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Unmanned Aircraft Systems (UAS) Communications
Published in R. Kurt Barnhart, Douglas M. Marshall, Eric J. Shappee, Introduction to Unmanned Aircraft Systems, 2021
Another important point about the radiated wave is that it may arrive at the receiver via a direct path but also other paths (multipaths) due to reflection, refraction, and diffraction. Reflection waves are caused when a signal bounces off objects before reaching the receiver, while refracted waves are caused when the waves pass through different media on their way to the receiver. Refracted waves can cause loss in signal amplitude while travelling through lossy media (media that dissipate EM energy). Diffraction is a phenomenon that occurs when a wave bends around a corner. Figure 13.5 is an illustration of these different wave propagation phenomena. Multipath signals can lead to both destructive and constructive interference with the direct signal. Multipath signals that arrive almost as the same time as the direct signal are more harmful and can cause more errors in the information received.
Codec Design
Published in Rajeshree Raut, Ranjit Sawant, Shriraghavan Madbushi, Cognitive Radio, 2020
Rajeshree Raut, Ranjit Sawant, Shriraghavan Madbushi
The word orthogonal indicates that there is a precise mathematical relationship between the frequencies of the carriers in the system. The available transmission bandwidth (BW) is divided by orthogonal overlapping of narrow band subchannel. To obtain high spectral efficiency, the frequency response of the subcarriers are overlapping and orthogonal thus the name OFDM. OFDM is resistant to multipath effects that hamper deployment of wireless broadband systems. Multipath is the phenomenon where a radio signal arrives at a receiver via two or more paths. These multiple occurrences of the signal interfere with each other, causing degradation in the overall received signal. Causes of multipath include reflections off objects and refraction. The key advantage of OFDM over single carrier modulation schemes is the ability to deliver higher BW efficiency. This allows the standard to deliver a higher throughput over the link, which is very desirable and cost-efficient.
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
There are a variety of processing techniques that can be used to mitigate the effects of small-scale fading. Some of these techniques require processing at the transmitter, some at the receiver, some both. Channel coding, equalization, antenna diversity combing, etc. are examples of techniques to mitigate multipath effects. Some of the more advanced techniques do not seek to mitigate the challenges posed by multipath fading, instead see multipath as offering a richly diverse environment that can be cleverly exploited to gain capacity enhancements. Examples of such techniques include space division multiplexing using intelligent antennas and multi-user diversity that exploits the uncorrelated fades experienced by different users to make capacity gains. TABLE 139.1 summarizes several of these signal processing techniques to mitigate multipath fading.
Travelling Wave Antenna Based MIMO for 5GHz WLAN Band Application With Pattern Diversity
Published in International Journal of Electronics, 2022
Vinay Sharma, Madhur Deo Upadhayay, Atul Vir Singh
The new multimedia wireless applications demand better coverage and high data rates. However, issues such as multipath fading and interference degrade the efficiency of wireless systems. These problems can be mitigated using MIMO antenna systems. MIMO introduces diversity (frequency, pattern and polarisation) that utilises fading to improve the signal to noise ratio (SNR) of the system (receiver). When wireless channels are less correlated, MIMO increases channel capacity using multiple antennas as front-end compact equipment without utilising extra spectrum and power (Jensen & Wallace, 2004). However, in a small space, it is a design challenge to pack a large number of antennas. As closely placed antennas will have either high mutual coupling or high correlation or both, which degrades antenna efficiency, channel capacity and SNR. Therefore, to design a compact MIMO system, the designer needs to reduce mutual coupling (Vaughan & Andersen, 1987).
Performance improvement of H-Shaped antenna with Zener diode for textile applications
Published in The Journal of The Textile Institute, 2022
Wearable antennas, on the other hand, may take advantage of all the available room on clothing to increase signal efficiency in wireless communications. Second, multi-path fading is one of the most serious issues in wireless communication when signal quality decreases when the cell terminal travels across a gap equivalent to a wavelength. Wearable antennas have seen a lot of press in recent years because of their ability to easily blend into clothes, which is a desirable attribute for hands-free applications and military applications that need low visibility. Antenna variety is a powerful tool for combating multipath fading. Antenna diversity, on the other hand, necessitates at least a half-wavelength separation between each antenna in the device. On limited form factor hand kept modules, this is not feasible, limiting the usage of antenna diversity. Antenna diversity, on the other side, may be seen on a wide scale with a body-wearing wireless device. Wearable textile antennas have also piqued the interest of the consumer electronics sector, as they meet the growing demands of the increasingly changing wireless environment. lightweight, efficient, durable, unobtrusive, and affordable wearable antennas with zero maintenance and no configuration specifications are attractive features popular to all applications.
Integrity monitoring using multi-GNSS pseudorange observations in the urban environment combining ARAIM and 3D city models
Published in Journal of Spatial Science, 2022
Ahmed El-Mowafy, Bing Xu, Li-Ta Hsu
A well-known challenge of urban GNSS positioning is the degraded GNSS signals. As shown in Figure 2, multipath is caused by multiple reception of signals from different transition paths due to the reflection by flat building surfaces. The NLOS, however, occurs where the direct line-of-sight signal is blocked and only the reflected signal is received. Multipath effect can be reduced or mitigated by sophisticated antenna designs or narrow correlator-based discriminator designs (Zhdanov et al. 2002). However, these techniques have no significant improvement against the errors caused by NLOS. Many attempts have been made to detect and mitigate the effect of NLOS reception. For instance, Jiang and Groves (2014) utilise carrier-to-noise ratio measurements of dual-polarisation to detect the NLOS, and (Groves and Jiang 2013, Hsu et al. 2017) applied consistency check methods. In recent years, 3D city models were rapidly developed to improve the performance of GNSS receivers in dense urban cities (Groves (2011). Satellite visibility prediction can also benefit from the usage of 3D city models (Miura et al. 2015).