Showing papers on "Relay published in 2011"
TL;DR: An exact characterization of the capacity of a network with nodes connected by deterministic channels is obtained, a natural generalization of the celebrated max-flow min-cut theorem for wired networks.
Abstract: In a wireless network with a single source and a single destination and an arbitrary number of relay nodes, what is the maximum rate of information flow achievable? We make progress on this long standing problem through a two-step approach. First, we propose a deterministic channel model which captures the key wireless properties of signal strength, broadcast and superposition. We obtain an exact characterization of the capacity of a network with nodes connected by such deterministic channels. This result is a natural generalization of the celebrated max-flow min-cut theorem for wired networks. Second, we use the insights obtained from the deterministic analysis to design a new quantize-map-and-forward scheme for Gaussian networks. In this scheme, each relay quantizes the received signal at the noise level and maps it to a random Gaussian codeword for forwarding, and the final destination decodes the source's message based on the received signal. We show that, in contrast to existing schemes, this scheme can achieve the cut-set upper bound to within a gap which is independent of the channel parameters. In the case of the relay channel with a single relay as well as the two-relay Gaussian diamond network, the gap is 1 bit/s/Hz. Moreover, the scheme is universal in the sense that the relays need no knowledge of the values of the channel parameters to (approximately) achieve the rate supportable by the network. We also present extensions of the results to multicast networks, half-duplex networks, and ergodic networks.
1,034 citations
TL;DR: Targeting at minimal interference power, a broad range of multiple-input multiple-output mitigation schemes are analyzed and the results confirm that self-interference can be mitigated effectively also in the presence of imperfect side information.
Abstract: Full-duplex relaying is more spectrally efficient than half-duplex relaying as only one channel use is needed per two hops. However, it is crucial to minimize relay self-interference to render full duplex feasible. For this purpose, we analyze a broad range of multiple-input multiple-output (MIMO) mitigation schemes: natural isolation, time-domain cancellation, and spatial suppression. Cancellation subtracts replicated interference signal from the relay input while suppression reserves spatial dimensions for receive and transmit filtering. Spatial suppression can be achieved by antenna subset selection, null-space projection, i.e., receiving and transmitting in orthogonal subspaces, or joint transmit and receive beam selection to support more spatial streams by choosing the minimum eigenmodes for overlapping subspaces. In addition, minimum mean square error (MMSE) filtering can be employed to maintain the desired signal quality, which is inherent for cancellation, and the combination of time- and spatial-domain processing may be better than either alone. Targeting at minimal interference power, we solve optimal filters for each scheme in the cases of joint, separate and independent design. The performance of mitigation schemes is evaluated and compared by simulations. The results confirm that self-interference can be mitigated effectively also in the presence of imperfect side information.
944 citations
TL;DR: This work proposes the combination of opportunistic mode selection and transmit power adaptation for maximizing instantaneous and average spectral efficiency after noting that the trade-off favors alternately the modes during operation.
Abstract: Focusing on two-antenna infrastructure relays employed for coverage extension, we develop hybrid techniques that switch opportunistically between full-duplex and half-duplex relaying modes. To rationalize the system design, the classic three-node full-duplex relay link is first amended by explicitly modeling residual relay self-interference, i.e., a loopback signal from the transmit antenna to the receive antenna remaining after cancellation. The motivation for opportunistic mode selection stems then from the fundamental trade-off determining the spectral efficiency: The half-duplex mode avoids inherently the self-interference at the cost of halving the end-to-end symbol rate while the full-duplex mode achieves full symbol rate but, in practice, suffers from residual interference even after cancellation. We propose the combination of opportunistic mode selection and transmit power adaptation for maximizing instantaneous and average spectral efficiency after noting that the trade-off favors alternately the modes during operation. The analysis covers both common relaying protocols (amplify-and-forward and decode-and-forward) as well as reflects the difference of downlink and uplink systems. The results show that opportunistic mode selection offers significant performance gain over system design that is confined to either mode without rationalization.
