TDD
Understanding TDD
TDD has become the dominant duplexing scheme for new wireless deployments, especially at higher frequencies where unpaired spectrum is abundant. The ability to dynamically adjust the UL/DL ratio and exploit channel reciprocity for Massive MIMO are compelling advantages.
TDD Operation
- Frame structure: Time is divided into frames with dedicated UL and DL slots.
- Guard period: A short gap between DL and UL to allow for propagation delay and switch settling.
- UL/DL ratio: Configurable. Common ratios: 7:3, 3:1, 4:1 (more DL for data-heavy services).
TDD Advantages
- Unpaired spectrum: No need for paired frequency bands. Simpler spectrum allocation.
- Channel reciprocity: UL channel measurement provides DL channel knowledge for beamforming (same frequency, reciprocal).
- Flexible UL/DL: Adjust ratio based on traffic demand.
Frequently Asked Questions
What is TDD?
TDD uses the same frequency for uplink and downlink, alternating in time. It uses unpaired spectrum with a switch instead of a duplexer. TDD is the dominant mode for 5G mid-band and mmWave, and for Wi-Fi.
Why does 5G prefer TDD?
5G uses TDD at higher frequencies because: unpaired spectrum is more available and cheaper, TDD enables channel reciprocity for Massive MIMO beamforming, and the UL/DL ratio can be optimized for asymmetric data traffic.
What is channel reciprocity?
In TDD, UL and DL use the same frequency, so the channel is reciprocal (same in both directions). The base station measures the UL channel and uses that knowledge to optimize DL beamforming. This eliminates the need for DL channel feedback from the user device.