Maximum Ratio Combing Research Articles

A Long-Distance Underwater Wireless Optical Link Enabled by a Solar Array With a Baseline Compensator

In this paper, a self-designed solar cell-based optical detector is proposed and demonstrated in a long-distance underwater wireless optical communication (UWOC) link. Benefitting from a large detection area, the detector could successfully capture optical signals with a received power density of 5.62 μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . To resist the baseline drift caused by the low-frequency cut-off of the system, an envelope detector-based baseline compensator is proposed. Comparing with digital solutions, the proposed compensator could be established in analog domain with common electronic components. With the compensator, a frequency domain equalizer (FDE) and a maximum ratio combing (MRC) algorithm, an 18 Mbps non-return-to-zero (NRZ) on-off keying (OOK) modulated link over an 80 m underwater channel is finally achieved. The proposed baseline compensator could compensate the low-frequency components effectively with low cost. Moreover, under a strong ambient light of approximately 1200 lux, the performance of the link shows little degradation. Our work shows a potential solution of long-distance UWOC system with ambient light tolerance, which is compatible with practical environment.

Wireless-Powered Relays Assisted Batteryless IoT Networks Empowered by Energy Beamforming

In this letter, we propose an energy beamforming empowered relaying scheme for a batteryless IoT network, where wireless-powered relays are deployed between the hybrid access point (HAP) and batteryless IoT devices to assist the uplink information transmission from the devices to the HAP. In particular, the HAP first exploits energy beamforming to efficiently transmit radio frequency (RF) signals to transfer energy to the relays and as the incident signals to enable the information backscattering of batteryless IoT devices. Then, each relay uses the harvested energy to forward the decoded signals from its corresponding batteryless IoT device to the HAP, where the maximum-ratio combing is used for further performance improvement. To maximize the network sum-rate, the joint optimization of energy beamforming vectors at the HAP, network time scheduling, power allocation at the relays, and relection coefficient at the users is investigated. As the formulated problem is non-convex, we propose an alternating optimization algorithm with the variable substitution and semi-definite relaxation (SDR) techniques to solve it efficiently. Specifically, we prove that the obtained energy beamforming matrices are always rank-one. Numerical results show that compared to the benchmark schemes, the proposed scheme can achieve a significant sum-rate gain.

Modeling and Analysis of Multi-Relay Cooperative Communications in C-V2X Networks

To compensate for the limitations of existing dedicated short range communications (DSRC), cellular vehicle-to-everything (C-V2X) has been proposed recently, which is also a promising technology for future intelligent transportation systems (ITS). Using stochastic geometry approach, this paper presents the modeling and analysis of success probability in multi-relay cooperative C-V2X networks. The spatial distribution of base stations (BSs) and vehicles in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbb {R}^{2}$ </tex-math></inline-formula> are modeled as a 2D Poisson point process (PPP) and a Poisson line Cox point process (PLCPP), respectively. We focus on the success probability of a source vehicle sending a message to the nearest destination vehicle assisted by the closest BS. Each vehicle is equipped with single antenna whereas each BS is equipped with multiple antennas, which act as independent relays. We consider two decoding schemes, i.e., selection combining (SC) and maximum ratio combing (MRC), and obtain the analytical expressions for joint success probability during two continuous time slots, taking into account the interference correlation (i.e., the spatial correlation of vehicle location). The analytical model is validated using Monte Carlo simulations in MATLAB, and the effects of major parameters on success probability are investigated.

Full‐Duplex UAV Legitimate Surveillance System against a Suspicious Source with Artificial Noise

We propose a legal full‐duplex unmanned aerial vehicle (UAV) surveillance system in the presence of the ground‐to‐ground suspicious link with antisurveillance technology. UAV performs passive surveillance and active jamming simultaneously, and the suspicious source with multiantenna employs artificial noise to avoid being monitored. In order to ensure effective surveilling, we adopt two beamforming schemes, namely, maximum ratio transmission (MRT)/receiving zero‐forcing (RZF) and transmitting zero‐forcing (TZF)/maximum ratio combing (MRC), for MIMO UAV. For the two beamforming schemes, we derive the surveilling nonoutage probability in a closed‐form expression and analyze the surveilling performance under different system environments. Monte Carlo (MC) simulation validates the correctness of the formula.

Joint Design and Performance Analysis of a Full-Duplex UAV Legitimate Surveillance System

In this paper, we propose a legitimate surveillance system, where a full-duplex unmanned aerial vehicle (UAV) legitimate monitor with simultaneous passive surveilling and active jamming is deployed to monitor a suspicious communication link between a dubious pair on the ground. Two different scenarios for the UAV, single-input single-output (SISO) and multiple-input multiple-output (MIMO), are studied. Three low-complexity linear beamforming schemes, transmit zero-forcing (TZF)/maximum ratio combing (MRC), maximum ratio transmission (MRT)/receive zero-forcing (RZF), and maximum ratio transmission (MRT)/maximum ratio combing (MRC) are considered for MIMO UAV. The surveilling non-outage probability is derived and analyzed, and optimal jamming power is obtained. Simulation and numerical results are used to validate the derivation.

