343 research outputs found
On Channel Estimation for 802.11p in Highly Time-Varying Vehicular Channels
Vehicular wireless channels are highly time-varying and the pilot pattern in
the 802.11p orthogonal frequency-division multiplexing frame has been shown to
be ill suited for long data packets. The high frame error rate in off-the-shelf
chipsets with noniterative receiver configurations is mostly due to the use of
outdated channel estimates for equalization. This paper deals with improving
the channel estimation in 802.11p systems using a cross layered approach, where
known data bits are inserted in the higher layers and a modified receiver makes
use of these bits as training data for improved channel estimation. We also
describe a noniterative receiver configuration for utilizing the additional
training bits and show through simulations that frame error rates close to the
case with perfect channel knowledge can be achieved.Comment: 6 pages, 11 figures, conferenc
An Iterative Receiver for OFDM With Sparsity-Based Parametric Channel Estimation
In this work we design a receiver that iteratively passes soft information
between the channel estimation and data decoding stages. The receiver
incorporates sparsity-based parametric channel estimation. State-of-the-art
sparsity-based iterative receivers simplify the channel estimation problem by
restricting the multipath delays to a grid. Our receiver does not impose such a
restriction. As a result it does not suffer from the leakage effect, which
destroys sparsity. Communication at near capacity rates in high SNR requires a
large modulation order. Due to the close proximity of modulation symbols in
such systems, the grid-based approximation is of insufficient accuracy. We show
numerically that a state-of-the-art iterative receiver with grid-based sparse
channel estimation exhibits a bit-error-rate floor in the high SNR regime. On
the contrary, our receiver performs very close to the perfect channel state
information bound for all SNR values. We also demonstrate both theoretically
and numerically that parametric channel estimation works well in dense
channels, i.e., when the number of multipath components is large and each
individual component cannot be resolved.Comment: Major revision, accepted for IEEE Transactions on Signal Processin
Efficient Downlink Channel Reconstruction for FDD Multi-Antenna Systems
In this paper, we propose an efficient downlink channel reconstruction scheme
for a frequency-division-duplex multi-antenna system by utilizing uplink
channel state information combined with limited feedback. Based on the spatial
reciprocity in a wireless channel, the downlink channel is reconstructed by
using frequency-independent parameters. We first estimate the gains, delays,
and angles during uplink sounding. The gains are then refined through downlink
training and sent back to the base station (BS). With limited overhead, the
refinement can substantially improve the accuracy of the downlink channel
reconstruction. The BS can then reconstruct the downlink channel with the
uplink-estimated delays and angles and the downlink-refined gains. We also
introduce and extend the Newtonized orthogonal matching pursuit (NOMP)
algorithm to detect the delays and gains in a multi-antenna multi-subcarrier
condition. The results of our analysis show that the extended NOMP algorithm
achieves high estimation accuracy. Simulations and over-the-air tests are
performed to assess the performance of the efficient downlink channel
reconstruction scheme. The results show that the reconstructed channel is close
to the practical channel and that the accuracy is enhanced when the number of
BS antennas increases, thereby highlighting that the promising application of
the proposed scheme in large-scale antenna array systems
On the distribution of an effective channel estimator for multi-cell massive MIMO
Accurate channel estimation is of utmost importance for massive MIMO systems to provide significant improvements in spectral and energy efficiency. In this work, we present a study on the distribution of a simple but yet effective and practical channel estimator for multi-cell massive MIMO systems suffering from pilot-contamination. The proposed channel estimator performs well under moderate to aggressive pilot contamination scenarios without previous knowledge of the inter-cell large-scale channel coefficients and noise power, asymptotically approximating the performance of the linear MMSE estimator as the number of antennas increases. We prove that the distribution of the proposed channel estimator can be accurately approximated by the circularly-symmetric complex normal distribution, when the number of antennas, M, deployed at the base station is greater than 10
Performance Analysis of Channel Extrapolation in FDD Massive MIMO Systems
Channel estimation for the downlink of frequency division duplex (FDD)
massive MIMO systems is well known to generate a large overhead as the amount
of training generally scales with the number of transmit antennas in a MIMO
system. In this paper, we consider the solution of extrapolating the channel
frequency response from uplink pilot estimates to the downlink frequency band,
which completely removes the training overhead. We first show that conventional
estimators fail to achieve reasonable accuracy. We propose instead to use
high-resolution channel estimation. We derive theoretical lower bounds (LB) for
the mean squared error (MSE) of the extrapolated channel. Assuming that the
paths are well separated, the LB is simplified in an expression that gives
considerable physical insight. It is then shown that the MSE is inversely
proportional to the number of receive antennas while the extrapolation
performance penalty scales with the square of the ratio of the frequency offset
and the training bandwidth. The channel extrapolation performance is validated
through numeric simulations and experimental measurements taken in an anechoic
chamber. Our main conclusion is that channel extrapolation is a viable solution
for FDD massive MIMO systems if accurate system calibration is performed and
favorable propagation conditions are present.Comment: arXiv admin note: substantial text overlap with arXiv:1902.0684
Advanced Channel Estimation Techniques for Multiple-Input Multiple-Output Multi-Carrier Systems in Doubly-Dispersive Channels
Flexible numerology of the physical layer has been introduced in the latest release of 5G new radio (NR) and the baseline waveform generation is chosen to be cyclic-prefix based orthogonal frequency division multiplexing (CP-OFDM). Thanks to the narrow subcarrier spacing and low complexity one tap equalization (EQ) of OFDM, it suits well to time-dispersive channels. For the upcoming 5G and beyond use-case scenarios, it is foreseen that the users might experience high mobility conditions. While the frame structure of the 5G NR is designed for long coherence times, the synchronization and channel estimation (CE) procedures are not fully and reliably covered for diverse applications.
