  This paper reviews recent advances in photonic-assisted radio-frequency
arbitrary waveform generation (RF-AWG), with emphasis on programmable
ultrabroadband microwave and millimeter-wave waveforms. The key enabling
components in these techniques are programmable optical pulse shaping,
frequency-to-time mapping via dispersive propagation, and high-speed
photodetection. The main advantages and challenges of several different
photonic RF-AWG schemes are discussed. We further review some proof-of-concept
demonstrations of ultrabroadband RF-AWG applications, including high-resolution
ranging and ultrabroadband non-line-of-sight channel compensation. Finally, we
present recent progress toward RF-AWG with increased time aperture and
time-bandwidth product.
