Deep Fading Research Articles

Optimized Scheme of Antenna Diversity for Radio Wave Coverage in Tunnel Environment

To suppress the deep fading of radio waves caused by the tunnel waveguide effect, this paper presents an optimized scheme for the spatial and polarization diversities of tunnel antennas. Through the correlation coefficient analysis of path loss curves obtained by antennas placed at different positions, the two antenna positions that generate path loss curves with the lowest correlation coefficient are found, and these two antennas are defined as a diversity antenna pair. Using this scheme, the spatial diversity properties of the transmitting antenna and receiving antenna, as well as the spatial-polarization combined diversity property of the transmitting antenna, are obtained. Furthermore, the impact of antenna polarization on the spatial diversity property is investigated. The performance of the proposed scheme for spatial and polarization diversities is evaluated in terms of the intensity and uniformity of the path loss. The simulation results illustrate that the proposed diversity optimization scheme can suppress the influence of the waveguide effect and achieve more uniform and flatter radio wave coverage in a tunnel environment.

Multi-beam coverage and beamforming technology for high altitude platform station communication system

Multi-beam antenna technology can effectively mitigate the impact of deep fading caused by multipath propagation on communication quality through spatial diversity and increase the reliability of wireless mobile communication systems. At the same time, the multi-beam antenna with beamforming algorithm as the key technology can realize spatial division multiple access (SDMA) by automatically adapting the weight vector of the antenna array, enhance the expected value, and suppress the interference. A novel beamforming technology increases the inner circle beam width and reduces the outer ring beam width to minimize the difference between the internal and external cell link SNR. This study considers reducing the number of cells by reasonably allocating beams, which satisfies both the requirements of coverage link quality and the difficulty of forming antenna beams. This research subject applies the scheme and strategy of improving multi-beam antenna technology in the same system. Under the requirement of cell coverage and partitioning, the related technologies and strategies combining multi-beamforming technology and space-time block code technology can obtain high coding gain and diversity gain without sacrificing bandwidth and effectivelyx improve the wireless systematic capacity.

Fundamental Limits of Dynamic Interference Management With Flexible Message Assignments and Separate Deep Fading Block Coding

The problem of interference management is considered in the context of a linear interference network that is subject to long term channel fluctuations due to shadow fading. The slow fading model used is one where communication takes place over blocks of time slots and each link in the network is subject independently to block erasure with probability $p$ . It is assumed that each receiver in the network is interested in one unique message, which is made available at $M$ transmitters. For the case where $M=1$ , the cell association problem is considered, and for $M>1$ , the problem of setting up the backhaul links for Coordinated Multi-Point (CoMP) transmission is investigated. In both cases, optimal schemes from a Degrees of Freedom (DoF) viewpoint are analyzed for the setting of no erasures, and new schemes are proposed with better average DoF performance at higher probabilities of erasure. Assuming separate coding over different deep fading erasure blocks, the average per user DoF for $M=1$ is characterized for every value of $p$ , and optimal message assignments are identified. For $M>1$ , it is first established that there is no strategy for assigning messages to transmitters in networks that is optimal for all values of $p$ . The optimal cooperative zero-forcing scheme for $M=2$ is then identified, and shown to be information-theoretically optimal when the size of the largest subnetwork that contains no erased links is at most five.

Cooperative Rate Splitting for MISO Broadcast Channel With User Relaying, and Performance Benefits Over Cooperative NOMA

Due to its promising performance in a wide range of practical scenarios, Rate-Splitting (RS) has recently received significant attention in academia for the downlink of communication systems. In this letter, we propose and analyse a Cooperative Rate-Splitting (CRS) strategy based on the three-node relay channel where the transmitter is equipped with multiple antennas. By splitting user messages and linearly precoding common and private streams at the transmitter, and opportunistically asking the relaying user to forward its decoded common message, CRS can efficiently cope with a wide range of propagation conditions (disparity of user channel strengths and directions) and compensate for the performance degradation due to deep fading. The precoder design and the resource allocation are optimized by solving the Weighted Sum Rate (WSR) maximization problem. Numerical results demonstrate that our proposed CRS scheme can achieve an explicit rate region improvement compared to its non-cooperative counterpart and other cooperative strategies (such as cooperative NOMA).

