Doppler Fading Research Articles

GPS L1CA/BDS B1I Multipath Channel Measurements and Modeling for Dynamic Land Vehicle in Shanghai Dense Urban Area

Global navigation satellite system multipath channel modeling is important for signal simulation and error mitigation in urban scenarios. However, there have not been sufficient studies on channel statistical models for dynamic land vehicles in eastern Asian cities. This paper presents both MULTIPATH and non-line-of-sight (NLOS) channel models for GPS L1CA and BDS B1I signals. They are based on an extensive field dataset collected in the Shanghai-Lujiazui area. In addition, a method is developed to estimate multipath components and NLOS signal parameters in a dynamic channel environment. It is found that the Gamma function best fits the distribution of MULTIPATH channel delays while the exponential function is better suited for the distribution of NLOS channel delays. Both MULTIPATH and NLOS channel signals follow a linear decline average power-delay trend and a zero-mean Gaussian distribution power scattering model. The Doppler fading frequency distribution for MULTIPATH and NLOS channels can be modeled by the Gaussian function except for the MULTIPATH channel signal at the low velocity range (0∼3 km/h), where an absolute exponential distribution is a better fit. The lifetime distribution for MULTIPATH and NLOS channels is represented by the generalized Pareto distribution function. These findings offer insights into the GNSS signal models in urban propagation channels and are essential to the development of accurate urban navigation systems.

BDS B1I multipath channel statistical model comparison between static and dynamic scenarios in dense urban canyon environment

Global Navigation Satellite System (GNSS) multipath channel models are fundamental and critical for signal simulation and receiver performance evaluation. They also aid the designing of suitable multipath error mitigation algorithms when the properties of multipath channel are available. However, there is insufficient existing research on BeiDou Navigation Satellite System (BDS) signal multipath channel models. In this study, multipath channel statistical models are established on the basis of extensive datasets of the BDS B1I signal. A multipath parameter estimation algorithm is designed to extract information of multipath rays from the intermediate frequency data. The delay, power loss, Doppler fading frequency, and lifetime distribution models for static and dynamic vehicle platforms are established and compared, and the effects of the satellite orbit type and platform speed on the models are analyzed. The results reveal the detailed distribution and variation characteristics of the multipath parameters and are valuable for the development of accurate urban navigation systems.

Evaluation of OFDM Channel Sounding Techniques with Three Modulation Sequences

This paper presents an evaluation of multicarrier channel sounding techniques using different random and pseudo-random sequences to modulate the OFDM sounding signal. The Random (Rand), Pseudo-Noise (PN) and Zadoff-Chu (ZC) were tested, both in laboratory simulations and in field measurements. For the laboratory simulations Matlab routines were used to generate OFDM signals modulated with each of the three sounding signals, that were then convoluted with a synthesized transfer function of a test channel with six multipath components and added Gaussian noise and Doppler fading. The resulting signals are then correlated with a copy of the original signal to provide the multipath power delay profiles. The root mean square deviation (RMSD) and the relationship between peak power and mean power (PAPR) were used as metrics for the comparison between the test transfer function and the simulation detected multipath delay profiles, showing slight advantages of the ZC sequence. The three sequences were then used in field measurements to characterize an urban channel at 700 MHz. In the field measurements, the ZC sequence showed the lowest detection threshold, allowing for the detection of a larger number of multipath components.

English

Fading is commonly used to describe the properties of the communication channel. Large efforts have been made to describe characteristics of the channel in wireless communication system. The performance of wireless signal propagation in a conventional environment needs Doppler fading channel schemes by assuming a perfect knowledge about the channel frequency response at both the transmitter and the receiver sides. Two models were used to depict the characteristics of Doppler fading channel in term of source velocity,  where the first  is free space transmission model consists of two antennas at a given distance (r) to calculate the received signal strength for transmission links at relative motion between the transmitter and receiver antennas. The second model is simulation based on Matlab (2013B) to compute and plot the received signal envelope taking into account the source velocity over the multipath fading. The analysis and employing the information of Doppler fading can enhance the characteristics of channel estimation.   Key words: Doppler, fading, frequency, receiver, transmitter.

