Half-duplex Relay Research Articles

Polar codes for soft decode-and-forward in half-duplex relay channels

Soft decode-and-forward (DF) can combine the advantages of both amplify-and-forward and hard DF in relay channels. In this paper, we propose a low-complexity soft DF scheme based on polar codes, which features two key techniques: a low-complexity cyclic redundancy check (CRC) aided list successive cancellation (CALSC) decoder and a soft information calculation method. At the relay node, a low-complexity CALSC decoder is designed to reduce the computational complexity by adjusting the list size according to the reliabilities of decoded bits. Based on the path probability metric of the CALSC decoder, we propose a method to compute the soft information of the decoded bits in CALSC. Simulation results show that our proposed scheme outperforms the soft DF based on low-density parity-check codes and the soft DF with belief propagation or soft cancellation decoder, especially in the case when the source-relay channel is at the high signal-to-ratio region.

A New Upper Bound on the Achievable Rate of Relay Channel With MIP-QF Strategy

Recently, a new mutual information preserving quantize-and-forward (MIP-QF) is proposed for the half-duplex relay channel, where the relay node does not need to decode the source signal correctly. In this paper, a new MIP-QF upper bound on the achievable rate of the half-duplex relay channel is proposed, which is shown to be lying between the compress-and-forward (CF) bound and the cut-set bound. For the half-duplex additive white Gaussian noise (AWGN) relay channel, the specific MIP-QF bound and the optimal duplex ratios of the MIP-QF upper bound and the MMSE-CF bound are also derived. In addition, we also show that the decode-and-forward (DF) bound can exceed the MIP-QF bound in some special cases that the relay is close to the source and far away from the destination. The validity and accuracy of the proposed theoretical results are verified by numerical results under the half-duplex AWGN relay channel.

Full-Duplex Regenerative Relaying and Energy-Efficiency Optimization Over Generalized Asymmetric Fading Channels

This paper is devoted to the end-to-end performance analysis, optimal power allocation (OPA), and energy-efficiency (EE) optimization of decode-and-forward (DF)-based full-duplex relaying (FDR) and half-duplex relaying (HDR) systems. Unlike existing analyses and works that assume simplified transmission over symmetric fading channels, we consider the more realistic case of asymmetric multipath fading and shadowing conditions. To this end, exact and asymptotic analytic expressions are first derived for the end-to-end outage probabilities (OPs) of the considered DF-FDR set ups. Based on these expressions, we then formulate the OPA and EE optimization problems under given end-to-end target OP and maximum total transmit power constraints. It is shown that OP in FDR systems is highly dependent upon the different fading parameters and that OPA provides substantial performance gains, particularly, when the relay self-interference (SI) level is strong. Finally, the FDR is shown to be more energy-efficient than its HDR counterpart, as energy savings beyond 50% are feasible even for moderate values of the SI levels, especially at larger link distances, under given total transmit power constraints and OP requirements.

A novel forwarding method for full-duplex two-way relay networks

An estimate-and-forward (EF) scheme for single-input single-output (SISO) and multiple-input multiple-output (MIMO) full-duplex two-way relay networks is proposed and analyzed. The relay estimates the received signal from two terminal nodes by a minimum mean squared error (MMSE) estimation and forwards a scaled version of the MMSE estimate to the destination. The proposed EF outperforms conventional amplify- and-forward (AF) and decode-and-forward (DF) across all signal-to-noise ratio (SNR) region. Because its computational complexity is high for relays with a large number of antennas (large MIMO) and/or high order constellations, an approximate EF scheme, called list EF, are thus proposed to reduce the computational complexity. The proposed list EF computes a candidate list for the MMSE estimate by using a sphere decoder, and it approaches the performance of the exact EF relay at a negligible performance loss. The proposed forwarding approach also could be used to other relay networks, such as half-duplex, one-way or massive MIMO relay networks.

