Maximal Ratio Combining Research Articles

Secrecy Outage Performance of Cognitive Radio Network with Selection Combining at Eavesdropper

Background: Based on the idea of cooperative communication, recently a lot of attention has been drawn to cooperative spectrum access for the secure information transmission in a Cognitive Radio Network (CRN). Security is one of the most important aspects of these networks, as due to their open and dynamic nature, they are extremely vulnerable to malicious behavior. Cooperative cognitive radio has emerged as a dynamic spectrum access technique, where an unlicensed (secondary) user is allowed to simultaneously access the licensed channels dedicated to a Primary User (PU), as long as the Quality of Service (QoS) of primary communication is not affected. Method: This paper investigates the secrecy outage performance of threshold-based cognitive decode-andforward relay network, with interference constraints from primary licensed user. Threshold-based relaying is considered where; the source message is successfully decoded by the relay, only if the received SNR satisfies the particular threshold. Outage probability expressions have been derived for the worst-case scenario, where only the eavesdropper can achieve the advantage of diversity. The Selection Combining (SC) diversity scheme is employed only at the secondary eavesdropper. Results: The system secrecy performance is better for SC diversity scheme at the eavesdropper than Maximal Ratio Combining (MRC) diversity scheme, as MRC has better diversity performance than SC. We have shown that the improvement in desired secrecy rate, predetermined threshold, eavesdropper channel quality and interference constraints affect the secrecy performance of the cognitive radio system. The outage probability decreases accordingly with an increase in the maximum tolerable interference level at primary destination. The outage probability of Optimal relay Selection (OS) scheme is derived for a multi-relay system, when either the Instantaneous Channel State Information (ICSI) or the Statistical Channel State Information (SCSI) is available. We have shown that the secrecy performance of the OS with ICSI of the system is better than with SCSI. Also, the OS improves the performance of the multi-relay system, when the number of relays is increased. Conclusion: The secrecy outage probability of threshold-based DF underlay cognitive relay network is evaluated. Both interference and maximum transmit power constraints are considered at secondary source and secondary relay. Also, the relay can successfully decode the message, only if it meets the pre-defined threshold. We have investigated the performance of MRC and SC diversity schemes at the secondary eavesdropper and have shown that the system secrecy performance is better for SC than MRC, as MRC has better diversity performance than SC. We have shown that the system secrecy performance is significantly affected by the required secrecy rate, pre-defined threshold, interference constraints and choice of diversity scheme (MRC/SC) at the eavesdropper. The outage probability of OS scheme is derived for a multi-relay system, when either the ICSI or the SCSI is available. We have shown that the secrecy performance of the OS with ICSI of the system is better than with SCSI. Also, the OS improves the performance of the multi-relay system, when the number of relays is increased.

Moment-Generating Function Based Symbol Error Probability Comparison of M-ary Modulation Techniques in Fading Environment

This paper analyses and compare Symbol Error Probability (SEP) of M-ary modulation in different fading environment. Maximal ratio combining (MRC) technique with N branch receiver diversity is taken into consideration for the analysis. Expression for probability of symbol error for M-ary frequency shift keying/M-ary phase shift keying/M-ary differential phase shift key/and M-Level Quadrature Amplitude Modulation are obtained through moment-generating function (MGF) approach. Conventional integration process has probability of uncertainty and erroneousness because of composite mathematical calculations and infinite integration bounds. MGF method is used to avoid complex numerical computation for calculation of SEP. Comparison of SEP for diverse values of Rician factor “K”, Nakagami-M factor, modulation order “M” and diversity order “N” has been carried out.

NOMA in Cooperative Communication Systems With Energy-Harvesting Nodes and Wireless Secure Transmission

In this paper, non-orthogonal multiple access (NOMA) in cooperative relay system is considered, where a source node communicates with a pair of energy harvesting (EH) user equipments through a multiple antennas relay node. A hybrid protocol is adopted at the relay, in which if the relay can successfully decode the signals, decode-and-forward (DF) protocol will be adopted to forward the signals to the users. Otherwise, amplify-and-forward (AF) protocol will be implemented. Assuming that the users adopt maximal ratio combining (MRC) to combine the received signals in the two cooperative phases, new explicit analytical expressions for the average sum-rate are derived when the relay works in, 1) AF mode, and 2) DF mode, in two scenarios when one user is the stronger in both cooperation phases, and when an alternative user is stronger in each phase. Then, the investigation is extended to the case where the relay is an untrusted node, and cooperative jamming technique is proposed to degrade the ability of the relay to decode the signals and enforce the relay to operate always in AF mode. For the untrusted relay scenario, new analytical expression for the average secrecy rate is derived. Monte Carlo simulations are provided to validate the analysis. The simulation results reveal that the location of the relay is the key parameter to achieve the best performance.

