Many wireless communication systems use a relay station for cooperative diversity or cell coverage extension. In this letter, an efficient partial single relay selection scheme is proposed for wireless communications. The conventional schemes such as the best harmonic mean and the threshold-based relay selection should know channel state informaion (CSI), or noise variance at all stations in advance. But the proposed scheme does not require any priori information. It uses a characteristic of the repeated signal pattern at candidates of the relay station. Simulation results show that the performance of proposed scheme is very close to the best harmonic mean relay selection scheme as one of the optimal relay selection schemes.

We theoretically analyze the performance of free-space optical (FSO) systems using cooperative-ARQ (C-ARQ), a joint scheme of automatic-repeat-request (ARQ) and cooperative diversity, over atmospheric turbulence channels. We also propose a modified C-ARQ (M-C-ARQ) scheme that allows relay nodes to store a copy of frames for the more efficient response to transmission failure so that both transmission delay and energy consumption can be improved. Using Markov chain-based analytical models for both schemes, the system performance is analytically studied in terms of frame-error rate, goodput and energy efficiency, which directly reflect the transmission delay and energy consumption. Numerical results confirm that the proposed schemes outperform conventional ones. In addition, we discuss cross-layer design strategies for selecting parameters in both physical and link layers in order to optimize the performance of FSO systems over different atmospheric turbulence conditions and channel distances.

Relay selection is a promising technique with which to achieve remarkable gains in multi-relay cooperative networks. Opportunistic relaying (OR) and selection cooperation (SC) are two major relay selection schemes for dual-hop decode-and-forward cooperative networks; they have been shown to be globally outage-optimal under an aggregate power constraint. However, due to channel fluctuations, the channel state information (CSI) used in the selection process may become outdated and differ from the CSI during the actual transmission of data. In this work, we study the effect of outdated CSI on OR and threshold-based SC (TSC) schemes under independent but not necessarily identically distributed Rayleigh fading channels. The source can possibly cooperate with the best relay for data transmission, with the destination performing maximal ratio combining of the signals from the source and the relay. In particular, we analyze the average symbol error probability (ASEP) of OR and TSC with outdated CSI by deriving approximate but tight closed-form expressions for the moment generating function of the end-to-end signal-to-noise ratio. We also investigate the asymptotic behavior of the ASEP. The results show that the diversity orders of OR and TSC reduce to one and two, respectively, due to the outdated CSI. However, TSC achieves full spatial diversity order when the relay-to-destination CSI is perfect. Finally, to verify the analytical results Monte Carlo simulations are performed, in which OR attains better ASEP than TSC in a perfect CSI scenario, while TSC is less susceptible to outdated CSI.

To provide robust wireless data transmission over fading channels, various schemes which involve the use of relays have been proposed. In some of those schemes, the relay chooses not to forward the received message if its reliability is deemed as too low. Some researchers refer to such schemes as selective decode-and-forward. Our work in this paper falls into such a category. More specifically speaking, the relay in our system is a censorial relay (a relay that performs censorial task). It evaluates the reliability, in terms of log likelihood ratio (LLR), of a received data bit (from the source). If its LLR magnitude is below some preset threshold, then it is censored (i.e. not sent to the destination). When the channel is Rayleigh faded, closed-form bit error rate (BER) expressions for the proposed system are derived for several scenarios. Those scenarios are differentiated by the availability of an energy detector (ED) and the various degrees of knowledge regarding the channel state information (CSI). Aided by those closed-form BER expressions, the system parameters can be efficiently optimized to achieve the minimum BER. Simulation results are observed to closely match theoretical values, as computed by the afore-mentioned closed-form BER expressions. As compared to some existing relay-assisted systems in which censoring is incorporated, the performance of our system is better in terms of BER when the same amount of CSI is exploited.

To accurately evaluate and manage future distributed wireless networks, it is indispensable to fully understand cooperative propagation channels. In this contribution, we propose cascaded multi-keyhole channel models for analyzing cooperative diversity wireless communications. The cascaded Wishart distribution is adopted to investigate the eigenvalue distribution of the multi-keyhole MIMO (multiple input multiple output) channel matrix, and the capacity performance is also presented for the wireless systems over such channels. A diversity order approximation method is proposed for better evaluating the eigenvalue and capacity distributions. The good match of analytical derivations and numerical simulations validates the proposed models and analysis methods. The proposed models can provide an important reference for the optimization and management of cooperative diversity wireless networks.

Cooperative relay selection, in which one of multiple relays is selected to retransmit the source signal to the destination, has received considerable attention in recent years, because it is a simple way to obtain cooperative diversity in wireless networks. The exact expression of outage probability for a decode-and-forward cooperative relay selection with multiple source and destination antennas over Rayleigh fading channels was recently derived in [9]. In this letter, we derive the exact expressions of outage probability and diversity-multiplexing tradeoff over independent and non-identically distributed Nakagami-m fading channels as an extension of [9]. We then analyze the effects of various parameters such as fading conditions, number of relays, and number of source and destination antennas on the outage probability.

