LTE - Long Term Evolution
LTE (Long Term Evolution) is the first 4G mobile technology standardized by 3GPP. It was defined in Rel-8 (2008) and extended in Rel-9 (2009).
Key Features
LTE revolutionized mobile networks with:
- All-IP architecture - no circuit-switched elements
- OFDMA modulation in downlink
- Flat architecture - fewer network nodes than 3G
- Low latency - under 10ms RTT
Network Architecture
Core Network - Evolved Packet Core (EPC)
EPC is the LTE core network, fully based on IP protocol. It replaced the hierarchical 3G architecture (SGSN/GGSN).
EPC Components
| Component | Full Name | Function |
|---|---|---|
| MME | Mobility Management Entity | Signaling, authentication, UE location tracking, S-GW/P-GW selection |
| S-GW | Serving Gateway | User data, anchor for inter-eNB handover |
| P-GW | PDN Gateway | IP address allocation, QoS, external network connectivity |
| HSS | Home Subscriber Server | Subscriber database, authentication data |
| PCRF | Policy and Charging Rules Function | QoS policies, charging rules |
📋 Specifications: TS 23.401 (EPC Architecture), TS 23.402 (Non-3GPP Access)
RAN - E-UTRAN
E-UTRAN (Evolved UTRAN) is the LTE radio access network.
Components
| Component | Description |
|---|---|
| eNodeB | Evolved NodeB - LTE base station, includes RNC functions from 3G |
| X2 interface | Interface between eNodeBs for handover |
| S1 interface | eNodeB ↔ EPC interface (S1-MME for signaling, S1-U for data) |
📋 Specifications: TS 36.300 (E-UTRAN Overall), TS 36.331 (RRC), TS 36.413 (S1AP), TS 36.423 (X2AP)
Physical Layer
LTE uses advanced modulation techniques for high spectral efficiency.
OFDMA (Downlink)
Orthogonal Frequency Division Multiple Access
- Band division into orthogonal subcarriers (15 kHz)
- Resource Block: 12 subcarriers × 0.5ms = 180 kHz × 0.5ms
- Resistance to multipath fading
- Flexible resource allocation
SC-FDMA (Uplink)
Single Carrier FDMA
- Lower PAPR (Peak-to-Average Power Ratio) than OFDMA
- Better for battery-powered devices
- DFT-precoded OFDMA
MIMO
Multiple Input Multiple Output
- Rel-8: 2×2, 4×4 MIMO
- Spatial multiplexing for higher throughput
- Transmit diversity for better coverage
Parameters
| Parameter | Value |
|---|---|
| Bandwidth | 1.4, 3, 5, 10, 15, 20 MHz |
| Max throughput DL | 150 Mbps (Cat 4), 300 Mbps (Cat 5) |
| Max throughput UL | 50 Mbps (Cat 4), 75 Mbps (Cat 5) |
| Latency | < 10ms (user plane) |
| Modulation | QPSK, 16QAM, 64QAM |
📋 Specifications: TS 36.211 (Physical Channels), TS 36.212 (Multiplexing), TS 36.213 (Physical Layer Procedures)
Frequency Bands
LTE operates in various frequency bands worldwide. Bands are designated by numbers (B1, B3, etc.) and operate in either FDD (paired spectrum) or TDD (unpaired spectrum) mode.
