Virtual multi-antenna arrays for estimating the bearing of radio transmitters
This talk will present a new method to estimate the bearing of a radio transmitter with a mobile, single-antenna receiver, and its implementation on a software-defined radio (SDR) testbed. The bearing estimation method relies on the following principle: by considering the signal received at several points along its trajectory, the receiver implicitly creates a virtual multi-antenna array (similarly to synthetic aperture radar systems) that can be used to estimate the bearing of the radio transmitter. Virtual multi-antenna arrays have two major differences with conventional multi-antenna arrays: 1) the position and orientation of each antenna in the virtual array depends on the movement of the receiver and must be estimated; and 2) the local oscillator (LO) offset between transmitter and receiver adds a phase offset to the signal received by each “virtual” antenna, which must be estimated and compensated for. The first problem is solved by using an inertial measurement unit (IMU), which can provide the relative position and orientation of the receiver for short time periods. The second problem is solved by expanding the signal model that is used in bearing estimation algorithms (such as MUSIC) to account for LO offset. The proposed system is implemented on a USRP-N210 SDR with an external high-end IMU and tested in an anechoic chamber. The SDR implementation is a mixture of FPGA and software development, while some parts of the data are processed off-line. The results show that our method is indeed feasible, with bearing estimation errors of only a few degrees. We then present the first efforts of an implementation on a USRP-E310 SDR which has an integrated, lower-quality IMU.