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We propose a novel interference alignment (IA) technique with receive antenna partitioning (RAP) for simultaneous wireless information and power transfer (SWIPT)-enabled downlink fog radio access networks (F-RANs). The basic idea of the proposed scheme is to apply a generalized downlink IA for each access point (AP) and to divide receive antennas at each user into two groups for information decoding (ID) and energy harvesting (EH), respectively. With the proposed scheme, an optimal receive antenna configuration at each user is obtained to maximize the weighted sum of achievable rate and the sum of harvested energy of the network. Also we develop an antenna configuration selection algorithm to obtain near-optimal performance with low computational complexity. Through extensive computer simulations, it is shown that the proposed scheme significantly improves a rate-energy region compared with existing techniques.

In this paper, we mathematically analyze bit-error rate (BER) and diversity order of physical-layer network coding (PNC) scheme in the two-way relay channel (TWRC) under the assumption of multiple antennas at relay node. We assume the quadrature phase shift keying (QPSK) modulation for simple data transmission in IoT applications, and max–min transmit antenna selection (TAS) scheme in the broadcast (BC) phase for achieving full diversity gain. In particular, we obtain the upper bound of BER at the multiple access (MA) phase, while deriving the exact BER performance at the BC phase. To the best our knowledge, the mathematical analysis in this paper is the first analytical results in the literature. Through computer simulations, the analytical results are validated.

Grant-free multiple access (GFMA) is an emerging technology to accommodate a massive number of devices for 6G-enabled Internet of Things (IoT) networks. The main advantages of GFMA are to efficiently reduce control signaling overhead for resource scheduling while improving resource efficiency. In this article, we propose a novel resource-hopping-based GFMA (RH-GFMA) framework with resource hopping schemes for providing massive connectivity in 6G cellular IoT networks, where each IoT device is allowed to access physical radio resources by using a preassigned resource hopping pattern without not only resource request but also grant procedure, which is the so-called “one-shot” noninteractive multiple access. We exploit three types of resource hopping schemes in the proposed RH-GFMA framework: 1) random hopping; 2) resource group hopping; and 3) Latin-square group hopping. We mathematically analyze the RH-GFMA system performance in terms of the hopping pattern collision probability, maximum allowable packet delay, and interference-over-thermal. Finally, we derive an accommodation capacity of the proposed RH-GFMA framework, which is defined as the expected number of IoT devices accommodated in a cell under a maximum allowable packet-delay requirement and an interference-over-thermal constraint. With the proposed GFMA, massive IoT devices are expected to be efficiently accommodated in 6G wireless networks, while satisfying strict latency and reliability requirements.

Unslotted ALOHA protocol has been adopted as a channel access mechanism in commercial low-power wide-area networks (LPWANs), such as Sigfox and long-range (LoRa) alliance. This work examines the throughput and random access (RA) delay distribution of unslotted ALOHA systems by considering exponential random backoff (ERB) or uniform random backoff (URB) algorithm. We further characterize the operating region of the systems as unsaturated stable, bistable, and saturated regions in terms of the new packet arrival and retransmission rates. To run the system stably with the maximum throughput, we propose a Bayesian online backoff algorithm that estimates the number of backlogged devices. Its performance is compared with other algorithms, such as particle filter (PF)-based algorithm, binary exponential backoff (BEB) algorithm, and the algorithm of exploiting exact backlog size information. Through extensive simulations, it is demonstrated that the performance of the proposed algorithm is very close to the upper bound and robust to time-varying traffic condition.

In this article, we investigate a secure uplink multiple-input multiple-output communication system, in which the multiple users transmit information to a legitimate access point (AP) when a passive eavesdropper exists. A secure multiple access method is proposed for single-antenna users by employing space–time line codes (STLCs), which enables noncoherent detection at the legitimate AP without channel state information (CSI) and achieves full spatial diversity. While extending the secure STLC scheme from the single-antenna to two-antenna users by combining the STLC with artificial noise (AN) injection, we design a low-complexity AN signals with the optimal power control scheme. The proposed AN is ensured to be eliminated after STLC decoding at the legitimate AP, while sustained in eavesdropper's received signals as interference. Theoretical sum secrecy rates are derived by considering time-varying fading channels with the uncertainty of CSI at the users, and the numerical results verify that the proposed STLC schemes outperform the existing secure transmission schemes in terms of the sum secrecy rate.