MMSE Equalization Based Performance of Chaotic Interleaving Scheme for Iterative IDMA System

Background: In this paper, a chaotic interleaving scheme is proposed for further performance improvement of Interleave Division Multiple Access (IDMA). Interleavers are used to generate permuted sequence of signals to be transmitted with optimum correlation. Methods: Random interleaving (RI) was the fundamental taxonomy and used to distinguish the different users in IDMA scheme. The logistic map based chaotic interleaving scheme is suggested for IDMA. The system is also combined with Minimum Mean Square Error (MMSE) equalizer to combat the effect of Inter Symbol Interference (ISI). Results: For the sake of simplicity, no pulse shaping is assumed at the transmitter and ISI channel is considered to be a fixed 3 tap channel. Simulations are performed in MATLAB. BPSK modulation is used. Furthermore, the results show that the IDMA with proposed logistic map based chaotic interleaver outperforms the traditional random interleaving scheme in terms of BER without the need of extra memory and computational complexity. Conclusion: The obtained results show the noticeable performance improvement of CI-IDMA with MMSE equalization in deep fading situations. Chaos based interleaver also offers other advantages such as less requirement of memory and small implementation complexity.

In-Network Collaboration for CDMA-Based Reliable Underwater Acoustic Communications

Achieving high throughput and reliability in underwater acoustic networks for transmitting distributed and large volume of data is a challenging task due to the bandwidth-limited and unpredictable nature of the acoustic channel. In a multinode network, such as in the Internet of Underwater Things, communication link efficiency varies dynamically: if the channel is not in good condition, e.g., when in deep fade, channel coding techniques may fail to deliver the information even with multiple retransmissions. Hence, an efficient and agile collaborative strategy is required to allocate appropriate resources to the communication links based on their statuses. The proposed solution adjusts the physical- and link-layer parameters collaboratively for a code-division multiple-access (CDMA)-based underwater network. An adaptive hybrid automatic repeat request solution is employed to guarantee reliable communications against errors in poor links. Results were validated using data collected from the LOON testbed—hosted at the North Atlantic Treaty Organization Science and Technology Organisation Centre for Maritime Research and Experimentation in La Spezia, Italy—and from the REP18-Atlantic sea trial conducted in September 2018 in Portuguese water.

Cooperation Techniques for a Cellular Internet of Unmanned Aerial Vehicles

UAVs are powerful Internet-of-Things components to provide sensing and communications in the air due to their advantages in mobility and flexibility. As aerial users, UAVs are envisioned to support various sensing applications in the next generation cellular systems, which have been studied by the 3GPP. However, the QoS of the cellular link between the UAV and the base station may not be guaranteed when UAVs are at the cell edge or experiencing deep fading. In this article, we first introduce the non-cooperative cellular Internet of UAVs. Then we propose a cooperative sense-and-send protocol, in which a UAV can upload sensory data with the help of a UAV relay, to provide a better communication QoS for the sensing tasks. Key techniques including trajectory design and radio resource management that support the cooperative cellular Internet of UAVs are presented in detail. Finally, the extended cooperative cellular Internet of UAVs is discussed for QoS improvement with some open issues, such as massive multiple-input multiple-output systems, millimeter-wave, and cognitive communications.

Performance evaluation of space time trellis coded MIMO for mobile communications

<p>The major enemy of wireless communication is multipath fading that highly degrades the received signal. Spatial diversity highly reduces deep fades of the wireless communication. Higher signal to noise ratio (SNR) requirement can be reduced by using space time coding. In this paper, the error performance of un-coded MIMO, different diversity combing methods, space time block codes and space time trellis codes are analyzed using different parameters including number of antennas, M-array modulations, trace criteria, rank and determinant. The simulation results have shown that the Bit Error Rate (BER) significantly reduces with increasing number of receiver antennas. However, the number of antennas that a mobile device can have is limited by its size and this is inconvenient for mobile communication. Hence, this paper suggests space time codes to mitigate multipath problems in mobile communication. So, BER performance of space time block codes scheme was evaluated by varying the number of transmitter and receiver antennas along with varying M-array PSK modulation orders. The results have shown that better BER is possible by integrating space time codes with spatial diversity. Finally, the error performance of space time trellis codes was evaluated by using trace, rank and determinant, and the simulation results depicted that better error performance is achieved using the proposed multipath reduction method.</p>

