Radar Testbed Characterization for Evaluation of Modulated Scatterer Concepts

Abstract

The following research explores the concepts of communication-embedded radar with an emphasis on radar operation and modulated scatterer concepts. Once firmly established the concept of communication via radar backscatter could be used in a variety of fields including, mass data collection, SAR calibration, and military communication. A radar testbed was developed and charactersized to enable experimental evaluation of communication via modulated scatterer concepts. The radar operates with a 1.84-GHz center frequency and 75-MHz bandwidth (later upgraded to a 235-MHz bandwidth). A dual-channel arbitrary waveform generator is loaded with used-defined complex baseband signals for frequency upconversion for transmission via a 22 dBi parabolic reflector antenna. Backscattered signals are received and frequency downconverted on four identical channels, each fed by a dipole antenna. A 4-channel data acquisition system digitizes and records the output video signal at 1-GSa/s per channel for signal analysis. The primary means of evaluating the radar testbed were loopback and freespace setups. The loopback setup consisted of a cabled inserted between transmitter and receiver to provide a controlled propagation environment. In this setup results were shown to be desirable, and easily explained by theory. Linear FM (chirp) waveforms were used which enabled pulse compression to reduce the peak signal power while preserving range resolution. After pulse compression via matched filter routines, amplitude, phase, and resolution were characterized and found to agree with theory. In extending the tests to freespace, it was seen that near targets could be seen and resolved coherently across the 4 channels. A prototype modulated scatterer constructed by a senior design group was further tested to evaluate the prototype's viability. This scatterer impresses a bit sequency on the radar wavefrom by modulating the scatterer's termination impedance between an open and short circuit at a rate determined by the bit sequence to be communicated (similar to frequency-shift keying). Connected via a cabled-loopback configuration, the prototype was shown to impress a bit sequence onto the backscatter of the transmit chirp and, through processing, the bit stream associated with the modulated scatterer was decoded successfully. Followon testing will evaluate freespace operation and techniques for decoding information from multiple devices in the field of view. This radar testbed will be used to experiementally evaluate various modulated scatterer concepts as well as other radar-related waveform and signal processing concepts in the future.