Optimal Power Allocation Factor Research Articles

Superimposed Training-Based Channel Estimation for MISO Optical-OFDM VLC

In this paper, we investigate a novel channel estimation (CE) method for multiple-input and single-output (MISO) systems in visible light communication (VLC). Direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) is commonly used in VLC where half of the available subcarriers are spent to guarantee a real-valued output after the inverse fast Fourier transform operation. Besides, dedicated subcarriers are typically used for CE, thus, many resources are wasted and the spectral efficiency is degraded. We propose a superimposed training approach for CE in MISO DCO-OFDM VLC scenarios. Analytical expressions of mean squared error (MSE) and spectral efficiency are derived when the least squares estimator is considered. This analysis is valid for outdoor and indoor scenarios. For the CE error, simulation results of MSE show a perfect match with analytical expressions. Moreover, results prove that this technique guarantees a larger spectral efficiency than previous schemes where dedicated pilots were used. Finally, the optimal data power allocation factor is also analytically derived.

Safeguarding UAV Communications Against Full-Duplex Active Eavesdropper

Unmanned aerial vehicle (UAV) wireless communication has recently been recognized to be inevitable and prevalent in the fifth-generation (5G) wireless networks. In this paper, we propose a secure transmission scheme for a wiretap channel, where a source communicates with a legitimate UAV in the presence of an eavesdropper. We consider the full-duplex active eavesdropper, which performs both eavesdropping and malicious jamming simultaneously. The source transmits artificial noise (AN) signals, in addition to information signals, to confuse this eavesdropper. By considering the ground-to-UAV channel model, we analyze the hybrid outage probability, which takes both the transmission outage probability and the secrecy outage probability into consideration. We further provide the asymptotic hybrid outage probability in a more compact form, where both the transmit power at the source and the jamming power at the eavesdropper become large with a fixed ratio. Through the analysis and the numerical results, we determine the optimal power allocation factor between information signals and AN signals as well as the operating height of UAV that minimize the hybrid outage probability. We also provide the most harmful antenna configuration of the eavesdropper to the UAV communications, and this paper can be a useful framework for the design of confidential UAV communication system.

Adaptive Power Allocation Scheme for Mobile NOMA Visible Light Communication System

Recently, due to its higher spectral efficiency and enhanced user experience, non-orthogonal multiple access (NOMA) has been widely studied in visible light communication (VLC) systems. As a main concern in NOMA-VLC systems, the power allocation scheme greatly affects the tradeoff between the total achievable data rate and user fairness. In this context, our main aim in this work was to find a more balanced power allocation scheme. To this end, an adaptive power allocation scheme based on multi-attribute decision making (MADM), which flexibly chooses between conventional power allocation or inverse power allocation (IPA) and the optimal power allocation factor, has been proposed. The concept of IPA is put forward for the first time and proves to be beneficial to achieving a higher total achievable data rate at the cost of user fairness. Moreover, considering users’ mobility along certain trajectories, we derived a fitting model of the optimal power allocation factor. The feasibility of the proposed adaptive scheme was verified through simulation and the fitting model was approximated to be the sum of three Gaussian functions.

Optimal Power Allocation Factor Based Real Time Energy Management Strategy for a Plug-in Hybrid Electric Vehicle

Optimal Power Allocation Factor Based Real Time Energy Management Strategy for a Plug-in Hybrid Electric Vehicle

Outage Performance of Cooperative Underlay CR-NOMA With Imperfect CSI

In this letter, a decode-and-forward cooperative underlay cognitive radio (CR) non-orthogonal multiple access (NOMA) network is studied. Exact closed-form expressions for the outage probability (OP) of two NOMA secondary destination users ( $U_{1}$ and $U_{2}$ ) are derived in the case of imperfect channel state information. Then, we find optimal power allocation factors for different distances of $U_{1}$ to satisfy the OP fairness for both users. Moreover, the proposed system model shows the superiority of the CR-NOMA compared with cooperative orthogonal multiple access. The derived analytical expressions are corroborated through Monte Carlo simulations.

