Half-Duplex FDD (HD-FDD) is one of the most commercially significant technical features in the eRedCap specification. It directly reduces the bill of materials cost of an eRedCap modem by eliminating the need for a simultaneous duplexer.

What is FDD?

Frequency Division Duplex (FDD) is the radio access mode used by most cellular networks globally. In FDD, separate frequency pairs are assigned for uplink (device to network) and downlink (network to device). In standard full-duplex FDD, the device transmits and receives simultaneously on these two frequencies.

Simultaneous transmission and reception requires a duplexer – a hardware component that isolates the transmit and receive radio chains from each other, preventing the transmitted signal from swamping the far weaker received signal. Duplexers are among the more expensive RF components in a cellular modem design.

What HD-FDD Changes

HD-FDD allows an eRedCap device to use FDD spectrum but transmit and receive at different times rather than simultaneously. The device switches between transmitting on the uplink frequency and receiving on the downlink frequency. Since the device is never doing both at once, the duplexer requirement is eliminated. The RF front end is simpler and cheaper.

The practical tradeoff is throughput – half-duplex operation means uplink and downlink share time rather than operating concurrently, which contributes to the ~10 Mbps peak downlink figure. For IoT applications that primarily receive occasional configuration updates or firmware and transmit periodic small data payloads, this is an acceptable tradeoff.

HD-FDD in Context

LTE Cat-1bis already used this principle – it was the single-antenna design (eliminating one of the two receive chains required for LTE Cat-1) that made Cat-1bis modules cheaper and smaller than their predecessors. eRedCap applies the same logic to the FDD duplexer, achieving the next step in complexity and cost reduction while maintaining 5G NR network access.

RedCap (Release 17) does not support HD-FDD. It is a Release 18 eRedCap-specific feature.