LTE Concepts

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This page is a DRAFT.

This page offers information about Long Term Evolution (LTE), its architecture and components. It describes the role of the eNodeB in the network and key concepts such as: E-UTRAN, Uu, X2, S1, MME/S-GW and EPC.

The LTE network overview

The Long Term Evolution Standard (LTE) is a wireless communications, high-speed data standard for mobile phones and data terminals. Its basis stands in the GSM/EDGE and UMTS/HSPA network technologies, with changes in terms of an increased capacity and higher speed by simplifying the core network and using a different radio interface.

The 3GPP (3rd Generation Partnership Project) developed the LTE standard in its Release 8 document series. Releases 9, 10 and 11 bring new features and enhancements, such as: carrier aggregation, enhanced downlink control channel, advanced MIMO technique and more. Release 12 delivered more enhancements, such as: FDD/TDD carrier aggregation, massive MIMO and beamforming or enhanced small cells and heterogeneous networks.

LTE uses either Frequency Division Duplex (FDD) or Time Division Duplex (TDD). While FDD makes use of separate bands to transmit uplink and downlink data, TDD uses time slots on the same frequency for both uplink and downlink. FDD and TDD LTE networks have been deployed on all continents.

LTE’s main advantages come from the following features:

  • increased carrier capacity of subscribers and coverage within a few decibels of the Shannon limit
  • high-speed data rates
  • reliable connectivity
  • cost-effectiveness

LTE’s performance can reach download rates of up to 299.6 Mbit/s and upload rates of up to 75.4 Mbit/s. It’s RAN latency is lower than 5ms latency for small IP packets in optimal conditions, and has a 2 up to 4 times improved spectral efficiency than previous communications technologies.

The LTE standard functions under the following parameters:

Frequency bands

Tables 5.5-1 and 5.6.1-1 from 3GPP TS 36.101 lists the LTE operating bands and frequencies.

Band number Type MHz Band name Uplink frequency Downlink frequency
1 FDD 2100 IMT 1920 - 1980
2 FDD 1900 PCS blocks A-F 1850 - 1910 1930 - 1990
3 FDD 1800 DCS 1710 - 1785 1805 -1880
4 FDD 1700 AWS blocks A-F (AWS-1) 1710 - 1755 2110 - 2155
5 FDD 850 CLR 824 - 849 869 - 894
7 FDD 2600 IMT-E 2500 - 2570 2620 - 2690
8 FDD 900 E-GSM 880 - 915 925 - 960
9 FDD 1800 Japan UMTS 1700 / Japan DCS 1749.9 - 1784.9 1844.9 - 1879.9
10 FDD 1700 Extended AWS blocks A-I 1710 - 1770 2110 - 2170
11 FDD 1500 Lower PDC 1427.9 - 1452.9 1475.9 - 1500.9
12 FDD 700 Lower SMH blocks A/B/C 698 - 716 728 - 746
13 FDD 700 Upper SMH block C 777 - 787 746 - 756
14 FDD 700 Upper SMH block D 788 - 798 758 - 768
17 FDD 700 Lower SMH blocks B/C 704 - 716 734 - 746
18 FDD 850 Japan lower 800 815 - 830 860 - 875
19 FDD 850 Japan upper 800 830 - 845 875 - 890
20 FDD 800 EU Digital Dividend 832 - 862 791 - 821
21 FDD 1500 Upper PDC 1447.9 - 1462.9 1495.5 - 1510.9
22 FDD 3500 3410 - 3500 3510 - 3600
23 FDD 2000 S-Band (AWS-4) 2000 - 2020 2180 - 2200
24 FDD 1600 L-Band (US) 1625.5 - 1660.5 1525 - 1559
25 FDD 1900 Extended PCS blocks A-G 1850 - 1915 1930 - 1995
26 FDD 850 Extended CLR 814 - 849 859 - 894
27 FDD 850 SMR 807 - 824 852 - 869
28 FDD 700 APT 703 - 748 758 - 803
29 FDD/CA 700 Lower SMH blocks D/E N/A 717 – 728
30 FDD 2300 WCS blocks A/B 2305 - 2315 2350 - 2360
31 FDD 450 452.5 - 457.5 462.5 - 467.5
32 FDD/CA 500 L-Band (EU) N/A 1452 – 1496
33 TDD 2100 IMT 1900 – 1920
34 TDD 2100 IMT 010 – 2025
35 TDD 1900 PCS (Uplink) 1850 – 1910
36 TDD 1900 PCS (Downlink) 930 – 1990
37 TDD 1900 PCS (Duplex spacing) 1910 – 1930
38 TDD 2600 IMT-E (Duplex Spacing) 2570 – 2620
39 TDD 1900 DCS-IMT gap 1880 – 1920
40 TDD 2300 2300 – 2400
41 TDD 2500 BRS / EBS 496 – 2690
42 TDD 3500 3400 – 3600
43 TDD 3700 3600 – 3800
44 TDD 700 APT 703 – 803
45 TDD 1500 L-Band (China) 1447 – 1467
46 TDD 5200 NII 5150 – 5925
65 FDD 2100 Extended IMT 1920 – 2010 2110 – 2200
66 FDD 1700 Extended AWS blocks A-J (AWS-1/AWS-3) 1710 – 1780
2110 – 2200
67 FDD / CA 700 EU 700 N/A 738 – 758