Retrieval of Vertical Air Motion in Precipitating Clouds Using Mie Scattering and Comparison with In Situ Measurements

AbstractFor the first time, the Mie notch retrieval technique is applied to airborne cloud Doppler radar observations in warm precipitating clouds to retrieve the vertical air velocity profile above the aircraft. The retrieval algorithm prescribed here accounts for two major sources of bias: aircraft motion and horizontal wind. The retrieval methodology is evaluated using the aircraft in situ vertical air velocity measurements. The standard deviations of the residuals for the retrieved and in situ measured data for an 18-s time segment are 0.21 and 0.24 m s−1, respectively; the mean difference between the two is 0.01 m s−1. For the studied cases, the total theoretical uncertainty is less than 0.19 m s−1 and the actual retrieval uncertainty is about 0.1 m s−1. These results demonstrate that the Mie notch technique combined with the bias removal procedure described in this paper can successfully retrieve vertical air velocity from airborne radar observations with low spectral broadening due to Doppler fading, which enables new opportunities in cloud and precipitation research. A separate spectral peak due to returns from the cloud droplets is also observed in the same radar Doppler spectra and is also used to retrieve vertical air motion. The vertical air velocities retrieved using the two different methods agree well with each other, and the correlation coefficient is as high as 0.996, which indicates that the spectral peak due to cloud droplets might provide another way to retrieve vertical air velocity in clouds when the Mie notch is not detected but the cloud droplets’ spectral peak is discernable.

Flexible Pilot Contamination Mitigation With Doppler PSD Alignment

Pilot contamination in the uplink (UL) can severely degrade the channel estimation quality at the base station (BS) in a massive multi-input multi-output (MIMO) system. Thus, it is critical to explore all possible avenues to enable more orthogonal resources for the users to transmit non-interfering UL pilots. In conventional designs, pilot orthogonality typically assumes constant channel gains over time, which limits the amount of orthogonal resources in the case of time-selective channels. To circumvent this constraint, in this paper, we show how to enable orthogonal multiplexing of pilots in the case of Doppler fading by aligning the power spectrum densities (PSD) of different users. From the derived PSD aligning rules, we can see multiple users can be sounded simultaneously without creating/suffering pilot contamination even when these users are experiencing time-varying channels. Furthermore, we provide analytical formulas characterizing the channel estimation mean square error (MSE) performance. Computer simulations further confirm us the PSD alignment can serve as one important decontamination mechanism for the UL pilots in massive MIMO.

Rate Prediction and Selection in LTE Systems Using Modified Source Encoding Techniques

In current wireless systems, the base-Station (eNodeB) tries to serve its user-equipment (UE) at the highest possible rate that the UE can reliably decode. The eNodeB obtains this rate information as a quantized feedback from the UE at time n and uses this, for rate selection till the next feedback is received at time n + {\delta}. The feedback received at n can become outdated before n + {\delta}, because of a) Doppler fading, and b) Change in the set of active interferers for a UE. Therefore rate prediction becomes essential. Since, the rates belong to a discrete set, we propose a discrete sequence prediction approach, wherein, frequency trees for the discrete sequences are built using source encoding algorithms like Prediction by Partial Match (PPM). Finding the optimal depth of the frequency tree used for prediction is cast as a model order selection problem. The rate sequence complexity is analysed to provide an upper bound on model order. Information-theoretic criteria are then used to solve the model order problem. Finally, two prediction algorithms are proposed, using the PPM with optimal model order and system level simulations demonstrate the improvement in packet loss and throughput due to these algorithms.

