The space-air-ground integrated network (SAGIN) paradigm has emerged as a pivotal enabler for the evolution of next-generation wireless systems. This article proposes a novel framework for covert communication in SAGINs, wherein a high-altitude platform (HAP), equipped with multiple antennas, serves terrestrial communication users (CUs) under the surveillance of multiple non-colluding wardens, with satellite assistance for warden location updates via space-air links. To safeguard the communication from detection by the wardens, the HAP employs artificial noise (AN) and robust beamforming techniques, addressing the challenges posed by imperfect channel state information (CSI) of the wardens. Subsequently, we formulate a non-convex optimization problem aimed at maximizing the number of served users, subject to stringent covertness constraints, satellite-HAP link outage probabilities, and maximum available power budgets. By employing ℓ0-norm relaxation, we convert the original problem into a mixed-integer optimization framework and develop a computationally efficient alternating optimization approach that combines bisection search, successive convex approximation (SCA), and semidefinite relaxation (SDR) techniques to tackle satellite power allocation, user scheduling, and beamforming design. Numerical simulations demonstrate that the proposed scheme significantly improves the user coverage while maintaining covertness, revealing a trade-off between covert communication and CU coverage capability in resource-constrained aerial-terrestrial environments, even under imperfect CSI conditions.

This paper investigates an integrated sensing and communication (ISAC) system employing delay-Doppler (DD) signaling. The sensing performance of both random and deterministic signaling schemes is evaluated based on the expected squared ambiguity function (AF), for which closed-form expressions are derived by leveraging the Zak transform-based orthogonal time-frequency space (Zak-OTFS) modulation framework. Our analysis highlights a key difference between the two signaling types: DD domain ISAC (DD-ISAC) with deterministic signaling yields a roughly periodic AF with prominent peaks and low sidelobes between adjacent peaks, whereas DD-ISAC with random signaling using a Quadrature Phase-Shift Keying (QPSK) constellation exhibits low sidelobe values periodically without prominent peaks. Furthermore, we demonstrate that DD-ISAC enables a flexible trade-off between delay and Doppler sidelobe levels by adjusting the number of delay and Doppler bins. The analytical findings are explicitly validated through numerical simulations.

In this paper, we propose a novel cross-domain channel estimation (CDCE) algorithm for orthogonal frequency division multiplexing (OFDM) systems, leveraging the unique characteristics of the delay-Doppler (DD) domain channel. Specifically, the proposed algorithm transforms the time-frequency (TF) domain pilot sequence of OFDM into the DD domain and applies a two-dimensional (2D) twisted-convolution for acquiring a coarse estimation of the underlying channel delay and Doppler. Then, the OFDM channel estimation is formulated as a sparse signal recovery problem in the TF domain according to the dictionary derived based on the obtained delay and Doppler estimates. Furthermore, a low-complexity ℓ1-regularized least-square estimator is proposed to effectively solve this problem. Moreover, we further develop a performance analysis framework of the proposed scheme based on the ambiguity function (AF) of the adopted pilot sequence. Our numerical results demonstrate noticeable estimation performance improvement compared to conventional OFDM channel estimation methods, particularly in the presence of high channel mobility.

In this paper, we propose a hybrid beamforming design for multiple target detection in an orthogonal time frequency space (OTFS)-based integrated sensing and communication (ISAC) multiple-input multiple-output (MIMO) system. The proposed hybrid beamformer allows spatial separation of the beams for communication and sensing, thereby eliminating inter-beam interference (IBI), while reducing the number of required radio frequency (RF) chains. More specifically, in addition to the beams allocated for communication users, multiple beams are assigned for target scanning and detection. By applying a combiner to the received echo signals, information about the target's existence, along with its angular, range, and Doppler characteristics, can be directly obtained across different RF chains. To shed light on the system performance, we analyze the signal-to-interference-plusnoise ratio (SINR) and discuss the effect of the beamformer in the on-grid and off-grid cases, respectively. Our simulation results indicate that accurate sensing can be achieved with integer delay and Doppler indices; however, in the cases of fractional delay and Doppler, the sensing accuracy depends on the resolution of these parameters.

