Free Space Path Research Articles

Performance enhancement of LEO-to-ground FSO systems using All-optical HAP-based relaying

In this paper, we propose to use all-optical high altitude platform (HAP)-based relaying technique to improve the performance of the free-space optical communication (FSO) system from a low-earth orbit (LEO) satellite to the ground station. Two all-optical relaying techniques are applied to HAP-based relay node including optical amplify-and-forward (OAF) and optical detect-amplify-and-forward (ODAF). A closed-form expression for bit error rate (BER) of the proposed relay-assisted LEO-to-ground FSO system under the effects of free space loss, path loss, and atmospheric turbulence is derived. We also carry out Monte-Carlo (M-C) simulation to verify the theoretical results in terms of BER and the quality of image transmission. The numerical results show that 28 dB power gain at BER of 10−9 can be achieved thanks to the use of HAP-based relaying. Also, lower BER that results in better quality of restored images at the ground station is demonstrated in this paper.

All-dielectric rod antenna array for terahertz communications

The terahertz band holds a potential for point-to-point short-range wireless communications at sub-terabit speed. To realize this potential, supporting antennas must have a wide bandwidth to sustain high data rate and must have high gain and low dissipation to compensate for the free space path loss that scales quadratically with frequency. Here we propose an all-dielectric rod antenna array with high radiation efficiency, high gain, and wide bandwidth. The proposed array is integral to a low-loss photonic crystal waveguide platform, and intrinsic silicon is the only constituent material for both the antenna and the feed to maintain the simplicity, compactness, and efficiency. Effective medium theory plays a key role in the antenna performance and integrability. An experimental validation with continuous-wave terahertz electronic systems confirms the minimum gain of 20 dBi across 315–390 GHz. A demonstration shows that a pair of such identical rod array antennas can handle bit-error-free transmission at the speed up to 10 Gbit/s. Further development of this antenna will build critical components for future terahertz communication systems.

Impact of Heterogeneous Fading Channels in Power Limited Cognitive Radio Networks

The tradeoff between decreasing the interference to primary user (PU) and increasing secondary users’ (SUs’) achievable throughput is an important problem in cognitive radio networks. Heterogeneous fading channels from PU to multiple SUs, PU’s traffic distribution, limited SU’s power and multiple SUs’ access contention impact both these two conflicting objectives. In this paper, we study the joint impact of these four factors on the tradeoff. More specifically, we consider that the channels from PU to SUs are exposed to non-identically independent free space path losses, PU’s traffic randomly arrives and departs from the channel, every SU’s average power consumption is limited, while multiple SUs contend to transmit. We first model the impact of these factors on SUs’ spectrum sensing and data transmission. Then, we formulate the tradeoff aiming at maximizing SUs’ aggregated throughput under two constraints: 1) interference probability to PU and 2) SUs’ average power consumption. To solve the optimization problem, we design a novel cluster based particle swarm optimization (C-PSO) algorithm. By iteratively updating the particles in a cluster based on the comparison of their fitnesses, the cluster converges to the optimal solution rapidly. Simulation results validate the feasibility of the C-PSO algorithm and the outperformance of our proposal compared against related contributions which consider the homogeneous fading channel. They also show how the optimal solution varies with path losses and PU’s traffic distribution.

Codebooks design and performance evaluation based on antenna array response and signal to noise ratio for mmWave communication

ABSTRACTCodebook design is a key point in beam-forming which is necessary for millimetre Wave (mmWave) communication to compensate its huge free space path loss. In beam-forming codebook, the transmitter and the receiver co-operate to select the optimum pre-designed code vectors to satisfy robust, efficient, and reliable transmission link between two pairs. In our study, we propose a new codebook structure based on unique phase states. Based on the Array Factor (AF) and Signal-to-Noise Ratio (SNR), the proposed codebook is compared and evaluated with the uniform weighting codebook, circular array codebook, and IEEE 802.15.3c standard codebook. The results show that the proposed codebook achieves comparable SNR and array directivity with the IEEE 802.15.3c, but higher peak and wide dynamic range of SNR and better array directivity compared with the uniform weighting and circular antenna codebooks. Thus, the proposed codebook is expected to improve signal quality and increase user throughput significantly.

