Interference mitigation has arguably become one of the most important issues in modern wireless system design. From a circuit design point of view, bandpass and bandstop filters are conventionally used to attenuate interfering signals outside the bandwidth of the desired signal, but have diminished effectiveness in the presence of in-band interferers. Spread spectrum communication schemes, such as CDMA or frequency hopping, has been used to eliminate in-band interference. However, existing spread spectrum solutions rely on digital signal processing for spreading and de-spreading. In the case of a strong in-band interferer/jammer, the front-end circuit would be desensitized, making it impossible to digitize or process the signal of interest.
In the DARPA Signal Processing at RF (SPAR) program, we are collaborating with Prof. Zhi Ding and Prof. Paul Hurst to work on a new spread spectrum communication architecture for suppressing strong in-band interference. In this architecture, an encoder is placed between the transmitter and the antenna to code the RF waveform with a pseudorandom (PN) sequence. On the receiver end, a correlator and a narrow-band filter are added between the antenna and the low noise amplifier (LNA). When the correlator PN sequence matches that of the transmitter, the original RF waveform can be reconstructed. An in-band interferer, on the other hand, will be spread in spectrum by the correlator. After passing through the narrow-band filter, the power presented at the input of the LNA will be attenuated by a ratio of the spread spectrum divided by the filter bandwidth. This scheme essentially performs CDMA directly at the RF frequency to prevent in-band interference from desensitizing the LNA, thus allowing reliable wireless communication in a hostile electromagnetic environment.
Working together with HRL Laboratories, we are using high-speed Gallium Nitride (GaN) transistors to implement the encoders and the correlators. We are currently in the Phase 3 of the program.