Time Division Duplex

The duplex separation of downlink (DL) and uplink (UL) is typically done either by using FDD or by using time division duplex (TDD).

Time Division Duplex

Air interface standards using TDD are for example LTE TDD and TD-SCDMA. To specify the TDD configuration of various air interfaces, click Project > Edit Project Parameters, click the Air Interface tab. Under Duplex Separation, from the drop-down list, select Duplex: TDD and click Settings.

Figure 1. The Duplex Properties TDD dialog.
Duplex switching

Switching between downlink and uplink channel in TDD mode can be specified to be fixed or adaptive. The adaptive mode allows switching between downlink and uplink according to the traffic load, either with or without taking into account the guard interval. In case of adaptive switching, always full availability is assumed for each direction (downlink or uplink). Therefore no further specifications are required in this case.

The TDD properties specified for each cell/carrier individually radio button is active when a 5G TDD air interface was selected when creating the project. In that case, TDD settings are defined individually for the specified carrier in each cell. This is done on the Carrier dialog when assigning a carrier to a transmitter. For 5G NR TDD, the Carrier dialog will offer fields for symbols per slot (number of downlink, uplink and flexible symbols).

Transmission Blocks / Frames / Sub-Frames
Both standards (with switching points in each sub-frame or with switching points only once or twice per frame) can be defined. Figure 1 shows the definition of 8 sub-frames completely allocated to the downlink (DL), one sub-frame completely allocated to the uplink (UL), and one specific sub-frame with 10:2:2 separation between DL:UL:GP where GP stands for guard period. The resulting overall ratios for downlink and uplink allocation are automatically computed (depending on the settings) and considered in the network simulation for each individual modulation and coding scheme.

Figure 2. Sketch of a sub frame.
MS : MS and BS : BS interference
Consider additional interference effects caused by other mobile stations or other base stations, respectively.