Physical Layer Network Research Articles

Multi-pair two-way relaying systems with physical layer network coding

Physical layer network coding (PNC) significantly improves the throughput of the two-way relay channel (TWRC). However, most of current work on PNC just focuses on one pair users, and there are limit work on multi-pair PNC, in which the main bottleneck on system performance is the interference caused by the remaining communicating pairs. In this paper, we consider a scenario where multiple pairs communicate with each other through one relay node. For a purpose of comparison, we present the analytical capacities of the multi-pair version of the existing TWRC schemes in closed forms. Considering more practical issues such as the direction distribution of incident signals, we propose a spatial multi-pair physical layer network coding scheme. The novelty includes: in the multiple access phase, the log likelihood ratio combination scheme is proposed, which is robust in bad communication conditions; in the broadcast phase, two adaptive group broadcasting schemes are proposed, which can efficiently overcome the near interference problem. Simulation results validate the superiority of the proposed algorithms.

Digital Optical Physical-Layer Network Coding for mm-Wave Radio-Over-Fiber Signals in Fiber-Wireless Networks

We demonstrate a digital Optical Physical-layer Network Coding (OPNC) for mm-wave fiber-wireless signals modulated with up to 2.5 Gb/s On/OFF Keyed (OOK) data. The proposed OPNC concept uses an all-optical XOR gate comprising a Semiconductor Optical Amplifier-Mach Zehnder Interferometer (SOA-MZI) with SOAs being driven at low moderate electrical currents in order to perform the all-optical encoding between the OOK envelopes of the data, ignoring the high-frequency Sub-Carrier (SC) signal. In this scheme, network coding is performed on-the-fly at the central office and the resultant packet is broadcasted at the client nodes, where the decoding takes place. We demonstrate experimental results of OPNC using OOK data signals modulated on a 10 GHz SC with the aid of a second all-optical XOR gate for the decoding process at the client's site, reporting error-free performance for both synchronous/asynchronous data packets. The scenario of all optical encoding for 60 GHz SC frequencies followed by electrical decoding at the end-users is evaluated via PHY-layer simulations. Going a step further and considering the network level, we present a performance improvement on the network throughput by using the proposed network coding.

Achievable degrees of freedom of MIMO two-way X relay channel with delayed CSIT

The previous work on interference alignment for multiple-input-multiple-output (MIMO) two-way X relay channel assumes perfect channel state information at the transmitter (CSIT), which is reasonable in slow fading channel. However, in fast fading scenario, this assumption is impractical. In this paper, assuming that each node has delayed CSIT, we study the achievable degrees of freedom (DOF) for MIMO two-way X relay channel in frequency division duplex (FDD) systems. Specifically, in the broadcast (BC) phase, we propose a new multiple-stage transmission (MST) scheme, which utilizes retrospective interference alignment for physical layer network coding (PLNC). We show that MST can achieve significant DOF gain and tremendous power gain over other schemes. When the number of antennas for each user, N, is smaller than the number of the relays, M, the time division multiple access (TDMA) scheme can only achieve an ergodic sum-rate increase by N bps/Hz for every increasing of 3 dB of signal-to-noise power ratio (SNR), while the proposed MST scheme can achieve an ergodic sum-rate increase by M bps/Hz.

An overview of 3GPP enhancements on machine to machine communications

The broad connection of devices to the Internet, known as the IoT or M2M, requires lowcost power-efficient global connectivity services. New physical layer solutions, MAC procedures, and network architectures are needed to evolve the current LTE cellular systems to meet the demands of IoT services. Several steps have been taken under the 3GPP to accomplish these objectives and are included in the upcoming 3GPP LTE standards release (3GPP Release 13). In this tutorial article, we present an overview of several features included in 3GPP to accommodate the needs of M2M communications, including changes in the physical layer such as enhanced machine type communications, and new MAC and higher-layer procedures provided by extended discontinuous reception. We also briefly discuss the narrowband IoT, which is in the development stage with a target completion date of June 2016.

