Handover mechanisms in next generation heterogeneous wireless ...

DYNAMIC NEIGHBOUR TRUST INFORMATION RETRIEVAL FOR GLOBAL ROAMING
where R n represents the maximum residence time of the mobile user in the nPOA
without handover. The location update cost and the power save mode registration cost
largely depend on the signalling process taking place in networks. The NTC update
interval may change over different access networks.
Taking different approaches, the PSMN and AOMN methods are intended to
complement each other in facilitating network trust information distribution among
neighbouring networks. The AOMN that relies on actual handover event can produce
the most accurate information about neighbouring relationship. In the AOMN, the
mobile users sacrifice their QoS and have the attached network obtain its neighbour
network trust pattern in return. In contrast, the PSMN derives neighbouring relationship
from analysing location update records. The mobile users selected for participating in
the PSMN sacrifice their power consumption instead.
4.5 Performance Evaluation
A random walk model [78] as shown in Figure 4.7 is introduced to simulate a multioperator
environment. It is assumed that the radio coverage of every network (denoted
as a cell in the model) is hexagonal shaped, and has six neighbouring cells. A mobile
user has equal probability of moving to any of the six neighbouring networks. The cell
residence time follows a Gamma distribution. The rings as marked in Figure 4.7 are
used to group cells. Each ring of cells is surrounded by the neighbouring outer ring of
cells, and is also adjacent to an inner ring of cells. The outmost ring of cells is referred
to as boundary ring or Ring n, and assumed to be in an absorbing state. A mobile user
entering the boundary ring will remain in that ring at all times. This can be explained by
the nomadic behaviour of a mobile user. Moreover, it is assumed that each network
belongs to an independent operator to make the handover task more challenging. Thus,
whenever a mobile user leaves a cell, a NTC exchange between two adjacent cells along
with a handover can be expected. The objective of the proposed NTC process is to have
every network to obtain its neighbour network trust pattern. According to the procedure
of Figure 4.4, this requires that a network has to exchange the NTC data with all of its 6
neighbouring networks at least once.
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