Two-Way Relay Protocol for Optical Wireless Scattering Communication

This letter addresses the half-duplex bidirectional relay protocol design and optimization for outdoor optical wireless scattering communication system. Such a relay system consists of two user nodes and one relay node. The relay node broadcasts to the user nodes using a predetermined power based on the number of photoelectrons received from the user nodes. Each user node can estimate the signal from the other node according to the combination of its own symbol and the broadcast signal from the relay. We consider two types of protocols based on the hard-decision and soft-forwarding at the relay node, named the hard-decision relaying protocol and the soft-forwarding relaying protocol, respectively. Simulation results show that the soft-forwarding relaying protocol can reach smaller bit error rate compared with the hard-decision relaying protocol.

Transmission Power Adaption for Full-Duplex Relay-Aided Device-to-Device Communication

Device-to-device (D2D) communications bring significant improvements of spectral efficiency by underlaying cellular networks. However, they also lead to a more deteriorative interference environment for cellular users, especially the users in severely deep fading or shadowing. In this paper, we investigate a relay-based communication scheme in cellular systems, where the D2D communications are exploited to aid the cellular downlink transmissions by acting as relay nodes with underlaying cellular networks. We modeled two-antenna infrastructure relays employed for D2D relay. The D2D transmitter is able to transmit and receive signals simultaneously over the same frequency band. Then we proposed an efficient power allocation algorithm for the base station (BS) and D2D relay to reduce the loopback interference which is inherent due to the two-antenna infrastructure in full-duplex (FD) mode. We derived the optimal power allocation problem in closed form under the independent power constraint. Simulation results show that the algorithm reduces the power consumption of D2D relay to the greatest extent and also guarantees cellular users’ minimum transmit rate. Moreover, it also outperforms the existing half-duplex (HD) relay mode in terms of achievable rate of D2D.

Two-way relaying schemes in full duplex cellular system

Recently, full duplex (FD) has been attracting great attention, due to its capability to double the spectral efficiency. In this paper, we focus on a FD wireless communication network, in which a FD base station (BS) and a half duplex (HD) mobile station (MS) exchange their information via a HD relay station (RS) within two phases. An amplify-and-forward (AF) relaying scheme and a decode-and-forward (DF) relaying scheme are proposed for the two-way relay network. In the proposed DF relaying scheme, the RS chooses the best DF relaying mode according to channel state information (CSI) to achieve the maximum capacity. Specifically, the RS can decode and forward data streams from both the BS and MS, or only one of them, or none of them, and thus obtaining a selection diversity gain. In order to analyze the performance of the proposed schemes, achievable rate regions and sum-capacities of the proposed schemes are derived in closed form. Numerical and simulation results show that the proposed relaying schemes provide significant capacity gain.

Capacity of the Gaussian Two-Hop Full-Duplex Relay Channel With Residual Self-Interference

In this paper, we investigate the capacity of the Gaussian two-hop full-duplex (FD) relay channel with residual self-interference. This channel is comprised of a source, an FD relay, and a destination, where a direct source-destination link does not exist and the FD relay is impaired by residual self-interference. We adopt the worst case linear self-interference model with respect to the channel capacity, and model the residual self-interference as a Gaussian random variable whose variance depends on the amplitude of the transmit symbol of the relay. For this channel, we derive the capacity and propose an explicit capacity-achieving coding scheme. Thereby, we show that the optimal input distribution at the source is Gaussian and its variance depends on the amplitude of the transmit symbol of the relay. On the other hand, the optimal input distribution at the relay is discrete or Gaussian, where the latter case occurs only when the relay-destination link is the bottleneck link. The derived capacity converges to the capacity of the two-hop ideal FD relay channel without self-interference and to the capacity of the two-hop half-duplex (HD) relay channel in the limiting cases when the residual self-interference is zero and infinite, respectively. Our numerical results show that significant performance gains are achieved with the proposed capacity-achieving coding scheme compared with the achievable rates of conventional HD relaying and/or conventional FD relaying.

