Decode‐and‐forward two‐way relaying protocol with one retransmission

This paper investigates the decode‐and‐forward two‐way relaying channel without direct link and proposes a protocol based on the physical‐layer network coding (PNC) protocol. The proposed protocol (termed ORT) introduces one retransmission into PNC, aiming at enhancing its outage performance. To manifest the merits of ORT, we compare it with PNC and the time‐division broadcast (TDBC) protocol, in terms of outage performance, expected rate, and diversity‐multiplexing tradeoff (DMT). Firstly, we derive the outage probability of the three protocols and then the expected rate. Secondly, asymptotic analysis is conducted to shed light on the diversity and coding gains. Finally, the DMT is obtained for the three protocols. The numerical results reveal the following: (i) that ORT performs better than PNC in both outage and expected rate performance when the nodes transmit with different powers. However, it has the same DMT performance with PNC; (ii) that ORT possesses improved DMT performance over TDBC whereas its expected rate is only better than the latter at medium to high signal‐to‐noise ratio. Copyright © 2012 John Wiley & Sons, Ltd.     

Nth best relay selection with network coding in two‐way relay systems

In this paper, we propose an Nth best relay selection (Nth‐RS) scheme for analog network coding in two‐way relay systems. In traditional two‐way single‐relay selection schemes, only the best one is selected to forward network‐coded signals. However, in practical applications, the best relay may be unavailable because of the scheduling or overload constraints. In this case, we investigate a more general scheme, where the Nth best but available relay is selected. To evaluate the transmission of reliability, the expression of outage probability in exponential–integral form and its asymptotic expression in closed form are presented. Moreover, the upper bound and lower bound of outage probability are also derived. The analysis reveals that the diversity order of Nth‐RS equals to (M ? N + 1), where M is the number of relay nodes, and the results are verified by simulations. In order to improve system performance, transmit power between sources and relay is optimally allocated to minimize the upper bound of outage probability under total power constraint. Simulation results show that Nth‐RS scheme with proposed power allocation can achieve substantial improvement over equal power allocation scheme. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance evaluation of multiuser diversity in multiuser two‐hop cooperative multi‐relay wireless networks using maximal ratio combining over Rayleigh fading channels

Multiuser diversity (MUD) cooperative wireless networks combine the features of the MIMO systems without confronting the physical layer constraints by providing multiple copies of the transmitted signal from the source to the destination with the help of the relay node. Cooperative wireless networks have attracted the full attention in the last few years and are implemented widely in many wireless communication systems to adapt for the fading impairments, provide higher data rates, and improve the performance of the wireless communication systems. In this paper, we present an informative study for the reason of evaluating the performance of the MUD in the multiuser two‐hop cooperative multi‐relay networks using maximal ratio combining. Furthermore, we derive tight closed‐form expressions of outage probability and symbol error probability for the amplify‐and‐forward and fixed decode‐and‐forward protocols with the MUD. Additionally, we conduct a simulation study to show to what extent our analytical and simulation results agree with each other. It is worthy to mention that our analytical and simulation results agree fairly with each other under high average signal‐to‐noise ratio, whereas they show that our proposed system with multiple relays provides significant improvements over those previously proposed systems having only one relay. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance analysis of time division broadcast protocol with incremental relaying and symmetric users

This paper investigates two‐way relaying with direct link transmission. We propose to combine incremental relaying (IR) with the time division broadcast protocol (TDBC) to form a new scheme, termed TDBC‐IR, to increase the spectral efficiency of TDBC. Assuming half‐duplex decode‐and‐forward relaying, the performance of three protocols, that is, TDBC, TDBC‐IR, and physical‐layer network coding (PNC), are studied and compared, in terms of outage probability, expected rate (ER), and diversity‐multiplexing tradeoff. First, we reveal that the outage probability performance of TDBC‐IR is the same as that of TDBC and better than that of PNC. Second, we compare the ER and diversity‐multiplexing tradeoff performance of the three protocols based on the derived outage probability. The numerical results reveal that (1) the ER of TDBC‐IR is greater than that of TDBC and PNC in the whole signal‐to‐noise ratio (SNR) region, that is TDBC‐IR has improved spectral efficiency performance; (2) the ER performance of TDBC is better than PNC in the low SNR region but worse in the high SNR region; (3) the maximum achievable diversity order of TDBC‐IR and TDBC are both two whereas that of PNC is one; (4) the maximum achievable multiplexing gain of TDBC‐IR is one, which is better than TDBC (its maximum achievable is 2/3) and achieves the maximum of PNC (its maximum achievable is one). Copyright © 2012 John Wiley & Sons, Ltd.     

