Handover Management in Mobile Satellite Systems

Handover Management in Mobile Satellite Systems

Due to the high mobility of low earth orbit (LEO) satellites, there is a significant number of handover attempts in a LEO-based mobile satellite communication system, causing a
high handover failure rate. This paper proposes to extend the period of which a handover request is valid, and thus rendering higher probability of successful handover. Satellite
communication service can be provided by geostationary earth orbit (GEO), medium earth orbit (MEO) or low earth orbit (LEO) satellites. Because of its much shorter distance from earth, lower power requirement and thus smaller mobile terminal (MT) size, LEO satellite system is a preferable choice. In this paper, only the LEO satellite system is considered. The satellite coverage area, or its footprint, is divided into a number of areas, each of which spotted by one of the satellite's multiple spotbeams, forming a cell. Since a LEO satellite is not located at a geosynchronous orbit, it is mobile with respect to a fixed point on earth. Hence an active MT may move from one cell to another and handover occurs. The ground velocity of the MT is ignored compared to the
much higher satellite velocity. Suppose the length of a cell is 400 km and the satellite moves at a velocity of 6.6 km/sec, the time taken for a MT to cross a cell, Tcell, is about 60 seconds. Thus handover is extremely frequent in this system. And it is probable that a call is dropped due to unsuccessful handover. Handover is prone to failure when the subsequent cell has no unused channel to offer. Drop call is a phenomenon where an ongoing call has to be discontinued, which the users find hard to tolerate with, making it a major technical issue.There have been some methods proposed to minimise handover failure. It is widely accepted that handover requests are to be prioritised over new call requests, either by allocating guard channels to the handover requests [1], or by queuing up the handover requests when all the channels in one cell are occupied [1] [2]. This is because dropping an ongoing call is less desirable than blocking a new call attempt. There are also proposals of making the handover request earlier, so that the request has longer time to wait for a free channel, thus reducing the handover failure rate [3] [4].
Gerard Maral et al. have proposed that a handover request is to be made to a cell as early as the MT enters the cell located right before the target cell [3]. In [4], the time of sending out a handover request during handover process was made available regardless of the location of MS in a cell. In all of these cases, a call somehow has to be terminated when the originating MT has crossed into the target cell and yet handover is not granted by the target cell. The termination is done since no service is provided by either the original cell or the target cell. In this paper it is proposed that in a similar situation, the call be only temporarily discontinued for a specific amount of time, before it is permanently terminated if there is still no available channel. Although no service is provided by both cells to the MT, its handover request which has been queued is ‘kept in view’ by the target cell. During this idle period, there is a chance that an originally occupied channel in the target cell is released. If this is the case, this channel is allocated to the suspended call and handover is completed. As a result, the handover failure rate is reduced.
Mathematical analysis has been carried out to verify the idea and the results are encouraging. For a user, he/she only experiences a short period of call discontinuity and is notified about the temporary discontinuity through a special tone. In terms of quality of service (QoS), this is more tolerable to the users compared to a drop call.

A handover management strategy is proposed to efficiently manage the channel resource of a cell in a multibeam mobile satellite system (MSS) and improves its service quality by reducing the interbeam handover failure rate (Phf) caused by limited number of communication channels. The Extended Queueing of Handover (EQH) technique extends the channel reservation time of a handover request into the adjacent cell that the user terminal (UT) is subsequently entering (destination cell). Both mathematical analysis and simulation show that EQH reduces Phf significantly, without compromising the new call
admission rate and efficiency of channel utilisation.

The footprint of a multibeam satellite is divided into cells where each cell is illuminated by a spotbeam. Interbeam handover is frequent in mobile satellite system (MSS) due to the high velocity of the satellite (about 7 km/h for a low earth orbit satellite). When a user terminal (UT) leaves from one cell and enters the adjacent one, a handover process must be completed for the sake of call continuity, where a
communication channel must be allocated to the UT by the adjacent cell (destination cell). In a channel resource limited system, handover is subject to failure when the destination cell has no idle channel to offer. In this case the call is dropped and this is intolerable to users. In order to provide higher handover quality, system operator has to allocate a larger portion of the channel resource to the ongoing call as compared to the new call. A method in use is by applying the blocked-calls-queued policy to the calls
requesting for handover (handover calls) [1]; and on the other hand sacrificing some new calls through the blocked-call-cleared policy. The longer a handover call stays in a queue, the higher chance of it being handed over successfully. Other methods that prioritise handover call over new call are: guard channelallocation [2], and channel reservation in advance [3] [4]. The compromise of new call causes inefficiency in channel utilisation because channels that are allocated to handover call cannot be taken up by new calls even though they are idle.