Half-duplex Relay Research Articles

When Are Dynamic Relaying Strategies Necessary in Half-Duplex Wireless Networks?

In this paper, we study a simple question: when are dynamic relaying strategies essential in optimizing the diversity-multiplexing tradeoff (DMT) in half-duplex wireless relay networks? This is motivated by apparently two contrasting results even for a simple three-node network with a single half-duplex relay. When all channels in the system are assumed to be independent and identically fading, a static schedule where the relay listens half the time and transmits half the time combined with quantize-map-and-forward (QMF) relaying is known to achieve the full-duplex performance. However, when there is no direct link between the source and the destination, a dynamic decode-and-forward (DDF) strategy is needed to achieve the optimal tradeoff. In this case, a static schedule is strictly suboptimal and the optimal tradeoff is significantly worse than the full-duplex performance. In this paper, we study the general case when the direct link is neither as strong as the other links nor fully nonexistent, and identify regimes where dynamic schedules are necessary and those where static schedules are enough. We identify four qualitatively different regimes for the single-relay channel, where the tradeoff between diversity and multiplexing is significantly different. We show that in all these regimes one of the above two strategies is sufficient to achieve the optimal tradeoff by developing a new upper bound on the best achievable tradeoff under channel state information available only at the receivers. A natural next question is whether these two strategies are sufficient to achieve the DMT of more general half-duplex wireless networks with a larger number of relays. We propose a generalization of the two existing schemes through a dynamic QMF (DQMF) strategy, where the relay listens for a fraction of time depending on received channel state information but not long enough to be able to decode. We show that such a DQMF strategy is needed to achieve the optimal DMT in a parallel channel with two relays, outperforming both DDF and static QMF strategies.

A Proposed Chaotic-Switched Turbo Coding Design and Its Application for Half-Duplex Relay Channel

Both reliability and security are two important subjects in modern digital communications, each with a variety of subdisciplines. In this paper we introduce a new proposed secure turbo coding system which combines chaotic dynamics and turbo coding reliability together. As we utilize the chaotic maps as a tool for hiding and securing the coding design in turbo coding system, this proposed system model can provide both data secrecy and data reliability in one process to combat problems in an insecure and unreliable data channel link. To support our research, we provide different schemes to design a chaotic secure reliable turbo coding system which we call chaotic-switched turbo coding schemes. In these schemes the design of turbo codes chaotically changed depending on one or more chaotic maps. Extensions of these chaotic-switched turbo coding schemes to half-duplex relay systems are also described. Results of simulations of these new secure turbo coding schemes are compared to classical turbo codes with the same coding parameters and the proposed system is able to achieve secured reasonable bit error rate performance when it is made to switch between different puncturing and design configuration parameters especially with low switching rates.

Relay-Assisted Opposite Directional Interference Alignment: Feasibility Condition and Achievable Degrees of Freedom

This letter proposes an opposite directional interference alignment (ODIA) algorithm for the general $K$ -user Gaussian interference channel with a half-duplex MIMO relay, where the numbers of transmit and receive antennas are arbitrary. By exploiting the additional space dimensions introduced by the MIMO relay, the algorithm requires only global channel state information (CSI) available at the relay to align the interference from the relay link along the opposite direction of the interference from the direct link. The ODIA feasibility involves the solvability of a linear system. We then derive a sufficient and necessary feasibility condition by using the properties of Kruskal-rank of Khatri-Rao product, and give a closed-form solution. Furthermore, the exact expression of the sum degrees of freedom (DoF) achieved through ODIA is provided and is also verified by simulation results.

Maximum Throughput Path Selection With Random Blockage for Indoor 60 GHz Relay Networks

Indoor communications in the 60 GHz band is capable of supporting multi-gigabit wireless access thanks to the abundant spectrum and the possibility of using dense antenna arrays. However, the high directivity and penetration loss make it vulnerable to blockage events, which can be frequent in indoor environments. Given network topology information in sufficient precision, we investigate the average throughput and outage probability when the connection between any two nodes can be established either via the line-of-sight (LOS) link, through a reflection link, or by a half-duplex relay node. We model the reflection link as an LOS with extra power loss and derive the closed-form expression for the relative reflection loss. For networks with a central coordinator and multiple relays, we also propose a generic algorithm, maximum throughput path selection (MTPS), to select the optimal path that maximizes the throughput. The complexity of the MTPS algorithm is O(n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) for networks equipped with n relays, whereas a brute-forced algorithm has complexity of O(n · n!). Numerical results show that increasing the number of relays can significantly increase the average throughput and decrease the outage probability, and resorting to reflection paths provides significant gains when the probability of link blockage is high.