Worldwide, LTE networks run on many bands and across a wide range of frequencies.

LTE networks use bandwidths between 1.4 to 20 Mhz.

Modulation schemes

LTE uses the following inner modulation schemes:

  • Downlink: QPSK, 16-QAM, 64-QAM, 256-QAM (Release 12)
  • Uplink: QPSK, 16-QAM, 64-QAM (depending on the UE)

A lower QAM is more robust against noise and interference, while a higher QAM offers a higher data rate.

The components of the LTE network

A standard LTE system architecture consists of an Evolved UMTS Terrestrial Radio Access Network, more commonly known as E-UTRAN, and the System Architecture Evolution, also known as SAE. SAE’s main component is the Evolved Packet Core, also known as an EPC.

The E-UTRAN is comprised of:

  • User Equipment (UEs)
  • evolved Node B basestations (eNodeBs)
  • the Evolved Universal Terrestrial Radio Access (E-UTRA)

The EPC is comprised of:

  • the Mobility Management Entity (MME)
  • the Serving Gateway (SGW)
  • the Packet Data Network Gateway (PGW)
  • the Home Subscriber Server (HSS)
  • the Access Network Discovery and Selection Function (ANDSF)
  • the Evolved Packet Data Gateway (ePDG)

All these components will be described in depth in the following sections.

LTE classic architecture.png

The EPC

The Evolved Packet Core (EPC) is the LTE core network. It is comprised of components that have the following functions: mobility management, authentication, quality of service, routing upload and download IP packets, IP address allocation, and more.

The EPC has a “flat” IP architecture that allows the network to handle a great amount of data traffic in an efficient and cost-effective manner.

The sections below will describe the EPC’s main elements.

The MME

The Mobility Management Entity (MME) handles all of the signaling exchanges between the UEs and the EPC, as well as those between the eNodeBs and the EPC. The signaling performed by the MME is also known as the NAS (Non-Access Stratum) signaling, as it done through the NAS protocol. The MME connects to the eNodeB through the S1-AP interface and performs authentication. It connects to the HSS and requests the authentication information for the subscriber trying to connect to the network.

The MME has the following functions:

  • Authentication – enables UEs to authenticate to the network by exchanging authentication information between the UE and the HSS.
  • Mobility management – allows the subscriber’s mobility within the network or across networks.
  • Location update – keeps track of the subscriber’s location within the network, and of it’s current state.
  • Bearer establishment – establishes bearers by deciding on a gateway router to the Internet if there are more gateways available.
  • Handover support – enables handover between eNodeBs (for handover on the S1 interface)

The MME is also responsible for allocating a gateway router to the Internet if there are more available.