Superimposed Pilot Optimization Design and Channel Estimation for Multiuser Massive MIMO Systems

For massive multiuser multiple input multiple output (MIMO) (MU-MIMO) systems, pilot contamination (PC) and data interference (DI) are two main factors that affect the accuracy of the superimposed pilot (SP) based channel estimation, provided that the channel coherence time is less than the total number of users. In this paper, to tackle the effect of the PC, we propose a block-diagonal Grassmannian line packing (GLP) approach, in which the specific sequences for different cells are first designed based on the GLP, and then are block-diagonally extended to form the SP matrices for the users in different cells. To also alleviate the effect of DI on the channel estimation, with the designed SP matrices, an iterative channel estimation (ICE) method based on Tikhonov regularization is presented. The impact that the proposed ICE method has on the signal-to-interference-plus-noise ratio with matched filtering receiver is theoretically analyzed. Moreover, the optimal power allocation factor to the SP by maximizing the spectral efficiency (SE) of the target cell is also derived. Numerical results show that the proposed ICE method with the designed SP matrices effectively mitigates the effects of the PC and DI, and hence, improves the accuracy of channel estimation and further the SE of the massive MU-MIMO systems.

Secrecy Performance of Untrusted Relay Systems With a Full-Duplex Jamming Destination

To improve the secrecy performance of the untrusted relay system with a multiple-antenna destination, a novel full-duplex destination jamming scheme with optimal antenna selection (FDJ-OAS) is proposed in this paper. The approximate closed-form expressions of the average achievable secrecy rate are derived for the proposed scheme. To attain more insightful results, the approximated closed-form expressions for the average achievable secrecy rate and the optimal power allocation factor for FDJ-OAS are derived in the case with a large-scale antenna array, and simple power allocation solutions are also provided in the high and low SNR regimes, respectively. Furthermore, we derive the asymptotic expressions for the secrecy outage probability under different asymptotic conditions for the proposed scheme, and the analysis results indicate that the secrecy diversity order of FDJ-OAS is significantly superior to that of the comparable schemes. Simulation results show that the analytical curves match well with the simulation results, and the advantage of FDJ-OAS is also clearly demonstrated through performance comparison.

Cooperative spectrum sharing in cognitive radio networks with energy accumulation: design and analysis

The authors propose an efficient spectrum sharing scheme in cooperative cognitive radio networks, where an energy-constrained secondary transmitter (ST) first scavenges radio frequency (RF) energy from the received primary signals, and then the ST assists the primary transmission to obtain the opportunity of spectrum access. Specifically, the ST can forward the primary signal with its own signal by adopting both the Alamouti coding technique and superposition scheme only if the harvested energy is sufficient while the primary data is decoded correctly by the ST. Otherwise, the ST will continue to harvest RF energy. The authors use the discrete Markov chain to model the processes of charging and discharging of the battery. Moreover, two different joint decoding and interference cancellation schemes are employed at the receivers to restore the desired data. Closed-form expressions of outage probabilities for both the primary and secondary systems are derived. Aiming to minimise the outage probability of the secondary system with guaranteeing the primary transmission, an optimal power allocation factor for the ST is determined by Monte-Carlo simulation. Numerical results demonstrate that the proposed scheme can effectively improve the transfer performance of the secondary system while realising the transfer requirement of the primary system.

Error rate and outage of dual‐hop DF relay system with selection combining over Rice fading

SummaryPerformance of dual‐hop decode‐and‐forward relay system with selection‐combining receiver is analyzed over Rice fading channels. The following closed‐form expressions of performance metrics are derived: moment generating function for selection‐combining receiver output signal‐to‐noise ratio, exact average bit error rate of noncoherent modulations, approximate average symbol error rate for coherent modulations, and outage probability. We also obtain simple asymptotic expressions for moment generating function, exact average bit error rate, average symbol error rate, and outage probability, which are useful to characterize the diversity order and the coding gain. The optimal power allocation analysis suggests that the optimal power allocation factor is independent of total signal‐to‐noise ratio and source‐to‐destination link fading parameters. The accuracy of the obtained analytical expressions are supported by computer simulation results.