674 citations
TL;DR: It is shown that the outage probability of cognitive relay networks is higher than that of conventional relay networks due to the interference constraint, and the decrease in outage probability achieved by increasing the selection diversity is not less than that in conventional relay Networks.
Abstract: This paper evaluates the outage probability of cognitive relay networks with cooperation between secondary users based on the underlay approach, while adhering to the interference constraint on the primary user, i.e., the limited amount of interference which the primary user can tolerate. A relay selection criterion, suitable for cognitive relay networks, is provided, and using it, we derive the outage probability. It is shown that the outage probability of cognitive relay networks is higher than that of conventional relay networks due to the interference constraint, and we quantify the increase. In addition, the outage probability is affected by the distance ratio of the interference link (between the secondary transmitter and the primary receiver) to the relaying link (between the secondary transmitter and the secondary receiver). We also prove that cognitive relay networks achieve the same full selection diversity order as conventional relay networks, and that the decrease in outage probability achieved by increasing the selection diversity (the number of relays) is not less than that in conventional relay networks.
521 citations
TL;DR: Detection performance of an energy detector used for cooperative spectrum sensing in a cognitive radio network is investigated over channels with both multipath fading and shadowing and can be extended to channels with Nakagami-m multipath fades and lognormal shadowing as well.
Abstract: Detection performance of an energy detector used for cooperative spectrum sensing in a cognitive radio network is investigated over channels with both multipath fading and shadowing. The analysis focuses on two fusion strategies: data fusion and decision fusion. Under data fusion, upper bounds for average detection probabilities are derived for four scenarios: 1) single cognitive relay; 2) multiple cognitive relays; 3) multiple cognitive relays with direct link; and 4) multi-hop cognitive relays. Under decision fusion, the exact detection and false alarm probabilities are derived under the generalized "k-out-of-n" fusion rule at the fusion center with consideration of errors in the reporting channel due to fading. The results are extended to a multi-hop network as well. Our analysis is validated by numerical and simulation results. Although this research focuses on Rayleigh multipath fading and lognormal shadowing, the analytical framework can be extended to channels with Nakagami-m multipath fading and lognormal shadowing as well.
512 citations
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TL;DR: In this article, a noisy network coding scheme for communicating messages between multiple sources and destinations over a general noisy network is presented, where the relays do not use Wyner-Ziv binning as in previous compress-forward schemes and each decoder performs simultaneous decoding of the received signals from all the blocks without uniquely decoding the compression indices.
Abstract: A noisy network coding scheme for communicating messages between multiple sources and destinations over a general noisy network is presented. For multi-message multicast networks, the scheme naturally generalizes network coding over noiseless networks by Ahlswede, Cai, Li, and Yeung, and compress-forward coding for the relay channel by Cover and El Gamal to discrete memoryless and Gaussian networks. The scheme also extends the results on coding for wireless relay networks and deterministic networks by Avestimehr, Diggavi, and Tse, and coding for wireless erasure networks by Dana, Gowaikar, Palanki, Hassibi, and Effros. The scheme involves lossy compression by the relay as in the compress-forward coding scheme for the relay channel. However, unlike previous compress-forward schemes in which independent messages are sent over multiple blocks, the same message is sent multiple times using independent codebooks as in the network coding scheme for cyclic networks. Furthermore, the relays do not use Wyner-Ziv binning as in previous compress-forward schemes, and each decoder performs simultaneous decoding of the received signals from all the blocks without uniquely decoding the compression indices. A consequence of this new scheme is that achievability is proved simply and more generally without resorting to time expansion to extend results for acyclic networks to networks with cycles. The noisy network coding scheme is then extended to general multi-message networks by combining it with decoding techniques for the interference channel. For the Gaussian multicast network, noisy network coding improves the previously established gap to the cutset bound. We also demonstrate through two popular Gaussian network examples that noisy network coding can outperform conventional compress-forward, amplify-forward, and hash-forward coding schemes.