Overcoming Half-Duplex Loss in Multi-Relay Networks: Multiple Relay Coded Cooperation for Optimal DMT

This paper studies diversity-multiplexing tradeoff (DMT) of decode-and-forward (DF)-based half-duplex relay networks without a direct path from source to destination. We propose a new multiple relay coded cooperation (MRCC) protocol to overcome the half-duplex loss in DF relay networks and show that the proposed MRCC protocol achieves the MISO/SIMO DMT upper bound as the number of assisting relays increases. The loss of multiplexing gain from the half-duplex constraint is recovered by coded cooperation and successive cancellation. Despite its notable gain in terms of DMT, the proposed MRCC protocol requires only limited feedback information/signaling for relay and operation mode selection. We also derive the achievable DMT of the proposed MRCC protocol when each node has multiple antennas. It is shown that the proposed MRCC protocol achieves K times diversity gain of the point-to-point N × N MIMO case. Compared with existing protocols, the MRCC protocol shows much higher diversity gain for almost all multiplexing gains. Furthermore, our numerical results verify that the proposed MRCC protocol outperforms existing protocols in terms of outage probability for the environments of practical interest.

Buffer-Aided Relaying for Two-Hop Secure Communication

We consider using a buffer-aided relay to enhance security for two-hop half-duplex relay networks with an external eavesdropper. We propose a link selection scheme that adapts reception and transmission time slots based on channel quality, while considering both the two-hop transmission efficiency and security. Closed-form expressions for the secrecy throughput and the secrecy outage probability (SOP) are derived, and the selection parameters are optimized to maximize the secrecy throughput or minimize the SOP. We then analyze two sub-optimal link selection schemes that in general only require a line search to solve the optimization problem, and we show that, under certain conditions, these approaches also admit closed-form solutions. All schemes are discussed in the context of two different scenarios where the relay either knows or does not know the channel to the legitimate receiver. In the former case, the relay adopts adaptive-rate transmission, whereas for the latter, it uses fixed-rate transmission. Numerical results show that buffer-aided relaying provides a significant improvement in security compared with conventional unbuffered relaying. Furthermore, the performance of the sub-optimal schemes is shown to approach the optimal one for certain ranges of signal-to-noise-ratio (SNR) or SOP constraints.

CAO-SIR: Channel Aware Ordered Successive Relaying

Cooperative communication suffers from multiplexing loss and low spectral efficiency due to the half duplex constraint of relays. To improve the multiplexing gain, successive relaying, which allows concurrent transmission of the source and relays, has been proposed. However, the severe inter-relay interference becomes a key challenge. In this paper, we propose a channel aware successive relaying protocol, also referred to as CAO-SIR, which is capable of thoroughly mitigating inter-relay interference by carefully adapting relays' transmission order and rate. In particular, a relay having a poorer link to the source is scheduled first to forward a message, the data rate of which is adapted to the link quality of the source-relay and relay-destination channels. By this means, each relay may decode the messages intended for the preceding relays, and then cancel these relays' interference in a low complexity which is equal to that of Decision Feedback Equalizer (DFE). To further optimize and analyze CAO-SIR, we present its equivalent parallel relay channel model, based upon which the adaptive relay selection and power allocation schemes are proposed. By employing M half duplex relays, CAO-SIR is capable of achieving an diversity-multiplexing tradeoff (DMT) given by d(r) = max (M + 1) 1- {(M+2/M+1r)} , (1 - r) , where d(r) and r denote the diversity and multiplexing gains, respectively. Its DMT asymptotically approaches the DMT upper bound achieved by (M + 1) × 1 MISO systems or M full duplex relays, when M is large.

