LTE Frame Structure

LTE needs a way to organize time and frequency so millions of users can share the same spectrum. The frame structure is that organizational system.

Think of it like a train schedule. Every train (data packet) needs a specific time slot and platform (frequency). Without this structure, chaos.

The Time Hierarchy

LTE divides time into a nested hierarchy. Each level zooms in further.

Radio Frame: The Big Picture

The radio frame is the largest time unit:

1 Radio Frame = 10 ms

This 10 ms window repeats forever. Everything in LTE is synchronized to this heartbeat.

Subframes: Dividing the Frame

Each radio frame contains 10 subframes:

Each subframe is the basic scheduling unit. The network decides what to send in each 1 ms subframe.

Time-Slots: Further Division

Each subframe splits into 2 time-slots:

So one radio frame has 20 time-slots total.

OFDM Symbols: The Atomic Unit

Each time-slot contains 7 OFDM symbols (with normal cyclic prefix):

Why 7? It comes from the math:

• Subcarrier spacing = 15 kHz
• Symbol duration = 1/15000 = 66.7 μs
• Add cyclic prefix time, and 7 symbols fit in 0.5 ms

Cyclic Prefix: The Guard Time

Remember the cyclic prefix from OFDM? It’s the guard time that prevents inter-symbol interference from multipath.

LTE offers two options:

Normal Cyclic Prefix

• CP duration: ~5.2 μs (first symbol) / ~4.7 μs (others)
• Symbols per slot: 7
• Use case: Most deployments, small to medium cells

The short guard time means more symbols fit in each slot, giving you higher throughput.

Extended Cyclic Prefix

• CP duration: ~16.7 μs
• Symbols per slot: 6
• Use case: Large cells, mountainous terrain, high multipath environments

The longer guard time wastes more capacity but provides better protection against multipath delay spread.

Rule of thumb: Normal CP handles delay spreads up to ~5 μs. Extended CP handles up to ~17 μs.

The Frequency Dimension

Now the other axis. LTE uses OFDM with a fixed subcarrier spacing:

Subcarrier spacing = 15 kHz

The total bandwidth determines how many subcarriers you get:

Resource Block: The Allocation Unit

Here’s the crucial concept. LTE allocates resources in 2D chunks called Resource Blocks (RB).

What’s in a Resource Block?

A Resource Block combines time and frequency:

1 RB = 12 subcarriers × 7 OFDM symbols

Breaking that down:

• 12 subcarriers × 15 kHz = 180 kHz bandwidth
• 7 symbols = 1 time-slot = 0.5 ms duration
• 84 Resource Elements total (12 × 7)

Resource Element: The Smallest Unit

A Resource Element (RE) is the atomic unit:

1 RE = 1 subcarrier × 1 OFDM symbol

This is the smallest addressable unit in LTE. Each RE carries one modulation symbol (QPSK, 16-QAM, or 64-QAM).

How Many Resource Blocks?

The number of RBs depends on your bandwidth:

The scheduler assigns RBs to users each subframe. More RBs = more capacity.

FDD vs TDD: Frame Types

LTE supports two duplexing modes that determine how uplink and downlink share the spectrum.

Type 1: FDD (Frequency Division Duplex)

FDD uses separate frequencies for uplink and downlink:

• Uplink and downlink transmit simultaneously
• Requires paired spectrum (two frequency bands)
• All 10 subframes available for each direction
• Most common in LTE deployments

Type 2: TDD (Time Division Duplex)

TDD uses the same frequency for both directions:

• Uplink and downlink take turns in time
• Uses unpaired spectrum (single frequency band)
• Some subframes for DL, some for UL, some special
• Flexible UL/DL ratio based on traffic patterns

Which is Better?

FDD dominates because paired spectrum was historically more available and the constant bandwidth simplifies planning.

Summary: Key Parameters

The frame structure is LTE’s universal clock. Every device, every tower, every packet follows this timing. It’s what makes coordinated multi-user access possible.