Long-reach underwater wireless optical communication with relaxed link alignment enabled by optical combination and arrayed sensitive receivers.

To extend the transmission distance and relax the strict alignment requirement of underwater wireless optical communication ((UWOC), we design and implement a UWOC system using a 3×1 fiber combiner and a high-sensitive multi-pixel photon counter (MPPC). The 50-m and 100-m transmission distances (corresponding to 24 attenuation lengths) are experimentally achieved with the data rates of 16.78 Mbps and 8.39 Mbps, respectively, in a 50-m standard swimming pool. Moreover, we also investigate and optimize the performance of misalignment tolerance of this system using two MPPCs as the detectors together with different diversity reception technologies. At the 50-m transmission distance, the maximum offset between the MPPC array and the light spot center can be extended to 9 m using the maximal ratio combining (MRC), while the maximum offset is 6 m when using single MPPC.

Opportunistic Access Point Selection for Mobile Edge Computing Networks

In this paper, we investigate a mobile edge computing (MEC) network with two computational access points (CAPs), where the source is equipped with multiple antennas and it has some computational tasks to be accomplished by the CAPs through Nakagami-m distributed wireless links. Since the MEC network involves both communication and computation, we first define the outage probability by taking into account the joint impact of latency and energy consumption. From this new definition, we then employ receiver antenna selection (RAS) or maximal ratio combining (MRC) at the receiver, and apply selection combining (SC) or switch-and-stay combining (SSC) protocol to choose a CAP to accomplish the computational task from the source. For both protocols along with the RAS and MRC, we further analyze the network performance by deriving new and easy-to-use analytical expressions for the outage probability over Nakagami-m fading channels, and study the impact of the network parameters on the outage performance. Furthermore, we provide the asymptotic outage probability in the low regime of noise power, from which we obtain some important insights on the system design. Finally, simulations and numerical results are demonstrated to verify the effectiveness of the proposed approach. It is shown that the number of transmit antenna and Nakagami parameter can help reduce the latency and energy consumption effectively, and the SSC protocol can achieve the same performance as the SC protocol with proper switching thresholds of latency and energy consumption.

Optimum Combiner for Spatially Correlated Nakagami-m Fading Channels

This paper proposes a novel optimum combiner that raises the time-averaged signal-to-interference-plus-noise ratio (SINR) for maximal-ratio combining (MRC) reception over spatially correlated Nakagami-m fading channels. By exploiting the principal component analysis (PCA) with eigenvalue decomposition (EVD) of the covariance matrix analytically evaluated offline, a complete set of orthonormal basis functions can be obtained. A decorrelation process analyzes and then projects the received signal into the space spanned by the basis functions. In accordance with the theorem of irrelevance, a principal component selection combining (PCSC) method is proposed to remove components in a few dimensions in which SINRs are considered low to raise the resulting time-averaged SINR on the subsequent MRC reception. The proposed technique also avoids noise enhancement occurring with MRC reception in the scenario where noises on different branches are correlated. The SINR distribution, level-crossing rate (LCR) and average fade duration (AFD) are derived. Based on a novel scattering model interpretation, a simulator consisting of spatially correlated Nakagami-m fading channels is developed according to the analytically evaluated covariance matrix. Computer simulations show that the proposed optimum combiner not only decreases interference and noise from the irrelevant subspace to achieve higher time-averaged SINR and lower AFD but also significantly reduces the complexity required for subsequent signal processing.

Performance and optimal analysis of time-switching energy harvesting protocol for MIMO full-duplex decode-and-forward wireless relay networks with various transmitter and receiver diversity techniques

In this paper, we consider a wireless multiple-input multiple-output (MIMO) full-duplex relaying (FDR) network, where a source node communicates with a destination node via the help of a decode-and-forward (DF) relay node. This relay node has the capability of harvesting the energy from radio frequency (RF) signals, in particular, from the source node via time-switching energy harvesting protocol. To enhance the quality of transmission, diversity techniques are applied at both transmitter and receiver. Specifically, transmit antenna selection (TAS) and maximal ratio transmission (MRT) schemes are employed at the transmitter, while both selection combining (SC) and maximal ratio combining (MRC) techniques are considered at the destination node. The main contribution of this paper is an in-depth performance and optimal analysis of the time-switching energy harvesting (EH) protocol by deriving the closed-form expressions of outage probability (OP), system throughput, and symbol error probability (SEP) of the considered system over Rayleigh fading channel and then solving the optimization problem of EH duration, where the instantaneous throughput is used as the objective function. It is also shown in this paper that although a portion of signal is used for power transfer, the obtained diversity gain of this system is still equal to the minimum number of the antennas configured at the source or at the destination. Finally, numerical results are presented to demonstrate the validity of our analysis and to show the advantages of MIMO-FDR system with RF EH.