Compute-and-Forward on a Multi-User Multi-Relay Channel

In this paper, we consider a system in which multiple users communicate with a destination with the help of multiple half-duplex relays. Based on the compute-and-forward scheme, each relay, instead of decoding the users' messages, decodes an integer-valued linear combination that relates the transmitted messages. Then, it forwards the linear combination towards the destination. Given these linear combinations, the destination may or may not recover the transmitted messages since the linear combinations are not always full rank. Therefore, we propose an algorithm where we optimize the precoding factor at the users such that the probability that the equations are full rank is increased and that the transmission rate is maximized. We show, through some numerical examples, the effectiveness of our algorithm and the advantage of performing precoding allocation at the users. Also, we show that this scheme can outperform standard relaying techniques in certain regimes.

양방향 복호전달 릴레이를 사용하는 레거시 OFDMA 시스템에서의 자원 할당 기법

OFDMA systems solves frequency selective fading problem and provides improved performance by optimal allocation of subcarriers and transmit power. Two-way relay systems provide improved spectral efficiency compared to that of the conventional half-duplex relay using bidirectional communications. In legacy OFDMA system such as WiBro, two-way DF relay utilization causes pilot re-assignment and impossibility of channel estimation and decoding at relay nodes by self-interference. In this paper, resource allocation schemes for legacy OFDMA systems with two-way DF relay are proposed. The proposed schemes allocate subcarriers considering destinations nodes which are connected to relay nodes as individual nodes which are directly connected to a base station. Subsequently, the proposed schemes compensate bandwidth loss due to orthogonal allocations by overlapped allocating unused subcarriers at other noes. Numerical simulations show that the proposed resource allocation schemes provide improved performance compared with orthogonal allocation.

Full Duplex Relay Based Cognitive Radio System with Physical Layer Network Coding

In this paper, a full duplex relay (FDR) based cognitive radio (CR) system is proposed with two secondary source nodes and two secondary destination nodes. The concept of physical layer network coding (PLNC) is applied on decode and forward (DF) FDR to ensure that both source nodes can transmit data to both destination nodes. Further, the availability of multiple spectrum bands enables CR system to decode source symbols at destination nodes. The major advantage of the proposed system with FDR and PLNC is that it requires only one time slot to send the data from both the source nodes to both the destination nodes. If FDR is replaced with half duplex relay (HDR), 2 time slots are required for the CR system. If PLNC is not applied at the DF relay, the CR system would require 2 or 4 time slots when FDR or HDR is employed, respectively. In Nakagami-$$m$$m fading channels environment, analytical expressions are derived for end-to-end outage performance of the proposed PLNC based system in the presence of echo interference at the FDR and with HDR in the place of FDR. The outage performance of the proposed CR system is analyzed for various scaling and shape parameters of Nakagami-$$m$$m fading channels.

List Decoding for Arbitrarily Varying Broadcast Channels With Receiver Side Information

In this paper, the discrete memoryless arbitrarily varying broadcast channel (AVBC) with receiver side information is studied and its random code and deterministic code capacity regions are derived for the average error criterion. In addition, it is analyzed for deterministic list codes and it is shown that the corresponding list capacity region displays a behavior, which is similar to Ahlswede's famous dichotomy result for the single-user arbitrarily varying channel: it either equals the random code capacity region or otherwise has an empty interior. This is characterized in terms of list sizes at the receivers and an appropriate concept of symmetrizability for the AVBC with receiver side information. The scenario studied here is motivated by the broadcast phase of bidirectional relaying, where a half-duplex relay node establishes a bidirectional communication between two other nodes using a decode-and-forward protocol. The relay decodes the messages both nodes have sent in the initial multiple access phase and broadcasts a re-encoded composition of them in the succeeding broadcast phase. Then, the broadcast phase corresponds to the AVBC with receiver side information, which differs from the classical broadcast channel, since both receivers can exploit their own messages from the previous phase as side information for decoding.

