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Hindawi Publishing CorporationEURASIP Journal on Wireless Communications and NetworkingVolume 2010, Article ID 693158, 2 pagesdoi:10.1155/2010/693158

EditorialPhysical-Layer Network Coding for WirelessCooperative Networks

Wen Chen,1 Xiaodai Dong,2 Pingyi Fan,3 Christoph Hausl,4 Tiffany Jing Li,5

Petar Popovski,6 and Meixia Tao1

1 Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China2 Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC, Canada V8W 2Y23 Department of Electronic Engineering, Tsinghua University, Beijing 100084, China

4 Institute for Communications Engineering, Technische Universitat Munchen, 80290 Munich, Germany5 Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, PA 18015, USA6Department of Electronic Systems, Aalborg University, 9220 Aalborg, Denmark

Correspondence should be addressed to Wen Chen, wenchen@sjtu.edu.cn

Received 28 October 2010; Accepted 28 October 2010

Copyright 2010 Wen Chen et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Cooperative communication is an intriguing topic that hasaroused a frenzy of heat in the wireless networking research.The notion of cooperative communication is to enable trans-mit and receive cooperation at the user level by exploitingthe broadcast nature of wireless radio waveform so thatthe overall system performance including power efficiencyand communication reliability can be significantly improved.However, due to the half-duplex constraint in practicalwireless relay systems, cooperative communication generallysuffers from loss in spectral efficiency. The inception of net-work coding provides a powerful remedy for improving thespectral efficiency and opens new possibilities for achievingthe end-to-end throughput optimality by allowing interme-diate network nodes to mix, demix, and remix the signalsreceived from multiple links for subsequent transmissions.

In addition, network coding fits naturally for the wirelesscooperative networks. Network coding may be implementedin the network layer, media/multiple access (MAC) layer, orthe physical layer. Specifically, physical-layer network coding(PLNC) encompasses a rich variety of signal processing andcoding techniques and may be exploited in combination withmultiple antennas, orthogonal frequency-division multiplex-ing (OFDM), channel coding, signal detection, and resourceallocation. Its excellent performance has been demonstratedin various relay-based communication scenarios, includingtwo-way communication, multiway relaying, multiple-accessrelaying, multicasting, and broadcasting, and active researchis continuing to explore new potentials.

This special issue aims to consolidate the latest researchadvances in physical-layer network coding in wireless coop-erative networks. The purpose is to seek new and originalcontributions addressing various aspects of PLNC, includ-ing, for example, fundamental limits, protocol design, codedesign, practical implementation issues, joint synchroniza-tion, modulation, channel coding and PLNC, combinedPLNC and diversity and multiplexing techniques, and cross-layer design and optimization.

In this special issue, we have received a total of 32 originalsubmissions, out of which 11 (34%) papers are accepted forpublication after peer review. We regret that we had to rejectmany good papers due to the limited number of papers thatcan be published in this special issue. The accepted paperscover a broad area of PLNC-related topics, including channel

estimation, protocol design, scheduling, resource allocation,and channel code design for network coded schemes.

To effectively decode a network code would inevitablyrequire the knowledge of at least partial channel informationof present channel estimation algorithms for two-way relaychannels. Both exploit the nature of reciprocal channels andboth consider OFDM transmission strategies, but approachthe problem from different aspects. In the paper entitledSuperimposed training-based joint CFO and channel esti-mation for CP-OFDM modulated two-way relay networks,the authors introduce superimposed pilots at the relay toenable the estimation of channels and carrier frequency offset(CFO) parameters. Three different estimators are proposed,

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as well as an iterative estimator, and the estimation erroris compared to the Cramer-Rao bound. In comparison,the paper Channel estimation for two-way relay OFDMnetworks studies the condition and method for designingoptimal block training sequences with explicit considerationof the peak-to-average power ratio (PAPR).

To design an eff

ective network coding protocol isan essential issue of implementing signal mixing in theintermediate network nodes and signal detection in thedestinations. We have included three papers discussingnetwork coding protocols in this special issue. The paperDesign criteria for hierarchical exclusive code with parameter-invariant decision regions for wireless 2-way relay channelfocuses on the design of a new PLNC scheme termed thehierarchical exclusive code, including the design criterion,the decision region, and the achievable performance. Thepaper The performance of network coding at the physi-cal layer with imperfect self-information removal discussesPLNC with denoising and evaluates the impact of residualself-information. The paper MIMO network coding-basedPHY/MAC protocol for replacement of CSMA/CA in efficienttwo-way multihop relay networks exploits PLNC with themultiple antenna technology to achieve efficient multipleaccess. The paper by Gacanin and Adachi considers thescenario of broadband two-way relaying with orthogonalfrequency division multiplexing (OFDM) and single carrierwith frequency domain equalization (SC-FDE) by using ana-log network coding (ANC) in frequency selective channels.The authors investigate the effects of imperfect removal ofself-information on the system performance.

The authors study the resource management issuein the context multiway relay and/or multicast PLNC.Beamforming-based physical layer network coding for non-regenerative multi-way relaying presents new beamformingrelaying strategies for multiway relaying that allow achievinghigh spectrum utilization with low interference. Jointpower allocation for multicast systems with physical-layernetwork coding discusses the allocation and optimizationof the transmit power. Joint NC-ARQ and AMC forQoS-guaranteed mobile multicast investigates incrementalnetwork coding through automatic repeat request (ARQ) toensure desired quality of service (QoS).

Joint design of LDPC coding and network coding canfurther improve the reliability of networks. There are twopapers devoted to the design of network codes by leveragingthe ideas and tools from channel coding. As the (sparse)

mixing operation of network coding finds natural connec-tion to low-density parity-check (LDPC) codes, both makeessential use of the LDPC coding structure. Specifically, thepaper by Cocco et al. exploits LDPC schemes in high-orderGalois fields, while the paper by Duyck et al. considers jointchannel-network coding through a unified graph structureand proposes the construction of an LDPC code that achievesfull diversity for multiple-access relaying.

Finally, Ma et al.s work in this PLNC special issueinvestigates network modulation through linear algebra.

The topics covered in these papers reveal only the tipof the iceberg, but they are certainly demonstrating excitingand diverse perspectives and methodologies in the study of

physical-layer network coding. We sincerely hope that thecollection of papers in this special issue will help enlightenmore and better research in this fruitful area of wirelessnetworks.

Acknowledgments

The authors thank all the authors for submitting theiroriginal work and all the reviewers for providing usefultechnical comments.

Wen ChenXiaodai Dong

Pingyi FanChristoph Hausl

Tiffany Jing LiPetar Popovski

Meixia Tao