In cellular wireless networks, it is very important to deal with Mobile station (MS) handoff between cells so that they can maintain a continuous and QoS-guaranteed service. There are four basic types of handoff protocols; network-controlled handoff (NCHO), mobile-assisted handoff (MAHO), soft handoff (SHO), and mobile-controlled handoff (MCHO). From NCHO to MCHO, the control protocols tend to decentralize the decision making process, which help shorten handoff delays; however, meanwhile, the decentralization makes the measurement information available to make a handoff decision also decreased. Next, we briefly introduce these four types of handoff mechanisms.



Network-controlled handoff

NCHO is a centralized handoff protocol, in which it is network that makes handoff decision based on measurements of the signal quality of mobile station (MS) at a number of based stations (BS). Specifically, if the MS is measured to have a weaker signal in its old cell, while a stronger signal in a neighbouring cell, then a handoff decision could be made by the network to switch BS from the old cell to the new cell. Such a type of handoff in general takes 100-200 ms and produces a noticeable ?interruption? in the conversation. However, overall delay of such a type of handoff is in general in the range of 5-10 s. Thus, this type of handoff is not suitable to a rapid changing environment and a high density of users due to the associated delay. NCHO is used in the first-generation analogue systems such as AMPS.

Mobile-assisted handoff

MAHO distributes the handoff decision process. It is MS makes measurements, and the Mobile switch centre (MSC) makes decisions on handoff. Compared to NCHO, this mechanism has more distributed control, thereby helping to improve the overall handoff delay, typically in the range of 1 s.

Soft handoff

SHO is often used in conjunction with MAHO. Rather than immediately terminating the connection between a MS and a BS. In the course of handoff, a new connection is established first between the MS and a new BS, while keeping the old connection between the MS and the old station. Only after the new connection can stably transmit data, the old connection is released. Thus, SHO is a ?make before break? mechanism. This mechanism helps ensure the service continuity, which is however at the cost of more capacity resource consumption during the handoff (as two connections are established simultaneously).

Mobile-controlled handoff

In contrast to NCHO, it is MS that totally control and make decisions on handoff in the MCHO approach. A MS keeps on measuring signal strength from all the surround base station (BS)s. If the MS find that there is a new BS who has a stronger signal than that of an old BS, it may consider to handoff from the old BS to the new BS given a certain signal threshold is reached. MCHO is the highest degree of handoff decentralization, thereby enabling it to have a very fast handoff speed, typically on the order of 0.1 s.

Summary

We have introduced four types of handoff mechanisms that are widely used in cellular wireless networks. From centralization to decentralization, network-controlled handoff (NCHO) shows the highest centralization with the network totally controlling and making decisions on handoff, while mobile-controlled handoff (MCHO) gives full flexibility to MS to allow it to make decisions on handoff. As a result, decentralized handoff mechanisms show advantages of very fast handoff speed, while centralized mechanisms generally take much longer time. In modern cellular wireless networks, decentralized handoff mechanisms are widely applied due to their good scalability and fast handoff speeds.

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