Integrated sensing and communication (ISAC) is set to transform data transmission and real-time sensing in sixth-generation (6G) networks. By offering high capacity, free-space optical (FSO) links can play a vital role as a backhaul solution, complementing radio frequency (RF) technologies such as millimeter wave (mmWave) to enhance 6G reliability. While adverse weather conditions such as fog can reduce the reliability of FSO links due to atmospheric attenuation, the back-scattered light it generates may also enable real-time sensing of atmospheric channel gain at the transmitter. This paper proposes a novel user offloading scheme utilizing an optical ISAC (O-ISAC) framework within a single-cell network, supported by a ground base station (GBS) and enhanced by an unmanned aerial vehicle (UAV). The UAV connects to the gateway via an FSO backhaul link, estimating the channel gain based on back-scattered light and dynamically optimizing its position to efficiently offload users in the downlink. Numerical results demonstrate the UAV’s effectiveness in optimizing its position under various weather conditions and show that the proposed deployment scheme outperforms existing UAV offloading methods. The framework is evaluated using a real dataset of hourly visibility measurements in Edinburgh, underscoring the significance of optical channel sensing for overall system performance and providing insights into how sensing impacts communication efficiency.

Integrated sensing and communication (ISAC) is expected to transform data transmission and real-time sensing, enhancing sixth-generation (6G) networks. Free-space optical (FSO) communication is a key 6G backhaul solution, complementing radio frequency (RF) technologies like millimeter wave (mmWave) for improved network reliability. However, adverse weather can significantly reduce FSO link reliability due to atmospheric attenuation. Such adverse weather conditions also increase the level of back-scattered light, potentially enabling the real-time sensing of the atmospheric channel gain at the transmitter side. Therefore, this paper proposes a novel optical ISAC (O-ISAC) framework, where the back-scattered light from the FSO communication signal is used as the sensing feedback signal. This O-ISAC framework is analyzed considering a single-cell network aided by an unmanned aerial vehicle (UAV) to support edge users. The UAV is connected to the gateway via a FSO backhaul link while estimating the FSO channel gain based on the back-scattered light and dynamically optimizing its trajectory. The aim of this adaptive O-ISAC system is to maximize the end-to-end network throughput of the edge users while considering FSO backhaul capacity and the UAV's directional antenna beamwidth and bandwidth allocation. Numerical results demonstrate that UAV can effectively optimize its trajectory by adjusting the antenna beamwidth and downlink bandwidth allocation at different weather conditions. The proposed framework is tested using hourly visibility data from Edinburgh, demonstrating that optical channel sensing is crucial for the system's overall performance.

In this paper, we investigate the capacity of the continuous spectrum of nonlinear frequency division multiplexing (NFDM) systems when data is encoded using b-modulation. We prove that the capacity-achieving distribution is unique, discrete in amplitude and uniform in phase, forming a support of finitely concentric shells. An algorithm is proposed to numerically calculate the optimal number of shells, their amplitudes and their probabilities such that the mutual information is maximized. An analytical lower bound on the capacity of the b-modulation channel is presented. Moreover, the mismatch capacity lower bound is calculated based on a set of realistic channel realizations, generated by the transmission of pulses over a long-haul fiber modeled by nonlinear Schrodinger equation (NLSE) and simulated by split step Fourier method. We also calculate the mutual information for discrete-points constellations. The numerical results show that the gap of mutual information is very small between these two approaches which proves that discrete-points distribution can be used as a tight lower bound instead of shell-based optimal distribution since it is a practical constellation. Finally, practical achievable rates are calculated based on some standard amplitude and phase shift keying (APSK) modulation schemes to show the achievable performance gains.

In this paper, we investigate the capacity of the$b- \mathbf{modulated}$nonlinear frequency division multiplexing (NFDM) systems. Recently, a tractable channel model was proposed for such optical fiber communication systems, describing the received$b-\mathbf{Modulated}$signal by an input-dependent complex Gaussian distributed noise. Considering this channel model, we prove that the capacity achieving input distribution has discrete amplitude, uniform independent phase (DAUIP). Noting that this distribution is supported on a finite number of concentric shells, we find a lower bound for the capacity by assuming a single shell support. The corresponding output distribution and mutual information expressions are derived in closed-form.