Indoor Corridor Wideband Radio Propagation Measurements and Channel Models for 5G Millimeter Wave Wireless Communications at 19 GHz, 28 GHz, and 38 GHz Bands

This paper presents millimeter wave (mmWave) measurements in an indoor environment. The high demands for the future applications in the 5G system require more capacity. In the microwave band below 6 GHz, most of the available bands are occupied; hence, the microwave band above 6 GHz and mmWave band can be used for the 5G system to cover the bandwidth required for all 5G applications. In this paper, the propagation characteristics at three different bands above 6 GHz (19, 28, and 38 GHz) are investigated in an indoor corridor environment for line of sight (LOS) and non‐LOS (NLOS) scenarios. Five different path loss models are studied for this environment, namely, close‐in (CI) free space path loss, floating‐intercept (FI), frequency attenuation (FA) path loss, alpha‐beta‐gamma (ABG), and close‐in free space reference distance with frequency weighting (CIF) models. Important statistical properties, such as power delay profile (PDP), root mean square (RMS) delay spread, and azimuth angle spread, are obtained and compared for different bands. The results for the path loss model found that the path loss exponent (PLE) and line slope values for all models are less than the free space path loss exponent of 2. The RMS delay spread for all bands is low for the LOS scenario, and only the directed path is contributed in some spatial locations. For the NLOS scenario, the angle of arrival (AOA) is extensively investigated, and the results indicated that the channel propagation for 5G using high directional antenna should be used in the beamforming technique to receive the signal and collect all multipath components from different angles in a particular mobile location.

Skema Lokalisasi Posisi Node Terdistribusi pada Lingkungan Free Space Path Loss

A wireless sensor network consists of interconnected nodes that exchange information and use shared resource in a wireless transmission medium. Sensor nodes are randomly deployed in observation area in static or moving term. During this situation, the position of each sensor nodes is required to be known to monitor the circumstances around the node according to the information collected by sensor. Localization is the process to determine the position of nodes. This process could be done in centralized or distributed manner. In this paper, a distributed localization mechanism is proposed, where the calculation of node position is carried out on the node itself. Trilateration method is employed to calculate the position of node based on estimated distance measured by Received Signal Strength Indicator (RSSI) technique using Zigbee module in Free-Space Path Loss (FSPL) outdoor area. The experiment result shows that, based on log-normal shadowing model, the path loss coefficient for observation area is 2.5443, whereas average estimated position error from three different measured nodes are 23.504 m, 17.369 m, and 17.95 m respectively. Each node needs 2.73 second to undertake localization process completely.

Performance Analysis of IEEE 802.11ad MAC Protocol

IEEE 802.11ad specifies a hybrid medium access control (MAC) protocol consisting of contention as well as noncontention-based channel access mechanisms. Further, it also employs directional antennas to compensate for the high freespace path loss observed in 60GHz frequency band. Therefore, it significantly differs from other IEEE 802.11(b/g/n/ac) MAC protocols and thus requires new methods to analyze its performance. In this paper, we propose a new analytical model for performance analysis of IEEE 802.11ad employing a threedimensional Markov chain considering all the features of IEEE 802.11ad medium access mechanisms including the presence of non-contention access and the different number of sectors due to the use of directional antennas. We show that the number of sectors has a high impact on the network throughput. We also show that the MAC packet delay is significantly affected by the duration of the contention period. Our results indicate that a suitable choice of the number of sectors and contention period can illustriously improve the channel utilization and MAC delay performance.

Received strength signal intensity performance analysis in wireless sensor network using Arduino platform and XBee wireless modules

Today, through the monitoring of agronomic variables, the wireless sensor networks are playing an increasingly important role in precision agriculture. Among the emerging technologies used to develop prototypes related to wireless sensor network, we find the Arduino platform and XBee radio modules from the DIGI Company. In this article, based on field tests, we conducted a comparative analysis of received strength signal intensity levels, calculation of path loss with “log-normal shadowing” and free-space path loss models. In addition, we measure packet loss for different transmission, distances and environments with respect to an “Arduino Mega” board, and radio modules XBee PRO S1 and XBee Pro S2. The tests for the packet loss and received strength signal intensity level show the best performance for the XBee Pro S2 in the indoor, outdoor, and rural scenarios.

