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Summary: Paper has considered multiple reconfigurable intelligent surfaces (RISs) assisted downlink non‐orthogonal multiple accesses (NOMA) system, referred to as MR‐NOMA. Multiple RISs create a line‐of‐sight (LOS) channel between BS and users to improve network coverage, especially in high frequencies and severe path attenuation scenarios. In MR‐NOMA, all RISs are in series and activated simultaneously; therefore, multiplicative beamforming gain from the inter‐RIS channels is achieved. Bit error probability (BEP) under the Rician fading channel is derived and compared with simulation results. It is observed that BEP performance depends on the number of passive reflecting elements, Rician factor, and positions of RISs panels in the MR‐NOMA. [ABSTRACT FROM AUTHOR]

Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles. [ABSTRACT FROM AUTHOR]

UAVs can be used as aerial relays to provide communication services in remote uncovered areas or dense environments with occasional high capacity demands. However, due to the low power of Internet-of-Things (IoT) devices, UAV-based IoT applications, such as precision agriculture and environment monitoring, may experience high shadowing or equipment failure, which degrades the communications’ quality between IoT devices and their gateways. To tackle this issue, we consider the long range (LoRa) communication technology. Specifically, we investigate the performance of LoRA-enabled aerial communications, where a LoRa gateway communicates with a distant IoT device through the assistance of an amplify-and-forward (AF) aerial relay. Under the assumption of shadowed Rician fading channels, we characterize at first the end-to-end LoRa communication link. Then, we derive an exact symbol error rate expression for the underlying system model. Finally, numerical results are presented to corroborate the efficacy of our derived expressions and provide valuable insights into the error performance of LoRa-enabled aerial networks. [ABSTRACT FROM AUTHOR]

In this letter, we investigate the ergodic capacity of the Relay-Reconfigurable Intelligent Surface (Relay-RIS) system exploiting the three-dimensional channel model, which is different from existing works ignoring antenna height. Specifically, we propose a novel three-dimensional statistical channel for the Relay-RIS system. The upper bound expressions of the ergodic capacity are deduced in diverse propagation environments. Meanwhile, the impact of channel condition and deployment location on the capacity improvement is quantified, and the minimum number of reflecting elements to achieve a given rate threshold is obtained. Simulation results verify the effectiveness of the Relay-RIS system and the three-dimensional deployment strategy. [ABSTRACT FROM AUTHOR]

We consider multiple unmanned aerial vehicles (UAVs) serving a density of ground terminals (GTs) as base stations. The objective is to minimize the outage probability of GT-to-UAV transmissions. Optimal placement of UAVs under different UAV altitude constraints and GT densities is studied. A practical variant of the Rician fading model, which has been developed by Azari et al. (2018) specifically for UAV systems, is used to model the communication channels. First, the structure of optimal deployments is determined when the common altitude constraint is large. For a wide class of GT densities, it is shown that all UAVs should be placed to the same location in an optimal deployment. A design implication is that one can use a single multi-antenna UAV as opposed to multiple single-antenna UAVs without loss of optimality. Next, using a random deployment argument, a general upper bound on the optimal outage probability is found for any density of GTs and any number of UAVs. Further, for any arbitrary user density, centralized and distributed numerical algorithms are designed using particle swarm optimization and modified gradient descent algorithms, respectively. Simulations show that our distributed algorithm provides almost optimal results even with considerably reduced communication and sensing ranges at the UAVs. [ABSTRACT FROM AUTHOR]

Several information-theoretic studies on channels with output quantization have identified the capacity-achieving input distributions for different fading channels with 1-bit in-phase and quadrature (I/Q) output quantization. However, an exact characterization of the capacity-achieving input distribution for channels with multi-bit phase quantization has not been provided. In this paper, we consider four different channel models with multi-bit phase quantization at the output and identify the optimal input distribution for each channel model. We first consider a complex Gaussian channel with $b$ -bit phase-quantized output and prove that the capacity-achieving distribution is a rotated $2^{b}$ -phase shift keying (PSK). The analysis is then extended to multiple fading scenarios. We show that the optimality of rotated $2^{b}$ -PSK continues to hold under noncoherent fast fading Rician channels with $b$ -bit phase quantization when line-of-sight (LoS) is present. When channel state information (CSI) is available at the receiver, we identify $\frac {2\pi }{2^{b}}$ -symmetry and constant amplitude as the necessary and sufficient conditions for the ergodic capacity-achieving input distribution; which a $2^{b}$ -PSK satisfies. Finally, an optimum power control scheme is presented which achieves ergodic capacity when CSI is also available at the transmitter. [ABSTRACT FROM AUTHOR]

The operation of point-to-point communication assisted with reconfigurable intelligent surface, where the multi-hop channels between the source and the destination are subject to Rician fading but are not necessarily independent, is investigated. Considering binary phase-shift keying signaling for the data transmission and optimal maximum-likelihood detection at the receiver, a system performance evaluation is conducted in terms of reliability and capacity. For such, the distributions of the magnitude of the product of the complex channel gains are first obtained for both, the case when the channel gains are independent and the case when they are correlated. Subsequently, expressions of the system’s error rate and the upper-bounds on the average capacity are obtained for the said correlated and uncorrelated channel cases. Numerical results corroborating the analysis are also presented. In particular, it is revealed that correlation between the channels has negligible effect on the error rate when the fading is Rayleigh distributed, and that an increase in the value of the Rician parameter pertaining to the channel between the receiver pair has reduced impact on the reduction of the error rate, which provides practical guidelines for system deployments with high reliability. [ABSTRACT FROM AUTHOR]

We investigate a multi-pair full-duplex (FD) two-way communication system assisted by a reconfigurable intelligent surface (RIS), where each user-pair exchanges information via RIS. Unlike most existing works related to FD systems with RIS, we take into account an inevitable electromagnetic interference (EMI) and also consider a practical Rician channel between RIS and users, and spatial correlation owing to size constraint at the RIS. In addition we consider low-cost hardware at RIS and users resulting in impairments. We first derive an approximate expression for the achievable sum-rate of the system. We then optimize RIS phases to achieve the optimum sum-rate using particle swarm optimization (PSO) algorithm. We, through the simulation results, show the optimality of PSO algorithm and the impact of EMI and hardware impairments on the sum-rate. [ABSTRACT FROM AUTHOR]

