82 research outputs found
Adaptive Multicell 3D Beamforming in Multi-Antenna Cellular Networks
We consider a cellular network with multi-antenna base stations (BSs) and
single-antenna users, multicell cooperation, imperfect channel state
information, and directional antennas each with a vertically adjustable beam.
We investigate the impact of the elevation angle of the BS antenna pattern,
denoted as tilt, on the performance of the considered network when employing
either a conventional single-cell transmission or a fully cooperative multicell
transmission. Using the results of this investigation, we propose a novel
hybrid multicell cooperation technique in which the intercell interference is
controlled via either cooperative beamforming in the horizontal plane or
coordinated beamfroming in the vertical plane of the wireless channel, denoted
as adaptive multicell 3D beamforming. The main idea is to divide the coverage
area into two disjoint vertical regions and adapt the multicell cooperation
strategy at the BSs when serving each region. A fair scheduler is used to share
the time-slots between the vertical regions. It is shown that the proposed
technique can achieve performance comparable to that of a fully cooperative
transmission but with a significantly lower complexity and signaling
requirements. To make the performance analysis computationally efficient,
analytical expressions for the user ergodic rates under different beamforming
strategies are also derived.Comment: Accepted for publication in IEEE Transaction on Vehicular Technolog
Interference Alignment and Cancellation in Wireless Communication Systems
The Shannon capacity of wireless networks has a fundamental importance for network information theory. This area has recently seen remarkable progress on a variety of problems including the capacity of interference networks, X networks, cellular networks, cooperative communication networks and cognitive radio networks. While each communication scenario has its own characteristics, a common reason of these recent developments is the new idea of interference alignment. The idea of interference alignment is to consolidate the interference into smaller dimensions of signal space at each receiver and use the remaining dimensions to transmit the desired signals without any interference. However, perfect alignment of interference requires certain assumptions, such as perfect channel state information at transmitter and receiver, perfect synchronization and feedback. Today’s wireless communication systems, on the other and, do not encounter such ideal conditions. In this thesis, we cover a breadth of topics of interference alignment and cancellation schemes in wireless communication systems such as multihop relay networks, multicell networks as well as cooperation and optimisation in such systems. Our main contributions in this thesis can be summarised as follows:
• We derive analytical expressions for an interference alignment scheme in a multihop relay network with imperfect channel state information, and investigate the impact of interference on such systems where interference could accumulate due to the misalignment at each hop.
• We also address the dimensionality problem in larger wireless communication systems such as multi-cellular systems. We propose precoding schemes based on maximising signal power over interference and noise. We show that these precoding vectors would dramatically improve the rates for multi-user cellular networks in both uplink and downlink, without requiring an excessive number of dimensions. Furthermore, we investigate how to improve the receivers which can mitigate interference more efficiently.
• We also propose partial cooperation in an interference alignment and cancellation scheme. This enables us to assess the merits of varying mixture of cooperative and non-cooperative users and the gains achievable while reducing the overhead of channel estimation. In addition to this, we analytically derive expressions for the additional interference caused by imperfect channel estimation in such cooperative systems. We also show the impact of imperfect channel estimation on cooperation gains.
• Furthermore, we propose jointly optimisation of interference alignment and cancellation for multi-user multi-cellular networks in both uplink and downlink. We find the optimum set of transceivers which minimise the mean square error at each base station. We demonstrate that optimised transceivers can outperform existing interference alignment and cancellation schemes.
