A method for receiving chirp signals at a receiver device according to a protocol in which each chirp signal has a gradient known to the receiver device, the method comprising: receiving a chirp signal having a first gradient g; generating a reference chirp signal having a second gradient g′, wherein the second gradient g′ differs from the first gradient g by a fixed value v; multiplying the reference chirp signal and the received chirp signal so as to form a mixed chirp signal; and detecting the received chirp signal by correlating the mixed chirp signal with a fixed gradient correlating chirp signal.