Orthogonal time frequency space (OTFS) modulation has received significant attention recently due to its superior performance compared to conventional multicarrier waveforms. However, symbol detection with OTFS is significantly more involved and typically operates on large signal blocks with intersymbol interference (ISI) in the delay-Doppler (DD) domain. In this paper, we investigate the performance of OTFS within the cross-domain iterative detection (CDID) framework. Specifically, three distinct CDID algorithms are presented and investigated, which estimate/detect the information symbols iteratively across the frequency and DD domains via passing either thea posteriorior extrinsic information using a full-sized or single-tap linear minimum mean square error (LMMSE) estimator. Building upon this framework, we study the average mean square error (MSE) for the considered CDID algorithms, where both the bias evolution and the state (variance) evolution are investigated. Particularly, we show that the proposed CDIDs can provide unbiased estimation under certain channel conditions. Furthermore, a fixed point exists in the state evolution when the estimation is unbiased, indicating that the algorithm’s convergence is guaranteed. More importantly, we reveal that passing thea posterioriinformation is more beneficial when the underlying channel has negligible Doppler spread while passing the extrinsic information is more suitable for non-negligible Doppler spread cases, where the frequency domain channel matrix lacks diagonal dominance. Our numerical results confirm our analytical findings and unveil the near-optimal error performance achieved by the proposed design.

We consider a downlink (DL) massive multiple-input multiple-output (MIMO) system, where different users have different mobility profiles. To support this system, we categorize the users into two disjoint groups according to their mobility profile and implement a hybrid orthogonal time frequency space (OTFS)/orthogonal frequency division multiplexing (OFDM) modulation scheme. Building upon this framework, two precoding designs, namely full-pilot zero-forcing (FZF) precoding and partial zero-forcing (PZF) precoding are considered. To shed light on the system performance, the spectral efficiency (SE) with a minimum-mean-square-error (MMSE)-successive interference cancellation (SIC) detector is investigated. Closed-form expressions for the SE are obtained using some tight mathematical approximations. To improve fairness among different users, we consider max-min power control for both precoding schemes based on the closed-form SE expression. However, by noting the large performance gap for different groups of users with PZF precoding, the per-user SE will be compromised when pursuing overall fairness. Therefore, we propose a weighted max-min power control scheme. By introducing a weighting coefficient, the trade-off between the per-user performance and fairness can be enhanced. Our numerical results confirm the theoretical analysis and reveal that with mobility-based grouping, the proposed hybrid OTFS/OFDM modulation significantly outperforms the conventional OFDM modulation for high-mobility users.

In this paper, we investigate the performance of the cross-domain iterative detection (CDID) framework with orthogonal time frequency space (OTFS) modulation, where two distinct CDID algorithms are presented. The proposed schemes estimate/detect the information symbols iteratively across the frequency domain and the delay-Doppler (DD) domain via passing either the a posteriori or extrinsic information. Building upon this framework, we investigate the error performance by considering the bias evolution and state evolution. Furthermore, we discuss their error performance in convergence and the DD domain error state lower bounds in each iteration. Specifically, we demonstrate that in convergence, the ultimate error performance of the CDID passing the a posteriori information can be characterized by two potential convergence points. In contrast, the ultimate error performance of the CDID passing the extrinsic information has only one convergence point, which, interestingly, aligns with the matched filter bound. Our numerical results confirm our analytical findings and unveil the promising error performance achieved by the proposed designs.

In this paper, we consider a downlink (DL) massive multiple-input multiple-output (MIMO) system, where different users have different mobility profiles. To support this system, we propose to use a hybrid orthogonal time frequency space (OTFS)/orthogonal frequency division multiplexing (OFDM) modulation scheme, where OTFS is applied for high-mobility users and OFDM is used for low-mobility users. Two precoding designs, namely full zero-forcing (FZF) precoding and partial zero-forcing (PZF) precoding, are considered and analyzed in terms of per-user spectral efficiency (SE). With FZF, interference among users is totally eliminated at the cost of high computational complexity, while PZF can be used to provide a trade-off between complexity and performance. To apply PZF precoding, users are grouped into two disjoint groups according to their mobility profile or channel gain. Then, zero-forcing (ZF) is utilized for high-mobility or strong channel gain users to completely cancel the inter-group interference, while maximum ratio transmission (MRT) is applied for low-mobility users or users with weak channel gain. To shed light on the system performance, the SE for high-mobility and low-mobility users with a minimum-mean-square-error (MMSE)-successive interference cancellation (SIC) detector is investigated. Our numerical results reveal that the PZF precoding with channel gain grouping can guarantee a similar quality of service for all users. In addition, with mobility-based grouping, the hybrid OTFS/OFDM modulation outperforms the conventional OFDM modulation for high-mobility users.