Phase Fading Research Articles

Current sensing using a Phase-Sensitive Optical Time Domain Reflectometer: Feasibility study

A novel method for distributed current sensing using an FBG-assisted Phase-OTDR with Mach-Zehnder Interferometer is proposed. The detrimental effect of the intrinsic linear birefringence of the sensing fiber is solved by calibration. An FBG pair is written at the two ends of the spun fiber coil to eliminate phase fading and increase the measurement accuracy. A simulation tool was developed to reveal the feasibility of the approach by investigating the impact of the detector noise as well as the effects of bending- and FBG-induced linear birefringence on the sensing performance.

Phase Stability Control of Optical Fiber Partial Discharge Ultrasonic Sensing System

Optic fiber interferometers are highly sensitive ultrasonic sensors for partial discharge detection. However, low-frequency vibration and environmental noise will disturb the sensors in the field, and cause a phase fading suppression effect that reduces sensitivity. This paper analyzed the problems existing in the phase feedback control system based on PZT, and an improved scheme incorporating a high-frequency carrier phase demodulation is proposed. Based on an acousto-optic modulator, the proposed phase feedback control system overcomes the phase fading suppression effect. A test is carried out on an ultrasonic calibration platform and a transformer oil discharge platform. The test results show that the stability of the improved phase demodulation system has been significantly improved, and meets the requirements of field applications. Compared with the signal-to-noise ratio at the time of phase fading of the system before the improvement, the signal-to-noise ratio of the improved system is improved by 69 dB.

Statistical Models of Trans-Ionospheric Radio Lines with Phase Fading of Signals

Statistical Models of Trans-Ionospheric Radio Lines with Phase Fading of Signals

On the phase fading effect in the dual-pulse heterodyne demodulated distributed acoustic sensing system.

Understanding signal fading effect is essential for the application of Rayleigh-scattering-based distributed acoustic fibre sensors (DASs) due to the nature of coherent beam interference within the pulse length. Statistical properties for the intensity of the Rayleigh backscattered light (i.e. intensity fading) and its impact on the sensitivity of DAS systems have been intensely studied over the last decades. Here we for the first time establish an analytical model for the phase signal retrieved from the dual-pulse heterodyne demodulated DAS system, which can be exploited to investigate the phase fading effect in this system. The developed model reveals that the phase fading phenomenon mainly originates from the randomness in the phase retardant of the Rayleigh scatters. The quantitatively resolved statistical features of the phase fading is confirmed by experimental results. Based on the analytical model, a noise figure is defined to characterize the global fading-induced noise level via taking into account contributions from all channels along the sensing fiber. The model also reproduces the anti-correlation relation between the power spectrum density of retrieved phase at the heterodyne frequency and the phase fading noise level. Following the analysis and the definition of the noise figure, an optimized real-time weighted-channel stack algorithm is developed to efficiently suppress the fading noise. Experimental results show that the algorithm can achieve a maximum noise figure reduction of 15.8 dB without increasing the system complexity.

Performance Improvement of Dual-Pulse Heterodyne Distributed Acoustic Sensor for Sound Detection.

Phase fading is fatal to the performance of distributed acoustic sensors (DASs) influencing its capability of distributed measurement as well as its noise level. Here, we report the experimental observation of a strong negative correlation between the relative power spectrum density (PSD) at the heterodyne frequency and the noise floor of the detected phase for the heterodyne demodulated distributed acoustic sensor (HD-DAS) system. We further propose a weighted-channel stack algorithm (WCSA) to alleviate the phase fading noise via an enhancement of the relative PSD at the heterodyne frequency. Experimental results show that the phase noise of the demodulated signal can be suppressed by 13.7 dB under optimal condition. As a potential application, we exploited the improved HD-DAS system to retrieve a piece of music lasted for 205 s, demonstrating the reliability of detecting wideband sound signal without distortion.

Research on Polarization and Phase Fading Compensation in Michelson Interferometer Based on 3 × 3 Coupler and Novel Probe with Built-in Faraday Rotator

A self-designed probe and a feedback control scheme based on the Michelson interferometer with a 3 × 3 fiber coupler are proposed. A 45° Faraday rotator is built into the self-designed probe, and a feedback control scheme is used to judge the direction of increase or decrease for the phase compensation, so as to solve the problems of polarization and phase fading. In addition, a result-normalized method is applied in a micro-vibration measurement experiment. The experimental interferometer system achieves a high frequency of 1 MHz micro-vibration. The normalized results keep stable with a maximum deviation from the mean of 1.9% when the power of light reflected back into the self-designed probe is altered. Applied research is carried out by detecting the displacement due to a photoacoustic wave. Therefore, the experimental interferometer system is available for the practical application of micro-displacement measurements, noncontact high-frequency detection, and point-by-point image scanning in biological tissue.

