Pilot Arrangement Research Articles

OFDM channel estimation and data detection with superimposed pilots

AbstractWe propose three iterative superimposed‐pilot based channel estimators for Orthogonal Frequency Division Multiplexing (OFDM) systems. Two are approximate maximum‐likelihood, derived by using a Taylor expansion of the conditional probability density function of the received signal or by approximating the OFDM time signal as Gaussian, and one is minimum‐mean square error. The complexity per iteration of these estimators is given by approximately O(NL2), O(N3) and O(NL), where N is the number of OFDM subcarriers and L is the channel length (time). Two direct (non‐iterative) data detectors are also derived by averaging the log likelihood function over the channel statistics. These detectors require minimising the cost metric in an integer space, and we suggest the use of the sphere decoder for them. The Cramér–Rao bound for superimposed pilot based channel estimation is derived, and this bound is achieved by the proposed estimators. The optimal pilot placement is shown to be the equally spaced distribution of pilots. The bit error rate of the proposed estimators is simulated for N = 32 OFDM system. Our estimators perform fairly close to a separated training scheme, but without any loss of spectral efficiency. Copyright © 2011 John Wiley & Sons, Ltd.

MIMO-OFDM Pilot Placement Algorithms for Wideband Indoor Communications

To facilitate coherent detection of orthogonal frequency division multiplexed transmissions, pilot symbols can be transmitted in some of the subcarriers. Their placement to minimise the mean squared error in channel estimation is considered. Such problems have been widely addressed previously for channels subject to wide sense stationary uncorrelated scattering processes. While the stationarity assumption is usually realistic, indoor wideband channels, for example, are subject to significantly correlated scattering processes. Furthermore, practical multiple antenna transmitters/receivers are bound to exhibit spatial correlations. This work considers pilot placement optimisation in a multiple antenna setting subject to arbitrary correlations in the delay and spatial domains. Noting that the naive approach incurs a prohibitive complexity, low complexity greedy solutions are developed. It is further proven that the conventional equi-spaced placement becomes optimal at high SNR even with correlations, when the correlation matrices are of full rank. A measurement campaign is conducted to estimate real-world channel statistics in an indoor wideband environment, and observe the applicability of developed algorithms. It is confirmed that practical channels are correlated to the extent of exhibiting rank deficiencies, and that the proposed placement algorithms result in improved channel estimation performance.

Pilot Placement for Time-Varying MIMO OFDM Channels with Virtual Subcarriers

The rapid time-variation of a fading multipath environment can impair the performance of multiple-input multiple-output orthogonal frequency division multiplexing (MIMO OFDM). This paper proposes a pilot placement method for MIMO OFDM systems under time-varying channels with the guard band. The time-varying channel is described by complex exponential basis expansion model (BEM). We discuss the least square (LS) channel estimation to obtain the minimum mean square error (MSE) and derive the pilot allocation that can satisfy the minimum MSE with regard to guard band in time-varying channels. It is shown that optimal pilot clusters can distribute non-uniformly in frequency domain and minimize the MSE. We generalize our scheme over G OFDM symbols and compare it with comb pilots. It is demonstrated that the proposed approach is more effective than previous work. Simulation results validate our theoretical analysis.

Underwater acoustic channel estimation based on sparse recovery algorithms

The authors consider underwater acoustic (UWA) channel estimation based on sparse recovery using the recently developed homotopy algorithm. The UWA communication system under consideration employs orthogonal frequency-division multiplexing (OFDM) and receiver preprocessing to compensate for the Doppler effect before channel estimation. The authors first extend the original homotopy algorithm which is for real-valued signals to the complex field. The authors then propose two enhancements to the sparse recovery-based UWA channel estimator by exploiting the UWA channel temporal correlations, including the use of a first-order Gauss–Markov model and the recursive least-squares algorithm for channel tracking. Moreover the authors propose a scheme to optimise the pilot placement over the OFDM subcarriers based on the discrete stochastic approximation. Simulation results show that the homotopy algorithm offers faster and more accurate UWA channel estimation performance than other sparse recovery methods, and the proposed enhancements and pilot placement optimisation offer further performance improvement.

Reducing ICI effect in OFDM system using low‐complexity Kalman filter based on comb‐type pilots arrangement

AbstractIn OFDM systems, time‐varying channels destroy orthogonality between subcarriers causing Inter‐Carrier Interference. To reduce this effect, a Kalman filter, as a benchmark, is used for channel estimation, based on comb‐type pilot arrangements of the OFDM system. An advantage of comb‐type pilot arrangements in channel estimation is the ability to track the variation of the channel caused by Doppler frequency. Kalman method has been proposed to estimate the channel frequency response (CFR) at the pilot locations, then CFR, at data subchannels, is obtained by means of interpolation between estimates at pilot locations. The low‐complexity Kalman method is introduced to reduce the complexity of the system while achieving the same BER/SNR. Different types of interpolations have been also compared such as Low‐pass, Spline‐cubic and Linear interpolation methods. The BER/SNR performance of BPSK modulation schemes are considered for time varying Rayleigh fading channels. Our results has shown that the low‐complexity Kalman estimation, used with the pilot arrangement and a suitable interpolation method, gives almost the same performance as that of the Kalman method specially for low SNR values and hence the effect of Doppler shift effect is controlled. Copyright © 2010 John Wiley & Sons, Ltd.

