Abstract Inter‐cell interference and smart jammer attacks significantly impair the performance of non‐orthogonal multiple access (NOMA) networks. This issue is particularly critical when considering strategic interactions with malicious actors. To address this challenge, the power allocation problem is framed in a two‐cell NOMA network as a sequential game. In this game, each base station acts as a leader, choosing a power allocation strategy, while the smart jammer acts as a follower, reacting optimally to the base stations' choices. To address this multi‐agent scenario, four multi‐agent reinforcement learning algorithms are proposed: Q‐learning based unselfish (QLU), deep QLU, hot booting deep QLU, and decreased state deep QLU. A game‐theoretic analysis that demonstrates the algorithms' convergence to the optimal network‐wide strategy with high probability is provided. Simulation results further confirm the superiority of our proposed algorithms compared to the Q‐learning‐based selfish NOMA power allocation method.

Wireless powered internet of thing (IoT) systems allow small IoT devices to operate without accompanying dedicated power sources. A well-known protocol for such networks utilizes the harvest and then transmit concept which involves wireless energy transfer (WET) followed by wireless information transfer (WIT). We formulate two optimization problems for wireless powered IoT systems to maximize weighted sum-rate for time division multiple access (TDMA) and non-orthogonal multiple access (NOMA) by optimizing the harvesting time and transmission time variables. First, we derive a semi-closed form solution,which achieves the global optimum, for both problems. The proposed approach is highly computationally efficient for large IoT networks. We prove that the scalar equations in both TDMA and NOMA maintain a unique solution which can be found via bisection. It is revealed that when the device’s circuit power consumption is negligible, NOMA outperforms TDMA. However, when devices consume large circuit power, TDMA is more efficient than NOMA. Numerical results determine a critical point where NOMA surpasses TDMA in weighted sum-rate if plotted versus WET transmit power. The critical point depends on the WET power, device’s circuit power consumption, conversion efficiency and saturation level of the (non-)linear energy harvester, and finally the number of devices and their associated weights.

We study the secrecy rate maximization problem in a millimeter wave (mmWave) network, consisting of a base station (BS), multiple intelligent reflecting surfaces (IRSs) (or reconfigurable intelligent surfaces (RISs)), multiple users, and a single eavesdropper. To ensure a fair secrecy rate among all the users, we adopt a max-min fairness criterion which results in a mixed integer problem. We first relax discrete IRSs phase shifts to the continuous ones. To cope with the non-convexity of the relaxed optimization problem, we leverage the penalty method and block coordinate descent approach to divide it into two sub-problems, which are solved by successive convex approximation (SCA). Then, we propose a low-complexity mapping algorithm where feasible IRSs phase shifts are obtained. Mathematical evaluation shows the convergence of sub-problems to a Karush-Kuhn-Tucker (KKT) point of the original ones. Furthermore, the convergence guarantee of the overall proposed algorithm and computational complexity are investigated. Finally, simulation results show our proposed algorithm outweighs the conventional solutions based on the semi-definite programming (SDP) in terms of convergence and secrecy rate, especially in a larger number of IRSs and phase shifts where SDP suffers from rank-one approximation. Maximum ratio transmission (MRT) and IRS-free systems are also considered as other benchmarks.

This paper investigates the optimum power allocation ratio of a jammer in order to ruin the sum spectral efficiency (SSE) of a user-centric cell-free network. In this regard, signal to interference plus noise ratio (SINR) is initially derived for each user in uplink, and then, a closed-form expression for SSE is provided. An optimization problem has been solved for two scenarios which finds the optimum power allocation that minimizes the SSE of the network, first for a single antenna jammer and next for a multi-antenna one. Additionally, the performance of the SSE has been studied in the presence of several single-antenna jammers in the proposed network and also one multi-antenna jammer. The result is compared with co-located multi-input multi-output (MIMO) network and it is found that unlike co-located MIMO, as the number of serving access points (APs) increases, the network becomes more robust against jamming. The effect of the jammer’s antenna on power allocation ratio and spectral efficiency is further presented. Moreover, the impact of the number of jammers and the number of serving APs on the SE has been surveyed.

Abstract In this paper, the problem of physical layer security in the uplink of millimeter‐wave massive multiple‐input multiple‐output (MIMO) networks is studied and a jamming detection and suppression method is proposed. The proposed method is based on directional information of the received signals at the base station antenna array. The proposed jamming detection method can accurately detect both the existence and direction of the jammer using the received pilot signals in the training phase. The obtained information is then exploited to develop a channel estimator that excludes the jammer's angular subspace from received training signals. The estimated channel information is then used for designing a combiner at the base station that is able to effectively cancel out the deliberate interference of the jammer. By numerical simulations, the performance of the proposed jamming detection method is evaluated in terms of correct detection probability and false alarm probability, and its effectiveness when the jammer's power is substantially lower than the user's power is shown. Also, the results show that the proposed jamming suppression method can achieve a very close spectral efficiency as the case of no jamming in the network.

