4G Mobile Broadband - LTE Network Architecture and Protocol Stack

ABSTRCT

The goal of the LTE standard is to create specifications for a new radio-access technology geared to higher data rates, low latency and greater spectrum efficiency. The spectrum efficiency target for the LTE system is three to four times higher than the current HSPA system. These aggressive spectral efficiency targets require using the technology envelope by employing advanced air-interface techniques such as low-PAPR orthogonal uplink multiple access based on SC-FDMA (single-carrier frequency division multiple access) MIMO multiple-input multiple-output multi-antenna technologies, inter-cell interference mitigation techniques, low latency channel structure and single-frequency network (SFN) broadcast. The researchers and engineers working on the standard come up with new innovative technology proposals and ideas for system performance improvement. Due to the highly aggressive standard development schedule, these researchers and engineers are generally unable to publish their proposals in conferences or journals, etc. In the standards development phase, the proposals go through extensive scrutiny with multiple sources evaluating and simulating the proposed technologies from system performance improvement and implementation complexity perspectives. Therefore, only the highest-quality proposals and ideas finally make into the standard.

Keywords: LTE Architecture, UDP, GDP, MIMO, MIME, MCCH, MBMS, QOS

1. INTRODUCTION

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 support of all services including voice through packet connections. The result in a highly simplified flatter architecture with only two types of node rarely 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.

2. LTE NETWORK ARCHITECTURE

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 Figure1. The S1 interface supports a many-to-many relationship between MME / GW and eNBs.

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) is defined. The S-GW acts as a local mobility anchor forwarding and receiving packets to and from the eNB servicing 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.

The MME is a signaling only entity and hence user IP packets do not go through the 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 reach ability including the control and implementation 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 such functions 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 the network side are now terminated in eNB.

Figure 1: Network Architecture

Figure 2: Functional split between eNB and MME / GW.

2.1 PROTOCOL STACK AND CONYTOL PLANE

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 Figure 4 shows the control plane protocol stack.

Figure 3: User plane protocol.

Figure 4: 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 functions such as EPS (evolved packet system) bearer management, authentication and security control, etc.

The S1 and X2 interface protocol stacks are shown in Figures 2.5 and 2.6 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

Figure 5: S1 interface user and control planes.

Figure 6: X2 interface user and control planes.

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.

3. QOS AND BEARER SERVICE ARCHITECTURE

Applications such as VoIP, web browsing, video telephony and video streaming have special QoS needs. Therefore, an important feature of any network network is the provision of a QoS mechanism to enable differentiation of packet flows based on QoS requirements. In EPS, QoS flows called EPS bearers are established between the UE and the P-GW as shown in Figure 7. A radio bearer transports the packets of an EPS bearer between a UE and an eNB. Each IP flow (eg VoIP) is associated with a different EPS bearer and the network can prioritize traffic accordingly.

Figure 7: EPS bearer service architecture.

When receiving an IP packet from the Internet, P-GW performances package classification based on certain predefined parameters and sends it an appropriate EPS bearer. Based on the EPS bearer, eNB maps packages to the appropriate radio QoS bearer. There is one-to-one mapping between an EPS bearer and a radio bearer.

4. LAYER 2 STRUCTURE

The layer 2 of LTE consist of three sub layers normally 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 sub layer provide the transport channels while the SAP between the MAC and RLC sub layers provide the logical channels. The MAC sub layer performances multiplexing of logical channels on to the transport channels.

The downlink and uplink layer 2 structures are given in Figures 8 and 9 respectively. The difference between downlink and uplink structures is that in the downlink, the MAC sub layer 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 sub layers 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).

The main services and functions of the RLC sub layers 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-acknowledgment 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 failure 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 notifiable and potential retransmissions and re-segmentation can be initiated.

Figure 8: Downlink layer 2 structure.

Figure 9: Uplink layer 2 structure.

The PDCP layer functions such as header compression and decompression, ciphering and in-sequence delivery and duplicate detection at handover for RLCAM, etc. The header compression and decompression is performed using the robust header compression (ROHC) protocol. 5.1 Downlink logical, transport and physical channels

4.1 DOWNLINK LOGICAL, TRANSPORT AND PHYSICAL CHANNELS

The relationship between downlink logical, transport and physical channels is shown in Figure 10. 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.

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 typically the common control channel (CCCH) and the restricted 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 continuous 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 (DLSCH).

