Undergraduate Communication Engineering

Introduction to LTE

Long Term Evolution (LTE) is the standard for high-speed wireless communication. Master the fundamentals of 4G mobile broadband technology, from OFDMA to MIMO systems.

100
Mbps Downlink
50
Mbps Uplink
10
ms Latency
20
MHz Bandwidth

1. LTE Overview & Evolution

What is LTE?

Long Term Evolution (LTE) is a standard for wireless broadband communication, representing the natural progression from 3G UMTS/HSPA networks. Despite being marketed as "4G", initial LTE releases (3GPP Release 8/9) are officially 3.9G or "pre-4G".

  • ▸ Developed by 3GPP (3rd Generation Partnership Project)
  • ▸ First deployed commercially in 2009 (TeliaSonera, Sweden)
  • ▸ All-IP packet-switched network (no circuit switching)

Key Advantages over 3G

Spectral Efficiency 3-4x better
Peak Data Rates 10-100x higher
Latency < 10 ms
Flexibility Scalable BW (1.4-20 MHz)

Mobile Network Evolution

1G
Analog
1980s
2G
GSM/CDMA
1990s
3G
UMTS/HSPA
2000s
LTE
4G
2009+
5G
NR
2019+

LTE Frequency Bands

LTE operates in both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes across various frequency bands.

Band 1 2100 MHz
FDD Band 1: 1920-1980 MHz (UL), 2110-2170 MHz (DL). Widely used in Europe and Asia for initial LTE deployments.
Band 3 1800 MHz
FDD Band 3: 1710-1785 MHz (UL), 1805-1880 MHz (DL). Most popular global LTE band, refarmed from GSM 1800.
Band 7 2600 MHz
FDD Band 7: 2500-2570 MHz (UL), 2620-2690 MHz (DL). Provides high capacity in dense urban areas.
Band 38 2600 TDD
TDD Band 38: 2570-2620 MHz. Used extensively in China (China Mobile) and other TDD deployments.
Band 40 2300 TDD
TDD Band 40: 2300-2400 MHz. Popular for indoor coverage and capacity augmentation.
FDD (Frequency Division Duplex)
TDD (Time Division Duplex)

2. LTE Network Architecture

E-UTRAN

Evolved UTRAN

The radio access network consisting only of eNodeBs (Evolved Node Bs).

  • • eNodeB: Base station handling radio resource management, scheduling, and handovers
  • • Simplified architecture (no RNC like in 3G)
  • • Direct connection to EPC via S1 interface
EPC

Evolved Packet Core

The all-IP core network handling user and control plane traffic.

  • • MME: Mobility Management Entity (control plane)
  • • S-GW: Serving Gateway (user plane anchor)
  • • P-GW: PDN Gateway (IP allocation, policy enforcement)
  • • PCRF: Policy and Charging Rules Function
UE

User Equipment

The mobile device containing the USIM and radio transceivers.

  • • Mobile Terminal: Radio and processing hardware
  • • USIM: Universal Subscriber Identity Module
  • • Supports multiple bandwidth configurations
  • • Category defines peak data rates (Cat 1-22)

LTE System Architecture Reference (EPS)

UE (User Equipment)
Uu
E-UTRAN (Evolved UTRAN)
eNodeB (Evolved Node B)
Handles: RRM, Scheduling, Compression, Encryption, Handover decisions
S1-MME
S1-U
EPC (Evolved Packet Core)
MME
Control Plane
S-GW
User Plane
P-GW
External PDN
PCRF
Policy Control
SGi
Internet (PDN)
IMS (VoLTE)
Corporate VPN
Uu
LTE-Uu air interface between UE and eNodeB. Uses OFDMA/SC-FDMA.
S1-MME
Control plane interface between eNodeB and MME. Uses SCTP.
S1-U
User plane interface between eNodeB and S-GW. Uses GTP-U over UDP/IP.
X2
Interface between eNodeBs for handover and interference coordination.

3. LTE Air Interface & Multiple Access

DL

OFDMA (Downlink)

Orthogonal Frequency Division Multiple Access divides the channel into multiple orthogonal subcarriers, allowing simultaneous transmission from the base station to multiple users.

