DF Relay Research Articles

Optimally Joint Subcarrier Pairing and Power Allocation for OFDM System with Multihop Symbol Level DF Relaying

Subcarrier pairing (SP) and power allocation (PA) can improve the channel capacity of the OFDM multi-hop relay system. Due to limitations of processing complexity and energy consumption, symbol-level relaying, which only regenerates the constellation symbols at relay nodes, is more practical than code-level relaying that requires full decoding and encoding. By modeling multi-hop symbol-level relaying as a multi-staged parallel binary symmetric channel, this paper introduces a jointly optimal SP and PA scheme which maximizes the end to end data rate. Analytical arguments are given to reveal the structures and properties of the optimal solution, and simulation results are presented to illustrate and justify the optimality.

Outage Analysis of Spectrum Sharing Cognitive DF Relay Networks Using Outdated CSI

We investigate the effect of the outdated channel state information (CSI) on the secondary relay-aided network's performance. We carry out the analysis of the outage probability as well as the diversity order of the secondary network (SN) considering three key constraints: 1) maximum transmit power at the secondary transmitter, 2) peak interference power at the primary receiver, and 3) outdated CSI at the secondary-to-primary and secondary-to-secondary links. The performance results show that the relay cluster position, the imperfect CSI of the interference links and of the second hop are the key optimization factors. We show that the correlation coefficient of the second hop link is the key optimization parameter of the diversity order.

Energy Efficient Relay Placement in Dual Hop 802.15.4 Networks

IEEE 802.15.4 has emerged as a popular standard for short range wireless sensor networks used in industrial, military, health, and environmental sectors. The limited lifetime of such networks is one of the critical design challenges. This paper examines the how relaying through intermediate sensor nodes can enhance the lifetime of an 802.15.4 network. In particular, novel energy consumption models for both AF and DF relays have been developed. Different relay gain scaling mechanisms and forwarding strategies under each of the relay categories were also considered and their energy efficiencies were compared. For every relaying protocol, it was found that there exists an optimum location where energy saving is maximum and this location is not necessarily different for different modes. In summary, it has been observed that the optimum location for AF relays is the equidistant point from source and destination. In contrast, the optimal location for DF relays is closer to source. The effect of different PHY level (outage probability, path loss) and MAC level parameters (frame length) on the energy efficiency are also studied.

Buffer-Aided Relaying With Adaptive Link Selection—Fixed and Mixed Rate Transmission

We consider a simple network consisting of a source, a half-duplex DF relay with a buffer, and a destination. We assume that the direct source-destination link is not available and all links undergo fading. We propose two new buffer-aided relaying schemes. In the first scheme, neither the source nor the relay have CSIT, and consequently, both nodes are forced to transmit with fixed rates. In contrast, in the second scheme, the source does not have CSIT and transmits with fixed rate but the relay has CSIT and adapts its transmission rate accordingly. In the absence of delay constraints, for both fixed rate and mixed rate transmission, we derive the throughput-optimal buffer-aided relaying protocols which select either the source or the relay for transmission based on the instantaneous SNRs of the source-relay and the relay-destination links. In addition, for the delay constrained case, we develop buffer-aided relaying protocols that achieve a predefined average delay. Compared to conventional relaying protocols, which select the transmitting node according to a predefined schedule independent of the link instantaneous SNRs, the proposed buffer-aided protocols with adaptive link selection achieve large performance gains. In particular, for fixed rate transmission, we show that the proposed protocol achieves a diversity gain of two as long as an average delay of more than three time slots can be afforded. Furthermore, for mixed rate transmission with an average delay of $E{T}$ time slots, a multiplexing gain of $r=1-1/(2E{T})$ is achieved. Hence, for mixed rate transmission, for sufficiently large average delays, buffer-aided half-duplex relaying with and without adaptive link selection does not suffer from a multiplexing gain loss compared to full-duplex relaying.

