5G FDD vs 5G TDD: Key Differences Explained

This article compares 5G FDD and 5G TDD, outlining the key differences between these two duplexing techniques used in 5G wireless networks.

FDD stands for Frequency Division Duplex, while TDD stands for Time Division Duplex.

Introduction

• Both FDD and TDD are spectrum usage techniques employed in mobile communication networks like Mobile WiMax, LTE, and 5G.
• In FDD, uplink and downlink transmissions occur simultaneously using different spectrum frequencies.
• In TDD, both uplink and downlink use the same spectrum frequencies but at different times.
• (Refer to this resource for more on FDD vs TDD in LTE wireless networks.)

• Figure 1 illustrates TDD and FDD topologies.
• As shown, FDD uses two different carrier frequencies (“Fc1” and “Fc2”) for uplink and downlink, respectively, at the same time (“t1”). The downlink utilizes the upper part of the spectrum, while the uplink uses the lower part, separated by a guard band (duplex gap).
• TDD uses the same carrier frequency (“Fc”) at different time instants (“t1” and “t2”) for uplink and downlink transmission.

In 28 GHz TDD mode, 5G-TF uses a frequency range from 27500.5 MHz (Low) to 28349.5 MHz (High), with a center frequency of 27925 MHz and a bandwidth of 850 MHz. This 5G band supports both downlink and uplink communications. Downlink refers to transmissions from the 5G Base Station (or NB) to the 5G UE (User Equipment) or mobile phone. Uplink refers to transmissions from the 5G UE to the 5G NB.

5G FDD

Figure 2 depicts a 5G FDD scenario. As shown, the same antenna array element interfaces with a diplexer. The RF diplexer separates transmit and receive frequencies for the Power Amplifier (PA) and Low Noise Amplifier (LNA), respectively. As previously stated, FDD uses two unique frequencies, one for downlink and another for uplink, enabling simultaneous communication in both directions.

The local oscillator in the 5G beamforming module uses frequencies ranging from 23.2 to 23.9 GHz. This LO frequency, when mixed with the received RF frequency, produces an Intermediate Frequency (IF) of about 4.4 GHz during down-conversion. This down-converted frequency is then fed to the LNA module.

Conversely, this LO frequency mixed with the IF frequency produces an RF frequency in the 28 GHz band during up-conversion. This up-converted frequency is then fed to the PA module.

5G TDD

Figure 3 illustrates a 5G TDD scenario.

As shown, the same antenna array element interfaces with SPDT (Single Pole Double Throw) Switches/Filters. TDD uses only one RF frequency for transmit/receive operations at different time instants. The SPDT switch interfaces the single antenna element with either the PA or the LNA, one at a time, at the same frequency.

The PA output is filtered before connecting to the SPDT switch. Similarly, the received RF signal is filtered before connecting to the LNA.

Both Figure 2 and Figure 3 represent parts of the beamforming module used in a 5G mobile phone or 5G smartphone.

5G FDD vs 5G TDD: Key Differences

Reference for Image 2 and Image 3:

Paper on “5G Cellular User Equipment: From Theory to Practical Hardware Design” Published on July 18, 2017, at IEEE Access by YIMING HUO1, XIAODAI DONG1, and WEI XU2