CSI Based Multiple Relay Selection and Transmit Power Saving Scheme for Underlay CRNs Using FRBS and Swarm Intelligence

In this article, a multiple relay selection (MRS) scheme for signal-to-noise ratio (SNR) enhancement is proposed for underlay relay-assisted cognitive radio networks (RCRNs). A secondary source-destination pair experiencing deep fading on direct path is assisted by amplify-and-forward (AF) relays in an underlay mode. In this energy-constrained scenario, the aim is to maximize the secondary network's end-to-end SNR through an intelligent power-saving method incorporated with MRS. In contrast to the prior relay selection (RS) schemes, the relay-selection factor is the difference of SNR of the source-relay link and corresponding relay-destination link for each relay along with its corresponding interference channel coefficient. The difference factor aims to achieve the SNR upper bound while performing minimum power amplification, eventually resulting in interference mitigation as well. The proposed algorithm has been implemented using Fuzzy Rule Based System (FRBS), Artificial Bee Colony (ABC) and Particle Swarm Optimization (PSO) and their performance has been compared through simulations.

Convolution Attack on Frequency Hopping by Full-Duplex Radios

In this paper, we propose a new adversarial attack on frequency-hopping-based wireless communication between two users, namely Alice and Bob. In this attack, the adversary referred to as Eve, instantaneously modifies the transmitted signal by Alice before forwarding it to Bob within the symbol period. We show that this attack forces Bob to incorporate Eve's signal in the decoding process; otherwise, treating it as noise would further degrade the performance akin to jamming. Through this attack, we show that Eve can convert a slow-fading channel between Alice and Bob to a rapid-fading one by modifying every transmitted symbol independently. As a result, neither pilot-assisted coherent detection techniques nor blind-detection methods are directly applicable as countermeasures. As potential mitigation strategies, we explore the applicability of frequency hopping along with on – off keying (OOK) and binary frequency-shift keying (BFSK) as modulation schemes. In the case of OOK, the attacker attempts to introduce deep fades on the tone carrying the information bit, whereas in the case of BFSK, the attacker pours comparable energy levels on the tones carrying bit-0 and bit-1, thereby degrading the performance. Based on extensive analyses and experimental results, we show that when using OOK, Bob must be equipped with a large number of receive antennas to reliably detect Alice's signal, and when using BFSK, Alice and Bob must agree upon a secret key to randomize the location of the tones carrying the bits, in addition to randomizing the carrier frequency of communication.

VariLoc: Path Loss Exponent Estimation and Localization Using Multi-Range Beaconing

The achievable accuracy of localization based on received signal strength measurements depends on the path loss exponent (PLE), which describes the multipath fading and shadowing characteristics of a channel. An inaccurate estimation of the PLE leads to inaccurate estimation of positions. In this letter, we introduce a new ranging technique, called VariLoc, which uses a multi-range beaconing approach to perform localization and PLE estimation. The main novelty of VariLoc is that it leverages the sensitivity level of a receiver device for sensing the channel and the transmission power, instead of the more commonly used RSSI measurements, which can be very noisy. This letter shows that VariLoc is robust against multipath fading and can estimate the PLE accurately, even in the presence of very deep fading. This letter further introduces a centroid localization algorithm that utilizes VariLoc to obtain estimates for both the PLE and the position of a target node. The performance of the proposed approach is compared with the state-of-the-art localization algorithms and Cramer–Rao bound using simulation and real-world experimentation using Bluetooth beacons.