While the eNodeB itself has handover capabilities, the MME transmits handover messages between eNodeBs when the X2 interface is not available.

The NAS signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service providers Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management.

The MME also supports lawful interception of signaling. It provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.

The S-GW

The S-GW (Serving Gateway) acts like an anchor for handover between neighboring eNodeBs routes and routes all the user data packets. The S-GW also handles mobility between LTE and other CS networks.

For idle state UEs, the S-GW maintains the UEs' context, and generates paging requests when the UE receives downlink data.

The S-GW also performs replication of the user traffic in case of lawful interception.

The P-GW

The P-GW (Packet Data Network Gateway) ensures the UE's connectivity to external packet data networks, acting like the point of exit and entry of traffic for the UE. A UE can be connected to more than one P-GW while accessing multiple PDNs.

The P-GW handles policy enforcement, user by user packet filtering, charging support, lawful interception and packet screening. It also acts like Another key role of the P-GW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).

The HSS

The Home Subscriber Server (HSS) is a central database that contains user-related and subscription-related information. The functions of the HSS include mobility management, call and session establishment support, user authentication and access authorization. The HSS is based on the Home Location Register (HLR) and the Authentication Center (AuC) of 2G and 3G networks.

The PCRF

The Policy and Charging Rules Function (PCRF), is a combination of the Charging Rules Function (CRF) and the Policy Decision Function (PDF), and ensures the service policy and sends Quality of Service (QoS) information for each session begun and accounting rule information. These policies are enforced in the eNodeB.

The PCEF

The Policy and Charging Enforcement Function (PCEF) performs policy enforcement and service data flow detection, allowing data flow through the implemented P-GW. It is also responsible for the QoS on IP packets in the P-GW. The PCEF enforces rules that allow data packets to pass through the gateway.

The role of the eNodeB

The eNodeB is a part of the E-UTRAN radio access network and is the component that allows UEs to connect to the LTE network. An eNodeB typically communicates with the UE, with other eNodeBs, and with the EPC through various interfaces: the Uu, X2 and S1. To learn more about these interfaces go to the section Interfaces of the eNodeB (hyperlink here).

The eNodeB performs the following functions:

  • Radio resource management, which includes:
    • radio bearer control – is responsible for the setup, maintenance and the release of radio bearers and its resource configuration
    • mobility management – handles the radio resource management for UEs in both idle and connected modes
    • admission control – allows or denies radio bearer setup requests
    • dynamic resource allocation, covering the release and allocation of radio resources in both the user plane and the control plane
  • Routing of user plane packets towards the S-GW
  • MME selection, which includes:
    • enabling the UE to be served by an MME while the UE is in the “attach” procedure
    • enabling the UE to be served by a different MME while being in a network
    • the establishment of the route towards an MME, based on the information provided by the UE when the routing information is not available
  • Packet compression and ciphering, which includes:
    • encryption and decryption of packets through ciphering algorithms
    • header compression for downlink packets and header decompression for uplink packets
  • Message scheduling and transmission, which includes:
    • the transmission of paging messages, OM messages or broadcast information via the Uu interface
    • the reception of broadcast information and paging messages from an MME and the OM messages from the operation and maintenance center

Interfaces of the eNodeB

LTE-Uu

The LTE-Uu is the radio interface that connects the UEs to the eNodeBs eNodeB with the UE. It handles all the signaling messages between the eNodeB and the MME as well as the data traffic between the UE and the S-GW.

The protocol stack has two planes:

  • the user plane – carries the data streams of interest to the user and distinguishes data streams to and from the circuit-switched, packet-switched and broadcast control domains
  • the control plane – carries the network’s signaling messages

The user plane protocol stacks of the LTE-Uu are:

  • PDCP
  • RLC
  • MAC
  • PHY

For the control plane, the protocol stack is comprised of:

  • NAS
  • RRC
  • RLC
  • MAC
  • PHY

For a more detailed explanation of all these protocols, go to the [/An_introduction_to_LTE##TheE-UTRANprotocolstack E-UTRAN protocol stack] section.