Identification and observation of the phase fading effect in phase-sensitive OTDR

We report a numerical analysis and direct measurement of the phase fading effect in phase-sensitive OTDR (Φ-OTDR) acoustic sensors. By demodulating the acoustic-induced phase information using a dual-pulse heterodyne Φ-OTDR, we show that the amplitude of the phase signal appears with ripple-like fluctuations over the fiber distance, even though the applied phase modulation has a constant amplitude. A generalized interference model for a Φ-OTDR acoustic sensor is presented, which identifies that this phase fading effect is mostly contributed by the random phase retardant (rather than the scattering amplitude) introduced through the distributed Rayleigh scattering process. Our analysis provides insight on the potential suppression of the phase fading noise in a phase-retrieved Φ-OTDR system.

<i>Amygdalus nana</i> L. reproductive structures features study when introduced in the North

As a result of Amygdalus nana plant flowers structure study, when introduced in the middle taiga subzone of the Komi Republic, its conformity to the species features of the plant is shown. The authors also note teratological changes, expressed in the abnormal development of sterile or fertile flower structures. The number of petals increases, their edges become wavy; stamens transform into petals, there is pistil underdevelopment, etc. The life cycle of A. nana flowers was traced in the conditions of the North, the duration of the phases of their development was determined (Phase I dense green bud, II phase of pink bud, III phase beginning of blossoming flower, IV phase full blossoming of the flower and V phase fading). The features of the fruiting process of A. nana are shown, where degeneration of fertilized ovules is noted, which is due to the possible violation of different stages of embryogenesis. In this case, a significant number of anomalies in the fertile parts of the flower, possibly, lead to a small number of fruits on plants. The analysis of the A. nana flower structure in the middle taiga subzone of the Komi Republic gives additional information on the intraspecific changes in the reproductive structures of this plant; the results obtained can serve as additional information for establishing anthocological differences within the taxon.

On Cooperation and Interference in the Weak Interference Regime

Handling interference is one of the main challenges in the design of wireless networks. In this paper, we study the application of cooperation for interference management in the weak interference (WI) regime, focusing on the Z-interference channel with a causal relay (Z-ICR), in which the channel coefficients are subject to ergodic phase fading, all transmission powers are finite, and the relay is full-duplex. The phase fading model represents many practical communications systems in which the transmission path impairments mainly affect the phase of the signal, such as non-coherent wireless communications and fiber optic channels. In order to provide a comprehensive understanding of the benefits of cooperation in the WI regime, we characterize, for the first time, two major performance measures for the ergodic phase fading Z-ICR in the WI regime: the sum-rate capacity and the maximal generalized degrees-of-freedom (GDoF). In the capacity analysis, we obtain conditions on the channel coefficients, subject to which the sum-rate capacity of the ergodic phase fading Z-ICR is achieved by treating interference as noise at each receiver, and explicitly state the corresponding sum-rate capacity. In the GDoF analysis, we derive conditions on the exponents of the magnitudes of the channel coefficients, under which treating interference as noise achieves the maximal GDoF, which is explicitly characterized as well. It is shown that under certain conditions on the channel coefficients, relaying strictly increases both the sum-rate capacity and the maximal GDoF of the ergodic phase fading Z-interference channel in the WI regime. Our results demonstrate for the first time the gains from relaying in the presence of interference, when interference is weak and the relay power is finite , both in increasing the sum-rate capacity and in increasing the maximal GDoF, compared with the channel without a relay.

Degrees of Freedom of Full-duplex Cellular Networks: Effect of Self-interference

It was recently shown that full-duplex (FD) operation at a base station (BS) can provide up to twice as many degrees of freedom (DoF) as a conventional half-duplex (HD) cellular network in the absence of self-interference. In practice, however, self-interference in an FD BS may not be eliminated completely due to imperfect cancellation, and it is not yet known whether FD operation can improve the sum DoF of cellular networks in the presence of residual self-interference. In this paper, we provide a complete characterization of the sum DoF in an FD-BS cellular network with self-interference, where the rank of the self-interference matrix is arbitrary. Specifically, we propose a self-interference cancellation scheme that maximizes the sum DoF, and we prove its optimality by deriving the matching upper bound. Our results show that even in the presence of residual self-interference, an FD-BS network can still outperform a conventional HD-BS network, and furthermore, the sum DoF coincides with that of an FD-BS network with no self-interference under certain conditions. We also derive an achievable sum rate under ergodic phase fading, showing that not only a sum-DoF gain but also a sum-rate gain can be obtained over the entire signal-to-noise ratio range even if residual self-interference exists.