Dynamic Spectrum Leasing for Bi-Directional Communication: Impact of Selfishness

In this paper, we propose a beamforming-based dynamic spectrum leasing (DSL) technique to improve the spectral utility of bi-directional communication of the legacy/primary spectrum users through the help of colocated secondary users. The secondary users help for a time interval to relay the data between two primary terminals using physical layer network coding and beamforming to attain bi-directional communication with high spectral utility. As a reimbursement, the secondary users, cognitive radios (CRs) in our case, get exclusive access to the primary spectrum for a certain duration. We use Nash bargaining to determine the optimal division of temporal resources between relaying and reimbursement. Moreover, we consider that a fraction of secondary nodes can act selfishly by not helping the primary, yet enjoy the reimbursement time. We measure the utility of the DSL scheme in terms of a metric called time-bandwidth product (TBP) ratio quantifying the number of bits transmitted in direct communication versus DSL. We show that if all secondary nodes act honestly, more than 17-fold increase in the TBP ratio is observed for a sparse CR network. However, in such a network, selfish behavior of CR nodes can reduce the gain by more than a factor of 2.

효율적 주파수사용을 위한 공간변조 물리계층 네트워크 코딩기법 제안

ABSTRACT Recently, the volume of mobile data traffic increases exponentially due to the emergence of various mobile services. In order to resolve the problem of mobile traffic increase, various new technologies have been devised. Especially, two-way relay communication in which two nodes can transfer data simultaneously through relay node, has gained lots of interests due to its capability to improve spectral efficiency. In this paper, we analyze the SM-PNC which combines Physical-layer Network Coding (PNC) and Spatial Modulation (SM) under two-way relay communication environment. Log-Likelihood Ratio (LLR) is considered and both separate decoding and direct decoding have been taken into account in performance analysis. Through performance evaluation, we have found that the bit error rate of the proposed scheme is improved compared to that of the conventional PNC scheme, especially when SNR is high and the number of antennas is large. 키워드 : 양방향 중계기술, 물리계층 네트워크 코딩, 공간변조, 로그 우도비 Key word :

A Random Channel Sounding Decision Feedback Receiver for Two-Way Relay Communication With Pilotless Orthogonal Signaling and Physical-Layer Network Coding

We propose a decision feedback (DFB) receiver at the relay of a two-phase (2P) two-way relay (TWR) communication system that employs pilotless orthogonal modulation (such as frequency-shift keying) in the uplink and physical-layer network coding over finite field in the downlink. The proposed relay receiver is able to attain a performance very close to that of an ideal coherent detector in the presence of time-selective Rayleigh fading and additive white Gaussian noise in the uplinks. It exploits the fact that when the uplink symbols from the users are different, then the fading gains affecting these symbols can be separated and individually tracked at the relay. In essence, the proposed receiver performs random channel sounding although no actual pilots are transmitted. The channel estimates obtained this way can then be subsequently used in a coherent detector to improve the reliability of the relay detected data. To ensure fast convergence, we propose to kick start the DFB receiver using a partial-coherent detector developed earlier by the authors. We compare the performance of the proposed system against a similar 2P-TWR system that employs differential phase-shift keying (DPSK) in the uplink and DFB multiple-symbol differential detection at the relay. We found that the proposed pilotless orthogonal modulation system can actually attain a significantly lower bit error rate (BER) than its DPSK counterpart. For static fading and a BER of $\mbox{10}^{-3}$ , the signal-to-noise ratio (SNR) gap between the two approaches is 1 dB in the binary case and 8 dB in the quaternary case. These gaps increase further with time-selective fading.

Multiuser Decoding Scheme for -user Fading Multiple-Access Channel Based on Physical-Layer Network Coding

We propose a new practical multiuser decoding scheme based on physical-layer network coding (PNC) for a $K$ -user fading multiple-access channel. In the scheme, the users encode their messages with a same practical channel code, and the coded and modulated signals are transmitted simultaneously. The receiver first reconstructs $K$ linear functions of the $K$ users’ messages based on PNC. Then all messages are recovered by solving the $K$ linear equations. Compared to the well-known iterative detection and decoding schemes, we show by numerical results that the proposed scheme achieves near interference-free bound performance without utilizing receiver iterations.