A Buffer-Aided Successive Opportunistic Relay Selection Scheme With Power Adaptation and Inter-Relay Interference Cancellation for Cooperative Diversity Systems

In this paper we consider a simple cooperative network consisting of a source, a destination and a cluster of decode-and-forward half-duplex relays. At each time-slot, the source and (possibly) one of the relays transmit a packet to another relay and the destination, respectively, resulting in inter-relay interference (IRI). In this work, with the aid of buffers at the relays, we mitigate the detrimental effect of IRI through interference cancellation. More specifically, we propose the min-power scheme that minimizes the total energy expenditure per time slot under an IRI cancellation scheme. Apart from minimizing the energy expenditure, the min-power selection scheme, also provides better throughput and lower outage probability than existing works in the literature. It is the first time that interference cancellation is combined with buffer-aided relays and power adaptation to mitigate the IRI and minimize the energy expenditure. The new relay selection policy is analyzed in terms of outage probability and diversity, by modeling the evolution of the relay buffers as a Markov Chain (MC). We construct the state transition matrix of the MC, and hence obtain the steady state with which we can characterize the outage probability. The proposed scheme outperforms relevant state-of-the-art relay selection schemes in terms of throughput, diversity and energy efficiency, as demonstrated via examples.

Design of training sequences and channel estimation for amplify-and-forward two-path relaying networks

Abstract This paper proposes a new channel estimation scheme for two-path relaying networks where two amplify-and-forward (AF) half-duplex relay nodes alternatively transmit the signals received from a source node. The channel estimation and interference cancellation would be mutually conditional in two-path relaying networks. To overcome such a difficulty, we design new training sequences and propose precoding matrices for use at the source and relays. Deriving the Cramer-Rao lower bound (CRLB), it is shown that the proposed estimator is efficient, i.e., the proposed channel estimation scheme achieves the CRLB. The performance of the proposed channel estimation is evaluated through the computer simulations where the validity of the theoretical analysis is also demonstrated.

Error Probability Analysis of a MIMO System With Noncoherent Relaying and MMSE Reception

This paper deals with symbol error probability (SEP) analysis of a multiple-input-multiple-output (MIMO) system with spatial multiplexing, which employs noncoherent (i.e., without channel state information at the relays) amplify-and-forward half-duplex relays and linear minimum mean square error (MMSE) equalization at the destination. A closed-form approximation of the SEP is derived, which holds for an arbitrary number of relays placed in different positions. Numerical results show that the obtained SEP approximation is very accurate if the number of relays is greater than two.

Relay Selection for Two-Way Full Duplex Relay Networks With Amplify-and-Forward Protocol

The full duplex (FD) technique, which allows the communication node to transmit and receive signals over the same frequency band simultaneously, has the potential to double the spectral efficiency in comparison with the traditional half duplex (HD) technique. However, self-interference, leaking from the FD node's transmission to its own reception, has the detrimental impact on the performance of FD communication. In this paper, we analyze and optimize the two-way FD relay system using amplify-and-forward protocol, when the multi-relay scenario is considered. The optimal relay selection scheme in maximizing the effective signal-to-interference and noise ratio is proposed, which significantly improves the system performance than a single relay network. Furthermore, to facilitate the comparisons with the traditional two-way HD relay, the analytical expressions of the two-way FD relay are derived in a closed form, including bit error rate (BER), ergodic capacity, and outage probability. Based on the analytical expressions, the optimal power allocation and the optimal choice of duplex mode, i.e., FD and HD, are obtained by minimizing the outage probability. Monte-Carlo simulations are fulfilled to verify the analytical expressions. The results reveal that the residual self-interference after interference suppression limits the performance of two-way FD relay: when the residual interference is small, the FD mode has lower BER/outage probability and higher ergodic capacity since it utilizes the resources effectively; otherwise, the HD mode achieves lower BER/outage probability and higher ergodic capacity since it can completely cancel the self-interference at the cost of lower resource utilization.

Resource Allocation for Two Source-Destination Pairs Sharing a Single Relay with a Buffer

In this paper, we obtain the optimal resource allocation scheme in order to maximize the achievable rate region in a dual-hop system that consists of two independent source-destination pairs sharing a single half-duplex relay. The relay decodes the received information and possesses buffers to enable storing the information temporarily before forwarding it to the respective destination. We consider both non-orthogonal transmission with successive interference cancellation at the receivers and orthogonal transmission. Also, we consider Gaussian block-fading channels and we assume that the channel state information is known and that no delay constraints are required. We show that, with the aid of buffering at the relay, joint user-and-hop scheduling is optimal and can enhance the achievable rate significantly. This is due to the joint exploitation of multiuser diversity and multihop diversity in the system. We provide closed-form expressions to characterize the average achievable rates in a generic form as functions of the statistical model of the channels. Furthermore, we consider sub-optimal schemes that exploit the diversity in the system partially and we provide numerical results to compare the different schemes and demonstrate the gains of the optimal one.

