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:
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:
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.
LTE uses the following inner modulation schemes:
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:
The EPC is comprised of:
All these components will be described in depth in the following sections.
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 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:
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 (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 (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 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 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 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 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
- 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
- encryption and decryption of packets through ciphering algorithms
- header compression for downlink packets and header decompression for uplink packets
- 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
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 protocol stacks of the LTE-Uu are:
For the control plane, the protocol stack is comprised of:
For a more detailed explanation of all these protocols, go to the [/An_introduction_to_LTE##TheE-UTRANprotocolstack E-UTRAN protocol stack] section.
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.
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.
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.
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.
Two or more eNodeBs can exchange signalling information through the X2 interface. The main roles of the X2 interface are the following:
Two or more eNodeBs exchange information related to load, interference or handover.
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 (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 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 2×2 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:
The interfacing layers to the EUTRAN protocol stack are:
• Internet Protocol (IP)
There are three main layers of channels in the LTE architecture:
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:
The three main entities of the OAM architecture are:
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.
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:
Others can be added depending on the the network’s need, such as:
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 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 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 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 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.