Fair resource allocation and DF relay selection for multiuser OFDMA-based cooperative networks

We investigate the joint problem of power and subcarrier allocation with relay selection for a multi-user multi-relay cellular system with selective relaying. We first formulate the optimal problem using Boolean-convex optimization with the objective of improving the average network capacity while providing proportional rate fairness to all users. We show that except for very small network instances the model is computationally very complex to solve. Then, we present a sub-optimal model by decomposing the joint problem into two sub-problems. This model proves to be a simple one, however, the sub-optimal design of the first sub-problem restricts the model from achieving near optimal solutions. Therefore, to achieve near optimal solutions, we propose an iterative two step method where near-optimal solutions can be achieved by iterating between the two steps. Numerical results show that for smaller networks this two step iterative method obtains optimal solutions in much less time. We also show that a selective relaying technique always achieves better performance over the always relaying technique.

Randomized Isometric Linear-Dispersion Space-Time Block Coding for the DF Relay Channel

This paper presents a randomized linear-dispersion space-time block code for decode-and-forward synchronous relays. The coding matrices are obtained as a set of columns (or rows) of randomly generated Haar-distributed unitary matrices. With respect to independent and identically distributed (i.i.d.)-generated codes, this particular isometric structure reduces the intersymbol interference generated within each relay. The gain over i.i.d. codes in terms of spectral efficiency is analyzed for both the LMMSE and the ML receivers under the assumption of frequency-flat quasi-static fading. In this setting, the spectral efficiency is a random quantity, since it depends on the random coding matrices. However, it is proven that the spectral efficiency converges in probability to a deterministic quantity when the dimensions of the matrices tend to infinity while keeping constant their ratio, i.e., the coding rate α. Consequently, when the random coding matrices are large enough, the presented system behaves as a deterministic one. This result is achieved by means of the rectangular R-transform, a powerful tool of free probability theory which allows determining the distribution of the singular values of a sum of rectangular matrices.

Performance Analysis of Multi-Antenna DF Relay Networks over Nakagami-m Fading Channels

In this letter, we present the performance of multi-antenna selective combining decode-and-forward (SC-DF) relay networks over independent and identically distributed (i.i.d) Nakagami-m fading channels with integer value of the fading severity parameter m. The outage probability, moment generation function (MGF) and symbol error probability are derived in closed-form using the SNR statistical characteristics. To simplify our analysis, we derive outage and symbol error probabilities at high SNRs over independent and non-identically distributed (i.n.i.d) Nakagami-m fading channels with no assumption on the fading severity parameter m .After that, we formulate the outage probability problem and optimize it with an approximated problem and then solve it analytically. Finally, for comparison with analytical results, we perform some Monte-Carlo simulations.

Performance Analysis of Two-User Cooperative Multiple Access Systems With DF Relaying and Superposition Modulation

In this paper, a two-user cooperative multiple access (CMA) system is considered. Using the superposition modulation scheme, we propose a low-complexity suboptimal detector at the destination to achieve the performance close to that of the optimal maximum-likelihood sequence detector (MLSD). For independent and non-identically distributed (i.n.d.) Rayleigh-faded channels, we derive error decision regions and present the exact bit-error rate (BER) expression in an integral form, which is validated by simulation results. We also approximate the decision regions and obtain the BER bound in closed form. Another efficient transmission scheme, i.e., the time-division broadcast (TDBC) protocol operating in three time slots, is also considered and analyzed. BER results show that the superposition modulation outperforms the others and can achieve both full diversity order and high coding gain.

Large-SNR Outage Analysis for the DF Relay Channel with Randomized Space-Time Block Coding

A simple half-duplex decode-and-forward relay channel is presented and analyzed. Relays access the common channel by means of a randomized linear-dispersion space-time block code which is flexible with respect to the number of relays and the coding rate α. When the dimensions of the linear dispersion matrices grow large, but with constant ratio α, the spectral efficiency of the system converges fast to a deterministic quantity. Simulation results show that this asymptotic value is an extremely good approximation of the finite reality, even for not-so-large codes. Then, this asymptotic spectral efficiency is used to characterize the outage probability in the high-SNR regime. With maximum-likelihood reception, the proposed randomized coding scheme is shown to achieve full diversity order L+1, with L the total number of relays. On the contrary, with the sub-optimal LMMSE receiver, relays add diversity to the system only if the coding rate is small enough.

