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This paper presents a comparative performance analysis of multiuser multirelay two-way and one-way amplify-and-forward relaying systems employing opportunistic transmission in the presence of channel estimation error and hardware impairments over Rayleigh fading channels. Considering such a practical scenario, we first deduce instantaneous end-to-end signal-to-noise-plus-distortion-and-error ratios at the end-user terminals. Then, we derive exact closed-form expressions of outage probability (OP) and expected spectral efficiency for both the systems. Further, we obtain simple asymptotic expressions at the high signal-to-noise ratio (SNR) regime, which facilitate the characterization of achievable diversity order of the systems. Thereafter, we validate our derived analytical expressions with the simulation results, which are sufficiently tight across the entire range of SNRs. Eventually, we optimize power allocation in order to minimize OP of the systems. Numerical results reveal the improvement in system outage when power allocation is in accordance with its optimal value. Findings of the paper suggest that full diversity order can be achieved only when the radio frequency front-end hardware is assumed to be perfect, while in practice the imperfections in hardware are inevitable and will reduce system diversity. Moreover, we demonstrate the influence of key parameters on the performance of the systems. [ABSTRACT FROM AUTHOR]

In this paper, we investigate security and energy efficiency of multicast communication in cognitive satellite terrestrial networks. To achieve high spectrum efficiency and low interference power transmissions in the cognitive terrestrial network, the rate splitting multiple access scheme (RSMA) is employed to prompt the massive access of the secondary users. In particular, we derive analytical and asymptotic closed-form expressions for secrecy outage probability of the satellite terrestrial network with eavesdropping, interfering, and imperfect channel state information. Hence, a beamforming (BF) scheme based on RSMA for secondary network is proposed to suppress eavesdropping, mitigate interference, and enhance energy efficiency of the proposed system model. Then, we transform non-convex optimization constraints into resoluble problems utilizing Taylor series expansion and successive convex approximation. Finally, simulation results corroborate the above theoretical analysis and highlight the superiority of the designed BF scheme on security and energy efficiency. [ABSTRACT FROM AUTHOR]

Thanks to the short wavelength at high frequency (HF), it is promising to combine massive multiple-input and multiple-output (MIMO) with HF communications. In particular, HF massive MIMO differs from the conventional MIMO in array structure and radio propagation. In this paper, we investigate the HF massive MIMO system with a compact planar array (CPA) deployed at the base station (BS) in the presence of electromagnetic mutual coupling (MC) for the asymptotic case under unlimited antenna elements assumption. Specifically, the HF sparse channel is analyzed asymptotically to show that the randomness characteristic of channel decreases in the asymptotic regime, which is conducive to the design of communication systems. Moreover, on the basis of the asymptotic analysis results, the optimal beamforming to maximize the minimum user signal-to-interference-plus-noise ratio (SINR) is derived by applying random matrix theory. Numerical results verify the accuracy of the asymptotic analysis, indicate the performance superiority of the proposed scheme to the traditional minimum mean squared error (MMSE) and maximum ratio transmission (MRT) schemes, and reveal the robustness of the proposal to electromagnetic MC. [ABSTRACT FROM AUTHOR]

Since optical links are easily blocked by obstacles in the environment, they have been considered difficult to directly perform wireless communications. However, reconfigurable intelligent surfaces (RISs), as a new type of digital coding meta-materials, can significantly improve the coverage of optical communication by establishing new links. We propose a controllable multi-branch wireless optical communication system based on optical RISs. By setting up multiple optical RISs in the environment, multiple artificial channels are built to improve the system performance and to reduce the outage probability. In this paper, we investigate three factors affecting channel coefficients, including beam jitter, RIS jitter, and obstruction probability. We derive closed-form probability density function of channel coefficients, asymptotic average bit-error rate (BER), and the outage probability for systems with multiple branches. Our analysis reveals that the probability density function contains an impulse function, which causes irreducible error rate and outage probability floors. Based on our numerical results, the BER and outage probability floor of the multi-branch system are significantly reduced compared with the single direct path system. Therefore, the optical RIS assisted multi-branch wireless communication is a promising solution against obstacles of optical channel. [ABSTRACT FROM AUTHOR]

