  Integrated sensing and communication (ISAC) is a key enabler of 6G. Unlike
communication radio links, the sensing signal requires to experience round
trips from many scatters. Therefore, sensing is more power-sensitive and faces
a severer multi-target interference. In this paper, the ISAC system employs
dedicated sensing signals, which can be reused as the communication reference
signal. This paper proposes to add time-frequency matched windows at both the
transmitting and receiving sides, which avoids mismatch loss and increases
energy efficiency. Discrete non-linear frequency modulation (DNLFM) is further
proposed to achieve both time-domain constant modulus and frequency-domain
arbitrary windowing weights. DNLFM uses very few Newton iterations and a simple
geometrically-equivalent method to generate, which greatly reduces the complex
numerical integral in the conventional method. Moreover, the spatial-domain
matched window is proposed to achieve low sidelobes. The simulation results
show that the proposed methods gain a higher energy efficiency than
conventional methods.
