Pilot Positions Research Articles

Block Distributed Compressive Sensing-Based Doubly Selective Channel Estimation and Pilot Design for Large-Scale MIMO Systems

The doubly selective (DS) channel estimation in the large-scale multiple-input multiple-output (MIMO) systems is a challenging problem due to the large number of the channel coefficients to be estimated, which requires unaffordable and prohibitive pilot overhead. In this paper, first we conduct the analysis about the common sparsity of the basis expansion model (BEM) coefficients among all the BEM orders and all the transmit–receive antenna pairs. Then, a novel pilot pattern is proposed, which inserts the guard pilots to deal with the intercarrier interference under the superimposed pilot pattern. Moreover, by exploiting the common sparsity of the BEM coefficients among different BEM orders and different antennas, we propose a block distributed compressive sensing-based DS channel estimator for the large-scale MIMO systems. Its structured sparsity leads to the reduction of the pilot overhead under the premise of guaranteeing the accuracy of the estimation. Furthermore, taking consideration of the block structure, a pilot design algorithm referred to as block discrete stochastic optimization is proposed. It optimizes the pilot positions by reducing the coherence among different blocks of the measurement matrix. Besides, a linear smoothing method is extended to large-scale MIMO systems to improve the accuracy of the estimation. Simulation results verify the performance gains of our proposed estimator and the pilot design algorithm compared with the existing schemes.

Dynamic wavelet-based pilot allocation algorithm for OFDM-based cognitive radio systems

In a cognitive radio system, the goal is to make better use of the radio electric spectrum, allowing non-licensed users access to those currently unused electromagnetic bands assigned to licensed users (LUs). This can be achieved using OFDM, where the non-licensed users must select the temporarily available subcarriers and turn off those subcarriers used by LUs in order to avoid interference. Hence, only a subset of the subcarriers can be used for data or pilot tone transmission. To this end, some pilot allocation algorithms have been proposed for this dynamic scenario, but they are designed in such away that an equispaced pilot placement is respected (as much as possible) while minimizing the mean squared error of the channel estimate. Nevertheless, this equispaced placement can lead to the use of an increased number of pilots in order to achieve a good channel estimation. In this work, a new pilot allocation algorithm based on wavelet transform is presented. The proposed algorithm uses the discrete wavelet transform to analyze the previous channel state information, taking the knowledge of the available subcarriers into account to provide a suboptimal solution for the pilot positions. This solution leads to a non-equispaced pilot placement, which improves the channel estimation and consequently, the system performance. Likewise, the introduced algorithm allows a reduction of the number of necessary pilots, which aids in increasing the data rate. Finally, simulation results corroborate the effectiveness of the algorithm in dynamic channel scenarios.

Flight Test Assessments of Pilot Workload, System Usability, and Situation Awareness of Tasar

Traffic Aware Strategic Aircrew Requests (TASAR) is a NASA-developed on-board automation concept intended to identify trajectory improvement opportunities clear of known traffic, weather, and airspace restrictions prior to the aircrew initiating a trajectory-change request to Air Traffic Control (ATC). The software implementation of the TASAR concept is the Traffic Aware Planner (TAP). A flight trial was conducted with commercial airline pilots flying in a Piaggio Avanti flight test aircraft, with the TAP software installed onboard. This paper focuses on the results of the subjective assessments of pilots collected during this flight trial, specifically, pilot workload, situation awareness, and system usability. These data were subjected to statistical analyses, and the results generally indicate the following: there were no statistical differences for any measure as a function of pilot position in the aircraft; pilot cognitive workload was reported as low; and system usability ratings were reported as high. Generally, ratings of comprehensibility, usefulness, and usability of TAP’s human machine interface (HMI) and features were high. These results will be used to further refine and improve the capabilities and features of the TAP HMI in preparation for operational use with NASA partner airlines.