Key Parameters:
  • • Subcarrier spacing: 15 kHz
  • • OFDM symbol duration: 66.7 μs + CP (4.7/5.2 μs)
  • • Cyclic Prefix (CP): Maintains orthogonality against multipath
  • • 12 subcarriers = 1 Resource Block (180 kHz)
Visualization: Subcarrier allocation over time-frequency grid
UL

SC-FDMA (Uplink)

Single Carrier FDMA provides lower Peak-to-Average Power Ratio (PAPR) compared to OFDMA, crucial for mobile device battery life and power amplifier efficiency.

Why SC-FDMA?
  • • Lower PAPR: ~3-6 dB better than OFDMA
  • • Efficient power amplifier operation
  • • Extended battery life for UEs
  • • Similar frequency domain processing as OFDMA
PAPR Comparison
OFDMA ~10-12 dB
SC-FDMA ~6-8 dB

LTE Resource Grid Structure

Resource Element (RE)
Smallest unit
1 Subcarrier × 1 OFDM Symbol
= 15 kHz × 66.7 μs
Resource Block (RB)
Allocation unit
12 Subcarriers × 1 Slot
= 180 kHz × 0.5 ms
= 84 Resource Elements (normal CP)
Resource Block Group
Scheduling unit
Contiguous RBs allocated
to a UE for data transmission
Time-Frequency Grid (1 Slot = 0.5 ms, 7 OFDM symbols)
Subcarrier 12
Subcarrier 11
Subcarrier 10
...
Subcarrier 2
Subcarrier 1
Sym 0
Sym 1
Sym 2
Sym 3
Sym 4
Sym 5
Sym 6
Reference Signal (RS)
Control Data (PDCCH)
User Data (PDSCH)
Synchronization (PSS/SSS)

Frame Structure Type 1 (FDD)

Radio Frame 10 ms
Subframe 0
1
2
3
4
5
6
7
8
9
• Subframes 0 & 5 carry synchronization signals (PSS/SSS)
• Subframe 0 carries PBCH (Physical Broadcast Channel)
• 10 subframes × 1 ms = 10 ms frame duration

Frame Structure Type 2 (TDD)

Special Subframe DwPTS + GP + UpPTS
D
S
U
U
U
D
S
U
U
U
Uplink-Downlink Configuration: Ratio of DL:UL subframes
• Config 0: 2:3 (DL heavy)
• Config 1: 4:4 (Balanced)
• Config 5: 9:1 (DL only)
Special subframe contains DwPTS (Downlink), GP (Guard Period), UpPTS (Uplink)

4. Physical Layer & Channels

LTE Channel Structure

Logical Channels
DTCH (Dedicated Traffic)
DCCH (Dedicated Control)
BCCH (Broadcast Control)
PCCH (Paging Control)
CCCH (Common Control)
What type of information?
Transport Channels
DL-SCH (Downlink Shared)
UL-SCH (Uplink Shared)
BCH (Broadcast)
PCH (Paging)
RACH (Random Access)
How is it transmitted?
Physical Channels
PDSCH (Physical DL Shared)
PUSCH (Physical UL Shared)
PBCH (Physical Broadcast)
PDCCH (Physical DL Control)
PUCCH (Physical UL Control)
Physical layer processing
↓ MAC Layer Mapping ↓

Reference Signals (RS)

  • • Cell-Specific RS: Used for channel estimation, sent in every downlink subframe
  • • UE-Specific RS (DM-RS): Dedicated reference signals for beamforming
  • • Sounding RS (SRS): Uplink reference for channel quality estimation

Synchronization Signals

  • • PSS (Primary Synchronization Signal):
    Slot 0 & 10, identifies cell identity within group (3 possibilities)
  • • SSS (Secondary Synchronization Signal):
    Slot 0 & 10, identifies cell group (168 possibilities)
  • • Total Cell IDs: 504 unique physical layer cell identities

Adaptive Modulation and Coding (AMC)

LTE dynamically selects modulation scheme and coding rate based on channel conditions (CQI - Channel Quality Indicator).

