A continuous-aperture array (CAPA)-based secure transmission framework is proposed to enhance physical layer security. Continuous current distributions, or beamformers, are designed to maximize the secrecy transmission rate under a power constraint and to minimize the required transmission power for achieving a specific target secrecy rate. On this basis, the fundamental secrecy performance limits achieved by CAPAs are analyzed by deriving closed-form expressions for the maximum secrecy rate (MSR) and minimum required power (MRP), along with the corresponding optimal current distributions. To provide further insights, asymptotic analyses are performed for the MSR and MRP, which reveals that i) for the MSR, the optimal current distribution simplifies to maximal ratio transmission (MRT) beamforming in the low-SNR regime and to zero-forcing (ZF) beamforming in the high-SNR regime; ii) for the MRP, the optimal current distribution simplifies to ZF beamforming in the high-SNR regime. The derived results are specialized to the typical array structures, e.g., planar CAPAs and planar spatially discrete arrays (SPDAs). The rate and power scaling laws are further analyzed by assuming an infinitely large CAPA. Numerical results demonstrate that: i) the proposed secure continuous beamforming design outperforms MRT and ZF beamforming in terms of both achievable secrecy rate and power efficiency; ii) CAPAs achieve superior secrecy performance compared to conventional SPDAs.

A continuous-aperture array (CAPA)-based integrated sensing and communications (ISAC) framework is proposed for both downlink and uplink scenarios. Within this framework, continuous operator-based signal models are employed to describe the sensing and communication processes. The performance of communication and sensing is analyzed using two information-theoretic metrics: the communication rate (CR) and the sensing rate (SR). 1) For downlink ISAC, three continuous beamforming designs are proposed: i) the communications-centric (C-C) design that maximizes the CR, ii) the sensing-centric (S-C) design that maximizes the SR, and iii) the Pareto-optimal design that characterizes the Pareto boundary of the CR-SR region. A low-complexity signal subspace-based approach is proposed to derive the closed-form optimal beamformers for the considered designs. On this basis, closed-form expressions are derived for the achievable CRs and SRs, and the downlink rate region achieved by CAPAs is characterized. 2) For uplink ISAC, the C-C and S-C successive interference cancellation-based methods are proposed to manage inter-functionality interference. Using the subspace approach closed-form expressions for the optimal detectors as well as the achievable CRs and SRs are derived. The uplink SR-CR region is characterized based on the time-sharing technique. Numerical results demonstrate that, for both downlink and uplink, CAPA-based ISAC achieves higher CRs and SRs as well as larger CR-SR regions compared to conventional spatially discrete array-based ISAC.

A framework of holographic MIMO (HMIMO) based integrated sensing and communications (ISAC), i.e., holographic ISAC (HISAC), is proposed. An accurate spherical wave-based model is utilized to characterize sensing link. The spacial correlation introduced by the densely spaced antennas of the HMIMO is incorporated when modeling the communication channel. Based on the proposed framework, closed-form expressions are derived for sensing rates (SRs), communication rates (CRs), and outage probabilities under different beamforming strategies to investigate fundamental information theoretical limits for HISAC. Further insights are gained by examining high signal-to-noise ratio slopes and diversity orders. A Pareto optimal design is proposed to characterize the attainable SR-CR region. Numerical results reveal that HISAC outperforms the conventional MIMO based ISAC and the HMIMO based frequency-division sensing and communications system in terms of both sensing and communications.

A closed-form expression for the secrecy capacity under near-field communications is derived and compared with its far-field counterpart. To gain further insights, the capacity scaling law is revealed by assuming an infinitely large transmit array and an infinitely high power. Both analytical and numerical results demonstrate that, different from the far-field scenario, i) nearfield communications expand the areas where secure transmission is feasible, specifically when the eavesdropper is located in the same direction as the intended receiver; ii) as the number of transmit antennas increases, the near-field secrecy capacity is capped at a finite value, adhering to the principle of energy conservation.