A massive number of Internet-of-Things (IoT) and machine-to-machine (M2M) communication devices generate various types of data traffic in cellular IoT networks: periodic or nonperiodic, bursty or sporadic, etc. In particular, bursty and nonperiodic traffic may cause an unexpected network congestion and temporary lack of radio resources. In order to effectively accommodate such bursty and nonperiodic traffic, we propose a novel recursive access class barring (R-ACB) technique to optimally utilize the available resources associated with the random access procedure (RAP) that consists of multiple steps in cellular IoT networks, while existing ACB schemes only considered the resource of the first step of RAP, i.e., the number of available preambles. The proposed R-ACB technique consists of two main parts: 1) online estimation of the number of active IoT/M2M devices who have data to transmit to an eNodeB and 2) adjustment of the ACB factor that indicates the probability that an active device sends a preamble to eNodeB. It is notable that the estimation and the adjustment recursively affect each other when R-ACB operates. In addition, we also propose mathematical models to analyze the performance of R-ACB in terms of total service time, average access delay, resource efficiency, and energy efficiency (EE). Through extensive computer simulations, we show that the proposed R-ACB technique outperforms the conventional ACB schemes.

Uplink power domain non-orthogonal multiple access (NOMA) random access (RA) system allows users to transmit their packet with one of target receive power (TRP) levels at the base station (BS). Then the BS can separate the users in simultaneous transmission with successive interference cancellation (SIC), when the users choose different TRP levels. It enjoys a higher throughput through supporting simultaneous transmissions that occur frequently in RA system. This letter examines how to incorporate uplink NOMA RA system with the existing Long-Term Evolution (LTE) RA procedure with minimal modifications. Furthermore, to optimize the proposed RA procedure, we propose an online control algorithm for backoff interval and analyze its performance particularly with focus on the average RA delay. The results show that the proposed RA procedure achieves $0.5896{L}$ of throughput for a total of ${L}$ RA preambles and guarantees the average RA delay by 1/( $0.5896{L} -\lambda $ ) for Poisson traffic with mean $\lambda $ .

We investigate the secrecy outage probability (SOP) of a downlink wiretap network consisting of a single legitimate base station (BS), multiple legitimate mobile stations (MSs), and multiple potential eavesdroppers (EVEs), where each EVE randomly attempts to overhear the data transmission with a certain probability. In particular, we consider an opportunistic feedback ( $\mathsf {OF}$ ) strategy in which each legitimate MS feeds their channel gain back to the BS for data reception only when its gain is greater than a certain threshold. We analyze a closed-form expression of the SOP under this strategy. As our main result, we demonstrate that the SOP of the $\mathsf {OF}$ strategy approaches that of the full feedback strategy as the number of legitimate MSs becomes large. It is worth noting that, for the first time, we mathematically characterize the SOP in this practical wiretap network setting having multiple potential EVEs.

In this paper, we investigate stability region of hybrid uplink non-orthogonal multiple access (NOMA) in conjunction with non-cooperative game, where two users independently send packets with transmit power control according to fading channel gain to the same radio resource so that their packets are received at a base station (BS) with one of two target receive powers (TRPs). The BS then tries to decode the packets with successive interference cancellation (SIC) technique in power domain. We consider a symmetric non-cooperative game where the users (or players) send packets with one of two TRPs or no transmission. We examine the convexity of stability region at the mixed-strategy Nash equilibrium (NE) of the game, where no one's queue grows to infinity. Furthermore, we investigate how the stability region changes depending on the parameters of users’ payoff.

Orbital angular momentum (OAM) multiplexing has recently been proposed as a solution to the ultimate goal of increasing the channel capacity of wireless communication links because of the existence of infinite orthogonal modes. It can be combined with the existing conventional multiplexing techniques to boost up the data rate multiple times for future wireless communication systems. We analyze the fundamental behavior of the channel capacity of OAM channels based on two concepts for the most common Laguerre-Gaussian OAM beams: paraxiality and orthogonality. We also confirm our general theory by applying the proposed method to the evanescing OAM beams, circular transverse electric beams. Our study is based on calculating the paraxial estimator of these OAM beams and relating it as a parameter that limits the degree of freedom in OAM channels. In particular, we investigate the dependency on physical parameters, such as transmitter and receiver sizes, in our calculation to define the capacity of OAM channels.

