What is HD?
HD (Half Duplex) is a two-way communication mode where data can flow in both directions between two nodes, but only one direction at a time — like a walkie-talkie. In contrast to full duplex (where simultaneous transmission and reception occur), half duplex requires the communicating devices to take turns. In 5G NR, half-duplex FDD is supported as a UE capability option that simplifies device RF design by eliminating the need for simultaneous transmit and receive chains, at the cost of reduced throughput.
How Does HD Work?
In half-duplex FDD operation, the UE does not transmit and receive simultaneously, even though the uplink and downlink use separate frequency bands. The network scheduler ensures that the UE is never assigned uplink and downlink resources at the same time, with a guard period for switching between transmit and receive modes. This eliminates the need for a high-performance duplexer — the most expensive and complex RF component in an FDD radio. Half-duplex TDD is inherent in TDD operation where uplink and downlink share the same frequency. 5G NR also supports self-interference cancellation research aimed at enabling full-duplex TDD in future releases.
Use Cases
Low-cost IoT and MTC devices with simplified RF design, NB-IoT and LTE-M modules, 5G NR UE operating in half-duplex FDD mode, push-to-talk communication systems, and cost-sensitive consumer devices.
3GPP / Standards Reference
3GPP TS 38.101-1 (NR UE radio transmission — half-duplex FDD operation), 3GPP TS 38.213 (NR physical layer procedures)
Related Terms
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