Analysis of Physical Layer Security via Co-operative Communication in Internet of Things

Co-operative communication in Internet of Things enable the co-existing heterogeneous wireless networks to co-operate with each other in order to facilitate network traffic, guarantee QoS requirements and to enable energy efficient secure communication even to most demanding users. Physical layer security approaches based on node cooperation promise secure communication even in the presence of an eavesdropper. The three main co-operative schemes that help in improving physical layer (PHY) security via co-operative communication are decode-and-forward (DF), amplify-and-forward (AF) and co-operative jamming (CJ). This work mainly focuses on the performance analysis of PHY layer security via two co-operative schemes (DF and AF) by taking all main fading phenomenon's like path loss, phase fading and shadow fading into consideration. Also a method has been discussed for enhancing the secrecy rate by using two heuristic algorithms: “hill-climbing” and “random-search”.

Capacity investigation of on–off keying in noncoherent channel settings at low SNR

AbstractOn–off keying (OOK) has repossessed much new research interest to realize green communication for establishing autonomous sensor networks. To realize ultra‐low power wireless design, we investigate the minimum energy per bit required for reliable communication of using OOK in a noncoherent channel setting where envelope detection is applied at the receiver. By defining different OOK channels with average transit power constraints, the achievability of the Shannon limit for both cases of using soft and hard decisions at the channel output is evaluated based on the analysis of the capacity per unit‐cost at low signal‐to‐noise ratio. We demonstrate that in phase fading using hard decisions cannot destroy the capacity only if extremely asymmetric OOK inputs are used with a properly chosen threshold. The corresponding pulse‐position modulation scheme is explicitly studied and demonstrated to be a Shannon‐type solution. Moreover, we also consider a slow Rayleigh fading scenario where the transmitter and receiver have no access to channel realizations. Throughput per unit‐cost results are developed to explore the tradeoff between power efficiency and channel quality for noncoherent OOK using soft and hard decisions. Copyright © 2015 John Wiley & Sons, Ltd.

Approximate Ergodic Capacity of a Class of Fading Two-User Two-Hop Networks

The fading AWGN two-user two-hop network is considered where the channel coefficients are independent and identically distributed (i.i.d.) according to a continuous distribution and vary over time. For a broad class of channel distributions, the ergodic sum capacity is characterized to within a constant number of bits/second/hertz, independent of the signal-to-noise ratio. The achievability follows from the analysis of an interference neutralization scheme where the relays are partitioned into M pairs, and interference is neutralized separately by each pair of relays. When M = 1, the proposed ergodic interference neutralization characterizes the ergodic sum capacity to within 4 bits/sec/Hz for i.i.d. uniform phase fading and approximately 4.7 bits/sec/Hz for i.i.d. Rayleigh fading. It is further shown that this gap can be tightened to 4 log π-4 bits/sec/Hz (approximately 2.6) for i.i.d. uniform phase fading and 4-4 log(3π/8) bits/sec/Hz (approximately 3.1) for i.i.d. Rayleigh fading in the limit of large M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> .

The Capacity Region of the Fading Interference Channel With a Relay in the Strong Interference Regime

The interference channel with a relay (ICR) is the fundamental building block of cooperation in wireless networks where there are multiple communicating pairs interfering with each other. This paper considers ICRs in which the links are subject to i.i.d. fading, and each node has channel state information (CSI) only on its incoming links (receive CSI). Two channel models are considered: phase fading and Rayleigh fading. Strong interference conditions are derived for the case where the links from the sources to the relay are good in the sense that the achievable region for decoding both messages at the relay contains the maximal achievable region at the destinations. This leads to the characterization of the capacity region for such scenarios. This is the first time the capacity region of the ICR is characterized for a nondegraded, noncognitive scenario, with a causal relay when all links are active.

Capacity Theorems for the Fading Interference Channel With a Relay and Feedback Links

Handling interference is one of the main challenges in the design of wireless networks. One of the key approaches to interference management is node cooperation, which can be classified into two main types: relaying and feedback. In this paper, we consider simultaneous application of both cooperation types in the presence of interference. We obtain exact characterization of the capacity regions for Rayleigh fading and phase fading interference channels with a relay and with feedback links, in the strong and very strong interference regimes. Four feedback configurations are considered: 1) feedback from both receivers to the relay, 2) feedback from each receiver to the relay and to one of the transmitters (either corresponding or opposite), 3) feedback from one of the receivers to the relay, and 4) feedback from one of the receivers to the relay and to one of the transmitters. Our results show that there is a strong motivation for incorporating relaying and feedback into wireless networks.

Symmetric Semideterministic Relay Networks With No Interference at the Relays

A symmetric semideterministic relay network is introduced, generalizing Aref networks. In the considered model, there is no interference at the relays, but the links from the source and the relays to the sink are interfering and nondeterministic. An achievable rate is determined for an arbitrary number of relays, based on symmetric relaying and simultaneous partial and backward decoding at the sink. The information expressions of the achievable rate match those of the standard cut-set bound, but the set of probability distributions is substantially different. We also study a wireless example of the defined network where nondeterministic links are replaced with fading AWGN channels, and obtain its ergodic capacity with phase fading when phase information is locally available at the sink.