An approximated channel model is proposed for direct signaling on the continuous spectrum of a nonlinear frequency division multiplexed (NFDM) communication system, describing the effect of noise and nonlinearity at the receiver. The optimal input distribution that maximizes the mutual information of the proposed approximated channel under the peak amplitude constraint is then studied. We present that, considering the input-dependency of the noise, the conventional amplitude-constrained constellation designs can be geometrically shaped to provide significant mutual information gains. However, it is observed that further probabilistic shaping and constellation size optimization can provide only limited additional gains beyond the best geometrically shaped benchmark scheme, i.e., 64 Amplitude Phase Shift Keying. Then, an approximated channel model that neglects the correlation between subcarriers is proposed for the matched filtered signaling system, based on which the input constellation is geometrically shaped. We demonstrate that although the inter-subcarrier interference in the filtered case is neglected in the channel model, the shaping of the matched filtered case can provide promising gains in mismatch capacity over the unfiltered scenario.

In this paper, we investigate the capacity of the continuous spectrum of nonlinear frequency division multiplexing (NFDM) systems when data is encoded using b-modulation. Recently, a tractable channel model is proposed for such optical fiber communication systems, describing the received b-modulated signal with an input-dependent complex Gaussian distributed noise. Considering this channel model, we prove that the capacity-achieving distribution is unique and discrete. A search algorithm is proposed to determine the optimal discrete input distribution that maximizes the mutual information. The numerical results show that the optimal input distributions form ring-based constellations. We compare these optimal designs with some conventional ring-based amplitude and phase shift keying (APSK) modulation schemes including 16 APSK, 32 APSK and 64 APSK, to show the achievable performance gains.

An approximated channel model is proposed for signalling on the continuous spectrum of a nonlinear frequency division multiplexing communication system, describing the effect of noise and nonlinearity at the receiver. The optimal input distribution that maximizes the mutual information of the proposed approximated channel under peak amplitude constraint is then studied. We show that, considering the input-dependency of noise, the conventional amplitude-constrained constellation designs can be shaped geometrically to show significant mutual information gains. However, it is observed that further probabilistic shaping and input distribution size optimization can only provide limited additional gains beyond the best geometrically shaped benchmark scheme, i.e., 64 Amplitude Phase Shift Keying.

Minimum energy required to achieve a distortion-noise profile, i.e., a function indicating the maximum allowed distortion value for each channel noise level, is studied for transmission of Gaussian sources over Gaussian channels. A general coding scheme is proposed to upper bound the minimum energy needed for any distortion-noise profile. Conversely, utilizing a family of lower bounds originally derived for broadcast channels with power constraints, the minimum required energy is lower bounded for any profile. As examples, linear, exponential, square-law, and staircase profiles are studied. It is shown that for the linear profile, as well as for any concave profile, uncoded transmission is optimal. As a negative result, it is shown that exponential profiles are not achievable with finite energy. For square-law and staircase profiles, upper bounds and lower bounds are derived and the gaps between them are studied.

In this paper, minimum energy required to achieve a distortion-noise profile is studied for robust transmission of Gaussian sources over Gaussian channels when there is side information about the source at the receiver. The distortion-noise profile is a function indicating the maximum allowed distortion value for each channel noise level and side information quality, where neither are known at the transmitter. It is shown here that uncoded transmission is optimal for (inversely) linear profiles. Turning then to staircase profiles, a proposed coding scheme is studied to obtain an upper bound to the minimum energy needed. Conversely, a general family of lower bounds is derived for the minimum required energy and compared against the upper bound.

Minimum power required to achieve a distortion-noise profile, i.e., a function indicating the maximum allowed distortion value for each noise level, is studied for the transmission of Gaussian sources over Gaussian channels under a regime of bandwidth approaching zero. A simple but instrumental lower bound to the minimum required power for a given profile is presented. For an upper bound, a dirty-paper based coding scheme is proposed and its power-distortion tradeoff is analyzed. Finally, upper and lower bounds to the minimum power is analyzed and compared for specific distortion-noise profiles, namely rational profiles with order one and two.

In order to compensate inefficiency of traditional macrocell base stations (MBS), femtocell base stations (FBS) are deployed in cell areas. This deployment can enhance the Quality of Service (QoS) for the users which have difficulty communicating with the MBS. However, the presence of FBSs causes interference for MBS that should be controlled. Guaranteed QoS such as delay-bounds is required in real-time video applications. Moreover, the rapid growth of mobile-video traffic and time varying nature of wireless channels make it difficult to guarantee stringent delay constraints. However, providing delay-bounds based on effective capacity looks more appropriate for unreliable channels. This paper proposes a power allocation scheme based on Nash Bargaining Solution(NBS). NBS is a cooperative solution for Nash bargaining competitive game, which leads to a fair resource allocation and also satisfies the Pareto efficiency. We derive NBS in a tractable closed form formula by taking into account the delay-bounds and interference constraints. Analyzing the simulation results demonstrates that our proposed solution reaches a high level of fairness among users.