Joint Channel Estimation and PTS to Reduce Peak-to-Average-Power Radio in OFDM Systems Without Side Information

In this letter, we propose a novel scheme to reduce the PAPR of OFDM signals, and its key idea is that the side information of partial transmit sequence is considered as a part of channel frequency response. Therefore, the proposed scheme does not need to reserve bits for submitting side information, resulting in an increase in the data rate. Furthermore, we propose a novel pilot arrangement, in which pilot tones are independently inserted into each subblock. Simulations show that the proposed scheme with the novel pilot arrangement could offer good performances of bit error rate and PAPR reduction.

Channel Estimation Based in Comb-Type Pilots Arrangement for OFDM System over Time Varying Channel

Orthogonal Frequency Division Multiplexing (OFDM) has been recently applied widely in wireless communication systems, due to its high data rate, transmission capability with high bandwidth, efficiency and its robustness to multipath delay .Channel estimation is an essential problem in OFDM system. Pilot-aided channel estimation has been used; a good choice of the pilot pattern should match the channel behavior both in time and frequency domains. We explored comb pilot arrangements. The advantage for comb type pilots arrangement in channel estimation is the ability to track the variation of the channel caused by doppler frequency, it is observed that the doppler effect can be reduced, and so this will increase the system mobility. Kalman and Least Square (LS) estimators have been proposed to estimate the Channel Frequency Response (CFR) at the pilots location, then CFR at data sub channels are obtained by mean of interpolation between estimates at pilot locations. Different types of interpolations have been used such as; low pass interpolation; spline cubic interpolation and linear interpolation. Kalman estimation has better performance than LS estimation. The estimators perform about the same for SNR lower than 10 dB. The performances of all schemes have been compared by finding Bit Error Rate (BER), where BPSK modulation scheme was used.

Channel Estimation in Wireless OFDM Systems With Irregular Pilot Distribution

This paper addresses pilot-assisted channel estimation for wireless orthogonal frequency division multiplexing (OFDM) systems with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irregular</i> pilot arrangements. Using nonuniform sampling techniques, we propose a novel channel estimator that is based on conjugate gradient iterations and features very low computational complexity. We investigate the impact of the pilot arrangement on the channel estimation mean square error and we provide a heuristic stopping criterion for early termination of the conjugate gradient iterations. One-dimensional and two-dimensional channel estimator implementations and applications of our method to multiple-antenna and multiuser OFDM systems are discussed. Extensive numerical simulations corroborate that the proposed method performs similarly to computationally much more expensive minimum mean square error estimators.

Frequency Domain Channel Estimation for Cooperative Communication Networks

In this correspondence, we deal with the problem of channel estimation in amplify-and-forward (AF) wideband cooperative relay-based networks. Two types of frequency domain channel estimation techniques are proposed and analyzed. First, a training based technique is presented for which an optimal pilot placement and power allocation strategy is described. Second, hybrid techniques are introduced in which both training as well as channel output correlation information is utilized for channel estimation. A theoretical performance study of the proposed algorithms is presented and closed-form expressions for the mean squared channel estimation error are provided. The presented theoretical analysis is verified via extensive Monte Carlo simulations.

A practical noise mitigation scheme for DVB-T/H system using delay spread estimation

In order to improve receiver performance in multipath channels, signal-to-noise ratio (SNR) should be maximized. In this paper, we propose and analyze several noise mitigation techniques for orthogonal frequency division multiplexing (OFDM) receivers, namely, the adaptive guard interval recycling algorithm and the adaptive interpolationbased channel estimation scheme. In realistic multipath environment, these techniques require accurate estimation of channel delay profile. Therefore, we also propose a robust and low-complexity channel impulse response analyzer for OFDM systems with lattice-type pilot arrangement. Extensive simulation results verify that the proposed techniques can improve bit error rate (BER) performance by maximizing SNR for a typical OFDM system such as digital video broadcasting -terrestrial and handheld (DVB-T/H) system1.