A holographic multiple-input multiple-output (MIMO)-based integrated sensing and communications (ISAC) framework is proposed for both downlink and uplink scenarios. The spatial correlation is incorporated into the communication channel modeling, while a spherical wave-based model is used to characterize the sensing link. By considering both instantaneous and statistical channel state information, closed-form expressions are derived for sensing rates (SRs), communication rates (CRs), and outage probabilities under various ISAC designs. This enables an investigation into the theoretical performance limits of the proposed holographic MIMO-based ISAC (HISAC) framework. Further insights are gained by examining the high signal-to-noise ratio (SNR) slopes and diversity orders. Specifically: I) for the downlink case, a sensing-centric (S-C) design and a communications-centric (C-C) design are investigated using different beamforming strategies, and a Pareto optimal design is proposed to characterize the attainable SR-CR region; II) for the uplink case, the S-C design and the C-C design differ in the interference cancellation order between the communication and sensing signals, with the rate region obtained through a time-sharing strategy. Numerical results are provided to demonstrate that HISAC systems outperform both conventional MIMO-based ISAC systems and holographic MIMO-based frequency-division sensing and communications systems, underscoring the superior performance of the HISAC framework.

The channel capacity of near-field (NF) communications is characterized by considering three types of line-of-sight multiuser channels: I) multiple access channel (MAC), II) broadcast channel (BC), and III) multicast channel (MC). For NF MAC and BC, closed-form expressions are derived for the sum-rate capacity as well as the capacity region under a two-user scenario. These results are further extended to scenarios with an arbitrary number of users. For NF MC, closed-form expressions are derived for the two-user channel capacity and the capacity upper bound with more users. Further insights are gleaned by exploring special cases, including scenarios with infinitely large array apertures, co-directional users, and linear arrays. For comparison, the MAC and BC sum-rates achieved by typical linear combiners and precoders are also analyzed. Theoretical and numerical results are presented and compared with far-field communications to demonstrate that: I) the NF capacity of these three channels converges to finite values rather than growing unboundedly as the number of array elements increases; II) the capacity of the MAC and BC with co-directional users can be improved by using the additional range dimensions in NF channels to reduce inter-user interference (IUI); and III) the MC capacity benefits less from the NF effect compared to the MAC and BC, as multicasting is less sensitive to IUI.

The secrecy performance in both near-field and far-field communications is analyzed using two fundamental metrics: the secrecy capacity under a power constraint and the minimum power requirement to achieve a specified secrecy rate target. 1) For the secrecy capacity, a closed-form expression is derived under a discrete-time memoryless setup. This expression is further analyzed under several far-field and near-field channel models, and the capacity scaling law is revealed by assuming an infinitely large transmit array and an infinitely high power. A novel concept of “depth of insecurity” is proposed to evaluate the secrecy performance achieved by near-field beamfocusing. It is demonstrated that increasing the number of transmit antennas reduces this depth and thus improves the secrecy performance. 2) Regarding the minimum required power, a closed-form expression is derived and analyzed within far-field and near-field scenarios. Asymptotic analyses are performed by setting the number of transmit antennas to infinity to unveil the power scaling law. Numerical results are provided to demonstrate that: i) compared to far-field communications, near-field communications expand the areas where secure transmission is feasible, specifically when the eavesdropper is located in the same direction as the intended receiver; ii) as the number of transmit antennas increases, neither the secrecy capacity nor the minimum required power scales or vanishes unboundedly, adhering to the principle of energy conservation.

The capacity limits of continuous-aperture array (CAPA)-based wireless communications are characterized. To this end, an analytically tractable transmission framework is established for both uplink and downlink CAPA systems. Based on this framework, closed-form expressions for the single-user channel capacity are derived. The results are further extended to a multiuser case by characterizing the capacity limits of a two-user channel and proposing the associated capacity-achieving decoding and encoding schemes. In the uplink case, the capacity-achieving detectors and sum-rate capacity are derived, and the capacity region is characterized. In the downlink case, the uplink-downlink duality is established by deriving the uplink-to-downlink and downlink-to-uplink transformations under the same power constraint, based on which the optimal source current distributions and the achieved sum-rate capacity and capacity region are characterized. For comparison, the uplink and downlink sum-rates achieved by the linear zero-forcing scheme are also analyzed. To gain further insights, several case studies are presented by specializing the derived results into various array structures, including the planar CAPA, linear CAPA, and planar spatially discrete array (SPDA). Numerical results are provided to reveal that the channel capacity achieved by CAPAs converges towards a finite upper bound as the aperture size increases; and CAPAs offer superior capacity over the conventional SPDAs.

