LTE Protocols:

Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS)

The NAS protocol is used on the control plane between the UE and the core network, the MME (Mobility Management Entity). The NAS layer is handling mobility related functions between the UE and the core network, like attach and tracking area update, authentication and security. It is also responsible for establishing and maintaining IP connectivity between the UE and the core network.

Radio Resource Control (RRC)

The RRC (Radio Resource Control) layer is the manager of the radio resource and the main controlling protocol in the Access Stratum. It is responsible for establishing the radio bearer and configuring the lower layers. It is also handling mobility related functions between the UE and the access network, like handover and measurements, broadcasting system information and paging.

Packet Data Convergence (PDCP) Specification

The PDCP (Packet Data Convergence Protocol) layer has functions for handling RRC (Radio Resource Control) messages originating from the control plane and IP (Internet Protocol) packets from the user plane. The functions for supporting the control plane is integrity protection and ciphering. For the user plane PDCP offers ciphering, header compression and reordering and retransmission during handover.

E-UTRA Radio Link Control (RLC) protocol specification

The RLC (Radio Link Control) layer is responsible for delivery of upper layer SDUs. RLC can work in 3 different modes, depending on the requirement of the data to be transported; Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). AM has functions for retransmission of erroneous received PDUs. The RLC working in UM or AM also has functions for segmentation and concatenation of  SDUs sent over the LTE air interface. For received PDUs RLC support functions for re-ordering, duplicate detection and reassembly.

E-UTRA MAC Protocol Specification

The MAC (Medium Access Control) layer multiplex data from different services (radio bearers) onto a MAC PDU, a transport block. The MAC layer maintain the negotiated QoS, Quality of Service for each radio bearer by instructing the layer above, RLC, about the amount of data to be transmitted from each radio bearer. An important task for the MAC layer is scheduling. The scheduler in the eNB controls the assignment of uplink and downlink radio resources.

E-UTRA Physical Channels and Modulation

The Physical layer is responsible for procedures and processing the data.

E-UTRA Physical Channels and Modulation  E-UTRA Multiplexing and channel coding

E-UTRA Physical Layer Procedures

Layer 1

The Layer 1 is defined in a bandwidth agnostic way based on resource blocks, allowing the LTE Layer 1 to adapt to various spectrum allocations. A resource block spans either 12 sub-carriers with a sub-carrier bandwidth of 15kHz or 24 sub-carriers with a sub-carrier bandwidth of 7.5kHz each over a slot duration of 0.5 ms.

The radio frame structure type 1 is used for FDD (for both full duplex and half duplex operation) and has a duration of 10 ms and consists of 20 slots with a slot duration of 0.5 ms. Two adjacent slots form one sub-frame of length 1 ms. The radio frame structure type 2 is used for TDD and consists of two half-frames with a duration of 5 ms each and containing each 8 slots of length 0.5 ms and three special fields (DwPTS, GP and UpPTS) which have configurable individual lengths and a total length of 1 ms. A sub-frame consists of two adjacent slots, except for sub-frames 1 and 6, which consist of DwPTS, GP and UpPTS. Both 5 ms and 10 ms switch-point periodicity are supported.

To support a Multimedia Broadcast and Multicast Service (MBMS), LTE offers the possibility to transmit Multicast/Broadcast over a Single Frequency Network (MBSFN), where a time-synchronized common waveform is transmitted from multiple cells for a given duration. MBSFN transmission enables highly efficient MBMS, allowing for over-the-air combining of multi-cell transmissions in the UE, where the cyclic prefix is utilized to cover the difference in the propagation delays, which makes the MBSFN transmission appear to the UE as a transmission from a single large cell. Transmission on a dedicated carrier for MBSFN with the possibility to use a longer CP with a sub-carrier bandwidth of 7.5 kHz is supported as well as transmission of MBSFN on a carrier with both MBMS transmissions and point-to-point transmissions using time division multiplexing.

Transmission with Multiple Input and Multiple Output antennas (MIMO) are supported with configurations in the downlink with two or four transmit antennas and two or four receive antennas, which allow for multi-layer transmissions with up to four streams.

Under Single-User MIMO, the base station allocates 1 or 2 streams to the selected user, in the case of Multi-User MIMO the allocation of different (1 or 2) streams is done to different users. This is supported in both UL and DL.

The physical channels defined in the downlink are:

Physical Downlink Shared Channel (PDSCH); Physical Multicast Channel (PMCH); Physical Downlink Control Channel (PDCCH); Physical Broadcast Channel (PBCH); Physical Control Format Indicator Channel (PCFICH); Physical Hybrid ARQ Indicator Channel (PHICH).

The physical channels defined in the uplink are:

Physical Random Access Channel (PRACH), Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH). In addition, signals are defined as reference signals, primary and secondary

synchronization signals.

The modulation schemes supported in the downlink and uplink are QPSK, 16QAM and 64QAM.

The channel coding scheme for transport blocks in LTE is Turbo Coding as for UTRA, with a coding rate of R=1/3, two 8-state constituent encoders and a contention-free Quadratic Permutation Polynomial (QPP) turbo code internal interleaver. Trellis termination is used for the turbo coding. Before the turbo coding, transport blocks are segmented into byte aligned segments with a maximum information block size of 6144 bits. Error detection is supported by the use of 24 bit CRC.

There are several Physical layer procedures involved with LTE operation:

Cell search; Power control; Uplink synchronization and Uplink timing control; Random access related procedures; HARQ related procedures.

Through the control of physical layer resources in the frequency domain as well as in the time and power domain, implicit support of interference coordination is provided in LTE.

S1 General aspects and principles

The S1-AP (S1 Application Protocol) protocol is used on the control plane between the eNB and the core network, the MME. It is used for signalling about the S1 bearer set-up and paging initiation.

X2 General aspects and principles

The X2-AP (X2 Application Protocol) protocol is used on the control plane between the eNBs on the X2 interface, which is optional. It is used for signalling about the X2 bearer set-up and forwarding of UE context during handover.

GTP-C

The GTPv2-C (GPRS Tunnelling Protocol version2 – Control Plane) protocol is used on the control plane between the LTE core nodes, MME, S-GW, PDN-GW. It is also used between LTE nodes and UMTS nodes. MME and S-GW to SGSN. It is used for signalling about the EPS bearer set-up and forwarding of UE context during MME change.