This paper proposes an amplify-and-forward-based massive multiple-input–multiple-output full-duplex relaying (AF-MM-FDR) protocol, where multiple source–destination pairs communicate simultaneously with a common full-duplex relay. A zero-forcing (ZF)-based relay transceiver design with semiblind gain control is presented, considering the problem of relay oscillation. The asymptotic expression of end-to-end signal-to-interference-plus-noise power (SINR) is derived under the general power scaling scheme. The scaling behaviors of echo interference (EI) and effective channel estimation error are determined. Scaling results show that the deployment of very large antenna arrays at the relay has the potential to eliminate the effect of EI due to the full-duplex operation, if the power scaling scheme is selected properly. Moreover, based on the asymptotic analysis, a low-complexity power control scheme is proposed to optimize the spectral efficiency of AF-MM-FDR. Simulation results show that the proposed power control scheme improves the spectral–energy efficiency tradeoff of AF-MM-FDR significantly. [ABSTRACT FROM AUTHOR]

Energy efficiency and its optimization constitute critical tasks in the design of low-power wireless networks. This paper is devoted to the error rate analysis and energy efficiency optimization of regenerative cooperative networks in the presence of multipath fading under spatial correlation. To this end, exact and asymptotic analytic expressions are first derived for the symbol error rate (SER) of $M$-ary quadrature amplitude and $M$-ary phase-shift keying modulations ( $M$-QAM and $M$- PSK), respectively, assuming a dual-hop decode-and-forward (DF) relay system, spatially correlated Nakagami- $m$ multipath fading, and maximum ratio combining (MRC) at the destination. The derived expressions are subsequently employed in quantifying the energy consumption of the considered system, incorporating both transmit energy and the energy consumed by the transceiver circuits, and in deriving the optimal power allocation (OPA) formulation for minimizing energy consumption under certain quality-of-service (QoS) requirements. A relatively harsh path-loss (PL) model, which also accounts for realistic device-to-device communications, is adopted in numerical evaluations, and various useful insights are provided for the design of future low-energy wireless networks deployments. Indicatively, it is shown that, depending on the degree of spatial correlation, severity of fading, transmission distance, relay location, and power allocation strategy, target performance can be achieved with much overall energy reduction compared with direct transmission (DT) reference. [ABSTRACT FROM AUTHOR]

This paper studies the channel estimation performance in massive multiple-input multiple-output (MIMO) systems with Ricean fading through considering the relative channel estimation error (RCEE) metric. The closed-form expressions of the probability distribution function (PDF) and cumulative distribution function of RCEE are derived, according to which the PDFs of RCEE with Rayleigh fading and pure line-of-sight (LoS) propagation are obtained as special cases. Through analyzing RCEE, it notes that as the number of base-station antennas M becomes unbounded, RCEE approaches a given constant. Moreover, the asymptotic analysis proves that, in high M-regime, the increasing rate of the RCEE scales in the order of √M for Rayleigh fading and pure LoS propagation, while the scaling order with general Ricean fading is capped by √M. All the above mentioned results are verified through Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel relay selection scheme for orthogonal frequency-division multiplexing systems by combining conventional bulk and per-subcarrier selection schemes and analyze its outage performance over equally spatially correlated channels. Specifically, the combined selection scheme selects only two relays at the first attempt and performs per-subcarrier selection over these two relays. We analyze the asymptotic outage performance of the combined selection scheme in the high signal-to-noise ratio (SNR) region and prove a generalized theorem. This theorem states that the combined selection can achieve an optimal outage probability equivalent to the per-subcarrier selection at a high SNR without using the full set of available relays for selection. This unique property is termed the equivalence principle, and it holds for all correlation conditions. To explore this principle, we consider three examples: decode-and-forward, fixed-gain amplify-and-forward (AF), and variable-gain AF relay systems. Furthermore, two extended applications, i.e., antenna selection and branch selection, are also considered to reveal the feasibility and the expandability of the equivalence principle. Our analysis is verified by Monte Carlo simulations. The proposed combined selection and the proved theorem provide a general and feasible solution to the tradeoff between system complexity and outage performance when relay selection is applied. [ABSTRACT FROM AUTHOR]

