Fading Simulator Research Articles

Characterization and Modeling of UHF Wireless Channel in Terrestrial SFN Environments: Urban Fading Profiles

This paper proposes several realistic fading channel models to describe the signal distortions that arise in single frequency network (SFN) environments. The proposed models characterize ultra-high frequency (UHF) wireless channels based on measurements from operating networks in metropolitan areas. In this paper, the unique features of SFN transmission are identified compared to the conventional, single antenna transmission channels. The impacts of urban propagation, tower configuration, and mobile and terrain effects are considered based on the tapped delay line structure. The proposed models can provide a useful reference to facilitate the SFN system design, network planning, verification, and receiver performance test. Free MATLAB source code of the fading simulator is available at <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">https://github.com/ETRI-KMOU/FadingChannelSimulator</uri> .

Measurement-Based Investigations of the NB-IoT Downlink Performance in Fading Channels

The letter presents experimental results of investigations on the narrowband Internet of Things (NB-IoT) performance under two predefined multipath channel models, related to the reflections-free scenario of an unobstructed signal path. The NB-IoT downlink channel performance was systematically tested for resilience to the multipath propagation phenomenon (naturally occurring in real-life environments of IoT devices deployment), with the most robust modulation-coding scheme enforced, i.e., quadrature phase shift keying (QPSK) modulation without repetitions. Each of these three radio fading channels was artificially generated in electromagnetically isolated laboratory facilities. The NB-IoT equipment setup, the fading simulator for emulating the fading channels as well as the infrastructure that allowed obtaining desirable maximum coupling loss (MCL) values, were described along with the presentation of the measurement scenarios. Analysis of results led to the formulation of practical takeaways regarding the reduction in the maximum pathloss of a fading-free channel, in response to fadings incurred on the NB-IoT downlink signal by Extended Pedestrian-A (EPA) and Extended Typical Urban (ETU) channels.

SOA-Based Multilevel Polarization Shift On–Off Keying Transmission for Free-Space Optical Communication

This paper proposes a semiconductor optical amplifier (SOA)-based multilevel polarization shift on–off keying (MPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, the MPS-OOK signal is modulated with a constant linear state of polarization (SOP) at the high-intensity level and various SOPs at the low-intensity level in order to improve the spectral efficiency (SE) with the transmitted power efficiency. At the receiver end, first, a polarization-independent SOA was introduced to optically suppress the turbulence-induced scintillation effect and equalize the intensities of the various SOPs in the deep gain saturation state without polarization distortion. Then, a linear polarizer (LP) with a high extinction ratio (ER) was deployed to convert the equalized SOPs into a known intensity. Finally, the converted MPS-OOK signal was detected using a single photodiode (PD) and distinguished using a multilevel fixed-threshold decision (M-FTD). The proposed technique was evaluated using experiments. A Mach–Zehnder modulator (MZM)-based fading simulator was introduced to emulate the turbulence-induced scintillation effect. The experimental results demonstrated that the scintillation effect was effectively mitigated and the SE was improved by up to 2 bit/s/Hz using the proposed four-level polarization shift on–off keying (4PS-OOK) transmission.

Enhancement of SOA-Based Scintillation Mitigation by PS-OOK Transmission in FSO Communication

This paper proposes an enhancement of semiconductor optical amplifier (SOA)-based scintillation mitigation by polarization shift on-off keying (PS-OOK) transmission in free-space optical (FSO) communication. The scintillation effect is a critical issue in a vertical FSO link. The issue of the extinction ratio (ER) degradation of the received OOK signal caused by the high dynamic gain frequency of the SOA limits the performance of the turbulence-induced scintillation effect mitigation using a gain-saturated and polarization-independent SOA. Therefore, a PS-OOK signal, which has the characteristics of orthogonal polarization of bits “1” and “0,” is transmitted at the transmitter end and a linear polarizer is deployed after the gain saturated SOA at the receiver end. Based on the polarization orthogonality of PS-OOK, the performance of the scintillation-effect suppression is significantly improved by the mitigation of the ER degradation by the blocking of the polarization of bit “0” of the received optical PS-OOK signal using the linear polarizer. The proposed scheme is experimentally verified utilizing the Mach-Zehnder modulator (MZM)-based fading simulator accommodated various turbulence channels. The experimental results illustrate that the scintillation effect is effectively suppressed by the proposed technique compared to the SOA-based conventional single-polarized OOK detection and balanced PS-OOK detection.