Novel Pilot Position Detection for SC-FDE Systems With Frequency Domain Pilot Multiplexing Technique

Frequency domain pilot multiplexing technique (FDPMT) has recently emerged as an appealing technique for channel estimation in single-carrier frequency domain equalization systems as it achieves high spectral efficiency at the expense of tiny signal distortion. In FDPMT, pilot positions are dynamically selected to achieve a relatively low level of signal distortion, rendering pilot position detection (PPD) indispensable at the receiver. In this letter, by exploiting both the prior known pilots and the statistical information of the data tones, we propose a novel PPD scheme based on the maximum-likelihood criterion. Simulation results show that our proposed PPD scheme significantly improves the PPD accuracy and outperforms the existing counterparts in terms of bit error rate.

Novel Pilot Position Selection and Signal Reconstruction Methods for Frequency Domain Pilot Multiplexing Techniques of SC-FDE

The frequency-domain pilot multiplexing technique (FDPMT) has recently emerged as an appealing technique for channel estimation in single-carrier frequency-domain equalization (SC-FDE) systems due to the high spectral efficiency it promises. However, the performance of the FDPMT system is significantly affected by the signal distortion level, which depends on the pilot position selection (PPS) strategy at the transmitter and the signal reconstruction (SR) ability of the receiver. In this paper, the relationship between the interference pattern and the loss of equally spaced tones is discovered. By exploiting this relationship, we propose a novel PPS method and a novel SR method. The proposed PPS method, which is based on the minimization of the most severe intersymbol interference (ISI) experienced by those subblocks decimated from the transmitted signal block, dispenses with a priori knowledge of the channel statistics and imposes very low computational cost to the transmitter. The proposed SR method, which is based on the maximum-likelihood criterion, can be carried out subblock by subblock in parallel and sample by sample within each subblock, saving the computational complexity and time consumption of the receiver. Simulation results in terms of bit error rate (BER) corroborate the superiority of both proposed methods over their existing counterparts in various channels of interest.

Novel Pilot Design and Signal Detection for SC-FDE Systems With Frequency-Domain Pilot Multiplexing Technique

The performance of single-carrier frequency-domain equalization systems with the frequency-domain pilot multiplexing technique is affected by the level of signal distortion, which highly depends on the pilot position selection (PPS) scheme at the transmitter and the signal detection (SD) scheme at the receiver. In this letter, by exploring the relationship between distorted symbols and decision boundaries, we propose a novel PPS scheme to confine the distorted symbols within their designated decision regions, which dispenses with a priori knowledge of the channel. Based on the maximum-likelihood criterion, we also propose a novel SD scheme to avoid error propagation, which operates on a symbol-by-symbol basis. Simulation results on bit error rate show that our proposed PPS scheme achieves almost the same performance as the optimal one and our proposed SD scheme outperforms the existing one in additive white Gaussian noise channels; in addition, both proposed schemes outperform their existing counterparts in frequency-selective Rayleigh fading channels.

Improvement of LTE Downlink Channel Estimation Performance by Using an Adaptive Pilot Pattern

This paper proposes an adaptive pilot pattern to improve channel estimation performance for LTE downlink system with high mobility. The downlink pilot positions are predefined in the time and frequency domain with fixed pilot pattern in LTE standard. However, that pilot structure is not efficient in a fast time varying channel, and leads to a decrease of channel estimation performance. The authors propose and evaluate the performance of LTE downlink channel estimation using an adaptive pilot scheme to optimally use pilot tones over time varying channels. It is shown that only seven bits of additional wide-band feedback per frame and per user are required to optimally support adaptive pilot pattern. Simulation results show that the proposed method allows high performance in terms of throughput and channel estimation error. This analysis shows that LTE downlink throughput could be increased over 4%.

Robust Pilot Detection Techniques for Channel Estimation and Symbol Detection in OFDM Systems

In this letter, we propose new frequency-domain pilot-multiplexing techniques (FDPMTs) for the channel estimation and equalization in orthogonal frequency-division multiplexing (OFDM) systems. A robust and effective pilot insertion and detection scheme is devised thereby. These pilot positions are optimally selected to minimize the distortion of the transmitted time-domain signal caused by the subcarrier-removal at the corresponding pilot positions. Besides, three different blind pilot-detection techniques are designed at the receiver without any a priori knowledge of the pilot positions, and the distorted data symbols can thus be iteratively reconstructed. Rigorous theoretical analysis and Monte Carlo simulation results both demonstrate that our proposed new OFDM system using dynamical pilot positions is more robust than the conventional OFDM system using the fixed pilot positions over multipath fading channels.