Ultrasonic Indexed Modulation and Multiple Access for Intra-Body Networks

Intra-body communication (IBC) plays a significant role in future health care systems. This article proposes a pulse-based ultrasonic index modulation (UsIM) to enable high data rate and low power IBCs. Two grouping-based UsIM strategies, namely ultrasonic grouped-chip indexed modulation (UsCIM) and ultrasonic chip-group indexed modulation (UsGIM), are proposed to implement multiple access for intra-body networks (IBNs). UsCIM uses the chip indices in a group to convey additional information bits for a biosensor node, while UsGIM uses the group indices and each node occupies one chip in a group. Three receivers, including the maximum likelihood (ML), the maximum ratio combination (MRC), and the energy detection (ED) receivers, are then proposed and studied. The theoretical bit-error rate (BER) upper bound for the ML receiver, and theoretical BER expressions for the MRC and ED receivers are derived. The system performance metrics in terms of complexity, throughput and energy efficiency are analyzed. The analytical and simulation results reveal that UsGIM achieves a lower BER than UsCIM under channel fading and multipath effect. Both UsCIM and UsGIM achieve higher throughput and energy efficiency than the existing IBC systems, and both strategies are important for energy-constrained implanted IBNs.

Is Massive MIMO Robust Against Distributed Jammers?

In this paper, we evaluate the uplink spectral efficiency (SE) of a single-cell massive multiple-input-multiple-output (MIMO) system with distributed jammers. We define four different attack scenarios and compare their impact on the massive MIMO system as well as on a conventional single-input-multiple-output (SIMO) system. More specifically, the jammers attack the base station (BS) during both the uplink training phase and data phase. The BS uses either least squares (LS) or linear minimum mean square error (LMMSE) estimators for channel estimation and utilizes either maximum-ratio-combining (MRC) or zero-forcing (ZF) decoding vectors. We show that ZF gives higher SE than MRC but, interestingly, the performance is unaffected by the choice of the estimators. The simulation results show that the performance loss percentage of massive MIMO is less than that of the SIMO system. Moreover, we consider two types of power control algorithms: jamming-aware and jamming-ignorant. In both cases, we consider the max-min and proportional fairness criteria to increase the uplink SE of massive MIMO systems. We notice numerically that max-min fairness is not a good option because if one user is strongly affected by the jamming, it will degrade the other users’ SE as well. On the other hand, proportional fairness improves the sum SE of the system compared with the full power transmission scenario.

Secrecy Outage Analysis for Satellite-Terrestrial Downlink Transmissions

This letter investigates the secrecy outage performance of satellite-to-terrestrial downlink transmissions consisting of one legitimate receiver ( ${D}$ ) and one eavesdropper ( ${E}$ ). To reflect the practical application scenarios, it is assumed that ${D}$ and ${E}$ are randomly distributed in the footprint of the satellite. Furthermore, in the considered downlink scenario, ${D}$ and ${E}$ are equipped with multiple antennas, and maximal ratio combining (MRC) technique is adopted at both of them to seek the maximum receiving diversity gain. To reveal the impacts of the randomness of the positions of ${D}$ and ${E}$ , and MRC scheme on the secrecy outage performance of the considered satellite-terrestrial system, the exact and asymptotic closed-form analytical expressions for secrecy outage probability are derived by using stochastic geometry method. Finally, simulation results are given to confirm the accuracy of our proposed analytical models.

On the performance of energy harvesting AF partial relay selection with TAS and outdated channel state information over identical channels

Energy scarcity has been known to be one of the most noticeable challenges in wireless communication system. In this paper, the performance of an energy harvesting based partial relay selection (PRS) cooperative system with transmit antenna selection (TAS) and outdated channel state information (CSI) is investigated. The system dual-hops links are assumed to follow Rayleigh distribution and the relay selection is based on outdated CSI of the first link. To realize the benefit of multiple antenna, the amplified-and-forward (AF) relay nodes then employs the TAS technique for signal transmission and signal reception is achieved at the destination through maximum ratio combining (MRC) scheme. Thus, the closed-form expression for the system equivalent end-to-end cumulative distribution function (CDF) is derived. Based on this, the analytical closed-form expressions for the outage probability, average bit error rate, and throughput for the delay-limited transmission mode are then obtained. The results illustrated that the energy harvesting time, relay distance, channel correlation coefficient, the number of relay transmit antennas and destination received antenna have significant effect on the system performance. Monte-carol simulation is employed to validate the accuracy of the derived expressions.

