| In cellular wireless networks, it is very
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| | with MAHO. Rather than immediately
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| important to deal with Mobile station
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| | terminating the connection between a MS
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| (MS) handoff between cells so that they
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| | and a BS. In the course of handoff, a new
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| can maintain a continuous and
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| | connection is established first between
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| QoS-guaranteed service. There are four
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| | the MS and a new BS, while keeping the
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| basic types of handoff protocols;
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| | old connection between the MS and the old
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| network-controlled handoff (NCHO),
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| | station. Only after the new connection
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| mobile-assisted handoff (MAHO), soft
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| | can stably transmit data, the old
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| handoff (SHO), and mobile-controlled
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| | connection is released. Thus, SHO is a
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| handoff (MCHO). From NCHO to MCHO, the
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| | "make before break" mechanism. This
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| control protocols tend to decentralize
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| | mechanism helps ensure the service
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| the decision making process, which help
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| | continuity, which is however at the cost
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| shorten handoff delays; however,
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| | of more capacity resource consumption
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| meanwhile, the decentralization makes the
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| | during the handoff (as two connections
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| measurement information available to make
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| | are established simultaneously).
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| a handoff decision also decreased. Next,
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| | Mobile-controlled handoff In contrast to
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| we briefly introduce these four types of
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| | NCHO, it is MS that totally control and
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| handoff mechanisms. Network-controlled
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| | make decisions on handoff in the MCHO
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| handoff NCHO is a centralized handoff
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| | approach. A MS keeps on measuring signal
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| protocol, in which it is network that
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| | strength from all the surround base
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| makes handoff decision based on
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| | station (BS)s. If the MS find that there
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| measurements of the signal quality of
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| | is a new BS who has a stronger signal
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| mobile station (MS) at a number of based
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| | than that of an old BS, it may consider
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| stations (BS). Specifically, if the MS is
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| | to handoff from the old BS to the new BS
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| measured to have a weaker signal in its
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| | given a certain signal threshold is
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| old cell, while a stronger signal in a
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| | reached. MCHO is the highest degree of
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| neighbouring cell, then a handoff
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| | handoff decentralization, thereby
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| decision could be made by the network to
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| | enabling it to have a very fast handoff
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| switch BS from the old cell to the new
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| | speed, typically on the order of 0.1 s.
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| cell. Such a type of handoff in general
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| | Summary We have introduced four types of
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| takes 100-200 ms and produces a
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| | handoff mechanisms that are widely used
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| noticeable "interruption" in the
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| | in cellular wireless networks. From
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| conversation. However, overall delay of
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| | centralization to decentralization,
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| such a type of handoff is in general in
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| | network-controlled handoff (NCHO) shows
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| the range of 5-10 s. Thus, this type of
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| | the highest centralization with the
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| handoff is not suitable to a rapid
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| | network totally controlling and making
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| changing environment and a high density
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| | decisions on handoff, while
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| of users due to the associated delay.
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| | mobile-controlled handoff (MCHO) gives
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| NCHO is used in the first-generation
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| | full flexibility to MS to allow it to
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| analogue systems such as AMPS.
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| | make decisions on handoff. As a result,
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| Mobile-assisted handoff MAHO distributes
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| | decentralized handoff mechanisms show
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| the handoff decision process. It is MS
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| | advantages of very fast handoff speed,
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| makes measurements, and the Mobile switch
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| | while centralized mechanisms generally
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| centre (MSC) makes decisions on handoff.
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| | take much longer time. In modern cellular
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| Compared to NCHO, this mechanism has more
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| | wireless networks, decentralized handoff
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| distributed control, thereby helping to
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| | mechanisms are widely applied due to
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| improve the overall handoff delay,
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| | their good scalability and fast handoff
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| typically in the range of 1 s. Soft
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| | speeds.
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| handoff SHO is often used in conjunction
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