This paper proposes an aggressive cuckoo search algorithm for optimum power allocation in a CDMA-based cellular network. To make the cuckoo search algorithm aggressive, adaptive parameters are used to vary the step size and probability of discovery. Furthermore, the Lévy flight is replaced with the Beta distribution to further improve the performance of the algorithm. To prove that the proposed algorithm is superior, the algorithm is tested on 23 benchmark test functions and its results are compared with those of 10 other standard optimization algorithms and 4 other advanced optimization algorithms. The performance of the proposed algorithm is proved via the statistical analysis of the results using the Wilcoxon rank-sum test. The proposed algorithm is then utilized in determining the optimal uplink power for multiple users in a CDMA-based cellular network in three different scenarios through Rician fading channels. The resultant allocated power should ensure that each mobile station meets its predetermined signal-to-interference-and-noise ratio while utilizing the least amount of power. [ABSTRACT FROM AUTHOR]

Summary: Spectral efficiency (SE) is one of the eminent requirements in 5G mobile networks. Cell‐free (CF) massive MIMO is deemed a key technology to provide substantial SE in 5G compared with the cellular and small cell approaches. Most of the prior studies have been focusing on the access points (APs) uniform distribution to assess the SE performance. However, 5G networks are typically dense, irregularly distributed, and mostly constrained by channel impairments. Therefore, considering a uniform distribution of APs is unrealistic. This paper considers a practical network distribution by taking into account the irregular and adaptive APs distribution based on the Poisson point process approach and over the Rician fading channels. Therein, the downlink (DL) SE of CF massive MIMO system is accurately investigated bearing in mind the AP's irregular distribution and fast channel variation for both perfect and imperfect channel state information (CSI) cases. The simulation results have shown that the DL SE of the CF massive MIMO system is considerably affected when considering the irregular deployment of APs compared with the uniform distribution, especially when the phase noise effect is tense. The SE gain performance is reduced by 37.9% in the DL transmissions, compared with the uniform model. Besides, the results have proven that the DL SE gain is remarkably improved when the APs are largely distributed within the network. However, the gained DL SE is affected when increasing the user's density and length of the uplink training period. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surface (RIS) has stimulated their potential applications for improving the performance of wireless communication networks. In this paper, the performance of RIS aided non‐orthogonal multiple access (NOMA) networks with hardware impairments over Rician fading channels is investigated. More specifically, the exact and asymptotic expressions of outage probability for a pair of users, that is, the nearby user n and distant user m are derived, where the imperfect successive interference cancellation (ipSIC) and perfect SIC (pSIC) are taken into consideration. According to the approximate analyses, the diversity orders of user n with ipSIC/pSIC and user m are obtained in the high signal‐to‐noise radio regime. It indicates that the diversity orders are in connection with reflecting elements and Rician factors except the channel ordering. The impact of these parameters on outage behaviors of RIS‐NOMA networks is also analysed. In addition, the system throughput of RIS‐NOMA networks with ipSIC/pSIC is surveyed in detail. Monte Carlo simulations are present to verify the correctness of theoretical analyses that: (1) The outage probability of user n with pSIC is superior to that of orthogonal user, while the outage probability of user m is inferior to that of orthogonal user and (2) as the number of reflecting elements and Rician factors increases, the outage behaviors of RIS‐NOMA networks are enhanced carefully. [ABSTRACT FROM AUTHOR]

Recently, academia and industry have shown keen interest in achieving ultra-reliable and low latency communication (URLLC) through short-packet communication to meet the strict demands concerning high reliability and latency for 5G and beyond applications. Un-manned aerial vehicle (UAV) has caught attention recently because of its cost, air time, and mobility, whereas, the non-orthogonal multiple access (NOMA) technique has proven effective in dense user network. Hence, a UAV based system with NOMA has been studied in this paper for remote coverage. In this system, devices communicate to the base station (BS) through a UAV relay where direct link from devices to BS is absent. As UAV has direct line of sight (LOS) to the remote devices, hence, all the analysis is done over a Rician fading channel. In this work, a closed-form expression of average block error rate (BLER) has been formulated for the given system model, which is used as performance metric to analyse the system performance. Moreover, exact BLER expression facilitates in optimization with partial channel state information (CSI) only, which reduces the latency and complexity. Furthermore, we derive an asymptotic expression for BLER in high signal to noise (SNR) regime. Also, a blocklength minimization problem is formulated and optimized with reliability constraints. Simulation results are presented to verify analytical work, as well as comparison of the results with orthogonal multiple access scheme are also shown. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) and cell-free (CF) massive multiple-input multiple-output (MIMO) are two promising technologies for realizing beyond-fifth generation (5G) networks. In this paper, we study the uplink spectral efficiency (SE) of a practical spatially correlated RISs-aided CF massive MIMO system over Rician fading channels. Specifically, we derive the closed-form expression for characterizing the uplink SE of the system, which shows that increasing the number of RIS elements can improve the system performance. Moreover, the results unveil that the spatial correlation of RIS has a significant impact on the system while leading to the best performance gain at a half wavelength spacing. Finally, the accuracy of our analytical results are verified by Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

In this work, we study the performance of a reconfigurable intelligent surface (RIS)-aided unmanned aerial vehicle (UAV) system where an RIS is installed on a UAV to reflect the signal to the ground user from the sky. More specifically, we present two different methods to denote the probability density functions (PDFs) of the instantaneous signal-to-noise ratio (SNR). With these two distributions, closed-form expressions for the outage probability, average bit error rate (BER), and average sum-rate are derived, respectively. Additionally, we develop the analysis on the optimal UAV altitude and location. Finally, numerical results are given to verify our analysis. [ABSTRACT FROM AUTHOR]