Enhancement of Spectral Efficiency FSO Links Using Linear Polarizer-Based MLPolSK Detection

This paper proposes the enhancement of spectral efficiency (SE) by linear polarizer based multilevel linear polarization shift keying (MLPolSK) detection in free-space optical (FSO) communication. At the transmitter end, in order to improve SE of FSO links, the various linear state of polarizations (SOPs) with an equal magnitude-spacing of x- or y-polarization are modulated for MLPolSK signal. At the receiver end, the cascaded polarization-independent semiconductor optical amplifier (SOA) and linear polarizer are introduced to detect MLPolSK signal. Polarization-independent SOA is applied to effectively mitigate the turbulence induced scintillation effect by the gain saturation without the distortion of polarization. Linear polarizer with high extinction ratio (ER) is used to transform various SOPs into the signal with an equal intensity-spacing of x- or y-polarization. Besides, the amplified spontaneous emission (ASE) noises are reduced by the polarization of ASE noises by linear polarizer. Finally, MLPolSK is detected by a single photodiode (PD) and decided by multilevel-thresholdings. In experiments, the performance of the proposed technique is evaluated under turbulence channels, which are emulated using Mach–Zehnder modulator (MZM)-based fading simulator. Experimental results show that the SE is improved up to 3 bit/s/Hz under the effective scintillation mitigation using the proposed technique.

High PDG-OA-Based MLPolSK Modulation for Spectral Efficient Free-Space Optical Communication

This paper proposes multilevel linear polarization shift keying (MLPolSK) modulation, based on a high polarization-dependent gain optical amplifier (PDG-OA), for spectral efficient free-space optical (FSO) communication. At the transmitter end, multiple linear states of polarization (SOPs) are adopted, and different SOPs are mapped for MLPolSK modulation according to the PDG-OA. First, in the receiver, scintillation is effectively mitigated by a polarization-independent semiconductor optical amplifier (SOA) with gain saturation. Next, various SOPs are transformed into different signal intensity levels by an OA with high PDG characteristics. Finally, transformed signal is directly detected by a single photodiode (PD) with multilevel-thresholds decision. The feasibility of the proposed technique is evaluated experimentally using a reflective semiconductor optical amplifier (RSOA), with a high PDG-up to 20 dB. A Mach–Zehnder modulator (MZM)-based fading simulator is used to accommodate lognormal distribution simulated turbulence effects into experiments. Experimental results illustrate that multilevel linear SOPs can be distinguished effectively by the proposed technique. Consequently, the MLPolSK detection is simplified, and the spectral efficiency (SE) is improved by up to 2 bit/s/Hz with effective scintillation mitigation in FSO.

Random phase and time of arrival statistics for circular disk scattering model

Fading simulators are tested by verifying the statistics of channel created by multipath environment. These statistics are an angle of arrival (AoA), time of arrival (ToA), and angle of deviation (AoD). In literature, fading simulators statistics are analysed by joint probability density function (pdf) of AoA and ToA and/or joint pdf of AoA and AoD. The multipath environment has been represented by various geometrical models. These fading simulators assume independency of random phase and angle of arrival. Continuously changing phase of multipath components affect the statistics which in turn causes fading. Because of this continuously changing phase of multipath components, at receiver side demodulator has to adjust the phase by trial and error method to recover the original time variant signal which is a tedious process. In this paper the circular disk scattering model is analysed by marginal pdf of ToA and random phase and it is proved that ToA is negative exponential and random phase is uniformly distributed which proves the dependency of ToA on random phase. Later, the simple relation has been derived between AoD and random phase which can directly help at receiver side for adjusting the deviation occurred in transmitted signal for all wireless applications.

Generalized Gamma Fading Simulation of Scintillation Disturbed GNSS Signals

The generalized gamma distribution includes as particular cases different well-known distributions, such as the Nakagami-m. We propose a computationally efficient technique to generate correlated time-series with generalized gamma distribution. This method is useful in the simulation of GNSS signals disturbed by ionospheric scintillation. The technique requires only knowledge of two parameters, besides the definition of a model for the amplitude power spectrum.

