Layer 2 Structure | LTE for 4G

The layer 2 of LTE consists of three sublayers namely medium access control, radio link control (RLC) and packet data convergence protocol (PDCP). The service access point (SAP) between the physical (PHY) layer and the MAC sublayer provide the transport channels while the SAP between the MAC and RLC sublayers provide the logical channels. The MAC sublayer performs multiplexing of logical channels on to the transport channels.

The downlink and uplink layer 2 structures are given in Figures 1 and 2 respectively. The difference between downlink and uplink structures is that in the downlink, the MAC sublayer also handles the priority among UEs in addition to priority handling among the logical channels of a single UE. The other functions performed by the MAC sublayers in both downlink and uplink include mapping between the logical and the transport channels, multiplexing of RLC packet data units (PDU), padding, transport format selection and hybrid ARQ (HARQ).

Figure 1: Downlink layer 2 structure.

Figure 2: Uplink layer 2 structure.

The main services and functions of the RLC sublayers include segmentation, ARQ in-sequence delivery and duplicate detection, etc. The in-sequence delivery of upper layer PDUs is not guaranteed at handover. The reliability of RLC can be configured to either acknowledge mode (AM) or un-acknowledge mode (UM) transfers. The UM mode can be used for radio bearers that can tolerate some loss. In AM mode, ARQ functionality of RLC retransmits transport blocks that fail recovery by HARQ. The recovery at HARQ may fail due to hybrid ARQ NACK to ACK error or because the maximum number of retransmission attempts is reached. In this case, the relevant transmitting ARQ entities are notified and potential retransmissions and re-segmentation can be initiated.

The PDCP layer performs functions such as header compression and decompression, ciphering and in-sequence delivery and duplicate detection at handover for RLC AM, etc. The header compression and decompression is performed using the robust header compression (ROHC) protocol .

Downlink logical, transport and physical channels

The relationship between downlink logical, transport and physical channels is shown in Figure 3. A logical channel is defined by the type of information it carriers. The logical channels are further divided into control channels and traffic channels. The control channels carry control-plane information, while traffic channels carry user-plane information.

Figure 3: Downlink logical, transport and physical channels mapping.

In the downlink, five control channels and two traffic channels are defined. The downlink control channel used for paging information transfer is referred to as the paging control channel (PCCH). This channel is used when the network has no knowledge about the location cell of the UE. The channel that carries system control information is referred to as the broadcast control channel (BCCH). Two channels namely the common control channel (CCCH) and the dedicated control channel (DCCH) can carry information between the network and the UE. The CCCH is used for UEs that have no RRC connection while DCCH is used for UEs that have an RRC connection. The control channel used for the transmission of MBMS control information is referred to as the multicast control channel (MCCH). The MCCH is used by only those UEs receiving MBMS.

The two traffic channels in the downlink are the dedicated traffic channel (DTCH) and the multicast traffic channel (MTCH). A DTCH is a point-to-point channel dedicated to a single UE for the transmission of user information. An MTCH is a point-to-multipoint channel used for the transmission of user traffic to UEs receiving MBMS.

The paging control channel is mapped to a transport channel referred to as paging channel (PCH). The PCH supports discontinuous reception (DRX) to enable UE power saving. A DRX cycle is indicated to the UE by the network. The BCCH is mapped to either a transport channel referred to as a broadcast channel (BCH) or to the downlink shared channel (DL-SCH). The BCH is characterized by a fixed pre-defined format as this is the first channel UE receives after acquiring synchronization to the cell. The MCCH and MTCH are either mapped to a transport channel called a multicast channel (MCH) or to the downlink shared channel (DL-SCH). The MCH supports MBSFN combining of MBMS transmission from multiple cells. The other logical channels mapped to DL-SCH include CCCH, DCCH and DTCH. The DL-SCH is characterized by support for adaptive modulation/coding, HARQ, power control, semi-static/dynamic resource allocation, DRX, MBMS transmission and multi-antenna technologies. All the four-downlink transport channels have the requirement to be broadcast in the entire coverage area of a cell.

The BCH is mapped to a physical channel referred to as physical broadcast channel (PBCH), which is transmitted over four subframes with 40 ms timing interval. The 40 ms timing is detected blindly without requiring any explicit signaling. Also, each subframe transmission of BCH is self-decodable and UEs with good channel conditions may not need to wait for reception of all the four subframes for PBCH decoding. The PCH and DL-SCH are mapped to a physical channel referred to as physical downlink shared channel (PDSCH). The multicast channel (MCH) is mapped to physical multicast channel (PMCH), which is the multi-cell MBSFN transmission channel.

The three stand-alone physical control channels are the physical control format indicator channel (PCFICH), the physical downlink control channel (PDCCH) and the physical hybrid ARQ indicator channel (PHICH). The PCFICH is transmitted every subframe and carries information on the number of OFDM symbols used for PDCCH. The PDCCH is used to inform the UEs about the resource allocation of PCH and DL-SCH as well as modulation, coding and hybrid ARQ information related to DL-SCH. A maximum of three or four OFDM symbols can be used for PDCCH. With dynamic indication of number of OFDM symbols used for PDCCH via PCFICH, the unused OFDM symbols among the three or four PDCCH OFDM symbols can be used for data transmission. The PHICH is used to carry hybrid ARQ ACK/NACK for uplink transmissions.