This paper presents techniques that are expected to improve the bit error rate (BER) performance of Unmanned Aerial Vehicles (UAVs) communication systems. The decode-and-forward (DF)-based cooperative multi-hop Rician network using space shift keying (SSK) modulation is employed for this purpose. The SSK modulation and cooperative communication are merged and used together. This enhances the BER performance for multiple-input multiple-output (MIMO) channels, thus accentuating its significance as an important technique for consistently reliable communication methodology while, at the same time, eliminating wastage of energy. The new technique is named as high performance (HP) due to not only BER performance improvement but also increase of the diversity of the communication system. This new model is further used for multi-hop networks and the resulting BER characteristics are assessed for possible enhanced performance in Rician channels. The paper also showcases the generalized-n-hop BER equation for various multi-hop DF-based cooperative communication systems in diverse UAV models. The Monte Carlo simulations further confirm the theoretical results that already exist for transmit-receive antennas. This work also underlines the fact that the number of hops in a communication system has no bearing whatsoever on the BER performance diversity value. [ABSTRACT FROM AUTHOR]

Although 5G networks have enabled mobile users to get a better experience, task scheduling remains challenging for massive Internet of Things (IoT) devices in remote areas. This paper investigates the task scheduling problem for delay-aware IoT applications in civil aircraft-augmented space-air-ground integrated networks (CAA-SAGIN), where the normalized sky access platforms (SAPs) can collect and forward the terrestrial tasks. Specifically, we first propose an access control scheme for a non-preemptive priority queuing system and a transmission control scheme with cross-layer optimization. Secondly, considering the uncertain distribution of the transmission numbers and generated data, we formulate a robust two-stage stochastic optimization problem of delay minimization. With the proposed robust task scheduling with risk aversion (RTS-RA) algorithm, the original problem can be decomposed into two subproblems, which can be further transformed into tractable semi-definite program (SDP) problems respectively. Simulation results show that the cross-layer optimization scheme can achieve a good tradeoff between delay and throughput. Also, the RTS-RA algorithm outperforms the exiting offloading schemes in terms of end-to-end delay, transmitted data, and energy consumption with lower computational complexity. [ABSTRACT FROM AUTHOR]

We consider the downlink of a relay-aided multi-cell massive multi-input multi-output (mMIMO) system, where in each cell the base station (BS) serves its users via multiple relays by employing non-orthogonal multiple access (NOMA). We model this system by considering spatially-correlated Rician-faded channels and their estimation errors. The users, consequently, perform imperfect successive interference cancellation (SIC). We derive a lower bound on the spectral efficiency (SE) of this system. We then optimize the non-convex global energy efficiency (GEE) metric, which is a fractional function of the optimization variables. We solve this problem by considering a low-complexity alternating minimization maximization approach, which splits a complex joint problem into multiple simpler convex surrogate sub-problems. We propose a novel surrogate function to exploit this framework, and analytically show that it satisfies the desirable properties of a valid surrogate function. We numerically show that 1) reusing the pilots in each cell, when the channel has sufficiently hardened, provides higher SE than using orthogonal pilots in all cells and 2) the proposed GEE algorithm provides similar GEE as that of an existing joint optimization framework, but with much less complexity. [ABSTRACT FROM AUTHOR]

In order to improve the overall energy efficiency of multi-hop relaying networks for the formation of unmanned surface vessel, and reduce the possibility of outage in communication, a distributed multi-hop relaying spectrum allocation algorithm is put forward based on maximizing energy efficiency. Firstly, for the sake of making sure that the possibility of inter node communication interruptions is kept within the threshold, an analysis on Rician fading channel is conducted to calculate the minimum transmitting power needed in networks. Then, the number of maximum subcarriers is calculated and the asynchronous distributed pricing algorithm is applied to select the candidate subcarriers group. After that, the sub gradient method is used, by solving KKT conditions, to select the subcarriers from the candidate subcarrier group and allocate the power, therefore, a sub optimal solution of the optimal allocation is obtained. Finally, the effectiveness of the proposed algorithm is verified by the simulation analysis on the reliability and energy efficiency of networks. [ABSTRACT FROM AUTHOR]

In this article, for a cell-free massive multi-input multi-output (MIMO) system assisted with intelligent reflective surface (IRS) panels, a generalized superimposed pilot (GSP) training scheme is proposed, where the available number of pilots are equal to the coherence time slots, and the transmitting data symbols are spread over the coherence time with the help of simple precoding. Further, in order to keep the system scalable, a low complexity approach is employed for processing, and the corresponding rate components are analyzed. It is shown that with careful design of precoding and number of data symbols, the GSP symbols can provide much better channel estimation and data detection performance, as compared to the regular pilot scheme and the conventional superimposed scheme. These results, verified via simulations, shows that the centralized processing improves the data detection performance than localized processing. The pilot contamination effect, is significantly reduced due to the availability of larger number of pilots, as compared to the regular pilots transmission. For four IRS panels in the system, the proposed scheme is shown to reduce the channel estimation errors by 74% approximately. [ABSTRACT FROM AUTHOR]

In this paper, we consider a multiple reconfigurable intelligent surface (RIS)-assisted system using positioning information (PI) to explore the potential of Doppler effect mitigation and spectral efficiency (SE) enhancement in high-speed communications (HSC) in the presence of hardware impairments (HWI). In particular, we first present a general multi-RIS-assisted system model for HSC with HWI. Then, based on PI, different phase shift optimization strategies are designed and compared for maximizing SE, eliminating Doppler spread, and maintaining a very low delay spread. Moreover, we compare the performance of different numbers of RISs with HWI in terms of SE and delay spread. Finally, we extend our channel model from line-of-sight to the Rician channel to demonstrate the effectiveness and robustness of our proposed scheme. Numerical results reveal that the HWI of RISs increases the delay spread, but has no impact on Doppler shift and spread. Additionally, the multiple RIS system not only suffers a more severe delay spread, but is limited in SE due to the HWI. When the number of RISs increases from 2 to 16, the range of average spectral efficiency and delay spread are from 4 to 4.6 Bit/s/Hz and from 0.7 μs to 2.5 μs, respectively. In contrast to conventional RIS-assisted systems that require channel estimation, the proposed PI-based RIS system offers simplicity without compromising effectiveness and robustness in both SE enhancement and Doppler mitigation. [ABSTRACT FROM AUTHOR]