Statistical Analysis of Modified Fading Simulator in Rayleigh Fading Environment

Statistical Analysis of Modified Fading Simulator in Rayleigh Fading Environment

Synthetic method to generate α‐µ distributed variants

Although the transformation method can essentially be used to generate any distribution, its application to the α-µ model involves inversion of the incomplete gamma function. Such inversion of a special function does not lead to a closed-form expression but is usually implemented using iterative methods, which heavily burden any simulation machine. By introducing a novel synthetic α-µ variant generation method, free from iterations and special functions, the proposed approach will be appropriate for building generalised real-time fading simulators.

A multi-criteria comparative analysis of different Rayleigh fading channel simulators

In this paper, an in-depth comparative analysis of the most accepted and approved Rayleigh fading channel simulators is presented. Our study addresses the three common methods for generating time-correlated Rayleigh fading processes: the inverse discrete Fourier transform (IDFT) method, the filtering white Gaussian noise (FWGN) method and the sum-of-sinusoids (SOS) method. Totally seven models, including the recently proposed models, are analyzed and compared with Clarke's reference model. We use four quantitative measures for judicious assessment of the quality of generated processes in terms of both first-order and second-order statistics. Additionally, these measures cover analysis in time domain as well as in frequency domain. An evaluation of the computational effort is also depicted. The simulation results suggest that the recently published Wang's model is very attractive and it outperforms all SOS-based models. We conclude that Zheng and Xiao's first model remains the best choice among Zheng and Xiao's family models. Several other important remarks and conclusions about the accuracy and the capability of the different Rayleigh fading simulators are driven from this study.

A general approach to generate Rayleigh fading simulators with correct statistical properties

ABSTRACTRayleigh fading channel is widely modeled using sum‐of‐sinusoids simulators. In the paper, the construction method of an accuracy simulator is of concern. The intrinsic relationships between path gain, angle of incoming wave (angle of arrival), and initial phase are analyzed. At the same time, rules of simulator construction were proposed. According to these rules, the second‐order statistical properties of designed simulators match to the desired ones perfectly. Furthermore, our method can be an elicitation to generate simulators with correct high‐order statistical properties. In addition, a realization based on the construction method is designed to example how to use these construction rules. The new realization can be used to generate multiple uncorrelated fading waveforms for frequency‐selective fading channels, multiple‐input multiple‐output channels, and diversity combining scenarios. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Frequency-selective Fading Simulator for Real-time Laboratory Measurements

A simple design for a frequency selective multipath fading simulator, intended for testing the performance of wireless communication system through laboratory measurements is presented. Proposed design is based on simplified three-path model, which can be cascaded in order to simulate larger number of paths. It operates at intermediate frequency and can be easily controlled by user during operation at particular frequency area, or can be adjusted in order to cover area of interest. Hardware realization of proposed solution is described in detail and good agreement of achieved results with the mathematical model of fading, on which the proposed solution is based, is reported.

Fast Simulation of Average Small-Scale Fading for Indoor Localization Applications

The majority of currently available wireless devices' localization systems are based on received signal strength (RSS) measurements. The input to the localization technique is configured by the average of a number of N instantaneous RSS samples; thereupon, during simulation of such techniques, it is necessary to generate N RSS samples and calculate the corresponding average, which results to increased computational cost and run time. A new technique for reducing computational cost and run time of localization techniques simulators is proposed based on directly sampling a probability distribution function (PDF) corresponding to the average RSS of N samples. However, PDFs of the average RSS cannot be readily calculated and often there is no analytical solution. A study based on goodness-of-fit tests and localization precision is presented herein in order to numerically evaluate the replacement of unknown average RSS PDFs with empirically yielded ones. Furthermore, an indoor propagation and localization techniques simulator has been developed employing the proposed technique. Numerical results demonstrate the applicability of the proposed approach in achieving fast simulation of average small-scale fading and its application to RSS-based localization techniques simulation.

Hardware Implementation of Nakagami and Weibull Variate Generators

An efficient implementation of Nakagami- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> and Weibull variate generators on a single field-programmable gate array (FPGA) is presented. The hardware model first generates a correlated Rayleigh fading variate sequence and then transforms it into a sequence of Nakagami- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> or Weibull fading variates. A biquad processor facilitates the compact implementation of a Rayleigh variate generator with arbitrary autocorrelation properties. A combination of logarithmic and linear domain segmentations along with piece-wise linear approximations is used to accurately implement the nonlinear numerical functions required to transform the correlated Rayleigh fading process into Nakagami-m or Weibull fading processes. When implemented on a Xilinx Virtex-5 5VSX240TFF1738-2 FPGA, the fading simulator uses only 1.6% of the configurable slices, 1.2% of the DSP48E modules and 3 block memories, while operating at 120 MHz, generating 120 million complex variates per second. The throughput can be increased up to 373 MHz with this FPGA if two separate clock sources are utilized.