Recently, to make the best use of limited and precious spectrum resources, spectrum sharing between satellite and cellular networks has received much interest. In this study, we mathematically analyze the success probability of a fixed (satellite) earth station (FES) based on a stochastic geometry framework. Both the FES and base stations (BSs) are assumed to be equipped with a directional antenna, and the location and the number of BSs are modeled based on the Poisson point process. Furthermore, an exclusion zone is considered, in which the BSs are prohibited from locating in a circular zone with a certain radius around the FES to protect it from severe interference from the cellular BSs. We validate the analytical results on the success probability of the cognitive satellite‐terrestrial network with directional antennas by comparing it using extensive computer simulations and show the effect of the exclusion zone on the success probability at the FES. It is shown that the exclusion zone‐based interference mitigation technique significantly improves the success probability as the exclusion zone increases.

In this paper, we propose a cooperative maximum-ratio transmission (C-MRT) technique with multi-antenna relay nodes for tactical mobile ad-hoc networks (MANETs) where both the location and the number of antennas of relay nodes are arbitrary. Each relay node is assumed to know both the wireless channel coefficient from the source node to itself and the wireless channel from itself to the destination node, which is known as local channel state information (CSI) assumption. Maximum-ratio combining (MRC) and MRT techniques are used for beamforming the transmit and receive signals of the relay node, respectively. In addition, we mathematically analyze the overall outage probability of the proposed technique with moment generating function (MGF). It is shown that the mathematical analysis on the overall outage probability of the proposed C-MRT technique is matched well with the simulation results especially in high transmit power regimes. It is also observed that the proposed technique outperforms other schemes such as phase steering (PS) and optimal relay selection (ORS) in terms of outage probability.

In military applications, mobile ad hoc networks are commonly used due to the autonomy, self-configuration, and flexibility. In addition, spectrum sharing-based communication protocols are being actively considered due to spectrum shortage. We propose a cooperative phase-steering (CPS) technique for a spectrum sharing-based military mobile ad hoc network which consists of a single secondary source (SS) node, multiple secondary relay (SR) nodes, a single secondary destination (SD) node, and multiple primary destination (PD) nodes. In the proposed technique, the SR nodes that succeeded the packet decoding cooperatively adjust the phase of their transmit signals such that the received signals at the SD node from the SR nodes are aligned to a certain angle, while the SR nodes control the transmit power such that the received power at all PD nodes is lower than a certain threshold. Through extensive computer simulations, it is shown that the proposed technique outperforms the conventional cooperative relaying scheme in terms of outage probability.

In 5G cellular networks, it is required to accommodate a massive number of Internet-of-Things (IoT) devices in a resource-efficient way. In this letter, we propose a random access with an inter-frame successive interference cancellation (RA-ISIC) for stationary IoT networks. In our proposed scheme, the BS does not discard the collided packets and attempts to recover them through inter-frame SIC operations. We evaluate the performance of our proposed scheme in terms of resource efficiency. Results show that a certain level of system load is required to maximize the efficiency of our proposed scheme, which can be achieved through a probabilistic retransmission (PR) policy. Consequently, our proposed scheme can effectively improve the resource efficiency and can be suitable to accommodate more IoT devices with a smaller amount of resources, compared to the conventional one.

This work proposes a random access system with uplink non-orthogonal multiple access (NOMA), where users transmit their packets by using the space-time line code (STLC) to fully exploit the spatial diversity without channel state information at the receiver. More specifically, users with a single antenna estimate channel gain from the base station (BS) with two antennas, encode the information using the STLC, and then transmit it using transmit power control so that the received power of their packets at the BS can be one of the predetermined values. The BS decodes the received packets via STLC decoding and the successive interference cancellation technique. Owing to spatial diversity, we show that the proposed scheme improves energy efficiency in comparison to the conventional system with a single antenna BS.