Figure 10: Downlink logical, transport and physical channels mapping.

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, MBM 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 sub frames with 40 ms timing interval. The 40 ms timing is detected blindly without requiring any explicit signaling. Also, each sub frame transmission of BCH is self-decodable and UEs with good channel conditions may not need to wait for reception of all the four sub frames 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 sub frame 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.

4.2 UPLINK LOGICAL, TRANSPORT AND PHYSICAL CHANNELS

The relationship between uplink logical, transport and physical channels is shown in Figure 2.11. 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.

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 the 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.

5. PROTOCOL STATES AND STATES TRANSITIONS

In the LTE system, two radio resource control (RRC) states previously RRC IDLE and RRC CONNECTED states are defined as described in Figure 2.12. A UE moves from RRC IDLE state to RRC CONNECTED state when an RRC connection is successfully established. A UE can move back from RRC CONNECTED to RRC IDLE state by releasing the RRC connection. In the RRC IDLE state, UE can receive broadcast / multicast data, monitors a paging channel to detect incoming calls, processes neighbor cell measurements and cell selection / reselection and acquires system information. Furthermore, in the RRC IDLE state, a UE specific DRX (discontinuous reception) cycle may be configured by upper layers to enable UE power savings. Also, mobility is controlled by the UE in the RRC IDLE
State.

In the RRC CONNECTED state, the transfer of uncast data to / from UE, and the transfer of broadcast or multicast data to UE can take place. At lower layers, the UE may be configured with a UE specific DRX / DTX (continuous transmission). Furthermore, UE monitors control channels associated with the shared data channel to determine if data is scheduled for it, provides channel quality feedback information, processes neighbor cell measurements and measurement reporting and acquires system information. Unlike the RRC IDLE state, the mobility is controlled by the network in this state.

Figure 11 Uplink logical, transport and physical channels mapping.

Figure 12: UE states and state transitions.

6. SEAMLESS MOBILITY SUPPORT

An important feature of a mobile wireless system such as LTE is support for seamless mobility across eNBs and across MME / GWs. Fast and seamless handovers (HO) is particularly important for delay-sensitive services such as VoIP. The handovers occur more frequently across eNBs than across core networks because the area covered by MME / GW serving a large number of eNBs is generally much larger than the area covered by a single eNB. The
signaling on X2 interface between eNBs is used for handover preparation. The S-GW acts as anchor for inter-eNB handovers.

In the LTE system, the network relies on the UE to detect the neighboring cells for handovers and therefore no neighbor cell information is signified from the network. For the search and measurement of inter-frequency neighboring cells, only the carrier frequencies need to be indicated. An example of active handover in an RRC CONNECTED state is shown in Figure 13 where a UE moves from the coverage area of ​​the source eNB (eNB1) to the coverage area of ​​the target eNB (eNB2). The handovers in the RRC CONNECTED state are network controlled and assisted by the UE. The UE sends a radio measurement report to the source eNB1 indicating that the signal quality on eNB2 is better than the signal quality on eNB1. As preparation for handover, the source eNB1 sends the coupling information and the UE context to the target eNB2 (HO request) [6] on the X2 interface. The target eNB2 may perform admission control dependent on the received EPS bearer QoS information. The target eNB configures the required resources according to the received EPS bearer QoS information and reserves a C-RNTI (cell radio network temporary identifier) ​​and optionally a RACH preamble.

Figure 13: Active handovers.

The C-RNTI provides a unique UE identification at the cell level identifying the RRC connection. When eNB2 signals to eNB1 that it is ready to perform the handover via HO ​​response message, eNB1 commands the UE (HO command) to change the radio bearer to eNB2. The UE receives the HO command with the necessary parameters (ie new C-RNTI, optionally dedicated RACH preamble, possible expiry time of the allocated RACH preamble, etc.) and is directed by the source eNB to perform the HO. The UE does not need to delay the handover execution for delivering the HARQ / ARQ responses to source eNB.

