Position Assisted Coordinate HARQ in LTE Systems for High Speed Railway

Voice over IP (VoIP) in LTE-R (LTE for Railway) will replace the circuit-switched (CS) voice service of the GSMR (GSM for Railway). However, it will face new problems in high speed railway scenario. In overlapping areas, the handover signaling (i.e. handover measurement and decision) is of higher priority than Hybrid Automatic Repeat reQuest (HARQ) retransmission so that the HARQ retransmission will be canceled when conflicts happen. If an user equipment (UE) is handed over to the target eNodeB (eNB), the HARQ retransmission or ACK/NACK of source eNB will not be received by the UE as normal, and the UE will wait for the potential retransmission and ACK/NACK. These particular problems will increase the VoIP packet error rate (PER) significantly. In order to solve these problems, a position assisted coordinate HARQ scheme in LTE system for high speed railway is proposed. With the help of trains position, the source eNB and target eNB share packets through coordination. If the handover is triggered, the UE will stop sending ACK/NACK. When the handover is completed, the UE sends the ACK of the last continuous packet to ensure lossless handover. Simulation results show that our proposal can reduce the VoIP PER in high speed railway scenario, and improve the system reliability, as well as VoIP capacity.