Handover mechanisms in next generation heterogeneous wireless ...

HANDOVER MANAGEMENT
The two schemes mentioned earlier use a centralised approach, in which one single
entity located at the network end, makes handover decision for mobile users. In contrast,
the Mobile-Controlled Handover (MCHO) uses a decentralised approach. The MCHO is
employed by both the European DECT and the North American PACS air interface
protocols [30]. In the MCHO, a MH is completely in control of handover process. The
MH keeps examining radio link quality including signal strength and interference levels
on all the available channels. A handover is initiated by the MH when the radio link
quality of the serving BS drops below a certain threshold. Since the MH is unaware of
other mobile users, it simply triggers a handover to a selected BS, the one with the
strongest signal strength (RSS). This type of handover control allows for faster
handover decision and is effective in reducing handover latencies for high mobility
inside micro-cellular systems [31].
A number of publications [31-33] have provided insight into various handover control
strategies for next generation wireless networks. Zhu et al. developed a policy-based
two-element model in [32] to analyse policy-based handover control. Two conceptual
elements: Policy Enforcement Point (PEP) and Policy Decision Point (PDP) were
introduced. According to whether two elements are located in the same entity (e.g. a
network node or a MH), two possible handover control architectures are identified: 1)
The PEP and PDP located in BS/AP for NCHO/MAHO; 2) The PEP and PDP located in
MH for MCHO. Calvagna et al. summarise the advantages and disadvantages of the
two different approaches in [31]. Aguiar et al. demonstrate in [33] how the networkcontrolled
and mobile-controlled approaches can be implemented for the future all-IPbased
4G networks using the knowledge obtained from practising Daidalos project.
Both approaches were proved to be viable in terms of scalability and QoS support if an
appropriate architecture like Daidalos is implemented.
Generally, the suitable handover control scheme for an integrated wireless infrastructure
is coupled with a specific integration architecture. The NCHO allows optimised
resources management to be applied in the network, but may need extensive
collaboration between network domains. This makes it harder, if not impossible, to deal
with inter-operator handover. The MCHO ensures that handover decision is made
timely towards maximising service quality by taking only individual needs into
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