Summary: Conventional‐generalized spatial modulation (C‐GSM) is an NT×NR multiple‐input multiple‐output (MIMO) spatial modulation (SM) scheme that transmits the same complex amplitude/phase modulation symbol from multiple active transmit antennas. By using two active transmit antennas during a transmission interval, C‐GSM is capable of achieving high spectral efficiencies. However, as the size of the signal constellation/domain increases, the minimum Euclidean distances (ED) between the symbols decrease. Hence, this leads to a degradation of the average bit error rate (ABER) performance of the scheme. Hence, motivated to improve the error performance of C‐GSM schemes, first, this study proposes a new scheme called Golden codeword‐based generalized spatial modulation (GCW‐GSM), which combines C‐GSM and the Golden code to take advantage of the benefits of both while avoiding their drawbacks. The proposed scheme uses conventional quadrature amplitude modulation (C‐QAM), and it is investigated over Rayleigh frequency‐flat fading channels with additive white Gaussian noise. Second, a closed‐form expression of the theoretical average bit error probability of the proposed GCW‐GSM scheme is formulated. Compared to simulation results, it proves to be increasingly tight at high signal‐to‐noise ratio values. Finally, the proposed scheme performance over the Rayleigh‐frequency‐flat fading channel is compared to that over the Rician channel. An improvement in the error performance of 6dB with spectral efficiency of 10bits/s/Hz is seen in 4×4 C‐16QAM GCW‐GSM over 4×4 C‐256QAM conventional‐spatial modulation and 4×4 C‐256QAM C‐GSM schemes at a bit error rate (BER) of 1×10−5. The 4×4 C‐16QAM GCW‐GSM scheme exhibits an improvement in the error performance of 2.9dB with spectral efficiency of 10bits/s/Hz over the 4×4 C‐64QAM conventional quadrature spatial modulation scheme at a BER of 1×10−5. Finally, the 4×4 16‐QAM GCW‐GSM under Rayleigh fading has a performance gain of 3 dB over 4×4 16‐QAM GCW‐GSM under Rician fading with a Rician K factor of 3. [ABSTRACT FROM AUTHOR]

Channel estimation plays a crucial role in intelligent reflecting surface (IRS)-aided communication systems, in which the channel state information must be acquired to configure the optimal IRS reflection coefficient. However, using large numbers of IRS elements induce large channel estimation overheads, which cause crucial problems in systems utilizing IRSs. In this study, we investigate the optimal pilot interval design for IRS-aided communication systems to reduce the channel estimation overheads. The channel coherent time depends on user equipment (UE) velocity. Therefore, the proposed method aims to find a pilot interval that balances accuracy of the IRS reflection coefficient with system throughput by considering UE velocity. In the proposed method, we identify the relationship between the mobility of the UE and complex spatial correlation of the channel matrix, and the transmission capacity in a time slot is formulated to maximize the achievable sum rate. Numerical results from simulations show that the proposed method adaptively adjusts the pilot interval under various communication environments. [ABSTRACT FROM AUTHOR]

Covert communication can hide the legitimate transmission from unauthorized eavesdropping. Benefiting from the deployment flexibility, unmanned aerial vehicles (UAVs) can be utilized to enhance communication confidentiality. In this correspondence, we consider a covert communication network with the aid of a full-duplex UAV relay, which is employed to help the transmission and confuse the warden. The warden adopts a radiometer to detect the covert transmission. We first find the optimal detection threshold and calculate the minimum detection error probability. Furthermore, a closed-form expression of outage probability via UAV relaying is derived over Rician fading. Then, a power optimization problem is formulated to maximize the effective convert throughput with covertness constraint. Numerical results illustrate that the cooperative jamming can disrupt the warden, and the optimal power tradeoff can guarantee the covert transmission effectively. [ABSTRACT FROM AUTHOR]

In this paper, an approximate closed-form probability density function expression for the Lognormal-Rician atmospheric turbulence with generalized pointing errors model is developed by using a series representation. The Kolmogorov-Smirnov (KS) goodness-of-fit statistical tool is employed to verify the accuracy of the proposed approximation. We demonstrate that a more efficient approximation can be achieved for the smaller jitter variances and nonzero boresight displacements. To reveal the importance of the proposed approximation, the approximate analytic expressions for the ergodic capacity and outage performance of single-input single-output free-space optical (FSO) systems are derived. It is observed that the pointing errors significantly degrade the performance of FSO links. By using the obtained asymptotic expressions, the impact of pointing errors on the ergodic capacity is considerably mitigated by selecting the optimum beam width at the transmitter side. We find that the optimum beam width is independent of atmospheric turbulence and the Pearson correlation coefficient in high signal-to-noise ratio conditions. All of the derived analytical results are confirmed by Monte Carlo simulation results. [ABSTRACT FROM AUTHOR]

