Subband Full Duplex Downlink-Uplink-Downlink (DUD) Operation

Description

Subband Full Duplex (SBFD) DUD operation is a Rel-19 feature that allows a gNB to simultaneously transmit downlink (DL) and receive uplink (UL) within the same carrier by dividing the carrier into three subbands: a lower DL subband, a middle UL subband, and an upper DL subband. This configuration is known as DUD (Downlink-Uplink-Downlink) because the UL subband is sandwiched between two DL subbands. It is defined in 3GPP TS 38.104 and TS 38.141, which specify the RF requirements and conformance testing for base stations supporting SBFD.

The DUD operation is particularly useful when the available spectrum is contiguous and the network wants to maximize downlink capacity while still providing uplink resources. By placing the UL subband in the middle, the two DL subbands can be used for downlink transmission, potentially with different beamforming or MIMO layers. The gNB must ensure sufficient guard bands between the subbands to avoid interference, and it must employ advanced filtering and self-interference cancellation to prevent its own DL transmissions from desensitizing the UL receiver.

From a scheduling perspective, the gNB can allocate DL resources in both the lower and upper subbands and UL resources in the middle subband on the same OFDM symbols. This allows for flexible adaptation to traffic demands: if downlink traffic increases, more resources can be assigned to the DL subbands; if uplink traffic increases, the UL subband can be widened or its scheduling priority increased. The DUD configuration is more complex than the DU configuration because it requires two guard bands and more sophisticated filtering, but it offers greater flexibility in spectrum utilization.

The DUD operation relies on the concept of SBFD symbols, which are OFDM symbols configured for simultaneous UL and DL within separate frequency subbands. The UE, however, operates in half-duplex mode within its assigned subband, meaning it either transmits in the UL subband or receives in one of the DL subbands, but not both simultaneously. This simplifies UE implementation while still achieving full-duplex at the base station.

Key challenges for SBFD DUD operation include self-interference, cross-link interference between adjacent gNBs, and the need for precise power control to avoid blocking. The 3GPP specifications define test cases to ensure that the base station meets requirements such as adjacent channel selectivity and blocking. Overall, SBFD DUD operation is a key enabler for 5G-Advanced networks to achieve higher spectral efficiency and support diverse service requirements.

Purpose & Motivation

SBFD DUD operation was developed to overcome the limitations of conventional TDD and the simpler DU configuration. While DU operation places the UL subband above the DL subband, DUD operation places the UL subband in the middle of two DL subbands, which allows for more flexible allocation of downlink resources on both sides of the uplink. This is particularly beneficial when the network needs to support high downlink throughput with multiple beams or when the spectrum is fragmented.

The motivation for DUD operation stems from the need to support asymmetric traffic patterns in 5G. In many scenarios, downlink traffic is much heavier than uplink, but uplink still requires low latency and reliable transmission. By having two DL subbands, the network can use one for widebeam coverage and the other for beamformed transmission, or it can aggregate carriers within the same band. The middle UL subband ensures that uplink resources are always available without waiting for a TDD switching point.

Previous approaches, such as static TDD, could not adapt to rapid changes in traffic, and dynamic TDD still required time-division multiplexing. SBFD DUD operation, introduced in Rel-19, enables true simultaneous transmission and reception, which improves spectral efficiency and reduces latency. It also facilitates new use cases like industrial IoT, where machines need to send small amounts of uplink data while receiving large downlink updates. The DUD configuration is a key part of the 3GPP's evolution toward full-duplex systems in 6G.