Scheduling on LTE Upload

The LTE UL scheduling is very similar to the DL scheduling, although the UL scheduler is a distinct entity. This section describes the difference from the DL scheduling only.

UL scheduling grants are indicated to the UE by transmitting all relevant UL scheduling information within the PDCCH. This is done by using a dedicated DCI type, DCI 0. Each UE has to monitor the PDCCH in every subframe for DCI types 0 scrambled with their RNTI. This does not apply for the case when power saving mode DRX is enabled, which switches off the UE's receiver periodically. UL resources are allocated without a designated PDCCH UL grant in the case of SPS or for non-adaptive HARQ retransmissions. A non-adaptive HARQ retransmission is triggered by the transmission of a Negative Acknowledgment (NACK) by the UE.

 
Figure 1: Example PDCCH message of DCI format 0 (uplink scheduling grant)

In the UL only localized scheduling is allowed, which means that an integer number of consecutive RBs is allocated to one UE. Furthermore, there is only one scheduling process per UE, thus there is not a dedicated scheduling process per radio bearer.

The UE feeds the UL scheduler with CQI, Buffer Status Reports (BSRs), ACKs/NACKs, and Scheduling Requests (SRs). BSRs indicate the fill level status of the current transmit buffer. This buffer status is reported in bins, quantizing the fill level in bytes. Figure 2 shows an example of a MAC BSR message.

 
Figure 2: Example MAC Buffer Status Report (BSR) message

LTE trial studies show that there are various cases of scheduled empty UL grants. This happens when the eNB assigns UL resources to a UE and the designated UE does not use those UL resources for an UL transmission. Operators should minimize such empty scheduled UL resource in order to optimize the usage of UL radio resources.