485 citations
TL;DR: This paper develops an algorithm for optimizing the performance of the ground-to-relay links through control of the UAV heading angle, and proposes a smart handoff algorithm that updates node and relay assignments as the topology of the network evolves.
Abstract: In this paper, we investigate a communication system in which unmanned aerial vehicles (UAVs) are used as relays between ground-based terminals and a network base station. We develop an algorithm for optimizing the performance of the ground-to-relay links through control of the UAV heading angle. To quantify link performance, we define the ergodic normalized transmission rate (ENTR) for the links between the ground nodes and the relay, and derive a closed-form expression for it in terms of the eigenvalues of the channel correlation matrix. We show that the ENTR can be approximated as a sinusoid with an offset that depends on the heading of the UAV. Using this observation, we develop a closed-form expression for the UAV heading that maximizes the uplink network data rate while keeping the rate of each individual link above a certain threshold. When the current UAV relay assignments cannot meet the minimum link requirements, we investigate the deployment and heading control problem for new UAV relays as they are added to the network, and propose a smart handoff algorithm that updates node and relay assignments as the topology of the network evolves.
378 citations
TL;DR: Simulation results show that the secrecy rate is dramatically increased when inactive nodes in the relay network participate in cooperativeJamming, and a GSVD-based cooperative jamming scheme is proposed that shows significant improvement in terms of secrecy rate compared to the approach without jamming.
Abstract: Secure communications can be impeded by eavesdroppers in conventional relay systems. This paper proposes cooperative jamming strategies for two-hop relay networks where the eavesdropper can wiretap the relay channels in both hops. In these approaches, the normally inactive nodes in the relay network can be used as cooperative jamming sources to confuse the eavesdropper. Linear precoding schemes are investigated for two scenarios where single or multiple data streams are transmitted via a decode-and-forward (DF) relay, under the assumption that global channel state information (CSI) is available. For the case of single data stream transmission, we derive closed-form jamming beamformers and the corresponding optimal power allocation. Generalized singular value decomposition (GSVD)-based secure relaying schemes are proposed for the transmission of multiple data streams. The optimal power allocation is found for the GSVD relaying scheme via geometric programming. Based on this result, a GSVD-based cooperative jamming scheme is proposed that shows significant improvement in terms of secrecy rate compared to the approach without jamming. Furthermore, the case involving an eavesdropper with unknown CSI is also investigated in this paper. Simulation results show that the secrecy rate is dramatically increased when inactive nodes in the relay network participate in cooperative jamming.
373 citations
TL;DR: This correspondence studies cooperative jamming to increase the physical layer security of a wiretap fading channel via distributed relays and shows that the optimization problem can be solved using a combination of convex optimization and a one-dimensional search.
Abstract: This correspondence studies cooperative jamming (CJ) to increase the physical layer security of a wiretap fading channel via distributed relays. We first provide the feasible conditions on the positiveness of the secrecy rate and then show that the optimization problem can be solved using a combination of convex optimization and a one-dimensional search. Distributed implementation to realize the CJ solution and extension to deal with per group relays' power constraints are discussed.
356 citations
01 Jan 2011
TL;DR: In this paper, the authors demonstrate relay attacks on Passive Keyless Entry and Start (PKES) systems used in modern cars and demonstrate that relaying the signal in one direction only (from the car to the key) is sufficient to perform the attack while the true distance between the key and car remains large.