Fat Containing HCC: Findings on CT and MRI Including Serial Contrast–Enhanced Imaging

The purpose of this article is to review the spectrum of computed tomography (CT) and magnetic resonance imaging (MRI) findings of fat containing hepatocellular carcinoma (HCC), including serial contrast-enhanced imaging. Imaging findings of 10 fat-containing HCCs on CT (n = 2) or MRI (n = 3) or on both CT and MRI (n = 5) were retrospectively reviewed in 9 patients. Both techniques included serial contrast enhanced imaging in arterial, portal venous, and late venous phases. On non-contrast CT, fat containing HCC was either homogeneously hypodense (n = 6) or of mixed density (n = 1). The density values ranged between -11 and 9 HU. On MRI, homogenous (n = 4) or heterogeneous (n = 4) signal loss was observed on T1-weighted out-of-phase images as compared to in-phase images. Enhancement patterns on serial contrast-enhanced CT and MRI included: arterial enhancement indistinguishable from the liver with venous wash out (n = 2), arterial capillary blush with venous phase fading (n = 2), and heterogeneous arterial enhancement with unenhanced foci and venous phase wash out of enhancements. Larger lesions had late capsular enhancement. Fat containing HCC has spectrum of imaging findings on CT and MRI. MRI with chemical shift technique depicts the fat content. Arterial contrast enhancement with venous washout or fading may help for the diagnosis of HCC in inconclusive cases.

The impact of CSI and power allocation on relay channel capacity and cooperation strategies

Capacity gains from transmitter and receiver cooperation are compared in a relay network where the cooperating nodes are close together. Under quasi-static phase fading, when all nodes have equal average transmit power along with full channel state information (CSI), it is shown that transmitter cooperation outperforms receiver cooperation, whereas the opposite is true when power is optimally allocated among the cooperating nodes but only CSI at the receiver (CSIR) is available. When the nodes have equal power with CSIR only, cooperative schemes are shown to offer no capacity improvement over non-cooperation under the same network power constraint. When the system is under optimal power allocation with full CSI, the decode-and-forward transmitter cooperation rate is close to its cut-set capacity upper bound, and outperforms compress-and-forward receiver cooperation. Under fast Rayleigh fading in the high SNR regime, similar conclusions follow. Cooperative systems provide resilience to fading in channel magnitudes; however, capacity becomes more sensitive to power allocation, and the cooperating nodes need to be closer together for the decode-and-forward scheme to be capacity-achieving. Moreover, to realize capacity improvement, full CSI is necessary in transmitter cooperation, while in receiver cooperation optimal power allocation is essential.

Multisource, Multidestination, Multirelay Wireless Networks

Networks with multiple source–destination pairs, involving possibly multicast, and where there are multiple nodes that can serve as potential relay nodes, are considered. A multisource, multirelay coding scheme is developed. In this scheme, each source's information is sent to its destination nodes via a multirelay route, with the multiple multirelay routes operating concurrently even when they intersect with each other, in the same spirit as code-division multiple access (CDMA). It is found that in the generalization to multiple sources, backward decoding achieves higher rates than sliding-window decoding. The routing structure where a joint backward decoding can be performed is characterized. The achievable rate region is found to combine aspects of both multiple relay and multiple access. Potential applications of this coding scheme to sensor networks are discussed. In particular, the exact capacity for the data downloading problem in sensor networks, where there are multiple sensor sources and one sink or collector node, is established for certain geometries when there is phase fading that is unknown to the transmitter.

Capacity Gain From Two-Transmitter and Two-Receiver Cooperation

Capacity improvement from transmitter and receiver cooperation is investigated in a two-transmitter, two-receiver network with phase fading and full channel state information (CSI) available at all terminals. The transmitters cooperate by first exchanging messages over an orthogonal transmitter cooperation channel, then encoding jointly with dirty-paper coding. The receivers cooperate by using Wyner-Ziv compress-and-forward over an analogous orthogonal receiver cooperation channel. To account for the cost of cooperation, the allocation of network power and bandwidth among the data and cooperation channels is studied. It is shown that transmitter cooperation outperforms receiver cooperation and improves capacity over noncooperative transmission under most operating conditions when the cooperation channel is strong. However, a weak cooperation channel limits the transmitter cooperation rate; in this case, receiver cooperation is more advantageous. Transmitter-and-receiver cooperation offers sizable additional capacity gain over transmitter-only cooperation at low signal-to-noise ratio (SNR), whereas at high SNR transmitter cooperation alone captures most of the cooperative capacity improvement.