Flexible Duplexing vs. Traditional Duplexing

This page compares flexible duplexing vs. traditional duplexing used in TDD (Time Division Duplexing) and FDD (Frequency Division Duplexing). It highlights the differences between them in the context of LTE and 5G, and explores the benefits of flexible duplexing.

Traditional Duplexing: TDD and FDD

As we know, FDD and TDD are popular duplexing techniques often used with multiple access schemes such as FDMA and TDMA. FDD has been dominant since the beginning of the mobile communication era. In 5G NR (New Radio), FDD is typically used for lower frequency bands, whereas TDD is used for frequencies above 10 GHz, particularly in very dense deployments.

What is FDD?

Figure 1: LTE FDD Topology

The figure above describes the FDD concept in an LTE system. As shown, a pair of frequencies are allocated dedicatedly for uplink and downlink directions.

Example of FDD:

• LTE band-13
  • Uplink (UEs to eNodeB) frequency (f1): 777 to 787MHz
  • Downlink (LTE eNodeB to UEs) frequency (f2): 746 to 756 MHz
• Radio frame duration: 10ms

The entire radio frame is used simultaneously over downlink and uplink directions. Both uplink and downlink will each have 10MHz bandwidth on which the entire frame will be used.

What is TDD?

Figure 2: TDD LTE Topology

The figure above describes the TDD concept in an LTE system. As shown, the same frequency (f1) is used for both uplink and downlink, but at different time slots.

Example of TDD:

• LTE band-33 (1900 to 1920MHz)
• The entire bandwidth of 20MHz is used for both eNodeB and UEs.
• Radio frame time slots 0 to 9 for UL/DL configuration of zero and 5ms DL/UL switch point periodicity: D,S,U,U,U,D,S,U,U,U (where D stands for downlink and U stands for uplink, and S for Special subframe).

Hence, the subframes of the entire radio frame are divided and used for both the uplink and downlink direction. Leading chipset manufacturers such as Ericsson, Altair semiconductor, and Qualcomm support both TDD and FDD versions of LTE on a single chip.

Flexible Duplexing in TDD and FDD

TDD and FDD-specific interference can occur in very dense deployments with low-power nodes (i.e., Base Stations and mobile devices).

Flexible duplexing refers to the dynamic assignment of transmission resources (i.e., time and frequency). This is very useful in very dense deployments requiring dynamic traffic variations.

Traditional TDD-based mobile technologies have restrictions on downlink/uplink configurations, as seen in TD-LTE or TDD-LTE. Flexible duplexing allows for very flexible and dynamic assignment of TDD resources.

Benefits of Flexible Duplexing

Following are the main advantages of flexible duplexing over traditional duplexing:

• Maximizes spectrum utilization: By dynamically adjusting resource allocation, flexible duplexing makes better use of available spectrum.
• Optimizes user experience: Adapting to real-time traffic demands results in a more responsive and efficient network, improving the user experience.
• Carrier aggregation across TDD and FDD is possible: Flexible duplexing enables more seamless integration and aggregation of different spectrum bands, further enhancing network capacity and performance.