Channels estimation in OFDM system over Rician fading channel based on comb-type pilots arrangement

Orthogonal frequency-division multiplexing (OFDM) is a transmission technique that is based on many orthogonal carriers that are transmitted simultaneously. Channel estimation techniques for OFDM systems, based on comb-type pilot arrangement, over frequency-selective Rician and time-varying fading channel are investigated. The advantage of comb-type pilot arrangement, in channel estimation, is the ability to track the variation in the channel, which is the main reason for inter-carrier interference modelled as an additive white Gaussian noise, leading to an increase in the noise level. The estimation of the channel at the pilot frequencies is based on least-square (LS) method. Several interpolation methods have been used to estimate the channel response at the data frequency. In this study, simulation results for Rician channel model were examined and bit error rate/signal-to-noise ratio performance, for various conditions, were considered, and the pilot arrangement was used with the LS estimation and low-pass interpolation techniques, leading to a reduction in the effect of Doppler frequency.

Analysis and Comparison of Time Replica and Time Linear Interpolation for Pilot Aided Channel Estimation in OFDM Systems

This paper analyzes and compares two time interpolators, i.e., time replica and time linear interpolator, for pilot aided channel estimation in orthogonal frequency division multiplexing (OFDM) systems. The mean square error (MSE) of two interpolators is theoretically derived for the general case. The equally spaced pilot arrangement is proposed as a special platform for these two time interpolators. Based on this proposed platform, the MSE of two time interpolators at the virtual pilot tones is derived analytically; moreover, the MSE of per channel estimator at the entire OFDM symbol based on per time interpolator is also derived. The effectiveness of the theoretical analysis is demonstrated by numerical simulation in both the time-invariant frequency-selective channel and the time varying frequency-selective channel.

Pilot Design for MIMO OFDM Systems With Virtual Carriers

We consider the problem of pilot design for channel estimation in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in the presence of virtual carriers. The design criterion is the average mean-square error (MSE) of the least square (LS) estimates of the channel frequency response over the data carriers. To maximize bandwidth efficiency, the number of pilots is set to its minimum which is equal to the number of unknown channel coefficients. Both the phase shift orthogonal and disjoint pilot schemes are investigated. For a given pilot placement, a closed-form expression for the pilot power distribution for the disjoint scheme is provided. It is found that the optimal power distribution allocates less power to the pilots that are close to the virtual carriers. Pilot placement is obtained using an exhaustive search. The latter is too complex in systems with large number of subcarriers. A suboptimum pilot placement design for the disjoint pilot scheme is presented. It is shown that, unlike fully loaded systems where the two pilot design schemes have similar performance, in the presence of virtual carriers the optimal disjoint scheme not only has lower complexity but also yields more accurate channel estimation and bit error rate performance.

Least-Squares Channel Estimation for Mobile OFDM Communication on Time-Varying Frequency-Selective Fading Channels

A least-squares (LS) channel estimation (CE) technique for mobile orthogonal frequency-division multiplexing (OFDM) communications over a rapidly time-varying frequency-selective fading channel is investigated in this paper. The proposed technique keeping the comb-type pilot arrangement can achieve a low error probability by accurately estimating channel impulse response (CIR) and effectively tracking rapid CIR time variations. The LS CE technique proposed here is conducted in the time domain (TD). Meanwhile, a generic estimator is serially performed block by block without assistance from a priori channel information and without increasing computational complexity. By taking advantage of linearly frequency-modulated (LFM) or pseudorandom signals transceived for jointly sounding pilot subchannels, the proposed LS CE can inherently perform pseudonoise (PN) matched filtering (MF) to suppress multipath interference (MPI) caused by frequency-selective fading and intercarrier interference (ICI) resulting from data subchannels. The optimality of the proposed technique is verified by taking Cramer-Rao lower bounds (CRLBs) into comparison both on noise- and interference-dominant signal-to-noise ratio (SNR) conditions. In addition, the dual optimality of the LFM and PN pilot symbols is verified for both TD and frequency-domain (FD) CEs. Furthermore, the proposed technique also exhibits good resistance against residual timing errors occurring with the discrete Fourier transform (DFT) demodulation. Extensive computer simulations in conjunction with statistical derivations show the superiority of the proposed technique.

MIMO-OFDM Channel Estimation in the Presence of Frequency Offsets

Optimal pilot design and placement for channel estimation in multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems with frequency offsets are considered. Both the single-frequency- offset case and the multiple-frequency-offset case are treated. We show that the constant-envelope (CE) condition is sufficient but not necessary for pilot design, and that pilots with multiple envelopes can also achieve the optimal performance in terms of the mean square error (MSE) minimization, provided that an additional constraint on the pilot placement is satisfied simultaneously. New pilot designs, which take into account the multiple-frequency-offset case, are proposed to eliminate inter- pilot-interference (IPI) and to optimize the MSE performance. The least-squares (LS) and linear minimum mean square error (LMMSE) channel estimators for the multiple-frequency- offset case are designed for uncorrelated and correlated MIMO-OFDM channels, respectively. The LMMSE estimator requires the channel covariance matrix. Both optimal adaptive pilot power allocation and suboptimal uniform pilot power allocation are developed for the proposed LMMSE estimator. The adaptive allocation performs 4 dB better than the uniform allocation in the high noise region, but they both perform identically in the low noise region. Performance comparisons are made against several previous pilot designs due to [1], [2]. The proposed LMMSE estimator significantly outperforms the LS estimator.