Stochastic Geometry Analysis for Mean Interference Power and Outage Probability in THz Networks

Mean interference power and probability of outage in the THz band (0.1–10 THz) networks are studied. The frequency band has potential for enabling future short range communication systems because of the large available spectrum resources. This can enable huge data rates, or on the other hand, large numbers of users sharing the resources. The latter case is closely related to the subject of this paper on interference modeling for dense THz networks with stochastic geometry. We use it to estimate the average behavior of random networks. The literature has shown convenient closed form solutions for the mean interference power in ultrahigh frequency band (UHF, 300 MHz – 3 GHz). Those are not always readily applicable for the THz band. This is especially the case when THz band is modeled with the molecular absorption and free space path loss. Still, the mean interference power does have closed form solutions in all cases, but in some, numerical approximations have to be used. We provide the derivation and analysis of the mean interference power and the outage probability. The results are verified with computer simulations.

Measurement and Analysis on Land-to-Ship Offshore Wireless Channel in 2.4 GHz

In this letter, measurement results of land-to-ship offshore wireless channels are presented. Large-scale and small-scale fading characteristics of 2.4 GHz frequency band are investigated by implementing narrowband channel measurement system between quay and a sailing coast patrol boat. For large-scale fading, free space path loss model, ITU-R P.1546 model, and two-ray model are compared with the empirical PL data. The results of an extensive measurement were used to determine that the existing ITU-R P.1546 model is inconsistent with the empirical PL data for offshore environment, whereas the two-ray model considering sea wave height agrees well with overall empirical data. Destructive and constructive interferences around distances of 200–500 m are observed from the empirical PL data, which ensures the validity of the two-ray model for offshore environment. For small-scale fading, the Kolmogorov–Smirnov test is utilized to verify goodness of fit, and it is shown that the result agrees best with the Rician distribution, compared with Nakagami-m or Rayleigh distributions.

RSSI Measurements of a GSM Signal within an Indoor Environment

Studies has suggested that the association between Received Signal Strength Indication (RSSI) and measured distance between the transmitter and receiver can be one of the key elements for ranging and localization technologies. In this study, measurements of RSSI and the distance between 1800MHz receiver and transmitter within an indoor environment were acquired, recorded and analyzed. Using the data, correlation for a select indoor environment was derived. By comparing the recorded distance values acquired at set RSSI with values calculated based on the Free Space Path Loss equation, provisional estimation of the indoor signal fading can be established. The experiment was the initial part of a three-stage work that will later entail the comparison to that of outdoor as well as clean room environments. The final result of the study hopes to achieve a simple yet improved indoor fading model experienced by mobile communication signals.

Received Power-Based Channel Estimation for Energy Beamforming in Multiple-Antenna RF Energy Transfer System

In this paper, we propose a channel estimation algorithm based on the received power measurements for the multi-antenna radio frequency (RF) energy transfer system. The energy beamforming is a key technology to resolve a low energy transfer efficiency problem of the RF energy transfer system, which is mainly caused by a free-space path loss. The channel gains between transmit and receive antennas should be estimated to use the energy beamforming technique. We propose a received power-based channel estimation scheme that can be implemented by means of a simple power meter at a receiver. In the proposed scheme, the receiver measures the received power of the RF signal and sends a feedback containing the received power measurements to the transmitter. We formulate a channel estimation problem, which is recast into a linear estimation problem by using complex vectorization. Then, this linear estimation problem is solved by the ordinary least squares method and the Kalman filter-based algorithm. We also propose a method to reduce the dimension of the estimation problem when there are a smaller number of receive antennas than transmit antennas.