Ergodic spectral efficiency of massive MIMO with correlated Rician channel and MRC detection based on LS and MMSE channel estimation

The authors study the spectral efficiency (SE) of a multi-cell massive multiple-input multiple-output (MIMO) system with a spatially correlated Rician channel. The correlation between least squares (LS) estimator and its error complicates SE analysis, since signal and interference components become cross-correlated, too. Minimum mean square error (MMSE) estimators do not suffer from this burden. In some previous works, a proper part of the signal is referred to interference, which makes them cross-uncorrelated, and leads to a SE lower bound. In their modified approach, the authors extract and refer the cross-correlated part of interference to the signal to attain this objective. Here, they use this approach for calculating the instantaneous SE of maximum ratio combining (MRC) detector under LS and MMSE estimation methods. They further derive closed-form approximations of their ergodic SE. This approach is also applicable to other linear channel estimators or data detectors. Numerical results show that achievable SE surpasses that of the previous approach. Moreover, they show that their approximation is close enough to Monte Carlo simulation results, especially at the high number of the base station antennas.

On the BEP analysis of M-QAM and R-QAM under cascaded double η-μ, κ-μ or α-μ fading channels

This article presents new expressions for the bit error probability (BEP) of the M-ary quadrature amplitude modulation (M-QAM) and rectangular QAM (R-QAM) scheme in a frequency non-selective channel model with cascaded double fading, characterized in the links by the distributions η-μ, κ-μ or α-μ. In this paper, the probability density function (PDF) of the signal-to-noise ratio (SNR) of the double link is written in terms of an integral of the product of the PDF of the envelope of the first link by the PDF of the SNR of the second link. With this procedure, expressions for the BEP of the M-QAM and R-QAM scheme, with and without a maximum ratio combiner (MRC) associated, are obtained by means of the general expressions of Cho and Yoon, calculated with the help of the Gauss-Laguerre quadrature. The new expressions are written in terms of the integral of the moment generating function (MGF) of the instantaneous SNR from the second link, which are simpler than using expressions involving infinite series in terms of special functions like hypergeometric functions and the Meijer G-function. In addition, insights regarding the expressions obtained and comments about their complexity are provided. Several curves for the BEP are presented, for different parameters that characterize the channel and different numbers of branches of the MRC. All analytical expressions obtained in this paper are corroborated by computer simulations performed with the Monte Carlo method.

Superimposed Pilots are Beneficial for Mitigating Pilot Contamination in Cell-Free Massive MIMO

In this letter, we present the first study for uplink cell-free massive multi-input multi-output (MIMO) systems with superimposed pilots (SPs). Closed-form linear minimum mean square error (LMMSE) channel estimation and achievable spectral efficiency (SE) expressions with maximal ratio combining (MRC) receiver filters are derived. The tractability of the closed-form results facilitates analyzing the effects of key design system parameters, such as the number of mobile terminals (MTs), the power balance factor between pilot and data symbols, and the length of coherence interval. Several significant insights are attained by assuming that the introduced pilot interference can or cannot be perfectly canceled from received data symbols. All the theoretical results are corroborated by simulation experiments.

Outage Probability of SIMO MRC Receivers With Correlated Poisson Field of Interferers

We study the performance of applying maximal ratio combining (MRC) in a single-input-multiple-output (SIMO) system with a Poisson field of interferers. Due to the correlated interference powers across different antennas, the derivation of the exact outage probability is highly involved. To tackle this problem, we take advantage of the mathematical property of the sub-Gaussian α-stable interference signal vector, and then derive an analytical lower bound of the outage probability, which is shown to be very tight through Monte Carlo simulations. Our results can be used to assist performance analysis and design of SIMO MRC receiver with random interferers.

A shared local oscillator spatial diversity PM-CO-OFDM systems based on group timing synchronization and diversity branch phase correction in satellite-to-ground optical communications

Due to the complexity and strong fluctuations of atmosphere turbulence, it is always a challenging task to mitigate atmospheric turbulence effects in space optical communications. In this paper, we demonstrate a shared local oscillator (LO) spatial diversity polarization multiplexing coherent optical orthogonal frequency division multiplexing (PM-CO-OFDM) system based on group timing synchronization and diversity branch phase correction in downstream low earth orbit satellite-to-ground (LEO-StG) optical communications. For the shared LO coherent receiver system, the maximal ratio combining (MRC) technique is used at the receiver to improve the system performance, and the diversity gain is found to depend on the relative phase offset (RPO) among the different branches. Simulation results under different turbulence channels not only demonstrate that the RPO can be effectively compensated by group timing synchronization and diversity branch phase correction, but also show the improvement in receiver sensitivity. Furthermore, the number of optimal branches is also investigated. The superiorities of such spatial diversity scheme make it highly desirable for potential application in LEO-StG optical communications.