Cooperative diversity using space-time codes offers effective space diversity with low complexity, but the scheme needs the space-time coding process in the relay nodes. We propose a simple cooperative relay scheme that uses space-time coding. In the scheme, the source node transmits the Alamouti coded signal sequences and the sink node receives the signal sequence via the two coordinated relay nodes. At the relay nodes, the operation procedure is just permutation and forwarding of the signal sequence. In the proposed scheme, none of the relay nodes need quadrature detection and space-time coding and the simple relay process offers effective space diversity. Moreover, simulations show the effectiveness of the proposed relay process by some simulations.

In this paper, we analyze the best relay selection scheme for the soft-decision-and-forward (SDF) cooperative networks with multiple relays. The term `best relay selection' implies that the relay having the largest end-to-end signal-to-noise ratio is selected to transmit in the second phase transmission. The approximate performances in terms of pairwise error probability (PEP) and bit error rate (BER) are analyzed and compared with the conventional multiple-relay transmission scheme where all the relays participate in the second phase transmission. Using the asymptotics of the Fox's H-function, the diversity orders of the best relay selection and conventional relay scheme for the SDF cooperative networks are derived. It is shown that both have the same full diversity order. The numerical results show that the best relay selection scheme outperforms the conventional one in terms of bit error rate.

The analytical derivation of the diversity-multiplexing tradeoff (DMT) for a half-duplex dynamic decode and forward (DDF) MIMO relay protocol has been regarded as an open problem. Recently, however, a minimization problem setting has been found, the solution of which corresponds to the DMT function for a half-duplex DDF MIMO relay protocol. In this paper, the DMT functions for three special half-duplex DDF MIMO relay protocols using two antennas at two of three nodes, source, relay, and destination nodes, and a single antenna at the other node are derived first. Then, the DMT function for a special half-duplex DDF MIMO relay protocol using two antennas at every node is derived. These DDF MIMO relay protocols are compared with one another and with some NAF MIMO relay protocols by simulation.

This work addresses the problem of a fast packet retransmission scheme intended for transporting delay-sensitive flows in a Cooperative Diversity (CD) environment. This cooperative fast retransmission scheme exploits the advantages of the CD environment and hybrid Automatic-Repeat-reQuest (ARQ), while allowing retransmission just one time via a cooperating user (i.e., partner) or via both the sender and the partner simultaneously. Complementary link packets are used for the retransmission whose policy can be adjusted on the basis of the qualities of channels among the sender, the partner and the receiver, as well as the application layer protocol data unit size, using the application throughput as the objective. For this scheme, we first derive the application throughput formulas which are then verified via simulations. Next, the CD-based optimized fast retransmission scheme is shown able to achieve better effective throughput (goodput) than other CD-based or non-CD-based ARQ schemes in various Nakagami-m slow-fading environments. As a result, the proposed scheme should be an excellent fast retransmission mechanism for real-time multimedia transport in many CD environments.

We consider Downlink (DL) scheduling for a multi-user cooperative cellular system with fixed relays. The conventional scheduling trend is to avoid interference by allocating orthogonal radio resources to each user, although simultaneous allocation of users on the same resource has been proven to be superior in, e.g., the broadcast channel. Therefore, we design a scheduler where in each frame, two selected relayed users are supported simultaneously through the Superposition Coding (SC) based scheme proposed in this paper. In this scheme, the messages destined to the two users are superposed in the modulation domain into three SC layers, allowing them to benefit from their high quality relayed links, thereby increasing the sum-rate. We derive the optimal power allocation over these three layers that maximizes the sum-rate under an equal rates' constraint. By integrating this scheme into the proposed scheduler, the simulation results show that our proposed SC scheduler provides high throughput and rate outage probability performance, indicating a significant fairness improvement. This validates the approach of simultaneous allocation versus orthogonal allocation in the cooperative cellular system.

This paper introduces a random selection cooperation scheme that takes the Decode-and-Forward (DF) approach to solve the unfairness problem in selection cooperation. Compared to previous work which obtained fairness but introduced performance loss, the proposed scheme guarantees fairness without performance loss. Its essence is to randomly select from the relays that can ensure the successful communication between the source and the destination, rather than to select the best relay. Both a theoretical analysis and simulation results confirm that the proposed scheme could achieve fairness and introduce no performance loss. We also discuss the conditions under which the proposed scheme is practical to implement.

In this letter, a multiuser cooperative network with multiple relays is introduced, and two decode-and-forward (DF) cooperation schemes are proposed aiming at outage-optimal and fair user scheduling, respectively. The outage probability and asymptotic expressions of symbol error probability (SEP) are derived to evaluate these two schemes. Analysis and simulations show that both schemes can achieve full diversity order, which is the combination of cooperative diversity and multiuser diversity.