S1

The S1 interface is described in the 3GPP TS 36.410 specification.

The S1 interface connects the E-UTRAN and the EPC for both the user and the control planes. It has two parts: the S1-AP, belonging to the control plane and the S1-U (GTP-U), belonging to the user plane.

The S1-AP connects the eNodeB to the MME and is based on IP transmission. It transmits signaling messages of the radio network layer of the E-UTRAN through the Stream Control Transmission Protocol (SCTP)/IP stack.

Therefore, when the eNodeB has to connect to an MME, it does so through the S1 interface seeking each MME node in the corresponding pool area. The next step is that of setting up the Transport Network Layer (TNL). One eNodeB and one MME can set up a single Stream Control Transmission Protocol (SCTP) connection. Once the TNL has been established, the eNodeB starts an S1 interface, which has the purpose of managing the configuration data for the operation exchange between the ENB and the MME.

S1 control protocol stack.png

As seen in the figure above, the S1 Application Protocol (S1-AP) is above the SCTP. Since there is not any other intermediary protocol between S1-AP and SCTP, the stack is simpler.

The S1-U connects the eNodeB to the S-GW through the GTP/UDP5/IP stack.

S1 userplane ps.png

In the user plane, the S1-U (GTP) is based on the GTP/UDP5/IP protocol stack from previous UMTS and GPRS networks. The GPRS Tunnelling Protocol User plane (GTP-U) is responsible for tunnelling the user plane bearers, acts as a reference point for inter-eNodeB handover, and allows intra-3GPP mobility.

X2

The X2 interface provides connectivity between two or more eNodeBs. There are two parts of the X2 interface, the X2-C, the interface between the control planes of eNodeBs, and the X2-U, the interface between the user planes of eNodeBs. The X2-C and the X2-U have the same structure as the S1 interface. as seen below. The only difference consists of the X2-AP replacing the S1-AP.

X2 protocol stack.png

Two or more eNodeBs can exchange signalling information through the X2 interface. The main roles of the X2 interface are the following:

  • Mobility management
  • Load management
  • Inter-cell interference management
  • Inter-eNodeB handover
  • Tracing function
  • X2 interface management and error handling

Two or more eNodeBs exchange information related to load, interference or handover.

KPI-related measurements

The key performance indicators (KPIs) in an LTE radio access network offer information related to the subscriber’s connection quality and the network’s performance. These indicators allow the network to offer subscribers a better service quality, and ensures an efficient resource allocation.

The eNodeB reports performance measurements. Network element managers calculate and analyze the performance measurements into KPIs.

To find out more about the network management system please refer to Operations and Management section.

The E-UTRAN

The E-UTRAN (Evolved Universal Terrestrial Radio Access Network) is comprised of UEs and eNodeBs. The UE can be a device such as: mobile phone, laptop, tablet, computer, etc., used by an subscriber for communication.

Its radio interface is the E-UTRA, the Evolved Universal Terrestrial Radio Access.

The E-UTRA

The E-UTRA is the air interface of an LTE network and is the equivalent of he UTRA air interface in UMTS networks.

The E-UTRA enables a latency decrease, allows high bandwidth capabilities and is optimized for packet data.

The E-UTRA uses Orthogonal frequency-division multiple access (OFDMA) in the downlink and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) in the uplink. OFDM splits data into small sub-carriers on neighboring frequencies, over a single channel. OFDM handles phenomena such as interference, noise or multipath significantly more efficiently than other modulation methods.

SC-FSMA is also a frequency division multiple access scheme and usually represents and alternative to OFDM. Its main advantage is a lower peak-to-average power ration, which is proven to be more efficient in networks where the transmit power is most important.

The E-UTRA also uses the MIMO technology and enables the simultaneous support of more users and a lower processing power required for each UE. In the case of a 2x2 MIMO antenna system, the two transmitters send different parts of the same data stream simultaneously, while the receivers have to piece them back together.