Enhancing physical-layer security via big-data-aided hybrid relay selection

The explosive growth in data traffic presents new challenges to the new generation of wireless communication systems, such as computing capabilities, spectrum efficiency and security. In this paper, we use the network structure, which is adaptable for the big data traffic, to improve the security of wireless networks. Specifically, a big-data aided hybrid relay selection scheme is designed and analyzed to enhance physical layer security. First, considering the ideal situation that an eavesdropper’s CSI (Channel State Information) is known to the legal nodes, we propose an optimal hybrid relay selection scheme consisting of the optimal mode selection scheme and the optimal relay selection scheme. In this case, we analyze the upper bound of an eavesdropper’s capacity in FD (Full-Duplex) mode and the secrecy outage probabilities of the optimal HD (Half-Duplex), FD, and hybrid relay selection schemes. Through the analysis of data, it is clear that the mode selection is decided by the self-interference of the FD technique. However, the instantaneous CSI of an eavesdropper is difficult to obtain due to the passive characteristic of eavesdroppers in practice. Therefore, a more practical hybrid relay selection scheme with only the channel distribution information of an eavesdropper is further studied, where a weighting factor is employed to guarantee that the hybrid mode is no worse than either the FD mode or HD mode when the self-interference grows. Finally, the simulation results show the improved security of our proposed scheme.

Joint Power Allocation and Strategy Selection for Half-Duplex Relay System

Decode and forward (DF) and compress and forward (CF) are two efficient cooperation strategies for relay networks. The combination of the DF and CF strategies is promising to improve the achievable rate of cooperative communication systems. To thoroughly aggregate the advantage of the DF and CF strategies, we put forward a joint power allocation and strategy selection (PASS) scheme for the half-duplex relay channel. The PASS scheme is shown to achieve a higher rate compared with a regular hybrid DF/CF scheme. The rate gain obtained in the static relay channel results from actively adjusting and optimizing the relay power, followed by selecting the better strategy between DF and CF. In particular, when the relay receives and transmits in sequential time slots, we characterize the positive rate improvement area of the PASS scheme analytically and give a near-optimal setting for approaching the best rate performance. The PASS scheme is extended to fading relay channels. We show that the rate increment obtained from the PASS scheme is further magnified by employing an advanced relay power allocation technique over channel states. The corresponding optimal power allocation is established based on the concavity of the rate achieved by the PASS scheme in static relay channel. Numerical results are presented to validate the analysis.

Optimal power allocation for a full-duplex multicarrier decode-forward relay system with or without direct link

This paper addresses power allocation problems for a dual-hop full-duplex multicarrier decode-forward relay system with or without a direct link from the source to the destination. The full-duplex relay has a residual self-interference proportional to its transmitted power. We consider two schemes of decode-forward at the relay: carrier-wise decode-forward (CDF) and group-wise decode-forward (GDF). For the CDF scheme, we consider problems of optimal power allocation subject to system-wise total power constraint, node-wise individual power constraint and system-wise rate constraint, respectively. All these problems are shown to be equivalent to convex problems, and fast algorithms for finding the exact solutions are developed. For the GDF scheme, we focus on the case of node-wise individual power constraint. This problem is non-convex for which we develop fast algorithms for finding locally optimal solutions. Using the algorithms developed in this paper, we are able to show that the system capacity with optimal power allocation based on either CDF or GDF is higher than that of the half-duplex relay (HDR) system at power levels where HDR outperforms the direct transmission via the direct link. Furthermore, the system capacity based on GDF is consistently higher than that of HDR for all power levels while both have the same degree of freedom. This paper also shows new insights of algorithmic development, which should be useful for other related problems.

Buffer-Aided Relaying With Discrete Transmission Rates for the Two-Hop Half-Duplex Relay Network

We consider the two-hop half-duplex (HD) relay network, where the source-to-relay and relay-to-destination links are impaired by block fading. The relay is equipped with a buffer, which enables the relay to receive or transmit in each time slot independent of previous time slots. As a practical constraint, source and relay can transmit only at rates taken from predefined and finite sets. Thereby, it is assumed that for each time slot, the instantaneous qualities of the two links are available. For this network, we derive the optimal scheduling of reception and transmission at the relay and the optimal rate selection at source and relay, such that the throughput is maximized. Since the optimal protocol introduces unbounded delay, we also propose a buffer-aided protocol, which limits the delay. For this delay-limited protocol, we study the achieved delay and throughput by modeling the queue at the buffer as a Markov chain. Our numerical results show that the throughputs achieved with the proposed buffer-aided protocols for discrete transmission rates are significantly larger than the throughputs achieved with conventional relaying protocols where the HD relay switches between reception and transmission in a strictly alternating manner.

