Gapped spectrum shaping for tandem-hopped radar/communications & cognitive sensing

Abstract

A non-repeating FMCW waveform was recently developed and experimentally demonstrated to provide a feasible instantiation of FM noise radar. This emission scheme was subsequently examined in terms of the impact of both stationary and hopped spectral gaps with the prospect of enabling in-band interference avoidance for cognitive sensing and possibly tandem hopped radar/communications. Here this gap-hopped spectrum framework is further explored with regard to the relation between the shaping of spectral gaps and the associated time sidelobe response. Experimental loopback measurements are shown that provide a sense of how this form of emission would operate on a real system.

Analysis of Spectral Notching in FM Noise Radar Using Measured Interference

“…3). It is now observed that each spectral notch has an achievable depth of only about 42-45 dB relative to the peak due to practical hardware limitations, similar to what was observed in [9]. Since the spectral notches no longer reach the measured noise floor, one can expect this more practical radar emission to impose more interference on other spectrum users.…”

“…It was noted in [8] that inclusion of spectral notches with sharp edges produce a sin(x)/x roll-off of the range sidelobes. This effect was partially mitigated [9] through tapering of the notch edges to allow for a gradual transition as…”

“…Note that this transmit power clearly corresponds to a low-power system and there are practical limitations to notch depth (e.g. see [9]) when operating in a high-power regime as discussed in Section 6. A notch of bandwidth B/16 is placed in the PRO-FMCW waveform spectrum centered on each interference source (see Fig.…”

“…3). Per (2) from [9], a spectral taper of bandwidth B/16 is also placed in the transition regions of each notch to minimize the growth of range sidelobes due to sharp spectral notches. To perform the simulated evaluation, a time-domain version of each interference + noise spectrum is superimposed with an attenuated version of the time-domain notched PRO-FMCW waveform to represent the received signal at the radar.…”

“…Further, while it is known that range sidelobes will degrade when spectral notches are incorporated into a waveform [7], the nonrepeated structure of PRO-FMCW provides inherent robustness to this effect due to the non-coherence of sidelobes across waveform segments in the coherent processing interval (CPI), and likewise for the pulsed PRO-FM case. It was shown in [8,9] that spectral notches can be readily incorporated into the waveform design process without adversely affecting physical realizability. In [10] it was even demonstrated that a multi-function radar/communication capability can be achieved via a tandem hopping arrangement.…”