Sight Channel Research Articles

Improving the Performance of HALE UAV Communication Link Through MIMO Cooperative Relay Strategy

High-altitude long endurance unmanned aerial vehicles (HALE UAVs) are military and strategic UAVs that fly above the ground in the stratosphere. The ability of stealth technology and long flight time, has led to use of these UAVs for various military missions such as interception and spying, control guidance, remote sensing, navigation, surveillance and others. In this paper, improving the performance of HALE UAVs communication through multiple input multiple output (MIMO) cooperative relay with amplitude-and-forward strategy is investigated. In clear sky (without rain), two-hop system with line of sight (LoS) channels from source to relay and relay to destination is considered. In LoS-MIMO channels, due to correlation between the sub-channels, neither high-rank MIMO channel nor maximum capacity are achieved. However, based on antennas optimum placement that provide orthogonally between the received signals, maximum capacity will be obtained. The proposed scheme in this paper dramatically increases capacity relative to LoS-SISO channel and dual-hop MIMO Rayleigh up to 6 b/s/Hz and 2 b/s/Hz respectively. Also, simulation results verify exactness analytical expressions. However, rain as one of the most important ambient conditions causes the signal to be scattered in different directions. Therefore, LoS channel is changed to fading (Rayleigh) channel by increasing rainfall. In this case, telecommunication range is proposed as a meaningful metric, and outage probability (Poutage) based on telecommunication range for N-hop channel is extracted. In rainy conditions, simulation results show, the telecommunication range dramatically increases by increasing the number of relay UAVs for specified outage probability so that in the long-range, the outage probability decreases up to 50% with the increasing number of relays.

Joint Trajectory and Resource Optimization for UAV-Enabled Relaying Systems

Unmanned aerial vehicles (UAVs) have attracted attentions due to their mobility and high possibility of the line of sight (LoS) channel. We can fully use these two properties by carefully optimizing the UAVs' trajectories and cooperating with some facilities. A UAV working as a mobile relay now attracts many interests due to its low cost and reliable performance. In this paper, we study a relaying system, where a UAV works as an aerial mobile relay to help some ground base stations send information to ground users periodically by using time division multiple access (TDMA). We aim to maximize the minimum average user rate through solving a non-convex and information causality constraints involved problem by jointly optimizing the UAV trajectory, nodes scheduling, and power allocation to ensure the fairness among all users. Finally, we propose an efficient iterative algorithm to solve a derived mixed-integer non-convex optimization problem to achieve this target by using block coordinate descent (BCD) and successive convex approximation (SCA) methods and prove the convergence of our algorithm. Simulations show the effectiveness of our proposed algorithm, some useful trade-offs and insights about the structure of our optimized trajectory, and the influence of two widely used trajectory initialization methods.

GMD-Based Hybrid Beamforming for Large Reconfigurable Intelligent Surface Assisted Millimeter-Wave Massive MIMO

Reconfigurable intelligent surface (RIS) is considered to be an energy-efficient approach to reshape the wireless environment for improved throughput. Its passive feature greatly reduces the energy consumption, which makes RIS a promising technique for enabling the future smart city. Existing beamforming designs for RIS mainly focus on optimizing the spectral efficiency for single carrier systems. To avoid the complicated bit allocation on different spatial domain subchannels in MIMO systems, in this paper, we propose a geometric mean decomposition-based beamforming for RIS-assisted millimeter wave (mmWave) hybrid MIMO systems so that multiple parallel data streams in the spatial domain can be considered to have the same channel gain. Specifically, by exploiting the common angular-domain sparsity of mmWave massive MIMO channels over different subcarriers, a simultaneous orthogonal match pursuit algorithm is utilized to obtain the optimal multiple beams from an oversampling 2D-DFT codebook. Moreover, by only leveraging the angle of arrival and angle of departure associated with the line of sight (LoS) channels, we further design the phase shifters for RIS by maximizing the array gain for LoS channel. Simulation results show that the proposed scheme can achieve better BER performance than conventional approaches. Our work is an initial attempt to discuss the broadband hybrid beamforming for RIS-assisted mmWave hybrid MIMO systems.

Performance analysis of non-line of sight visible light communication systems

Visible Light Communication (VLC) has been emerged as an alternative technology to the radio frequency communication in the last few years. In this paper, a novel approach for the performance analysis of VLC system over non-line of sight (Non-Los) channel is proposed. In particular, VLC system considers the single order reflection points which are randomly distributed over the walls and there is no direct line of sight (LOS) path between transmitter and receiver. The mathematical expression for the probability of error of the proposed system is derived using Gaussian approximation. The proposed Non-Los VLC system offers a comparable performance to that of the conventional LOS VLC system when the number of reflection points is sufficiently large. The performance of the proposed system is investigated for the parameters of interest. Simulation results verify the analytical results.

