Performance Analysis of Zero-Padded Sequences for Joint Communications and Sensing

Self-interference (SI) is a known but critical issue at the receiver side of a monostatic radar in joint communications and sensing (JCAS) systems, and this is particularly true for high spectral-efficient waveforms. Full-duplex transceivers are usually assumed in the literature, but it is arguable if they can be taken for granted in many devices. We propose a different approach, by eliminating the SI using zero-padded (ZP) orthogonal frequency-division multiplexing (OFDM) and single-carrier frequency-division multiple access (SC-FDMA), instead of the more widespread cyclic-prefix (CP)-OFDM. ZP-sequences do not need full-duplex for the monostatic radar operation, as there is no SI between the transmit and receive antennas during the guard interval (GI), which can be used for radar detection. We derive the required radar receiver processing for ZP-sequences and CP-OFDM in time and frequency domains, respectively, to show that when the SI in JCAS is high, ZP-sequences can be beneficial, depending on the target range. Furthermore, we prove analytically that the low peak-to-average power ratio (PAPR) of ZP-SC-FDMA, when compared to ZP-OFDM, is beneficial in time-domain radar processing. This is demonstrated also by means of numerical simulation with ROC curves, for all the candidate waveforms in both coherent and incoherent receiver processing.

I. Introduction

Joint communications and sensing (JCAS) is likely to be one of the key features in 6G wireless networks [1], promising a more efficient use of hardware and spectrum. Besides, it will allow the creation of a plethora of new applications that can profit from the new offering of radar as-a-service (RaaS). Integrated communications will also improve the radar performance, since this will enable sensor fusion using connected distributed radars [2], as well as facilitate interference management by means of better coordination [3]. The communications networks will also profit from sensing data, for instance by using this information to improve beam allocation [4]. Intensive research efforts have been devoted for developing this technology in the last decade, witnessing a booming interest from both academia and industry on the joint system, due to its significant potentials in emerging networks.