📡
QPSK
2 bits/symbol
Robust, poor coverage
📶
16-QAM
4 bits/symbol
Balanced
📶📶
64-QAM
6 bits/symbol
High throughput
🚀
256-QAM
8 bits/symbol
LTE-A only, near eNB

5. Advanced LTE Features

MIMO (Multiple Input Multiple Output)

Spatial Multiplexing

MIMO uses multiple antennas at both transmitter and receiver to improve data throughput and link reliability without increasing bandwidth.

2×2
2×2 MIMO
2 transmit, 2 receive antennas. ~50% capacity gain.
4×4
4×4 MIMO
4 transmit, 4 receive antennas. Up to 4x throughput.
8×8
8×8 MIMO (LTE-A)
8 layers spatial multiplexing. Massive throughput.
Transmission Modes (TM):
  • TM1: Single antenna port
  • TM2: Transmit diversity (SFBC)
  • TM3: Open-loop spatial multiplexing
  • TM4: Closed-loop spatial multiplexing
  • TM6: Single layer closed-loop
  • TM7: Single antenna port (beamforming)
MIMO 2×2 Configuration
TX1
TX2
eNodeB
Multipath Channel
RX1
RX2
UE

Carrier Aggregation (CA)

LTE-A Feature

Carrier Aggregation allows combining multiple component carriers (CC) across contiguous or non-contiguous frequency bands to increase bandwidth and data rates.

Intra-band CA
Contiguous
CC1+CC2+CC3
Intra-band CA
Non-contiguous
Gap between
Inter-band CA
Different bands
Band 3 + Band 7
5
Component Carriers max
100
MHz Total Bandwidth
3 Gbps
Peak DL Throughput

LTE vs LTE-Advanced Comparison

Feature LTE (3GPP R8) LTE-Advanced (R10+)
Peak Downlink 300 Mbps 1 Gbps
Peak Uplink 75 Mbps 500 Mbps
Max Bandwidth 20 MHz 100 MHz (CA)
MIMO Layers Up to 4×4 Up to 8×8
Modulation DL 64-QAM 256-QAM
Carrier Aggregation ❌ No ✅ Yes (5 CCs)
Heterogeneous Networks Basic Enhanced (eICIC)
Relay Nodes ❌ No ✅ Yes

LTE Throughput Calculator

Calculate theoretical peak data rates based on LTE parameters

Configuration

0.1 0.93 0.95
0% 14% 30%

Results

0
Mbps Theoretical Peak
Resource Blocks 100
Subcarriers 1200
REs per ms 16800
Spectral Efficiency 0
Formula: Throughput = N_RB × 12 × 14 × 1000 × N_layers × bits_per_symbol × coding_rate × (1 - overhead) / 10^6
Where: 12 subcarriers/RB, 14 OFDM symbols/slot (normal CP), 1000 slots/second, 2 slots/subframe

6. Key Concepts Summary

Flat Architecture

LTE eliminates the RNC (Radio Network Controller) from 3G, reducing latency and simplifying the network with direct eNodeB to EPC connection.

All-IP Network

LTE is the first cellular standard designed as a pure packet-switched network. Circuit-switched voice is handled via VoLTE (Voice over LTE) or CSFB.

Flexible Bandwidth

Unlike previous standards with fixed 5 MHz channels, LTE supports scalable bandwidth from 1.4 MHz to 20 MHz, enabling flexible spectrum deployment.

FDD vs TDD

LTE supports both Frequency Division Duplex (separate frequencies for UL/DL) and Time Division Duplex (same frequency, different time slots).

Self-Organizing Networks

SON capabilities allow automatic configuration, optimization, and healing of the network, reducing operational costs and deployment time.

QoS Management

EPS bearers provide differentiated QoS with guaranteed bit rates (GBR) for real-time services and non-GBR for best-effort data.

Study Checklist

Understand LTE network architecture (E-UTRAN and EPC components)
Explain the difference between OFDMA and SC-FDMA
Describe the resource grid structure (RE, RB, Subframe)
Understand channel mapping (Logical → Transport → Physical)
Explain MIMO concepts and transmission modes
Calculate throughput using the provided formula
Compare LTE with LTE-Advanced features
Understand FDD vs TDD frame structures
Progress 0/8 completed