  The discussion on using zero padding (ZP) instead of a cyclic prefix (CP) for
enhancing channel estimation and equalization performance is a recurring topic
in waveform design for future wireless systems that high spectral efficiency
and location awareness are the key factors. This is particularly true for
orthogonal signals, such as orthogonal frequency-division multiplexing (OFDM).
ZP-OFDM is appealing for joint communications and sensing (JCS) in 6G networks
because it takes the advantage of both OFDM and pulse radar. In term of
communication, ZP-OFDM compared to CP-OFDM, has higher power efficiency and
lower bit error rate (BER). However, time synchronization is challenging in
ZP-OFDM systems due to the lack of CP. In terms of sensing, ZP facilitates
ranging methods, such as time-sum-of-arrival (TSOA). In this paper, we propose
a moment-based timing offset (TO) estimator for multiple-input multiple-output
(MIMO) ZP-OFDM system without the need for pilots. We then introduce the which
significantly improves the estimation accuracy of the previous estimator. We
show that the proposed method asymptotically reaches the maximum likelihood
(ML) estimator. Simulation results show very high probability of lock-in for
the proposed estimators under various practical scenarios.