Abstract: We demonstrate relay attacks on Passive Keyless Entry and Start (PKES) systems used in modern cars. We build two efficient and inexpensive attack realizations, wired and wireless physical-layer relays, that allow the attacker to enter and start a car by relaying messages between the car and the smart key. Our relays are completely independent of the modulation, protocol, or presence of strong authentication and encryption. We perform an extensive evaluation on 10 car models from 8 manufacturers. Our results show that relaying the signal in one direction only (from the car to the key) is sufficient to perform the attack while the true distance between the key and car remains large (tested up to 50 meters, non line-of-sight). We also show that, with our setup, the smart key can be excited from up to 8 meters. This removes the need for the attacker to get close to the key in order to establish the relay. We further analyze and discuss critical system characteristics. Given the generality of the relay attack and the number of evaluated systems, it is likely that all PKES systems based on similar designs are also vulnerable to the same attack. Finally, we propose immediate mitigation measures that minimize the risk of relay attacks as well as recent solutions that may prevent relay attacks while preserving the convenience of use, for which PKES systems were initially introduced.
316 citations
TL;DR: This letter reports a performance analysis of a dual-hop relay system composed of asymmetric radio-frequency and free-space optics (RF/FSO) links, based on the fact that FSO links can provide even wider bandwidths as compared to RF ones.
Abstract: This letter reports a performance analysis of a dual-hop relay system composed of asymmetric radio-frequency and free-space optics (RF/FSO) links. This approach is based on the fact that FSO links can provide even wider bandwidths as compared to RF ones. In particular, an exact closed-form expression for the end-to-end outage probability of the RF/FSO relay links is derived. Overall, the RF/FSO links show a slightly worse performance than the RF/RF links, but the performance gap is gradually reduced as the signal-to-noise ratio (SNR) increases. Our mathematical analysis results were verified by exactly matching Monte Carlo simulation results.
TL;DR: In this article, the authors proposed a protection strategy based on microprocessor-based relays for lowvoltage microgrids, which is to a large extent independent of the fault current magnitude and the mode of operation.
Abstract: One of the major challenges associated with microgrid protection is to devise an appropriate protection strategy that is effective in the grid-connected as well as islanded mode of operation. This paper proposes a protection strategy based on microprocessor-based relays for low-voltage microgrids. Further, the structure of a new relay enabling the proposed protection strategy is presented. One of the salient feature of the developed protection scheme is that it does not require communications or adaptive protective devices. Moreover, it is to a large extent independent of the fault current magnitude and the mode of operation. Transient time-domain simulation studies are conducted to demonstrate the effectiveness of the proposed protection strategy and its enabling relay, using the PSCAD/EMTDC software package.
TL;DR: An optimal approach based on the dual method and a suboptimal approach are developed to reduce complexity while maintaining reasonable performance in cognitive radio (CR) systems.
Abstract: In this paper, we investigate joint relay selection and power allocation to maximize system throughput with limited interference to licensed (primary) users in cognitive radio (CR) systems. As these two problems are coupled together, we first develop an optimal approach based on the dual method and then propose a suboptimal approach to reduce complexity while maintaining reasonable performance. From our simulation results, the proposed approaches can increase the system throughput by over 50%.
TL;DR: Simulation results indicate that the proposed RS-AF scheme with OPA yields considerable performance improvement over an equal-power-allocation scheme, particularly with a large number of relay nodes.
Abstract: In this paper, we propose a relay selection amplify-and-forward (RS-AF) protocol in general bidirectional relay networks with two sources and N relays. In the proposed scheme, the two sources first simultaneously transmit to all the relays, and then, a single relay with a minimum sum symbol error rate (SER) will be selected to broadcast the received signals back to both sources. To facilitate the selection process, we propose a simple suboptimal min-max criterion for relay selection, where a single relay that minimizes the maximum SER of two source nodes will be selected. Simulation results show that the proposed min-max selection has almost the same performance as the optimal selection with lower complexity. We also present a simple asymptotic SER expression and make a comparison with the conventional all-participate AF relaying scheme. The analytical results are verified through simulations. To improve the system performance, optimal power allocation (OPA) between the sources and the relay is determined based on the asymptotic SER. Simulation results indicate that the proposed RS-AF scheme with OPA yields considerable performance improvement over an equal-power-allocation scheme, particularly with a large number of relay nodes.
