Relay Link Research Articles

Differential Quadrature Amplitude Modulation and Relay Selection with Detect-and-Forward Cooperative Relaying

In this paper, we consider a cooperative communication system with differential modulation and relay-selection (DM---RS) in multi-relay networks, where the best relay is selected to forward the source node's signal to the destination node with detect-and-forward (DetF) protocol. Unlike the conventional DM---RS---DetF scheme using phase shift keying (PSK), we propose a DM---RS---DetF scheme with quadrature amplitude modulation (QAM). Since the differential PSK scheme cannot be applied to the QAM constellations directly, we firstly develop the modulation and demodulation methods for the differential QAM scheme. Then, we derive a closed-form approximation and an upper bound of the symbol error rate for the DQAM---RS---DetF scheme over independent Rayleigh fading channels, by using the approximated signal-to-noise ratio of the equivalent relay link. Simulations results verify the validity of the analytical results and further show that when the modulation order $$M>8$$ in Rayleigh fading channels, the DQAM---RS---DetF scheme outperforms the DPSK---RS---DetF scheme.

Energy-efficient power allocation for non-regenerative OFDM relay links

We consider a two-hop non-regenerative (amplify-and-forward) relay link with one source, one destination and one relay, in which orthogonal frequency division multiplexing (OFDM) is used. The power allocation (PA) to maximize the energy efficiency for the whole link is investigated under quality of service (Qos) requirement and separate power constraints. An approximation method with low complexity is first proposed to convert the PA problem into a quasi-concave problem, which can obtain a tight lower bound in high SNR region. Furthermore, a more accurate approximation method with higher complexity is raised, which can approach the actual performance both in the low SNR and high SNR regions. We also introduce subcarrier pairing scheme to further enhance the energy efficiency for the link. Simulation results are shown to compare the performance of the two proposed methods.

Power allocation and relay selection for AF two-path successive relaying networks

Two-path or successive relaying, which aims to establish two relay links transmitting different information symbols in adjacent time slots, has recently emerged as an attractive wireless communication protocol to improve the spectral efficiency in half-duplex cooperative systems. In this paper, we investigate power allocation and relay selection techniques for amplify-and-forward two-path successive relaying networks. Our approach is based on the maximization of the received SNR subject to a total power budget consumed by the source and the relay assisting this specific transmission. Two scenarios including with and without direct link are considered here. We show that the main problem has a closed-form solution and only requires a few amounts of feedback bits to be broadcasted. Numerical results reveal that the proposed approaches are more insensitive to the inter-relay interference and robust to channel estimation errors; meanwhile, they perform better than the existing schemes. Copyright © 2013 John Wiley & Sons, Ltd.

Performance Evaluation of Cooperative ML-IDMA Communications with DF Protocols

Relaying and cooperative diversity allow multiple wireless radios to effectively share their antennas and create a virtual antenna array, thereby leveraging the spatial diversity benefits of multiple-input multiple-output antenna systems. In this paper, we consider the bit error rate performance analysis of a cooperative relay communication system on Multi-Layer-IDMA (ML-IDMA) using Maximal-Ratio-Combining (MRC) technique; we examine the effect of layers number on the performance and derive the average bit error probability of the Decode-and-Forward (DF) relay schemes by using the closed-form relay link Signal-to-Noise Ratio (SNR). Based on the analysis, we show that from the simulation results, degradation in the performance is observed when increasing number of layers but on the other hand we saved in the Band Width (BW).

Performance Evaluation of Cooperative Multi-Layer IDMA Communications with Amplify-and-Forward Protocol

In this paper, we consider the Bit Error Rate (BER) performance analysis of a cooperative relay communication system for Multi-Layer IDMA (ML-IDMA) using Maximal-Ratio-Combining (MRC) technique. We examine the effect of layers number on the performance and derive the average bit error probability of the Amplify-and-Forward (AF) relay scheme by using the closed-form relay link Signal-to-Noise Ratio (SNR). The proposed system has been simulated to evaluate the performance of ML-IDMA with different numbers of layers and different number of relays in the ML-IDMA cooperative environment. The simulation results show that the BER performance has been improved by about 4dB in the case of double layer (K=2) IDMA system with 4-relays, also, the saving in the band width is 50%. Finally, the BER performance has been degraded with increasing in the number of layers and the proposed system has improved the band width by about 1/K.

