Distributed BLAST with relay selection for multi‐antenna AF relaying

By examining the diversity–multiplexing tradeoff (DMT) of an amplify‐and‐forward (AF) multi‐antenna multi‐relay network, a procedure to select the optimal number of relays and relay candidates is established. Subsequently, a distributed BLAST transmission scheme in conjunction with successive nulling and cancellation at destination is proposed to achieve the optimal DMT. Illustrative results indicate that the proposed method can significantly enhance the outage probability performance, and for a selected number of relays in use, the performance enhancement can be further improved by increasing the number of available relays in the AF network. Copyright © 2010 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.     

Exact SEP and performance bounds for orthogonal space‐time block codes in cooperative multiantenna AF relaying networks

In this paper, we derive a moment generating function (MGF) for dual‐hop (DH) amplify‐and‐forward (AF) relaying networks, in which all nodes have an arbitrary number of antennas, with orthogonal space‐time block code (OSTBC) transmissions over Rayleigh fading channels. We present an exact error rate expression based on the derived MGF and another analytical approach to derive achievable performance bounds as closed‐forms of symbol error rate, outage probability, and normalized channel capacity. Furthermore, we derive the asymptotic behavior of symbol error rate and outage probability. From this asymptotic behavior, it is shown that the diversity order and its dependence on antenna configurations can be explicitly determined. Simulation results are also presented to verify their accuracy by comparing with numerical results and to provide an insight to the relationship between relaying networks' antenna configuration and diversity order. It is confirmed that the transmit antenna gain of the source node and the receive antenna gain of the relay node can be obtained only when the relay is close to the destination, and then, the transmit antenna gain of the relay node and the receive antenna gain of the destination node can be obtained only when the relay is close to the source.     

Randomized cyclic delay code for cooperative communication systems

We deal here with distributed cooperative communication systems in which a single source transmits its information with the help of multiple relays to a destination. Specifically, we consider decode and forward relay networks based on orthogonal frequency division multiplexing (OFDM). Most previous work needs a priori information for antenna allocation to obtain cooperative diversity advantages. We propose a simple method that does not require additional information in order to coordinate relay transmission. Our main proposal is to retransmit randomly cyclic-shifted sequences of the received symbol at relays. Simulation results verify that full gain in diversity is achieved when the number of subcarriers is sufficiently large.     

Error performance for relaying protocols with multiple decode‐and‐forward relays

This paper investigates the error performance of three relaying protocols with multiple decode‐and‐forward relays. In the first protocol, relays that can decode correctly will forward the signals from source. Nevertheless, selection cooperation (SC) and opportunistic relaying (OR) are adopted to select only a single relay to forward in the other two protocols, respectively. At sufficiently high signal‐to‐noise ratio, the upper bounds on bit error probability are derived for three protocols, where the developments apply for various channel fading models. Simulation results are provided to verify the tightness of the analytical bounds, and the performance comparisons among different relaying protocols are presented. Copyright © 2010 John Wiley & Sons, Ltd.     

Mixed AF and DF cooperative relay systems and their performance bounds analyses

This paper analyzes the performance bounds of a wireless relay system consisting of several relay stations working on both amplifier‐and‐forward (AF) and decode‐and‐forward (DF) protocols. We want to study the outage probability behavior of the proposed mixed AF and DF relay systems under independent Nakagami‐m fading channels. In particular, we will derive the lower and upper bounds of outage probability of the mixed AF and DF relay systems based on maximal ratio combining diversity reception. The results give optimal configuration of AF and DF relays under a specific channel condition, thus helping us to design an optimized mixed AF and DF relay system in a generic fading environment. The trade‐off between complexity and performance is discussed in this paper. In addition, we will use computer simulations to verify the effectiveness of the proposed mixed AF and DF relay configurations. Finally, the power allocation issues for such a mixed AF and DF relay system will also be discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Packet splitting and adaptive modulation based on CSI of time domain for decode‐and‐forward cooperative OFDM systems in different channel model