Superdirective arrays have been extensively studied because of their considerable potential accompanied, unfortunately, by a high sensitivity to random errors that affect the responses and positions of array elements. However, the statistics of their actual beam pattern (BP) has never been systematically investigated. This paper shows that the Rician probability density function (PDF), sometimes adopted to study the impact of errors in conventional arrays, is a valid approximation for superdirective BP statistics only where some mathematical terms are negligible. The paper also shows that this is the case for all linear end-fire arrays considered. A similar study is proposed concerning the correlation between BP lobes, showing that for the superdirective arrays considered the lobes, especially non-adjacent ones, are almost independent. Furthermore, knowledge of the PDF of the actual BP allows one to define quantile BP functions, whose probability of being exceeded, at any point, is fixed. Combining the lobes’ independence with quantile BP functions, an empirical equation for the probability that the entire actual BP will not exceed a quantile function over an interval larger than a given size is obtained. This new knowledge and these tools make it possible to devise new methods to design robust superdirective arrays via optimization goals with clearer and more relevant statistical meaning. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RIS) have been actively researched as a potential technique for future wireless communications, which intelligently ameliorate the signal propagation environment. In the conventional design, each RIS element configures and reflects its received signal independently of all other RIS elements, which results in a diagonal phase shift matrix. By contrast, we propose a novel RIS architecture, where the incident signal impinging on one element can be reflected from another element after an appropriate phase shift adjustment, which increases the flexibility in the design of RIS phase shifts, hence, potentially improving the system performance. The resultant RIS phase shift matrix also has off-diagonal elements, as opposed to the pure diagonal structure of the conventional design. Compared to the state-of-art fully-connected/group-connected RIS structures, our proposed RIS architecture has lower complexity, while attaining a higher channel gain than the group-connected RIS structure, and approaching that of the fully-connected RIS structure. We formulate and solve the problem of maximizing the achievable rate of our proposed RIS architecture by jointly optimizing the transmit beamforming and the non-diagonal phase shift matrix based on alternating optimization and semi-define relaxation (SDR) methods. Moreover, the closed-form expressions of the channel gain, the outage probability and bit error ratio (BER) are derived. Simulation results demonstrate that our proposed RIS architecture results in an improved performance in terms of the achievable rate compared to the conventional architecture, both in single-user as well as in multi-user scenarios. [ABSTRACT FROM AUTHOR]

This paper investigates the outage performance of simultaneous wireless information and power transfer (SWIPT)-enabled relay networks with the decode-and-forward relaying protocol, where the effect of the energy triggering threshold at the relay on the system performance is considered. The closed-form expressions of the system outage probability and throughput are derived in Rician channel fading. Monte Carlo Simulation method is used to verify the accuracy of the derived closed-form expressions. The effects of some system parameters on the system performances are discussed via simulations, which show that the system outage probability increases with the increase of the minimum transmission rate required by the users and also decreases with the increase of the energy conversion efficiency. Besides, the system throughput increaseswith the increment of the transmit power of the source node, as well as the energy conversion efficiency. Additionally, the outage performance of the system with the equal two-hop distance is better than that of the system with unequal two-hop distance. [ABSTRACT FROM AUTHOR]

In this paper, we consider an imperfect hardware hybrid satellite-terrestrial network, where the satellite communication with a ground user equipment (UE) is aided by the multiple amplify-and-forward 3-D mobile unmanned aerial vehicle (UAV) relays. Herein, we consider that all transceiver nodes are corrupted by the radio frequency hardware impairments (RFHI). Furthermore, a stochastic mixed mobility model is employed to characterize the instantaneous location of 3-D mobile UAV relays in a cylindrical cell with UE lying at its center on the ground plane. Taking into account the aggregate RFHI model for satellite and UAV relay transceivers and the random 3-D distance-based path loss for UAV relay–UE links, we investigate the outage probability and corresponding asymptotic outage behavior of the system under an opportunistic relay selection scheme in a unified form for shadowed-Rician satellite links’ channels and Nakagami-m as well as Rician terrestrial links’ channels. We corroborate theoretical analysis by simulations. [ABSTRACT FROM AUTHOR]

In this article, the achievable sum rate and the energy efficiency (EE) are investigated for a multiuser relay-aided massive multiple-input–multiple-output (MIMO) downlink. Low-resolution digital-to-analog converters (DACs) are equipped at both the base station (BS) and the relay station (RS), and the amplify-and-forward protocol is adopted at the RS. Under the Rician fading channel, closed-form approximate expressions for the achievable sum rate are derived with perfect and imperfect channel state information. A more general power law is extracted to save transmit power without reducing the achievable sum rate, and a local optimal power allocation scheme is proposed to improve the channel capacity of active users. Then, the tradeoff between the achievable sum rate and the EE is discussed. The numerical results show that, due to the quantization noise, the transmit power has a limited increase in the achievable rate. In addition, it is more valuable to improve the DAC quantization bit at the RS when the number of BS antennas is larger than that of RS antennas. In addition, under strong line-of-sight channels or high pilot transmit power conditions, the channel estimation accuracy is higher, and the best tradeoff between the achievable sum rate and the EE can be obtained when the DAC quantization bit is 4. [ABSTRACT FROM AUTHOR]

In this article, we study a dual-hop mixed power line communication and radio-frequency communication (PLC/RF) system, where the connection between the PLC link and the RF link is made by a decode-and-forward or amplify-and-forward relay. It is assumed that the PLC channel is affected by both additive background noise and Poisson–Gaussian impulsive noise which suffer from log-normal fading, while the RF link undergoes generalized Rician fading. Based on this model, analytical expressions for the outage probability (OP), average bit error rate (BER), and the average channel capacity (ACC) are derived. Furthermore, asymptotic analyses for the OP and average BER as well as an upper bound expression for the ACC are presented. Illustrative numerical results are presented along with insightful discussions. [ABSTRACT FROM AUTHOR]

The impacts of channel estimation errors, inter-cell interference, phase adjustment cost, and computation cost on an intelligent reflecting surface (IRS)-assisted system are severe in practice but have been ignored for simplicity in most existing works. In this paper, we investigate a multi-antenna base station (BS) serving a single-antenna user with the help of a multi-element IRS in a multi-cell network with inter-cell interference. We consider imperfect channel state information (CSI) at the BS, i.e., imperfect CSIT, and focus on the robust optimization of the BS’s instantaneous CSI-adaptive beamforming and the IRS’s quasi-static phase shifts in two scenarios. In the scenario of coding over many slots, we formulate a robust optimization problem to maximize the user’s ergodic rate. In the scenario of coding within each slot, we formulate a robust optimization problem to maximize the user’s average goodput under the successful transmission probability constraints. The robust optimization problems are challenging two-timescale stochastic non-convex problems. In both scenarios, we obtain closed-form robust beamforming designs for any given phase shifts and more tractable stochastic non-convex approximate problems only for the phase shifts. Besides, we propose an iterative algorithm to obtain a Karush-Kuhn-Tucker (KKT) point of each of the stochastic problems for the phase shifts. It is worth noting that the proposed methods offer closed-form robust instantaneous CSI-adaptive beamforming designs which can promptly adapt to rapid CSI changes over slots and robust quasi-static phase shift designs of low computation and phase adjustment costs in the presence of imperfect CSIT and inter-cell interference. Numerical results further demonstrate the notable gains of the proposed robust joint designs over existing ones and reveal the practical values of the proposed solutions. [ABSTRACT FROM AUTHOR]

