Radio Protocol Architecture

The radio protocol architecture for LTE can be separated into Control Plane architecture and User Plane architecture, as shown in Figure 10 (Tutorialspoint, 2015):

Figure 10: LTE radio protocol architecture (Tutorialspoint, 2015).

At User Plane side, the application creates data packets that are processed by protocols such as TCP, UDP and IP, while in the Control Plane the radio resource control (RRC) protocol writes the signalling messages that are exchanged between the eNodeB and the UE. In both cases, the information is processed by the Packet Data Convergence Protocol (PDCP), the Radio Link Control (RLC) protocol and the medium access control (MAC) protocol, before being passed to the physical layer for transmission, see Figure 11.

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Control Plane

The Control Plane consists of protocols for control and support of the User Plane functions (3GPP TS 23.401 v12.2.0, 09-2013):

 Controlling the evolved UMTS Terrestrial Radio Access (E-UTRA) network access connections, such as attaching to and detaching from E-UTRAN;

 Controlling the attributes of an established network access connection, such as activation of an IP address;

 Controlling the routing path of an established network connection in order to support user mobility;

 Controlling the assignment of network resources to meet changing user demands. Figure 11 refers to the control plane protocol stack between the UE and the MME. Additional variants can also be considered (i.e.: MME-MME, SGW-PDN GW, MME-HSS), which are outside the scope of this deliverable. For detailed information on these variants please refer to 3GPP TS 23.401 (3GPP TS 23.401 v12.2.0, 09-2013).

Figure 11: Control Plane UE – MME (3GPP TS 23.401 v12.2.0, 09-2013).

User Plane

The User Plane carries the network's user’s traffic. The most common scenario used for User Plane and its protocol stack is depicted in Figure 12:

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The main functions for each of the LTE layers are briefly described in Table 1.

Table 1: Summary of the main functions in each LTE layer.

Layer description

Non Access Stratum (NAS) Protocols

NAS is specified in 3GPP TS 24.301 (3GPP TS 24.301, 12-2013) and forms the highest stratum of the control plane between the UE and the MME.

 NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW.

Radio Resource Control (RRC)

RRC is specified in 3GPP TS 36.331 (3GPP TS 36.331 v12.5.0, 03- 2015). The main services and functions of the RRC sublayer include:

 Broadcast of system information related to the non-access stratum (NAS);

 Broadcast of system information related to the access stratum (AS);

 Paging;

 Establishment, maintenance and release of an RRC connection between the UE and E-UTRAN;

 Security functions including key management;

 Establishment, configuration, maintenance and release of point- to-point Radio Bearers;

 Mobility functions;

 QoS management functions;

 UE measurement reporting and control of the reporting;  NAS direct message transfer to/from NAS from/to UE.

Packet Data Convergence Control (PDCP)

PDCP is specified in 3GPP TS 36.323 (3GPP TS 36.323 v12.3.0, 03- 2015) and the main services and functions for the user plane include:

 Header compression and decompression: Robust Header Compression (ROHC) only;

 Transfer of user data;

 In-sequence delivery of upper layer protocol data units (PDUs) at PDCP re-establishment procedure for RLC acknowledged mode (AM);

 Duplicate detection of lower layer Service Data Units (SDUs) at PDCP re-establishment procedure for RLC AM;

 Retransmission of PDCP SDUs at handover for RLC AM;  Ciphering and deciphering;

 Timer-based SDU discard in uplink.

 The main services and functions of the PDCP for the control plane include:

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 Transfer of control plane data.

Radio Link Control (RLC)

RLC is specified in 3GPP TS 36.322 (3GPP TS 36.322 v12.2.0, 03- 2015). The main services and functions of the RLC sublayer include:

 Transfer of upper layer PDUs;

 Error Correction through ARQ (only for AM data transfer);  Concatenation, segmentation and reassembly of RLC SDUs (only

for unacknowledged mode (UM) and AM data transfer);

 Re-segmentation of RLC data PDUs (only for AM data transfer);  In sequence delivery of upper layer PDUs (only for UM and AM

data transfer);

 Duplicate detection (only for UM and AM data transfer);  Protocol error detection and recovery;

 RLC SDU discard (only for UM and AM data transfer);  RLC re-establishment.

Medium Access Layer (MAC)

MAC is specified in 3GPP TS 36.321 (3GPP TS 36.321 v12.5.0, 03- 2015) and the main services and functions of the MAC sublayer include:

 Mapping between logical channels and transport channels;  Multiplexing/demultiplexing of MAC SDUs belonging to one or

different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels;  Scheduling information reporting;

 Error correction through Hybrid ARQ (HARQ);  Priority handling between logical channels of one UE;

 Priority handling between UEs by means of dynamic scheduling;  Transport format selection;

 Padding.

Physical Layer (Layer 1)

Air Interface Physical Layer is specified in 3GPP TS 36.201 (3GPP TS 36.201 v12.2.0, 03-2015), 3GPP TS 36.211 (3GPP TS 36.211 v12.5.0, 03- 2015), 3GPP TS 36.212 (3GPP TS 36.212 v12.4.0, 03-2015), 3GPP TS 36.213 (3GPP TS 36.213 v12.5.0, 03-2015) and 3GPP TS 36.214 (3GPP TS 36.214 v12.2.0, 03-2015).

The LTE air interface physical layer offers data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service:

 Error detection on the transport channel and indication to higher layers;

 FEC encoding/decoding of the transport channel;  Hybrid ARQ soft-combining;

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 Rate matching of the coded transport channel to physical channels;

 Mapping of the coded transport channel onto physical channels;  Power weighting of physical channels;

 Modulation and demodulation of physical channels;  Frequency and time synchronisation;

 Radio characteristics measurements and indication to higher layers;

 Multiple Input Multiple Output (MIMO) antenna processing;  Transmit Diversity (TX diversity);

 Beamforming;  RF processing.