An Improved HF Channel Wideband Detection Method Based on Scattering Function
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Fading characterizes the effect of random superposition of signal copies received from different propagation paths. These signal replicas may add together constructively or destructively, which leads to a large variation in received signal strength. Multipath fading manifests itself in a much smaller spatial scale than path loss and shadowing, and therefore, is called the small-scale effect of wireless channel. In this chapter, we first develop the general model for multipath fading channels. Then, we discuss the classification of fading channels based on their time domain and frequency domain characteristics. Finally, we present simplified models for two important types of multipath fading channels, which are widely used in the design and analysis of digital wireless transmission systems.
Fading distribution
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Multipath effect is an important factor that affects capability of TTC system at low elevation angles.The principle of multipath effect and the mathematical model of multipath fading are introduced in this paper.While electromagnetic wave travels between transmitting and receiving antennas,it has one direct path and several reflective paths.It will produce multipath fading when reflective signal counterbalances direct signal.Based on the mathematical model,the conditions in which the multipath fading appears and the influences of multipath fading on TTC system are analyzed in this paper.The position where multipath fading appears depends on the altitudes of the both antennas and the distance between them.The intensity of multipath fading depends on the reflectance of surface.Some methods are summarized to avoid and overcome multipath fading.
Fading distribution
Rake receiver
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Multipath fading occurs in any environment where there is multipath propagation and there is some movement of elements within the radio communications system. This may include the radio transmitter or receiver position, or in the elements that give rise to the reflections. The multipath fading can often be relatively deep, i.e. the signals fade completely away, whereas at other times the fading may not cause the signal to fall below a useable strength. In this paper we discuss a model for extended Suzuki process (type II) and an appropriate deterministic model, and we present and analyze simulation results based on these proposed models. IndexTerms: Fading channels, mobile radiochannels, wireless communications,extendedSuzukiprocesses.
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Fading distribution
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Multipath fading heavily contributes to the unreliability of wireless links, causing fairly large deviations from link quality predictions based on path loss models; its impact on wireless sensor networks is considerable. Although analytical models provide a probabilistic description, multipath fading is a deterministic phenomenon. Moreover, in the case of static nodes, fading is time-invariant. We illustrate its spatial nature with experimental evidence obtained using lower-end sensing node hardware. We also show the limitations of the supposed immunity of wideband radios to multipath fading in indoor deployments.
Fading distribution
Rake receiver
Wideband
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Due to surface and bottom space constraints of the underwater acoustic channel, multi-path fading occurs and causes degradation of communication. Multi-path fading depending on the maximum delay time(Tm) and symbol period(Ts) can be divided into two kinds of channels. In this paper, we propose Anti-Multipath Fading Underwater Acoustic Communication(AMF-UAC) System. This system estimates the kind of channel and distinguishes flat fading (Tm<Ts) from frequency selective fading (Tm>Ts) under the ocean environmental variability. After checking the channel status, this system selects a mitigation technique depending on the type of multipath fading. In order to verify AMF-UAC system performance, we test transmission of image using 43.2kbit of gray image through the multi-path fading channel. Test results show that the number of bit errors is reduced from 300 to 10 under flat fading channel and from 20,000 to 90 under frequency selective fading channel when the reference SNR is 14dB.
Fading distribution
Channel state information
Underwater acoustic communication
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Mobile wireless sensors in indoor environments will experience multipath fading, causing rapid variations in the capacity of the radio link. We present a strategy that increases the throughput by modifying the trajectory of the sensor so it spends more time at positions where the capacity is high. While doing so, it still maintains some desired average velocity. Our approach includes closed-loop estimation of the parameters of the fading, which may change when moving between rooms. We prove stability of the feedback system and illustrate its behavior through simulations. Finally, we demonstrate robustness to errors in the channel model.
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Shadow mapping
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We investigate the performance of maximal ratio combining (MRC) systems operating in a non-Gaussian multipath fading environment. Specifically, we model the fading process by a spherically invariant random process (SIRP), which has many Gaussian-like properties and has been shown to be appropriate for modeling the multipath fading phenomenon in some practical fading environments. Closed-form expressions are derived for the average error probability of coherent binary modulation in the presence of multiple, equal-power non-Gaussian interferers in a non-Gaussian multipath fading environment.
Fading distribution
Maximal-ratio combining
Rake receiver
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Multipath fading occurs in any environment where there is multipath propagation and there is some movement of elements within the radio communications system. This may include the radio transmitter or receiver position, or in the elements that give rise to the reflections. The multipath fading can often be relatively deep, i.e. the signals fade completely away, whereas at other times the fading may not cause the signal to fall below a useable strength. Multipath fading may also cause distortion to the radio signal. As the various paths that can be taken by the signals vary in length, the signal transmitted at a particular instance will arrive at the receiver over a spread of times. This can cause problems with phase distortion and inter symbol interference when data transmissions are made. As a result, it may be necessary to incorporate features within the radio communications system that enables the effects of these problems to be minimized. This paper analyses the effects of various types of multipath fading in wireless transmission system.
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Fading distribution
Channel state information
Multipath interference
Distortion (music)
Diversity scheme
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OF THE THESIS Fading Correlation of Co-located Transmitters by Prashant Jadhav Thesis Director: Prof. Yanyong Zhang Fading or attenuation of a signal due to environment is a phenomenon often encountered in wireless communications. It is expected that co-located transmitters i.e. transmitters placed very close to each other show a high signal fading correlation due to the presence of similar fading environment. In this thesis, we present an experimental study of this phenomenon. Correlation of received signal strengths obtained from co-located transmitters in dynamically varying environments indicate that the large scale signal variations (shadow fading) are highly correlated while the small scale variations (multipath or fast fading) show a low correlation. Highly correlated large scale variations suggest a presence of same large shadowing elements in the transmit-receive path while a low correlation among the multipath variations is due to mutual coupling between the antennas at very close distances. This has two implications: it suggests that shadow fading variations can serve as an indicator of the co-location of closely spaced transmitters while the multipath variations cannot. However, low multipath signal correlations suggest that antenna diversity could be investigated for implementation in mobile handsets.
Fading distribution
SIGNAL (programming language)
Shadow mapping
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