Principles of Modern Radar - Volume 2 1891121537

414 CHAPTER 9 Adaptive Digital BeamformingFIGURE 9-12Array factor andsubarray pattern fora 3:1 overlappedsubarrayarchitecture.Relative Directivity (dB)0–10–20–30–40–50Subarray PatternArray FactorRelative Directivity (dB)0–10–20–30–40–50Subarray PatternFull Pattern–60–1–0.8 –0.6 –0.4 –0.20Sine Space Angle(a)0.2 0.4 0.6 0.8 1–60–1 –0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1Sine Space Angle(b)the effect of reducing the peak sidelobes and filling in the grating nulls. The overlappedarchitecture applies low sidelobe weighting both within the subarray and at the subarrayoutputs, which causes the both subarray pattern and the array factor to have low sidelobes,as shown in Figure 9-12. This results in a composite antenna pattern that has very lowsidelobes and is more robust to perturbations in the subarray-level weighting.9.2.3 Benefits of Digital BeamformingThe primary motivation for implementing DBF in a phased array radar is to provide multiplespatial channels in the digital computer for advanced signal processing algorithms toexploit. Enhanced functionality enabled by the DBF architecture can include the following:• Adaptive nulling of jammers• Multiple simultaneous beams on receive• High-resolution angle estimation• Enhanced dynamic range• Reduced phase noise on transmit• Digital time delay on transmit or receive• Array healingMost of this chapter focuses on adaptive jammer cancellation, but this section will brieflydescribe forming multiple simultaneous beams with a DBF architecture.9.2.4 Multiple Simultaneous Beams on ReceiveOnce the subarray data are available in the digital computer, they can then be processed inmultiple ways without impacting radar resources. For example, by applying the appropriatephase weightings to the data from each subarray, beams that are offset in angle from theoriginal beam location used to set the element-level phase shifters can be synthesized inthe digital computer.For a uniform linear array (ULA) with the subarray architecture shown in Figure 9-13,the beampattern response as a function of the sine space angle coordinate and wavelength