Uplink logical, transport and physical channels

The relationship between uplink logical, transport and physical channels is shown in Figure 4 In the uplink two control channels and a single traffic channel is defined. As for the downlink, common control channel (CCCH) and dedicated control channel (DCCH) are used to carry information between the network and the UE. The CCCH is used for UEs having no RRC connection while DCCH is used for UEs having an RRC connection. Similar to downlink, dedicated traffic channel (DTCH) is a point-to-point channel dedicated to a single UE for transmission of user information. All the three uplink logical channels are mapped to a transport channel named uplink shared channel (UL-SCH). The UL-SCH supports adaptive modulation/coding, HARQ, power control and semi-static/dynamic resource allocation.

Figure 4: Uplink logical, transport and physical channels mapping.

Another transport channel defined for the uplink is referred to as the random access channel (RACH), which can be used for transmission of limited control information from a UE with possibility of collisions with transmissions from other UEs. The RACH is mapped to physical random access channel (PRACH), which carries the random access preamble.

The UL-SCH transport channel is mapped to physical uplink shared channel (PUSCH). A stand-alone uplink physical channel referred to as physical uplink control channel (PUCCH) is used to carry downlink channel quality indication (CQI) reports, scheduling request (SR) and hybrid ARQ ACK/NACK for downlink transmissions.

Network Architecture and Protocols | LTE for 4G

The LTE network architecture is designed with the goal of supporting packet-switched traffic with seamless mobility, quality of service (QoS) and minimal latency. A packet-switched approach allows for the supporting of all services including voice through packet connections. The result in a highly simplified flatter architecture with only two types of node namely evolved Node-B (eNB) and mobility management entity/gateway (MME/GW). This is in contrast to many more network nodes in the current hierarchical network architecture of the 3G system. One major change is that the radio network controller (RNC) is eliminated from the data path and its functions are now incorporated in eNB. Some of the benefits of a single node in the access network are reduced latency and the distribution of the RNC processing load into multiple eNBs. The elimination of the RNC in the access network was possible partly because the LTE system does not support macro-diversity or soft-handoff.

We discuss network architecture designs for both unicast and broadcast traffic, QoS architecture and mobility management in the access network. We also briefly discuss layer 2 structure and different logical, transport and physical channels along with their mapping.

All the network interfaces are based on IP protocols. The eNBs are interconnected by means of an X2 interface and to the MME/GW entity by means of an S1 interface as shown in Figure 1. The S1 interface supports a many-to-many relationship between MME/GW and eNBs.

Figure 1: Network architecture.

Figure 1: Network architecture.

The functional split between eNB and MME/GW is shown in Figure 2. Two logical gateway entities namely the serving gateway (S-GW) and the packet data network gateway (P-GW) are defined. The S-GW acts as a local mobility anchor forwarding and receiving packets to and from the eNB serving the UE. The P-GW interfaces with external packet data networks (PDNs) such as the Internet and the IMS. The P-GW also performs several IP functions such as address allocation, policy enforcement, packet filtering and routing.

Figure 2.2: Functional split between eNB and MME/GW.

Figure 2.2: Functional split between eNB and MME/GW.

The MME is a signaling only entity and hence user IP packets do not go through MME. An advantage of a separate network entity for signaling is that the network capacity for signaling and traffic can grow independently. The main functions of MME are idle-mode UE reachability including the control and execution of paging retransmission, tracking area list management, roaming, authentication, authorization, P-GW/S-GW selection, bearer management including dedicated bearer establishment, security negotiations and NAS signaling, etc.

Evolved Node-B implements Node-B functions as well as protocols traditionally implemented in RNC. The main functions of eNB are header compression, ciphering and reliable delivery of packets. On the control side, eNB incorporates functions such as admission control and radio resource management. Some of the benefits of a single node in the access network are reduced latency and the distribution of RNC processing load into multiple eNBs.

The user plane protocol stack is given in Figure 3. We note that packet data convergence protocol (PDCP) and radio link control (RLC) layers traditionally terminated in RNC on the network side are now terminated in eNB.

Figure 3: User plane protocol.

Figure 3: User plane protocol.

Figure 4 shows the control plane protocol stack. We note that RRC functionality traditionally implemented in RNC is now incorporated into eNB. The RLC and MAC layers perform the same functions as they do for the user plane. The functions performed by the RRC include system information broadcast, paging, radio bearer control, RRC connection management, mobility functions and UE measurement reporting and control. The non-access stratum (NAS) protocol terminated in the MME on the network side and at the UE on the terminal side performs functions such as EPS (evolved packet system) bearer management, authentication and security control, etc.

Figure 4: Control plane protocol stack.

Figure 4: Control plane protocol stack.

The S1 and X2 interface protocol stacks are shown in Figures 5 and 6 respectively. We note that similar protocols are used on these two interfaces. The S1 user plane interface (S1-U) is defined between the eNB and the S-GW. The S1-U interface uses GTP-U (GPRS tunneling protocol – user data tunneling) on UDP/IP transport and provides non-guaranteed delivery of user plane PDUs between the eNB and the S-GW. The GTP-U is a relatively simple IP based tunneling protocol that permits many tunnels between each set of end points. The S1 control plane interface (S1-MME) is defined as being between the eNB and the MME. Similar to the user plane, the transport network layer is built on IP transport and for the reliable transport of signaling messages SCTP (stream control transmission protocol) is used on top of IP. The SCTP protocol operates analogously to TCP ensuring reliable, in-sequence transport of messages with congestion control . The application layer signaling protocols are referred to as S1 application protocol (S1-AP) and X2 application protocol (X2-AP) for S1 and X2 interface control planes respectively.

Figure 5: S1 interface user and control planes.

Figure 5: S1 interface user and control planes.

Figure 6: X2 interface user and control planes.

Figure 6: X2 interface user and control planes.