In this paper, an $M\times 4$ double space-time line code (D-STLC) is designed for a system with $M$ -transmit and four-receive antennas whose channel state information is available at the transmitter. By delivering two independent STLC data streams simultaneously, both spatial diversity and multiplexing gains are obtained, like a $4\times M$ double space-time transmit diversity (DSTTD) system that transmits two space-time block coded data streams. It is analytically shown that the decoding signal-to-interference-plus-noise ratios of D-STLC and DSTTD are identical to each other, and furthermore, it is numerically verified that the proposed D-STLC significantly outperforms a conventional $M\times 4$ STLC scheme and other existing spatial-diversity and spatial-multiplexing schemes, such as precoded space-time block code and precoded spatial multiplexing, in terms of achievable rate and bit-error-rate performance as the signal-to-noise ratio and/or transmission data rate increase.

In this paper, we propose an enhanced random access (RA) with a distributed pilot orthogonalization (ERA-DPO) for mitigating packet collisions in Step3 during the RA procedure in cellular internet-of-things (IoT) networks. Even though each IoT device experiences a preamble collision in Step1, it can avoid the packet collision in Step3 by differentiating uplink pilot with the proposed technique, which provides an opportunity for the collided packets to be separated in spatial domain by multiple receive antennas. We also mathematically analyze the performance of the proposed scheme in terms of packet collision probability, throughput, and resource efficiency, based on a Markov chain model. Simulation results reveal that the proposed scheme significantly reduces the packet collision probability and increases resource efficiency compared to the conventional scheme.

Recently, there has been a rapid increase in the number of (small-cell) base stations (BSs) to support the massive amount of mobile data traffic and rapidly increasing number of mobile devices in beyond 5G (B5G) wireless communication systems or Internet of Things (IoT) networks. However, many of these BSs tend to waste a considerable amount of energy to support such data traffic and mobile devices. Therefore, the development of an efficient BS status control algorithm is important for realizing energy-efficient IoT networks. To reduce network energy consumption, we herein propose a density clustering-based BS control algorithm for energy-efficient IoT networks (DeCoNet). DBSCAN (Density-Based Spatial Clustering of Applications with Noise) and OPTICS (Ordering Points To Identify the Clustering Structure) are utilized for partitioning high and low user-density regions. To find the effective number of BSs and their appropriate locations considering user-density differences, we utilize parameters obtained after applying density clustering algorithms to derive the thinning radius that is used to adjust the status of BSs in overall cellular IoT networks. Specifically, the average reachability-distance of each cluster in OPTICS and the distance between the outermost border users of each cluster in DBSCAN are used to obtain the radius of each cluster region. Through extensive computer simulations, we show that the proposed algorithms outperform the conventional algorithms in terms of average area throughput, energy efficiency, energy per information bit, and power consumption per unit area.

In this paper, we suggest an RFID tag antenna for ultrahigh frequency band for radio frequency identification for drug runout management and a float motion detection that can be used in hospitals and fishing. The RFID tag antenna works as a sensor which warns of drug runout when the drug inside the drip chamber is completely consumed, while it does not work if the drug remains inside the drop chamber. Also, the antenna which attached to the fishing float propagates signals when it is exposed to air, but it does not work when submerged in water. The fabricated antenna has an average omnidirectional reading range of 10.65 m in the horizontal plane.

In this paper, we model and characterize interference of directional unmanned aerial vehicle (UAV) networks based on stochastic geometry, where each UAV is equipped with a directional antenna and it communicates with another UAV that is located in the three dimensional (3D) space. In particular, the 3D location of UAVs is assumed to be uniformly distributed in a certain volume, which is modeled by Poisson point process. Given a beamwidth, we first design an ideal 3D directional antenna beam pattern with a constant gain of both main-lobe and side-lobe. To model the interference in the UAV network, we analyze the effect of elevation and azimuth between a typical UAV receiver and an interfering UAV transmitter with spherical coordinate system. Then, we investigate distribution of the aggregate interference at a typical UAV receiver from multiple UAVs in terms of side-lobe gain, beamwidth, height of UAV, and distance of a UAV transmitter–receiver pair. Keywords: UAV networks, Poisson point process (PPP), 3D directional antenna, Interference, Stochastic geometry