Patent•
23 Sep 2011
TL;DR: In this paper, an embodiment method for power control in a multi-hop communications system includes transmitting a power usage pattern for each relay node in a subset of relay nodes served by a communications controller.
Abstract: An embodiment method for power control in a multi-hop communications system includes transmitting a power usage pattern for each relay node in a subset of relay nodes served by a communications controller, where the power usage pattern specifies transmit power levels for the relay node while the relay node is operating in a power control mode. The method also includes receiving channel measurements of access links between the relay nodes in the subset of relay nodes and subscriber equipment served by the relay nodes, determining backhaul link transmit power levels and access link transmit power levels based on the channel measurements of access links and channel measurements of backhaul links between the communications controller and the relay nodes, and transmitting the access link power levels to the subset of relay nodes.
21 Apr 2011
TL;DR: In this paper, the authors investigate a relay effect to extend the energy transfer distance and show that the efficiency of power transfer can be improved significantly using one or more relay resonators.
Abstract: Wireless power transfer using strongly coupled electromagnetic resonators is a recently explored technology. Although this technology is able to transmit electrical energy over a much longer distance than traditional near field methods, in some applications, its effective distance is still insufficient. In this paper, we investigate a relay effect to extend the energy transfer distance. Theoretical analysis is performed based on a set of coupled-mode equations. Experiments are conducted to confirm the theoretical results and demonstrate the effectiveness of the relay approach. Our results show that the efficiency of power transfer can be improved significantly using one or more relay resonators. This approach significantly improves the performance of the present two-resonator system and allows a curved path in space to be defined for wireless power transfer using smaller resonators.
TL;DR: The outage performance is investigated for a cognitive relay system with underlay spectrum sharing model and it is shown that the dependence among the received SNRs is introduced due to the interference power constraint.
Abstract: The outage performance is investigated for a cognitive relay system with underlay spectrum sharing model. It is shown that the dependence among the received SNRs is introduced due to the interference power constraint. The joint probability of the received SNRs is derived. By considering the dependence for the case of null decoding set, a tight lower bound on the outage probability is obtained. Simulation results are presented to verify the theoretical analyses.
TL;DR: A comprehensive treatment of eight possible half-duplex bi-directional relaying protocols in Gaussian noise is provided, obtaining their relative performance under different SNR and relay geometries.
Abstract: In a bi-directional relay channel, two nodes wish to exchange independent messages over a shared wireless half-duplex channel with the help of a relay. In this paper, we derive achievable rate regions for four new half-duplex protocols and compare these to four existing half-duplex protocols and outer bounds. In time, our protocols consist of either two or three phases. In the two phase protocols, both users simultaneously transmit during the first phase and the relay alone transmits during the second phase, while in the three phase protocol the two users sequentially transmit followed by a transmission from the relay. The relay may forward information in one of four manners; we outline existing amplify and forward (AF), decode and forward (DF), lattice based, and compress and forward (CF) relaying schemes and introduce the novel mixed forward scheme. The latter is a combination of CF in one direction and DF in the other. We derive achievable rate regions for the CF and Mixed relaying schemes for the two and three phase protocols. We provide a comprehensive treatment of eight possible half-duplex bi-directional relaying protocols in Gaussian noise, obtaining their relative performance under different SNR and relay geometries.
TL;DR: It is shown that user cooperation can increase the achievable secrecy region and an achievable scheme is proposed that combines Marton's coding scheme for broadcast channels and Cover and El Gamal's compress-and-forward scheme for relay channels.
Abstract: We investigate the effects of user cooperation on the secrecy of broadcast channels by considering a cooperative relay broadcast channel. We show that user cooperation can increase the achievable secrecy region. We propose an achievable scheme that combines Marton's coding scheme for broadcast channels and Cover and El Gamal's compress-and-forward scheme for relay channels. We derive outer bounds for the rate-equivocation region using auxiliary random variables for single-letterization. Finally, we consider a Gaussian channel and show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation.
