In cellular wireless networks, it is very important to handle 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 handoff (NCHO), mobile assisted handoff (MAHO), soft handoff (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 measurement information available to make a handover decision also decreased. Below, we briefly introduce these four types of handover mechanisms.
Network Controlled Transfer
NCHO is a centralized handoff protocol, where the handoff decision is made by the network based on the mobile station (MS) signal quality measurements at various 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 switch the BS from the old cell to the new cell. . This type of handover typically takes 100 to 200 ms and produces a noticeable “break” in the conversation. However, the total delay of 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 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 more distributed control, which helps improve the overall handover delay, typically in the 1 s range.
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 the handover, a new connection is first established between the MS and a new BS, while the old connection between the MS and the old station is maintained. Only after the new connection can stably transmit data 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 has the cost of a higher consumption of capacity resources during the handover (since two connections are established simultaneously).
Mobile controlled transfer
Unlike NCHO, it is the MS that fully controls and makes handover decisions in the MCHO approach. An MS continues to measure the signal strength of all surrounding base stations (BS). If the MS finds that there is a new BS that has a stronger signal than an old BS, it may consider switching from the old BS to the new one given that a certain signal threshold is reached. MCHO is the highest degree of handoff 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 handoff mechanisms that are widely used in cellular wireless networks. From centralization to decentralization, Network Controlled Handover (NCHO) shows the highest centralization with the network fully controlling and making decisions on the handover, while Mobile Controlled Handover (MCHO) gives full flexibility to the MS to allow make transfer decisions. As a result, decentralized handover mechanisms show the advantages of a very fast handover speed, while centralized 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.