The research on alternative multi-carrier waveforms has brought up valuable results in terms of spectral efficiency, applications coexistence and flexibility. Nevertheless, the receiver design becomes more challenging for multiple-input multiple-output (MIMO) non-orthogonal multi-carriers because the receiver must deal with multiple dimensions of interference. This thesis aims to deliver accurate pilot-aided estimations of the wireless channel for coherent detection. Considering a MIMO non-orthogonal multi-carrier, e.g. generalized frequency division multiplexing (GFDM), we initially derive the classical and Bayesian estimators for rich multi-path fading channels, where we theoretically assess the choice of pilot design. Moreover, the well time- and frequency-localization of the pilots in non-orthogonal multi-carriers allows to reuse their energy from cyclic-prefix (CP). Taking advantage of this feature, we derive an iterative approach for joint CE and EQ of MIMO systems. Furthermore, exploiting the block-circularity of GFDM, we comprehensively analyze the complexity aspects, and propose a solution for low complexity implementation.
Assuming very high mobility use-cases where the channel varies within the symbol duration, further considerations, particularly the channel coherence time must be taken into account. A promising candidate that is fully independent of the multi-carrier choice is unique word (UW) transmission, where the CP of random nature is replaced by a deterministic sequence. This feature, allows per-block synchronization and channel estimation for robust transmission over extremely doubly-dispersive channels. In this thesis, we propose a novel approach to extend the UW-based physical layer design to MIMO systems and we provide an in-depth study of their out-of-band emission, synchronization, CE and EQ procedures. Via theoretical derivations and simulation results, and comparisons with respect to the state-of-the-art CP-OFDM systems, we show that the proposed UW-based frame design facilitates robust transmission over extremely doubly-dispersive channels.:1 Introduction 1
1.1 Multi-Carrier Waveforms 1
1.2 MIMO Systems 3
1.3 Contributions and Thesis Structure 4
1.4 Notations 6
2 State-of-the-art and Fundamentals 9
2.1 Linear Systems and Problem Statement 9
2.2 GFDM Modulation 11
2.3 MIMO Wireless Channel 12
2.4 Classical and Bayesian Channel Estimation in MIMO OFDM Systems 15
2.5 UW-Based Transmission in SISO Systems 17
2.6 Summary 19
3 Channel Estimation for MIMO Non-Orthogonal Waveforms 21
3.1 Classical and Bayesian Channel Estimation in MIMO GFDM Systems 22
3.1.1 MIMO LS Channel Estimation 23
3.1.2 MIMO LMMSE Channel Estimation 24
3.1.3 Simulation Results 25
3.2 Basic Pilot Designs for GFDM Channel Estimation 29
3.2.1 LS/HM Channel Estimation 31
3.2.2 LMMSE Channel Estimation for GFDM 32
3.2.3 Error Characterization 33
3.2.4 Simulation Results 36
3.3 Interference-Free Pilot Insertion for MIMO GFDM Channel Estimation 39
3.3.1 Interference-Free Pilot Insertion 39
3.3.2 Pilot Observation 40
3.3.3 Complexity 41
3.3.4 Simulation Results 41
3.4 Bayesian Pilot- and CP-aided Channel Estimation in MIMO NonOrthogonal Multi-Carriers 45
3.4.1 Review on System Model 46
3.4.2 Single-Input-Single-Output Systems 47
3.4.3 Extension to MIMO 50
3.4.4 Application to GFDM 51
3.4.5 Joint Channel Estimation and Equalization via LMMSE Parallel Interference Cancellation 57
3.4.6 Complexity Analysis 61
3.4.7 Simulation Results 61
3.5 Pilot- and CP-aided Channel Estimation in Time-Varying Scenarios 67
3.5.1 Adaptive Filtering based on Wiener-Hopf Approac 68
3.5.2 Simulation Results 69
3.6 Summary 72
4 Design of UW-Based Transmission for MIMO Multi-Carriers 73
4.1 Frame Design, Efficiency and Overhead Analysis 74
4.1.1 Illustrative Scenario 74
4.1.2 CP vs. UW Efficiency Analysis 76
4.1.3 Numerical Results 77
4.2 Sequences for UW and OOB Radiation 78
4.2.1 Orthogonal Polyphase Sequences 79
4.2.2 Waveform Engineering for UW Sequences combined with GFDM 79
4.2.3 Simulation Results for OOB Emission of UW-GFDM 81
4.3 Synchronization 82
4.3.1 Transmission over a Centralized MIMO Wireless Channel 82
4.3.2 Coarse Time Acquisition 83
4.3.3 CFO Estimation and Removal 85
4.3.4 Fine Time Acquisition 86
4.3.5 Simulation Results 88
4.4 Channel Estimation 92
4.4.1 MIMO UW-based LMMSE CE 92
4.4.2 Adaptive Filtering 93
4.4.3 Circular UW Transmission 94
4.4.4 Simulation Results 95
4.5 Equalization with Imperfect Channel Knowledge 96
4.5.1 UW-Free Equalization 97
4.5.2 Simulation Results 99
4.6 Summary 102
5 Conclusions and Perspectives 103
5.1 Main Outcomes in Short 103
5.2 Open Challenges 105
A Complementary Materials 107
A.1 Linear Algebra Identities 107
A.2 Proof of lower triangular Toeplitz channel matrix being defective 108
A.3 Calculation of noise-plus-interference covariance matrix for Pilot- and CPaided CE 108
A.4 Bock diagonalization of the effective channel for GFDM 109
A.5 Detailed complexity analysis of Sec. 3.4 109
A.6 CRLB derivations for the pdf (4.24) 113
A.7 Proof that (4.45) emulates a circular CIR at the receiver 11
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