Medium Access Control Functions

The layer-2 functions in LTE are classified into medium access control, radio link control, and packet data convergence protocol functions. The MAC sublayer is responsible for data transfer and logical channel multiplexing, HARQ retransmissions and uplink/downlink scheduling, random access procedure, and maintenance of uplink timing. It is also responsible for multiplexing/de-multiplexing of data across multiple component carriers when carrier aggregation is used. From physical layer perspective, MAC sublayer provides and receives services in the form of transport channels. The data in a transport channel is organized into transport blocks. By varying the transport format of the transport blocks, the MAC sublayer can realize different data rates and reliability levels. MAC sublayer receives the RLC SDUs mapped to various logical channels in the downlink and generates the MAC PDUs that further become the transport blocks in the physical layer. In this chapter, we take a top-down systematic approach to study and understand various MAC sublayer constituent functional components and services in LTE/LTE-Advanced and how those functions interact and interoperate with other protocol layers.