Outage Probability of Decode‐and‐Forward Relaying Systems with Efficient Partial Relay Selection in Nakagami Fading Channels

Recently, efficient partial relay selection (e‐PRS) was proposed as an enhanced version of PRS. In comparing e‐PRS, PRS, and the best relay selection (BRS), there is a tradeoff between complexity and performance; that is, the complexity for PRS, e‐PRS, and BRS is low to high, respectively, but vice versa for performance. In this paper, we study the outage probability for e‐PRS in decode‐and‐forward (DF) relaying systems over non‐identical Nakagami‐m fading channels, where the fading parameter m is an integer. In particular, we provide closed‐form expressions of the exact outage probability and asymptotic outage probability for e‐PRS in DF relaying systems. Numerical results show that e‐PRS achieves similar outage performance to that of BRS for a low or medium signal‐to‐noise ratio, a high fading parameter, a small number of relays, and a large difference between the average channel powers for the first and the second hops.     

Physical‐layer network coding with limited feedback using orthogonal space–time block codes

This paper introduces limited feedback technique into physical‐layer network coding (PLNC) scheme, which is the most spectrally efficient protocol in two‐way relay channels, consisted of one relay and two end nodes (sources). Decode‐and‐forward (DF) and partial‐decode‐and‐forward (PDF) strategies are considered for PLNC, and all nodes are assumed to have two antennas to allow transmission by Alamouti's orthogonal space–time block code to provide diversity. In DF, by limited feedback, one of the sources is informed about instantaneous channel state information (CSI) to increase the bit error rate (BER) performance at relay. The closed‐form upper and lower bounds on the bit error probability are derived for binary phase‐shift keying (BPSK) and quadrature PSK (QPSK) modulations and approved via computer simulations. In PDF strategy, each source has to know CSI between relay and the other source for decoding, which causes extra protocol complexity. Moreover, for the system in which all nodes have two antennas, classical PDF strategy does not satisfy orthogonality at the end nodes. Therefore, in this paper, a modified‐PDF (MPDF) strategy with limited feedback is proposed. In MPDF, for decoding at the end nodes, differential phase information between channel fading coefficients having maximum amplitudes is fed back to the sources by relay. This approach enables single‐symbol decoding, besides full diversity, and sources do not need to know CSI between relay and the other source. It is shown via computer simulations that MPDF strategy provides significantly better BER performance than the classical PDF for BPSK and QPSK modulations.Copyright © 2012 John Wiley & Sons, Ltd.     

Performance analysis of AF OFDM system using multiple relay in presence of nonlinear‐PA over inid Nakagami‐m fading

In this paper, performance of an orthogonal frequency division multiplexing–based variable‐gain amplify and forward cooperative system using multiple relay with relay selection is analyzed over independent but not necessarily identically distributed frequency selective Nakagami‐m fading channels. For the analysis, nonlinear power amplifier is considered at the relay, and selection combining is adopted at destination node. Closed‐form expressions of the outage probability for various threshold signal‐to‐noise ratio (SNR) values and average symbol error rate for M‐ary quadrature amplitude modulation techniques are derived for the considered system. Further, the outage probability analysis is performed in high SNR regime to obtain the diversity order. Furthermore, impact of different fading parameters, multiple relay, and nonlinear power amplifier is highlighted on the outage probability and asymptotic outage probability for various threshold SNRs and on the average symbol error rate for various quadrature amplitude modulation constellations. The derived analytical expressions are generalized for various fading environments while considering the integer‐valued fading parameters. Finally, all the analytical results are verified through the Monte Carlo simulations for various SNR levels and system configurations.     

