Link sense: Beyond wireless carrier sensing

Carrier sensing is the act of listening on a wireless channel before initiating a transmission. It is intrinsic to modern random access protocols to avoid collisions. However, carrier sense is only a binary indicator of whether the channel is idle or busy. Knowing why it is busy (i.e., which sender-receiver pair is currently communicating) would be very useful in scheduling concurrent transmissions. While this information could be decoded from the frame header, it is not always feasible especially when the listener is outside the decoding (but within the sensing) range. Even if the listener is within decoding range, it may not be able to decode the header because of interference, or perhaps because it was sleeping. We propose LinkSense that leverages OFDM subcarriers to embed a per-link signature in each transmission. With LinkSense, a listener can decide to sense the channel at any given time, and identify which links are active in the vicinity. The listener need not be within decoding range of the active links, and can sense the channel only for a few OFDM symbol durations. We explore the feasibility of LinkSense and implement/validate it using USRPs/GNU Radios.