Subband Full Duplex Symbol

Description

An SBFD (Subband Full Duplex) symbol is a physical layer concept introduced in 3GPP Rel-19 that enables simultaneous uplink (UL) and downlink (DL) transmission within the same OFDM symbol. Unlike conventional TDD symbols, which are dedicated to either DL or UL, an SBFD symbol is partitioned into separate non-overlapping frequency subbands: one or more subbands are used for DL transmission from the gNB to the UE, and one or more subbands are used for UL reception from the UE to the gNB. This allows the base station to transmit and receive at the same time on the same carrier, effectively achieving full-duplex operation at the gNB while the UE remains half-duplex.

The SBFD symbol 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 symbol structure is based on the OFDM numerology of the carrier, with each subband occupying a contiguous set of resource blocks (RBs). The gNB must ensure that the subbands are separated by guard bands to prevent interference between the DL and UL transmissions. The guard bands are typically a few RBs wide and are not used for data transmission.

From a scheduling perspective, the gNB can allocate DL resources in the DL subbands and UL resources in the UL subbands on the same SBFD symbol. This is done by the scheduler, which considers the channel conditions, traffic demands, and interference levels. The UE, operating in half-duplex mode, is assigned to either a DL subband or a UL subband for that symbol, depending on whether it is receiving or transmitting. The UE does not need to support simultaneous transmission and reception, which simplifies its implementation.

The SBFD symbol is a key enabler for subband full duplex operation, which includes configurations like DU (Downlink-Uplink) and DUD (Downlink-Uplink-Downlink). In DU operation, the SBFD symbol has one DL subband and one UL subband, with the UL subband higher in frequency. In DUD operation, the SBFD symbol has two DL subbands and one UL subband in the middle. The SBFD symbol can also be used in other configurations as defined by 3GPP.

Key challenges for SBFD symbols include self-interference from the gNB's own DL transmission into its UL receiver, cross-link interference between gNBs, and the need for advanced beamforming and interference cancellation. The 3GPP specifications define test cases to ensure that the base station meets requirements such as adjacent channel leakage ratio (ACLR) and blocking. Overall, the SBFD symbol is a fundamental innovation that allows 5G-Advanced networks to achieve higher spectral efficiency and lower latency.

Purpose & Motivation

The SBFD symbol was created to enable simultaneous uplink and downlink transmission on a single carrier, which is not possible with conventional TDD or FDD. In TDD, the same carrier is time-multiplexed between DL and UL, which means that at any given time, the carrier is used for only one direction. This leads to inefficient use of spectrum, especially when traffic is asymmetric. FDD uses separate carriers for DL and UL, which requires paired spectrum and is not always available. The SBFD symbol solves this problem by allowing the carrier to be partitioned into frequency subbands, so that DL and UL can occur simultaneously.

The motivation for SBFD symbols comes from the need to support new services in 5G-Advanced, such as extended reality (XR), industrial IoT, and cloud gaming, which require high downlink throughput and low-latency uplink. These services often have asymmetric traffic patterns, and the ability to transmit DL and UL simultaneously can significantly reduce latency and improve user experience. Additionally, SBFD symbols can improve spectral efficiency by allowing the network to use the full carrier bandwidth for both directions, rather than alternating.

Previous approaches, such as flexible TDD and dynamic TDD, allowed some adaptation but still could not achieve true simultaneous transmission. The SBFD symbol, introduced in Rel-19, represents a paradigm shift by leveraging advances in antenna isolation, digital cancellation, and filtering. It is a key step toward full-duplex systems in 6G, where both the base station and the UE may eventually operate in full-duplex mode. The SBFD symbol is thus a foundational element for the evolution of 5G and beyond.