Performance analysis of two‐way wireless‐powered Amplify‐and‐Forward relaying in the presence of co‐channel interference

In this paper, we investigate the performance assessment of a bidirectional relaying system using energy harvesting techniques. We assume independent and nonidentically distributed (i.n.i.d.) Nakagami‐m fading channels where the amplify‐and‐forward relay is subject to co‐channel interference (CCI) due to transmissions of other transmitters. Two different scenarios, namely, scenario I and scenario II are evaluated. In scenario I, both end‐sources provide the required energy for the relay, whereas the relay also harvests energy from the co‐channel interferes. Then, in the first phase of cooperation, both end‐sources send the information to the relay, and after amplifying the received signal, relay transfers information to the appropriate destination in the second time‐slot. In the scenario II, both end‐sources harvest energy from the relay. After that, the information cooperative transmission is done similar to the first scenario. For both considered scenarios, tight closed‐form expressions of outage probability, symbol error probability, ergodic capacity, and throughput are obtained at arbitrary signal‐to‐noise‐ratios (SNRs). To get more insights, simplified high SNR results for both scenarios are also deduced where the diversity orders are obtained. Monte Carlo simulation results are presented to validate the correctness of our proposed analysis. Our results explicitly demonstrate that the first scenario has a better performance than the second one in the medium and high SNR region, whereas the second scenario outperforms the first one in the low SNR regime.     

Performance analysis of spectrally efficient amplify‐and‐forward opportunistic relaying scheme for adaptive cooperative wireless systems

In this paper, we propose an adaptive amplify‐and‐forward (AF) relaying scheme that selects the best relay among the available relay nodes opportunistically to cooperate with a source node for improvement of the spectral efficiency. This improvement can be achieved by introducing a policy that gives the useful cooperative regions and defines a switching threshold signal‐to‐noise ratio that guarantees the bit error rate (BER) of cooperative transmission is below the target. We model all links as independent non‐identically distributed Rayleigh fading channels. We then derive closed‐form expressions for the average spectral efficiency, average BER, and outage probability when an upper bound for the signal‐to‐noise ratio of the end‐to‐end relay path is applied and adaptive discrete rate is considered. Numerical and simulation results show that the proposed scheme, compared with the outage‐based AF incremental relaying, AF fixed relaying, and the conventional direct transmission, can achieve the maximum average spectral efficiency while maintaining the average BER and outage probability. Copyright © 2013 John Wiley & Sons, Ltd.     

BER‐modified regenerative protocols for OFDM‐based two‐way wireless relay channel

It is well known that symbol‐level regenerative relay protocols suffer the error propagation problem because receiver decodes blindly and overlooks the probability of relay forwarding wrong bits. In a two‐way relay networks, the problem still exists in both network coding (decode‐and‐forward) and physical network coding (denoise‐and‐forward) protocols. For today's widely adopted wide band Orthogonal frequency‐division multiplexing (OFDM) systems, error propagation will dramatically restrict the system's end‐to‐end performance especially when frequency selective fading exists. In this paper, we propose a bit error rate (BER) modified decoding algorithm for these OFDM‐based two‐way symbol‐level regenerative relay strategies. By confining the confidence level of demodulated soft information according to the likelihood of relay having made an error on each bit, this proposed algorithm significantly boosts the end‐to‐end BER performance of the system. Copyright © 2015 John Wiley & Sons, Ltd.     

Performance analysis of hybrid transmit antenna selection/maximal‐ratio transmission in Nakagami‐m fading channels

Hybrid diversity systems have been of great importance because they provide better diversity orders and robustness to the fading effects of wireless communication systems. This paper focuses on the performance analysis of multiple‐input gle‐output systems that employ combined transmit antenna selection (TAS)/maximal‐ratio transmission (MRT) techniques (i.e., hybrid TAS/MRT). The probability density function, the moment generating function and the n ^th order moments of the output signal‐to‐noise ratio of the investigated diversity scheme are derived for independent identically distributed flat Nakagami‐m fading channels. The system capacity of the hybrid TAS/MRT scheme is examined from the outage probability perspective. Exact bit/symbol error rate (BER/SER) expressions for binary frequency shift keying, M‐ary phase shift keying and square M‐ary quadrature amplitude modulation signals are derived by using the moment generating function‐based analysis method. By deriving the upper bounds for BER/SER expressions, it is also shown that the investigated systems achieve full diversity orders at high signal‐to‐noise ratios. Also, by Monte Carlo simulations, analytical performance results are validated and the effect of feedback delay, channel estimation error and feedback quantization error on BER/SER performances are examined. Copyright © 2011 John Wiley & Sons, Ltd.     

