Adaptive NC-OFDM Waveforms With Reduced Autocorrelation Sidelobes For Simultaneous Sensing and Communications

This paper presents an optimal and adaptive scheme to adjust OFDM waveforms for sensing while accommodating arbitrary communication data blocks. The study is motivated by the challenge of integrating simultaneous ranging and communications in an ISAC (Integrated Sensing and Communications) network. Traditional dual-function OFDM waveforms are communication-centric and often fail to ensure desirable sensing properties, such as low autocorrelation sidelobes. Moreover, precise ranging typically requires a large bandwidth, which is difficult to achieve in civilian ISAC applications. To address this, we explore the use of Non-Contiguous Orthogonal Frequency-Division Multiplexing (NC-OFDM) waveforms. NC-OFDM enables ISAC systems to achieve a large effective bandwidth while coexisting with narrower-band applications if waveform synthesis ensures both spectral nulling in stop bands and sidelobe suppression in waveform autocorrelation. To tackle these challenges, this paper proposes and evaluates a practical NC-OFDM waveform optimization strategy for simultaneous sensing (or ranging) and communication. Given an incoming data block, a mask code over all NC-OFDM subcarriers is optimally synthesized based on predefined objectives. This optimized code (or a set of complex-valued weights) is transmitted alongside the data, enabling proper demodulation at communication receivers. The optimization problem is formulated as a convex Semidefinite Programming (SDP) problem, which can be efficiently solved numerically. Simulations validate the proposed approach, demonstrating consistent performance improvements with minimal impact on data transmission efficiency.