Network-Coded Two-Way Relaying in Spectrum-Sharing Systems With Quality-of-Service Requirements

We investigate the performance of a dual-hop two-way cognitive radio system, where the secondary users (SUs) exchange information in an underlay mode with the assistance of a half-duplex relay utilizing physical-layer network coding over finite GF(2). Moreover, we consider a practical scenario of interference from the primary users (PUs) affecting the relay and source nodes. The analysis provides a generalization of previous works as it considers an extended transmission system where the channels can consist of a combination of independent and identically distributed (i.i.d.) and independent but non-identically distributed (i.n.i.d.) Nakagami- $m$ fading models. In addition, unlike prior works, this paper focuses on the performance of both the PUs and the SUs. Closed-form expressions for the symbol error probability (SEP) and the outage probability of the intended PU are obtained. In addition, we derive exact closed-form expressions for the SEP with consideration of special cases of practical interest (e.g., no interference power, interference limited, and single dominant interference cases) for the SUs. Furthermore, an upper bound on the achievable rate of the secondary system is provided. Subsequently, a closed-form approximating expression for the SEP of the secondary system at high signal-to-noise ratios (SNRs) is obtained. Simulation results are provided and attest to the accuracy of the analytical results.

The Approximate Capacity of the MIMO Relay Channel

Capacity bounds are studied for the multiple-antenna complex Gaussian relay channel with $t_{1}$ transmitting antennas at the sender, $r_{2}$ receiving and $t_{2}$ transmitting antennas at the relay, and $r_{3}$ receiving antennas at the receiver. It is shown that the partial decode–forward coding scheme achieves within $\min (t_{1},r_{2})$ bits from the cutset bound and at least one half of the cutset bound, establishing a good approximate characterization of the capacity. A similar additive gap of $\min (t_{1} + t_{2}, r_{3}) + r_{2}$ bits is shown to be achieved by the compress–forward coding scheme. The corresponding results for time-division half-duplex relay channels are also established.

Achievable Degrees of Freedom for the Two-Cell Two-Hop MIMO Interference Channel With Half-Duplex Relays

We consider the two-cell two-hop multiple-input–multiple-output interference channel with half-duplex relays, where each source group having $M$ single antenna users communicates with the corresponding destination with $M$ antennas via two relays, each having $M$ antennas. For such a channel, by exploiting three time slots, the previously known achievable degrees of freedom (DoF) is $2M/3$ regardless of whether the half-duplex relays have global channel state information (CSI) for the first hop. In this paper, we show that using $n\geq 3$ time slots, the achievable DoF is $(n-1)M/n$ , which is higher than the previous result of $2M/3$ DoF for the case of $n\geq 4$ . The achievability is shown by a new relaying protocol, which combines the alternate transmission strategy with an interference cancellation technique. A major implication of the derived result is that a normalized DoF of one can be achieved asymptotically without requiring global CSI at the source and relay nodes.

A Practical Feasibility Study of a Novel Strategy for the Gaussian Half-Duplex Relay Channel

This paper presents a practical feasibility study of a novel two-phase three-part-message strategy for half-duplex relaying, which features superposition coding and interference-aware cancellation decoding. Aiming to analyze the performance of the proposed scheme in the non-asymptotic regime, this paper evaluates the spectral efficiency with finite block-length and discrete constellation signaling and compares it with the theoretical performance of Gaussian codes with asymptotically large block-lengths. The performance evaluation is carried out on an LTE simulation test bench. During each transmission phase, the modulation and coding scheme is adapted to the channel link qualities to enhance the overall spectral efficiency. A single-antenna source and relay, and a multi-antenna destination are assumed. The static Gaussian and two frequency selective channel models are considered for the proposed scheme. A spectral efficiency comparison with a baseline scheme (non-cooperative two-hop transmission, i.e., the source-destination link is absent) and with the point-to-point transmission strategy (no relay) is presented. The results confirm that physical-layer cooperation and multi-antennas are critical for performance enhancement in heterogeneous networks. Moreover, they show that physical layer cooperation advantages are within practical reach with existing LTE coded-modulation and interference-mitigation techniques, which are prevalent in modern user-equipment.