Multicast Gain for IPTV Transmission in WiMAX Multi-hop Relay Networks

Due to characteristics such as: multicast support, wide coverage range, high bandwidth and mobility support, WiMAX could be a leading solution to deliver bandwidth-hungry IPTV services to mobile users. Mobile multi-hop relay (MMR) was proposed as an amendment of the current mobile WiMAX standard to provide relaying capabilities. Relays are deployed as a cost-effective solution to extent coverage area and increase throughput without deploying expensive base stations. In this paper, we introduce multicast gain as a performance criterion, which can be used for computing multicast efficiency. In order to quantify the benefits of multicasting, we analyze multicast transmission for delivery of live TV channels in MMR networks, and determine the gain of access and relay links. The quantification of the efficiency of multicasting can be used by IPTV service providers in resource provisioning, access control or decision support mechanisms in delivery of different TV channels.

Energy allocation for green multiple relay cooperative communication

Cooperative communication achieves diversity through spatially separated cooperating nodes, which are battery powered in most applications. Therefore the energy consumption must be minimized without compromising the quality of service. In this context, we present a novel energy allocation scheme for multiple relay nodes that results in efficient cooperative multiple-input multiple-output (MIMO) communication. Considering channel path loss, the total transmission energy is distributed between the source and the relay nodes. The energy distribution ratio between the relay and direct link is optimized such that the quality of received signal is maintained with minimum total transmission energy consumption. We calculate the energy distribution ratio analytically and verified it through computer simulation. With the new energy allocation scheme, the system also obtains an increased channel capacity as compared to the cooperative scheme with conventional equal energy allocation and the non-cooperative scheme. Optimal relay positioning with the proposed energy allocation scheme is also explored to maximize the capacity.

Resource sharing in LTE‐Advanced relay networks: uplink system performance analysis

ABSTRACTRelay‐enhanced networks are expected to fulfil the demanding coverage and capacity requirements in a cost‐efficient way. Type 1 inband relaying has been standardised as an integral part of the Third Generation Partnership Project (3GPP) Long‐Term Evolution Release 10 and beyond (LTE‐Advanced). This type of relay nodes (RNs) supports a relaying mode where the RN to donor evolved node B (donor eNB, DeNB) link (relay link, a.k.a. backhaul link) transmission is time‐division multiplexed with the RN‐served user equipments (RUEs) to RN link (access link) transmission, whereas macrocell‐served user equipments (MUEs) share the same resources with the RNs at DeNB. Hence, system performance depends strongly on the resource sharing strategy among and within the links. Further, the set of subframes assigned for the relay link transmission is semi‐statically configured and thus a dynamic reconfiguration to adapt to fast‐changing system conditions (e.g. RN cell load) is not viable. Besides, in order to fully exploit the benefits of relaying, the inter‐cell interference, which is increased because of the presence of RNs, should be limited via a proper power control (PC) scheme on each link. Therefore, an optimisation of both the resource sharing and PC strategy is required to enhance the overall performance of relay networks. In order to tackle these issues, we employ a statistic‐based over‐provisioned backhaul subframe allocation to be utilised for flexible co‐scheduling of RNs and MUEs at the DeNB. In addition, we propose a combination of RN scheduling based on the number of RUEs and user throughput throttling achieving max–min fairness. Performance analysis of various resource sharing techniques along with PC optimisation is then carried out within the LTE‐Advanced uplink framework in urban and suburban scenarios. Comprehensive results show that the proposed schemes achieve significant throughput gains and high system fairness with substantially increased flexibility in resource allocation. Copyright © 2012 John Wiley & Sons, Ltd.

