Handover Performance Restricted Call Admission Control  Algorithm for HAP Communications Systems

High altitude platform communications systems possess the advantages of both terrestrial wireless communications system and satellite communications system, and will play an important role in future communications infrastructure. In this paper, geographical information based call admission control (CAC) schemes are discussed. Call density restricted CAC (DR-CAC) scheme and handover time interval restricted CAC (TR-CAC) scheme are analyzed. We present a handover Performance Restricted CAC (PR-CAC) scheme, in which the handover dropping probability introduced by a new call arrival is calculated using the geographical information, and a handover performance threshold is set to restrict the handover performance and to improve the call blocking performance. Simulation results show that the call blocking probability for the modified TR-CAC scheme is 13.6% lower than that for the original scheme with a fixed threshold of handover dropping probability. In the application of the TR-CAC scheme, the handover interval time threshold can be adapted according to the current traffic to minimize the call blocking probability while meeting the handover dropping performance. The PR-CAC scheme can keep the handover dropping performance to a specific threshold. Moreover, the PR-CAC scheme can achieve better call blocking performance than the other schemes mentioned in this paper, with an improvement of 25.3% compared with time based channel reservation algorithm, and with an improvement of 6.5% compared with TR-CAC.