A Hybrid-Grant Random Access Scheme in Massive MIMO Systems for IoT

The grant-free random access (RA) can minimize the access delay but also brings severe data transmission interferences. To overcome this defect, we propose a new RA scheme that inserts a base station (BS) broadcasting message after user equipments (UEs) transmitting pilots. In this scheme, UEs can determine whether they have colliders by resolving the broadcasting message, and only non-colliding UEs can transmit data in the following step while colliding UEs keep silent. By doing this, the data interferences from colliding UEs are eliminated without costing much extra time. Since this BS broadcasting message is also used in the legacy grant-based RA, we call the new RA scheme as a hybrid-grant RA. We investigate the hybrid-grant RA in massive multiple-input multiple-output (MIMO) systems and obtain a tight closed-form approximation of the spectral efficiency with maximum-ratio-combing (MRC) and zero-forcing (ZF) receivers, respectively. Via simulation, we find that our proposed hybrid-grant RA can obtain a significant gain on the spectral efficiency compared with grant-free RA, especially for ZF receivers. In particular, this gain grows rapidly as the UE number goes up, which means the hybrid-grant RA is more suitable for the system with large amount of UEs and it is a typical scenario in future communications networks. Moreover, we also analyze the optimal pilot length and UE activation probability that maximize the spectral efficiency, which can be used as references for the practical application of the proposed hybrid-grant RA.

Performance Comparison between Fountain Codes-Based Secure MIMO Protocols with and without Using Non-Orthogonal Multiple Access

In this paper, we propose and evaluate the performance of fountain codes (FCs) based secure transmission protocols in multiple-input-multiple-output (MIMO) wireless systems, in presence of a passive eavesdropper. In the proposed protocols, a source selects its best antenna to transmit fountain encoded packets to a destination that employs selection combining (SC) or maximal ratio combing (MRC) to enhance reliability of the decoding. The transmission is terminated when the destination has a required number of the encoded packets to reconstruct the original data of the source. Similarly, the eavesdropper also has the ability to recover the source data if it can intercept a sufficient number of the encoded packets. To reduce the number of time slots used, the source can employ non-orthogonal multiple access (NOMA) to send two encoded packets to the destination at each time slot. For performance analysis, exact formulas of average number of time slots (TS) and intercept probability (IP) over Rayleigh fading channel are derived and then verified by Monte-Carlo simulations. The results presented that the protocol using NOMA not only reduces TS but also obtains lower IP at medium and high transmit signal-to-noise ratios (SNRs), as compared with the corresponding protocol without using NOMA.

Performance Enhancement of Multi-Antenna Relay Link in LTE Het-Net

In this paper, the relay link performance for an Amplify and Forward (AF) multi-antenna relay based heterogeneous LTE (Long Term Evolution) network is enhanced with two novel techniques, the Golden codes and Error Vector Magnitude (EVM) based Constellation Combiner (ECC). The performance evaluation of the proposed co-operative LTE Het-Net is done in a real time indoor environment using Wireless open Access Research Platform (WARP). The proposed techniques for the LTE Het-Net are compared with traditional Alamouti Scheme with Maximal Ratio Combing (MRC). From the obtained results it is seen that the LTE network with proposed techniques gives a 12% improvement in EVM than the conventional LTE network.

Cognitive Tropospheric Scatter Communication

Tropospheric scatter as a promising communication for beyond line-of-sight links between remote geographic areas, regains its predominance especially in military applications recently when considering microwave and cable systems are not feasible. In this paper, we propose cognitive troposcatter (short for tropospheric scatter) communication, which aims to improve the quality and capacity of troposcatter communication. Hybrid cognitive troposcatter communication scenarios and cognitive system architecture are established at first. Physical awareness objects including link geometry, nonhomogeneities, and meteorological conditions, are studied and then optimal working frequency has been calculated for specific link scenarios. More importantly, we investigate the potential of applying cognitive radio techniques in troposcatter communication due to the requirement of expanding services and capacity. Focusing on maximal ratio combing (MRC), we derive exact series-form expression of detection probability for diversity reception over Nakagami-m channel. Finally, we evaluate the performance of detection based on MRC techniques over fading channel. Numerical and simulation results demonstrate diversity promotes the detection performance in case of rain, low signal-to-noise ratio, and high antenna elevation.

