The MAC layer provides services through SAPs to the upper layer as all other sublayers of layer 2. The layer above MAC is the RLC layer; the lower layer is the physical layer which provides services to the MAC. In the case of the MAC, SAPs to the RLC layer are logical channels. Logical channels are used by higher layers to differentiate between logical connections which may use different metrics, for example, in terms of quality or delay, and so on. Furthermore, logical channels are used to distinguish control plane connections, either CCCHs or DCCHs, from user plane connections (DTCHs).
Services provided by the physical layer to the MAC layer are granted via another type of SAP. SAPs between the MAC and the physical layer are transport channels. Transport channels match data units to physical channels in which data is supposed to be transmitted. One exception is the PCH which is multiplexed into the PDSCH identified with the P-RNTI =0xFFFE.
Multiplexing of data units from logical channels to transport channels is one of the tasks of the MAC layer. Logical channels are differentiated with LCIDs. Tables 1 and 2 show the defined LCIDs and their values for DL and UL respectively. A CCCH always has LCID =0. Other UE dedicated channels start with LCID = 1.
Identity of the logical channel
UE contention resolution identity
Timing advance command
Identity of the logical channel
Power headroom report
A MAC PDU consists of a MAC payload part and a MAC header part. The MAC payload conveys multiple units of MAC control elements and MAC SDUs from higher layers. Therefore, the MAC header is also divided into sub-headers depending on the units carried in the MAC payload as MAC sub-headers describe the MAC payload units. There are various possible combinations of MAC control elements, MAC SDUs, and MAC padding derivatives. An example of a MAC PDU with a combination of MAC sub-headers, MAC control elements, and MAC SDUs in the payload section is depicted in Figure 1.
Figure 1: Example of MAC PDU consisting of MAC header, MAC control elements, MAC SDUs, and padding (TS36.321). Reproduced with permission from © 3GPP™
Logical channels which are multiplexed to transport channels are prioritized by the scheduling algorithm. The scheduling algorithm decides what to schedule on which physical resources as described in detail for DL scheduling. There is only one MAC entity per UE; thus, the UL within the UE has one MAC entity and the eNB executes multiple parallel MAC entities in the DL direction in case the eNB has to handle multiple UEs.
The MAC layer implements a soft combining N -process stop-and-wait FEC and detection mechanism, or HARQ (Hybrid Automatic Repeat Request). Transport blocks are protected with a FEC algorithm known as turbo codes. Soft combining means not correctly decoded blocks are not acknowledged in order to conduct a retransmission, but the previous received not decoded block is held in a soft buffer to be recombined with the new retransmission. This process of soft combining two or more receptions increases the chance that the last received retransmission can be decoded error-free.