Error Analysis of a Conceptual Cloud Doppler Stereoradar with Polarization Diversity for Better Understanding Space Applications

AbstractAn error budget analysis is performed for retrieval of along-track winds based on the design of a spaceborne Doppler radar using polarization diversity. The analysis is conducted within the framework of a case study of an Atlantic hurricane. The proposed concept consists of either a Ka-band or W-band stereoradar mounted on an LEO satellite equipped with both nadir- and forward-viewing beams and with an optional cross-scanning capability. Such a radar design is intended for observing the microphysical and dynamical structures of cloud systems, including disturbed mesoscale convective systems. Because of the high winds involved in such weather phenomena and because of the Doppler fading introduced by platform motion, polarization diversity is adopted. The simulation framework enables a breakdown of the Doppler velocity measurement error budget into its most important components, that is, nonuniform beamfilling, multiple scattering, and inherent signal noise. The impact of each of these error terms on the total error depends on the adopted integration length, the number of scanned tracks, and the specifics of the radar. This allows for optimally selecting an integration length suitable for minimizing the total rms velocity error. The analysis shows that the use of a large antenna could achieve impressive measurement accuracy of the along-line-of-sight wind velocities. Notably, this would be the case for integration lengths longer than 3 km, even when carrying out cross-track scanning for up to 17 separate tracks. Examples of retrieved along-track wind fields also reveal that the large antenna configurations are capable of identifying and quantifying the foremost dynamic features (e.g., vertical wind shear and convergence/divergence regions).

부반송파 할당방식에 따른 OFDM 시스템의 성능 분석

OFDM technique uses multiple sub-carriers for the data transmission. Therefore, inter carrier interference is generated because of nonlinear high power amplifier and carrier frequency offset. Wireless OFDM transmission over Doppler fading channels also causes inter carrier interference. The interference increases the bit error rate in receiver. Sub-carrier allocation methods in LTE and WiMAX standards are different. The performance of OFDM systems using different sub-carrier allocation, gauged by the bit error rate, is analyzed considering the nonlinear high power amplifier, carrier frequency offset and Doppler fading channels.

Performance Analysis of Carrier Frequency Offset Estimation Techniques for OFDM Systems

In this study we will study the effects of Carrier Frequency Offset (CFO) upon Signal to Noise Ratio (SNR) for an OFDM system. We will also present performance analysis of two CFO estimation schemes: time domain Cyclic Prefix (CP) based estimation scheme and frequency domain based Moose scheme, Simulation results show that frequency domain based scheme can achieve better performance in term of Mean Square Error (MSE) than the time domain CP based scheme. Both schemes are analyzed under Doppler fading.

DOMUS: DOppler MUltiple-Scattering Simulator

Multiple scattering (MS) strongly affects CloudSat's W-band cloud-profiling radar (CPR) reflectivity when the satellite is overpassing moderate and heavy precipitation systems. With the upcoming deployment of W-band Doppler radars in space-Earth Clouds, Aerosols, and Radiation Explorer's CPR in primis-and the goal of retrieving vertical motions within convective systems, there is an urgent need to assess the impact of MS onto the Doppler signatures. A Monte Carlo code capable of simulating the reflectivity enhancement due to higher orders of scattering has been extended to include the Doppler effects. This paper presents the main guidelines for the inclusion of the Doppler analysis into the Monte Carlo scheme. To our knowledge, this simulator is the first one capable of simulating realistic Doppler signals in the presence of MS. The case studies are first presented in uniform beam-filling conditions for the profiles extracted from a cloud-resolving model simulation of deep convection (i.e., 1-D profiles are used to characterize a stratified atmosphere). The simulations demonstrate that, at ranges where MS contributions affect the overall radar signal, two main features appear as the following: 1) The Doppler spectrum tends to broaden with increasing MS enhancement, adding up to the single-scattering (SS) Doppler fading due to the satellite motion; and 2) the mean Doppler of the backscattered signal departs from the mean Doppler determined by the combined effect of the vertical-wind and hydrometeor-terminal velocities at all range bins below the altitude where the MS contribution significantly overcomes the SS. The simulator can be run in nonuniform-beam-filling conditions as well (i.e., a 3-D field is used to characterize the atmosphere at scales smaller than the radar resolution). With its cutting-edge capabilities, it provides a unique tool for the evaluation of the performances of the upcoming high-frequency spaceborne Doppler radars.

