Classification of VHD algorithms

There are various ways to classify VHD algorithms. I have chosen to divide VHD algorithms into four groups based on the handover decision criteria used and the methods used to process these.

RSS based algorithms: RSS is used as the main handover decision criterion in this group. Various strategies have been developed to compare the RSS of the current point of attachment with that of the candidate point of attachment. In RSS based horizontal handover

decision strategies are classified into the following six subcategories: relative RSS, relative RSS with threshold, relative RSS with hysteresis, relative RSS with hysteresis and threshold, and prediction techniques. For VHD, relative RSS is not applicable, since the RSS from different types of networks can not be compared directly due to the disparity of the technologies involved. For example, separate thresholds for each network. Furthermore, other network parameters such as bandwidth are usually combined with RSS in the VHD process.

Bandwidth based algorithms: Available bandwidth for a mobile terminal is the main criterion in this group. In some algorithms, both bandwidth and RSS information are used in the decision process. Depending on whether RSS or bandwidth is the main criterion considered, an algorithm is classified either as RSS based or bandwidth based.

Cost function based algorithms: This class of algorithms combine metrics such as monetary cost, security, bandwidth and power consumption in a cost function, and the handover decision is made by comparing the result of this function for the candidate networks. Different weights are assigned to different input metrics depending on the network conditions and user preferences.

Combination algorithms: These VHD algorithms attempt to use a richer set of inputs than the others for making handover decisions. When a large number of inputs are used, it is usually very difficult or impossible to develop analytical formulations of handover decision processes. Due to this reason, researchers apply machine learning techniques to formulate the processes. A literature survey reveals that fuzzy logic and artificial neural networks based techniques are popular choices. Fuzzy logic systems allow human experts’ qualitative thinking to be encoded as algorithms to improve the overall efficiency. Examples of applying this approach into VHD can be found. If there is a comprehensive set of input-desired output patterns available, artificial neural networks can be trained to create handover decision algorithms. It is also possible to create adaptive versions of these algorithms. By using continuous and real-time learning processes, the systems can monitor their performance and modify their own structure to create highly effective handover decision algorithms.

VHD criteria

Several parameters as shown in Fig. 1 have been proposed for use in the VHD algorithms.

We briefly explain each of them below.

Received signal strength (RSS) is the most widely used criterion because it is easy to measure and is directly related to the service quality. There is a close relationship between the RSS  readings and the distance from the mobile terminal to its point of attachment. Majority of existing horizontal handover algorithms use RSS as the main decision criterion, and RSS is an important criterion for VHD algorithms as well.

Network connection time refers to the duration that a mobile terminal remains connected to a point of attachment. Determining the network connection time is very important for choosing the right moment to trigger a handover so that the service quality could be maintained at a satisfactory level. For example, a handover done too early from a WLAN to a cellular network would waste network resources while being too late would result in a handover failure. Determining the network connection time is also important for reducing the number of superfluous handovers, as handing over to a target network with potentially short connection time should be discouraged. The network connection time is related to a mobile terminal’s location and velocity. Both the distance from the mobile terminal to its point of attachment and the velocity of the mobile terminal affect the RSS at the mobile terminal. The variation of the RSS then determines the time for which the mobile terminal stays connected to a particular network. Network connection time is especially important for VHD algorithms because heterogeneous networks usually have different sizes of network coverage.

Available bandwidth is a measure of available data communication resources expressed in bit/s. It is a good indicator of traffic conditions in the access network and is especially important for delay-sensitive applications.

Power consumption becomes a critical issue especially if a mobile terminal’s battery is low. In such situations, it would be preferable to handover to a point of attachment which would help extending valuable battery life

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Monetary cost: For different networks, there would be different charging policies, therefore, in some situations the cost of a network service should be taken into consideration in making handover decisions.

Security: For some applications, confidentiality or integrity of the transmitted data can be critical. For this reason, a network with higher security level may be chosen over another one which would provide lower level of data security.

User preferences: A user’s personal preference towards an access network could lead to the selection of one type of network over the other candidates. 

Handover Processes

Handover is the process of maintaining a user’s active sessions when a mobile terminal changes its connection point to the access network (called ‘‘point of attachment”), for example, a base station or an access point. Depending on the access network that each point of attachment belongs to, the handover can be either horizontal or vertical. 

A horizontal handover takes place between points of attachment supporting the same network technology,for example, between two neighbouring base stations of a cellular network. On the other hand, a vertical handover occurs between points of attachment supporting different network technologies, for example, between an IEEE 802.11 access point and a cellular network base station.

A handover process can be split into three stages: handover decision, radio link transfer and channel assignment. Handover decision involves the selection of the target point of attachment and the time of the handover. Radio link transfer is the task of forming links to the new point of attachment, and channel assignment deals with the allocation of channel resources. VHD algorithms help mobile terminals to choose the best network to connect to among all the available candidates. Here, we only focus on the research efforts and recent developments on improving the efficiency of VHD process. In contrast to horizontal handover decision algorithms which mainly consider RSS as the only decision criterion, for VHD algorithms, criteria such as cost of services, power consumption and velocity of the mobile terminal may need to be taken into consideration to maximize user satisfaction