Doubly selective channel estimation for OFDM modulated amplify-and-forward relay networks using superimposed training

This article is concerned with the problem of superimposed training (ST)-aided channel estimation for orthogonal frequency division multiplexingmodulated amplify-and-forward relay networks in doubly selective environment. A ‘subblockwise’ linear assumption-based channel model is proposed to represent the mobile-to-mobile time- and frequency-selective channels. We then propose a novel ST strategy that allows the destination node to separately obtain the channel information of the source → relay link and the relay → destination link, from which the optimal ST signals are derived by minimizing the channel mean-square-error. To enhance the performance of channel estimation, a subblock tracking-based low-complexity decision feedback approach is introduced to iteratively mitigate the unknown data interference. Finally, extensive numerical results are provided to corroborate the proposed studies.

Capacity Theorems for the Fading Interference Channel With a Relay and Feedback Links

Handling interference is one of the main challenges in the design of wireless networks. One of the key approaches to interference management is node cooperation, which can be classified into two main types: relaying and feedback. In this paper, we consider simultaneous application of both cooperation types in the presence of interference. We obtain exact characterization of the capacity regions for Rayleigh fading and phase fading interference channels with a relay and with feedback links, in the strong and very strong interference regimes. Four feedback configurations are considered: 1) feedback from both receivers to the relay, 2) feedback from each receiver to the relay and to one of the transmitters (either corresponding or opposite), 3) feedback from one of the receivers to the relay, and 4) feedback from one of the receivers to the relay and to one of the transmitters. Our results show that there is a strong motivation for incorporating relaying and feedback into wireless networks.

Queuing analyses and statistically bounded delay control for two‐hop green wireless relay transmissions

SUMMARYAlthough the wireless relay transmission has been recognized as one of the effective approaches in extending the coverage area and improving the network capacity, the delay control over the two‐hop wireless channels toward end users’ QoS satisfaction is still challenging in green wireless network designs. In this paper, we propose the QoS‐driven delay control schemes over two‐hop wireless relay links to statistically upper bound the total queuing delay for the decode‐and‐forward relay transmissions. Specifically, we first conduct asymptotic queuing‐delay analyses for the relay transmissions. Then, we show that an efficient approach for statistical delay guarantees is to make the exponentially decaying speed of the complementary cumulative distribution functions of the queueing delay at both hops identical. We further derive that this target needs to be attained through asymmetric resource allocations over the two hops. Motivated by this fact, we formulate two optimization problems aiming at satisfying the specified delay‐bound violation probability for the relay transmission, while minimizing the power consumption toward green wireless networking. One framework is for the frequency‐division duplex relay mode and the other is for time‐division duplex relay mode. In the frequency‐division duplex relay mode, the source and relay nodes use a different transmit power. For the time‐division duplex relay mode, the source and relay nodes use time‐slots with different length but with the same instantaneous power. We solve for the optimal solutions for both of the aforementioned frameworks. Also conducted is a set of simulation results to show the impact of the QoS requirements, traffic load, and position of the relay node on the resource allocation over wireless relay links. Copyright © 2012 John Wiley & Sons, Ltd.

Throughput Improvement by Joint Relay Selection and Link Scheduling in Relay-Assisted Cellular Networks

We consider joint relay selection and link scheduling to maximize the network throughput in relay-assisted cellular networks. The spatial reuse is leveraged by scheduling multiple links to simultaneously transmit. The coupling among relay selection, link scheduling, and the interference that is introduced by simultaneous transmissions makes this problem hard to solve. We summarize spatial reuse into two forms. The first form of spatial reuse exists among second-hop links, where relay stations transmit to mobile users. The second form of spatial reuse exists between second- and first-hop links, where the base station transmits to relay stations or mobile users. A framework is proposed to decouple the joint problem into the following two subproblems: 1) a frame segmentation problem and 2) a relay selection problem. Under this framework, we propose two algorithms for either only the first form of spatial reuse exists or both forms of spatial reuse exist. Numerical results show that, with the first form of spatial reuse, the performance of the proposed heuristic relay selection algorithm is very close to the optimum. In the given scenario, when both forms of spatial reuse exist and the proposed heuristic frame segmentation algorithm is applied, the throughput is improved by up to more than 50% compared with the case without spatial reuse.

