Radio Protocol Architecture in LTE
The EUTRAN radio protocol model specifies the protocols terminated between UE and eNB. The protocol stack follows the standard guidelines for radio protocol architectures (ITU-R M1035) and is thus quite similar to the WCDMA protocol stack of UMTS.
The protocol stack defines three layers: the physical layer (layer 1), data link and access layer (layer 2) and layer 3 hosting the access stratum and non-access stratum control protocols as well as the application level software (e.g. IP stack).
Physical layer: The physical layer forms the complete layer 1 of the protocol stack and provides the basic bit transmission functionality over air. In LTE the physical layer is driven by OFDMA in the downlink and SC-FDMA in the uplink. FDD and TDD mode can be combined (depends on UE capabilities) in the same physical layer. The physical layer uses physical channels to transmit data over the radio path. Physical channels are dynamically mapped to the available resources (physical resource blocks and antenna ports). To higher layers the physical layer offers its data transmission functionality via transport channels. Like in UMTS a transport channel is a block oriented transmission service with certain characteristics regarding bit rates, delay, collision risk and reliability. Note that in contrast to 3G WCDMA or even 2G GSM there are no dedicated transport or physical channels anymore, as all resource mapping is dynamically driven by the scheduler.
MAC (Medium Access Control): MAC is the lowest layer 2 protocol and its main function is to drive the transport channels. From higher layers MAC is fed with logical channels which are in one-to-one correspondence with radio bearers. Each logical channel is given a priority and MAC has to multiplex logical channel data onto transport channels. In the receiving direction obviously demultiplexing of logical channels from transport channels must take place. Further functions of MAC will be collision handling and explicit UE identification. An important function for the performance is the HARQ functionality which is official part of MAC and available for some transport channel types.
RLC (Radio Link Control): Each radio bearer possesses one RLC instance
working in either of the three modes: UM (Unacknowledged), AM (Acknowledged) or TM (Transparent). Which mode is chosen depends on the purpose of the radio bearer. RLC can thus enhance the radio bearer with ARQ (Automatic Retransmission on reQuest) using sequence numbered data frames and status reports to trigger retransmission. Note that it shall be possible to trigger retransmissions also via the HARQ entity in MAC. The second functionality of RLC is the segmentation and reassembly that divides higher layer data or concatenates higher layer data into data chunks suitable for transport over transport channels which allow a certain set of transport block sizes.
PDCP (Packet Data Convergence Protocol): Each radio bearer also uses one PDCP instance. PDCP is responsible for header compression (ROHC RObust
Header Compression; RFC 3095) and ciphering/deciphering. Obviously header
compression makes sense for IP datagram’s, but not for signaling. Thus the PDCP entities for signaling radio bearers will usually do ciphering/deciphering only.
RRC (Radio Resource Control): RRC is the access stratum specific control
protocol for EUTRAN. It will provide the required messages for channel management, measurement control and reporting, etc.
NAS Protocols: The NAS protocol is running between UE and MME and thus
must be transparently transferred via EUTRAN. It sits on top of RRC, which
provides the required carrier messages for NAS transfer.