LTE Frame Structure of FDD & TDD in detail | Techlteworld

Описание: LTE Frame Structure – FDD & TDD in Detail

Introduction

LTE (Long-Term Evolution) is a high-speed wireless communication standard renowned for its low latency and increased data rates compared to its predecessors. To organize data transmission efficiently, LTE utilizes a meticulously designed frame structure. This structure exists in two primary modes:

FDD (Frequency Division Duplex): Employs separate frequency bands for downlink (transmission from the base station to the device) and uplink (transmission from the device to the base station) communications.
TDD (Time Division Duplex): Utilizes a single frequency band, alternating between downlink and uplink transmissions within specific time intervals.
Understanding the frame structures for both FDD and TDD modes is essential for comprehending LTE's core operational principles.

Fundamental LTE Concepts

Before diving into the specifics of frame structures, let's review some key LTE concepts:

Subframes: The smallest unit of time in the LTE frame structure is a subframe, lasting 1 millisecond (ms).
Slots: Each subframe is divided into two slots with a duration of 0.5 ms each.
Symbols: Each slot comprises multiple OFDM (Orthogonal Frequency-Division Multiplexing) symbols. The number of symbols within a slot depends on the cyclic prefix configuration (normal or extended).
Resource Blocks (RBs): The fundamental unit of resource allocation in LTE. An RB is constituted by 12 subcarriers over a single slot (0.5ms).
FDD Frame Structure

Let's break down the frame structure of LTE FDD:

Radio Frame: In FDD, the largest time division is a radio frame, lasting 10ms. A single radio frame consists of 10 subframes.
Subframes: As mentioned, each subframe has a duration of 1 ms.
Slots: Every subframe contains two slots of 0.5 ms each. Since a radio frame has 10 subframes, there are a total of 20 slots in a radio frame.
Resource Blocks: Each slot carries a specific number of RBs, creating a grid-like structure for resource allocation.
Diagram of FDD Frame Structure
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FDD LTE Frame Structure

Key Points about FDD

Separate frequency bands for downlink and uplink avoid interference.
Simultaneous transmission of downlink and uplink data is feasible.
FDD is usually employed in scenarios with symmetrical traffic demands (similar downlink and uplink data volumes).
TDD Frame Structure

Now, let's explore the structure of LTE in TDD mode:

Radio Frame: Similar to FDD, a radio frame in TDD has a duration of 10 ms, also divided into 10 subframes.
Half Frame: A unique feature in TDD is the half-frame of 5ms. Each half-frame consists of 5 subframes.
Switching Point Periodicity: This parameter determines the interval between downlink and uplink. It can be different lengths based on configuration.
Subframe Configurations: TDD defines special configurations for subframes, indicating their transmission direction:
D: Downlink
U: Uplink
S: Special subframe (used for synchronization, broadcast, and paging)
Diagram of TDD Frame Structure
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TDD LTE Frame Structure

Key Points about TDD

Downlink and uplink use the same frequency band, allowing flexible capacity allocation based on actual traffic requirements.
TDD is beneficial in situations with asymmetric traffic patterns (e.g., more downlink than uplink).
Offers greater flexibility for network operators than FDD in fine-tuning capacity and coverage.

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