Before diving into TDD and FDD, it is important to understand the broader concept of duplexing — how a wireless device manages two-way communication (transmit + receive).

Duplexing defines whether a device can transmit (UL) and receive (DL) simultaneously or not.

There are two fundamental types:

• Half Duplex
• Full Duplex
   

These concepts apply across walkie-talkies, LTE/5G radios, Wi-Fi, satellite communication, Bluetooth, and even IoT devices.

A Half Duplex device can either transmit OR receive at a given moment—but not both simultaneously.

Think of it like a walkie-talkie:

• You press the button → you talk (TX mode)
• You release the button → you listen (RX mode)

Wireless systems alternate between UL and DL using time.

• One direction at a time
• Communication is sequential, not parallel
• Requires switching time between TX and RX
• Lower hardware complexity
• Often used in low-power or low-cost devices
   

• Walkie-talkies
• Push-to-talk radios
• NFC
• HD-Wi-Fi modes
• IoT sensors
• Satellite terminals using TDD
• 5G NR TDD (at the UE perspective)
  UE generally cannot TX and RX simultaneously (except advanced designs)
   

Half duplex is ideal when:

• UL and DL do not need to be simultaneous
• Cost must be minimized
• Spectrum usage can be sequential
   

A Full Duplex device can transmit and receive simultaneously on:

• the same channel, or
• different channels

Meaning UL and DL happen at the same time.

A telephone system is full duplex:

• You speak
• You hear the other person
• Simultaneously
   

• Simultaneous transmission and reception
• No switching time
• Potentially doubles spectral efficiency
• Requires strong isolation between TX and RX
• Much more complex radio hardware
   

This is the classic FDD model:

• One frequency = DL
• Another frequency = UL
• Radio uses filters and duplexers to cancel self-interference

This is how LTE FDD and 5G FDD work.

This is called In-Band Full Duplex (IBFD).

TX and RX occur on the same frequency at the same time.

Your own transmitter’s power is millions of times stronger than the signal you’re trying to receive.
So the receiver needs:

• Self-interference cancellation
• Analog domain cancellation
• Digital domain cancellation
• Antenna isolation

This is still research-stage, not used commercially in 5G or LTE UEs. 

Because UL and DL occur at different times, TDD devices naturally behave half-duplex.

Because UL and DL occur simultaneously on different frequencies, FDD behaves as full duplex at the UE.

A possible 6G feature:
Simultaneous TX/RX on the same frequency, if self-interference is solved.

FDD uses two separate frequency bands:

• One band for uplink
• One band for downlink
   

Both UL and DL occur simultaneously.

Example in LTE:

• DL at 1800–1850 MHz
• UL at 1710–1785 MHz

A fixed duplex spacing ensures they never overlap.

Real-time applications (VoLTE, emergency calls) benefit from constant uplink+downlink.

No switching gaps → ideal for conversational services.

Supports legacy networks where UL ≈ DL load.

Most macro-cell infrastructure is built on FDD bands.

Smartphone traffic is DL-heavy (video, browsing), so fixed UL bandwidth wastes spectrum.

Spectrum is scarce and expensive.

FDD needs duplexers & filters → costlier UE and BS.

Wide paired bands are rare at high frequencies.

TDD uses the same frequency band, but switches UL/DL over time:

| DL | DL | DL | UL | UL | DL | UL | ...

UL and DL occur in separate time slots.

NR-5G uses a highly flexible TDD frame structure—dynamic UL/DL switching based on traffic demand (You can think of it as first 0.7 sec for DL and then 0.3 secs for UL on the same frequency. Network decides this DL/UL distribution based on how much data is scheduled).

Only one spectrum block required.

DL:UL ratio can be dynamically adjusted (e.g., 7:1 for 5G mobile broadband).

TDD enables UL/DL channel reciprocity → reduces CSI feedback overhead.

Paired spectrum at those bands is practically nonexistent.

Switching between UL and DL consumes overhead → slight efficiency loss.

If operators are not synchronized, DL from one cell may interfere with UL of another.

DL transmissions may overpower UL receivers.

Fast-moving users may see UL/DL switching inconsistencies.

5G’s most advanced bands—n78 (3.5 GHz), n79 (4.9 GHz), FR2 (mmWave)—are TDD-only because:

• They offer large contiguous bandwidth (unpaired)
• Massive MIMO needs channel reciprocity
• Traffic is DL-heavy
• Beamforming becomes simpler
   

FDD is still used in low-band (<1 GHz) for wide coverage and mobility.