Mid-Infrared Gas Sensor Based on Mutually Injection Locked Quantum Cascade Lasers

We present experimental investigations of a novel mid-infrared gas sensor relying on a laser system consisting of two quantum cascade sources mutually coupled with each other. The free-space path between the two lasers contains the sensing cell which is filled with the gas of interest. This laser system proves to be a very interesting dynamical system which can offer enhanced detectivity when compared to a conventional direct absorption spectrometer utilizing a laser, the gas cell, and a detector. We present comprehensive investigations of the operation regimes under which the laser system behaves as an efficient sensor and demonstrate its dependence on critical parameters such as the coupling strength between the two lasers and their bias currents. The experimental study is supported by numerical simulations, which provide a theoretical understanding of the underlying mechanisms. A six fold improvement of the sensitivity in a N2O absorption spectroscopy experiment is demonstrated in comparison to a conventional direct absorption spectrometer. This sensitivity improvement is exhibited for a wide range of N2O concentrations.

Steerable Robot-assisted Micromanipulation in the Middle Ear: Preliminary Feasibility Evaluation.

The use of a robotic manipulator with a dexterously orientable gripper will expand the ability of middle ear surgeons to perform precise tasks and access otherwise challenging anatomic regions. Middle ear surgery presents unique challenges because of the constrained operative space and limited access to certain anatomic regions. A custom-designed robot with a sideways-reaching gripper was used to evaluate feasibility of manipulation tasks in different middle-ear anatomical zones. Reachable workspace within the middle ear, accuracy of free-space path following, and tool steadiness were compared between robotic telemanipulation and manual control. Preliminarily assessments of the robot's clinical utility included: 1) touching the round window niche, Eustachian tube orifice, and sinus tympani; 2) placing a stapes prosthesis; 3) removal of mockup diseased tissue in the sinus tympani. The reachable workspace in the middle ear was considerably greater with the robot as compared with manual manipulation using a Rosen needle. In a simple path-tracing task outside the ear, robotic telemanipulation was associated with significantly reduced error. Within the middle ear, the robot contributed to steadier movement, but longer task completion time. The gripper successfully placed a 4.5 mm piston prosthesis, accessed the round window niche, Eustachian tube orifice, and removed mockup disease from the sinus tympani. This study demonstrates that robotic assistance using steerable tools allows surgeons to access challenging anatomic regions of the middle ear. Coordinated and accurate manipulation is evidenced by motion analyses and completion of feasibility tasks within the middle ear.

Evaluating Path Loss by Extended Squitter Signals for Aeronautical Surveillance

For the successful deployment of the aeronautical surveillance systems, it is important to select the appropriate path loss model that is used in the coverage design. While the flight experiment is a traditional approach to evaluate the path loss models, the passive measurement of the 1090-MHz extended squitters (1090 ESs) broadcast by target of opportunity is a new complementary approach. This letter examines the feasibility of the 1090 ES measurement for evaluating the aeronautical path loss models. First, the effect of the alternate transmission using two transponder antennas is removed to extract the effect of the propagation channel. Then, the measurement result during the flight experiment is compared to the reference values given by free-space path loss in order to show the relevance of using the 1090 ES.

Characterization of 300-GHz Wireless Channel on a Computer Motherboard

This paper presents characterization of 300-GHz wireless channel on a computer motherboard, where several different measurement scenarios have been considered. Results indicate that the presence of the ground plane and/or vertical parallel-plate structures in the channel introduces multipath that, if constructively superimposed, may create a path loss lower than free-space propagation path loss. In addition, it has been found that a few centimeters of vertical misalignment between the transmitter and the receiver result in a path loss greater than 5 dB. Furthermore, the results indicate that vertical components such as dual in-line memory modules with reflection coefficient close to one for all incident angles can be used to create directed non-line-of-sight (LoS) links with wide coherence bandwidth. Finally, for LoS propagation obstructed by large objects, such as a heatsink, the path-loss exponent of 1.77 is found, while the rotating fan causes periodic fading in the received power, indicating that channel equalization techniques will be needed to overcome deep fades. All these results indicate that optimal communications can be achieved by carefully positioning the antennas with respect to the motherboard layout.

Synchronization of Clocks Through 12 km of Strongly Turbulent Air Over a City.

