426 research outputs found
Space time transceiver design over multipath fading channels
Imperial Users onl
Estimation of Sparse MIMO Channels with Common Support
We consider the problem of estimating sparse communication channels in the
MIMO context. In small to medium bandwidth communications, as in the current
standards for OFDM and CDMA communication systems (with bandwidth up to 20
MHz), such channels are individually sparse and at the same time share a common
support set. Since the underlying physical channels are inherently
continuous-time, we propose a parametric sparse estimation technique based on
finite rate of innovation (FRI) principles. Parametric estimation is especially
relevant to MIMO communications as it allows for a robust estimation and
concise description of the channels. The core of the algorithm is a
generalization of conventional spectral estimation methods to multiple input
signals with common support. We show the application of our technique for
channel estimation in OFDM (uniformly/contiguous DFT pilots) and CDMA downlink
(Walsh-Hadamard coded schemes). In the presence of additive white Gaussian
noise, theoretical lower bounds on the estimation of SCS channel parameters in
Rayleigh fading conditions are derived. Finally, an analytical spatial channel
model is derived, and simulations on this model in the OFDM setting show the
symbol error rate (SER) is reduced by a factor 2 (0 dB of SNR) to 5 (high SNR)
compared to standard non-parametric methods - e.g. lowpass interpolation.Comment: 12 pages / 7 figures. Submitted to IEEE Transactions on Communicatio
Channel Estimation for Massive MIMO-OFDM Systems by Tracking the Joint Angle-Delay Subspace
In this paper, we propose joint angle-delay subspace based channel estimation in single cell for broadband massive multiple-input and multiple-output (MIMO) systems employing orthogonal frequency division multiplexing (OFDM) modulation. Based on a parametric channel model, we present a new concept of the joint angle-delay subspace which can be tracked by the low-complexity low-rank adaptive filtering (LORAF) algorithm. Then, we investigate an interference-free transmission condition that the joint angle-delay subspaces of the users reusing the same pilots are non-overlapping. Since the channel statistics are usually unknown, we develop a robust minimum mean square error (MMSE) estimator under the worst precondition of pilot decontamination, considering that the joint angle-delay subspaces of the interfering users fully overlap. Furthermore, motivated by the interference-free transmission criteria, we present a novel low-complexity greedy pilot scheduling algorithm to avoid the problem of initial value sensitivity. Simulation results show that the joint angle-delay subspace can be estimated effectively, and the proposed pilot reuse scheme combined with robust MMSE channel estimation offers significant performance gains
Cyclic Prefix-Free MC-CDMA Arrayed MIMO Communication Systems
The objective of this thesis is to investigate MC-CDMA MIMO systems where
the antenna array geometry is taken into consideration. In most MC-CDMA
systems, cyclic pre xes, which reduce the spectral e¢ ciency, are used. In order
to improve the spectral efficiency, this research study is focused on cyclic pre x-
free MC-CDMA MIMO architectures.
Initially, space-time wireless channel models are developed by considering the
spatio-temporal mechanisms of the radio channel, such as multipath propaga-
tion. The spatio-temporal channel models are based on the concept of the array
manifold vector, which enables the parametric modelling of the channel.
The array manifold vector is extended to the multi-carrier space-time array
(MC-STAR) manifold matrix which enables the use of spatio-temporal signal
processing techniques. Based on the modelling, a new cyclic pre x-free MC-
CDMA arrayed MIMO communication system is proposed and its performance
is compared with a representative existing system. Furthermore, a MUSIC-type
algorithm is then developed for the estimation of the channel parameters of the
received signal.
This proposed cyclic pre x-free MC-CDMA arrayed MIMO system is then
extended to consider the effects of spatial diffusion in the wireless channel. Spatial
diffusion is an important channel impairment which is often ignored and the
failure to consider such effects leads to less than satisfactory performance. A
subspace-based approach is proposed for the estimation of the channel parameters
and spatial spread and reception of the desired signal.
Finally, the problem of joint optimization of the transmit and receive beam-
forming weights in the downlink of a cyclic pre x-free MC-CDMA arrayed MIMO
communication system is investigated. A subcarrier-cooperative approach is used
for the transmit beamforming so that there is greater flexibility in the allocation
of channel symbols. The resulting optimization problem, with a per-antenna
transmit power constraint, is solved by the Lagrange multiplier method and an
iterative algorithm is proposed
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
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