On the Subtleties of -PAM Linear Physical-Layer Network Coding

This paper investigates various subtleties of applying linear physical-layer network coding (PNC) with q-level pulse amplitude modulation (q-PAM) in two-way relay channels (TWRC). A critical issue is how the PNC system performs when the received powers from the two users at the relay are imbalanced. In particular, how would the PNC system perform under slight power imbalance that is inevitable in practice, even when power control is applied? To answer these questions, this paper presents a comprehensive analysis of q-PAM PNC. Our contributions are as follows: 1) We give a systematic way to obtain the analytical relationship between the minimum distance of the signal constellation induced by the superimposed signals of the two users (a key performance determining factor) and the channel-gain ratio of the two users, for all q. In particular, we show how the minimum distance changes in a piecewise linear fashion as the channel-gain ratio varies. 2) We show that the performance of q-PAM PNC is highly sensitive to imbalanced received powers from the two users at the relay, even when the power imbalance is slight (e.g., the residual power imbalance in a power-controlled system). This sensitivity problem is exacerbated as q increases, calling into question the robustness of high-order modulated PNC. 3) We propose an asynchronized PNC system in which the symbol arrival times of the two users at the relay are deliberately made to be asynchronous. We show that such asynchronized PNC, when operated with a belief propagation (BP) decoder, can remove the sensitivity problem, allowing a robust high-order modulated PNC system to be built.

Novel self-interference suppression schemes based on Dempster-Shafer theory with network coding in two-way full-duplex MIMO relay

A two-way full-duplex multiple-input multiple-output (MIMO) relay provides effective spectral efficiency and system capacity improvement. However, self-interference (SI) as a serious problem limits the development of a full-duplex relay. In this paper, the novel SI suppression schemes based on the Dempster-Shafer (DS) evidence theory are proposed to suppress SI in the two-way full-duplex MIMO relay. DS evidence theory can appropriately characterize uncertainty and make full use of multiple evidence source information by DS combination rule to obtain reliable decisions. The first proposed DS network coding (DS-NC) scheme adopts DS evidence theory to detect the signal of each source node, considering SI suppression in the basic probability assignment computation in the multiple access phase. Moreover, different from the DS-NC scheme, the further proposed DS physical-layer network coding (DS-PNC) scheme considers SI suppression from the vector space perspective and combines the PNC mapping rule with the DS theory to obtain a network-coded signal without estimating each source node signal. In the broadcast phase, antenna selection is adopted at the relay and DS evidence theory is also applied at each source node for SI suppression. Meanwhile, the effects of imperfect SI channel estimation and the intended channel estimation on the proposed schemes are also studied. Finally, the proposed DS-PNC scheme is studied with the SI signal model which considers analog radio frequency cancellation, the effects of I/Q modulator imbalances, and power amplifier nonlinearity of the transmitter. Simulation results reveal that the proposed DS-PNC scheme is a little more sensitive to the channel estimation errors than the traditional spatial-domain schemes. However, with the perfect channel state information, the proposed DS-NC with one iteration and DS-PNC schemes outperform traditional spatial-domain schemes. Furthermore, considering the SI signal model with nonlinear distortion and radio frequency cancellation, the DS-PNC scheme is also superior to the traditional schemes. Meanwhile, the DS-PNC scheme is more robust to the SI and achieves the same diversity gain as the ideal cancellation.

