Semi persistent scheduling

Every VoIP packet is received / sent every 20ms when the user is talking whereas in silence period, discontinuous transmission (DTX) is used to reduce the transmission rate. Also, in order to sustain voice quality, silent insertion descriptor (SID) packet arrives every 160ms. The frequent arrival/transmission of VoIP packet means large control overhead for lower layers (L1/L2) in the radio protocol stack. To deal with this issue, semi persistent scheduling plays an important role.

Scheduling is a mechanism where UE requests eNB for the resource allocation during each transmission time interval (TTI). If UE has some data that it needs to transmit continuously, it will request eNB every TTI for the resource allocation. This scheduling type is dynamic scheduling. The advantage of dynamic scheduling is flexibility and diversity of resource allocation but as mentioned, this results in huge L1/L2 load which in turn means inefficient use of scarce radio resources.

In case of semi persistent scheduling, eNB can assign predefined chunk of radio resources for VoIP users with interval of 20ms. Therefore, UE is not required to request resources each TTI, saving control plan overhead. This scheduling is semi-persistent in the sense that eNB can change the resource allocation type or location if required for link adaptation or other factors.