Time‐frequency MSE analysis of linear channel estimation methods for the LTE downlink

AbstractOrthogonal frequency‐division multiplexing (OFDM) is widely used in modern communication systems thanks to the ease of its implementation as well as to a simple design of channel estimation methods. Pilot‐based schemes are commonly employed to recover channel information and implement efficient channel equalisation. From the symbols transmitted over the pilot carriers, the channel coefficients for a whole OFDM are estimated. In this paper, we propose a method to compute in a closed form the mean square estimation error of different linear approximation and interpolation algorithms in the time‐frequency domain. Specifically, by exploiting the linear relationship between the estimated channel coefficients for the whole frame and those relative to the pilots, the proposed approach permits to obtain the mean square error relative to all OFDM symbols in a frame. The method can be used for any linear frequency/time interpolation/approximation method and for any choice of pilot positions and channel statistics. Thanks to our derivations, several pilot‐aided channel estimation and interpolation algorithms can be compared without resorting to burdensome computer simulations. In order to demonstrate the effectiveness of the method, the downlink of the long term evolution system is taken as a case study: some popular channel estimators have been selected and the best strategy in a realistic wireless scenario is investigated. The performance of the method is also verified by computing, by means of simulations, bit error rate (BER) curves for the long term evolution (LTE) multiple input multiple output (MIMO) downlink, using 2 × 2 and 4 × 4 Alamouti space‐frequency coding configurations, in both uncoded and coded communications. Copyright © 2014 John Wiley & Sons, Ltd.

Interference cancellation aided channel estimation for OFDM/OQAM system

Orthogonal frequency division multiplexing offset-QAM (OFDM/OQAM) is a promising multicarrier transmission technique for its merits such as operating without cyclic prefix (CP) and flexibility of introducing well time-frequency localized pulse shapes. However, under dispersive channel condition, the intrinsic inter-symbol and inter-carrier interference arises in OFDM/OQAM system. Especially, the surrounding data symbols will cause interferences to the pilot symbols, leading to channel estimation error and degradation of system performance. In this paper, an efficient pilot structure and a channel estimation method are proposed for OFDM/OQAM system. The basic principal is to rebuild the transmitted symbols around the considered pilot position through pre-decision, then calculate and cancel the interference from the received signal. Analysis and simulation results show that the proposed scheme outperforms conventional ones with respect to both channel estimation accuracy and bit error rate (BER) performance while with less pilot consumption.

A new pilot design for NC‐OFDM systems under power constraints

ABSTRACTThe pilot design is one of the key issues that has to be solved in order to improve the performance of non‐continuous orthogonal frequency division multiplexing (NC‐OFDM) systems in cognitive radio context. Equally‐spaced and equally‐powered pilot tones that are the optimal pilot design for conventional OFDM systems cannot be applicable to NC‐OFDM systems with the non‐continuous(NC) spectrum. In NC‐OFDM systems, the secondary users(SUs) communicate to each other by spectrum mapping the primary users(PUs)’ band to protect the PUs from interference. So the SUs’ available subcarriers’ positions and power allocation will be changed with PUs’ active state and interference constraints. The NC available spectrum and power distribution bring new challenges on system design, which make the pattern of pilots in the frequency domain vulnerable. Several recent researches have studied the optimal or suboptimal pilot designs under equally‐powered assumption. In this paper, we only focus on the comb‐type pilot design under uneven power distribution hypothesis for the NC‐OFDM transmission. Firstly, based on maximising each available subcarrier’ power under PUs’ interference temperature constraints, the pilot power constraints have been discussed, then the formula between pilot positions and pilot powers have been derived by minimising the channel estimation mean square error; moreover, a suboptimal pilot design under the pilot power constraints is also presented. The proposed pilot design cannot only obtain better channel estimation performance compared with previous pilot designs, but also consider the PUs’ communication reliability. Simulation results demonstrate the effectiveness of the proposed method. Copyright © 2013 John Wiley & Sons, Ltd.

