What is FDD, TDD and Half Duplex FDD ?


One important part of LTE requirements in terms of spectrum flexibility is the possibility to deploy LTE-based radio-access in both paired and unpaired spectrum, (i.e., LTE should support both FDD and TDD based duplex arrangements).

Explanation of the Figure

  • Frequency Division Duplex (FDD), as illustrated in the left part of figure above, implies that downlink and uplink transmission take place in different, sufficiently separated frequency bands.
  • Time Division Duplex (TDD), as illustrated in the right part of figure above, implies that downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
  • Both half-duplex and full-duplex FDD are supported.

Half Duplex FDD

In half-duplex FDD, the terminal cannot transmit and receive at the same time, whereas in full duplex FDD there is no such limitation. Thus, from a terminal perspective, half duplex FDD is similar to TDD in the sense that reception and transmission are separated in the time domain. Conversely, from a base station perspective, there is no fundamental difference between half duplex and full duplex FDD as the base station can transmit in the downlink in all sub frames. The main benefit of half duplex FDD is that no duplex filter is required in the terminal, thereby simplifying the implementation, especially for multi-band terminals in situations with a narrow frequency separation between uplink and downlink.

Frame Structure Difference

The support for both TDD, half and full-duplex FDD is handled by two very similar frame structures: frame structure type 1 for half and full-duplex FDD and frame structure type 2 for TDD. Apart from some smaller differences in the frame structure and mapping of some physical signals, the overall physical layer processing (e.g. channel coding, modulation and multi-antenna schemes) is identical for FDD and TDD.