In this paper, we propose a virtual full-duplex cooperative non-orthogonal multiple access (NOMA) framework for a downlink two-hop network assisted by multiple half-duplex decode–and–forward (DF) relay stations (RSs). In the proposed virtual full-duplex framework, the RSs except for a selected RS to forward the received signal suffer from inter-RS interference since both the BS and the selected RS transmit the super-imposed signal simultaneously. To address this problem, we propose an RS selection algorithm with adaptive inter-RS interference management and we mathematically attain the closed-form outage probability, which is challenging in general. In addition, we investigate diversity-multiplexing tradeoff (DMT) performance of a modified version of the proposed RS selection algorithm based on a discrete Markov chain, which adaptively resets the successive transmission. Simulation results show that the proposed RS selection algorithm outperforms the conventional cooperative NOMA algorithm in terms of both the outage probability and the DMT, which has the best performance reported in the literature.

In power domain non-orthogonal multiple access (NOMA) used for uplink random access (RA) systems, users need to control transmit power such that the received power at the base station can be one of the predetermined values. Thus, selecting one of the predetermined values incurs different transmission costs. Furthermore, users can often withdraw their (re)transmission, which involves a waiting (or regret) cost. This letter analyzes ${N}$ -user non-cooperative game of uplink NOMA RA systems, where waiting cost is taken into account. As results, we show the condition, under which a unique mixed-strategy Nash equilibrium (MNE) exists and its efficiency with respect to social welfare maximization (SWM).

We mathematically derive an upper bound of bit-error rate (BER) of uplink non-orthogonal multiple access (NOMA) systems with quadrature phase shift keying (QPSK) modulation in fading channels. In particular, we exploit the joint maximum-likelihood (ML) detector at a base station (BS) with multiple antennas since it results in the optimal BER performance of a super-imposed QPSK modulated symbols from multiple users. In particular, we obtain a closed-form integral of Q-function with Erlang distributed random variable to derive the BER. We also obtain diversity order of the uplink NOMA systems. Through extensive computer simulations, we validate that our analytical results match well with the simulation results especially in high signal-to-noise ratio (SNR) regime.

This letter investigates uplink random access (RA) systems based on power-domain non-orthogonal multiple access (NOMA) with multichannel. In NOMA RA systems, users transmit their packet with transmit power by inverting their channel gain, i.e., channel inversion, such that the received power of their packet at the base station (BS) can be one of the predefined values. While this facilitates the decoding process based on successive interference cancellation (SIC), it can incur excessive transmit power consumption upon low channel gains. To reduce transmit power (improve energy efficiency), this letter considers multichannel selection diversity, where users choose one channel whose channel gain is the maximum of all. We characterize the stability of multichannel NOMA RA systems with the cusp catastrophe and analyze diversity gain in terms of energy efficiency.

In this letter, we propose a novel multi-input multi-output (MIMO) interference coordination (IC) technique for uplink fog radio access networks (F-RANs), consisting of two multi-antenna remote radio heads (RRHs) connected to a single fog access point (F-AP) and user equipments (UEs) equipped with different numbers of antennas. It is assumed that UEs are scheduled with an asynchronous manner for each RRH. With the proposed MIMO IC technique, the interference dimension at each RRH is dynamically adjusted via transmit beamforming at the selected UEs based on both currently scheduled UEs and their antenna configurations. To the best of our knowledge, the MIMO IC exploiting heterogeneous multi-antennas at UEs is first proposed in this letter for the F-RANs. Numerical results show that the proposed technique outperforms the conventional schemes in terms of sum rates.

In this correspondence paper, a novel machine learning (clustering) based blind decoding method is proposed for the space–time line code (STLC) systems without the information of modulation size and channels. The number of clusters, which is equivalent to the modulation size, is estimated from the received signals by using a $k$ -mean cluster validation metrics, such as silhouette score and Davies–Bouldin index, and the cluster indices are directly mapped to the transmitted binary information bits. To improve the clustering performance, received signal normalization and the initial centroids of the clusters are designed by exploiting the combined STLC signal structure. From the numerical results, it is verified that the proposed blind decoding method can achieve near-coherent decoding performance with either small-size modulation, low noise at the receiver, or a large number of transmit antennas.

