SBFD

SBFD is a duplexing trick that loosens one of TDD's fundamental limits. In normal TDD, a slot is either downlink or uplink — you can't do both at once on the same carrier, so uplink-heavy or latency-sensitive traffic waits for an uplink slot to come around. SBFD instead lets a cell transmit and receive simultaneously by splitting the carrier into subbands and assigning some to downlink and some to uplink within the same TDD slot. The benefits are lower latency (uplink doesn't have to wait for its turn) and better uplink coverage and capacity.

The reason it isn't free — and why it's a Release 18 study/early feature rather than a flipped switch — is self-interference. A base station transmitting on the downlink subband while listening on the uplink subband is, in effect, shouting and trying to hear a whisper at the same time; the strong downlink leaks into the sensitive uplink receiver. Making SBFD work needs serious self-interference cancellation, careful subband guard design, and management of interference between neighbouring cells. It's a promising direction for flexible duplexing, but the implementation challenge is real, which keeps it early-stage.