After receiving the HO command, the UE performances synchronization to the target eNB and accesses the target cell via the random access channel (RACH) following a contention-free procedure if a restricted RACH preamble was allocated in the HO command or following a contention-based procedure if no dedicated preamble was allocated. The network responds with uplink resource allocation and timing advance to be applied by the UE. When the UE has successfully accessed the target cell, the UE sends the HO confirm message (C-RNTI) along with an uplink buffer status report indicating that the handover procedure is completed for the UE. After receiving the HO confirm message, the target eNB sends a path switch message to the MME to inform that the UE has changed cell. The MME sends a user plane update message to the S-GW. The S-GW switches the downlink data path to the target eNB and sends one or more "end marker" packages on the old path to the source eNB and then releases any user-plane / TNL resources towards the source eNB. Then S-GW sends a user plane update response message to the MME. Then the MME confirms the path switch message from the target eNB with the path switch response message. After the path switch response message is received from the MME, the target eNB informs success of HO to the source eNB by sending release source message to the source eNB and triggers the release of resources. On receiving the release source message, the source eNB can release radio and C-plane related sources associated with the UE context.

During handover preparation U-plane tunnels can be established between the source ENB and the target eNB. There is one tunnel established for uplink data forwarding and another one for downlink data forwarding for each EPS bearer for which data forwarding is applied. During handover execution, user data can be forwarded from the source eNB to the target eNB. Forwarding of downlink user data from the source to the target eNB should take place in order as long as packets are received at the source eNB or the source eNB buffer is exhausted.

For mobility management in the RRC IDLE state, concept of tracking area (TA) is introduced. A tracking area generally covers multiple eNBs as described in Figure 2.14. The tracking area identity (TAI) information indicating which TA an eNB belongs to is broadcast as part of system information. A UE can detect change of tracking area when it receives a different TAI than in its current cell. The UE updates the MME with its new TA information as it moves across TAs. When P-GW receives data for a UE, it buffers the packages and queries the MME for the UE's location. Then the MME will page the UE in its most current TA. A UE can be registered in multiple TAs simultaneously. This enables power saving at the UE under conditions of high mobility because it does not need to constantly update its location with the MME. This feature also minimizes load on TA boundaries.

8. MULTICAST BROADCAST SYSTEM ARCHITECTURE

In the LTE system, the MBMS either use a single-cell transmission or a multi-cell transmission. In single-cell transmission, MBMS is transmitted only in the coverage of a specific cell and therefore combining MBMS transmission from multiple cells is not supported. The single-cell MBMS transmission is performed on DL-SCH and hence uses the same network architecture as the unicast traffic.

Figure 14: Tracking area update for UE in RRC IDLE state.

The MTCH and MCCH are mapped on DL-SCH for point-to-multipoint transmission and scheduling is done by the eNB. The UEs can be allocated dedicated uplink feedback channels identical to those used in unicast transmission, which enables HARQ ACK / NACK and CQI feedback. The HARQ retransmissions are made using a group (service specific) RNTI (radio network temporary identifier) ​​in a time frame that is co-ordinated with the original MTCH transmission. All UEs receiving MBMS are able to receive the retransmissions and combine with the original transmissions at the HARQ level. The UEs that are associated with dedicated uplink feedback channel are in RRC CONNECTED state. In order to avoid unnecessary MBMS transmission on MTCH in a cell where there is no MBMS user, network can detect presence of users interested in the MBMS service by polling or through UE service request.

The multi-cell transmission for the evolved multimedia broadcast multicast service (MBMS) is realized by transmitting an identical waveform at the same time from multiple cells. In this case, MTCH and MCCH are mapped to MCH for point-to-multipoint transmission. This multi-cell transmission mode is referred to as multicast broadcast single frequency network (eMBSFN) as described in detail in Chapter 17. An MBSFN transmission from multiple cells within an MBSFN area is indicated as a single transmission by the UE. An MBSFN area compresses a group of cells within an MBSFN synchronization area of ​​a network that is co-ordinate to achieve MBSFN transmission. An MBSFN synchronization area is defined as an area of ​​the network in which all eNBs can be synchronized and perform MBSFN transmission. An MBMS service area may consist of multiple MBSFN areas. A cell within an MBSFN synchronization area may form part of multiple SFN areas each characterized by different content and set of participating cells.

Figure 15. The eMBMS service area and MBSFN areas.

An example of MBMS service area consistent of two MBSFN areas, area A and area B, is depicted in Figure 2.15. The MBSFNA area consists of cells A1-A5, cell AB1 and AB2. The MBSFN area consists of cells B1-B5, cell AB1 and AB2. The cells AB1 and AB2 are part of both MBSFN area A and area B. The cell B5 is part of area B but does not contribute to MBSFN transmission. Such a cell is referred to as MBSFN area reserved cell. The MBSFN area reserved cell may be allowed to transmit for other services on the resources allocated for the MBSFN but at a restricted power. The MBSFN synchronization area, the MBSFN area and reserved cells can be semi-static configured by O & M.