Performance of BER with Different Diversity Techniques for Millimeter-Wave Communication System

In a communications system, a diversity technique is used to enhance the reliability of a message signal by using at least two channels with different characteristics. In this paper, all four possible scenarios are considered: Single-input and singleoutput (SISO), single input and multi-output (SIMO), multipleinput and single-output (MISO), Multiple-input and multiple output (MIMO) systems. Antenna arrays will be used to reduce BER and improve the performance of the system using array gain in the line of sight channel for 60 GHz frequency in an indoor scenario. Single input and single output are investigated before analysis of multiple inputs and multiple output channel in the line of sight (LOS) and multipath propagation. MATLAB simulation has been performed using BPSK modulation. The comparative studies show that the performance of the MIMO diversity technique is more reliable in terms of BER to improve the performance and efficiency of the communication system.

Energy-Based Timing Estimation and Artificial Neural Network Based Ranging Error Mitigation in mm-Wave Ranging Systems Using Statistics Fingerprint Analysis

Non-line-of-sight (NLOS) and dense multipath problems are the major challenges for the millimeter wave (mm-wave) indoor ranging systems. To acquire time of arrival (TOA) estimate accurately in such a poor environment, an improved statistics fingerprint analysis algorithm for energy-based timing estimation and artificial neural network (ANN) based ranging error mitigation is presented in this paper. The developed algorithm can obtain TOA estimate accurately by measuring the kurtosis, skewness, standard deviation, minimum slope, and gradient of the received mm-wave pulses. ANN is employed to mitigate the ranging error based on the obtained nonlinear regression between the thresholds and the analyzed characteristics of mm-wave pulses. The presented numerical simulation results indicate the proposed algorithm can achieve significant performance improvements in both line of sight and NLOS channels of the IEEE 802.15.3c standard, as compared to conventional algorithms.

Experiment and Analysis for Air-to-Ground Channel in 400 MHz Band

Background/ Objectives: Recently, air-to-ground wireless communication systems have been used in various equipment such as unmanned aerial vehicle (UAV). In this paper, we analyze the air-to-ground channel in 400 MHz band with experiment using light aircraft. Methods/ Statistical analysis: The characteristics of the air-to-ground channel are analyzed as follows. We generate and store a pilot signal in commercial vector signal generator and an aircraft equipped with the signal generator takes off. As a pilot signal, Zadoff-Chu sequence is used for its good autocorrelation characteristics. In the ground station, the received waveform is stored by using spectrum analyzer. When an airplane approaches near the ground station, airplane transmits the pilot signal and the ground station begins to store the received signals. After completing the signal storing, air-to-ground channel analysis is performed. Findings: From the experiments, we obtained several hundreds of received waveforms, and those waveforms are used for statistical air-to-ground channel analysis. According to the analysis results, most of the measurement channels are single path or line of sight channels. However, in a certain measurement, multiple paths or non-line of sight channels are observed. Those results confirm that multipath environments exist at certain air-to-ground condition. The maximum number of multipath components is 4 and the maximum delay spread is 0.4 μsec. Therefore, when designing a wireless communication system considering air-to-ground environments, the design should consider 4-path fading channels and maximum delay spread over 0.4 μsec. When designing air-to-ground wireless communication systems such as aircraft and drone systems, the experimental and analyzed channels in this paper might be useful. Improvements/ Applications: In this paper, we analyze the air-to-ground channel characteristics of a mountainous area in Korea with field experiment using light aircraft. The analyzed channel characteristics are helpful to those who want to design wireless communication systems between aircraft to ground or UAV to ground. In addition, the analyzed channel can be used for verification of compatibility for the existing communication systems to air-to-ground applications.

Optimal ZF Precoder for MU massive MIMO Systems over Ricean Channel with per Antenna Power Allocation

Multiuser massive multiple-input multiple-output systems have the potential to increase the data rate. However, with a large base station (BS) antenna, the non-square channel matrix restricts the zero-forcing (ZF) precoder rotations to obtain the best optimal solution with the per-antenna power allocation. In this paper, we propose the beamforming and lattice reduction (LR) approach to restrain the channel matrix and transform the lattice of the channel vectors to be near orthogonal. Numerical results show that the LR-based ZF precoder outperforms other ZF precoder schemes, such as, the norm approximation of the beamforming matrix. In particular, the sum rate of the proposed optimal ZF precoder requires a small number of BS antenna. Subsequently, with the strong line of sight (LoS) channel, the optimal power allocations in the subchannels depend on the dominance of the users in order to achieve substantial multiplexing and diversity gains. Specifically, the Ricean channel gain with the water-filling allocation at high SNR is non-negligible.