The E-UTRAN protocol stack

There are various protocol layers in the E-UTRAN:

  • Radio Resource Control (RRC) – handles the broadcast system information related to the access stratum and transport of the non-access stratum (NAS) messages, paging, establishment and release of the RRC connection, security key management, handover, UE measurements related to inter-system (inter-RAT) mobility, QoS, etc.
  • Packet Data Convergence Protocol (PDCP) – provides transport of its data with ciphering and integrity protection. For the IP layer’s transport of IP packets, with ROHC header compression, it provides ciphering, duplicate detection and retransmission of its own SDUs during handover (the last two functions depend of the RLC mode in-sequence delivery).
  • Radio Link Control (RLC) – transports the PDCP’s PDUs. Depending on this mode it can provide: ARQ error correction, segmentation/concatenation of PDUs, reordering for in-sequence delivery, duplicate detection etc.
  • Medium Access Control (MAC) – offers a set of logical channels to the RLC sublayer that it multiplexes into the physical layer transport channels. It also manages the HARQ error correction, handles the prioritization of the logical channels for the same UE and the dynamic scheduling between UEs, etc.
  • Physical layer – carries all the information from the MAC transport channels over the air interface. It takes care of the link adaptation (AMC), power control, cell search (for initial synchronization and handover purposes) and other measurements (inside the LTE system and between systems) for the RRC layer.

The interfacing layers to the EUTRAN protocol stack are:

  • Non Acess Stratum (NAS) – is the protocol between the UE and the MME on the network side (outside of E-UTRAN). It is responsible for some of the following functions: mobility management, UE authentication and security control.

• Internet Protocol (IP)

Below you can find the functional protocol split between the UE, the eNodeB and the MME in the control plane.

Cplane ProtocolSplit.png

Here is the the functional protocol split between the UE, the eNodeB and the MME in the user plane.

Uplane ProtocolSplit.png

Channel Architecture

There are three main layers of channels in the LTE architecture:

  • Logical channels – send data from the RLC to the MAC layer
  • Transport channels – send the the logical channel data from the MAC to the PHY
  • Physical channels – send the physical channel data from the PHY to the UE

The E-UTRAN downlink channels

ENodeB DownlinkChannels.png

Logical channels

  • Paging Control Channel (PCCH) – broadcasts notifications related to paging information and system information changes. This channel is typically used for paging when the UE’s location is unknown by the network.
  • Broadcast Control Channel (BCCH) – transmits system control information.
  • Common Control Channel (CCCH) – transmits control information between the UE and the core network when there is not an RRC connection between them.
  • Dedicated Control Channel (DCCH) – broadcasts control information between the UE and the core network when there is an RRC connection between them.
  • Dedicated Traffic Channel (DTCH) – transmits user information and is assigned to a single UE.
  • Multicast Control Channel (MCCH) – sends data traffic from the network to the UE.
  • Multicast Traffic Channel (MTCH) – sends data from the core network only to the UEs that receive Multimedia Broadcast/Multicast Service (MBMS).

Transport Channels

  • Paging Channel (PCH) – transmits the cell's coverage area, supports UE discontinuous reception; carries PCCH information.
  • Broadcast Channel (BCH) – transmits the cell's coverage area; carries BCCH information.
  • Downlink Shared Channel (DL-SCH) – sends downlink data, and supports dynamic link adaptation, H-ARQ, dynamic and semi-persistent resource allocation, MBMS transmission and UE discontinuous reception; it is shared by multiple logical channels to transmit their information (BCCH, CCCH, DCCH, DTCH, MCCH, MTCH).
  • Multicast Channel (MCH) – sends the cell's coverage area, supports semi-persistent resource allocation and Multicast/Broadcast Single Frequency Network (MBSFN) transmission; carries MCCH and MTCH information.