Comparison of different channel estimation approaches for Block‐IFDMA

AbstractIn this paper, channel estimation for Discrete Fourier Transform (DFT) precoded Orthogonal Frequency Division Multiple Access (OFDMA) with block‐interleaved subcarrier allocation per user, which is denoted as Block‐Interleaved Frequency Division Multiple Access (B‐IFDMA), is considered. Recent investigations are presented for new pilot insertion methods which exhibit different impact on the Peak‐to‐Average Power Ratio (PAPR) of the transmit signal and offer different possibilities of positioning the pilots in frequency domain. Each of these pilot insertion methods is combined with a corresponding algorithm to estimate the Channel Variations in Frequency Domain (CVFD). The Channel Variations in Time Domain (CVTD) are estimated by an improved Decision Directed Channel Estimation (DDCE) with joint iterative Wiener filtering which reduces the error propagation by considering certain neighbouring symbols jointly in each iteration step. The DDCE is compared to a Wiener interpolation filter for the estimation of CVTD. The combinations of pilot insertion method, estimation algorithm for CVFD and CVTD are compared in terms of the PAPR and the Mean Square Error (MSE) performance. Thereby, the impact of different pilot positions in frequency domain and time domain is investigated. Copyright © 2010 John Wiley & Sons, Ltd.

A simple pilot position detection technique for channel estimation of SC-FDE

This letter investigates the frequency domain pilot multiplexing technique (FDPMT) for channel estimation of single-carrier frequency domain equalization (SC-FDE) and proposes a simple pilot position detection technique to blindly estimate the pilot positions in the frequency domain by using the noise subspace. Simulation results show that the proposed technique is robust to multipath fading channels with exponential power delay profile.

Pilot Position Selection and Detection for Channel Estimation of SC-FDE

In this letter, we propose a simple pilot position selection/detection technique for channel estimation of single- carrier frequency domain equalization (SC-FDE). Unlike the conventional channel estimation techniques such as frequency domain superimposed pilot technique (FDSPT), the proposed scheme selects the pilot positions to minimize the distortion of original signals caused by the loss of useful data tones in frequency domain. The corresponding receiver structure is also presented, where the pilot positions are blindly detected and the distorted data symbols are iteratively reconstructed. Simulation results show that the proposed system gives better BER performance than the FDSPT and approaches the lower bound of SC-FDE.

Frequency domain pilot multiplexing technique for channel estimation of SC-FDE

A frequency domain pilot multiplexing technique for channel estimation of single-carrier frequency domain equalisation (SC-FDE) is proposed. The proposed scheme selects pilot positions to minimise the pre-imposed distortion caused by the loss of useful data tones. The corresponding receiver structure is also presented, where the pilot positions are blindly detected and the distorted data symbols are iteratively reconstructed. Simulation results show that the proposed system approaches the lower bound of SC-FDE.

Noise reduction for channel estimation based on pilot-block averaging in DVB-T receivers

This paper proposes a noise-reduction method to improve the channel-estimation performance in digital video broadcasting-terrestrial (DVB-T) receivers. In DVB-T, the channel estimation is carried out through two processes of pilot signal estimation and channel coefficient interpolation and thus the performance of the channel estimation is seriously affected by the accuracy of the interpolation. Since the accuracy of the interpolation depends on signal-to-noise ratio, it is important to reduce noise existing at the reference coefficients for the interpolation. To achieve the reduction of noise, we use pilot-insertion type approximation in four consecutive OFDM symbols, which construct one block with the same channel coefficients at the same pilot positions. Considering the fade rate of channels, we average the estimated reference, channel coefficients corresponding to the same subcarriers in two or more blocks. Computer simulation shows that the proposed method improves the estimation performance by more than 1 dB in terms of the symbol error rate obtained after equalization.