Theoretical Outage Analysis of Nonlinear OFDM Systems with AF Relays

In this work, a performance analysis of the outage probability of a cooperative diversity OFDM system is presented, considering the distortions introduced by nonlinear power amplifiers (PAs) at the source and relay nodes. A closed form expression for the instantaneous SNR is developed and analytical expressions for the outage probability are derived, considering an amplify-and-forward (AF) relay and Rayleigh fading. Both the maximum ratio combing (MRC) and selection combining (SC) methods are considered. The derived expressions show how some PA parameters affect the outage probability and diversity order. Computer simulation results illustrate and confirm the proposed theoretical expressions under different scenarios.

Outage performance of cognitive DF relay networks with nonidentical Rayleigh fading channels and maximal ratio combining

In the expression of end-to-end signal-to-noise ratio (SNR) of cognitive relay networks, the channel gaina of the interference links (from secondary nodes to primary nodes in the networks) are usually shared by multiple items in the expression. These shared variables lead to the so called correlation issue, which results in the higher complexity of outage performance analysis for such networks. Till now, for cognitive relay networks with decode-and-forward (DF) relay scheme, we have not found works obtaining explicit closed-form expression of exact outage probability (OP) meanwhile treating the correlation issue completely. In this paper, for cognitive DF relay networks with maximal ratio combing (MRC) and independent non-identical Rayleigh fading channels, we obtain a closed-form expression of exact OP taking consideration of the correlation issue completely. Additionally, by shrinking or expanding the triangular integral region to suitable rectangular regions, we obtain simpler closed-form expressions of the lower and upper bounds of both OP and symbol error probability (SER). Furthermore, the asymptotic expressions of OP and SER are also obtained. The correctness of our analysis results are verified through numerical simulations. Both analysis results and simulation results show that how the correlation issue is treated can affect the coding gain whereas has no effect on the diversity gain of the network. Simulation results also indicate that, exact OP and SER are bounded from both sides by the lower bound and the upper one tightly, whereas the upper bounds of OP and SER are more tighter than the lower bounds.

Moment Generating Function Based Performance Analysis of Maximal-Ratio Combining Diversity Receivers in the Generalized-K Fading Channels

In this paper, we have analyzed the performance of maximal ratio combing (MRC) diversity receiver of the wireless communication systems over the composite fading environment, which is modelled by using the generalized-K distribution. However, this distribution has been considered as a versatile distribution for the precise modelling of a great variety of the short-term fading in conjunction with the long-term fading (shadow fading) channel conditions. In this proposed analysis, we have derived the mathematical expression for the moment generating function (MGF) of the generalized-K fading channel model that is used to evaluate a novel closed-form expression of the average bit error rate for (BER) the binary phase-shift keying /binary frequency-shift keying and average symbol error rate (SER) for the rectangular quadrature amplitude modulation scheme. We have also derived the mathematical expressions for the outage probability as well as the channel capacity for the generalized-K fading channel model.

Achievable Transmission Rate of the Secondary User in Cognitive Radio Networks with Hybrid Spectrum Access Strategy

A hybrid spectrum access strategy which is different from traditional underlay or overlay strategy is proposed for cognitive radio networks. In proposed strategy, the secondary users (SU) cooperatively sense the state of the primary user (PU) in a given spectrum band. Based on the sensing results and an additional requirement on the bit error rate, the SU adapts its transmit power and modulation level. Specifically, if the PU is inactive, the SU allocates its power subject to a peak transmit power constraint. Else if the PU is active, the average interference power constraint would be further imposed to protect the primary link. We assume that the maximal ratio combing (MRC) technique is available at the secondary receiver. Imperfect spectrum sensing and imperfect estimations of the channel power gain from the secondary transmitter to primary receiver are also considered. Achievable transmission rate of the SU over Rayleigh fading channel is formulated as an optimization problem and solved by using Lagrange dual decomposition method. Simulation results demonstrate that the transmission rate of the SU with hybrid strategy is significantly improved when compared with the traditional ones.

Analysis of Optimal Spectral Efficiency with Adaptive Modulation Coding and Cooperative ARQ Protocols

In this paper aiming at maximizing the spectral efficiency, a cross-layer design which combines adaptive modulation and coding (AMC) at the physical layer and truncated automatic repeat request (ARQ) protocol at the data link layer in cooperation system is proposed. In order to analyze the system performance, we first derive a closed form expression of probability density function (PDF) of the received signal-to-noise (SNR) using maximum ratio combing (MRC) at the destination node, the theoretical results and simulation show that the new design outperforms the traditional AMC system especially at low SNR.