Phase ambiguity diminishing space-time trellis codes

Phase ambiguities affect any joint decoding and channel estimation algorithm over fading channels, making codes that mitigate this problem of considerable interest. This work presents a design that transforms a Space-Time Trellis Code (STTC), prone to carrier phase ambiguities, into a Phase Ambiguity Diminishing STTC (PAD-STTC) that is robust to such impairment. A PAD-STTC can be considered as a Multiple STTC (MSTTC) with special constellation mappings. The design criteria for these mappings are developed in this work by trading among the PAD property, diversity, and coding gain. Two PAD-STTC structures termed PAD1-STTC and PAD2-STTC are proposed, where PAD2-STTC increases the number of states over the constituent STTC. Computer simulation results indicate that PAD-STTCs can solve the phase ambiguity problem and provide a larger diversity gain than the constituent STTC over high Doppler fading channels. Moreover, with the same constituent STTC, the PAD2-STTC outperforms the PAD1-STTC at the expense of increased complexity.

Switching rates of two-branch selection diversity in correlated Doppler fading

The switching rate of a correlated dual branch selection diversity combiner in Rayleigh and Rician fading is derived in noise-free conditions. Balanced and unbalanced branches are considered. Numerical results are presented for a space-diversity system with horizontally spaced omnidirectional antennas at a mobile station.

Data detection and kalman estimation for multiple space-time trellis codes

This work presents a joint channel estimation and data detection algorithm over high Doppler fading channels, for space-time trellis codes (STTCs) having more than one code vector per trellis branch. Such codes are referred to as multiple STTCs (MSTTCs). This algorithm belongs to the per-survivor processing (PSP) family, where a survivor sequence associated with a trellis state can be considered as the data sequence aiding per-path channel estimation in a joint detection and estimation procedure.We propose a smoothed data detection technique that utilizes past, present and future received symbols to increase the probability of the survivor being a correct data sequence. A symbol by symbol maximum a posteriori probability (MAP) criterion, with a fixed delay D, is employed for data detection, combined with a Kalman Predictor (KP) for channel estimation. This novel algorithm, called smoothed data detection and Kalman estimation (SDD-KE), provides a significant performance gain when D > 0 over D = 0, with a linear increase in complexity when D increases. Comparison with the delayed mixture Kalman filtering (D-MKF) technique shows that the SDD-KE algorithm provides important performance and complexity advantages.

Space-frequency-Doppler coded OFDM over the time-varying Doppler fading channels

In this paper, space-frequency-Doppler coded OFDM (SFDO-OFDM) scheme over the time-varying Doppler fading channels via the time-frequency duality is proposed. Based on the basis expansion model (BEM) and the time-frequency duality, through the circulant matrix diagonalized processing, the nonlinear time-varying Doppler fading channel is dually converted to the virtual frequency-selective linear channels. With OFDM module, subgrouping the subcarriers in OFDM through the block matrix method and fatherly general complex orthogonal coding (GCOD) on each corresponding block subcarriers, SFDO-OFDM codes for the general multiple input multiple output (MIMO) is thus constructed. And concatenating it with the signal constellation precoding, full maximum diversity gains including the inherent Doppler fading are achieved. Theoretical analysis and corroborating simulation results demonstrate that, comparing with existing Doppler coding alternatives, the proposed scheme can effectively and robustly combat the Doppler fading with high bandwidth efficiency and even lower bit error ratio (BER).