Scintillation model of laser beam propagation in satellite-to-ground bidirectional atmospheric channels

This paper discusses a scintillation model of laser beam propagation in satellite-to-ground bidirectional atmospheric channels. The frequency characteristics of the downlink were theoretically derived on the basis of measurements in low Earth orbit satellite-to-ground laser communication experiments. The speckle patterns were averaged and the frequency response of the received optical signal was filtered by a telescope aperture. The model parameters were obtained by fitting the results to the model. This paper introduces scaling factors for the uplink and extends the theory for the downlink to that for the uplink. The proposed model can generate time-varying optical signals based on the von Kármán spectrum for space-to-ground laser links. The scintillation index was estimated using the modified Hufnagel–Valley model, which was obtained from real measurements. The probability density function was fitted by the estimated scintillation index and compared to the gamma–gamma distribution under strong turbulence conditions. The scaling factor for the root mean square wind speed was newly introduced to fit the frequency spectra for the uplink. The simulation results are presented in this paper. The proposed scintillation model can contribute to improving the fading simulation of satellite-to-ground communication links as well as add to the future discussion of standards, like those proposed by the Consultative Committee for Space Data Systems.

Development of a MIMO-OTA System with Simplified Configuration

This paper introduces our development of a MIMO-OTA system with simplified configuration. The key element of our proposal is the adoption of an antenna branch-controlled configuration for generating multipath delayed waves. The signal processing is carried out on IF band signal with an FPGA in a fading-emulator-type MIMO-OTA measurement system. The proposed scheme is largely different from available system configurations for the fading simulator method of constructing the OTA test environment. We describe the principle of the proposed scheme, channel model incorporated in the system, basic configuration of the developed system, and its performance.

Sensing Methods for Detecting Analog Television Signals

This paper introduces a unified method of spectrum sensing for all existing analog television (TV) signals including NTSC, PAL and SECAM. We propose a correlation based method (CBM) with a single reference signal for sensing any analog TV signals. In addition we also propose an improved energy detection method. The CBM approach has been implemented in a hardware prototype specially designed for participating in Singapore TV white space (WS) test trial conducted by Infocomm Development Authority (IDA) of the Singapore government. Analytical and simulation results of the CBM method will be presented in the paper, as well as hardware testing results for sensing various analog TV signals. Both AWGN and fading channels will be considered. It is shown that the theoretical results closely match with those from simulations. Sensing performance of the hardware prototype will also be presented in fading environment by using a fading simulator. We present performance of the proposed techniques in terms of probability of false alarm, probability of detection, sensing time etc. We also present a comparative study of the various techniques.

Short-term fading simulation using artificial neural networks

A novel method for short-term fading simulation is presented in this paper. Proposed solution is based on artificial neural network principles. In order to obtain an adequate training data set, extensive measurements of the received electric field strength were carried out in an indoor environment, in a nonline-of-sight scenario. As quality assessment of the proposed method, performance comparisons were made with existing Rayleigh fading simulation methods. The statistical analysis of the short-term fading sequence obtained by the proposed method has shown better correspondence with measurement results as compared to other analyzed methods.

Modelling and simulation of Rayleigh fading, path loss, and shadowing fading for wireless mobile networks

In this paper, we first derive two explicit mathematical expressions of bit error rates (BERs) with Rayleigh fading for differential phase shift keying (DPSK) and quadrature amplitude modulation (QAM), and establish corresponding simulation models to realize the effect of Rayleigh fading. Next, we propose a mathematical expression to define the effect of shadowing fading and an approach to computing it. Then, we optimize the effect of path loss through adapting the value of the path loss exponent to different environments. Finally, using a packet-oriented simulation tool, OPtimized Network Engineering Tool (OPNET), we realize the effects of Rayleigh fading and shadowing fading, optimize the effect of path loss, and investigate the integrated impact of these effects on the performance of wireless mobile networks including wireless local area networks (WLANs) and mobile ad hoc networks (MANETs). Simulation results not only coincide with the experimental results in present literature, but also show that these effects exert a non-negligible impact on the performance of wireless mobile networks. Accordingly, our new models and approaches make the present over-optimistic simulation results more accurate and reasonable.