TDD vs FDD for LTE

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- everything RF

Aug 17, 2020

Time-division duplexing (TDD) is a communication method where both the transmitter and receiver use the same frequency band but transmit and receive traffic at different times.

Frequency Division Duplexing (FDD) is a full-duplex method that uses two different frequencies for transmit and receive operations.

SpectrumTransmit and Receive functions occur in the same frequency band at different timesTransmit and Receive functions occur simultaneously at different frequencies in the same channel
Channel Reciprocity
Channel propagation is the same in both directions as the same frequency is used for both operations
Channel characteristics are different in both directions as different frequencies are used for Tx and Rx
Uplink / Downlink Asymmetry
It is possible to dynamically change the Uplink and Downlink capacity ratio to match demand
Uplink / Downlink capacity is determined by the frequency allocation set out by the regulatory authorities. It is therefore not possible to make dynamic changes to match capacity. Regulatory changes would normally be required and capacity is normally allocated so that it is the same in either direction.
Guard Period/BandGuard period is required to ensure uplink and downlink transmissions do not clash. Guard periods limit capacity. Larger guard periods are usually required if distances are increased to accommodate larger propagation times.
Guard bands are required to provide sufficient isolation between uplink and downlink transmission. In FDD large guard bands do not impact capacity.
Discontinuous Transmission
Since Uplink and Downlink transmissions are done at different times, there is a discontinuity during transmissions. This can degrade the performance of the RF power amplifier in the transmitter.
Since different frequencies are used Uplink and Downlink transmissions can be done simultaneously so there are no discontinuities.
Cross Slot Interference
Base stations need to be synchronized w.r.t the uplink and downlink transmission times. If neighboring base stations use different uplink and downlink assignments and share the same channel, then interference may occur between cells.
Not Applicable
Hardware CostLower cost as no diplexer is needed to isolate the transmitter and receiver. Cost of user equipment is of major importance for wireless infrastructure installations.
A Diplexer is needed which results in a higher cost.

Click here to learn more about TDD.

Click here to learn more about FDD.