Joint sensing and communication (JSAC) has become increasingly popular in recent years due to spectrum scarcity, hardware limitations, power constraints, and the emergence of applications that require both communication and sensing capabilities. As such, the coexistence of both functionalities remains a challenge for current systems. Therefore, this article proposes a novel orthogonal time-frequency space (OTFS)-chirp waveform. The proposed waveform exploits the sparsity of linear chirps in delay-Doppler domain to multiplex OTFS and chirp in orthogonal manner. This enables simultaneous high-data rate communication and low-complexity accurate sensing simultaneously. Furthermore, we propose a multiuser JSAC scheme where multiple orthogonal chirps (OCs) are assigned to different users within the same OTFS-Chirp waveform. On top of that, a novel chirp-based channel estimation technique is proposed for OTFS systems. The proposed method leverages the sparsity of chirp in both delay-Doppler and Fresnel domains. This approach alleviates the pilot guard overhead and reduces the peak-to-average power ratio (PAPR) of the transmitted signal. The effectiveness of the proposed JSAC waveform is verified by the conducted numerical results that agree with the developed analysis and validate that the proposed waveform design can achieve accurate low-complexity radar parameter estimation while preserving high-data rates.

The upcoming sixth generation (6G) wireless networks are designed to support advanced applications that the current air interface based on orthogonal frequency division multiplexing (OFDM) cannot accommodate, necessitating innovations in waveform design. Orthogonal time frequency space (OTFS) has emerged as a promising alternative, offering enhanced performance particularly in high-mobility channel environments. However, deploying fundamentally different waveforms over the same frequencies fails to exploit the mutual relationships between them. In this paper, a forward-compatible yet backward-compatible coexistence framework is proposed, where various OFDM waveform options are incorporated while seamlessly integrating OTFS within the same network. The proposed coexistence paradigm involves utilizing a specially designed spreading matrix in the delay-Doppler (DD) domain to process OTFS data. The matrix is tailored to establish orthogonality between OTFS and OFDM waveforms, effectively reducing cross-interference when both coexist within the same time-frequency (TF) resources. Our results demonstrate up to a 6 dB improvement in bit error rate (BER) performance for the OTFS waveform compared to other coexistence schemes. The proposed approach also shows greater interference resilience and approaches the upper bounds of achievable rates. By utilizing the full bandwidth and time duration, it enhances range and velocity resolutions, leading to improved sensing performance. These findings are supported by mathematical analysis and extensive simulations for uplink scenarios.

As wireless technology advances toward the sixth generation (6G) communication systems, ambient backscatter communication (AmBC) has emerged as a key enabler for low-cost internet of things (IoT) networks. This paper introduces a novel joint localization and communication (JLAC) technique based on orthogonal time sequency multiplexing (OTSM), specifically designed for passive backscatter devices (BDs). The proposed design enhances spectral efficiency by supporting broadband direct link communication, reduces synchronization requirements by leveraging the spreading characteristics of OTSM carriers, and ensures reliable BC even in the presence of strong direct link interference (DLI) without the need for successive interference cancellation (SIC). The approach utilizes pilot carriers that are protected by a guard in the delay-sequency (DS) domain to achieve diversity gains along the delay domain while ensuring zero interference from either the data or the direct link. The scheme also exploits the pilot signal for localization, using received signal strength (RSSI) from BDs to localize the transmitter. Furthermore, the scheme minimizes the energy consumption at the BDs by reducing the switching rate (SR) by a factor ofM. Extensive analysis and simulations demonstrate the proposed OTSM-based JLAC system’s robustness to DLI, synchronization mismatches, power efficiency, and low complexity characteristics.

The recently popularized ISAC paradigm attempts carry out both communication and sensing functionalities uses the same time-frequency resources to combat the scarcity of these resources. However, high-resolution range and velocity radars require wideband long-duration transmission, which implies complex, costly receivers to sample at a high-frequency rate. In this paper, we propose an orthogonal time-frequency space (OTFS)-based ISAC system which enables achieving highly accurate range-velocity profiles without the need for large bandwidth transmissions or long-duration frames. This approach relaxes the constraints on bandwidth and time while still providing precise sensing information. The proposed scheme exploits a single OTFS carrier with rectangular pulse shaping as a pilot to estimate both simultaneous accruing delay and Doppler, thereby determining range and velocity, respectively. By leveraging the sidelobes of the physical pulse shape of the pilot signal, we propose an algorithm that allows the detection of the range and the velocity of radar targets beyond the resolution limitation set by the time duration and the bandwidth of the transmitted signal. The conducted simulation results along with the real experimental results demonstrate that the proposed design can achieve accurate low-complexity radar parameter estimation.

Joint sensing and communication (JSAC) systems are becoming attractive technologies since they can map the radio environment while performing communication using the same frequency bands. This is achieved by radar signal processing of the received signal that is composed of a known waveform. However, varying the waveform used for JSAC will create a trade-off between throughput and computational complexity. In this paper, we propose a joint orthogonal time-frequency space (OTFS)-frequency modulated continuous wave (FMCW) waveform design to perform JSAC to achieve high data rates due to OTFS and low-complex simple radar receiver thanks to FMCW. This is done by exploiting the simultaneous locality property of the FMCW in both time-frequency and delay-Doppler domains to orthogonally superimpose OTFS and FMCW, and use them for communication and sensing, respectively. Then, we provide an analysis of the computational complexity of the proposed design. The conducted simulation results demonstrate that the proposed waveform design can achieve accurate low-complexity radar parameters estimation while preserving high data rates.

In this work, novel physical layer security (PLS) schemes are proposed for orthogonal frequency-division multiplexing (OFDM) to secure both data and pilots in multiple-input multiple-output (MIMO) systems. The majority of previous studies focus on only securing the data without considering the security of the pilots used for channel estimation. However, the leakage of channel state information (CSI) from a legitimate node to an eavesdropper allows the latter to acquire knowledge about the channel of the legitimate nodes. To this end, we propose adaptive and flexible PLS algorithms which can 1) secure data, 2) secure pilots, and 3) jointly secure both data and pilots. Particularly, minimum-phase all-pass channel decomposition is exploited, where the proposed algorithms use the all-pass component to provide security without harming the performance of the legitimate user. In the analysis for data security, we evaluate the secrecy under correlated and uncorrelated eavesdropping channels via closed-form bit error rate (BER) formulas. For pilot security, we analyze the estimated channel’s normalized mean squared error (NMSE) performance. The simulation results and theoretical analysis demonstrate that the proposed algorithms can effectively enhance the communication secrecy of the overall system.

Reconfigurable intelligent surface (RIS) has recently drawn significant attention in wireless communication technologies. However, identifying, modeling, and estimating the RIS channel in multiple-input multiple-output (MIMO) systems are considered challenging in recent studies. In this paper, a disassembled channel estimation framework for the RIS-MIMO system is proposed based on the eigenvalue decomposition (EVD) concept to separate the cascaded channel links and estimate each link separately. This estimation is based on modeling the RIS-MIMO channel as a keyhole MIMO system model. Numerical results show that the proposed estimation method has a low estimation time overhead while providing less estimation error.