Cooperative communications obtain the transmission and channel diversity gains by using the relay node. However, since cooperative communications transmit the redundancy signal to obtain the transmission diversity gain, the transmission rate is degraded. Moreover, since cooperative communications add the interference in the relay node, the diversity gain is also degraded. The packet splitting has been proposed based on the channel state information of the time domain to obtain the good system performance without the redundancy signal. Moreover, the adaptive modulation has been proposed to improve the transmission rate. In this paper, we propose the combination method with the packet splitting and the adaptive modulation based on the channel state information of the time domain to improve the bit error rate and throughput performances for decode‐and‐forward cooperative orthogonal frequency division multiplexing systems in the different channel model. From the computer simulation results, we determine the optimum weight and threshold for the proposed method. Moreover, the proposed method shows the good bit error rate and throughput performances.     

Optimal power‐aware relay placement for cooperative wireless sensor networks

We study the problem of optimizing the symbol error probability (SEP) performance of cluster‐based cooperative wireless sensor networks. Recent studies in literature show that an efficient relay selection protocol based on simple geographical information of the nodes to execute cooperative diversity can significantly improve the SEP performance at the destination of such networks. As well, similar line of research on optimal power allocation (for the source and relay nodes) can be found in literature. However, to achieve the best SEP performance at the destination of a cooperative wireless sensor network, joint optimization of power allocation and relay placement should be accomplished. To this aim, we reformulate the SEP of a multi‐hop cooperative communication in a general form and optimize transmitted power level and relay placement simultaneously. This analysis is developed for both amplify‐and‐forward and decode‐and‐forward relaying protocols. Simulation results demonstrate that the proposed joint optimization can effectively improve the SEP performance of the network. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance and diversity analysis of decode‐and‐forward cooperative system over Nakagami‐m fading channels

In this paper, we derive the upper bound of bit‐error rate (BER) performance and diversity gain for a decode‐and‐forward (DF) cooperative system. Either maximal‐ratio combining (MRC) or cooperative MRC (C‐MRC) is employed at the receiver in the presence of independent, non‐identical Nakagami‐m fading. Both analytical and simulation results show that C‐MRC takes advantage of spatial diversity more efficiently and thus achieves the same or better performance and diversity order as compared to MRC. Copyright © 2009 John Wiley & Sons, Ltd.     

Interference‐resistant cooperative wireless networks based on complementary codes

Spatial diversity in wireless networks can be attained by exploiting the broadcast nature of wireless transmission without the need of multiple antennas in individual device, leading to the implementation of cooperative communication. While most prior works focused on the single source—destination scenario, it should be more realistic to consider how to induce cooperation among multiple source‐destination pairs assisted by multiple relays. In such a case, multiple access interference (MAI) may present due to asynchronous transmissions of the users and relays. In this paper, a cooperative network architecture based on orthogonal complementary (OC) codes inherently immune to MAI is proposed. To efficiently utilize the scarce radio spectrum and codes, a centralized medium access control (MAC) protocol is proposed to coordinate the code assignment and channel access among users and relays. We theoretically analyze the bit error rate (BER) performance of the proposed OC coded cooperative network over multipath Rayleigh fading channel. The performance gain resulted from different numbers of relays is investigated, and compared with a time division multiple access (TDMA) based cooperative scheme. We show that the proposed OC coded cooperative network performs well in the presence of timing offset, and thus is well suited for asynchronous uplink transmission with cooperative relaying. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance analysis of multi‐antenna relay communication systems with MRC

The performance of multi‐antenna relay communication systems is investigated in this letter. The source broadcasts the signal to all the relays and the destination. Decode‐and‐forward scheme is adopted at the relays, and the destination employs maximum ratio combining (MRC) technique to maximize the received signal‐to‐noise ratio. Closed‐form expressions of outage probability are derived in the MRC case and minimum routes MRC case. An adaptive grouping algorithm of relay antennas is also presented under the constraint of total antenna number. Simulation results show that the analytical curves agree with the simulated ones very well, and the performance of the proposed grouping algorithm is very close to the upper bound mentioned in other papers. Copyright © 2012 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.     