Common FDD Bands
| Band | Uplink (MHz) | Downlink (MHz) | Bandwidth | Primary Region |
|---|---|---|---|---|
| B1 | 1920-1980 | 2110-2170 | 60 MHz | Global |
| B3 | 1710-1785 | 1805-1880 | 75 MHz | Europe, APAC |
| B7 | 2500-2570 | 2620-2690 | 70 MHz | Europe, LATAM |
| B8 | 880-915 | 925-960 | 35 MHz | Europe, APAC |
| B20 | 832-862 | 791-821 | 30 MHz | Europe (Digital Dividend) |
| B28 | 703-748 | 758-803 | 45 MHz | APAC, LATAM |
Common TDD Bands
| Band | Frequency (MHz) | Bandwidth | Primary Region |
|---|---|---|---|
| B38 | 2570-2620 | 50 MHz | Europe, APAC |
| B40 | 2300-2400 | 100 MHz | China, India |
| B41 | 2496-2690 | 194 MHz | USA, China |
| B42 | 3400-3600 | 200 MHz | Europe |
| B43 | 3600-3800 | 200 MHz | Europe |
FDD vs TDD
| Aspect | FDD | TDD |
|---|---|---|
| Spectrum | Paired (separate UL/DL) | Unpaired (shared) |
| Guard | Frequency guard band | Time guard period |
| Latency | Lower (simultaneous TX/RX) | Higher (switching) |
| Flexibility | Fixed UL/DL ratio | Configurable ratio |
| Use case | Voice, symmetric data | Asymmetric data (more DL) |
📋 Specifications: TS 36.101 (UE Radio Transmission), TS 36.104 (BS Radio Transmission)
UE Categories
UE (User Equipment) categories define device capabilities including maximum data rates, MIMO support, and modulation schemes.
Rel-8/9 Categories
| Category | Max DL | Max UL | DL MIMO | Modulation |
|---|---|---|---|---|
| Cat 1 | 10 Mbps | 5 Mbps | 1×1 | 64QAM DL, 16QAM UL |
| Cat 2 | 50 Mbps | 25 Mbps | 2×2 | 64QAM DL, 16QAM UL |
| Cat 3 | 100 Mbps | 50 Mbps | 2×2 | 64QAM DL, 16QAM UL |
| Cat 4 | 150 Mbps | 50 Mbps | 2×2 | 64QAM DL, 16QAM UL |
| Cat 5 | 300 Mbps | 75 Mbps | 4×4 | 64QAM DL, 64QAM UL |
LTE-Advanced Categories (Rel-10+)
| Category | Max DL | Max UL | Features | Introduced |
|---|---|---|---|---|
| Cat 6 | 300 Mbps | 50 Mbps | 2CC CA, 2×2 MIMO | Rel-10 |
| Cat 9 | 450 Mbps | 50 Mbps | 3CC CA, 2×2 MIMO | Rel-11 |
| Cat 12 | 600 Mbps | 100 Mbps | 3CC CA, 4×4 MIMO | Rel-12 |
| Cat 16 | 1 Gbps | 150 Mbps | 4CC CA, 4×4 MIMO | Rel-12 |
IoT Categories (Rel-13+)
| Category | Max DL | Max UL | Features | Use Case |
|---|---|---|---|---|
| Cat M1 | 1 Mbps | 1 Mbps | Half-duplex, PSM | Smart meters, wearables |
| Cat M2 | 4 Mbps | 7 Mbps | VoLTE capable | Industrial IoT |
| Cat NB1 | 26 kbps | 66 kbps | Ultra-low power | Sensors, trackers |
| Cat NB2 | 127 kbps | 159 kbps | Positioning | Asset tracking |
📋 Specifications: TS 36.306 (UE Radio Access Capabilities)
Services
QoS (Quality of Service)
LTE introduced bearer-based QoS:
- Default Bearer - always active, best effort
- Dedicated Bearer - for services with guaranteed QoS
- QCI (QoS Class Identifier) - 9 standardized classes
IMS Integration
LTE is designed for integration with IMS (IP Multimedia Subsystem):
- All services over IP
- VoLTE for voice calling
- Video calling, RCS
📋 Specifications: TS 23.228 (IMS Architecture)
Security
EPS-AKA
Evolved Packet System - Authentication and Key Agreement
- Mutual authentication of UE and network
- Key derivation for encryption and integrity
Encryption and Integrity
| Layer | Encryption | Integrity |
|---|---|---|
| NAS | AES (EEA2), SNOW (EEA1) | AES (EIA2), SNOW (EIA1) |
| AS (RRC) | AES, SNOW | AES, SNOW |
| AS (UP) | AES, SNOW | - |
📋 Specifications: TS 33.401 (EPS Security Architecture)
Key Procedures
LTE defines several critical procedures for network operation.
