4G
LTE
Long Term Evolution
Rel-8 (2008)Rel-9 (2009)
Overview
Long Term Evolution (LTE) is 3GPP's fourth-generation radio access technology, standardized starting in Release 8. It replaced the WCDMA/HSPA air interface with OFDMA in the downlink and SC-FDMA in the uplink, and flattened the radio access network by merging the Node B and RNC functions into a single element, the eNodeB, connected directly to an all-IP core network (the Evolved Packet Core).
LTE was designed for packet-switched data from the outset, with voice delivered either via circuit-switched fallback (CSFB) to 2G/3G or, from Release 9 onward, natively over IP via VoLTE. It remains the dominant global radio access technology and continues to anchor many non-standalone 5G deployments (EN-DC).
Key Features
- OFDMA downlink / SC-FDMA uplink air interface with scalable 1.4–20 MHz channel bandwidths
- Flat, all-IP network architecture: eNodeB connects directly to the EPC over S1
- Peak theoretical rates of 300 Mbps downlink / 75 Mbps uplink (2x2 MIMO, 20 MHz, Rel-8)
- Sub-frame-based scheduling with 1 ms TTI for low-latency link adaptation
- Support for FDD and TDD duplexing within a single core specification set
- Self-organizing network (SON) capabilities for automated configuration and optimization
Network Architecture
E-UTRAN consists solely of eNodeBs, which handle all radio-related functions and connect to neighboring eNodeBs over X2 and to the Evolved Packet Core over S1.
| Component | Full Name | Function |
|---|---|---|
| eNodeB | Evolved Node B | Radio resource management, scheduling, mobility control, header compression, ciphering |
| MME | Mobility Management Entity | NAS signaling, idle-mode tracking, bearer management, authentication |
| S-GW | Serving Gateway | Local mobility anchor, packet routing/forwarding, lawful interception |
| P-GW | PDN Gateway | UE IP address allocation, packet filtering, policy enforcement, external PDN anchor |
| HSS | Home Subscriber Server | Master subscriber database for authentication and subscription profiles |
| PCRF | Policy and Charging Rules Function | Dynamic policy and charging control decisions |
Physical Layer
| Parameter | Value |
|---|---|
| Downlink multiple access | OFDMA |
| Uplink multiple access | SC-FDMA |
| Subcarrier spacing | 15 kHz |
| Slot / subframe / frame | 0.5 ms slot, 1 ms subframe, 10 ms frame |
| Channel bandwidths | 1.4, 3, 5, 10, 15, 20 MHz |
| Modulation | QPSK, 16-QAM, 64-QAM (256-QAM from Rel-12 for downlink) |
| MIMO | Up to 4x4 (8x8 in later releases) with codebook-based precoding |
Frequency Bands
FDD Bands (selected)
| Band | Uplink (MHz) | Downlink (MHz) | Region |
|---|---|---|---|
| B1 | 1920–1980 | 2110–2170 | Global (2100 MHz) |
| B3 | 1710–1785 | 1805–1880 | Europe / APAC (1800 MHz) |
| B5 | 824–849 | 869–894 | Americas (850 MHz) |
| B7 | 2500–2570 | 2620–2690 | Europe / APAC (2600 MHz) |
| B13 | 777–787 | 746–756 | North America (700 MHz) |
| B20 | 832–862 | 791–821 | Europe (800 MHz digital dividend) |
TDD Bands (selected)
| Band | Frequency (MHz) | Region |
|---|---|---|
| B38 | 2570–2620 | Global |
| B40 | 2300–2400 | APAC |
| B41 | 2496–2690 | North America |
| B42 | 3400–3600 | Global (also 5G n78 overlap) |
UE Categories
| Category | DL Peak Rate | UL Peak Rate | MIMO Layers |
|---|---|---|---|
| Cat 1 | 10.3 Mbps | 5.2 Mbps | 1 |
| Cat 3 | 102 Mbps | 51 Mbps | 2 |
| Cat 4 | 150.8 Mbps | 51 Mbps | 2 |
| Cat 6 | 301.5 Mbps | 51 Mbps | 2–4 (with CA) |
| Cat 9 | 452.2 Mbps | 51 Mbps | 2–4 (with CA) |
| Cat M1 (eMTC) | 1 Mbps | 1 Mbps | 1 |
Services
- Mobile broadband data over default and dedicated EPS bearers with QCI-based QoS