Grant-free multiple access (GFMA) protocol has been regarded as a key element to support sporadic traffic generated from massive internet-of-things (IoT) networks. In GFMA protocol, each IoT device transmits data packets without grant from a base station (BS) via pre-reserved uplink resources. Packet collisions inherently occur when multiple IoT devices transmit packets by using the same radio resource, but the collision effect can be alleviated with multi-packet reception (MPR) capability of the BS. Since a number of studies have focused on improving the physical layer performance such as bit error rate, they may be hard to provide intuitions from the MAC layer perspective when a number of IoT devices sporadically generate uplink packets and attempt the GFMA. In this paper, we thoroughly investigate the GFMA from the MAC layer perspective. We provide an analytical framework based on a Markov chain to capture the performance of the GFMA in terms of packet transmission success probability, ergodic throughput, and access delay. Through simulations, we validate our analytical framework and verify the necessity of adopting MPR technique for supporting a massive number of IoT devices generating sporadic traffic.

In this paper, we investigate two user scheduling algorithms (optimal user and threshold-based user scheduling algorithms) when we consider potential eavesdroppers in an uplink wiretap network. The optimal user scheduling (OUS) algorithm selects the user who has the maximum secrecy rate, based on channel feedback from all users. On the other hand, the threshold-based user scheduling (TUS) algorithm first considers the information leakage from the users to potential eavesdroppers and then selects the user among candidates who satisfy a threshold criterion on the information leakage. The OUS algorithm shows an optimal performance in terms of secrecy rate, but the TUS algorithm can achieve secrecy rate comparable with the OUS algorithm with reduced feedback overhead. For main contributions, we mathematically analyze the asymptotic behavior of the achievable secrecy rate of two scheduling algorithms when the signal-to-noise ratio (SNR) approaches to infinity. Further, we derive the approximated secrecy rate of the TUS algorithm and propose criteria to determine threshold values which maximize the achievable secrecy rate of the TUS algorithm. We verify our analytical results through simulations. We perform an extra simulation to investigate the effect of channel estimation error in the wiretap links on the average secrecy rate. Due to different scheduling principles in OUS and TUS schemes, the TUS scheme yields robustness against the channel estimation error in the wiretap links, compared with the OUS schemes.

In this paper, we propose four transmit power control strategies for the underlay device-to-device (D2D) communications, in which the spectral efficiency (SE) of the D2D communications is maximized while the amount of interference caused to a base station (BS) is kept less than a predefined threshold. To this end, we first propose a centralized power control strategy based on instantaneous and global channel state information (CSI) by formulating a convex optimization problem. Then, three distributed power control strategies are taken into account in which each D2D pair adjusts its transmit power in a distributed manner based on interference price and its local CSI, which significantly reduces the signaling overhead. In the distributed strategies, the interference price can be determined based on (1) the instantaneous local CSI; (2) the statistics of the local CSI (average power); and (3) the number of the D2D pairs without any CSI knowledge. Through extensive computer simulations, we show that the performances of the proposed strategies optimally adjust the transmit power of the D2D communications. Especially, we find that the distributed power control strategies can achieve almost the same SE with the centralized strategy with much lower signaling and control overhead.

In this paper, the performance of several spoofing mitigation techniques using multiple receive antennas is compared and analyzed under various spoofing attack scenarios. We consider three different spoofing attack scenarios in this paper. In the first scenario, the received power of the spoofing signal is 5 dB higher than that of the satellite signal and the spoofing signal has a completely different code phase and doppler frequency from the satellite signal at the GNSS receiver. In the second scenario, the received power of the spoofing signal is 5 dB higher than that of the satellite signal and the spoofing signal has 0.45 chip delay in the code phase of the satellite signal at the GNSS receiver. In the third scenario, the received power of the spoofing signal is 0-2 dB higher than that of the satellite signal and the spoofing signal has 0.45 chip delay in the code phase of the satellite signal at the GNSS receiver. The uniform circular array consists of 4, 5, or 7 elements. The software-based GPS L1 C / A signal generator and the spoofing signals generator is designed. The software-based GPS receiver for anti-spoofing techniques based on array antenna is also designed. As indicators for performance, the array antenna radiation pattern, the accuracy of DoA estimation, C/N0 after applying the anti-spoofing technique, and code tracking error are used. From the results, it is confirmed that the accuracy of the DoA estimation increases as the number of antennas increases when the received power of the spoofing signal is sufficiently higher than that of the satellite signal. However, the performance of the spoofing mitigation technique becomes deteriorated when the received power of the spoofing signal is similar to that of the satellite signal even though the number of antenna elements is large enough.

