  Integrated Sensing and Communication (ISAC) is one of the key pillars
envisioned for 6G wireless systems. ISAC systems combine communication and
sensing functionalities over a single waveform, with full resource sharing. In
particular, waveform design for legacy Orthogonal Frequency Division
Multiplexing (OFDM) systems consists of a suitable time-frequency resource
allocation policy balancing between communication and sensing performance. Over
time and/or frequency, having unused resources leads to an ambiguity function
with high sidelobes that significantly affect the performance of ISAC for OFDM
waveforms. This paper proposes an OFDM-based ISAC waveform design that takes
into account communication and resource occupancy constraints. The proposed
method minimizes the Cram\'er-Rao Bound (CRB) on delay and Doppler estimation
for two closely spaced targets. Moreover, the paper addresses the
under-sampling issue by interpolating the estimated sensing channel based on
matrix completion via Schatten $p$-norm approximation. Numerical results show
that the proposed waveform outperforms the state-of-the-art methods.