On the training of MIMO-OFDM channels with least square channel estimation and linear interpolation

The effect of pilot arrangement is investigated on the performance of trained MIMO-OFDM channels where fading is time-flat in a block of coherence-time duration but frequency-selective. The MIMO-OFDM channel is considered to be trained by using least square channel estimation on pilot subcarriers and by applying linear interpolation on data subcarriers. The achievable rate of such a system is derived employing the generalized mutual information approach. Numerical examples are presented examining the optimal density of pilots in the two-dimensional time-frequency domain.

Detection of Hierarchically-Modulated Data for Advanced T-DMB Receivers

This paper proposes an efficient detection method of hierarchically-modulated data for an advanced terrestrial digital multimedia broadcasting (AT-DMB) receiver. Hierarchical modulation, where low-priority (LP) symbols are hierarchically added to the existing high-priority (HP) symbols, has been considered to be useful for increasing the data rate in AT-DMB systems. The distance between two neighboring LP symbols is limited by the noise margin of HP symbols and thus should be shortened to minimize the effect on the detection of HP symbols. To cope with the poor condition of LP symbols, we introduce a coherent detection method based on coded decision-directed (CDD) channel estimation (CE). CDD-CE may provide an accurate estimation result even under the infrequent pilot arrangement of the T-DMB system. Additionally, we propose a moving average (MA) method in frequency direction to reduce noise existing in the estimated channel coefficients. Simulation results show that the proposed method can be a good solution to successfully detect LP data.

Planning and implementing a collaborative clinical placement for medical, nursing and allied health students: A qualitative study

Background: Clinical placements have been traditionally offered on a profession specific basis, and as a result, we have a good understanding of salient issues related to their development and delivery. We know less about the planning and implementation of collaborative clinical placements.Aims: This paper presents key findings from a qualitative study that explored the collaborative processes connected to an interprofessional planning group who created and implemented a clinical placement for medical, nursing and allied health students.Methods: An ethnographic approach was employed to explore the successes and challenges connected with the planning group's interprofessional work. Interviews, observations and documents were gathered over two years to obtain a comprehensive understanding of this placement.Results: The study found that while the planning group achieved a number of successes in their work including the implementation of a well-received pilot placement, their enthusiasm for the placement created a number of challenges. In particular, it resulted in them neglecting their roles, responsibilities and collaborative group processes, which created difficulties in their ability to work together. In addition, a turnover of members, changes in management and a hospital reorganization inhibited the group's collaborative work.Conclusions: Collaboration around the planning and implementation of interprofessional placements is a complex venture. In striving for success in this work, planning groups need to focus their attention on both internal group-based factors as well external organizational factors.

The Collaborative Model of Fieldwork Education: Implementation of the model in a regional hospital rehabilitation setting

Aim: This paper describes the implementation of a Collaborative Model of Fieldwork Education in a regional hospital occupational therapy department. Methods: The literature on models of fieldwork education for occupational therapy students is reviewed, and an approach to the implementation of the collaborative model with three students to one clinical educator is described after piloting of this model's recommendations, arising from the pilot placement, are proposed. Conclusions: The implementation of a Collaborative Model of Fieldwork Education requires careful planning, close links with the university fieldwork team and a willingness on the behalf of occupational therapist clinical educators to explore alternative approaches to the provision of fieldwork education.

OPTIMAL TRAINING DESIGN FOR MIMO–OFDM SYSTEMS UNDER SPATIALLY CORRELATED DOUBLY SELECTIVE FADING CHANNELS

In this paper, we design an optimal training scheme for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems under spatially correlated time- and frequency- (doubly) selective fading channels. We first develop the optimal pilot symbols and placement of pilot clusters with respect to the minimum mean square error (MMSE) of the linear channel estimate. We then derive the optimal power allocation for pilot symbols in a two-water-level way: by maximizing the averaged capacity lower bound, how much power to be allocated for training is determined subject to the global water level (or the constraint of total transmit power); subsequently, pouring power to the pilot symbols with an approximately optimal water-filling scheme subject to the local water level (or the constraint of assigned power for training). In addition, for a particular OFDM size, the optimal number of pilot clusters is derived by maximizing the capacity lower bound and by minimizing the channel estimate's MMSE.