Recently, reconfigurable intelligent surfaces (RISs) have received much interest from both academia and industry due to their flexibility and cost-effectiveness in adjusting the phase and amplitude of wireless signals with low-cost passive reflecting elements. In particular, many RIS-aided techniques have been proposed to improve both data rate and energy efficiency for 6G wireless communication systems. In this paper, we propose a novel RIS-based channel randomization (RCR) technique for improving physical-layer security (PLS) for a time-division duplex (TDD) downlink cellular wire-tap network which consists of a single base station (BS) with multiple antennas, multiple legitimate pieces of user equipment (UE), multiple eavesdroppers (EVEs), and multiple RISs. We assume that only a line-of-sight (LOS) channel exists among the BS, the RISs, and the UE due to propagation characteristics of tera-hertz (THz) spectrum bands that may be used in 6G wireless communication systems. In the proposed technique, each RIS first pseudo-randomly generates multiple reflection matrices and utilizes them for both pilot signal duration (PSD) in uplink and data transmission duration (DTD) in downlink. Then, the BS estimates wireless channels of UE with reflection matrices of all RISs and selects the UE that has the best secrecy rate for each reflection matrix generated. It is shown herein that the proposed technique outperforms the conventional techniques in terms of achievable secrecy rates.

In this paper, we propose a novel intelligent reflecting surface (IRS) based wireless channel confusion technique for energy efficient jamming. Since IRS consists of passive reflecting elements, its amplitude and phase can be changed rapidly. Due to its rapid alternation of passive elements, IRS makes legitimate receiver to acquire wrong channel state information (CSI) by changing effective channel of legitimate network so that the channel at pilot signal slot and at data signal slots are different. As a result, channel confusion technique can effectively degrade the communication performance of legitimate network even legitimate transmitter uses much higher transmit power. The jamming performance of proposed IRS based channel confusion technique is evaluated by extensive computer simulation. Moreover, it is shown that proposed IRS based channel confusion technique is better jamming technique than Gaussian jamming technique.

In this paper, we propose a novel opportunistic interference alignment technique for spectrum-shared radar and uplink cellular communication systems where both systems are equipped with multiple antennas. In the proposed OIA technique, the radar system sends its signal so that the radar signal is received into interference space at base stations (BSs) of the cellular system, while each uplink user (UE) generates its transmit beamforming vector so that communication signals are received within interference space at the radar receiver. Moreover, to achieve better sum-rate performance of the cellular communication system, the BS selects the UEs which results in sufficiently small interference to other cells for the uplink communication. With the proposed OIA technique, detection performance of the radar system is protected, while the communication system achieves satisfactory sum-rate performance. Through extensive computer simulations, we show that the performances of both radar and communication systems with the proposed technique significantly outperform a conventional null-space projection based spectrum sharing scheme.

With the growth of the number of Internet of Things (IoT) devices, a wide range of wireless sensor networks (WSNs) will be deployed for various applications. In general, WSNs are constrained by limitations in spectrum and energy resources. In order to circumvent these technical challenges, we propose a novel cooperative phase-steering (CPS) technique with a simple on-off power control for generic spectrum sharing-based WSNs, which consists of a single secondary source (SS) node, multiple secondary relay (SR) nodes, a single secondary destination (SD) node, and multiple primary destination (PD) nodes. In the proposed technique, each SR node that succeeds in packet decoding from the SS and for which its interference power to the PD nodes is lower than a certain threshold is allowed to transmit the signal to the SD node. All SR nodes that are allowed to transmit signals to the SD node adjust the phase of their transmit signal such that the phase of received signals at the SD node from the SR nodes is aligned to a certain angle. Moreover, we mathematically analyze the outage probability of the proposed scheme. Our analytical and simulation results show that the proposed technique outperforms the conventional cooperative relaying schemes in terms of outage probability. Through extensive computer simulations, it is shown that the analytical results match well with the simulated outage probability as a lower bound.