Multiple Access MmWave Design for UAV-Aided 5G Communications

Unmanned aerial vehicles (UAVs) have tremendous potential to improve wireless network capacity, but are challenging to operate in ultra-dense networks, mainly due to the strong interference received from the dominated lineof- sight channels of the UAVs. By forming multiple highly directional beams, millimeter-wave (mmWave) communication technology allows concurrent user transmissions via beam-division multiple access (BDMA), and thus has emerged as a promising solution to mitigate interference for the fifth generation (5G) UAV communication. However, due to the limited number of beams generated in practical mmWave communication systems, conventional BDMA cannot meet the ever increasing capacity requirement. A new multiple access technique is intensely desired. In this article, we integrate mmWave communication with UAV-aided 5G ultra-dense networks, and design a novel link-adaptive constellation- division multiple access (CoDMA) technique. We discuss key challenges in efficient multiple access technique design, and then investigate design principles on new multiplexing methods and beamwidth optimization for interference management in UAV-aided dynamic networks. We further apply flexible constellation division in rateless codes, and put forward the system-level design of CoMDA with beamwidth adaptation to unleash the multiplexing gain. Finally, we show that our design can successfully enable multiple-user access within a single beam without causing any intra-beam interference while efficiently mitigating interference from adjacent beams. We also demonstrate that the proposed design is adaptive to the UAV network dynamics, and can greatly improve the system throughput.

A Spatially Consistent MIMO Channel Model With Adjustable K Factor

In the area of research on massive multiple-input multiple-output (MIMO), two assumptions on the wireless channel dominate channel modeling. Either, a rich scattering environment is assumed and the channel is modeled as i.i.d. Rayleigh fading, or, a line of sight (LOS) channel is assumed, enabling geometric channel modeling under a farfield assumption. However, either of these assumptions represents an extreme case that is unlikely to be observed in practice. While there is a variety of MIMO channel models in literature, most of them, and even very popular geometry based stochastic channel models, are not spatially consistent. This is especially problematic for technologies in which channel correlation of adjacent users is an important factor, such as massive MIMO. In this work, we introduce a simple but spatially consistent MIMO channel model based on multiple scattering theory. Our proposed channel model allows to adjust the Rician $K$ factor by controlling the number and strength of scattering elements. This allows to perform spatially consistent simulations of wireless communications systems for a large range of scattering environments in between an i.i.d. Rayleigh fading assumption and pure LOS channels. A statistical analysis in terms of the Rician $K$ factor for the introduced model is provided and verified by simulations. By comparison to other channel models, we show that non spatially consistent channel models lead to an underestimation of inter-user correlation and therefore to an overestimation of achievable sum rate.

A photodiode array and Langmuir probe for characterizing plasma in GLAST-III tokamak device

A newly developed spectroscopic system based on linear photodiodes array has been installed on upgraded Glass Spherical Tokamak (GLAST-III) for spatial as well as temporal characterization of hydrogen discharge through light emission. The Spectral range of each silicon photodiode is from 300 nm to 1100 nm with response time of 10 ns and active area of 5 mm2(circular). The light from the plasma is collected through holes along 4 lines of sight channels with spatial resolution of about 5 cm passing from entire poloidal cross section. The photodiode's signals located at position of 10 and 14 cm from inboard side show fluctuations in the central plasma region. Moreover, the sequence of plasma lighting shows that plasma instigates from the central resonant field region and then expands outwards. At lower pressure, outboard movement of the plasma is slower suggesting better plasma confinement. In addition to photodiode array, an Ocean spectrometer (HR2000+) has been used to record the visible spectrum over the selected range (597–703 nm) with a spectral resolution of 0.15 nm. The studies have been conducted during initial phase of plasma formation for two different hydrogen gas fill pressures. Langmuir triple probe (LTP) is used to get time-resolved information on plasma parameters in the edge region. The time evolution of whole discharge including microwave pre-ionization phase and current formation phase has been demonstrated by temporal profiles of light emission and plasma floating potential.

Direct Position Determination of Unknown Signals in the Presence of Multipath Propagation.