With the proliferation of fifth-generation (5G) mobile communication wireless networks, the investigation into the performance of physical layer secrecy is increasingly becoming the center of attention of recent studies. Physical layer security (PLS) is the pivotal notion of enhancing the secrecy of mobile communication wireless networks against eavesdropping by utilizing the intrinsic randomness of the wireless channel. In this study, we focus on the information-theoretic secrecy perspective in which authorized users convey their information to each other through a quasi-static channel and adversary users are obtaining this secret information through illegitimate wiretap quasi-static channel, where it is assumed that all the channels are represented as double shadowed Rician distributed. In this context, analytical solutions for the expressions of various physical layer secrecy metrics include the strictly positive secrecy capacity (SPSC) and the lower bound on secure outage probability (SOPL) are procured in closed-form. In addition, another physical layer secrecy metric, i.e., average secrecy capacity (ASC) is also investigated and determined in analytical closed-form. The effect of double shadowing on the performance of PLS is investigated. It is found that severer shadowing improves the secrecy performance. Our results also show that the legitimate users can communicate secretly when the legitimate channel link is superior to illegitimate channel link. [ABSTRACT FROM AUTHOR]

Channel estimation (CE) is a crucial phase in wireless communication systems, especially in cell-free (CF) massive multiple input multiple output (M-MIMO) since it is a dynamic wireless network. Therefore, this work is introduced to study CE for the CF M-MIMO system in the uplink phase, wherein the performance of different estimators are evaluated, discussed, and compared in various situations. We assume the scenario in which each access point has prior knowledge of the channel statistics. The phase-aware-minimum mean square error (PA-MMSE) estimator, the non-phase-aware-MMSE (NPA-MMSE) estimator, and the least-squares estimator are the three estimators which are exploited in this work. Besides, we consider the Rician fading channel in which the line-of-sight path is realized with a phase-shift that models the users' mobility where the considered phase-shift follows a uniform distribution. On the other hand, the mean-squared error metric is employed in order to evaluate the performance of each estimator, where an analytical and simulated result is provided for the PA-MMSE estimator and the NPA-MMSE estimator in order to assert our numerical results. [ABSTRACT FROM AUTHOR]

Intelligent omni surface (IOS) technology that can provide both reflection and refraction for signals impinging on its surface, was recently proposed to overcome the coverage area limitation of reconfigurable intelligent surfaces (RISs). This letter investigates the efficacy of IOS in the context of physical layer security (PLS) over wireless channels. In particular, we consider a scenario where the IOS is utilized to enhance the secrecy performance of a legitimate receiver in the presence of a multi-antenna eavesdropper. In addition, artificial noise (AN) aided beamforming is implemented to provide additional security robustness. The resultant optimization problem is non-convex and difficult to solve. Accordingly, the block coordinate descent (BCD) optimization approach is adopted, and the Lagrangian dual method is applied to reduce the complexity of the AN-aided beamforming design. Furthermore, the reflecting and refracting phase shifts are optimized via the quadratically constrained quadratic programming (QCQP) method. Simulation results validate the efficacy of the proposed algorithm and confirm the superiority of IOS over traditional RIS. [ABSTRACT FROM AUTHOR]

Optimizing the current distribution of an evenly spaced antenna array has shown to be an efficient approach for reducing side lobe levels. In this article, the Tchebyscheff distribution-based antenna array synthesis approach is combined with an adaptive signal processing algorithm for beamforming and side lobe level reduction in smart antennas in various fading situations. The performance of smart antennas in uniformly spaced linear, planar, circular, and semi-circular arrays are evaluated. The presence of Rayleigh and Rician channels is examined in the network. The least mean square (LMS) and normalised least mean square (NLMS) algorithms are applied as adaptive algorithms. In fading environments, the NLMS algorithm with Tchebyscheff distribution outperforms than the LMS algorithm with Tchebyscheff distribution, with a side lobe level decrease of 11.23 dB. The lowest side lobe achieved with the NLMS algorithm with Tchebyscheff distribution is − 45.59 dB for uniform planar array. [ABSTRACT FROM AUTHOR]

This paper considers an uplink reconfigurable intelligent surface (RIS)-aided massive multiple-input multiple-output (MIMO) system, where the phase shifts of the RIS are designed relying on statistical channel state information (CSI). Considering the complex environment, the general Rician channel model is adopted for both the users-RIS links and RIS-BS links. We first derive the closed-form approximate expressions for the achievable rate which holds for arbitrary numbers of base station (BS) antennas and RIS elements. Then, we utilize the derived expressions to provide some insights, including the asymptotic rate performance, the power scaling laws, and the impacts of various system parameters on the achievable rate. We also tackle the sum-rate maximization and the minimum user rate maximization problems by optimizing the phase shifts at the RIS based on genetic algorithm (GA). Finally, extensive simulations are provided to validate the benefits by integrating RIS into conventional massive MIMO systems. Our simulations also demonstrate the feasibility of deploying large-size but low-resolution RIS in massive MIMO systems. [ABSTRACT FROM AUTHOR]

This letter investigates the influence of the number of repetitions adopted in the NB-IoT standard to increase cellular coverage. We characterize the SINR in both uplink and downlink channels of ultra-dense networks, which is then used to derive the coverage probability and the NB-IoT’s capacity. The performance results show the effectiveness of the number of repetitions to reduce the block error rate for different densities of transmitters. [ABSTRACT FROM AUTHOR]