We design a jamming-resistant receiver scheme to enhance the robustness of a massive MIMO uplink system against jamming. We assume that a jammer attacks the system both in the pilot and data transmission phases. The key feature of the proposed scheme is that, in the pilot phase, the base station estimates not only the legitimate channel, but also the jamming channel by exploiting a purposely unused pilot sequence. The jamming channel estimate is used to construct linear receiver filters that reject the impact of the jamming signal. The performance of the proposed scheme is analytically evaluated using the asymptotic properties of massive MIMO. The best regularized zero-forcing receiver and the optimal power allocations for the legitimate system and the jammer are also studied. Numerical results are provided to verify our analysis and show that the proposed scheme greatly improves the achievable rates, as compared with conventional receivers. Interestingly, the proposed scheme works particularly well under strong jamming attacks, since the improved estimate of the jamming channel outweighs the extra jamming power.

In this paper, we perform a coverage analysis of a cellular massive multiple-input multiple-output (MaMIMO) network which adopts 3D beamforming. In contrast to the previous works on 3D beamforming which assume that all users are placed on the ground (i.e., in a 2D environment), we consider a more practical scenario where the users of the network are dropped in a 3D environment with different heights. In this scenario, by adopting a stochastic geometry framework, first we calculate the coverage probability of the network as a function of the users height. Then considering this coverage probability as the objective function, the optimum tilt angle of the base stations antenna pattern is found and the effect of users' heights on this tilt angle is investigated. Our numerical results show that by taking the users' heights into account and using the proposed method, a considerable improvement is achieved in the performance of the network compared to other similar 3D beamforming methods that ignore the users' height distribution.

Most of the beamforming designs that have been proposed in literature for relay networks often assume flat fading channels with perfect channel state information. However, in practice, the channel is likely to be frequency selective. On the other hand, in many cases, having the perfect channel state information is impossible. In this paper, we investigate the robust filter and forward beamforming for a relay network consisting a transmitter, a receiver and R relay nodes. All channels between the transmitter, relays and the receiver are assumed to be frequency selective with imperfect channel state information and it is also assumed that the channel uncertainty is bounded by a spherical region. Our aim in this paper is to minimize the total transmit power of the relays subject to a constraint on the worst signal-to-interference-plus-noise ratio (SINR). To solve the resulting non convex problem, we use a two-stage approach that considers the channel error. The robust beamforming can be obtained by KKT conditions, an auxiliary variable and S-procedure. Simulation results demonstrate the efficiency of the proposed method.

In this paper, we study the design of Distributed Space-Time Coding (DSTC) for two-way relay networks, where two users have bidirectional communication in the presence of two relay nodes and a direct link. Two new DSTC methods based on Amplify-and-Forward (AF) and Decode-and-Forward (DF) relaying are proposed that provide full transmission rate in the network. We evaluate the performance of the proposed methods and compare them by computer simulations. Our simulations show that both schemes achieve the maximum diversity and in addition, the proposed DF-based method outperforms the AF-based one.

A cognitive radio (CR) network is a secondary network operating in the frequency band and geographical area of a primary network to which the spectrum is licensed. In this paper, we consider a primary broadcasting network consisting of a transmitter and a number of primary users (PUs). The secondary users (SUs) make opportunistic unlicensed spectrum access to communicate with their base station (BS). With this system model, a joint power allocation and beamforming scheme is proposed in the uplink of the CR network to minimize the maximum interference to the PUs in two different scenarios. In the first scenario, the primary transmitter does not cooperate with the CR network and it transmits with a fixed power, whereas it cooperates in the joint power allocation and beamforming scheme and adjusts its transmission power accordingly, in the second scenario. In the former case, our proposed joint power allocation and beamforming scheme finds the beamforming weights and transmission power values under quality of service (QoS) constraints of all SUs, taking into account their maximum transmission power constraints. In the latter case, the proposed scheme finds the beamforming weights of the BS and transmission powers of primary transmitter and SUs under their target signal-to-interference-plus-noise ratios (SINRs) and maximum power constraints. Our approach employs Genetic Algorithm (GA) to determine optimum transmission power values and beamforming weight vectors. Simulation results show the performance of the proposed scheme.

This paper proposes two different transmit precoding schemes for downlink of overloaded CDMA systems i.e. CDMA systems in which the number of users is more than the processing gain of the system. These schemes are based on user grouping technique. In this technique, K users in the system are partitioned into G groups and a unique spreading sequence is assigned to each group. Using multiple antenna in the base station transmitter, we can separate users in each group by their different spatial characteristics. This separation is performed by some linear spatial pre-processing techniques in the transmitter. In addition, to suppress the multiple access interference between users in different groups, we use temporal linear transmit precoding method. Using precoding method in the base station's transmitter makes the mobile receivers very simple and low cost. We present the bit error rate (BER) performance of the proposed schemes in a flat fading channel by computer simulations. It is demonstrated that a considerable improvement in the performance and increasing in the system capacity will be obtained by using the proposed group transmission schemes