Linear MMSE Processing Design for 3-Phase Two-Way Cooperative MIMO Relay Systems

This letter addresses the joint linear processing issues for 3-phase two way cooperative MIMO relay systems, aiming to minimize the mean squared error (MMSE). Considering the difficulty to acquire the channel state information (CSI) of the direct link at relay node, we first derive an iterative linear processing scheme for the relay node ignoring the direct link between two source nodes. Then the receive processing matrix combining the signal from direct link and relay link can be easily given according to MMSE orthogonality principle. Finally by comparing other schemes that can accomplish bidirectional data exchange, simulation results show that our proposed 3-phase cooperative scheme can achieve better tradeoff of link reliability and spectrum efficiency especially when the direct link is in medium quality.

Relay-assisted hierarchical adaptation scheme for multi-user scalable video delivery to heterogeneous mobile devices

In multi-user video (MUV) delivery scenarios, the available resources of receiver devices, such as processing capability, link packet error rate (PER), and bandwidth, are usually different. We propose a relay-assisted hierarchical adaptation (RHA) scheme to maximize the total perceptual quality of all users when transmitting video streams coded via scalable video coding (SVC). First, MUV bitstreams are adaptively extracted under the constraints of network bandwidth and individual decoding capacity. Next, the relay links are introduced as substitutes of possible bad direct links for packets retransmissions. Approximately equal opportunity of transmission is allocated to each stream while the packets inside a stream are scheduled according to their priorities. The priorities are determined by the links states and packets loss distortions. Simulation results show that our RHA scheme has significant performance improvements compared with other schemes.

Analysis and performance of network decoding strategies for cooperative network coding

In this work, we present two new low-complexity network decoding strategies for cooperative network coding in a multiple-access relay channel scenario. For these two strategies, Selection and Soft Combining and Majority Vote Network Decoding, along with the optimal joint network decoding, we derive expressions for bit error probability performance as a function of the signal-to-noise ratio (SNR) of the different Rayleigh fading links, and show the tightness of the derived bounds through simulation results. The two proposed schemes provide a similar bit error probability (BEP) performance compared to the optimal scheme, while having significantly lower complexity. Further, we study the effect of user pairing on the error performance by considering different SNRs on the user and relay links towards the destination. It is also shown that the error performance of the different schemes follows the same trend for a given user pairing strategy.

Relay Handover and Link Adaptation Design for Fixed Relays in IMT-Advanced Using a New Markov Chain Model

Fixed relay networks will be an integrated component of future International Mobile Telecommunications (IMT)-Advanced. In this paper, we propose a new approach to relay handover and link adaptation. Our approach is built on a new Markov chain model (MCM) that comprehensively characterizes different relay protocols and quantifies their quality-of-service (QoS) measures, such as packet drop rate and latency, and spectral efficiency. Our relay handover and link adaptation scheme combats channel fluctuations while satisfying QoS requirements. It also accommodates multiple relay stations, supports multiplexing and diversity, and copes with mutually related and time-dependent transmissions. Analytical results, which were validated by simulations, show that our scheme can reduce the packet loss by up to three orders of magnitude. It also decreases the packet delay by up to 18% and improves the throughput by up to 10%.