Cooperative quadrature spatial modulation with Euclidean distance and capacity optimized antenna selection

In this study, new cooperative‐quadrature spatial modulation techniques based on Euclidean distance and capacity optimized antenna selection techniques (EDAS‐CQSM and COAS‐CQSM) are proposed for cooperative communication systems. The considered cooperative system consists of three terminals that are the source terminal (ST), relay terminal (RT), and destination terminal (DT). The decode‐and‐forward (DF) cooperative technique is considered to retransmit the signal at the RT. In order to improve the performance of the conventional QSM demodulator in the RT and DT, both antenna selection techniques have been used in both the ST and RT to reduce performance degradation due to error propagation of the DF scheme. Therefore, the overall performance of the considered CQSM system has been increased. Furthermore, computer simulations have shown that the proposed EDAS/COAS‐CQSM systems have better the performances than the cooperative spatial modulation (CSM), CQSM, EDAS‐CSM, COAS‐CSM, and traditional DF‐based cooperative system with the same bit rate.     

Performance analysis of switch‐and‐stay combining in two‐way relay systems with analog network coding and time‐division broadcast protocols

In this paper, we consider switch‐and‐stay combining (SSC) in two‐way relay systems with two amplify‐and‐forward relays, one of which is activated to assist the information exchange between the two sources. The system operates in either analog network coding (ANC) protocol where the communication is only achieved with the help of the active relay or time‐division broadcast (TDBC) protocol where the direct link between two sources can be utilized to exploit more diversity gain. In both cases, we study the outage probability and bit error rate (BER) for Rayleigh fading channels. In particular, we derive closed‐form lower bounds for the outage probability and the average BER, which remain tight for different fading conditions. We also present asymptotic analysis for both the outage probability and the average BER at high signal‐to‐noise ratio. It is shown that SSC can achieve the full diversity order in two‐way relay systems for both ANC and TDBC protocols with proper switching thresholds. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance analysis of MIMO cognitive amplify‐and‐forward relay networks with orthogonal space–time block codes

In this paper, we study the performance of multiple‐input multiple‐output cognitive amplify‐and‐forward relay networks using orthogonal space–time block coding over independent Nakagami‐m fading. It is assumed that both the direct transmission and the relaying transmission from the secondary transmitter to the secondary receiver are applicable. In order to process the received signals from these links, selection combining is adopted at the secondary receiver. To evaluate the system performance, an expression for the outage probability valid for an arbitrary number of transceiver antennas is presented. We also derive a tight approximation for the symbol error rate to quantify the error probability. In addition, the asymptotic performance in the high signal‐to‐noise ratio regime is investigated to render insights into the diversity behavior of the considered networks. To reveal the effect of network parameters on the system performance in terms of outage probability and symbol error rate, selected numerical results are presented. In particular, these results show that the performance of the system is enhanced when increasing the number of antennas at the transceivers of the secondary network. However, increasing the number of antennas at the primary receiver leads to a degradation in the secondary system performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance analysis of wireless multihop diversity systems

We study the performance of multihop diversity systems with non‐regenerative relays over independent and non‐identical Rayleigh fading channels. The analysis is based on the evaluation of the instantaneous end‐to‐end signal‐to‐noise ratio (SNR), depending on the type of the relay and the diversity scheme used. A closed‐form expression is derived for the average end‐to‐end SNR, when fixed‐gain relays and a maximal ratio combiner are used; also, an analytical expression formula for the average symbol‐error rate (ASER) for the above case is presented. The results show that, as expected, multihop diversity systems outperform conventional telecommunication systems in terms of ASER when the same amount of energy is assumed to be consumed in both cases. Copyright © 2008 John Wiley & Sons, Ltd.     

