Beamforming optimization for ISAC interference suppression via range-domain design in mixed near/far-field scenarios

Abstract Conventional far-field, angle-only ISAC beamforming cannot separate communication users from sensing targets that share the same angular sector. This paper introduces a range-domain interference-suppression principle for hybrid near/far-field ISAC, exploiting the asymmetry that near-field beams focus in both range and angle (2D) while far-field beams focus only in angle (1D). On this basis, we derive a bi-directional mixed near/far-field SINR model with range-dependent steering vectors absent from conventional formulations, and prove, via a closed-form single-user analysis, a wavefront-curvature-mismatch effect: under full angular overlap, range-induced channel decorrelation yields an interference-suppression gain that scales with array aperture and inverse user range, opening a new spatial degree of freedom unavailable in pure far- or near-field designs. The resulting non-convex sum-rate problem is solved by an SOCP-based WMMSE–SCA procedure with guaranteed KKT-point convergence. At SNR $$ = 10$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>=</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:math> dB the proposed scheme attains 9.0 bps/Hz, surpassing MRT and Random baselines by about $$60\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>60</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> and $$108\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>108</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> under the 8 dB per-user and 25 dB radar SINR constraints, with the advantage robust across system parameters.