Variable‐rate adaptive modulation with optimum switching thresholds for cooperative systems with relay selection

In this paper, the performance of variable‐rate adaptive modulation schemes in the amplify‐and‐forward cooperative systems with relay selection is analyzed over Rayleigh fading channels. We consider constant power and discrete‐rate adaptive multi‐level modulation techniques. The switching levels required for discrete‐rate adaptive modulation have been determined for two schemes, namely fixed switching levels and optimum switching levels, both respecting a target bit error rate requirements, where in the later scheme, the switching levels are optimally determined in a way that the average spectral efficiency of the system is maximized. Two M‐ary modulation schemes, namely quadrature amplitude modulation and phase shift keying, are considered. Closed‐form expressions are derived for three performance metrics, namely average spectral efficiency, outage probability, and average bit error rate, for two cases: independent and identically distributed fading relay links and independent and non‐identically distributed links. It is shown that, compared with using fixed switching levels, employing optimum switching levels provides a slight improvement in the spectral efficiency and moderate improvements in the signal‐to‐noise ratio gain and in the outage probability of the system. It is also shown that compared with the independent and identically distributed links, independent and non‐identically distributed relay links yield a slight increase in the signal‐to‐noise ratio gain and a slight decrease in the diversity order of the system. Copyright © 2011 John Wiley & Sons, Ltd.     

Opportunistic Decode‐and‐Forward Cooperation in Mixed Rayleigh and Rician Fading Channels

We consider a two‐hop multiple‐relay network implemented with opportunistic decode‐and‐forward cooperative strategy, where the first hop and second hop links experience different fading (Rayleigh and Rician). We derive the exact expressions of end‐to‐end outage probability and analyze the approximate results in high signal‐to‐noise ratio region. The analysis shows that the same diversity order can be achieved even in different mixed fading environments. Simulation results are provided to verify our analysis.     

Study on cognitive DF relaying cooperation with the mutual interference between primary and secondary users over Nakagami‐m fading channels

In this paper, the cognitive relay cooperation (CRC) wireless communication systems are investigated over Nakagami‐m fading channels. The decode‐and‐forward (DF) relay is employed to assist the communications between cognitive source and destination. Especially, to achieve full diversity order, we consider the case in which there is a direct path between cognitive source and destination. Besides the interference at primary users (PUs) created by secondary users (SUs), the interference at SUs created by PUs is also considered. For the interested CRC systems, we first achieve the exact expression for the CDF of the equivalent end‐to‐end signal‐to‐interference ratio (SIR) of CRC systems. Then, with the exact CDF, the exact average symbol error ratio (SER) and outage performance of CRC systems are achieved. The derivation is of significance, by which we can obtain a detailed knowledge about CRC systems. Though a single integral included in the derivation, it can be calculated numerically by employing some mathematical tools such as Matlab. At the same time, to obtain the insight and highlight the effect of system parameters on the considered CRC systems, by using the high SIR approximation, we obtain the asymptotic closed‐form expression of CDF as well as the ones of average SER and outage probability. From the asymptotic results, we can find the main factors that dominate the performance of CRC systems. The presented simulation results for outage probability and average SER show the derivations and simulations are in agreement. Moreover, in high SIR the achieved asymptotic results match well the exact ones. As a result, in high SIR we can employ the asymptotic closed‐form solutions to evaluate the exact performance of CRC systems. This can reduce greatly the implementation complexity. Besides this, the simulations also show that the diversity order is dominated by the fading severities of the secondary systems, i.e. the diversity order be proportional to the summation of the minimum fading severity between the two hops and that of the direct link. In contrast, the parameters of the primary systems only affect the coding gain, not the diversity gain. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance analysis of cooperative multi‐carrier relay‐based UAV networks over generalized fading channels

The outage probability in a network of cooperative unmanned airborne vehicles (UAVs) over generalized fading channels is studied analytically using finite mixture with expectation maximization technique. A relay‐based topology with one ground control unit (GCU) is considered, where the cooperative UAVs can communicate with the GCU directly or through relay. The application the UAV is assigned for specifies the minimum required transmission rate the UAV should achieve. The outage probability of the system is defined as the probability that either the transmission rate over any of the links drops below a predefined minimum threshold for that link or the Relay‐GCU link is not able to transmit the aggregate data from all relayed UAVs and the minimum rate required by the relay UAV itself. Throughout the paper, expressions for the outage probability and the average achievable bit rate of a cooperative multi‐carrier system are derived over generalized fading channels. Finite mixture with expectation maximization algorithm is utilized to derive a simple approximate expression for the probability density function (pdf) of the achievable bit rate assuming adaptive M‐ary quadrature amplitude modulation (M‐QAM). This pdf is used to derive closed‐form expressions for the outage probability and the average bit rate. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance analysis of MIMO systems with multiuser diversity

