THESIS
2016
xv, 122 pages : illustrations ; 30 cm
Abstract
Future 5th generation (5G) wireless networks aim at supporting 1000 times higher spectral efficiency
as compared with current network deployments. To meet this requirement, network
densification is regarded as one of the dominant themes for wireless evolution. Network
densification is realized by deploying small access nodes over the conventional macrocell
networks, generating heterogeneous networks (HetNets). Nevertheless, deploying HetNets
faces many challenges. One of the key challenges is the incursion of the interference between
different tiers, namely, the cross-tier interference. A great amount of research has been
launched on exploring effective methods to manage the interference in HetNets. Unfortunately,
the optimal spectrum access scheme, which is one of the most effe...[
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Future 5th generation (5G) wireless networks aim at supporting 1000 times higher spectral efficiency
as compared with current network deployments. To meet this requirement, network
densification is regarded as one of the dominant themes for wireless evolution. Network
densification is realized by deploying small access nodes over the conventional macrocell
networks, generating heterogeneous networks (HetNets). Nevertheless, deploying HetNets
faces many challenges. One of the key challenges is the incursion of the interference between
different tiers, namely, the cross-tier interference. A great amount of research has been
launched on exploring effective methods to manage the interference in HetNets. Unfortunately,
the optimal spectrum access scheme, which is one of the most effective interference
management methods for HetNets, remains unknown. In this thesis, we shall investigate the
optimal spectrum access scheme for macro-femto HetNets.
Firstly, we propose a general cognitive spectrum access framework, where the idea of cognitive
cross-tier spectrum access is introduced into the conventional spectrum access scheme
to improve the area spectral efficiency (ASE) of macro-femto HetNets. With this proposed
framework, the optimal probability for femtocells to access the idle macro-tier spectrum can
be determined. This cross-tier spectrum access probability provides the degree of freedom
(DoF) to flexibly offload data traffic between the macrocell and femtocell tiers. The maximum
ASE obtained with this optimal cross-tier spectrum access probability is much larger
than that achieved in previous works. Moreover, the proposed general spectrum access framework
provides high feasibility since it is applicable for different base station models.
Next, two kinds of optimal cognitive spectrum access schemes for macro-femto HetNets
are determined. Specifically, each optimal spectrum access scheme consists of three parts:
spectrum deployment, cognitive cross-tier spectrum access and co-tier spectrum access.
Corresponding to these three components, there are three DoFs that we can manipulate to
maximize the ASE of macro-femto HetNets. We determine the optimal F-ALOHA based
overlay and underlay cognitive spectrum access schemes for macro-femto HetNets by optimizing
the three DoFs, where the spectrum overlay and underlay are two spectrum sharing
techniques. With the derived optimal three DoFs, the ASE of macro-femto HetNets can be
maximized. Moreover, many meaningful results are obtained after determining the optimal
spectrum access schemes. As a simple example, we find that cochannel deployment is the optimal spectrum deployment for sparse macro-femto HetNets while partial orthogonal deployment
is optimal for dense networks.
To explore the dominate factor in the ASE of macro-femto HetNets, we further investigate
the parameters involved in the expression of ASE based on the derived optimal three DoFs in
the proposed overlay and underlay spectrum access schemes. Analytical results show that the
factor which dominates the ASE of macro-femto HetNets is the number of active femto-links
per unit area. In addition, the maximum ASE of macro-femto HetNets can be obtained with
the optimal number of active femto-links per unit area, which is closely related to the femtocell
intensity. Therefore, to maximize the ASE of macro-femto HetNets, the optimal number
of active femto-links supported in each unit area should be achieved. Simulation results shall
validate all the analytical results and demonstrate the effectiveness of our proposed optimal
cognitive spectrum access schemes.
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