Beamwidth-Invariant and Independent Multi-Beam Time-Modulated Antenna Arrays With Stepped Amplitude-Phase Modulation

Pseudorandom time-modulated antenna array (TMA) offers high design degrees of freedom for low-complexity beamforming. However, existing pseudorandom modulation schemes usually rely on subarray partitioning to generate multiple independent beams, resulting in a trade-off between beamwidth, array size, and the number of beams. This article proposes a novel pseudorandom modulation technique, named stepped amplitude-phase (SAP) modulation, for beamwidth-invariant and independent harmonic beamforming. By superimposing and discretizing only the complex-weighted carrier signals of the wanted harmonic orders, the proposed technique allows independent control of multiple harmonic beams under a single radio-frequency chain. Since each antenna element in this case produces multi-harmonic spectra with arbitrary amplitude and phase weightings, the beamwidth is decoupled from the number of beams for a fixed array size. A pattern synthesis approach is then developed to analytically determine the instantaneous modulation statuses for a specified number of beams, beam-scanning angles, and sidelobe levels (SLLs). It benefits the real-time design of modulation waveforms based on a field-programmable gate array (FPGA). The effectiveness of the proposed approach is verified through a Ku-band 16-element U-slot microstrip linear array.