Achieving the capacity of half-duplex degraded relay channels using polar coding

In this paper, a novel transmission protocol based on polar coding is proposed for the half-duplex degraded relay channel. In the proposed protocol, referred to as the partial message relaying, the relay only needs to forward a part of the decoded source message that the destination needs according to the exquisite nested structure of polar codes. Theoretically, it is proved that the scheme can achieve the capacity of the half-duplex relay channel under the decode-and-forward (DF) cooperation strategy while enjoying low encoding/decoding complexity. Practically, in order to minimize the global transmission power, the optimization of the power allocation is performed between the source and the relay by using information theoretic tools. Furthermore, a joint iterative soft parallel interference cancellation receiver structure is developed to suit to the proposed scheme. Simulation results show that the proposed scheme outperforms the conventional scheme designed by low-density parity-check (LDPC) codes.

Gaussian Half-Duplex Relay Networks: Improved Constant Gap and Connections With the Assignment Problem

This paper considers a Gaussian relay network where a source transmits a message to a destination with the help of N half-duplex relays. The information theoretic cut-set upper bound to the capacity is shown to be achieved to within 1.96(N+2) bits by noisy network coding, thereby reducing the previously known gap. This gap is obtained as a special case of a more general constant gap result for Gaussian half-duplex multicast networks. It is then shown that the generalized degrees-of-freedom of this network is the solution of a linear program, where the coefficients of the linear inequality constraints are proved to be the solution of several linear programs referred as the assignment problem in graph theory, for which efficient numerical algorithms exist. The optimal schedule, that is, the optimal value of the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sup> possible transmit-receive configuration states for the relays, is investigated and known results for diamond networks are extended to general relay networks. It is shown, for the case of N=2 relays, that only N+1=3 out of the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sup> =4 possible states have a strictly positive probability and suffice to characterize the capacity to within a constant gap. Extensive experimental results show that, for a general N -relay network with N≤8 , the optimal schedule has at most N+1 states with a strictly positive probability. As an extension of a conjecture presented for diamond networks, it is conjectured that this result holds for any half-duplex relay network and any number of relays. Finally, a network with N=2 relays is studied in detail to illustrate the channel conditions under which selecting the best relay is not optimal, and to highlight the nature of the rate gain due to multiple relays.

Generalized maximum likelihood noncoherent block detection for decode-and-forward relay systems

This paper considers noncoherent cooperative decode-and-forward (DF) half-duplex multi-branch relay systems. Each relay branch is modeled as a probabilistic transition system at the last hop, and thus it can be considered as a relaying chain comprising multi-hop relays. An approximation to the generalized maximum likelihood (ML) noncoherent block detection is derived for uncoded M-ary modulation in a faded noisy environment. In particular, the derived noncoherent block detection in a noiseless case is equivalent to a multichannel reception with full diversity. Furthermore, the generalized detection is extended specifically to block coded M-ary phase shift keying (MPSK) modulation. For a DF three node relay system using block coded quadrature phase shift keying (QPSK), simulation results are provided to examine the end-to-end error performance of the noncoherent detection with considering the effects of network geometry and power allocation, respectively. It is shown that under a fixed power allocation, a proper relay placement can yield near full diversity for large signal-to-noise ratio.

Buffer-aided cooperative communications: opportunities and challenges

Cooperative communication can increase the throughput and/or extend the coverage of wireless networks. However, in conventional cooperative networks, half-duplex relays transmit and receive under a prefixed schedule, which does not allow them to exploit the best receiving and transmitting channels, thus limiting performance. Recently, new protocols have been proposed that circumvent this problem by making use of the additional flexibility offered by relays with buffers. Compared to conventional relaying protocols, these buffer-aided protocols provide significant gains in terms of throughput, diversity, and signal-to-noise ratio. This article outlines several buffer-aided relaying protocols for different network topologies, including one-way single- and multi-relay networks as well as two-way single-relay networks. Moreover, some practical challenges inherent to buffer-aided relaying, such as increased delay and complexity, and topics for future research are discussed.