4G/Long Term Evolution (LTE), VOLTE

4G/ Long Term Evolution (LTE)

LTE stands for Long Term Evolution and is a registered trademark owned by ETSI (European Telecommunications Standards Institute) for the wireless data communications technology and a development of the GSM/UMTS standards.

  • Both radio and core network evolution
  • All-IP packet-switched architecture
  • Standardised by 3GPP
  • (+) Lower CAPEX ans OPEX involved

LTE evolved from an earlier 3GPP system known as the Universal Mobile Telecommunication System (UMTS), which in turn evolved from the Global System for Mobile Communications (GSM). Also it is aligned with 4G (fourth-generation mobile)

It is backward compatible with GSM/EDGE/UMTS/CDMA/WCDMA systems on existing 2G and 3G spectrum, even hand-over and roaming to existing mobile networks.

Motivation for evolution – Wireless/cellular technology standards are constantly evolving for better efficiency and performance.LTE evolved as a result of rapid increase of mobile data usage. Applications such as voice over IP (VOIP), streaming multimedia, videoconferencing , cellular modemetc.

It provides packet-switched traffic with seamless mobility and higher qos than predecessors. Also high data rate, throughput, low latency and packet optimized radioaccess technology on flexible bandwidth deployments.

Timeline of Evolution 

  • GSM  : calls  on circuit switching ( CS ) between 2 parties for communication. Dedicated circuits are used for voice and SMS.
  • GPRS : packet switching (PS) is introduced for data services
  • UMTS / 3G : network elements begin evolving into PS . No changes to core.
  • EPC / LTE/VOLTE : No circuit switched domain at all .

LTE Performance

Peak Data Rate

  • uplink – 75Mbps(20MHz bandwidth)
  • downlink – 150 Mbps(UE Category 4, 2×2 MIMO, 20MHz bandwidth), 300 Mbps(UE category 5, 4×4 MIMO, 20MHz bandwidth)

Carrier bandwidth : Range from 1.4 MHz up to 20 MHz. Ultimately bandwidth used by carrier depends on frequency band and the amount of spectrum available with a network operator.

Mobility 350 km/h

Coverage 5 – 100km with slight degradation after 30km

LTE architecture supports hard QoS and guaranteed bit rate (GBR) for radio bearers.

Technology used in LTE

All interfaces between network nodes are IP based in LTE.
Duplexing – Time Division Duplex (TDD) , Frequency Division Duplex (FDD) and half duplex FD

MIMO ( Multiple Input Multiple Output ) transmissions Allows the base station to transmit several data streams over the same carrier simultaneously.

Multiple Access Schemes

  • uplink: SC-FDMA (Single Carrier Frequency Division Multiple Access) 50Mbps+ (20MHz spectrum)
  • downlink: OFDM (Orthogonal Frequency Division Multiple Access) 100Mbps+ (20MHz spectrum)
  • Multi-Antenna Technology, Multi-user collaborative MIMO for Uplink and TxAA, spatial multiplexing, CDD, max 4×4 array for downlink

Modulation Schemes : QPSK, 16QAM, 64QAM(optional)

4G/ LTE Architecture

Primarily composed of 3 parts UE, E-UTRANand EPC.

1. User Equipment (UE)

  • Mobile Termination (MT)
  • Terminal Equipment (TE) 
  • Universal Integrated Circuit Card (UICC) : also known as the SIM card for LTE equipments. It runs an application known as the Universal Subscriber Identity Module (USIM).

2. Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)

handles the radio communications between the mobile and the evolved packet core. High level representation for  eNodeB or eNB

Role of eNB : sends and receives radio transmissions to all the mobiles using the analogue and digital signal processing functions of the LTE air interface. eNB also controls the low-level operation of all its mobiles, by sending them signalling messages such as handover commands.

3. Evolved Packet Core (EPC)

This sub system resembles IMS environment.

SGW (Serving Gateway) for routing and forwarding of user data packets

Packet Data Network (PDN) Gateway (P-GW) communicates with the outside world simillar to GGSN ( GPRS support node ) and SGSN ( serving GPRS support node ) in UMTS and GSM.

Home Subscriber Server (HSS) is a central database that contains information about all the network operator’s subscribers. Almost simillar to HLR/AAA in 2G /3G architcture.

Mobility management entity (MME) controls the high-level operation.

LTE Roaming Service

For a roming user in Visited-PLMN, the user is connected with the E-UTRAN, MME and S-GW of the visited LTE network. However, LTE/SAE allows the P-GW of either the visited or the home network to be used, as shown in below:

For roaming prepaid charging, accounting flows are made to access prepaid customer data, via P-Gateways or CSCF in an IMS environment.

VoLTE ( Voice over LTE)

A-SBC (Access Session Border Controller) consists of P-CSCF and ALG/AGW. This connects access network (LTE) to IMS core.

  • Connect IP networks, including IPv4 and IPv6 interworking, NAT traversal, etc.
  • Security
    • DDos prevention
    • Topology hiding
    • Encryption
    • P-CSCF maintains the security associations between itself and the UE
  • QoS control
  • trancoding
  • media service handling using Application layer gateway (ALG) access gateway (AGW)

Core IMS has

  • I-CSCF (Interrogating Call Session Control Function) which provides Location service to find the correct S-CSCF for each subscriber
    •  for peer networks the I-CSCF is the first point of contact.
  • S-CSCF (Serving Call Session Control Function) SIP session management and routing
    • connect to HSS for policies
    • invokes Application Servers (TAS, IPSMGW)

Telephony Application Server (TAS) is the application layer of the telecommunication system which adds intelligience and business logic to the platform. We can design call flows and usecases such as

  • address normalization
  • call diverting/ forwarding/ forking
  • Smart screening/ barring( whitelisting , greylisting , blacklisting)
  • It connects to MRF – Media Resource Function
    • media mixer or as a media server for tones, announcements

To proivide compatibility with 2G , 3G and prior systems ths archietcture has

IPSMGW enable support for SMS over SIP 

MGCF – Media Gateway Control Function to support Circuit switched network

BGCF – Breakout Gateway Control Function to find routing based on ENUM/DNS (e.g. PSTN number)

Interop between 3G and LTE user endpoints

Interoperation between IMS networks

Interoperation between multiple IMS networks and LTE operators

Interconnect Session Border Controller (I-SBC) handles the boundary where service providers interconnect and exchange inbound outbound SIP sessions. It consists of

  • Interconnect Border Control Function (IBCF)
    • Inter-Working Function (IWF)
  • Transition Gateway (TrGW)

Voice over Wifi and WebRTC with LTE

Voice over Wifi and WebRTC with LTE

TWAG or ePDG gateway is used to integrate the Wireless LAN access network into the Mobile Network Packet Core – EPC Network.

TWAG (Trusted Wireless Access Gateway) in the Wi-Fi core provides trusted access to the UE( User Equipment). The TWAG is then connected directly to the P-GW (Packet Gateway) in the Evolved Packet Core (EPC).


Advanced features planned for LTE include

  • LTE devices capable of CAT6 speeds (Category 6 )
  • Increased peak data rate – downlink 3 Gbps, Uplink 1.5 Gbps ( 1 Gbps = 1000 Mbps)
  • Spectral efficiency from 16bps/Hz in R8 to 30 bps/Hz in R10
  • Carrier Aggregation (CA)
  • Enhanced use of multi-antenna techniques
  • Support for Relay Nodes (RN)


Also read about previous generations of telecom namely 2 G and 3G

5G and IMS

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