Duplex Systems transmission: LTE-FDD and LTE-TDD

FDD (Frequency Division Duplex) is a scheme of transmitting and receiving signals, this system allows full duplex communication using two different frequencies, one for the downlink and one for uplink, maintaining a separation band between said frequencies in order no overlap of channels, it makes that the FDD spectral efficiency is not very good. However, one advantage of this multiplexing scheme is that it does not introduce additional delays or latency.

On the other hand, TDD (Time Division Duplex) is another multiplexing technique for communication that uses a single channel or frequency, for information transmission. In this case, the transmission and reception is performed by the same frequency but with differences in time and a temporal separation between the two directions of communication, making more efficient use of spectrum. The TDD multiplexing takes a temporary assignment to the communication directions, and the guard time, which makes it more sensitive to delays and latency.

Some comparisons:

• Regarding FDD distance has better characteristics at longer distances than TDD, so TDD has more acceptation in scenarios where distances are shorter.

• The signal propagation also has different characteristics, since TDD uses a single frequency, this makes the channel as such, presents the same characteristics in transmission and reception.

• In FDD each channel has different propagation characteristics depending on the frequency used.

• The frequency difference of the FDD channels, causes that the capacity of each channel depends on the frequency allocated by regulatory authorities.

• In TDD is easier to make a dynamic distribution of the uplink and downlink capacity in order to meet the demand characteristics of resources.

Figure # 1 shows the frequency assignments set by 3GPP for the use of TDD and FDD:

Figure # 1: Frequency bands for LTE FDD and TDD [1].