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