Design and Analysis of Heterogeneous Physical Layer Network Coding

In this paper, physical layer network coding with heterogeneous modulations (HePNC) is proposed for the asymmetric two-way relay channel (TWRC) scenario. The existing PNC solutions using the same modulation for signals transmitted from two source nodes may not be desirable for practical situations when traffic loads exchanged between the sources are unequal and channel conditions of source-relay links are heterogeneous. HePNC includes two stages: multiple access (MA) and broadcast (BC) stages. In the MA stage, the two source nodes transmit to the relay simultaneously with heterogeneous modulations selected according to the channel conditions and the ratio of traffic loads exchanged between the sources, and then the signals superimposed at the relay are mapped to a network-coded symbol by a mapping function adaptively; in the BC stage, the relay broadcasts the network-coded symbol back to both sources with a modulation selected according to the bottleneck link’s channel condition. We present three HePNC designs, including QPSK-BPSK, 8PSK-BPSK and 16QAM-BPSK HePNC. How to design and optimize the mapping function is investigated and the error performance of QPSK-BPSK HePNC is analyzed. We further study the HePNC system performance, throughput upper bound and energy efficiency. Extensive simulations demonstrated that the proposed HePNC can substantially enhance the throughput and energy efficiency compared with the existing PNC.

Asynchronous Physical-layer Network Coding Scheme for Broadband Two-Way Relay Channels

In OFDM based two-way relay channels, carrier frequency offsets (CFOs) between relay and terminal nodes introduce severe inter-carrier interference which degrades the performance of traditional physical-layer network coding. In this paper, we present a two-step asynchronous physical-layer network coding scheme, which consists of an interference cancelation step and a network coding mapping step, to deal with the frequency asynchrony in two-way OFDM relay system. We show through the average signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER) that the proposed scheme can efficiently map the received superimposed signal, which is corrupted by the CFOs, into the exclusive OR of two terminals' transmitted symbols. Moreover, a simple upper bound of the average SINR for the proposed scheme is derived. Through the simulation results, it is shown that the exact average SINR curve of the proposed scheme is very close to the upper bound even when the normalized CFO is as large as 0.15. Finally, we show that the average SINR and BER performances of the proposed scheme outperform the reference schemes considerably.

The impact of capacity growth in national telecommunications networks.

This paper discusses both UK-based and global Internet data bandwidth growth, beginning with historical data for the BT network. We examine the time variations in consumer behaviour and how this is statistically aggregated into larger traffic loads on national core fibre communications networks. The random nature of consumer Internet behaviour, where very few consumers require maximum bandwidth simultaneously, provides the opportunity for a significant statistical gain. The paper looks at predictions for how this growth might continue over the next 10-20 years, giving estimates for the amount of bandwidth that networks should support in the future. The paper then explains how national networks are designed to accommodate these traffic levels, and the various network roles, including access, metro and core, are described. The physical layer network is put into the context of how the packet and service layers are designed and the applications and location of content are also included in an overall network overview. The specific role of content servers in alleviating core network traffic loads is highlighted. The status of the relevant transmission technologies in the access, metro and core is given, showing that these technologies, with adequate research, should be sufficient to provide bandwidth for consumers in the next 10-20 years.

Nonbinary Channel Coded Physical Layer Network Coding Over Modulo-Sum Algebraic Ring Structures

In this letter, we extend the conventional (finite-field based) channel-coded wireless physical layer network coding (WNC) into a more general setup based on the algebraic rings. The key idea is to enable a joint WNC-channel coding over an appropriate algebraic structure, which then in turn allows to employ some advanced WNC functions (e.g., modulo-sum with superior performance) jointly with nonbinary channel coding at the relaying nodes in the network. We justify the suitability of ring structures for coded WNC systems, and show that a particular solution with modulo-sum WNC function can significantly outperform the conventional (finite-field based) implementation. In addition, we compare the achieved performance of the proposed (coded WNC) processing with several practical and information-theoretical benchmarks to substantiate its performance gains.

Source and physical‐layer network coding for correlated two‐way relaying

In this paper, we study a half-duplex two-way relay channel (TWRC) with correlated sources exchanging bidirectional information. In the case, when both sources have the knowledge of correlation statistics, a source compression with physical-layer network coding (SCPNC) scheme is proposed to perform the distributed compression at each source node. When only the relay has the knowledge of correlation statistics, we propose a relay compression with physical-layer network coding (RCPNC) scheme to compress the bidirectional messages at the relay. The closed-form block error rate (BLER) expressions of both schemes are derived and verified through simulations. It is shown that the proposed schemes achieve considerable improvements in both error performance and throughput compared with the conventional non-compression scheme in correlated two-way relay networks (CTWRNs).