• Finally, we consider power adaptation and user selection schemes. The simulation results indicate that user selection and power adaptation techniques based on estimated rates can improve the overall system performance significantly
A Novel Antenna Selection Scheme for Spatially Correlated Massive MIMO Uplinks with Imperfect Channel Estimation
We propose a new antenna selection scheme for a massive MIMO system with a
single user terminal and a base station with a large number of antennas. We
consider a practical scenario where there is a realistic correlation among the
antennas and imperfect channel estimation at the receiver side. The proposed
scheme exploits the sparsity of the channel matrix for the effective selection
of a limited number of antennas. To this end, we compute a sparse channel
matrix by minimising the mean squared error. This optimisation problem is then
solved by the well-known orthogonal matching pursuit algorithm. Widely used
models for spatial correlation among the antennas and channel estimation errors
are considered in this work. Simulation results demonstrate that when the
impacts of spatial correlation and imperfect channel estimation introduced, the
proposed scheme in the paper can significantly reduce complexity of the
receiver, without degrading the system performance compared to the maximum
ratio combining.Comment: in Proc. IEEE 81st Vehicular Technology Conference (VTC), May 2015, 6
pages, 5 figure
D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies
This document provides the most recent updates on the technical contributions and research
challenges focused in WP3. Each Technology Component (TeC) has been evaluated
under possible uniform assessment framework of WP3 which is based on the simulation guidelines
of WP6. The performance assessment is supported by the simulation results which are in their
mature and stable state. An update on the Most Promising Technology Approaches (MPTAs)
and their associated TeCs is the main focus of this document. Based on the input of all the TeCs in WP3, a consolidated view of WP3 on the role of multinode/multi-antenna transmission
technologies in 5G systems has also been provided. This consolidated view is further
supported in this document by the presentation of the impact of MPTAs on METIS scenarios
and the addressed METIS goals.Aziz, D.; Baracca, P.; De Carvalho, E.; Fantini, R.; Rajatheva, N.; Popovski, P.; Sørensen, JH.... (2015). D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies. http://hdl.handle.net/10251/7675
Millimeter Wave Systems for Wireless Cellular Communications
This thesis considers channel estimation and multiuser (MU) data transmission
for massive MIMO systems with fully digital/hybrid structures in mmWave
channels. It contains three main contributions. In this thesis, we first
propose a tone-based linear search algorithm to facilitate the estimation of
angle-of-arrivals of the strongest components as well as scattering components
of the users at the base station (BS) with fully digital structure. Our results
show that the proposed maximum-ratio transmission (MRT) based on the strongest
components can achieve a higher data rate than that of the conventional MRT,
under the same mean squared errors (MSE). Second, we develop a low-complexity
channel estimation and beamformer/precoder design scheme for hybrid mmWave
systems. In addition, the proposed scheme applies to both non-sparse and sparse
mmWave channel environments. We then leverage the proposed scheme to
investigate the downlink achievable rate performance. The results show that the
proposed scheme obtains a considerable achievable rate of fully digital
systems. Taking into account the effect of various types of errors, we
investigate the achievable rate performance degradation of the considered
scheme. Third, we extend our proposed scheme to a multi-cell MU mmWave MIMO
network. We derive the closed-form approximation of the normalized MSE of
channel estimation under pilot contamination over Rician fading channels.
Furthermore, we derive a tight closed-form approximation and the scaling law of
the average achievable rate. Our results unveil that channel estimation errors,
the intra-cell interference, and the inter-cell interference caused by pilot
contamination over Rician fading channels can be efficiently mitigated by
simply increasing the number of antennas equipped at the desired BS.Comment: Thesi
Novel transmission and beamforming strategies for multiuser MIMO with various CSIT types
In multiuser multi-antenna wireless systems, the transmission and beamforming strategies that achieve the sum rate capacity depend critically on the acquisition of perfect Channel State Information at the Transmitter (CSIT).
Accordingly, a high-rate low-latency feedback link between the receiver and the transmitter is required to keep the latter accurately and instantaneously informed about the CSI.
In realistic wireless systems, however, only imperfect CSIT is achievable due to pilot contamination, estimation error, limited feedback and delay, etc.
As an intermediate solution, this thesis investigates novel transmission strategies suitable for various imperfect CSIT scenarios and the associated beamforming techniques to optimise the rate performance.
First, we consider a two-user Multiple-Input-Single-Output (MISO) Broadcast Channel (BC) under statistical and delayed CSIT.
We mainly focus on linear beamforming and power allocation designs for ergodic sum rate maximisation.
The proposed designs enable higher sum rate than the conventional designs.
Interestingly, we propose a novel transmission framework which makes better use of statistical and delayed CSIT and smoothly bridges between statistical CSIT-based strategies and delayed CSIT-based strategies.
Second, we consider a multiuser massive MIMO system under partial and statistical CSIT.
In order to tackle multiuser interference incurred by partial CSIT, a Rate-Splitting (RS) transmission strategy has been proposed recently.
We generalise the idea of RS into the large-scale array.
By further exploiting statistical CSIT, we propose a novel framework Hierarchical-Rate-Splitting that is particularly suited to massive MIMO systems.
Third, we consider a multiuser Millimetre Wave (mmWave) system with hybrid analog/digital precoding under statistical and quantised CSIT.
We leverage statistical CSIT to design digital precoder for interference mitigation while all feedback overhead is reserved for precise analog beamforming.
For very limited feedback and/or very sparse channels, the proposed precoding scheme yields higher sum rate than the conventional precoding schemes under a fixed total feedback constraint.
Moreover, a RS transmission strategy is introduced to further tackle the multiuser interference, enabling remarkable saving in feedback overhead compared with conventional transmission strategies.
Finally, we investigate the downlink hybrid precoding for physical layer multicasting with a limited number of RF chains.
We propose a low complexity algorithm to compute the analog precoder that achieves near-optimal max-min performance.
Moreover, we derive a simple condition under which the hybrid precoding driven by a limited number of RF chains incurs no loss of optimality with respect to the fully digital precoding case.Open Acces
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