Posted Content•
TL;DR: In this article, the authors considered the problem of two-hop relaying in the presence of energy harvesting nodes and identified the optimal offline transmission scheme for energy harvesting source and relay when the relay operates in the full-duplex mode.
Abstract: Communication nodes with the ability to harvest energy from the environment have the potential to operate beyond the timeframe limited by the finite capacity of their batteries; and accordingly, to extend the overall network lifetime. However, the optimization of the communication system in the presence of energy harvesting devices requires a new paradigm in terms of power allocation since the energy becomes available over time. In this paper, we consider the problem of two-hop relaying in the presence of energy harvesting nodes. We identify the optimal offline transmission scheme for energy harvesting source and relay when the relay operates in the full-duplex mode. In the case of a half-duplex relay, we provide the optimal transmission scheme when the source has a single energy packet.
Patent•
19 Dec 2011
TL;DR: In this article, a method and system for improving the security of storing digital data in a memory or its delivery as a message over the Internet from a sender to a receiver using one or more hops is disclosed.
Abstract: Method and system for improving the security of storing digital data in a memory or its delivery as a message over the Internet from a sender to a receiver using one or more hops is disclosed. The message is split at the sender into multiple overlapping or non-overlapping slices according to a slicing scheme, and the slices are encapsulated in packets each destined to a different relay server as an intermediate node according to a delivery scheme. The relay servers relay the received slices to another other relay server or to the receiver. Upon receiving all the packets containing all the slices, the receiver combines the slices reversing the slicing scheme, whereby reconstructing the message sent.
TL;DR: This paper considers a multiple input multiple output (MIMO) two-way relay channel, where each relay node has one or more antennas and a compress and forward strategy is proposed and its DM-tradeoff is derived.
Abstract: In a two-way relay channel, two sources use one or more relay nodes to exchange data with each other. This paper considers a multiple input multiple output (MIMO) two-way relay channel, where each relay node has one or more antennas. Optimal relay transmission strategies for the two-way relay channel are derived to maximize the achievable rate with amplify and forward (AF) at each relay and to achieve the optimal diversity-multiplexing tradeoff (DM-tradeoff). To maximize the achievable rate with AF, an iterative algorithm is proposed which solves a power minimization problem subject to minimum signal-to-interference-and-noise ratio constraints at every step. The power minimization problem is nonconvex. The Karush Kuhn Tucker conditions, however, are shown to be sufficient for optimality. Capacity scaling law of the two-way relay channel with increasing number of relays is also established by deriving a lower and upper bound on the capacity region of the two-way relay channel. To achieve the optimal DM-tradeoff, a compress and forward strategy is proposed and its DM-tradeoff is derived. For the full-duplex two-way relay channel, the proposed strategy achieves the optimal DM-tradeoff, while for the half-duplex case the proposed strategy is shown to achieve the optimal DM-tradeoff under some conditions.
Patent•
14 Jun 2011TL;DR: In this paper, the authors propose an autoconfigurable virtual base station (VBS) for tree-topology multi-hop relay networks, where the VBS is auto-configurable.
Abstract: In a wireless multi-hop relay network arranged in a tree topology, the base station and one or more relay stations are associated as a virtual base station (VBS). The base station and each relay station have a unique virtual base station identifier (VBS-ID) associated with the path defined by the base station and the one or more relay stations. a relay station in the branch uses its VBS-ID for communicating with an attached subscriber station (e.g. a mobile station) such that communications between the base station and subscriber station occur via the VBS. Subscriber station data communications are relayed between the base station and the one or more relay stations over the VBS via a tunnel connection. The VBS is autoconfigurable. Mobility for subscriber stations and relay stations is provided through reconfiguration of VBS's.
