In cellular wireless networks, it is very important to deal with mobile station (MS) handover between cells so that they can maintain continuous service and guaranteed quality of service. There are four basic types of handover protocols; network controlled handover (NCHO), mobile assisted handover (MAHO), continuous handover (SHO) and mobile controlled handover (MCHO). From NCHO to MCHO, control protocols tend to decentralize the decision-making process, which helps shorten handover delays; however, in the meantime, decentralization makes metering information available to make a handover decision also decreased. We briefly introduce these four types of handover mechanisms below.
Network controlled transfer
NCHO is a centralized handover protocol, in which the handover decision is made by the network based on mobile station (MS) signal quality measurements at multiple base stations (BS). Specifically, if the MS is measured to have a weaker signal in its old cell, while a stronger signal in a neighboring cell, then the network could make a handover decision to change the BS from the old cell to the new one. This type of handover typically takes 100 to 200 ms and produces a noticeable “break” in the conversation. However, the total delay for this type of handover is generally in the range of 5 to 10 s. Therefore, this type of handover is not suitable for a rapidly changing environment and high user density due to the associated delay. NCHO is used in the first generation analog systems like AMPS.
Mobile Assisted Transfer
MAHO distributes the transfer decision process. The MS performs the measurements, and the mobile switching center (MSC) makes handover decisions. Compared to NCHO, this mechanism has a more distributed control, which helps to improve the overall handover delay, typically in the range of 1 s.
soft hand
SHO is often used in conjunction with MAHO. Instead of immediately terminating the connection between an MS and a BS. In the course of handover, a new connection between the MS and a new BS is first established, while the old connection between the MS and the old station is maintained. Only after the new connection can transmit data stably is the old connection released. Therefore, SHO is a “make before you break” mechanism. This mechanism helps to guarantee the continuity of the service, which however comes at the cost of a greater consumption of capacity resources during the handover (since two connections are established simultaneously).
Mobile Controlled Transfer
Unlike NCHO, it is the MSs who fully control and make decisions about the handover in the MCHO approach. An MS continues to measure the signal strength of all surrounding base stations (BSs). If the MS finds that there is a new BS that has a stronger signal than an old BS, it may consider moving from the old BS to the new one given that a certain signal threshold is reached. MCHO is the highest degree of handover decentralization, which allows it to have a very fast handover speed, typically on the order of 0.1 s.
Summary
We have introduced four types of handover mechanisms that are widely used in cellular wireless networks. From centralization to decentralization, Network Controlled Handoff (NCHO) shows the highest centralization with the network fully controlling and making decisions on the handover, while Mobile Controlled Handoff (MCHO) gives full flexibility to MS to allow it to make transfer decisions. As a result, decentralized handover mechanisms show the advantages of very fast handover speed, while centralized handover mechanisms generally take much longer. In modern cellular wireless networks, decentralized handover mechanisms are widely applied due to their good scalability and fast handover speeds.
This article was contributed by http://www.ylesstech.com.