WHAT IS THE PURPOSE OF THE MAC LAYER IN LTE?

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.