TL;DR: The results indicate that the ROCPAD relay can reliably detect islanding conditions in large microgrid structures and the widely used rate of change of frequency relays (ROCOF) is found working effectively in the situations where ROCOF fails.
Abstract: The paper presents a new approach for islanding detection in distributed generation (DG) using rate of change of phase angle difference (ROCPAD). The process starts with retrieving the voltage and current signals at the DG end and estimating the phasors (amplitude, phase, and frequency) using synchronous transformation based algorithm. The next step is to compute the phase angle difference and the ROCPAD for registering islanding detection. The proposed algorithm is compared with the widely used rate of change of frequency relays (ROCOF) and found working effectively in the situations where ROCOF fails. The proposed ROCPAD relay is tested on standard microgrid structures with possible variations in operating parameters and the results indicate that the ROCPAD relay can reliably detect islanding conditions in large microgrids.
TL;DR: A theoretical, scaled, 32-bit MEM relay-based adder, with a single-bit functionality demonstrated by the measured circuits, is found to offer a factor of ten energy efficiency gain over an optimized CMOS adder for sub-20 MOPS throughputs at a moderate increase in area.
Abstract: This work presents measured results from test chips containing circuits implemented with micro-electro-mechanical (MEM) relays. The relay circuits designed on these test chips illustrate a range of important functions necessary for the implementation of integrated VLSI systems and lend insight into circuit design techniques optimized for the physical properties of these devices. To explore these techniques a hybrid electro-mechanical model of the relays' electrical and mechanical characteristics has been developed, correlated to measurements, and then also applied to predict MEM relay performance if the technology were scaled to a 90 nm technology node. A theoretical, scaled, 32-bit MEM relay-based adder, with a single-bit functionality demonstrated by the measured circuits, is found to offer a factor of ten energy efficiency gain over an optimized CMOS adder for sub-20 MOPS throughputs at a moderate increase in area.
TL;DR: Three low-complexity relay-selection strategies, namely, selective amplify and forward, selective decode and forward (S-DF), and amplified and forward with partial relay selection (PRS-AF) in a spectrum-sharing scenario are studied and the diversity and coding gains are derived and compared.
Abstract: Three low-complexity relay-selection strategies, namely, selective amplify and forward (S-AF), selective decode and forward (S-DF), and amplify and forward with partial relay selection (PRS-AF) in a spectrum-sharing scenario are studied. First, we consider a scenario where perfect channel state information (CSI) is available. For these scenarios, the respective asymptotic outage behaviors of the secondary systems are analyzed, from which the diversity and coding gains are derived and compared. Unlike the coding gain, which is shown to be very sensitive with the position of the primary receiver, the diversity gain of the secondary system is the same as the nonspectrum-sharing system. In addition, depending on the cooperative strategy employed, an increase in the number of relays may lead to severe loss of the coding gain. Afterwards, the impacts of imperfect CSI regarding the interference and transmit channels on the outage behavior of the secondary systems are analyzed. On one hand, the imperfect CSI concerning the interference channels only affects the outage performance of the primary system, whereas it has no effect on the diversity gain of the secondary system. On the other hand, the imperfect CSI concerning the transmit channels of the secondary systems may reduce the diversity gain of the three relay-selection strategies to unity, which is validated by both theoretical and numerical results.
TL;DR: This letter presents an optimal joint relay selection (RS) and power allocation scheme for two-way relay networks which aim to establish a communication link between two transceivers with the help of one relay.
Abstract: In this letter, we present an optimal joint relay selection (RS) and power allocation scheme for two-way relay networks which aim to establish a communication link between two transceivers with the help of one relay. Our approach is based on the maximization of the smaller of the received signal-to-noise-ratios (SNRs) of the two transceivers under a total transmit power budget. We show that this problem has a closed-form solution and requires only a single integer parameter (i.e, the index of the optimally selected relay) to be broadcasted to all relays. We also show that for large values of the total transmit power, the selection criterion can be approximated as the harmonic mean of the amplitudes of the relays' local channel coefficients. We evaluate the performance of our scheme numerically.
