Principles of Modern Radar - Volume 2 1891121537

9.3 Adaptive Jammer Cancellation 4339.3.5.1 Cancellation RatioThe cancellation ratio (CR) is defined as the ratio of the jammer power without adaptivecancellation to the jammer power with cancellationCR J = J uJ a= wH 0 R J w 0w H a R J w a(9.40)Cancellation ratio provides a measure of how well the jamming is suppressed by adaptivecancellation. Typically, CR is used as a metric to evaluate how well the hardware inthe receive chain is calibrated and matched to support the desired level of cancellation.Unfortunately, CR as previously defined cannot be measured directly on real hardwareimplementations because the jammer cannot be measured without the noise also beingpresent.An alternative form of the CR that can be directly measured on hardware isCR J+N = J u + N uJ a + N a(9.41)This version of the cancellation ratio converges to the jammer-only version when J u ,J a >>N u ,N a , but it puts a ceiling on the cancellation ratio as J a becomes very small. The jammeronlycancellation ratio can be inferred from the jammer-plus-noise cancellation ratio ifthe adaptive weights that are computed when the jammer is turn on are stored and thenapplied to noise-only data when the jammer is turned off to estimate the adapted noisepower (N a ). Then CR J can be estimated by subtracting N u and N a from the numeratorand denominator of CR J+N , respectively. This approach works under the conditions thatthe jammer and noise signals are uncorrelated and that the noise power is stationary overthe time the measurements are taken.9.3.5.2 ResidueThe residue is usually defined as the output power of the beamformer divided by the noisepower and typically does not include target signalResidue = J + NN(9.42)The residue can be computed with and without adaption to show how well the jammer isbeing suppressed. If the jammer is perfectly cancelled, then the residue will be 0 dB. Theresidue is useful for comparing adaptive cancellation approaches based on the amount ofjamming that remains after cancellation.9.3.5.3 SINRSINR is the ratio of the signal power to the sum of the jammer and noise powers.SINR =SJ + N(9.43)This provides a better system-level measure of performance because it also captures theimpact of the adaptive beamformer on the signal of interest. Cancellation ratio and residuemeasure only what happens to the jammer, so if the adaptive beamformer suppresses thesignal along with the jammer that will not be revealed by those metrics. SINR, on the other