An investigation of S-DF cooperative communication protocol over keyhole fading channel

This paper investigates the keyhole or pinhole effect on the pairwise error probability (PEP) performance of the multiple-input multiple-output (MIMO) space–time block-code (STBC) based selective decode and forward (S-DF) protocol. The closed form PEP expressions are derived for several configurations in terms of number of hops, phases, and relays over Nakagami-m fading channel, with perfect and imperfect channel state information (CSI), and with and without the keyhole condition. Further, a framework is developed for deriving the diversity order (DO) for each configuration. It demonstrates that full DO for cooperation protocol can be achieved when there is a knowledge of perfect CSI. A convex optimization framework is formulated for obtaining the optimal source–relay power allocation factors which significantly improve the end-to-end reliability of the system under power constraint scenarios. Monte Carlo simulations are performed for both equal and optimal power allocation factors and results show the substantial improvement in PEP performance with an increase in the value of the shape parameter and relay to destination (RD) link channel variance. Results show an improved PEP performance by increasing the RD link channel variance in comparison to increasing the source to relay (SR) link channel variance. Also PEP performance degrades for keyhole Nakagami-m fading channel as compared to Nakagami-m fading channel. Simulation results are in close agreement with the analytical results at high signal to noise ratio (SNR) regimes.

Performance of Regenerative Relay-Assisted D2D Communication in Mixed Fading Channels

Performance of device-to-device communication using selective decode-and-forward relaying is analyzed in mixed two-wave with diffuse power and Nakagami- $m$ fading channels. A relay that successfully decodes the source information forwards it to the destination. The destination device combines the direct message signal from the source and its copy via the relay using maximal-ratio combining. Novel closed-form expressions for exact and asymptotic average bit error rates are derived. Furthermore, diversity order, the coding gain, and the optimal power allocation factor are obtained for better system insights. The analysis has been verified through Monte Carlo simulations.

Single Relay Selection and Power Allocation Based on Energy Efficiency in Cooperative Cognitive Radio Network

In cooperative cognitive network (CCNs), relay selection generally depends on the channel state information (CSI). However, over-reliance on the relay with good channel condition may result in excessive energy consumption and lower network lifetime. Most previous works focused on improving system capacity without considering energy efficiency and residual energy information (REI). In this paper, a joint single relay selection and power allocation scheme based on energy efficiency is proposed. First, we propose a novel single relay selection scheme considering both REI and CSI to balance the energy consumption and performance. Then, we formulate a maximizing energy efficiency convex problem as the optimization model under limited total transmission power of secondary users (SUs) and power constraints to primary users (PUs). According to the convex optimization theory and the Karush-Kuhn-Tucker (KKT) condition, we obtain the optimal power allocation factor and the maximum energy efficiency. It is shown through the simulation that new scheme can dynamically select the appropriate relay and adjust the power allocation. Our scheme can effectively reduce the energy consumption and improves the system lifetime.

Asymptotic SER Analysis and Optimal Power Sharing for Dual-Phase and Multi-Phase Multiple-Relay Cooperative Systems