Considering limited pilot and storage resources in satellite communications, we propose an efficient correlation-based reception scheme consisting of a “parallel” frequency-phase estimation (PFPE) mode and a data blockwise compensation (DBC) scheme. In specific, the PFPE mode provides an autocorrelation (AC) frequency offset estimator and a maximum-likelihood (ML) phase offset estimator using a special but heuristic AC value; the DBC scheme uses a small amount of cross-correlation (CC) based compensation values to eliminate the impact of residual offsets on each data block. Simulation results show that with a small pilot overhead the proposed reception scheme can weaken “frequency-to-phase coupling” and achieve good error performance with a small demand for storage. [ABSTRACT FROM AUTHOR]

This paper investigates the secure transmission in downlink cell-free massive multiple-input multiple-output (MIMO) systems in the presence of an active multi-antenna eavesdropper (Eve) over Rician fading channels, assuming that each access point (AP) possesses multiple antennas which are connected with low-resolution digital-to-analog converters (DACs). Closed-form expressions of the achievable secrecy rate relied on the additive quantization noise model are derived. Based on these analytical results, we quantify the impacts of key system parameters, such as the antenna array number, DAC resolution, Rician $\mathcal K$ -factor, and balance factor between data and artificial noise power on secrecy enhancement. Several interesting insights are attained by assuming that Eve can or cannot perfectly remove inter-mobile-terminal interference. Moreover, we also propose a power control algorithm that maximizes the achievable secrecy rate, which can be represented as a series of second-order-cone programs for which efficient solvers exist. All the theoretical analyses and the effectiveness of the proposed algorithm are corroborated by simulation experiments. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the performance of an RIS-aided wireless communication system subject to outdated channel state information that may operate in both the near- and far-field regions. In particular, we take two RIS deployment strategies into consideration: (i) the centralized deployment, where all the reflecting elements are installed on a single RIS and (ii) the distributed deployment, where the same number of reflecting elements are placed on multiple RISs. For both deployment strategies, we derive accurate closed-form approximations for the ergodic capacity, and we introduce tight upper and lower bounds for the ergodic capacity to obtain useful design insights. From this analysis, we unveil that an increase of the transmit power, the Rician- $K$ factor, the accuracy of the channel state information and the number of reflecting elements help improve the system performance. Moreover, we prove that the centralized RIS-aided deployment may achieve a higher ergodic capacity as compared with the distributed RIS-aided deployment when the RIS is located near the base station or near the user. In different setups, on the other hand, we prove that the distributed deployment outperforms the centralized deployment. Finally, the analytical results are verified by using Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Intelligent reflecting surface (IRS) has emerged as a promising paradigm to improve the capacity and reliability of a wireless communication system by smartly reconfiguring the wireless propagation environment. To achieve the promising gains of IRS, the acquisition of the channel state information (CSI) is essential, which however is practically difficult since the IRS does not employ any transmit/receive radio frequency (RF) chains in general and it has limited signal processing capability. In this paper, we study the uplink channel estimation problem for an IRS-aided multiuser single-input multi-output (SIMO) system. The existing channel estimation approach for IRS-aided multiuser systems mainly consists of three phases, where the direct channels from the base station (BS) to all the users, the reflected channel from the BS to a typical user via the IRS, and the other reflected channels are estimated sequentially based on the estimation results of the previous phases. However, this approach will lead to a serious error propagation issue, i.e., the channel estimation errors in the first and second phases will deteriorate the estimation performance in the second and third phases. To resolve this difficulty, we propose a novel two-phase channel estimation (2PCE) strategy which is able to alleviate the negative effects caused by error propagation and enhance the channel estimation performance with the same amount of channel training overhead as in the existing approach. Specifically, in the first phase, the direct and reflected channels associated with a typical user are estimated simultaneously by varying the reflection patterns at the IRS, such that the estimation errors of the direct channel associated with this typical user will not affect the estimation of the corresponding reflected channel. In the second phase, we estimate the CSI associated with the other users and demonstrate that by properly designing the pilot symbols of the users and the reflection patterns at the IRS, the direct and reflected channels associated with each user can also be estimated simultaneously, which helps to reduce the error propagation. Moreover, the asymptotic mean squared error (MSE) of the proposed 2PCE strategy is analyzed when the least-square (LS) channel estimation method is employed, and we show that the 2PCE strategy can outperform the existing approach. Finally, extensive simulation results are presented to validate the effectiveness of our proposed channel estimation strategy. [ABSTRACT FROM AUTHOR]

The Rician ${K}$ -factor has been extensively used to characterize indoor and outdoor wireless propagation channels based on experimental and analytical models. In this paper, the $K$ -factor statistics are derived analytically and experimentally validated for LOS and NLOS scenarios. Receiver operating characteristics (ROC) are formulated based on the derived detection statistics such that an optimal decision threshold can be selected to better enhance LOS/NLOS detection accuracy compared to usual methods. The proposed approach is shown to have a greater LOS/NLOS discrimination accuracy than other state-of-the-art approaches. Furthermore, there is evidence that a second metric can be obtained which characterizes the current environment and provides a confidence value, which can be used to inform further decisions. [ABSTRACT FROM AUTHOR]

Wireless networks have been widely utilized in industries, where wireless links are challenged by the severe nonstationary Rician fading channel, which requires online link quality estimation to support high-quality wireless services. However, most traditional Rician estimation approaches are designed for channel measurements and work only with nonmodulated symbols. Then, the online Rician estimation usually requires a priori aiding pilots or known modulation order to cancel the modulation interference. This article proposes a nondata-aided method with redundant Gaussian mixture model (GMM). The convergence paradigm of GMM with redundant subcomponents has been analyzed, guided by which the redundant subcomponents can be iteratively discriminated to approach the global optimization. By further adopting the constellation constraint, the probability to identify the redundant subcomponent is significantly increased. As a result, accurate estimation of the Rician parameters can be achieved without additional overhead. Experiments illustrate not only the feasibility but also the near-optimal accuracy. [ABSTRACT FROM AUTHOR]