On the Performance of Dual-Hop Systems with Multiple Antennas: Effects of Spatial Correlation, Keyhole, and Co-Channel Interference

In this paper, taking into account realistic propagation conditions, namely, spatial correlation, keyhole channels, and unequal-power co-channel interference, we investigate the performance of a wireless relay network where all the nodes are equipped with multiple antennas. Considering channel state information assisted amplify-and-forward protocol, we present analytical expressions for the symbol error rate (SER) and outage probability. More specifically, we first derive the SER expressions of a relay system with orthogonal space-time block coding (OSTBC) over correlated/keyhole fading channels. We also analyze the outage probability of interference corrupted relay systems with maximal ratio combing (MRC) at the receiver as well as multiple-input multiple-output MRC (MIMO MRC). Numerical results are given to illustrate and verify the analytical results.

A NOVEL MGF BASED ANALYSIS OF CHANNEL CAPACITY OF GENERALIZED-K FADING WITH MAXIMAL-RATIO COMBINING DIVERSITY

In this paper, we have analyzed the channel capacity by using the maximal-ratio combing (MRC) diversity scheme for communication systems operating over a composite fading environment modeled by the Generalized-K distribution at the receiver. For the Generalized-K fading channel with arbitrary values for small and large scale fading parameters, we have derived a closed-form expression for the moment generating function (MGF) of the received signal-to-noise ratio (SNR) and utilized it to obtain a novel closed-form expressions for the channel capacity under difierent adaptive transmission schemes. The result of the proposed methods is compared with other reported literature to support the analysis.

Propagation Channel Modeling in the Mixture of NLOS and LOS Environments for MIMO-MRC System and Its Application to ITS-IVC

This paper presents a Multiple-Input Multiple-Output (MIMO) propagation model for independent and identically distributed (i.i.d.) channels in the mixture of none-Line-of-Sight (NLOS) and Line-of-Sight (LOS) environments. The derived model enables to evaluate the system statistical characteristics of Signal-to-Noise-Ratio (SNR) for MIMO transmission based on Maximal Ratio Combing (MRC). An application example applying the model in 2×2 configuration to ITS Inter-Vehicle Communication (IVC) system is introduced. We clarify the effectiveness of the proposed model by comparisons of both computer simulations and measurement results of a field experiment. We also use the model to show the better performance of SNR when applying MIMO to IVC system than SISO and SIMO.

On Diversity Reception Over Fading Channels With Impulsive Noise

In this paper, we analyze the performance of different diversity combining techniques over fading channels with impulsive noise. We use Middleton's Class A model for the noise distribution and adopt two noise models, which assume dependent and independent noise components on each branch. We systematically analyze the performance of maximum ratio combing (MRC), equal gain combining (EGC), selection combining (SC), and post-detection combining (PDC) under these impulsive noise models, and derive insightful lower and upper bounds. We show that even under impulsive noise, the diversity order is retained for each combining scheme. However, we also show that under both models, there is a fundamental tradeoff between diversity gain and coding gain. Under the independent noise model, PDC is shown to combat impulsive noise more effectively than MRC, EGC, and SC. Our simulation results also corroborate our analysis.

Differential detection of GMSK signals with postdetection MRC over correlated and unbalanced Nakagami-m fading channels

Differential detection with postdetection diversity reception of Gaussian minimum shift keying (GMSK) signals has become increasingly important owing to its widespread use in wireless communications. Its error analysis, however, is not available in the literature. The authors derive a bit-error probability expression for the differential detection of GMSK signals with post-detection maximal ratio combing (MRC) in additive white Gaussian noise and slow frequency-nonselective arbitrarily correlated and unbalanced Nakagami-m fading channels, in which the diversity branches can have unequal signal-to-noise ratios (SNRs) as well as different severity parameters m. The effects of arbitrary values of the fading severity parameter m and the arbitrary correlation between the unbalanced L diversity channels are considered. The derived expression can be easily computed via numerical integration routines, and hence, can be usefully exploited in the performance evaluation of digital mobile radio systems.

OCDD/CDMA: a new DS/CDMA with orthonormal code diversity detection

The paper presents a new spread spectrum communication system called orthonormal code diversity detection (OCDD)/CDMA system based on the novel concept of orthonormal-basis diversity which is a generalization of the existing spread spectrum diversity concepts such as path diversity and frequency diversity. The OCDD/CDMA system is similar to the conventional DS/CDMA system in the transmitter structure, but is different in the receiver structure as it employs the extended orthonormal basis-function set which is the union of the Walsh basis-functions multiplied by the PN sequences and, optionally, their delayed replicas. The received signal is matched to the extended basis functions, and the matched signal components are combined together after individual channel compensation. The proposed OCDD/CDMA system exhibits the bit error performance which is much improved over the conventional DS/CDMA system using maximal ratio combing. In addition, it is robust to the chip timing error, which becomes more crucial in the future DS/CDMA systems having a higher data rate and smaller chip interval. From the simulation results, we confirm that the OCDD/CDMA system is a unique spread spectrum communication technique that can effectively increase the diversity utilization in the slowly fading channel, overcoming the inherent problems in the DS/CDMA and OFDM/CDMA systems.