In this paper, we present a comprehensive performance analysis for multiple‐input multiple‐output (MIMO) systems with multiuser diversity over Rayleigh fading channels. We derive exact closed‐form expressions of the outage probability and the average bit error rate (BER) for different MIMO schemes, including the selective combining (SC), maximum ratio combining (MRC) and space‐time block codes (STBC). We also provide the explicit upper bounds on the BER performance. Finally, the mathematical formalism is verified by numeric results that study the interaction between the antenna diversity and the multiuser diversity. It is observed that the system performance is deteriorated as the number of transmit antennas increases in multiuser scenario, which is unlike the case in single‐user systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Comparative performance analysis of cooperative and multi dual‐hop relay networks using MGF approach

In this paper, we analyzed and compared the performance of cooperative diversity systems such as cooperative and multi dual‐hop networks with non‐regenerative relay nodes. The contributions of this study are twofolds. Firstly, analytical expressions of outage probability P_out and average symbol error rate ASER are derived using moment generating function (MGF) analysis of the received SNR with the assumption that the channel experiences Weibull fading and the best relay selection is used. Then, using the analytical results, comparative performance evaluation of cooperative and multi dual‐hop relay networks is done for varying number of relay nodes and different receive diversity schemes such as maximal ratio combining (MRC) and selection combining (SC). The results show that the cooperative relay network has better performance than a multi dual‐hop relay network in terms of P_out and ASER. The results also show that the multi dual‐hop network can achieve the same performance as the cooperative network with the requirement that it needs the deployment of three times more relay nodes.     

Performance analysis of dual switched diversity over correlated Weibull fading channels with co‐channel interference

This paper studies the performance of switch and stay combining (SSC) diversity in the presence of co‐channel interference over correlated Weibull fading channels. SSC diversity based on signal‐to‐interference ratio (SIR) is a low‐complexity and a very efficient technique that reduces fading and co‐channel interference influence. New closed‐form expressions for the probability density function and cumulative distribution function of the output SIR's are derived. These formulas are used in a detailed analysis of the average output SIR and outage probability. The influence of fading severity and correlation coefficient on the optimum switching threshold and system performance is investigated. Monte Carlo simulations are performed to verify obtained theoretical results and determine average bit error rate in detecting binary phase‐shift keying (BPSK), differential BPSK and quadrature amplitude modulation signals. Copyright © 2011 John Wiley & Sons, Ltd.     

Cognitive amplify‐and‐forward relay networks with beamforming under primary user power constraint over Nakagami‐m fading channels

In this paper, we analyze the performance of cognitive amplify‐and‐forward (AF) relay networks with beamforming under the peak interference power constraint of the primary user (PU). We focus on the scenario that beamforming is applied at the multi‐antenna secondary transmitter and receiver. Also, the secondary relay network operates in channel state information‐assisted AF mode, and the signals undergo independent Nakagami‐m fading. In particular, closed‐form expressions for the outage probability and symbol error rate (SER) of the considered network over Nakagami‐m fading are presented. More importantly, asymptotic closed‐form expressions for the outage probability and SER are derived. These tractable closed‐form expressions for the network performance readily enable us to evaluate and examine the impact of network parameters on the system performance. Specifically, the impact of the number of antennas, the fading severity parameters, the channel mean powers, and the peak interference power is addressed. The asymptotic analysis manifests that the peak interference power constraint imposed on the secondary relay network has no effect on the diversity gain. However, the coding gain is affected by the fading parameters of the links from the primary receiver to the secondary relay network. Copyright © 2012 John Wiley & Sons, Ltd.     

Single Relay Selection for Bidirectional Cooperative Networks with Physical‐Layer Network Coding

To serve the growing demand of the bidirectional information exchange, we propose a single relay selection (RS) scheme for physical‐layer network coding (PNC) in a bidirectional cooperative network consisting of two sources and multiple relays. This RS scheme selects a single best relay by maximizing the bottleneck of the capacity region of both information flows in the bidirectional network. We show that the proposed RS rule minimizes the outage probability and that it can be used as a performance benchmark for any RS rules with PNC. We derive a closed‐form exact expression of the outage probability for the proposed RS rule and show that it achieves full diversity gain. Finally, numerical results demonstrate the validity of our analysis.