Gaussian Z-Interference Channel With a Relay Link: Achievability Region and Asymptotic Sum Capacity

This paper studies a Gaussian Z-interference channel with a rate-limited digital relay link from one receiver to another. Achievable rate regions are derived based on a combination of a Han–Kobayashi common-private power-splitting technique and either a compress-and-forward relay strategy or a decode-and-forward strategy for interference subtraction at the other end. For the Gaussian Z-interference channel with a digital link from the interference-free receiver to the interfered receiver, the capacity region is established in the strong interference regime; an achievable rate region is established in the weak interference regime. In the weak interference regime, the decode-and-forward strategy is shown to be asymptotically sum-capacity achieving in the high signal-to-noise ratio and high interference-to-noise ratio limit. In this case, each relay bit asymptotically improves the sum capacity by one bit. For the Gaussian Z-interference channel with a digital link from the interfered receiver to the interference-free receiver, the capacity region is established in the strong interference regime; achievable rate regions are established in the moderately strong and weak interference regimes. In addition, the asymptotic sum capacity is established in the limit of a large relay link rate. In this case, the sum capacity improvement due to the digital link is bounded by half a bit when the interference link is weaker than a certain threshold, but the sum capacity improvement becomes unbounded when the interference link is strong.

Performance analysis of selective cooperation with fixed gain relays in Nakagami-[formula omitted] channels

Selecting the best relay using the maximum signal to noise ratio (SNR) among all the relays ready to cooperate saves system resources and utilizes the available bandwidth more efficiently compared to the regular all-relay cooperation. In this paper, we analyze the performance of the best relay selection scheme with fixed gain relays operating in Nakagami-m channels. We first derive the probability density function (PDF) of upper bounded end-to-end SNR of the relay link. Using this PDF, we derive some key performance parameters for the system including average bit error probability and average channel capacity. The analytical results are verified through Monte Carlo simulations.

Joint optimization of source and relay precoding in non‐regenerative MIMO relay systems

ABSTRACTWe investigate the problem of precoding optimization in an amplify‐and‐forward multiple‐input‐multiple‐output relay system. Most reported works on this problem focus chiefly on the design of relay precoder without simultaneously optimizing the direct link. In this paper, we propose a method for joint source/relay precoder design, taking both direct and relay links into account. Our design is based on maximizing the mutual information (MI) under limited transmission power constraints at the source and relay, respectively. We first formulate a constrained optimization problem before relaxing the original cost function for tractability and derive a MI lower bound. This elaborate bound can asymptotically approach the exact expression of MI in an iterative fashion. In contrast to previous strategies, we then prove that the optimal structure of the source and relay precoders jointly convert the multiple‐input‐multiple‐output relay channel into a bank of single‐input‐single‐output relay channels without having to assume a beamforming structure to simplify the derivation. Specifically, the linear precoding design problem degenerates into power loading among multiple single‐input‐single‐output relay channels. Applying standard Lagrange technique results in a scalar convex optimization, and it can be readily solved by iterative water filling. Numerical examples demonstrate that the proposed scheme, either exploiting partial or full channel state information, significantly outperforms the existing methods. Copyright © 2012 John Wiley & Sons, Ltd.

Joint Zero-Forcing Based Precoder Design for QoS-Aware Power Allocation in MIMO Cooperative Cellular Network

We study a cellular system scenario where multiple data streams originating from a base station (BS) targeted to multiple cell-edge mobile stations (MSs) are transmitted via pre-installed cooperative relay stations (RSs) with multiple antennas. Our objective is to guarantee quality-of-service (QoS) in terms of predefined signal-to-noise ratio at such users within the transmit power budgets at BS and RSs while minimizing total transmit power. We propose a novel precoder design method for power allocation between multiple data streams at BS and RS by using joint zero-forcing strategy in order to avoid multiuser interference (MUI) in the signal received by MSs via both the direct and relay links. We also propose low-complexity suboptimal power allocation algorithm. We focus on analyzing the significance of direct link transmission in providing QoS to cell-edge MSs specially when RS is not situated directly between BS and MSs. Simulation results show that considering direct link and using the proposed scheme in such case significantly improves system outage performance compared to existing schemes in the literature which do not consider direct link.