Interference Mitigation for Ultrareliable Low-Latency Wireless Communication

This paper proposes interference mitigation techniques for provisioning ultrareliable low-latency wireless communication in an industrial automation setting, where multiple transmissions from controllers to actuators interfere with each other. Channel fading and interference are key impairments in wireless communication. This paper leverages the recently proposed ``Occupy CoW'' protocol that efficiently exploits the broadcast opportunity and spatial diversity through a two-hop cooperative communication strategy among distributed receivers to combat deep fading, but points out that because this protocol avoids interference by frequency division orthogonal transmission, it is not scalable in terms of bandwidth required for achieving ultrareliability, when multiple controllers simultaneously communicate with multiple actuators (akin to the downlink of a multicell network). The main observation of this paper is that full frequency reuse in the first phase, together with successive decoding and cancellation of interference, can improve the performance of this strategy notably. We propose two protocols depending on whether interference cancellation or avoidance is implemented in the second phase, and show that both outperform Occupy CoW in terms of the required bandwidth and power for achieving ultrareliability at practical values of the transmit power.

Enhanced Repeat Transmission Technique for Statistical Signal Transmission

Statistical signal transmission (SST) is a developing technology, which can carry additional data over conventional signal streams without extra bandwidth sacrifice. A latest research provided an automatic repeat transmission technique for SST, which can largely strengthen SST's detection performance by identifying and handling deep fading situations. In this paper, we discover a novel negative effect, i.e., high divergence of channel coefficients (HDCC) effect for SST. Such an effect brings in a performance bottleneck for SST while it cannot be well tackled by the latest repeat transmission technique. Motivated by this circumstance, we analyze the essential cause of HDCC effect, and design an enhanced repeat transmission technique for SST accordingly. The proposed method can not only well address the impact of HDCC effect, but also absorb the advantages of the latest repeat transmission technique. Numerical results demonstrate that the proposed method obviously improve the detection performance upon existing techniques.

Fading-aligned OFDM with index modulation for mMTC services

5th generation (5G) of wireless networking is coming with diverse use cases, such as enhanced-Mobile BroadBand (eMBB), Ultra Reliable and Low Latency Communications (URLLC), and massive Machine Type Communications (mMTC). As a result, 5G wireless networks require flexible physical layer solutions through new radio access technologies (RATs). At this point, orthogonal frequency division multiplexing with index modulation (OFDM-IM) appears a flexible solution to satisfy the diverse user demands. Considering the strict requirements of mMTC services, such as low throughput, low power consumption, and low cost design, we propose fading-aligned OFDM-IM for more spectrum- and energy-efficient communication. In the proposed method, inactive subcarriers in OFDM-IM are cleverly utilized to avoid deep fading sub-channels, because the deep fading of the active subcarriers decreases bit error rate (BER) performance significantly. Computer simulation results demonstrate that more than 10 dB gain is obtained for a reference BER value of 10−4 at the same spectral efficiency with conventional OFDM. Moreover, the proposed method is compared with convolutional coded (CC) OFDM at the same spectral efficiency, and it is shown that the proposed scheme performs better in terms of BER performance. Furthermore, theoretical error performance of the proposed method is investigated to support our computer simulations.

Improvement of Noise Uncertainty and Signal-To-Noise Ratio Wall in Spectrum Sensing Based on Optimal Stochastic Resonance.

Noise uncertainty and signal-to-noise ratio (SNR) wall are two very serious problems in spectrum sensing of cognitive radio (CR) networks, which restrict the applications of some conventional spectrum sensing methods especially under low SNR circumstances. In this study, an optimal dynamic stochastic resonance (SR) processing method is introduced to improve the SNR of the receiving signal under certain conditions. By using the proposed method, the SNR wall can be enhanced and the sampling complexity can be reduced, accordingly the noise uncertainty of the received signal can also be decreased. Based on the well-studied overdamped bistable SR system, the theoretical analyses and the computer simulations verify the effectiveness of the proposed approach. It can extend the application scenes of the conventional energy detection especially under some serious wireless conditions especially low SNR circumstances such as deep wireless signal fading, signal shadowing and multipath fading.

SU Aur: A deep fading event in Visible and near-infrared bands

In three seasons of 2015-2018 we carried out a series of visible and near infrared (NIR) photometry of SU Aur. In course of this photometric monitoring we detected an event of a deep fading of the star in spring of 2018. In this paper we present preliminary analysis of our photometry.

