TDD vs FDD: Advantages and Disadvantages

This page covers the advantages and disadvantages of TDD and FDD. It mentions TDD advantages, TDD disadvantages, FDD advantages, and FDD disadvantages.

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

What is TDD?

Image alt: TDD LTE topology

Introduction: To understand the concept of TDD, let’s take the example of an LTE system where a user (UE) and a Base station (eNB) are communicating as shown in Figure 1.

TDD or Time Division Duplex uses the same frequency ‘f1’ for uplink (from UE to eNB) and downlink (from eNB to UE) transmissions. It uses different time slots for both uplink and downlink transmissions. TDD requires a single channel for both uplink and downlink, hence it is a spectrum-efficient topology.

The TDD system provides a provision to allocate any number of time slots for uplink and downlink directions. Hence, it is used for asymmetric transmissions requiring different data rates in both directions. TDD uses a Guard period between downlink and uplink transmissions. It is also referred to as TTG and RTG in WiMAX/LTE systems.

Benefits or Advantages of TDD

The following are the benefits or advantages of TDD:

• It does not use paired spectrum. Hence, it benefits operators in terms of efficient usage of the spectrum.
• It is used for dynamic resource requirements based on application and quality of service. This is possible due to dynamic allocation of time slots without changing the bandwidth once allocated. Hence, TDD is best suited for unpaired spectrum scenarios requiring asymmetric data rates.
• Due to channel reciprocity, channel equalizer algorithms are simpler in TDD compared to FDD. Hence, UE can benefit in AAS (Advanced Antenna System) and beamforming-based wireless systems.

Drawbacks or Disadvantages of TDD

The following are the disadvantages of TDD:

• As TDD operates based on allocated time slots, it requires stringent phase/time synchronization to avoid interference between UL (Uplink) and DL (Downlink) transmissions.
• Uplink and downlink transmissions occur at different time instants at the same carrier frequency. As transmissions are not continuous, the required data rates cannot be achieved as compared to FDD at similar distances from the Base Station (or eNB).
• As TDD supports lesser distances compared to FDD, it needs more base stations to achieve a given coverage area.
• Due to the requirements of more Base stations (or eNBs), deployment and operating costs are higher in TDD.

What is FDD?

Image alt: LTE FDD topology

Introduction: FDD or Frequency Division Duplex uses different frequencies ‘f1’ and ‘f2’ for uplink and downlink, respectively. This is shown in Figure 2.

The system can use the same time slots for both uplink and downlink transmissions as the frequencies are different. As FDD requires a pair of channels for both uplink and downlink, it is not spectrum efficient.

FDD uses a guard band between uplink and downlink bands similar to TDD, which uses a time delay interval.

Benefits or Advantages of FDD

The following are the benefits or advantages of FDD:

• It is a proven technology for voice traffic. It is designed for symmetric traffic and does not require guard time like TDD.
• It uses a paired spectrum on a continuous basis for both directions and hence it can achieve higher rates for similar distances as a TDD system.
• Due to the above, the FDD system requires fewer base stations (or eNBs) compared to TDD as it covers larger distances with the same rates as TDD.
• Due to the requirements of fewer Base Stations, overall deployment, operation, and maintenance costs are less.

Drawbacks or Disadvantages of FDD

The following are the disadvantages of FDD:

• In FDD, frequencies are allocated dedicatedly. This leads to a wastage of spectrum when it is not used. Moreover, a guard band is used between uplink and downlink to avoid interference; this part is wasted as it is not used for useful traffic.
• FDD cannot be deployed where the spectrum is un-paired.
• Though it saves on the number of Base Station requirements, hardware costs associated with FDD are higher.