LTE VoLTE : Semi-persistent Scheduling

Voice over LTE uses SPS scheduling at the MAC Scheduler for optimal handling of Voice.Dynamic scheduling can also be used for Voice transmission in LTE but that is not very efficient. Voice packet are normally AMR encoded and data rate is 12.2 kbps support. Packets are expected at the interval of 20 ms. Only one new tranmission from eNodeB happens for a particular UE in the interval of 1 ms. No segmentation or fragmentation happens for SPS packets at RLC. 2-3 PRBs per UE is normally allocated for SPS scheduling. SPS Scheduling requires resources to be pre-allocated for the UE. DRX long duration is used for the optimal transmission. The DRX onduration time is matched with the UE peristent resource scheduling occasion. Retransmission scheduled dynamicall based on the HARQ ACK/NACK. UE keeps awake to listen to ACK/NACK in a particular tti and retranmismit if it sees NACK. In SPS Scheduling the resources are automatically released after maximun no of SPS scheduling opportunity. It is normally configured as 8 (20*8 ms) as silent Insertion Detection is 160ms. No other UEs can be allocated when a particular UE is allocated a SPS resource at particular SPS occassion. SR Prohibit Timer is also used in conjunction with SPS scheduling. After sending SR, SR is prohibited for this duration because SPS schedulred resources may be on the way. SPS grant is allocated by providing a particular pattern in PDCCH and similalry it is removed using a pattern. For IMS Volte qci 1 and qci 5 is used.