A device is provided, which includes one or more antennas and a circuit. The circuit is configured to receive a packet from a transmitter device through the antennas, and obtain channel-state information from the received packet. The circuit includes one or more filters to filter out or recognize the received packet that complies with one or more preset conditions. The circuit uses the channel-state information of the received packet, which complies with the one or more preset conditions, to estimate a status of a channel between the transmitter device and the device.