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].

The Propagation of radio signals has a number of inherent problems of electromagnetic waves, of which LTE or 4G is not exempt. Aspects such as absorption, refraction, diffraction, fading, and other properties can cause serious problems of coverage, interference, and general degradation of the transmitted signal. Some of these parameters are largely unpredictable, mainly those arising from meteorological phenomena or the ground, and therefore, fewer steps may be taken in such cases.

Moreover, the free space loss (FSL) is directly related to the system operating frequency and the distance between transmitter and receiver. The case of frequency is a parameter already defined for this type of technology by the International Telecommunication Union [1], also depends on the frequency assignments of each country, and therefore it is not something that operators can easily handle to increase the coverage of their mobile systems.

The case of multipath is a problem that is associated with the spread of radio beam by several pathways, supported by the phenomena of reflection, diffraction and scattering. This type of phenomenon produces a difference in the distance that a beam that travels with sight line over another who has suffered the effect of reflection (to name one example) and consequently there is a difference between the arrival times of the signals and the receiver, which may cause the overlap of the signals at the receiving antenna causing inter-symbol interference (ISI), or too low power to be demodulated, it is also possible that even a given beam incident on the receiving antenna as shown in the following Figure # 1.

Figure # 1: Multipath example.