Subband Full Duplex Downlink-Uplink (DU) Operation

Description

Subband Full Duplex (SBFD) DU operation is a Rel-19 feature that allows a base station (gNB) to transmit downlink (DL) and receive uplink (UL) simultaneously within the same carrier by partitioning the carrier bandwidth into non-overlapping frequency subbands. In the DU configuration, exactly one SBFD UL subband and one SBFD DL subband are configured, with the UL subband placed higher in frequency than the DL subband. This arrangement is particularly suited to scenarios where the lower part of the carrier is used for downlink and the upper part for uplink, allowing the network to schedule DL and UL transmissions concurrently on the same OFDM symbols.

The architecture builds on the concept of SBFD symbols, which are OFDM symbols configured for simultaneous UL reception and DL transmission within separate frequency subbands. The gNB uses a duplexer or advanced filtering to isolate the transmit and receive paths, while the UE operates in half-duplex mode within its assigned subband. The DU operation is defined in 3GPP TS 38.104 (Base Station radio transmission and reception) and TS 38.141 (Base Station conformance testing), which specify the RF requirements, including adjacent channel leakage ratio (ACLR), spurious emissions, and receiver blocking, to ensure coexistence with other systems.

From a scheduling perspective, the gNB can allocate DL resources in the lower subband and UL resources in the upper subband on the same symbol. This increases the effective utilization of the TDD carrier, which traditionally alternates between DL and UL in time. By enabling simultaneous transmission, SBFD reduces the latency associated with TDD switching and improves the overall throughput, especially for latency-sensitive applications such as URLLC and industrial IoT.

The DU operation is one of several SBFD configurations; the DUD (Downlink-Uplink-Downlink) operation places the UL subband in the middle of two DL subbands. The choice between DU and DUD depends on the available spectrum, interference environment, and the desired DL/UL traffic ratio. The DU configuration is simpler in terms of guard bands and filtering, as it requires only one transition between DL and UL subbands.

Key challenges for SBFD DU operation include self-interference from the gNB's own transmission into its receiver, cross-link interference between gNBs, and the need for advanced beamforming and interference cancellation techniques. The 3GPP specifications define test cases and requirements to ensure that the base station can operate with acceptable performance. Overall, SBFD DU operation represents a significant step toward more flexible and efficient use of spectrum in 5G-Advanced and future networks.

Purpose & Motivation

SBFD DU operation was created to address the inherent inefficiency of conventional TDD systems, where the same carrier cannot be used for simultaneous downlink and uplink transmission. In traditional TDD, the network must alternate between DL and UL in time, which leads to underutilization of spectrum during low-traffic periods and increased latency due to switching gaps. As 5G networks evolve to support diverse services with varying DL/UL traffic patterns, the static TDD configuration becomes a bottleneck.

The DU configuration specifically solves the problem of asymmetric traffic where downlink demand is higher than uplink, but uplink still requires low latency. By placing the UL subband above the DL subband, the network can prioritize downlink capacity in the lower part of the carrier while still providing dedicated uplink resources in the upper part. This is particularly useful for applications like video streaming, where downlink dominates, but occasional uplink feedback is needed.

Previous approaches, such as flexible TDD and dynamic TDD, allowed some adaptation but still could not achieve true simultaneous transmission. SBFD DU operation, introduced in Rel-19, enables full duplex at the base station, which is a paradigm shift. It leverages advances in antenna isolation, digital cancellation, and filtering to make simultaneous transmission feasible. The motivation is to improve spectral efficiency, reduce latency, and support new use cases like extended reality (XR) and industrial automation, where both high downlink throughput and low-latency uplink are required.