Methods, systems, and devices for wireless communication are described. A transmitting device may obtaining channel access after a clear channel assessment (CCA) operation. The transmitting device may then determine an offset between a boundary of a symbol and a particular time at which the channel access is obtained. The transmitting device may generate a channel reservation (CR) preamble based on the offset and transmit the CR preamble. The transmitting device may also generate the CR preamble based on occupancy of sub-bands or beams used by the transmitting device. A receiving device may monitor for a CR message based on the CR preamble.