Physical channels

  • Physical Broadcast Channel (PBCH) – broadcasts the basic system information within the eNodeB that allow UEs to access the network. It transmits parameters in a Master Information Block that is 14 bits each and is split into four 10 ms frames, thus being repeated every 40 ms; carries BCH information.
  • Physical Downlink Shared Channel (PDSCH) – handles the L1 data traffic transmission. The data is transmitted in transport blocks once every 1 ms. It also transmits paging messages or system information blocks; carries DL-SCH and PCH information.
  • Physical Multicast Channel (PMCH) – transmits Multimedia Broadcast and Multicast Services (MBMS); carries MCH information.
  • Physical Downlink Control Channel (PDCCH) – responsible for handling downlink allocation information, uplink allocation grants for the terminal, paging indications or system information.
  • Physical Hybrid ARQ Indicator Channel (PHICH) – carries the acknowledgement signals from the uplink transmissions.

The E-UTRAN uplink channels

ENodeB UplinkChannels.png

Logical channels

  • Common Control Channel (CCCH) – transmits control information between the UE and the core network when there is not an RRC connection between them.
  • Dedicated Control Channel (DCCH) – broadcasts control information between the UE and the core network when there is an RRC connection between them.
  • Dedicated Traffic Channel (DTCH) – a channel dedicated to a single UE that transmits user data.

Transport channels

  • Random Access Channel (RACH) – sends a small amount of data and information about state changes.
  • Uplink Shared Channel (UL-SCH) – offers support for beamforming and supports H-ARQ, dynamic link adaptation and dynamic and semi-persistent resource allocation; carries information from multiple logical channels (CCCH, DCCH, DTCH).

Physical channels

  • Physical Random Access Channel (PRACH) – does the initial access when the UE losses its uplink synchronization; carries RACH information.
  • Physical Uplink Shared Channel (PUSCH) – carries the L1 uplink transport data with the control information; carries UL-SCH information.
  • Physical Uplink Control Channel (PUCCH) – carries control information.

Operations and Management

The Operations and Management (OAM) is a fundamental piece of the LTE network. Operations and Management's architecture is specified in the 3GPP specifications.

The OAM network has five main functions related to:

  • fault
  • configuration
  • accounting
  • performance
  • security

The three main entities of the OAM architecture are:

  • network elements – manage multiple eNodeBs
  • element managers (EM) – manage a collection of elements of the same type (MMEs, S-GWs, P-GWs, etc.)
  • network managers (NMs) – manage multiple element managers

The element management system (EMS) is responsible of the functions of each network element. However, the EMS does not manage the traffic between network elements.

The EMS is the key element for enforcing LTE quality of service (QoS) demands.

The network management system (NMS) offers a wide array of network management information, ranging from the elements' capabilities, automation, malfunction information, etc.

The element management system scales up with the increase of LTE network components, and can be integrated to work with OSS and BSS systems.

KPI measurements

KPIs are indicators allow the network to offer subscribers a better service quality, and ensures an efficient resource allocation. Overall, KPIs are responsible with evaluating the LTE network's performance.

As specified in the 3GPP TS 32.451 document,there are several types of KPI parameters that are integral to any LTE network, depending on the target they measure:

  • Accessibility
  • Retainability
  • Integrity
  • Availability
  • Mobility

Others can be added depending on the the network's need, such as:

  • Utilization
  • Traffic
  • Latency
Accessibility

Accessibility is a measurement that allows operators to know information related to the mobile services accessibility for the subscriber. The measurement is performed through E-UTRAN's E-RAB service.

Retainability

Retainability measures how many times a service was interrupted or dropped during use, thus preventing the subscriber to benefit from it or making it difficult for the operator to charge for it. Therefore, a high retainability is very important from a business stand point.

The measurement is performed through E-UTRAN's E-RAB service.

Integrity

Integrity measures the high or low quality of a service while the subscriber is using it.

The measurement is performed through E-UTRAN's delivery of IP packets.

Availability

Availability measures a service's availability for the subscriber.

The measurement is performed by determining the percentage of time that the service was available for the subscribers served by a specific cell. The measurement can also aggregate data from more cells or from the whole network.

Mobility

Mobility measures how many times a service was interrupted or dropped during a subscriber's handover or mobility from on cell to another.

The measurement is performed in the E-UTRAN and will include Intra E-UTRAN and Inter RAT handovers.