A novel geolocation architecture, termed “Multiple Transponders and Multiple Receivers for Multiple Emitters Positioning System (MTRE)” is proposed in this paper. Existing Direct Position Determination (DPD) methods take advantage of a rather simple channel assumption (line of sight channels with complex path attenuations) and a simplified MUltiple SIgnal Classification (MUSIC) algorithm cost function to avoid the high dimension searching. We point out that the simplified assumption and cost function reduce the positioning accuracy because of the singularity of the array manifold in a multi-path environment. We present a DPD model for unknown signals in the presence of Multi-path Propagation (MP-DPD) in this paper. MP-DPD adds non-negative real path attenuation constraints to avoid the mistake caused by the singularity of the array manifold. The Multi-path Propagation MUSIC (MP-MUSIC) method and the Active Set Algorithm (ASA) are designed to reduce the dimension of searching. A Multi-path Propagation Maximum Likelihood (MP-ML) method is proposed in addition to overcome the limitation of MP-MUSIC in the sense of a time-sensitive application. An iterative algorithm and an approach of initial value setting are given to make the MP-ML time consumption acceptable. Numerical results validate the performances improvement of MP-MUSIC and MP-ML. A closed form of the Cramér–Rao Lower Bound (CRLB) is derived as a benchmark to evaluate the performances of MP-MUSIC and MP-ML.

Design and Performance Evaluation of MB-MIMO-UWB for Different Transmission Techniques

This work represents a design of multiple input multiple output multi band ultra wide band (MIMO-MB-UWB) system by using four antennas at the transmitter and eight antennas at the receiver with the combination of orthogonal space time block code encoders and minimum mean square (MMSE) equalizes. The proposed system used for a short range wireless communication and is one of the most promising technologies for generating a high data-rate. It will likewise take on a very important role in next generation wireless communication system where everyone will be plugged in at anywhere, anytime, hence use of 4 * 8 antennas enables to increase capacity and data-speed of the system by four times and also supplies an efficient wireless link as the receiver has multiple choice to commit the best data by comparing the quality of meeting data. The functioning and efficiency of the system are valued along the basis of bit-error-rate (BER, transmitted–received signal, constellation plot and MMSE performance plot for binary phase shift keying, quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM, 256-QAM modulation schemes under CM1 line of sight (LOS) channel and CM2 non-line of sight (N-LOS) channel condition of the range between 0 and 4 m. Outcomes show that with use of MMSE equalizer, inter symbol interference is reduced, especially for a high order modulation system which further raises the operation of the proposed scheme. On the basis of BER It is also noted, that QPSK is the best modulation scheme for LOS condition and 16-QAM is the best for N-LOS condition. Additionally, proposed work will also be helpful in academics and for the different researchers and technologists who are operating in the UWB system, because a complete illustration of simulation will be exemplified and explained.

Low complexity link level performance prediction for SIMO systems

In this paper a low-cost link level performance prediction technique is proposed for a single input and multiple output system. Receiver link level abstraction is used in system level simulations of large networks in order to reduce their complexity. Usually, a single lookup table is employed in link level abstraction to predict a receiver's performance under different channel conditions. In the presented work, the mean frame error rate curve of different diverse channels is proposed as the reference for performance prediction in single input multiple output systems. Its generation involves compression of the received code word into a single quality measure based on the postdetection signal to noise ratio values using nonlinear capacity, exponential, and mutual mapping functions. The overall performance difference between simulated and predicted frame error rates shows that the proposed scheme gives very good performance approximations under different modulation and coding schemes, clearly outperforming the classical line of sight channel lookup table.

A 60GHz Wireless Cooperative Communication System Based on Switching Beamforming

The challenge of penetrating obstacles along with impact from weak multipath effects makes 60GHz signal very difficult to be transmitted in non-line of sight (NLOS) channel. So 60GHz system is vulnerable to obstructions and thus likely results in link interruption. While the application of cooperative technology to solve link blockage problemin 60GHz system should consider the characteristic of directional transmission for 60GHz signal. Therefore in this paper a system is proposed to solve the link blockage problem in 60GHz NLOS communication environment based on the concept of cooperation and also the beamforming technology, which is the basis of directional transmission for 60GHz communication system. The process of anti-blockage solution with cooperative communication is presented in detail, and the fast switching and recovery schemes are well designed. The theoretical values of symbol error rate (SER) using decode and forward (DF) cooperation and amplify and forward (AF) cooperation are presented respectively when the common channel interference exists. Simulation results demonstrate that the performance based on DF cooperation is better than the performance based on AF cooperation when directional transmission is used.

