In low earth orbit satellite (LEO-Sat) communication systems based on orthogonal time frequency space (OTFS) modulation, existing channel estimation methods usually use high-power pilots, leading to a high peak-to-average power ratio (PAPR). In this paper, we propose a comb-type pilot based frame structure by inserting multiple identical Zadoff-Chu (ZC) sequences along the Doppler dimension. The ZC sequences are arranged at equal intervals in the delay dimension. The power of pilots is consistent with the power of data symbols, which can achieve low PAPR at the transmitter. At the receiver, based on the analyzed power distribution of the correlation function, we derive the power threshold to identify the received pilots. Then, an iterative ZC-sequences based channel estimation algorithm is proposed aided by data detection and interference cancellation. Through iterations, the data interference can be alleviated and estimation accuracy can be improved. Simulation results demonstrate that the proposed scheme can achieve superior channel estimation performance with low PAPR in the time domain.

In order to support colourful multimedia services with strict quality-of-service (QoS) requirements of user equipments (UEs), the space-air-ground integrated networks (SAGIN) can be taken as a promising approach to enhance network capacity. Among them, millimeter wave (mmWave) and edge caching promise to significantly improve the SAGIN performance due to the advantage in rich bandwidth resource and low latency, respectively. In this paper, we investigate the joint caching and resource allocation for multimedia services in SAGIN, where multimedia content requests can be simultaneously served by multiple access points (APs). Considering the delay-constraint of multimedia services, we then formulate a mixed-integer non-linear programming (MINLP) problem aiming at minimizing the service delay, which involves jointly optimizing coded caching (CC), power allocation (PA) and UEs-to-APs association (UA). We propose to find the optimal solution by employing an alternating iteration optimization framework. The optimal CC and PA problems are firstly addressed by utilizing convex optimization technology. Then, two many-to-many swap matching algorithms are developed to slove the UA subproblem effectively. Numerical results demonstrate that our proposed algorithms can substantially reduce the service delay over other benchmarks.

Federated Learning (FL) empowers wireless intelligent applications, by leveraging distributed data of edge clients for training without compromising privacy. Client selection is inevitable in FL, since clients have diversified learning capabilities arising from heterogeneous computing and communication resources. Existing methods like fair-selection and dropping-straggler are either inefficient or unfair (resulting in a less effective trained model). Therefore, we propose FedAoI, an Age-of-Information (AoI) based client selection policy. FedAoI ensures fairness by allowing all clients, including stragglers, to submit their model updates while maintaining high training efficiency by keeping round completion times short. This trade-off is achieved by minimizing Peak-AoI (PAoI), the interval between a client's consecutive participations. An optimization problem is formulated by minimizing the Expected-Weighted-Sum-of-PAoI. This NP-hard problem is addressed with a two-step sub-optimal algorithm, PriorS. It first calculates client priority in a round using Lyapunov optimization and then selects the highest-priority clients through G-FPFC (Greedy minimization of the round weighted-sum-of-PAoI with First-Priority-First-Considered). Simulation results demonstrate that, compared to fair-selection, FedAoI improves average efficiency by 83.8% and achieves an average model accuracy of 97.3% (or at the cost of averaging 2.7% degradation in model accuracy). Compared to dropping-straggler, FedAoI reduces the average model accuracy degradation from 9.5% to 2.7%.

In this paper, we study a secure communication scheme assisted by multiple distributed friendly users over fading channels. To defeat a malicious user, friendly users transmit artificial noise continuously regardless of whether the transmitter is sending signals or not. The idle nodes in wireless networks can be selected as friendly users to assist in achieving secure communications. Due to the different positions of friendly users relative to the malicious user and receiver, the assisting abilities of friendly users in secure communications are different. To explore the potential of friendly users, a metric factor is proposed in this paper to evaluate the assisting abilities of friendly users. Based on the evaluating results, an optimum user selection and power allocation scheme is designed to maximize the secure throughput under the power limit and secure constraint. Compared with the benchmark scheme, the proposed secure communication scheme is easy to implement. Moreover, according to the numerical results, the proposed scheme can improve the secure throughput by more than 16% compared with the benchmark scheme when the secure constraint is larger than 0.5.

Resource management is important for wireless networks. However, most model-driven resource allocation algorithms are limited by computational complexity and suboptimal spectral efficiency. This paper focuses on learning-based resource management to improve spectral efficiency in multicellular networks with imperfect channel state information (CSI). Considering imperfect CSI, the joint subchannel selection and power allocation problem can be formulated as a probability-constrained non-convex optimization problem. By means of parameter transformation, the non-convex optimization problem with probabilistic constraints is first transformed into a non-probabilistic optimization problem. Then, to solve this problem, a dual-module network based on dueling deep Q-network and deep deterministic policy gradient algorithm is proposed to maximize spectral efficiency. Simulation results show that the proposed dual-module network outperforms model-driven optimization algorithms such as fractional programming and existing deep reinforcement learning algorithms in terms of spectral efficiency.

Orthogonal matching pursuit (OMP) and its improved algorithms are widely used as hybrid precoding solutions in millimeter wave massive MIMO systems. The existing modified hybrid precoding schemes reduce computational complexity, however, they cause the loss of spectral efficiency to some extent. In this paper, we propose a novel generalized orthogonal matching pursuit (gOMP) algorithm based on order-recursive least squares (ORLS) in order to balance both computational cost and spectral efficiency. Compared to OMP algorithm, the maximum iteration of gOMP-ORLS algorithm can be reduced by choosing more than one vector in each iteration. Meanwhile, it has much lower implementation complexity due to avoiding matrix inversion operation. More importantly, the realized spectral efficiency of the proposed algorithm is higher than other existing algorithms. The simulation results as well as our detailed analysis demonstrate that a) the proposed gOMP-ORLS algorithm can achieve the approximately same spectral efficiency as OMP algorithm; b) it can reduce computational complexity and improve precoding efficiency prominently.