A novel relay selection algorithm using ant colony optimization with artificial noise for secrecy enhancement in cooperative networks

Cooperative communication is one of the fastest growing research areas of today. It can efficiently mitigate the effect of shadowing and fading with the help of relays and proper relay selection technique. In this paper, a novel relay selection scheme combined with artificial noise (AN) is devised to enhance the secrecy of cooperative networks with amplify‐and‐forward scheme, over Rayleigh fading channels in the presence of a passive eavesdropper. The probability of path selection of ant colony optimization algorithm is used for selecting the best relay with high end‐to‐end signal‐to‐noise ratio. The probability of choosing a path depends on the significance of channel gain (G) and fading coefficients (h). The proposed algorithm finds the best relay in the following wireless scenarios: when (i) both channel gain and fading coefficients are significant; (ii) only fading coefficients are significant; and (iii) only channel gain is significant. Because the direct links between source and destination and source and eavesdropper are considered, AN along with the information is sent by both the source and the selected relay. The performance is evaluated based on secrecy rate (R_s); for the relays randomly placed between the source and destination and for different eavesdropper's location. The results show that the proposed relay selection scheme achieves better secrecy for different wireless scenarios compared with traditional schemes. With the help of AN, the secrecy rate can be made positive even when the eavesdropper lies near to source.     

Secure Connectivity Probability of Multi‐hop Clustered Randomize‐and‐Forward Networks

This work investigates secure cluster‐aided multi‐hop randomize‐and‐forward networks. We present a hop‐by‐hop multi‐hop transmission scheme with relay selection, which evaluates for each cluster the relays that can securely receive the message. We propose an analytical model to derive the secure connectivity probability (SCP) of the hop‐by‐hop transmission scheme. For comparison, we also analyze SCPs of traditional end‐to‐end transmission schemes with two relay‐selection policies. We perform simulations, and our analytical results verify that the proposed hop‐by‐hop scheme is superior to end‐to‐end schemes, especially with a large number of hops or high eavesdropper channel quality. Numerical results also show that the proposed hop‐by‐hop scheme achieves near‐optimal performance in terms of the SCP.     

Energy Savings in OFDM Systems through Cooperative Relaying

Energy savings in orthogonal frequency division multiplexing (OFDM) systems is an active research area. In order to achieve a solution, we propose a new cooperative relaying scheme operated on a per subcarrier basis. This scheme improves the bit error rate (BER) performance of the conventional signal‐to‐noise ratio (SNR)‐based selection relaying scheme by substituting SNR with symbol error probability (SEP) to evaluate the received signal quality at the relay more reliably. Since the cooperative relaying provides spatial diversity gain for each subcarrier, thus statistically enhancing the reliability of subcarriers at the destination, the total number of lost subcarriers due to deep fading is reduced. In other words, cooperative relaying can alleviate error symbols in a codeword so that the error correction capability of forward error correction codes can be fully exploited to improve the BER performance (or save transmission energy at a target BER). Monte‐Carlo simulations validate the proposed approach.     

Outage performance of relay selection with spatially random relays using RF energy harvesting

To enable green wireless networks, one appealing approach is to deploy energy harvesting (EH) relays to assist the source transmission. Unlike conventional relays relying on fixed power supplies, EH relays make use of the energy collected from the RF radiation of the source node, and thus, they do not introduce extra energy cost to the network. This paper presents an analytical study to assess the efficacy of EH relays when the one with the maximal end‐to‐end signal‐to‐noise ratio is selected to perform data relaying while others perform EH. Because the action (either harvesting energy or forwarding data) of one EH relay affects those of others, exact performance analysis is not tractable. Additionally, relay density and positions may be random, which further complicates the analysis. Our analysis is conducted based on the hypothesis that each EH relay has an equal chance to be selected. This hypothesis allows for analytical tractability and is of importance to EH relays because otherwise some may drain their batteries fast. We identify the conditions under which the aforementioned hypothesis is valid. Our analysis also considers two variants of amplify‐and‐forward relays with and without using channel state information. Numerical results are presented to validate the analysis accuracy along with extensive discussions on the impact of numerous system parameters. Copyright © 2015 John Wiley & Sons, Ltd.