Advantages and Disadvantages of TDD and FDD

This page covers 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?

TDD LTE topology

To understand concept of TDD, we will take example of LTE system where in user (i.e. UE) and Base station (i.e. eNB) are communicating as shown in the figure-1. TDD or Time Division Duplex uses 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 single channel for both uplink and downlink, hence it is spectrum efficient topology. TDD system provides 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 the directions. TDD uses Guard period between downlink and uplink transmissions. It is also referred as TTG and RTG in wimax/lte systems.

Benefits or advantages of TDD

Following are the benefits or advantages of TDD:
➨It does not use paired spectrum. Hence it benefits operators in terms of efficient usage of 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 simple in TDD compare to FDD. Hence UE can be benefitted in AAS (Advanced Antenna System) and beamforming based wireless systems.

Drawbacks or disadvantages of TDD

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 same carrier frequency. As transmissions are not continuous, the required data rates can not be achieved as compare to FDD at similar distances from Base Station (or eNB).
➨As TDD supports lesser distances compare to FDD, it needs more base stations to achieve given coverage area.
➨Due to requirements of more Base stations (or eNBs) , deployment and operating costs are higher in TDD.

What is FDD?

LTE FDD topology

FDD or Frequency Division Duplex uses different frequencies 'f1' and 'f2' for uplink and downlink respectively. This is shown in the figure-2. The system can use same time slots for both uplink and downlink transmissions as frequencies are different.

As FDD requires pair of channels for both uplink and downlink, it is not spectrum efficient. FDD uses guard band between uplink and downlink bands similar to TDD which uses time delay interval.

Benefits or advantages of FDD

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

Drawbacks or disadvantages of FDD

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

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