01 Dec 2011
TL;DR: This paper identifies the optimal offline transmission scheme for energy harvesting source and relay when the relay operates in the full-duplex mode and provides the optimal transmission scheme when the source has a single energy packet.
Abstract: Communication nodes with the ability to harvest energy from the environment have the potential to operate beyond the timeframe limited by the finite capacity of their batteries; and accordingly, to extend the overall network lifetime. However, the optimization of the communication system in the presence of energy harvesting devices requires a new paradigm in terms of power allocation since the energy becomes available over time. In this paper, we consider the problem of two-hop relaying in the presence of energy harvesting nodes. We identify the optimal offline transmission scheme for energy harvesting source and relay when the relay operates in the full-duplex mode. In the case of a half-duplex relay, we provide the optimal transmission scheme when the source has a single energy packet.
TL;DR: This paper develops an analytical model with a generic radio channel model to fully characterize the access probability and connectivity probability performance in a vehicular relay network considering both one-hop and two-hop communications between a vehicle and the infrastructure.
Abstract: IEEE 802.11p and 1609 standards are currently under development to support Vehicle-to-Vehicle and Vehicle-to-Infrastructure communications in vehicular networks. For infrastructure-based vehicular relay networks, access probability is an important measure which indicates how well an arbitrary vehicle can access the infrastructure, i.e. a base station (BS). On the other hand, connectivity probability, i.e. the probability that all the vehicles are connected to the infrastructure, indicates the service coverage performance of a vehicular relay network. In this paper, we develop an analytical model with a generic radio channel model to fully characterize the access probability and connectivity probability performance in a vehicular relay network considering both one-hop (direct access) and two-hop (via a relay) communications between a vehicle and the infrastructure. Specifically, we derive close-form equations for calculating these two probabilities. Our analytical results, validated by simulations, reveal the tradeoffs between key system parameters, such as inter-BS distance, vehicle density, transmission ranges of a BS and a vehicle, and their collective impact on access probability and connectivity probability under different communication channel models. These results and new knowledge about vehicular relay networks will enable network designers and operators to effectively improve network planning, deployment and resource management.
TL;DR: Simulation results show that the proposed algorithms almost achieve the optimal weighted sum rate and outperform the existing methods in various channel conditions.
Abstract: In this paper, a point-to-point orthogonal-frequency- division multiplexing (OFDM) system with a decode-and- forward (DF) relay is considered. The transmission consists of two hops. The source transmits in the first hop, and the relay transmits in the second hop. Each hop occupies one time slot. The relay is half-duplex, and capable of decoding the message on a particular subcarrier in one time slot, and re-encoding and forwarding it on a different subcarrier in the next time slot. Thus, each message is transmitted on a pair of subcarriers in two hops. It is assumed that the destination is capable of combining the signals from the source and the relay pertaining to the same message. The goal is to maximize the weighted sum rate of the system by jointly optimizing subcarrier pairing and power allocation on each subcarrier in each hop. The weighting of the rates is to take into account the fact that different subcarriers may carry signals for different services. Both total and individual power constraints for the source and the relay are investigated. For the situations where the relay does not transmit on some subcarriers because doing so does not improve the weighted sum rate, we further allow the source to transmit new messages on these idle subcarriers. To the best of our knowledge, such a joint optimization inclusive of the destination combining has not been discussed in the literature. The problem is first formulated as a mixed integer programming problem. It is then transformed to a convex optimization problem by continuous relaxation, and solved in the dual domain. Based on the optimization results, algorithms to achieve feasible solutions are also proposed. Simulation results show that the proposed algorithms almost achieve the optimal weighted sum rate and outperform the existing methods in various channel conditions.