Initial Attach
The process when UE connects to network for the first time:
- RRC Connection Setup - UE → eNodeB (random access, RRC)
- Attach Request - UE → MME (via NAS signaling)
- Authentication - MME ↔ HSS (EPS-AKA)
- Security Mode - MME → UE (encryption activation)
- Location Update - MME → HSS
- Create Session - MME → S-GW → P-GW (GTP-C)
- Bearer Setup - eNodeB configures radio bearer
- Attach Accept - MME → UE
📋 Specifications: TS 23.401 §5.3.2 (Attach Procedure)
Handover Types
| Type | Path | Use Case |
|---|---|---|
| X2 Handover | Source eNB → Target eNB | Intra-LTE, low latency |
| S1 Handover | Source eNB → MME → Target eNB | Inter-MME, different TAC |
| Inter-RAT | LTE → 3G/2G | Coverage fallback |
X2 Handover Steps:
- Measurement Report (UE → Source eNB)
- Handover Request (Source → Target eNB via X2)
- Handover Request Ack (Target → Source)
- RRC Connection Reconfiguration (Source → UE)
- Random Access (UE → Target eNB)
- Path Switch Request (Target eNB → MME)
📋 Specifications: TS 36.300 §10 (Mobility), TS 23.401 §5.5 (Handover)
Paging
Network-initiated procedure to reach idle UE:
- MME determines Tracking Area(s) to page
- MME → eNodeBs: Paging message
- eNodeBs broadcast on PCH (Paging Channel)
- UE monitors PCH in DRX cycles
- UE responds with Service Request
📋 Specifications: TS 36.304 (Idle Mode Procedures)
Power Saving
LTE includes mechanisms to extend battery life, especially important for IoT devices.
DRX (Discontinuous Reception)
UE periodically wakes up to check for paging:
| Parameter | Idle Mode | Connected Mode |
|---|---|---|
| Cycle | 320ms - 2.56s | 10ms - 2.56s |
| Purpose | Save battery | Reduce monitoring |
| Config by | SIB2 | RRC Reconfiguration |
eDRX (Extended DRX) - Rel-13
Longer sleep cycles for IoT:
| Mode | Max Cycle | Use Case |
|---|---|---|
| Idle eDRX | 43.69 min | Infrequent updates |
| Connected eDRX | 10.24s | Bursty traffic |
PSM (Power Saving Mode) - Rel-12
Deep sleep between transmissions:
- UE remains registered but unreachable
- Timer T3324: Active time after data
- Timer T3412: Periodic TAU (up to 413 days)
- Wake up: Mobile Originated only
| State | Reachable | Power | Duration |
|---|---|---|---|
| Connected | Yes | High | Active data |
| Idle | Yes (paging) | Medium | DRX cycles |
| PSM | No | Ultra-low | Hours to days |
📋 Specifications: TS 23.682 (MTC Architecture), TS 24.301 (NAS)
Interworking
LTE is designed to work with other access technologies.