The MBMS architecture for multi-cell transmission is described in Figure 2.16. The multicell multicast coordination entity (MCE) is a logical entity, which means it can also be part of another network element such as eNB. The MCE performances functions such as the allocation of the radio resources used by all eNBs in the MBSFN area as well as determining the radio configuration including the modulation and coding scheme. The MBMS GW is also a logical entity which main function is sending / broadcasting MBMS packets with the SYNC protocol to each eNB transmitting the service. The MBMS GW hosts the PDCP layer of the user plane and uses IP multicast for forwarding MBMS user data to eNBs.

The eNBs are connected to eMBMS GW via a pure user plane interface M1. As M1 is a pure user plane interface, no control plane application part is defined for this interface. Two control plane interfaces M2 and M3 are defined. The application part on M2 interface conveys radio configuration data for the multi-cell transmission mode eNBs. The application part on M3 interface between MBMS GW and MCE performances MBMS session control signaling on EPS bearer level that includes procedures such as session start and stop.

An important requirement for multi-cell MBMS service transmission is MBMS content synchronization to enable MBSFN operation. The eMBMS user plane architecture for content synchronization is described in Figure 2.17. A SYNC protocol layer is defined on the transport network layer (TNL) to support the content synchronization mechanism. The SYNC protocol carries additional information that enables eNBs to identify the timing for radio frame transmission as well as detect packet loss.

Figure 16: eMBMS logical architecture.

Figure 17: The eMBMS user plane architecture for content synchronization.

The eNBs participating in multicell MBMS transmission are required to comply with content synchronization mechanism. An eNB transmitting only in single-cell service is not required to comply with the stringent timing requirements indicated by SYNC protocol. In case PDCP is used for header compression, it is located in eMBMS GW. The UEs receiving MTCH transmissions and taking part in at least one MBMS feedback scheme need to be in an RRC CONNECTED state. On the other hand, UEs receiving MTCH transmissions without taking part in an MBMS feedback mechanism can be in either an RRC IDLE or an RRC CONNECTED state. For receiving single-cell transmission of MTCH, a UE may need to be in RRC CONNECTED state. The signaling by which a UE is triggered to move to RRC CONNECTED state solely for single-cell reception purposes is carried on MCCH.

8. SUMMARY
The LTE system is based on highly simplified network architecture with only two types of nodes typically eNode-B and MME / GW. Fundamentally, it is a flattened architecture that enables simplified network design while still supporting seamless mobility and advanced QoS mechanisms. This is a major change relative to traditional wireless networks with many more network nodes using hierarchical network architecture. The simplification of network was
partly possible because LTE system does not support macro-diversity or soft-handoff and since it does not require a RNC in the access network for macro-diversity combining. Many of the other RNC functions are incorporated into the eNB. The QoS logical connections are provided between the UE and the gateway enabling differentiation of IP flows and meeting the requirements for low-latency applications.

A separate architecture optimized for multi-cell multicast and broadcast is provided, which consist of two logical nodes normally the multicast co-ordination entity (MCE) and the MBMS gateway. The MCE allocates radio resources as well as determines the radio configuration to be used by all eNBs in the MBSFN area. The MBMS gateway broadcasts MBMS packets with the SYNC protocol to each eNB transmitting the service. The MBMS gateway uses IP multicast for forwarding MBMS user data to eNBs. The layer 2 and radio resource control protocols are designed to enable reliable delivery of data, ciphering, header compression and UE power savings.

9. REFERENCES

[1] 3GPPTS 36.300 V8.4.0, Evolved Universal Terrestrial Radio Access Network (E-UTRA): Overall Description.

[2] 3GPP TS 29.060 V8.3.0, GPRS Tunneling Protocol (GTP) Across the Gn and Gp Interface.

[3] IETF RFC 4960, Stream Control Transmission Protocol.

[4] IETF RFC 3095, RObust Header Compression (ROHC): Framework and Four Profiles: RTP, UDP, ESP, and uncompressed.

[5] 3GPP TS 36.331 V8.1.0, Radio Resource Control (RRC) Protocol Specification.

[6] 3GPP TR 23.882 V1.15.1, 3GPP System Architecture Evolution (SAE): Report on Technical Options and Conclusions.