- VoLTE — voice delivered as IMS-anchored VoIP over a dedicated GBR bearer
- SMS over IP (SGs or IMS)
- eMBMS — evolved Multimedia Broadcast Multicast Service for broadcast video/data
- Circuit-switched fallback (CSFB) to UTRAN/GERAN for voice on networks without VoLTE
Security
| Layer | Algorithm Family | Purpose |
|---|---|---|
| NAS / AS ciphering | EEA0, 128-EEA1 (SNOW 3G), 128-EEA2 (AES), 128-EEA3 (ZUC) | User and signaling plane confidentiality |
| NAS / AS integrity | EIA1, EIA2, EIA3 | Signaling message integrity protection |
| Authentication | EPS-AKA | Mutual authentication between UE and network using USIM credentials |
| Key hierarchy | K → CK/IK → KASME → KeNB | Layered key derivation isolating radio keys from core keys |
Key Procedures
Attach
- UE sends Attach Request (NAS) piggybacked on RRC Connection Setup Complete
- eNodeB forwards Initial UE Message to MME over S1AP
- MME authenticates the UE (EPS-AKA) and establishes NAS security
- MME creates a default EPS bearer via Create Session Request/Response with the S-GW/P-GW
- MME sends Initial Context Setup Request; eNodeB configures the radio bearer via RRC Reconfiguration
- UE completes Attach with Attach Complete, now in EMM-REGISTERED / ECM-CONNECTED state
Handover (X2-based)
- Source eNodeB triggers handover based on UE measurement reports (A3 event)
- Source eNodeB sends Handover Request to target eNodeB over X2AP
- Target eNodeB admits resources and returns Handover Request Acknowledge
- Source eNodeB commands the UE via RRC Reconfiguration with mobilityControlInfo
- UE synchronizes to the target cell and sends RRC Reconfiguration Complete
- Target eNodeB signals Path Switch Request to the MME to update the S1-U path
Paging
- Downlink data or NAS signaling arrives for an idle UE
- MME sends Paging over S1AP to all eNodeBs in the UE's tracking area
- eNodeBs broadcast a Paging message referencing the UE's S-TMSI
- UE initiates RRC Connection Establishment (Service Request) in response
Power Saving
| Mechanism | Description | Introduced |
|---|---|---|
| DRX (Connected) | UE sleeps between scheduled monitoring occasions while RRC_CONNECTED | Rel-8 |
| DRX (Idle) | UE monitors paging occasions on a configurable cycle while idle | Rel-8 |
| PSM | Power Saving Mode — UE deregisters from the radio but stays registered with the core between activity bursts | Rel-12 |
| eDRX | Extended DRX cycles up to ~43 minutes for infrequent, delay-tolerant traffic | Rel-13 |
Interworking
| Aspect | LTE / EPC | UMTS / GPRS |
|---|---|---|
| Core network | Evolved Packet Core (flat, all-IP) | GPRS core (SGSN/GGSN) |
| Handover interface | S3/S4 (Gn/Gp-based interworking) | Iu / Gb |
| Voice | VoLTE (IMS) or CSFB to 2G/3G | Native circuit-switched voice |
| Mobility procedure | Tracking Area Update (TAU) | Routing Area Update (RAU) |
Extensions for Later Releases
- Rel-9: Dual-layer beamforming, eMBMS, SON self-healing enhancements.
- Rel-10 (LTE-Advanced): Carrier aggregation, relay nodes, enhanced MIMO — see the LTE-Advanced technology page.
- Rel-12: Dual connectivity, D2D/ProSe, PSM for IoT power savings.
- Rel-13 (LTE-Advanced Pro): NB-IoT, LAA, massive MIMO groundwork.
Comparison with Previous Generations
| Aspect | GPRS/EDGE (2.5G) | UMTS/HSPA (3G) | LTE (4G) |
|---|---|---|---|
| Access | TDMA | WCDMA | OFDMA/SC-FDMA |
| Peak DL rate | ~236 kbps (EDGE) | ~42 Mbps (HSPA+) | ~300 Mbps (Rel-8, 2x2 20 MHz) |
| Core network | GPRS core (SGSN/GGSN) | UMTS core (evolved GPRS core) | EPC (flat, all-IP) |
| RAN architecture | BSC/BTS | RNC/Node B | eNodeB only (flat) |