Decode‐and‐forward polar coding scheme for receive diversity: a relay partially perfect retransmission for half‐duplex wireless relay channels

Recently, it is shown that the channel polarisation phenomenon of Arikan's polar codes with their original multilevel construction happens in several levels with perfect, partially perfect and useless bit channels. On the basis of this construction, this study aims to illustrate design principles and decoding steps of cooperative decode-and-forward polar coding schemes for half-duplex relay channel. In these schemes, the authors consider the use of relay retransmission of partially perfect bit channels in time-division process. The source node transmits its polar-coded information to both the relay and the destination nodes during the first time slot. For the second transmission period, the relay first decodes the source signal then after reconstruction of the polarisation matrix, the extracted partially perfect bit channels are recoded using another polar code and finally transmitted to the destination node. At destination, the signals received from both source and relay nodes are processed by using multi-stage successive cancellation decoding for retrieving the original information bits. They demonstrate via numerical results that by carefully constructed polarisation matrix at relay node, their cooperative polar coding schemes can achieve better performances than other previous schemes.

Design and Implementation of a Full-Duplex Pipelined MAC Protocol for Multihop Wireless Networks

In multihop wireless networks, data packets are forwarded from a source node to a destination node through intermediate relay nodes. With half-duplex relay nodes, the end-to-end delay performance of a multihop network degrades as the number of hops increases, because the relay nodes cannot receive and transmit at the same time. Full-duplex relay nodes can reduce their per-hop delay by starting to forward a packet before the whole packet is received. In this paper, we propose a pipelined medium access control (PiMAC) protocol, which enables the relay nodes on a multihop path to simultaneously transmit and receive packets for full-duplex forwarding. For pipelined transmission over a multihop path, it is important to suppress both the self-interference of each relay node with the full-duplex capability and the intra-flow interference from the next relay nodes on the same path. In the PiMAC protocol, each relay node can suppress both the self- and intra-flow interference for full-duplex relaying on the multihop path by estimating the channel coefficients and delays of the interference during a multihop channel acquisition phase. To evaluate the performance of the PiMAC protocol, we carried out extensive simulations and software-defined radio-based experiments.

Opportunistic Network Decoupling with Virtual Full-Duplex Operation in Multi-Source Interfering Relay Networks

We introduce a new achievability scheme, termed opportunistic network decoupling (OND), operating in virtual full-duplex mode. In the scheme, a novel relay scheduling strategy is utilized in the $K\times N\times K$ channel with interfering relays, consisting of $K$ source--destination pairs and $N$ half-duplex relays in-between them. A subset of relays using alternate relaying is opportunistically selected in terms of producing the minimum total interference level, thereby resulting in network decoupling. As our main result, it is shown that under a certain relay scaling condition, the OND protocol achieves $K$ degrees of freedom even in the presence of interfering links among relays. Numerical evaluation is also shown to validate the performance of the proposed OND. Our protocol basically operates in a fully distributed fashion along with local channel state information, thereby resulting in a relatively easy implementation.

Buffer-Aided Relaying Improves Throughput of Full-Duplex Relay Networks With Fixed-Rate Transmissions

The concept of buffer-aided relaying has recently emerged as a viable solution for improving the performance of wireless networks with half-duplex relays. In this letter, we show that buffer-aided relaying is also beneficial to wireless networks with ideal full-duplex (FD) relays. To this end, we investigate the two-hop FD relay network, which is comprised of a source, an FD relay equipped with a buffer, and a destination where a direct source–destination link does not exist. For this network, we assume that the source and the relay select their transmission rates from discrete sets of available transmission rates, and propose a corresponding buffer-aided relaying scheme, which maximizes the throughput. Our numerical results show that the proposed buffer-aided relaying scheme provides the significant throughput gains compared with conventional FD relaying schemes, which do not employ buffers at the relays.