A radio access node or wireless device performs LBT on a channel and dynamically determines a transmission starting point on the channel, based on whether the LBT succeeds before or after a slot boundary between a slot n and a previous slot n−1. In response to determining that the LBT succeeds before the slot boundary, a transmission scheduled for the slot n is extended to start earlier, in the previous slot n−1. Otherwise, in response to determining that the LBT succeeds after the slot boundary, either a transmission scheduled for a next slot n+1 is extended to start earlier, in the slot n, or a transmission scheduled for the a second mini-slot of the slot n is extended to start earlier, in a first mini-slot of the slot n.