The asymptotic high-signal-to-noise ratio (SNR) symbol error rate (SER) performance of selective decode-and-forward cooperative multiple relay-aided wireless systems is derived for $M$ -PSK and $M$ -QAM modulations. The proposed analysis considers both dual-phase and multi-phase relaying protocols. Furthermore, analytical results are also presented for optimal power allocation both at the source and at each of the relays, since it significantly influences the performance of cooperative communication. A novel aspect of the proposed framework is that the SER and optimal power allocation results derived are applicable to diverse fading channels such as $\eta -\mu $ , $\kappa -\mu $ , and shadowed-Rician scenarios and each for non-identical fading for the source–destination, source–relay (SR), and relay–destination (RD) links. The applicability of the proposed framework is also demonstrated for diverse PHY layer schemes, such as multiple-input multiple-output-orthogonal space–time block codes, cooperative beamforming, joint transmit/receive antenna selection, and free-space optical scenarios. This high-SNR analysis provides the valuable insights into the impact of the diversity orders of the SR and RD links on the end-to-end SER as well as on the optimal power allocation factors both in the dual-phase and multi-phase protocols of various fading channels and schemes. Our simulation results verify the analytical results derived.

Power Allocation Strategy of Maximizing Secrecy Rate for Secure Directional Modulation Networks

In this paper, given the beamforming vector of confidential messages and artificial noise (AN) projection matrix and total power constraint, a power allocation (PA) strategy of maximizing secrecy rate (Max-SR) is proposed for secure directional modulation (DM) networks. By the method of Lagrange multiplier, the analytic expression of the proposed PA strategy is derived. To confirm the benefit from the Max-SR-based PA strategy, we take the null-space projection (NSP) beamforming scheme as an example and derive its closed-form expression of optimal PA strategy. From simulation results, we find the following facts: in the medium and high signal-to-noise-ratio (SNR) regions, compared with three typical PA parameters such $\beta=0.1, 0.5$, and $0.9$, the optimal PA shows a substantial SR performance gain with maximum gain percent up to more than $60\%$. Additionally, as the PA factor increases from 0 to 1, the achievable SR increases accordingly in the low SNR region whereas it first increases and then decreases in the medium and high SNR regions, where the SR can be approximately viewed as a convex function of the PA factor. Finally, as the number of antennas increases, the optimal PA factor becomes large and tends to one in the medium and high SNR region. In other words, the contribution of AN to SR can be trivial in such a situation.

Modeling and Throughput Analysis of an ADO-OFDM Based Relay-Assisted VLC System for 5G Networks

Visible light communication (VLC) is a powerful supplement to the fifth generation (5G) mobile communications. In VLC system, the adoption of orthogonal frequency division multiplexing (OFDM) and relaying technique can greatly improve system performance. In this paper, we proposed a novel relay-assisted VLC system based on asymmetrically clipped direct current biased optical OFDM (ADO-OFDM) for 5G networks. An amplify-and-forward relay is used in the proposed system to forward the signals from source terminal and transmit its own signals simultaneously. The signals from source terminal are allocated to even subcarriers, while the signals from relay terminal are allocated to odd subcarriers. Then, these two parts of signals are combined to constitute ADO-OFDM signals. The signals on odd subcarriers don’t interfere with the signals on even subcarriers, while the interference of the signals on even subcarriers to the signals on odd subcarriers can be eliminated by the noise estimation in ADO-OFDM system. Finally, both signals can be recovered at the receiver. We derive the formula of system throughput and investigate the impacts of two power allocation factors on system throughput. Simulation results show that the proposed system makes full use of the relay terminal and improves system throughput. In addition, simulation results also demonstrate that there are optimal power allocation factors to achieve maximum throughput.

On the Performance of Full-Duplex Two-Way Relay Channels With Spatial Modulation

In this paper, the spatial modulation (SM) technique is employed at the source and relay nodes in a full-duplex two-way relay channel (FD-TWRC) to support spectral-efficient bi-directional communications while guaranteeing a low cost implementation. Maximum likelihood detectors are employed at each node that is subject to an intrinsic self-loop interference. We first propose a tight upper bound on the average bit error probability (ABEP). Then, based on the ABEP upper bound, an asymptotic ABEP expression is derived in the high signal-to-noise ratio (SNR) regime. Exploiting the asymptotic ABEP, an exact SNR threshold for the selection between FD-TWRC-SM and half-duplex (HD)-TWRC-SM is derived in a closed form, which sheds light on when it is beneficial to select the FD (or HD) mode. In addition, the power allocation (PA) among sources and relay is investigated, through which an optimal PA factor in terms of ABEP is obtained. All analytical results derived in this paper are verified by Monte Carlo simulations, from which some new insights are obtained on the performance of FD-TWRC-SM.

