Duplex Methods | Radio Interface Basics

A valuable communication feature is bidirectional communication between peers. Feedback is the major difference to unicast or broadcast systems, such as TV or radio transmission. Bidirectional information transfer can occur either simultaneously or consecutively. Both in data networks and especially for human communication, simultaneous bidirectional communication is very basic and essential. Systems with the nature of simultaneous bidirectional information transfer are full-duplex systems, in contrast to half-duplex systems, which allow only one-by-one bidirectional communication.

A full-duplex application visible to the user does not need to imply a full-duplex transmission scheme on lower layers. In mobile networks two basic methods of multiplexing and handling the duplex streams are used. Here duplex means the sense of UL and DL transmission from the handset to the base station and vice versa.

Frequency Division Duplex (FDD) divides the available frequency spectrum in a frequency range dedicated to UL transmission and a separate range for DL transmission only. A guard band is used between the frequency bands allocated for the UL and DL direction in order to prevent UL and DL interference. Figure 1 depicts a divided frequency spectrum as is typically used with FDD.

 
Figure 1: FDD system

Time Division Duplex (TDD) makes use of the same frequency resources for both transmission directions. This method divides the time domain into slots allocated for UL and DL transmission as shown in Figure 2. A guard interval is implemented to prevent UL and DL interference, especially for larger cells with a longer propagation delay spread (larger cells have a longer duration behavior until all echo reflections of the transmitted signal are received) and cases of non-ideal UL synchronization.

 
Figure 2: TDD system with example duplex slot structure

TDD defines various slot configurations of switching between UL and DL transmission. Different configurations are possible depending on the mixture between UL and DL traffic; usually the UL data volume is much smaller compared to DL data consumption. A change in transmission resources for the UL and DL direction are just one manner of soft system configuration with TDD, instead of FDD, which uses fixed frequency resources.

TDD systems have the advantage of a synchronous channel between the UL and DL direction because the wireless channel characteristic is frequency dependent and TDD uses the same frequency band for UL and DL transmission. This is especially interesting in systems which use frequency-selective scheduling and smart antennas (MIMO (Multiple Input, Multiple Output) transmission with multiple antennas) as they are defined in LTE. The base station needs to know the reception conditions of the UE in order to make an efficient frequency resource scheduling decision and for selecting the best antenna configuration for multiple antenna layer precoding.