We demonstrate real-time, femtosecond-level clock synchronization across a low-lying, strongly turbulent, 12-km horizontal air path by optical two-way time transfer. For this long horizontal free-space path, the integrated turbulence extends well into the strong turbulence regime corresponding to multiple scattering with a Rytov variance up to 7 and with the number of signal interruptions exceeding 100 per second. Nevertheless, optical two-way time transfer is used to synchronize a remote clock to a master clock with femtosecond-level agreement and with a relative time deviation dropping as low as a few hundred attoseconds. Synchronization is shown for a remote clock based on either an optical or microwave oscillator and using either tip-tilt or adaptive-optics free-space optical terminals. The performance is unaltered from optical two-way time transfer in weak turbulence across short links. These results confirm that the two-way reciprocity of the free-space time-of-flight is maintained both under strong turbulence and with the use of adaptive optics. The demonstrated robustness of optical two-way time transfer against strong turbulence and its compatibility with adaptive optics is encouraging for future femtosecond clock synchronization over very long distance ground-to-air free-space paths.

Directional Radio Propagation Path Loss Models for Millimeter-Wave Wireless Networks in the 28-, 60-, and 73-GHz Bands

Fifth-generation (5G) cellular systems are likely to operate in the centimeter-wave (3–30 GHz) and millimeter-wave (30–300 GHz) frequency bands, where a vast amount of underutilized bandwidth exists world-wide. To assist in the research and development of these emerging wireless systems, a myriad of measurement studies have been conducted to characterize path loss in urban environments at these frequencies. The standard theoretical free space (FS) and Stanford University Interim (SUI) empirical path loss models were recently modified to fit path loss models obtained from measurements performed at 28 GHz and 38 GHz, using simple correction factors. In this paper, we provide similar correction factors for models at 60 GHz and 73 GHz. By imparting slope correction factors on the FS and SUI path loss models to closely match the close-in (CI) free space reference distance path loss models, millimeter-wave path loss can be accurately estimated (with popular models) for 5G cellular planning at 60 GHz and 73 GHz. Additionally, new millimeter-wave beam combining path loss models are provided at 28 GHz and 73 GHz by considering the simultaneous combination of signals from multiple antenna pointing directions between the transmitter and receiver that result in the strongest received power. Such directional channel models are important for future adaptive array systems at millimeter-wave frequencies.

Preliminary Analyses on Measured Received Signals of Orbiting RazakSAT Satellite

RazakSAT satellite transmits both X-Band and S-band frequencies. The main objective of the whole research is to revise free space path attenuation. The preliminary findings in this study will contribute for final research works in the near future which includes the revision of free space path attenuation. Ultimately, a new and more reliable free space path loss formulation will be developed. In this particular study, the purpose is to find linear relationship between received signal from the satellite and the distance with the ground station. The second purpose is to derive linear relationship between the received signal and the elevation angle. As a start, S-band satellite data are selected for 3 months out of a year data samples. All satellite data samples for this study are filtered with rain gage data and weather radar data to ensure no rain and no cloud conditions. All derived linear formulations are presented in this paper.

RF channel modelling and multi‐hop routing for wireless sensor networks located on oil rigs

This study presents a combined analytical and empirical model for predicting the signal loss effects of metallic drilling rig structures on radio-frequency electromagnetic waves used by wireless sensor networks (WSNs) installed on a drilling rig. The model is based on the combination of free space path loss and the excess loss caused by the metallic structure separately. The authors combine both losses to predict the overall loss of signal strength. The model has been validated against field data collected from multiple drilling rigs. Further modification of the model to include the effects of different signal frequencies is under way. They present simulation results from OMNeT++ based on their model, to establish the packet loss and energy consumption expected for a real WSN. They also present a novel dynamic multi-hop routing protocol, which improves network performance by removing the constraints of single-hop forwarding. The algorithm directs packets to their destination via a selected node within a routing ‘cluster’. By combining their channel model and routing protocol, they are able to achieve 100% packet success, while setting transmit power levels appropriately to achieve the longest possible network lifetime.