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5G SA FDD vs TDD Configurations, Field Deployments

  • Writer: Venkateshu
    Venkateshu
  • May 16
  • 4 min read

Introduction

5G New Radio (NR) introduces flexible spectrum utilization across Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes. In a 5G Standalone (SA) architecture, these duplexing modes significantly impact network performance, latency, coverage, and scheduling strategy. This article breaks down the differences between FDD and TDD in 5G SA, focusing on protocol parameters and real-world deployment practices.

 

1.      Duplexing Basics

duplexing refers to the method used to enable bi-directional (UL and DL) communication between the user equipment (UE) and the base station (gNB). 5G supports two primary duplexing modes:

Frequency Division Duplex (FDD)

  • FDD separates uplink and downlink transmissions by assigning them to two different frequency bands.

  • Full-Duplex Transmission: Since both UL and DL occur at the same time on different frequencies, it enables simultaneous transmission and reception.

Time Division Duplex (TDD)

  • TDD uses the same frequency band for UL and DL, but separates them in the time domain using time slots.

  • Half-Duplex Operation: UE cannot transmit and receive at the same time—DL and UL alternate in time.

Feature

FDD

TDD

Duplexing Method

Uplink and downlink on separate frequencies

Uplink and downlink share same frequency but separated in time

Spectrum Use

Paired

Unpaired

Channel Reciprocity

Not applicable

Supports UL-DL channel reciprocity (useful for beamforming)

Guard Periods

Not required

Required between DL and UL slots

Scheduling

Asynchronous

Highly synchronized

Basic Frame Structure in 5G NR

FDD Frame Structure

  • Uses a paired spectrum—separate frequencies for UL and DL.

  • Downlink and uplink transmissions occur simultaneously.

  • Frame duration: 10 ms, consisting of 10 subframes.

  • Subcarrier Spacing (SCS): Typically 15 kHz or 30 kHz.

  • No guard periods needed.


 

TDD Frame Structure

  • Uses a single unpaired frequency shared for both UL and DL.

  • Requires synchronization and guard periods (GPs).

  • Slot-based division of DL, UL, and GP within a frame.

  • SCS: Typically 30 kHz or 60 kHz.

  • Frame is also 10 ms with 14 OFDM symbols per slot.

 

TDD Slot Formats: Semi-Static and Dynamic

Semi-Static TDD Slot Format

  • Configured via TDD-UL-DL-ConfigurationCommon in SIB1.

  • Same slot configuration for all UEs.

  • Typically used for synchronized, planned deployments.

Example Configuration (Band n78, 100 MHz, SCS = 30 kHz):

  • DL: 7 slots, GP: 2 slots, UL: 5 slots

Slot Index

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Direction

DL

DL

DL

DL

DL

DL

DL

GP

GP

UL

UL

UL

UL

UL

 Benefits: Predictable scheduling, low complexity.

Dynamic TDD Slot Format

  • Configured dynamically via DCI format 2_0.

  • Slot direction (DL/UL) can change based on traffic demands.

  • Used in advanced deployments with traffic-aware scheduling.

Example Use Case:

  • DL-heavy application (e.g., video streaming): more slots dynamically allocated to DL.

  • UL-heavy case (e.g., live streaming from UE): dynamic UL slot assignments.

Benefits:

  • Improved resource utilization.

  • Adaptive QoS support.

  • Requires more sophisticated gNB scheduling logic and UE support.

                                                                                                                                       

2. Use in 5G SA Deployment

FDD Usage

  • Low and Mid Bands (e.g., n1, n5, n28)Widely used for coverage and mobility due to better propagation.

  • 5G SA FDD enables:

    o   Extended coverage (rural, suburban)

    o   VoNR over wide areas

    o   Efficient anchor for EN-DC or dual connectivity

TDD Usage

  • Mid and High Bands (e.g., n78, n77, n41, n258)Preferred for capacity due to larger contiguous bandwidths (40–100 MHz+)

  • 5G SA TDD enables:

    o   Massive MIMO and beamforming

    o   High-throughput use cases (eMBB, FWA)

    o   Indoor small cell and hotspot deployments

 

3. Physical Layer and Numerology

Numerology

Parameter

FDD

TDD

Subcarrier Spacing (SCS)

Common: 15 kHz, 30 kHz

Common: 30 kHz, 60 kHz

FR1 (Sub-6 GHz)

Supported

Supported

FR2 (mmWave)

Rare

Preferred (e.g., n257, n261)

Bandwidth Allocation

  • FDD: Each direction has fixed bandwidth (e.g., 10 MHz DL + 10 MHz UL)

  • TDD: A single bandwidth (e.g., 100 MHz) is shared between DL and UL using a slot pattern

 

4. Slot Configuration (TDD Specific)

TDD systems rely on DL/UL slot configurations defined in the TDD-UL-DL-ConfigurationCommon IE (from SIB1):

TDD-UL-DL-ConfigurationCommon ::= SEQUENCE {

  referenceSubcarrierSpacing   ENUMERATED {kHz15, kHz30, ...},

  pattern1                     TDD-UL-DL-Pattern,

  pattern2                     TDD-UL-DL-Pattern OPTIONAL

}

TDD Slot Pattern Example (n78, 100 MHz, SCS 30 kHz):

  

Symbol Index

0–3

4–7

8–10

11–13

Direction

DL

DL

GP

UL

 

  • DL: Downlink symbols

  • GP: Guard period

  • UL: Uplink symbols

 

5. RRC and MAC Layer Differences

 

Layer

Parameter

FDD

TDD

RRC

tdd-UL-DL-ConfigCommon

Not used

Mandatory

RRC

servingCellConfigCommon

Carries DL/UL ARFCNs

Used in both

MAC

Scheduling Type

UL grants, DL assignments

Synchronized slot-based

MAC

BSR (Buffer Status Reporting)

Asynchronous

More sensitive to timing

6. Deployment Insights and Field Practices

FDD in Field

  • Used for anchor carriers, especially where LTE refarming is done (e.g., n1, n5, n3)

  • Important for VoNR and fallback reliability

  • Usually configured as initialBWP due to better SNR

TDD in Field

  • Used for high-capacity hotspots

  • Massive MIMO antennas (64T64R) used in urban deployments

  • Synchronized across cell sites using GPS or PTP

 

Scheduling Strategy

  • FDD: Better suited for latency-sensitive applications

  • TDD: More efficient in DL-heavy applications (e.g., video streaming), but needs slot planning

 

7. Dynamic Spectrum Sharing (DSS)

  • FDD bands often support DSS (e.g., LTE + 5G NR in Band n1)

  • Allows gradual migration to SA 5G using legacy spectrum

  • Coexistence with LTE impacts throughput, optimized using features like rate matching

 

Summary Table

Category

FDD

TDD

Frequency

Paired

Unpaired

Bands

n1, n3, n28

n78, n77, n41

Best Use Case

Coverage, VoNR

Capacity, eMBB

Numerology

15/30 kHz

30/60 kHz

DL/UL Config

Fixed

Dynamic

Deployment

Macro cells, rural

Urban, dense small cells

Protocol Dependencies

Standard

TDD-UL-DL Config IEs

References

  1. 3GPP TS 38.331 – NR RRC Protocol Specification

  2. 3GPP TS 38.304 – Cell Selection and Reselection Procedures

  3. 3GPP TS 38.213 – Physical Layer Procedures for Control

  4. 3GPP TS 38.211 – Physical Channels and Modulation

  5. 3GPP TS 38.101-1/2 – UE Radio Access Capabilities

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