The MAC layer functionalities in LTE are listed below such as:
- Hybrid ARQ (HARQ)
- Priority handling (scheduling)
- Transport format selection
- Discontinuous Reception (DRX) control etc.
MAC Layer Support
- The MAC layer does not support in-order delivery to Radio Link Control (RLC).
- HARQ re transmissions lead to MAC Protocol Data Units (PDUs) being received in a different order than they are sent.
- Due to the lack of MAC sequence numbers, it is up to the RLC receivers to restore the original sequence and to provide in-order delivery to higher layers.
- The MAC layer supports the ARQ in the RLC layer with certain triggers if residual HARQ errors are detected, e.g., if the maximum number of HARQ transmissions has been reached.
- MAC also allows flows from a single user to be multiplexed. Correspondingly, the MAC header carries multiplexing information which is used to de-multiplex RLC PDUs to different flows.
Structure of MAC
- The MAC PDU structure format is shown in Figure below.
- The header contains a logical channel id (LCID) and the length in bytes (L) for each contained Service Data Unit (SDU) or MAC control element, except for the last SDU for which no L field is present.
- The length of the L field can be 15 or 7 bit and is indicated by a flag (F).
- An extension bit (E) indicates if further header fields are present or if an SDU/MAC control element follows.
- MAC control elements are used to carry control information such as buffer status information and time alignment information. Each MAC control element type and padding are indicated by a corresponding LCID value. In case of MAC control elements with pre-defined sizes, the L and F fields in the header are omitted.