LTE ↔ 3G/2G
| Mechanism | Description | Direction |
|---|---|---|
| CSFB | CS Fallback for voice | LTE → 2G/3G |
| SRVCC | Voice call continuity | LTE → 2G/3G (during call) |
| PS HO | Packet-switched handover | LTE ↔ 3G |
SRVCC Flow:
- VoLTE call active on LTE
- Coverage degradation detected
- SRVCC triggered by eNodeB
- Call transferred to CS domain
- Voice continues on 2G/3G
LTE ↔ WiFi
| Feature | Release | Description |
|---|---|---|
| ANDSF | Rel-8 | Access Network Discovery, policy-based selection |
| MAPCON | Rel-10 | Multi-Access PDN Connectivity |
| IFOM | Rel-10 | IP Flow Mobility between LTE/WiFi |
| LWA | Rel-13 | LTE-WiFi Aggregation (combine throughput) |
| LWIP | Rel-13 | LTE-WLAN Integration with IPsec |
LTE ↔ 5G (Rel-15+)
| Mode | Description |
|---|---|
| NSA Option 3 | 5G NR + LTE EPC (EN-DC) |
| NSA Option 7 | 5G NR + 5GC + LTE |
| SA | Standalone 5G NR + 5GC |
EN-DC (E-UTRAN NR Dual Connectivity):
- LTE as anchor (Master Node)
- NR as secondary (Secondary Node)
- Split bearer or SCG bearer options
- Common deployment for initial 5G
📋 Specifications: TS 23.402 (Non-3GPP Access), TS 37.340 (Multi-RAT Dual Connectivity)
Rel-9 Extensions
Rel-9 (2009) brought significant LTE extensions:
HeNB - Home eNodeB (Femtocells)
Small base stations for home/enterprise use:
- Connection via DSL/cable
- HeNB Gateway for scaling
- CSG (Closed Subscriber Group) for access control
📋 Specifications: TS 22.220, TS 25.467
eMBMS - Evolved Multimedia Broadcast Multicast Service
Efficient content distribution to multiple users:
- TV broadcasting
- Software updates
- Emergency alerts
- Single Frequency Network (SFN)
📋 Specifications: TS 23.246, TS 36.440
Location Services (LCS)
Positioning services for LTE:
- E-CID (Enhanced Cell ID)
- OTDOA (Observed Time Difference of Arrival)
- A-GNSS (Assisted GNSS)
📋 Specifications: TS 36.305
VoLTE
Voice over LTE via IMS:
- HD Voice quality (AMR-WB)
- Faster call setup than CS
- Simultaneous voice + data
📋 Specifications: TS 23.228, IR.92 (GSMA)
CSFB - Circuit Switched Fallback
Fallback to 2G/3G for voice calls:
- For operators without IMS/VoLTE
- Transitional solution
📋 Specifications: TS 23.272
SRVCC - Single Radio Voice Call Continuity
Handover of VoLTE call to CS domain:
- Seamless transition when losing LTE coverage
📋 Specifications: TS 23.216
Related Work Items
Key 3GPP work items that defined LTE:
Rel-8 Core Work Items
| Work Item | Title | TSG |
|---|---|---|
| LTE-SAE | Long Term Evolution - System Architecture Evolution | RAN, SA |
| RP-080137 | LTE Physical Layer | RAN1 |
| RP-080138 | LTE Layer 2 and RRC | RAN2 |
| SP-080344 | EPC Architecture | SA2 |
Rel-9 Work Items
| Work Item | Title | TSG |
|---|---|---|
| RP-090360 | Home eNodeB (HeNB) | RAN |
| SP-090133 | eMBMS Enhancements | SA |
| SP-090139 | SRVCC from LTE to 3G/2G | SA2 |
| SP-090150 | CS Fallback | SA2 |
Study Items
| Study Item | Title | Output |
|---|---|---|
| TR 25.913 | Requirements for E-UTRA | Requirements |
| TR 25.912 | Feasibility study for E-UTRA | Feasibility |
| TR 23.882 | 3GPP SAE Architecture | Architecture |
| TR 36.913 | Requirements for LTE-Advanced | Rel-10+ |
📋 Work Items: Browse 3GPP Work Plan for complete list
Comparison with Previous Generations
| Feature | 2G (GSM) | 3G (UMTS) | 4G (LTE) |
|---|---|---|---|
| Access | TDMA/FDMA | WCDMA | OFDMA/SC-FDMA |
| Max DL | 384 kbps | 42 Mbps (HSPA+) | 300 Mbps |
| Latency | ~300ms | ~100ms | ~10ms |
| Core | CS + PS | CS + PS | PS only (EPC) |
| IP | Tunnel | Tunnel | Native |
Timeline
- 2004 - LTE study started in 3GPP
- 2008 - Rel-8 frozen (first LTE standard)
- 2009 - Rel-9 frozen (HeNB, eMBMS, VoLTE)
- 2009 - First commercial LTE network (TeliaSonera, Stockholm)
- 2010 - LTE expansion worldwide