Capacity limits of near-field (NF) multiple access channel (MAC) is characterized. Closed-form expressions are derived for the sum-rate capacity and the capacity region under a two-user scenario. These results are further extended to scenarios with an arbitrary number of users. Further insights are gleaned by exploring special cases, including scenarios with infinitely large array apertures and co-directional users. Theoretical and numerical results are presented and compared with far-field communications to demonstrate that: i) the NF capacity of the MAC converges to finite values rather than growing unboundedly as the number of array elements increases; ii) the MAC capacity with co-directional users can be improved by using the additional range dimensions in NF channels to reduce inter-user interference.

In this paper, a near-field ISAC framework is proposed with an accurate channel model, in which the loss caused by effective aperture and polarization mismatch are considered. Based on the proposed model, sensing and communication (S&C) performance are analyzed in terms of three different designs: the communications-centric design, the sensing-centric design, and the Pareto optimal design. Within each design, sensing rates (SRs) and communication rates (CRs) are derived. Moreover, the attainable SR-CR regions of the near-field ISAC are characterized. Numerical results reveal that 1) the adopted channel model is more accurate than the conventional models within near field; 2) ISAC achieves a more extensive rate region than the conventional frequency-division S&C.

This paper proposes a cluster-based Non-orthogonal multiple access framework for integrated sensing and communications (ISAC). Based on the proposed model, we investigate three typical ISAC precoding designs: sensing-centric design, communications-centric design, and Pareto optimal design. Under each scenario, the sensing rate (SR), communications rate (CR), and their asymptotic expressions in high signal-to-noise ratio (SNR) regime are derived. High-SNR slopes are also obtained to gain better insights. Finally, the SR-CR rate regions achieved by ISAC and the conventional frequency-division sensing and communications (FDSAC) are studied. Numerical results reveal that ISAC outperforms FDSAC in terms of both SR and CR, and is able to achieve a more extensive rate region.

This paper proposes a framework of multiple-input multiple-output based non-orthogonal multiple access for Integrated sensing and communications (ISAC), which incorporates signal alignment to enhance system performance. Sensing rate (SR) and communication rate (CR) are derived under three different precoding designs: sensing-centric design, communications-centric design, and Pareto optimal design. Moreover, the SR-CR regions achieved by ISAC and frequency-division sensing and communications (FDSAC) are studied. Numerical results reveal that ISAC outperforms FDSAC in terms of both SR and CR and can achieve a broader rate region, clearly showcasing its superiority over the conventional FDSAC.

Integrated Sensing and Communications (ISAC) surpasses the conventional frequency-division sensing and communications (FDSAC) in terms of spectrum, energy, and hardware efficiency, with potential for greater enhancement through integration of non-orthogonal multiple access (NOMA). Leveraging these advantages, a multiple-input multiple-output NOMA-ISAC framework is proposed in this paper, in which the technique of signal alignment is adopted. The performance of the proposed framework for both downlink and uplink is analyzed. 1) The downlink ISAC is investigated under three different precoding designs: a sensing-centric (S-C) design, a communications-centric (C-C) design, and a Pareto optimal design. 2) For the uplink case, two scenarios are investigated: a S-C design and a C-C design, which vary based on the order of interference cancellation between the communication and sensing signals. In each of these scenarios, key performance metrics including sensing rate (SR), communication rate (CR), and outage probability are investigated. For a deeper understanding, the asymptotic performance of the system in the high signal-to-noise ratio (SNR) region is also explored, with a focus on the high-SNR slope and diversity order. Finally, the SR-CR rate regions achieved by ISAC and FDSAC are studied. Numerical results reveal that in both downlink and uplink cases, ISAC outperforms FDSAC in terms of sensing and communications performance and is capable of achieving a broader rate region, clearly showcasing its superiority.