A Low-Complexity Belief Propagation Based Decoding Scheme for Polar Codes - Decodability Detection and Early Stopping Prediction

In the 5G communication systems, polar code has been adapted as the control channel coding solution in the enhanced mobile broadband (eMBB) scenario. Although different decoding schemes, including belief propagation (BP) and successive cancellation (SC) based algorithms, have been proposed, the decoding complexity as well as the latency are still significant. To address this critical issue, several low-complexity schemes, e.g., the use of simplified decoding operation and stop the decoding operation in earlier stage, have been proposed recently. However, conventional early stopping strategies have to check a pre-defined metric in each iteration, and the associated decoding delay is significant. In this paper, to address this challenge, we proposed a low-complexity BP based decoding scheme, which contains the decodability detection stage and the early stopping prediction stage. The decodability detection stage can identify the codewords in the deep channel fading environment and eliminate the unnecessary decoding operations to reduce the decoding complexity, while the early stopping prediction stage can directly predict the required number of iterations rather than checking the metric in each iteration to avoid the associated decoding delay. Through the above two approaches, our proposed scheme is shown to achieve 71% decoding delay reduction and maintain the same decoding performance as traditional BP, G-matrix, MinLLR schemes.

Impact of Interbranch Correlation on Multichannel Spectrum Sensing With SC and SSC Diversity Combining Schemes

Multiantenna receivers are often deployed in cognitive radio systems for accurate spectrum sensing. However, correlation among signals received by multiple antennas in these receivers is often ignored which yields unrealistic results. In this paper, the effect of this correlation is accurately quantified by deriving analytical expressions for the average probability of detection. Alternative simpler expressions are also derived. These are done for selection combining (SC) and switch and stay combining (SSC) diversity techniques in dual arbitrarily correlated Nakagami- $m$ fading channels. Then, it is repeated for triple exponentially and identically correlated Nakagami- $m$ fading channels with SC diversity technique. Analysis results show that the interbranch correlation impacts the detector performance significantly, especially in deep fading scenarios. Also, SC outperforms SSC as expected. However, the difference between them becomes very small in low fading and highly correlated scenarios, which indicates that the simpler SSC scheme can as well be deployed in such situations.

Cross-polarization effect of radio waves propagation by forest vegetation in wireless communication systems on transport

During the movement of various types of transport (auto, rail) in forest, radio communications in the VHF / UHF bands are sharply disturbed, which is associated with attenuation and a change in the polarization plane (cross polarization effect) of radio waves in the forest. Therefore, this paper reviews the problem of the appearance of the cross-polarization effect appearance in the through propagation of VHF/UHF radio waves in forests. The parameters of cross-polarization are analyzed and the functional dependences XPD {f, ζF, rF}, XPR {f, ζF, rF} are evaluated on the operating frequency, forest vegetation properties and the distance traveled within the forest from the transmitting antenna to the receiving antenna. Rerouted by forward and backward vegetation elements, radio signals contain many cross-polarization components that can have levels comparable to the main (useful) component. An increase in cross-polarization components can be observed in the "forward" region, which can be formed by successively decreasing the useful component, ultimately causing a significant decrease in the XPD value. As a result, the relatively high cross-polarization component can consecutively reduce the level of the useful component, while the cross-polarization effect is random, and the XPD value with fluctuating distance between the transmitting and receiving stations begins to fluctuate relative to XPD = 0. Thus, the influence of forest vegetation on the propagation conditions in VHF/UHF ranges is not only manifested in a random variation of the attenuation coefficient, but also leads to a cross-polarization effect, which ultimately causes deep fading of the radio signal at the receiving point.This work can be used in assessing the fluctuations of received signals due to the occurrence of cross-polarization during transport traffic in the forest.

Log-normal fluctuation channel model of short distance in tunnels

Due to the serious interferences in the short distance of long narrow tunnels, the intensity of the electromagnetic wave is unstable and fluctuated greatly. The propagation path of each wave interferes seriously that lead to deep fading. This deep fading is of regularity, namely the quasi-periodic fluctuation in our work. By means of ray tracing method, the fluctuation period is proportional to the wave length, and is inversely to the tunnel size. Further, the parameters of the fluctuation-function are derived, which is introduced to the log normal distribution model to build the log-normal fluctuation model of tunnels. The simulation analysis and experimental results show that the log-normal fluctuation model can reduce the mean square error of fitting to the actual measurement data, and characterize of the deep fading of the path loss.