Secure Multiuser Communications in Wireless Sensor Networks with TAS and Cooperative Jamming.

In this paper, we investigate the secure transmission in wireless sensor networks (WSNs) consisting of one multiple-antenna base station (BS), multiple single-antenna legitimate users, one single-antenna eavesdropper and one multiple-antenna cooperative jammer. In an effort to reduce the scheduling complexity and extend the battery lifetime of the sensor nodes, the switch-and-stay combining (SSC) scheduling scheme is exploited over the sensor nodes. Meanwhile, transmit antenna selection (TAS) is employed at the BS and cooperative jamming (CJ) is adopted at the jammer node, aiming at achieving a satisfactory secrecy performance. Moreover, depending on whether the jammer node has the global channel state information (CSI) of both the legitimate channel and the eavesdropper’s channel, it explores a zero-forcing beamforming (ZFB) scheme or a null-space artificial noise (NAN) scheme to confound the eavesdropper while avoiding the interference to the legitimate user. Building on this, we propose two novel hybrid secure transmission schemes, termed TAS-SSC-ZFB and TAS-SSC-NAN, for WSNs. We then derive the exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput of both schemes to characterize the secrecy performance. Using these closed-form expressions, we further determine the optimal switching threshold and obtain the optimal power allocation factor between the BS and jammer node for both schemes to minimize the secrecy outage probability, while the optimal secrecy rate is decided to maximize the effective secrecy throughput for both schemes. Numerical results are provided to verify the theoretical analysis and illustrate the impact of key system parameters on the secrecy performance.

Optimised power allocation to maximise secure rate in energy harvesting relay network

For an energy harvesting relay network, an optimal secure transmission scheme is proposed. In the case of limited harvesting energy and total power, destination-based jamming technique is applied to protect the confidential information from relay's jamming and eavesdropping. Moreover, the penalty function algorithm is utilised to obtain the optimal power allocation factor to maximise secure rate. The impacts of power allocation factor and energy transformation efficiency on the system secure rate are analysed. Simulation results validate the correctness and efficiency of the proposed scheme.

Power Allocation and Performance Comparison of Full Duplex Dual Hop Relaying Protocols

This paper provides two optimal power allocation (OPA) strategies for full duplex (FD) relay networks with amplify and forward (AF) protocol. Two strategies are designed based on statistical channel state information (S-CSI) and instantaneous CSI (I-CSI) to minimize the system outage probability under total source and relay power constraint. For the S-CSI scheme, the OPA factor is found through an exhaustive search while, for the I-CSI scheme, the OPA factor is given in a closed-form expression. To evaluate quickly the system performance with two proposed schemes, we derive the tight closed-form lower and upper bounds on the system outage for S-CSI based OPA scheme and the asymptotic performance for I-CSI based OPA scheme. Furthermore, how much difference in performance between AF and decode and forward (DF) relaying with OPA is also verified through our numerical results.

Performance Analysis of Cognitive Spectrum Sharing with Time Division Broadcast Protocol

We investigate the performance of a cognitive spectrum sharing system where a best selected secondary user (SU) assists two-way communication between two primary users (PUs) by employing time division broadcast over Rayleigh channels. We derive tight lower-bound expressions for the overall outage probability (OOP) of both the primary and secondary systems of the considered scheme. We also provide valuable insights into the achievable diversity order, cell coverage, and optimal power allocation factor to ensure spectrum sharing. Numerical and simulation results corroborate the theoretical analysis and illustrate the effectiveness of the considered scheme.