The 5G cellular network supports massive Machine Type Communication (mMTC) for Wireless Sensor Network (WSN) application. In this paper, High Altitude Platforms (HAPs) is used as a replacement for Base Station (BS). So that the cluster head (CH) from every cluster will send information owned to the HAPs by using the Power Domain Non-Orthogonal Multiple Access (PD NOMA) as a multiple access technique. PD NOMA uses the Successive Interference Cancellation (SIC) technique on the receiver side. SIC process is proven effective for detecting PD NOMA signal by sorting the received signal strength and then decoding it. However, error from the prioritized signal that has high decoding has a tremendous impact on the prioritized signal that has a way lower decoding, and this error can then further spread with the SIC process. In this paper, we propose a Convolutional Neural Network (CNN) approach to decode information from multiple CH without performing traditional communication signal processing. The simulation is already done by the Rician channel with 11 CH that is connected to the HAP. From the series of simulations that have been done, we can see that the CNN used to replace the conventional SIC on the uplink PD NOMA can detect NOMA signals without the use of conventional signal processing. The CH node nearest to the HAP requires a lower SNR than the CH node farthest from the HAP to achieve BER = 10-4 in both conventional uplink PD NOMA and uplink PD NOMA with CNN. Uplink PD NOMA with CNN has a lower complexity than conventional uplink PD NOMA. [ABSTRACT FROM AUTHOR]

In this paper, we present unified performance analysis framework of distributed transmit beamforming (DTB) with residual phase errors which arise from various sources such as imperfect synchronization and channel feedback quantization. More specifically, we derive statistics of the receive signal-to-noise ratio of DTB in Rayleigh and Rician fading channels in the presence of residual phase errors. In the case of residual phase errors caused by channel feedback quantization, we show that the distribution of residual phase errors can be modeled by uniform distribution in Rayleigh fading channels and Gaussian distribution in Rician fading channels. Also, our results reveal that a small number of bits are sufficient for quantized feedback to achieve comparable performance to DTB with perfect channel feedback. Finally, capitalizing on the derived statistics, we present closed-form expressions for the outage probability and the ergodic capacity of DTB with quantized channel feedback. [ABSTRACT FROM AUTHOR]

We introduce a comprehensive statistical characterization of the multipath wireless channel built as a superposition of scattered waves with random phases. We consider an arbitrary number $N$ of specular (dominant) components plus an undetermined number of other weak diffusely propagating waves. Our approach covers the cases in which the specular components have constant amplitudes, as well as when these components experience random fluctuations. We show that this class of fading models can be expressed in terms of a continuous mixture of an underlying Rician (or Rician shadowed) fading model, averaged over the phase distributions of the specular waves. The proposed model parameters can be adjusted to tailor the statistical distribution of the received radio signal power to a wide variety of wireless scenarios, some of which are not covered by other state-of-the-art stochastic wireless channel models. In this regard, we verify that the proposed models accurately fit experimental measurements for which their multi-modality cannot be properly captured by other current stochastic models. It is shown that the fluctuations of the specular components have a detrimental impact on performance, and it is formally demonstrated that the lower error rate is obtained when the signal power is concentrated on a single specular component, regardless of whether it fluctuates or not. [ABSTRACT FROM AUTHOR]

Supporting reliable and seamless wireless connectivity for unmanned aerial vehicles (UAVs) has recently become a critical requirement to enable various different use cases of UAVs. Due to their widespread deployment footprint, cellular networks can support beyond visual line of sight (BVLOS) communications for UAVs. In this paper, we consider cellular connected UAVs (C-UAVs) that are served by massive multiple-input-multiple-output (MIMO) links to extend coverage range, while also improving physical layer security and authentication. We consider Rician channel and propose a novel linear precoder design for transmitting data and artificial noise (AN). We derive the closed-form expression of the ergodic secrecy rate of C-UAVs for both conventional and proposed precoder designs. In addition, we obtain the optimal power splitting factor that divides the power between data and AN by asymptotic analysis. Then, we apply the proposed precoder design in the fingerprint embedding authentication framework, where the goal is to minimize the probability of detection of the authentication tag at an eavesdropper. In simulation results, we show the superiority of the proposed precoder in both secrecy rate and the authentication probability considering moderate and large number of antenna massive MIMO scenarios. [ABSTRACT FROM AUTHOR]

This work focuses on the impact of finite-blocklength (FBL) regimes on performance of hybrid automatic repeat request (HARQ) protocols under the proposed conditions of outdated channel state information (oCSI) severity, generalised-Rician fading and maximal-ratio-combining deployment. The outage probability (OP)|defined as the probability of erroneous HARQ short-packet transmissions and the OP is a key performance indicator in this work|will be derived in compact form under the proposed conditions. Compact expressions are obtained, which depict the OP under these practical conditions and present certain trade-off among latency, blocklength and the OP. A brief review of existing results is performed. Penalty of increasing information capacity and the dominance of system coherence are shown, which justifies this work. [ABSTRACT FROM AUTHOR]

This paper deals with the secrecy performance analysis of a multicast network over mixed fading scenarios in which a cluster of passive eavesdroppers is trying to overhear the secret transmission. Our key contribution is to prevent this malicious attack of the illegitimate receivers. Rayleigh/ Rician mixed fading channels are considered to model alternately the multicast/ eavesdropper and eavesdropper/ multicast channels as such mixed fading scenarios are often encountered in cellular communication where only one link (either multicast or eavesdropper) undergo a line-of-sight propagation path. At first, we derive the probability density functions for the single-input-multiple-output multicast scenarios and then the secrecy analysis is carried out by obtaining closed-form expressions for the performance matrices such as the probability of non-zero secrecy multicast capacity, ergodic secrecy multicast capacity, and secure outage probability for multicasting. The derived expressions are beneficial to investigate how the antenna diversity can combat the detrimental impact of fading as well as the number of multicast users and eavesdroppers, and improve the secrecy level to the acceptable limit. Moreover, the best secure scenario in terms of the secrecy parameters is obtained when the multicast channels undergo Rician fading whereas the eavesdropper channels experience Rayleigh fading. Finally, the analytical expressions are justified via the Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]