This paper considers a cooperative device-to-device (D2D) system with non-orthogonal multiple access (NOMA). We assume that the base station (BS) can simultaneously communicate with all users to satisfy the full information transmission requirement. In order to characterize the impact of the weak channel and different decoding schemes, two novel decoding strategies are introduced: single signal decoding scheme and maximum ratio combining (MRC) decoding scheme, respectively. With the single signal decoding scheme, the users decode the received signals immediately after the receptions from the BS. On the other hand, the MRC decoding scheme jointly decodes the received signals via MRC until the corresponding phase comes and the users jointly decode the received signals by employing MRC. Considering Rayleigh fading channels, the ergodic sum-rate (SR), outage probability and outage capacity of the proposed D2D-NOMA system are analyzed. Moreover, approximate expressions for the ergodic SR are also provided with a negligible performance loss. Numerical results demonstrate that the ergodic SR and outage probability of the proposed D2D-NOMA scheme overwhelm that of the conventional NOMA schemes. Furthermore, it is also revealed that the system performance including the ergodic SR and outage probability are limited by the weak channel for both the single signal decoding scheme and conventional NOMA schemes, but not for the MRC decoding scheme.

This letter investigates an unmanned aerial vehicle (UAV)-assisted satellite-terrestrial communication network. Specifically, a three-dimensional mobile UAV helps to enhance coverage and simultaneously enable secure communication between the satellite and ground users with the internal eavesdropper. Moreover, non-orthogonal multiple access protocol is adopted, such that the spectral efficiency can be further improved. Aiming to maximize the secrecy rate, a joint cooperative jamming, transmit power, and UAV trajectory optimization problem is proposed. To deal with its non-convexity, we propose an iterative optimization algorithm that first optimally addresses the jamming and transmit power optimization in close forms and then uses an iterative relaxation technique to design the UAV trajectory for the given power. Simulation results highlight the critical role of the mobile UAV and significantly confirm the effectiveness of the proposed algorithm over the benchmark schemes.

This paper investigates a novel joint content caching and energy harvesting scheme to enhance unmanned aerial vehicle (UAV) communications in Internet of Things non-orthogonal multiple access (NOMA) network, where a UAV is acted as an aerial relay to serve users on demand. In particular, a neoteric gravitational search-based multi-constraint optimization algorithm is designed to maximize the throughput of the served users by jointly optimizing power allocation, energy harvesting, and time scheduling schemes. The algorithm engages two main characteristics: 1) classification of multiple constraints, which is for reducing the complexity of the problem solving process due to multiple iterations, and 2) design of force sets, which aims to enhance the global search for the optimal solution at the feasible region boundary. Finally, simulations are provided to show the effective convergence and superiority of the proposed algorithm.

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) has been studied as an alternative waveform for post-orthogonal frequency division multiplexing (OFDM) wireless communications. However, due to the intrinsic imaginary inter-carrier/inter-symbol interference of FBMC/OQAM signals, accurate channel estimation and equalization remain open issues for FBMC/OQAM systems in dispersive channels. In this article, two new signal models of the analysis filter bank (AFB) output are first derived in the absence of any channel assumption and in the presence of a relaxed channel assumption, respectively. Based on the new-built models, initial channel estimation with least squares (LS) (IniCE-LS) is presented and its output is used as the initial condition of recursive weighted LS (RWLS) estimation; moreover, non-iterative and iterative channel equalization combined with RWLS channel estimation (i.e., N-ICE-RWLS and ICE-RWLS) are proposed by using the real-time reconstructed FBMC/OQAM data symbols as pilots. The proposed N-ICE-RWLS and ICE-RWLS schemes not only allow to improve channel estimation and equalization performance, but also achieve high transmission efficiency. Considering the requirements of practical application, we further develop the low-complexity implementation methods of N-ICE-RWLS and ICE-RWLS. All these schemes are validated by analyzing the channel estimation accuracy and bit error rate (BER) over various frequency selective channels.

Considering the limited bandwidth of underwater wireless communications with radio frequency and acoustic waves and the constant increasing need for underwater high-speed data transmission, underwater wireless optical communication (UWOC) has been regarded as an attractive and viable alternative. However, UWOCs suffer from degradation attributed to underwater attenuation, oceanic turbulence, and pointing errors in oceanic channel. Therefore, performance enhancement technique plays a key role in the design and deployment of a UWOC system. In this paper, we propose to improve the performance of a UWOC by transmitting with pointing adjustable beam arrays (PABA). In a PABA, we can adjust pointing directions of every single beam by reflectors. To decrease the path loss and turbulence-induced scintillations and weaken the influence of pointing errors in UWOCs, we make the one central beam and several surrounding beams in a PABA point toward the center of a receiver aperture, and make the remaining surrounding beams in a PABA point toward several target points located symmetrically on a ring with an adjustable radius around the center of a receiver aperture. This adjustable radius is set according to the average pointing errors in every specific UWOC link. To estimate the average pointing errors more accurate, we calculate pointing errors in view of both beam wander caused by oceanic turbulence and the misaligned displacements resulted by the changes in the position and attitude of an underwater platform. Finally, we validate the effect of PABA on improving the performance of UWOCs by simulations.

With the widespread use of Light Emitting Diode (LED) in vehicle lights and traffic lights, LED-based Visible Light Communication (VLC) has been regarded as an alternative candidate for data transmission for connected vehicles. In order to enhance the performance of non-uniform dynamic VLC, where the luminous intensity distribution is not uniform and the received optical power changes frequently with vehicle motion, we propose to integrate receiver spatial and angular diversity with VLC for connected vehicles. We take the position and the posture of the receiver into consideration since the performance of VLC varies significantly according to the spatial relationship between the receiver and the transmitter. In this paper, firstly, we present a Vehicle-to-Vehicle VLC (V2V-VLC) model and a Vehicle-to-Infrastructure VLC (V2I-VLC) model according to the spatial relationship model. Then, we present a layout scheme of receivers. In our scheme, we can not only achieve diversity gains but also obtain a forward total 180 degrees Field of View (FOV) and a backward total 180 degrees FOV respectively. Finally, we simulate and analyze the Bit Error Rate (BER) in both turning and lane change scenarios. Simulation results show that receiver spatial and angular diversity can significantly decrease the BER of VLC equipped vehicles.