dFSK: distributed frequency shift keying modulation in dense sensor networks

We consider the problem of reachback communication in wireless sensor networks: multiple sensors are deployed on a field, and they collect local measurements of some random process which then need to be encoded and reproduced at a remote location. In this paper we present the design of a distributed modulation scheme for reachback, dubbed dFSK (for distributed frequency shift keying). In dFSK, all nodes in the network first agree on a common stream of bits to send. Then, nodes listen to a few transmissions by other nodes to form an estimate of when to schedule their own transmissions. The aggregate waveform that results from superimposing a large number of these weak transmissions has a pre-specified set of zero-crossings. It is through the location of these zeros that information is conveyed to the far receiver. This mode of operation is analogous to that of standard FSK, where it is through spectral properties of the transmitted symbol (and hence closely related to the rate of its zero-crossings) that information is conveyed to the far receiver.