Disclosed are systems, apparatuses, processes, and computer-readable media for wireless communications. For example, a network device can transmit, to a group of passive devices, a continuous wave (CW) signal. The network device can receive, from each passive device of the group of passive devices, at least one backscatter signal, based on the CW signal, at a respective frequency shift. The network device can estimate, based on the at least one backscatter signal, channel information for each passive device of the group of passive devices. The network device can then determine, based on the channel information, a channel for transmitting an energy signal to at least one passive device of the group of passive devices for energy harvesting.