Ad Hoc Routing (AHR) was proposed to replace optimal routing in cluster-based multihop networks since it offers lower implementation complexity. However, this complexity reduction comes at the cost of an increase in the required transmission power. In addition, when the conventional distributed relay selection is applied to implement AHR, another increase in the required transmission power occurs due to the receiver selection error. In this paper, Ad Hoc Cooperative Routing (AHCR) that integrates the cooperative transmission with AHR is presented to reduce the difference between the required transmission power of AHR and that of optimal routing. Besides, Distributed Ad Hoc Cooperative Routing (DAHCR) scheme 1 that combines the cooperative transmission with AHR is proposed to reduce the difference between the required transmission power of DAHR and that of AHR. We then address the problem of DAHCR scheme 1 and propose DAHCR scheme 2. Simulation results show that the required transmission power of AHCR and DAHCR scheme 1 is less than that of AHR and DAHR, respectively. In addition, DAHCR scheme 2 further reduces the required transmission power of DAHCR scheme 1. On the other hand, DAHCR scheme 1 increases the complexity by 43% compared to DAHR. Besides, DAHCR scheme 2 increases the complexity by 1.97% compared to DAHCR scheme 1.

In this paper, the performance of the soft-decision-and-forward (SDF) protocol in the cooperative communication network with one source, one relay, and one destination, where each node has two transmit and receive antennas, is analyzed in terms of the bit error rate (BER) obtained from the pairwise error probability (PEP). Using the moment generating function and Q-function approximation, the PEP of SDF protocol is calculated and we confirm that the SDF with two antennas achieves the full diversity order. For the slow-varying Rayleigh fading channel, the optimal power allocation ratio can be determined so as to minimize the average PEP (or BER). Due to the difficulty of deriving the optimal value analytically, an alternative strategy of maximizing the product signal-to-noise ratio (SNR) of direct and relay links, which we call the suboptimal power allocation, is considered. Through a numerical analysis, we show that the performance gap between the suboptimal and optimal power allocation strategies is negligible in the high SNR region.

Recent advances in cooperative communication and wireless Network Coding (NC) may lead to huge performance gains in relay systems. In this context, we focus on the two-way relay scenario, where two nodes exchange information via a common relay. We design a practical Superposition Coding (SC) based NC scheme for Decode-and-Forward (DF) half-duplex relaying, where the goal is to increase the achievable rate. By taking advantage of the direct link and by providing a suboptimal yet efficient power division among the superposed layers, our proposed SC two-way relaying scheme outperforms the reference schemes, including the well-known 3-step DF-NC scheme and the capacity of 2-step schemes for a large set of SNRs, while approaching closely the performance bound.

The selection cooperation is a basic and attractive scheme of cooperative diversity in the multiple relays scenario. Most previous schemes of selection cooperation consist only one relay-stage in which one relay is selected to retransmit, and the signal from the selected relay is not utilized by other relays. In this paper, we introduce a two relay-stage selection cooperation scheme. The performance can be improved by letting all other relays to utilize the signal from the first selected relay to make another selection and retransmission in the second relay-stage. We derive the closed-form expression of the outage probability of the proposed scheme in the high SNR regime. Both theoretical and numerical results suggest that the proposed scheme can reduce the outage probability compared with the traditional scheme with only one relay-stage. Furthermore, we demonstrate that more than two relay-stage can not further reduce the outage probability. We also study the dependence of the proposed scheme on stage lengths and topology, and analyze the increased overhead.

This paper proposes cooperative coding using cyclic delay diversity (CDD) for OFDM systems. The cooperative diversity is combined with channel coding while CDD is applied to the cooperative transmission of the multiple relays to improve the beneficial effects of the cooperating relays. Analyses of frame error probability (FEP) and the average channel power of the proposed scheme are shown. Simulation results show the frame error rate (FER) of the proposed scheme. The proposed scheme provides not only a simple code design and low system complexity compared to conventional space-time processing, but better FER and diversity gain compared to direct transmission and conventional cooperative coding without CDD.

We propose 3-hop cooperative diversity using QOSTBC (Quasi-Orthogonal Space-Time Block Code), which offers 3-hop cooperative diversity without signal separation in relay nodes. The key of our proposed scheme is encoding signal sequence in different signal unit according to relay stage. This letter explains details of the proposed scheme and shows that it offers interference reduction among streams and space diversity gain by result of simulations.

In this letter, we analyze the error performance of a code combining based cooperative diversity protocol. For coded transmission schemes, code-combining can obtain a near optimal low rate code by combining repeated codewords. An analytical method for evaluating the performances of such scheme is presented. We develop a closed form expression for pairwise error probability and tight upper bounds for bit error rate (BER) and frame error rate (FER) under Rayleigh fading environment. The analytical upper bounds are verified with simulation results.