Performance study of wavelength diversity serial relay OFDM FSO system over exponentiated Weibull channels

This paper proposes a wavelength diversity decode-and-forward (DF) serial relay orthogonal frequency division multiplexing (OFDM) free-space optical (FSO) communication system. The proposed system performance is evaluated by the quadrature phase-shift keying (QPSK) as an OFDM signal subcarrier mapping method, considering the maximum ratio combining (MRC) and equal gain combining (EGC) schemes, over exponentiated Weibull (EW) channels. The Monte Carlo (MC) method is used to simulate the system performance. The closed-form expressions of outage probability and average symbol error rate (SER) are also derived considering the joint effect of atmospheric turbulence, path loss, and pointing errors. In addition, the effects of the number of operating wavelength, receiving aperture diameter, number of relay hops, and link length between relays on the outage performance of an FSO communication system, and the effect of the MRC and EGC schemes the average SER are analyzed through simulations. The MC simulations verify the correctness of the numerical results. The results show that the proposed system can concurrently expand the communication range and improve system performance.

Physical layer security for multiuser multi‐eavesdropper multi‐input multi‐output (MIMO) system in the presence of imperfect feedback

SummaryIn this work, we analyze the physical layer security (PLS) for a multiuser (MUS) multi‐input multi‐output multi‐eavesdropper (MIMOME) wireless network using selection transmission (ST) at the base station (BS) with maximum ratio combining (MRC) at both of the receivers, receivers of legitimate users (LUs) as well as the receivers of illegitimate users over an uncorrelated Rayleigh fading channel. In this scenario, multiple users communicate to a BS at the same time multiple eavesdropper user may intercept the transmission. Next, we drive exact closed‐form expression for probability of nonzero secrecy capacity (PNZSC) and secrecy outage probability (SOP) in the presence of imperfect channel state information (ICSI). At last, we present the simulation results to authenticate the analytical results and analyze the impact of ICSI on PLS of MUS‐MIMOME system.

Approximate ASER analysis of MIMO TAS/MRC networks over Weibull fading channels

In this paper, the average symbol error rate (ASER) evaluation of multiple-input-multiple-output (MIMO) systems with transmit antenna selection (TAS) and maximal ratio combining (MRC) are analysed under Weibull fading conditions. The impact of additive white Gaussian noise (AWGN) and additive white generalized Gaussian noise (AWGGN) on the ASER evaluation of the MIMO TAS/MRC networks are considered. Closed form approximate and asymptotic ASER expressions of the considered network with AWGN for different quadrature amplitude modulation techniques are derived based on the probability density function approach. The closed-form approximate and asymptotic ASER expressions for the AWGGN case are also obtained. In addition, a comprehensive analysis of the ASER performance for the MIMO TAS/MRC networks is demonstrated by varying the fading parameter values, numbers of transmitter-receiver antennas and constellation size. Finally, the obtained theoretical results are confirmed through the exact simulation results.

Secrecy analysis of multiple‐input multiple‐output underlay cognitive radio networks with energy harvesting

AbstractThe cognitive radio (CR) technique is an encouraging approach to deal with spectrum scarcity in wireless networks. However, this technique raises the issues of secrecy and privacy in the CR network. In this work, a multiple‐input multiple‐output underlay CR network is considered, which consists of a primary network, a secondary network, and an eavesdropper. All the primary, secondary as well aseavesdropper's nodes are assumed to have multiple antennas with radio frequency energy harvesters to enhance their spectral and energy efficiency properties. The eavesdropper node manipulates it's simultaneous wireless information and power transfer architecture in a pursuit to intercept the secure communication between the cognitive transmitter and the cognitive receiver. This technique and considered network are combined with the traditional selection combining (SC) and maximal ratio combining (MRC) scheme and the results are compared. The Monte Carlo simulation is done for the validation of results. The main aim is to drive the closed‐form analytical expression for exact and asymptotic secrecy outage probability. The present results show that the SC scheme at eavesdropper provides better secrecy in comparison to the MRC technique and also the number of antennas affects the secrecy of the system. Moreover, the impact of power splitter is considered and also the optimal value of power splitter is derived to maximize information decoding.