. This technology has drawn attention as a means of implementing reliable high capacity outdoor systems that cannot be implemented by conventional fiber optics. It has emerged recently as an efficient solution to match the larger bandwidth and high data rates requirement of the upcoming wireless communication systems. However, although FSO (free space optics) system has many appealing features, it has rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In the presence of such type of performance impairments, the received signal exhibits random intensity fluctuations, which increase the BER (bit error-rate), where the severe weather conditions can have a detrimental impact on the performance, which may result in an inadequate availability. The MIMO wireless optical procedure, in which the spatial dimensions are used to improve the reliability and spectral efficiency of point-to-point links, provides a promising approach to mitigate turbulence effects due to its powerful performance enhancing capabilities.

In this paper, we investigate a terrestrial atmospheric FSO communication system operating under the influence of strong atmospheric turbulences. Additionally, the MIMO technique with equal gain combining (EGC) is used in this work to enhance the data rate of the proposed system. Atmospheric turbulence impacts are modeled as a lognormal channel with due regard for geometric losses. With the use of NRZ line coding, an FSO highly sensitive receiver using either avalanche photodetector (APD) or PIN is designed and simulated for best system performance. The preference is achieved by using Bessel and Gaussian filters. It has been found that APD receiver using Gaussian filter is suitable for long-range links with APD gain value of 3. Also, the selection of APD gain is critical to the system performance. In addition, the optimal value of APD gain required for best system performance decreases as the size of the MIMO technique increases. The achievable performance improvements including received power levels, BER and Q-factor are also discussed. The results show that the system with optical amplifier at the transmitter gives an optimum performance. In addition the system performance is enhanced in most weather conditions by using an amplified 2×2 MIMO-FSO system with booster amplifier.

The results of experimental studies showing the applicability of short-range radar in the millimeter wavelength range for remote identification of explosive liquids in closed dielectric containers are presented. This study presents radiometric study of hazardous liquids based on an example of gasolines and diesel fuels. It has been shown experimentally that radiometric studies allow remote studies and object identification at distances of up to several meters. The variety of physical and chemical properties of liquid fuels does not allow their identification using the measurement results of only the permittivity and loss tangent from one measurement in one frequency range. The use of a thermal portrait of liquid fuels in closed dielectric containers for their identification is more illustrative and informative. The dependence of the spatial temperature portrait of the container with liquid on the polarization of the received signal is experimentally shown. The use of different polarizations of the received signal and the difference in the spatial temperature portraits of the liquid in the container provide additional information in order to increase the probability of correct identification of the liquid. Thermal portraits obtained with linear scanning for gasolines of different manufacturers differ more strongly, in comparison with thermal portraits of diesel fuels. Due to the modification of the measuring system, the total time for scanning and obtaining the output experimental data is 42 s. The standard deviation of the obtained experimental data did not exceed 3.6%.

A new compact design of CPW-fed wideband (WB) spiral-shaped slot antenna is proposed. The proposed antenna has a compact size with overall dimensions 37-33 mm and is fabricated on FR4 substrate with dielectric constant ɛ_r = 4.4 and thickness h = 1.6 mm. With the different length of the spiral-shaped slots, simulated and experimental results of the antenna are suitable for WB operations. The–10 dB bandwidth of the WB antenna from measurement is approximately 115.2% (2.36–8.53 GHz). The proposed antenna provides nearly omni-directional radiation characteristics. The new antenna configuration operates in several different bands: 2.4, 3.5, 5.2, 5.5, and 5.8 GHz covering 2.4/5.2/5.8 GHz WLAN bands and 2.5/3.5/5.5 GHz WiMAX bands. The results for S₁₁, far-field H- and E-plane radiation patterns and gain of the proposed antennas are presented and discussed. The agreement between measured results and full-wave simulation validates the feasible configuration of the proposed antennas.