Methods, systems, and devices for wireless communication are described. A first wireless communications device may transmit, to one or more second wireless communications devices, a first message that triggers transmission of a second message by each of the one or more second wireless communications devices, the first message allocating to each of the one or more second wireless communications devices a respective frequency-based resource unit for transmission of the second message, each of the respective frequency-based resource units spanning a portion of a channel bandwidth of a channel. The first wireless communications device may perform channel sensing measurements on the second message received from each of the one or more second wireless communications devices. The first wireless communications device may estimate the channel based on the channel sensing measurements performed on the respective second messages received on the respective frequency-based resource units.