The DL scheduler is spit into 3 functional parts:

1.The Static Scheduler:
Which assigns a fixed amount of Transport Blocks as well as PDCCH and PDSCH resources for the BCCH over the DL-SCH Transport Channel.Those resources are permanently allocated.

2.The Semi-static Scheduler:
Which assigns Transport Blocks as well as PDCCH and PDSCH resources for PCCH and CCCH over the PCH and DL PDSCH SCH Transport Channels. The semi-static scheduler also assigns a regular set of Transport Blocks for all established VoIP bearers.

3.The Dynamic Scheduler:
Which assigns Transport Blocks as well as PDCCH and PDSCH resources for DCCH & DTCH over the DL-SCH Transport Channels.
The dynamic scheduler is also in charge of sending the MAC Control Timing Advance control messages in order to keep the UE in the connected mode, synchronized with the network

The LTE DL scheduler is composed of 2 main algorithms:

A pre-booking stage which reserve resources over the PDSCH for the static and semistatic schedulers. A scheduling stage which assign the resources over the PDSCH for effective traffic.