With HPB, the BTS transmit power is adjusted to the maximum before the BSC sends a handover command to the MS. In addition, the BTS transmit power is not adjusted during the handover to ensure the success of the handover.
When the receive level of an MS drops rapidly, a handover occurs. In this case, the BSC cannot adjust the transmit power of the MS and BTS in time. The MS may fail to receive the handover command, thus leading to the call drop.