Half-Duplex Frequency Division Duplex

Description

Half-Duplex Frequency Division Duplex (HD-FDD) is a duplexing method used in cellular systems like LTE and NR where uplink and downlink communications occur on separate, paired frequency bands (as in standard FDD), but the User Equipment (UE) is not required to transmit and receive simultaneously. Instead, the UE operates in a time-division manner within the FDD spectrum, alternating between transmission and reception phases. This requires a guard period for the UE's radio to switch between transmit and receive modes.

At the architecture level, an HD-FDD UE has a simplified RF front-end compared to a Full-Duplex FDD (FD-FDD) UE. An FD-FDD UE requires a high-performance duplexer—a filter that isolates the powerful outgoing transmit signal from the sensitive incoming receive signal, as both are active concurrently on adjacent frequencies. An HD-FDD UE can use a simpler switch or a lower-performance duplexer, as the transmit and receive paths are never active at the exact same time. This reduction in RF component cost and complexity is the primary advantage.

From a protocol perspective, the base station (eNodeB in LTE, gNB in NR) must be aware of which UEs are HD-FDD capable. The network scheduling must ensure that it does not schedule an HD-FDD UE for an uplink transmission at the same time it is scheduled for a downlink reception. In practice, the standard allows the UE to define a specific HD-FDD pattern or the network can configure it via higher-layer signaling. The switching time between Tx and Rx states is specified in the RF requirements (e.g., TS 36.101, 38.101) and consumes a small portion of the subframe, which is accounted for as a guard period. This guard period represents a minor loss in spectral efficiency, traded for the significant reduction in UE cost. HD-FDD operation is particularly relevant for LTE and NR in bands traditionally used for FDD systems, enabling low-cost IoT devices (like LTE-M, NB-IoT in FDD bands) and low-tier handsets to leverage FDD spectrum.

Purpose & Motivation

HD-FDD was introduced to lower the cost and power consumption of UEs, especially for machine-type communication (MTC) and low-end consumer devices, while still allowing operation in FDD spectrum. Traditional FD-FDD UEs require expensive and physically large duplexer filters to provide sufficient isolation between the closely spaced transmit and receive frequencies. These components are a significant part of the RF Bill of Materials.

By relaxing the requirement for simultaneous transmission and reception, HD-FDD eliminates the need for this high-performance duplexer. A cheaper switch or filter can be used instead. This was a critical enabler for the LTE Cat-1, and later Cat-M1 (LTE-M) and NB-IoT devices, which are designed for ultra-low cost and long battery life for IoT applications. It allowed these technologies to be deployed in existing FDD bands owned by operators worldwide without requiring UE costs to remain prohibitively high. The trade-off is a slight reduction in peak data rates and scheduling flexibility, as the UE cannot be 'always listening' to the downlink while it transmits. However, for many IoT applications with intermittent, small data transfers, this trade-off is perfectly acceptable and results in a more optimized total cost of ownership.