VDL mode 4 is a digital data link operating in the VHF band, its mainly use is for the aviation industry. VDL4 can as an example provide with positioning data, speed information of aircrafts or vehicles equipped with a VDL4 transponder.
A connection between the ground system and the airborne system is called a point to point connection, which can be used for various applications. This data link needs to be transferred between ground stations during flights in order maintain the connection, which is called handoff.
The handoff process needs to be quick enough to not drop the link and at the same time a low rate of handoffs is desirable. The data link is regarded as a narrow resource and link management data for handoff is considered as overhead.
This thesis studies how to make the handoff procedure optimal with respect to involved aspects. Previous research of handoff algorithms and models of the VHF-channel are treated. Standardized parameters and procedures in VDL4 and are explored in order to find an optimal solution for the handoff procedure in VDL4.
The studied topics are analyzed and it is concluded to suggest an algorithm based on an adaptive hysteresis including signal quality and positioning data provided in VDL4. Standardized parameters which could be useful in the handoff procedure are commented, since the VDL4 standards are under development.
Source: Linköping University
Author: Andersson, Rickard