A low-complexity fading simulator for underwater acoustic communication channels

Shallow water is an extremely complicated place for communications, and realistic channel models can provide an effective tool for system design. This paper proposes a low-complexity simulator that generates channel impulse responses for broadband frequency-selective acoustic channels. It uses a sum-of-sinusoids (SoS) method to generate Doppler fading impulse responses for wide-sense stationary correlated scattering (WSSCS) Ricean fading channels. The line-of-sight component is obtained by G0(t,τ0)=A0exp[2π(1+ε0)ftcosα0+φ0δ(τ−τ0 ) and the scatterers by Gs(t,τ)=∑n=1NAnexp{j[2πft∑m= 1M(1+εm)cosαm,n+ψnδ(τ−τn), where the random parameters An, αm,n, ψn, and τn are, respectively, the attenuation coefficients, impinging angles, initial phases, and path delays. The Doppler scaling is (1+εm)f, where f is the carrier frequency and εm depends on the speed of sound propagation in water and the velocities of the transmitter, receiver, and wind. Nonuniform probability density functions are selected for impinging angles αm,n to account for the nonisotropic waveguide effect in shallow-water channels. The proposed simulator is a general stochastic model that does not require site-specific information. It only uses 6N+4 random variables and requires O(N) multiplications and additions, where N is on the order of several tens. This low-complexity simulator can be easily implemented by real-time digital signal processors.

Doppler Velocity from Sea Surface on the Spaceborne and Airborne Weather Radars

The Doppler operation applied to the sea surface is studied for the spaceborne and airborne weather radars. In the space mission, a small amount of beam misalignment can cause large contamination of a platform velocity into a measured Doppler velocity. In this paper, theoretical bases for retrieving water droplet velocity via the Doppler signal from a sea surface are considered for pulse-pair operation in the nadir direction. On a fast-moving platform, Doppler fading is so crucial that the finite beamwidth of an antenna is taken into account in the formalism based on a scalar Kirchhoff theory. Correlation signals are derived for both the coherent and incoherent scatterings that have substantial differences in Doppler velocity. Furthermore, the incoherent Doppler velocity is shown to include a path-difference term, which is characteristic of the incoherent surface scattering, set apart from the incoherent body scattering. Applying the retrieval methodology to the Doppler signal from a ground surface is also discussed.

On uplink cdma cell capacity: mutual coupling and scattering effects on beamforming

It has been shown that code-division multiple-access (CDMA) systems that employ digital beamforming and base station antenna arrays have the potential to increase capacity significantly. Therefore, accurate performance prediction of such systems is important. We propose to take the electromagnetic behavior of the base station antenna array into account, as well as its impact on wireless channel propagation. Specifically, the wideband channel introduces scattering, while the mobile environment causes Doppler fading, which in turn degrades power controllability. We develop a more accurate performance analysis of antenna arrays, where the performance degradation in digital beamforming, due to the combination of mutual coupling, scatter and imperfect power control, and its impact on uplink CDMA system capacity is quantified. A Rayleigh fading amplitude with varying angle-of-arrival spread is assumed, and maximum signal-to-noise ratio beamforming weights are used. These weights are further correlated with mutual coupling at the base station array. Despite the degradation due to the combination of mutual coupling, scattering, and imperfect power control, significant capacity increases are possible.

On uplink CDMA cell capacity: mutual coupling and scattering effects on beamforming

It has been shown that code-division multiple-access (CDMA) systems that employ digital beamforming and base station antenna arrays have the potential to increase capacity significantly. Therefore, accurate performance prediction of such systems is important. We propose to take the electromagnetic behavior of the base station antenna array into account, as well as its impact on wireless channel propagation. Specifically, the wideband channel introduces scattering, while the mobile environment causes Doppler fading, which in turn degrades power controllability. We develop a more accurate performance analysis of antenna arrays, where the performance degradation in digital beamforming, due to the combination of mutual coupling, scatter and imperfect power control, and its impact on uplink CDMA system capacity is quantified. A Rayleigh fading amplitude with varying angle-of-arrival spread is assumed, and maximum signal-to-noise ratio beamforming weights are used. These weights are further correlated with mutual coupling at the base station array. Despite the degradation due to the combination of mutual coupling, scattering, and imperfect power control, significant capacity increases are possible.

Performance degradation of differential space-time modulation in fast frequency flat fading Rayleigh MIMO channels

The performance degradation of differential unitary space-time modulation of any data rate, RD, due to time selectivity is quantified for any number of transmit antennas, M, for fast frequency flat fading Rayleigh multiple-input multiple-output (MIMO) channels. The degradation bounds related to the Doppler fading parameter are tabulated and demonstrated for varying RD and M.