Tracking of time misalignments for OFOM systems in multipath fading channels

In this paper, new time offset tracking techniques for OFDM are proposed without devoting specific pilot symbols for synchronization. This can be attained since, in coherent OFDM, channel estimation is usually performed employing pilot symbol assisted modulation (PSAM), based on inserting known symbols in the time-frequency grid. We show how this two dimensional embedded signalling can also be used for tracking time-frequency offsets. A two-stage procedure is proposed, focusing this work on the time correction step. The ML estimator has been derived, but though optimal, may not be practical due to its complexity. Hence, a frequency-domain low-complex estimator has been proposed, based on only the samples at pilot positions. Also, the stability of the generated estimate can be improved by averaging over a few number of OFDM symbols. This estimator has been proven to be efficient against time dispersive channels.

Senior clinical nurse specialist pilot position in Hong Kong.

This article reports findings of a consultancy project commissioned by Hospital Authority, Hong Kong, to explore the scope of service, role competence, and contribution to patient service of senior-position clinical nurse specialists. Three clinical nurse specialists were selected for this project. The consultant guided and monitored effects of initiatives proposed by each clinical nurse specialist. The evaluation strategies were dependent on the nature of the projects proposed by the senior clinical nurse specialist. Data were collected through nonparticipant observation and interviews to address the overall consultancy project aims. This study shows that the role components of clinical nurse specialists were common across different countries. The value of the clinical nurse specialist to initiate strategies to promote cost-effective and quality care were confirmed, but the career development of these specialists remains a concern in a healthcare environment with constraining resources.

Frequency offset correction for coherent OFDM in wireless systems

In this paper, a new frequency offset correction approach for OFDM is proposed to track a wider range of offsets than conventional proposals, without devoting specific pilot symbols for synchronization. This can be attained since, in coherent OFDM, channel estimation is usually performed employing pilot-symbol assisted-modulation (PSAM), based on inserting known symbols in the time-frequency grid. We show how this two-dimensional embedded signalling can also be used for frequency synchronization, with no need of additional synchronization symbols which reduce the data-rate. The ML estimator has been derived, but also, a frequency-domain low-complex estimator has been proposed, based on only the samples at pilot positions. Additionally, a novel decision-guided strategy is presented, working jointly with the channel-estimation signalling and reducing the variance of the estimation considerably. Also, the stability of the scheme is shown to get significantly better by averaging over a few OFDM symbols. This estimator is efficient against AWGN and multipath fading channels. Simulation results demonstrate that the presented frequency offset correction scheme provides the accuracy required in practical applications for OFDM wireless systems.

Retaining Women in Collegiate Aviation by Implementing Learning Style Considerations

Women are clearly underrepresented in aviation. Research must be accomplished to determine which factors influence women, once they have indicated a serious interest in an aviation career, to stay in collegiate aviation programs or to leave. Addressing the issue of women’s retention in aviation is one way to help address the growing commercial pilot shortage, while moving toward gender equity in this critical, national industry. Projected shortages in the commercial pilot population, coupled with the low representation of women in career pilot positions, suggest that aviation education and training institutions should re-examine the structure and organization of the aviation knowledge transfer process. Classroom enhancements could improve education methods to make them more efficient from the perspectives of increased knowledge retention, improved application to broader subjects, and reduced loss to attrition of viable pilot candidates to enter the commercial pilot workforce. This study examines how aviation education can best serve the aviation student’s learning style needs. The study looks at learning style theory, from the viewpoint of the wide diversity of aviation learners who are dominantly visual, auditory, or hands-on, tactile, or kinesthetic learners, and how women’s learning styles are pivotal to their success and retention in collegiate aviation. By exploring how people learn best, and then providing those learners with the tools to maximize their dominant learning styles, the next generation of pilots, both women and men, should be better prepared to enter the aviation industry and help reduce the projected commercial pilot shortages.