Application to Wireless Networking: Tracking Sources in Real Time

Abstract

In this chapter, we begin to make the connection between the radiolocation proper, that we have discussed so far, and the data communication within which the radiolocation is meant to function. In other words, we want to start using our radiolocator in real wireless networking.

We have said this already, but let us repeat the fundamental requirements: Radiolocation in digital communication should be done on a per-packet basis, and it should be done at the pace of the communication itself, i.e., in real time. Also, by radiolocation, we consistently mean location of sources which, although legitimate interlocutors within some wireless standard, do not cooperate by informing us of their location; the only thing that they reveal about their position is the electromagnetic wave that they transmit. True radiolocation is, therefore, an out-of-channel process, independent of the content of the communication, and relying entirely on the properties of the radio wave. This is fundamentally different from either the GPS, which can be viewed as a form of range-finding with the help of collaborative beacons (satellites) (see e.g. Kaplan and Hegarty 2006), or from any schemes in which the transmitter embeds its own location data into the transmissions. The latter typically amount to broadcasting the location obtained from one’s own GPS receiver.

Unlike analog radio communication, digital communication is sporadic and interrupted, with data bundled into carrier bursts (packets) of finite duration, typically under a millisecond. Requirement that each and every packet be separately radiolocated is important for security applications: If a source does not want to be located, it will naturally seek to send few packets, and send them sporadically, at irregular intervals. Radiolocation methods that rely on continuous traffic to reconstruct the source’s location will be easily foiled by such simple stratagems.