  Integrated sensing and communication (ISAC) is a key enabling technique for
future wireless networks owing to its efficient hardware and spectrum
utilization. In this paper, we focus on dual-functional waveform design for a
multi-input multi-output (MIMO) orthogonal frequency division multiplexing
(OFDM) ISAC system, which is considered to be a promising solution for
practical deployment. Since the dual-functional waveform carries communication
information, its random nature leads to high range-Doppler sidelobes in the
ambiguity function, which in turn degrades radar sensing performance. To
suppress range-Doppler sidelobes, we propose a novel symbol-level precoding
(SLP) based waveform design for MIMO-OFDM ISAC systems by fully exploiting the
temporal degrees of freedom (DoFs). Our goal is to minimize the range-Doppler
integrated sidelobe level (ISL) while satisfying the constraints of target
illumination power, multi-user communication quality of service (QoS), and
constant-modulus transmission. To solve the resulting non-convex waveform
design problem, we develop an efficient algorithm using the
majorization-minimization (MM) and alternative direction method of multipliers
(ADMM) methods. Simulation results show that the proposed waveform has
significantly reduced range-Doppler sidelobes compared with signals designed
only for communications and other baselines. In addition, the proposed waveform
design achieves target detection and estimation performance close to that
achievable by waveforms designed only for radar, which demonstrates the
superiority of the proposed SLP-based ISAC approach.