Optimal Duplex Mode for DF Relay in Terms of the Outage Probability

This paper deals with a full-duplex relay (FDR) system over Rayleigh fading channels. The exact outage probability of FDR is derived as a closed form to consider interferences from full duplex. Then, we obtain the conditions of the signal-to-noise ratio (SNR) and the signal to interface ratios (SIRs) for cases of FDR showing a lower outage probability than that of the half-duplex relay (HDR) system under the target outage probability. According to this condition, FDR is superior to HDR with lower SIRs in the low-SNR region rather than in the high-SNR region. In addition, the target outage probability is only satisfied when the SNR and SIRs are within the boundaries. These boundaries vary due to the target rate, the channel states of each link, and the target outage probability.

Cooperative Communications for Reliable Data Transport with Fountain Codes

Providing efficient and reliable data transport is a challenging problem for a variety of emerging applications which require reliable data packet delivery in wireless networks. In this paper, we propose to incorporate fountain codes at transport layer in the notion of cooperative relay communications to provide reliability and robustness for data transmission in wireless networks. Our basic idea is to exploit the joint merits of fountain codes and cooperative relay communications. We first derive the achievable rate of cooperative communications with fountain codes based on a general 3-node relay model and find that substantial improvement can be achieved compared with direct transmission and conventional AF and DF relay approaches. Inspired by this finding, w e develop two cooperative communication strateg ies and analyze their performance. Numerical results show that our proposed approach es can achieve significant performance improvement in terms of data transport efficiency. In addition, the proposed approach es exhibit strong robustness to packet losses on wireless links for data transfer .

On the DMT-Optimality of Nondynamic DF Relaying in Asymmetric Nakagami Fading Channels

The optimality of nondynamic DF relaying over a general three-node Nakagami fading channel in terms of the diversity-multiplexing tradeoff (DMT) is established. In particular, when the fading of the relay-source link is less severe than that of the relay-destination link, both orthogonal and nonorthogonal DF protocols are shown to be DMT-optimal over certain ranges of the multiplexing gains. It is also shown that when the multiplexing gains are sufficiently high, the DMT of the DF protocols is completely independent of the statistical properties of the relay-destination link.

Joint optimization of power and distance to minimize outage probability in transparent DF relay network

AbstractIn this paper, we jointly optimize the position of the relay node and transmission power ratio of the source node over the relay node to minimize the outage probability, which can provide guidance for IEEE 802.16j transparent relay node configuration. The result indicates that the outage probability is minimum when the relay is positioned at the midway between the source and the destination, and the transmission power ratio of the source node over the relay node is 11:9. Copyright © 2009 John Wiley & Sons, Ltd.

Closed-Form Solution of Outage Capacity for Cooperative DF and AF Relay Network

Cooperative relaying permits one or more relays to transmit a signal from the source to the destination, thereby increasing network coverage and spectral efficiency. The performance of cooperative relaying is often measured as outage probability. However, appropriate measure for the channel quality is outage capacity. Although the outage probability for cooperative relaying protocol has been analyzed before, very little research has been addressed for the outage capacity. This paper is the first of its kind to derive a closed-form analytical solution of outage capacity using fixed decode and forward relaying and amplify and forward relaying in independent but non identically distributed (i.n.i.d.) Rayleigh fading channels, considering channel coefficients known to the receiver side. The analytical results show a tradeoff between the SNR and the number of relays for a specific outage capacity. A comparison between decode and forward relaying and amplify and forward relaying shows that decode and forward relaying outperforms amplify and forward relaying for a large number of relays.