From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

4.4 TD-LTE Air Interface

Most initial LTE deployments around the world use FDD, which means that data is transmitted and received simultaneously on two separate channels. In addition, a TDD variant of the LTE air interface has been specified. It is used, for example, in China. Apart from using a single channel for both uplink and downlink transmissions, most other parameters of the air interface are identical to the FDD implementation described earlier. Some differences exist, however, which are described in this section.

As Time Division Long-Term Evolution (TD-LTE) only uses a single channel, a transmission gap is required when switching from transmission mode to reception mode. This is also referred to as the guard period and its length is determined by the time it takes to switch the operation mode of the transceiver unit and the maximum timing advance that can be encountered in a cell. The timing advance influences the guard period because the more distant a mobile device is from the center of the cell, the earlier it has to start its transmissions so that they are synchronized with the transmissions of devices that are closer to the center of the cell. The guard period has to be long enough to accommodate such earlier transmissions in relation to mobile devices closer to the cell. The guard period can be up to 10 OFDM symbol durations. In urban environments with a cell radius of 2 km or less, a single OFDM symbol duration is usually sufficient and only little capacity is ...