Radio Protocols for LTE and LTE-Advanced

Chapter 6

Medium Access Control (MAC)

3GPP provides the MAC specification in [1]. As the name suggests, the Medium Access Control (MAC) layer controls the upper layers' access to the communication medium, which is the PHY layer. The MAC layer is connected to the PHY layer below through transport channels, and the MAC layer is connected to the RLC layer above through logical channels. Thus, the MAC layer decides which logical channels can access the transport channels at a given time, and performs multiplexing and de-multiplexing of the data between them.

The MAC layer basically provides the radio resource allocation service and the data transfer service to the upper layer. As part of the radio resource allocation service the MAC layer performs procedures such as logical channel prioritization, power headroom reporting, the handling of UL grant and DL assignment, and so on. As part of the data transfer service, the MAC layer performs procedures such as scheduling requests, buffer status reporting, random access, and HARQ.

In addition, the MAC layer handles the Semi-Persistent Scheduling (SPS) procedure and the Discontinuous Reception (DRX) procedure. The SPS procedure is used to increase the cell capacity for a voice service, and the DRX procedure is used to reduce the power consumption of the UE.

6.1 MAC Functions and Services

As the layer in charge of controlling radio resource, connecting the RLC layer and the PHY layer, and transferring data, the following functions are supported ...