3 research outputs found
Corrector Operator to Enhance Accuracy and Reliability of Neural Operator Surrogates of Nonlinear Variational Boundary-Value Problems
This work focuses on developing methods for approximating the solution
operators of a class of parametric partial differential equations via neural
operators. Neural operators have several challenges, including the issue of
generating appropriate training data, cost-accuracy trade-offs, and nontrivial
hyperparameter tuning. The unpredictability of the accuracy of neural operators
impacts their applications in downstream problems of inference, optimization,
and control. A framework is proposed based on the linear variational problem
that gives the correction to the prediction furnished by neural operators. The
operator associated with the corrector problem is referred to as the corrector
operator. Numerical results involving a nonlinear diffusion model in two
dimensions with PCANet-type neural operators show almost two orders of increase
in the accuracy of approximations when neural operators are corrected using the
proposed scheme. Further, topology optimization involving a nonlinear diffusion
model is considered to highlight the limitations of neural operators and the
efficacy of the correction scheme. Optimizers with neural operator surrogates
are seen to make significant errors (as high as 80 percent). However, the
errors are much lower (below 7 percent) when neural operators are corrected
following the proposed method.Comment: 34 pages, 14 figure
Interference modelling and management for cognitive radio networks
Radio spectrum is becoming increasingly scarce as more and more devices go wireless.
Meanwhile, studies indicate that the assigned spectrum is not fully utilised.
Cognitive radio (CR) technology is envisioned to be a promising solution to address
the imbalance between spectrum scarcity and spectrum underutilisation. It improves
the spectrum utilisation by reusing the unused or underutilised spectrum owned by
incumbent systems (primary systems). With the introduction of CR networks, two
types of interference originating from CR networks are introduced. They are the interference
from CR to primary networks (CR-primary interference) and the interference
among spectrum-sharing CR nodes (CR-CR interference). The interference should be
well controlled and managed in order not to jeopardise the operation of the primary
network and to improve the performance of CR systems. This thesis investigates the
interference in CR networks by modelling and mitigating the CR-primary interference
and analysing the CR-CR interference channels.
Firstly, the CR-primary interference is modelled for multiple CR nodes sharing the
spectrum with the primary system. The probability density functions of CR-primary
interference are derived for CR networks adopting different interference management
schemes. The relationship between CR operating parameters and the resulting CRprimary
interference is investigated. It sheds light on the deployment of CR networks
to better protect the primary system.
Secondly, various interference mitigation techniques that are applicable to CR networks
are reviewed. Two novel precoding schemes for CR multiple-input multipleoutput
(MIMO) systems are proposed to mitigate the CR-primary interference and
maximise the CR throughput. To further reduce the CR-primary interference, we also
approach interference mitigation from a cross-layer perspective by jointly considering
channel allocation in the media access control layer and precoding in the physical
layer of CR MIMO systems.
Finally, we analyse the underlying interference channels among spectrum-sharing CR
users when they interfere with each other. The Pareto rate region for multi-user MIMO
interference systems is characterised. Various rate region convexification schemes are
examined to convexify the rate region. Then, game theory is applied to the interference
system to coordinate the operation of each CR user. Nash bargaining over MIMO
interference systems is characterised as well.
The research presented in this thesis reveals the impact of CR operation on the resulting
CR-primary network, how to mitigate the CR-primary interference and how
to coordinate the spectrum-sharing CR users. It forms the fundamental basis for interference
management in CR systems and consequently gives insights into the design
and deployment of CR networks