Channel direction information probing for multi-antenna cognitive radio system

This work studies the problem of channel direction information (CDI) probing for multi-antenna cognitive radio system. The CDI of the channel from the secondary transmitter (ST) to primary receiver (PR) is elementary information in designing the beamforming at the ST for mitigating the interference to the PR. However, lacking the explicit cooperation between primary and secondary systems, the CDI has to be acquired by probing at the ST, which is challenging. To solve this, we consider the line of sight (LoS) channel between the ST and the PR, and propose one CDI probing scheme for the ST. Specifically, the ST sends two types of probing signals by beamforming towards an interested region where both the secondary receiver (SR) and the PR are located and then actively learns the hidden feedback information from the primary system to acquire the CDI. The proposed scheme has a closed-form solution, and avoids the iteration between the probing and acquisition, which is desirable for practical system. Moreover, we show that the proposed probing scheme can be extended for primary systems working under multi-access channel and broadcasting channel. Simulation results demonstrate that the proposed scheme can improve the accuracy of the acquired CDI at the ST in cognitive ratio system remarkably.

English

This paper analyzes the power ratio between the directed line of sight configuration links increased by reducing the receiver Field of View (FOV). Line of Sight systems employ high degree of directionality of the transmitter and receiver and uninterrupted LOS. LOS link design reduces multipath distortion and increases power efficiency. The performance of the link relies upon the existence of an uninterrupted LOS path between the transmitter and receiver. This can be quantified by the Rician factor. The Field of View of the receiver is related to the collection area of a lens and the photo detector area. The FOV of the detector is smaller than the radiation incident angle. It is an observation that the concentrator gain or channel gain increases when the value of FOV reduces. In this paper, the line of sight channel gain is calculated at different values of FOV.

Performance Evaluation of Adaptive Indoor Matched Rake Receiver Using Multiple-Combining Techniques

This paper discusses the enhancement of the wireless rake receiver for high speed and short distance indoor ultra wideband (UWB) propagation with line-of sight (LOS) and non line-of sight (NLOS) channel models. The proposed matched rake receiver uses three main combining techniques, maximum ratio combining (MRC), equal gain combining (EGC), and selective combining (SC) to capture most of the energy of the multi-path components (MPCs). When the wireless communication systems work with high capacity and high speed in transmission and reception scenarios, there will be a serious challenge defined as inter-symbol interference (ISI) during the reception process. The ISI causes increasing in the bit error rate (BER) when the wireless communication systems work with high bit rate propagation. The matched rake receiver scheme was designed to suppress ISI by maximizing the signal to noise ratio (SNR) before constructing the desired signal in decision circuit and effectively the system enhancement is improved. After adding additive white Gaussian noise (AWGN) to the received signal, the improvement is cleared comparing with the theoretical results that has no AWGN. During the comparison of the simulation results, MRC partial rake receiver of less complexity showed better performance than the EGC and SC rake receivers.

Design and practical deployment of a network-centric remotely piloted aircraft system

Remotely piloted aircraft systems (RPASs) are gaining more are more relevance during the last decade since more applications are being enabled by lighter planes with increasing autonomy, higher ceilings, and more powerful transmission technologies. The integration of the RPAS as part of network-centric warfare would be a very important milestone to achieve because of the huge amount of information and capabilities that all these aircrafts can incorporate into the global scheme. This integration is easier for handheld (short-range) RPASs since their communications are typically based on digital transmission like WiFi or WiMAX, but it may not be as obvious for bigger RPASs (long-range, e.g., tactical or medium/high altitude systems) because their line of sight communications are frequently based on analog transmissions. This implies indirect integration into network-centric warfare by means of the ground control station (satellite communications, when available, may suffer notorious delay for certain applications). This article presents a recent practical experience, including flight test campaigns, deploying an all- IP communication architecture into one of the most relevant Spanish tactical RPASs, the SIVA, used by both the Spanish Army and the Spanish Air Force for the last 10 years. This deployment enables cost-effective integration of this RPAS (and its natural successor, the MILANO, a medium- altitude RPAS) into the network-centric warfare by means of direct TCP/IP transmissions over a long-range digital line of sight channel combined with satellite communications for beyond line of sight operations. The proposed design includes network-level security over the radio interfaces, automatic data link selection, support of remote video terminals, and access connectivity toward external IPv6 networks.

Codebook design for uplink of densely small cells

The shrinking of cell-size brings significant changes to the wireless uplink of densely small cells (DSCs). A codebook design is proposed that utilizes the strong line of sight (LoS) channel component existing in a DSC system for uplink of the DSC system. To further improve the uplink performance, the high-rank codebook is designed based on singular value decomposition (SVD) due to the unnecessary preservation of strict constant modulus in the DSC system. And according to the simulation result, the proposed codebook leads to significant sum-rate gain and appreciable block error rate (BLER) performance improvement in the DSC system.