In this paper, we derive closed-form exact and asymptotic expressions for the symbol error rate (SER) and channel capacity when communicating over Fox's $H$-function fading channel. The SER expressions are obtained for numerous practically employed modulation schemes in case of single-branch and three multiple-branch diversity receivers: maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC). The derived exact expressions are given in terms of the univariate and multivariate Fox's $H$-functions for which we provide a portable and efficient Python code. Since Fox's $H$-function fading channel represents the most generalized fading model ever presented in the literature, the derived expressions subsume most of those previously presented for all the known simple and composite fading models. Moreover, easy-to-compute asymptotic expansions are provided to easily study the behavior of the SER and channel capacity at high values of the average signal-to-noise ratio (SNR). The asymptotic expansions are also useful in comparing different modulation schemes and receiver diversity combiners. Numerical and simulation results are also provided to support the mathematical analysis and prove the validity of the obtained expressions. [ABSTRACT FROM AUTHOR]

Two-stage beamforming is a transmit strategy that uses two types of beamformers to reduce the feedback overhead of frequency-division-duplexing massive multiple-input multiple-output systems that are spatially correlated. In this paper, we present large system analysis of two-stage beamforming when a transmitter has limited channel information from feedback and when regularized-zero-forcing (RZF) is used as a second-stage beamformer. We consider two random-vector-quantization-based feedback schemes that can be respectively applied when users have perfect channel information or perfect effective channel information. For each feedback method, we analyze both expected signal-to-interference-plus-noise ratio (SINR) and expected rate loss and then characterize their bounds as a function of the number of feedback bits. From the characterization, we reveal that the sum rate of two-stage beamforming is very sensitive to the regularization parameter of RZF, especially when the number of feedback bits is limited. Motivated by this, we derive the optimal regularization parameter that maximizes the SINR of two-stage beamforming with limited feedback. Using simulations, we verify the tightness of the characterized bounds and quantify how the use of the optimal regularization parameter improves the sum rate of two-stage beamforming. [ABSTRACT FROM AUTHOR]

Three low-complexity relay-selection strategies, namely, selective amplify and forward (S-AF), selective decode and forward (S-DF), and amplify and forward with partial relay selection (PRS-AF) in a spectrum-sharing scenario are studied. First, we consider a scenario where perfect channel state information (CSI) is available. For these scenarios, the respective asymptotic outage behaviors of the secondary systems are analyzed, from which the diversity and coding gains are derived and compared. Unlike the coding gain, which is shown to be very sensitive with the position of the primary receiver, the diversity gain of the secondary system is the same as the nonspectrum-sharing system. In addition, depending on the cooperative strategy employed, an increase in the number of relays may lead to severe loss of the coding gain. Afterwards, the impacts of imperfect CSI regarding the interference and transmit channels on the outage behavior of the secondary systems are analyzed. On one hand, the imperfect CSI concerning the interference channels only affects the outage performance of the primary system, whereas it has no effect on the diversity gain of the secondary system. On the other hand, the imperfect CSI concerning the transmit channels of the secondary systems may reduce the diversity gain of the three relay-selection strategies to unity, which is validated by both theoretical and numerical results. [ABSTRACT FROM PUBLISHER]

In this correspondence, we analyze the effects of radio frequency impairments and channel estimation error in a cellular decode-and-forward relay network, where multiple antennas are deployed at the base station (BS). Here, a single relay that has the best end-to-end signal-to-noise-plus-distortion-and-error ratio is selected among the available relays, which decode the BS information correctly in each transmission session. Furthermore, we derive an exact closed-form expression for outage probability and expected spectral efficiency in multiuser and multirelay scenario. In addition, simple asymptotic expression at high signal-to-noise ratio (SNR) regime is also obtained, which facilitates the characterization of the achievable diversity order of the practical cellular relay network. To validate the derived analytical expressions, we have presented simulation results, which are sufficiently tight across the entire range of SNRs. Finally, we examine the diversity order for the ideal as well as practical relaying systems. [ABSTRACT FROM AUTHOR]

In this correspondence, we propose an efficient hybrid beamforming algorithm for multipair two-way relaying systems with a large number of antenna elements. We first investigate an asymptotic behavior of the fully digital (FD) case where the number of radio frequency (RF) chains equals that of antenna elements. Our analysis reveals that the optimal RF filters can be determined by the array response vectors in the asymptotic region without consideration of the baseband (BB) filters. From this observation, we provide a new RF filter design based on the Gram–Schmidt process. Also, in order to manage interpair interference, we design BB filters based on the weighted sum minimum mean square error for given RF filters. Our simulation results demonstrate that as the number of antenna elements grows, our method achieves the optimal FD performance with much reduced computational complexity. [ABSTRACT FROM AUTHOR]