In cellular Internet-of-Things (IoT) systems, system overload may occur during a random access (RA) procedure under a limited number of preamble resources and physical uplink shared channel (PUSCH) resources especially when there exist massive IoT devices in a cell. In order to resolve the system overload, the commercial system like 3GPP LTE adopted a group paging (GP)-based uplink access technique, but it has been known that the performance of the GP-based technique drastically degrades as the number of devices increases. In this paper, we first propose a dynamic access control (DAC) mechanism for the GP-based cellular massive IoT system, which dynamically adjusts RA-attempting probability by considering not only the number of available preambles but also the number of available PUSCH resources. We also intelligently combine the proposed DAC mechanism with an early preamble collision detection technique to further improve the RA performance of the cellular IoT system. Through extensive computer simulations, we show that the proposed DAC mechanism outperforms the conventional access control mechanisms, which consider only the number of available preamble resources, in terms of GP completion time, PUSCH resource efficiency, transmission efficiency, and energy efficiency.

The reactive symbol-level jamming (SLJ) technique is a simple but practical technique to disable or disrupt malicious communication links. After sensing the transmission and detecting the digital modulation scheme of the malicious transmitter, a communication node with the reactive SLJ technique, called a reactive jammer, generates random digital modulation symbols and then sends them to the malicious receiver. In this correspondence, we mathematically analyze both uncoded and coded bit error rate (BER) at the malicious receiver under not only reactive SLJ, but also Gaussian SLJ techniques. In particular, we consider the partial jamming scenario, in which a portion of symbols in a data frame are affected by the SLJ, which is a practical scenario under the reactive jamming techniques. We also consider the effect of imperfect power control and channel estimation error of the reactive jammer on the BER performance at the malicious receiver.

This letter applies non-orthogonal multiple access to uplink random access systems, where the transmit power from each user equipment (UE) is received at the base station as one of $L$ target (received) powers. More precisely, if a backlogged UE’s channel gain is greater than a threshold, it randomly aims at one of the target powers with channel inversion. Then, the BS decodes the received signals with successive interference cancellation. This letter investigates the performance of this system in terms of throughput, access delay, energy efficiency, and bistability.

Machine nodes (MNs) such as monitoring devices and utility devices can be handled with machine-to-machine (M2M) group communications in commercial 3GPP LTE networks. However, the current LTE networks are designed for human-oriented communication, i.e., human-to-human (H2H) communication, as a main service. Hence, the M2M group communications may utilize a limited amount of radio resource in the LTE networks so that they do not degrade the quality-of-services (QoS) of the H2H communications. Under the resource limitation, the MNs in the M2M communication group may suffer resource contentions when they simultaneously send uplink data packets to a base station (BS). In this paper, we first mathematically model the overall procedure of the M2M group communications. Then, we also optimize the system parameters for the M2M group communications to maximize resource utilization of the LTE networks and minimize the packet transmission delay, while satisfying the resource constraint.

We investigate an interference management problem in multi-carrier cognitive radio systems. To this end, we propose a new framework for protecting primary users such that the primary user can shape the envelope of interference on each subcarrier. Furthermore, we introduce the concept of minimum individual interference budget that the primary user should allow on each subcarrier so that the secondary user can receive a certain quality of service. We develop a joint optimal transmit power and interference budget allocation algorithm for the primary user, which maximizes the throughput of the primary user. Through simulations, we show that our approach can provide better protection for primary users than the existing approach.

본 논문은 기계학습 기반의 레이다 기술의 연구 동향을 요약한다. 특히, 본 논문에서는 레이다 시스템, 레이다 안테나, 레이다 송수신장치, 레이다 신호처리기 등 각 분야에 적용된 기계학습기술에 관하여 분석하고 연구의 발전 방향에 관하여 논한다.