Physical‐layer network coding with multi‐antenna transceivers in interference limited environments

In this study, the authors first analyse system performance of beamforming amplify-and-forward two-way relay networks with physical-layer network coding under the impact of co-channel interference from multiple surrounding terminals and then propose the associated power allocation strategies. The performance of the two-way communications in terms of outage probability, symbol error rate (SER), and total ergodic channel capacity of the system is quantified. Asymptotic performance analysis for sufficiently high signal-to-noise ratio is also provided to obtain further valuable insights into system designs. Based on the analysis, power allocation strategies to minimise the asymptotic outage probability and SER as well as to maximise the ergodic channel capacity under the total power constraint are developed. The numerical results show that the proposed power allocation approaches outperform equal power allocation given the same total power budget and other system parameters.

Cooperative mechanism for PNC two-hop slotted ALOHA network

In this paper, we propose a buffer-information-aided (BIA) request to send (RTS)/clear to send (CTS) mechanism to improve the throughput performance of physical-layer network coding (PNC) in a slotted-ALOHA two-hop relay network. In the proposed BIA mechanism, the relay node collects the information of the destination address of the first several packets at the sending buffers of the client nodes, and determines the transmission client nodes at the next time slot. Therefore, the collision probability can be decreased and the proportion of PNC transmissions among the total transmissions is improved. Simulation results show that compared with the traditional RTS/CTS mechanism, the proposed cooperative mechanism can greatly improve the PNC network throughput.

Flexible and re-configurable optical three-input XOR logic gate of phase-modulated signals with multicast functionality for potential application in optical physical-layer network coding.

Optical logic gate, especially exclusive-or (XOR) gate, plays important role in accomplishing photonic computing and various network functionalities in future optical networks. On the other hand, optical multicast is another indispensable functionality to efficiently deliver information in optical networks. In this paper, for the first time, we propose and experimentally demonstrate a flexible optical three-input XOR gate scheme for multiple input phase-modulated signals with a 1-to-2 multicast functionality for each XOR operation using four-wave mixing (FWM) effect in single piece of highly-nonlinear fiber (HNLF). Through FWM in HNLF, all of the possible XOR operations among input signals could be simultaneously realized by sharing a single piece of HNLF. By selecting the obtained XOR components using a followed wavelength selective component, the number of XOR gates and the participant light in XOR operations could be flexibly configured. The re-configurability of the proposed XOR gate and the function integration of the optical logic gate and multicast in single device offer the flexibility in network design and improve the network efficiency. We experimentally demonstrate flexible 3-input XOR gate for four 10-Gbaud binary phase-shift keying signals with a multicast scale of 2. Error-free operations for the obtained XOR results are achieved. Potential application of the integrated XOR and multicast function in network coding is also discussed.

Fixed-gain AF PLNC over cascaded Nakagami-m fading channels for vehicular communications

We propose an inter-vehicle communication system and investigate its performance. Due to employing physical layer network coding (PLNC) with fixed gain amplify-and-forward (AF) scheme on relay, our system is more practical in terms of implementation complexity than the ones using variable gain. Analytic results are derived under the cascaded Nakagami-m fading channel model assumption covering cascaded Rayleigh, double Nakagami-m, Generalized-K and conventional cellular channel models as well. As we investigate performance of the system, first we derive exact cumulative density functions (CDF) of the end-to-end signal-to-noise-ratio. Using this CDF, we obtain the exact closed-form outage probability. Then we derive the exact closed-form symbol error rate (SER) expression for various modulation types. Finally, we verify the analytic results by comparing with computer simulations. Our results show that the outage probability and SER performance decrease when the cascading degrees of the channels increase.

A coalitional graph game framework for network coding-aided D2D communication

The application of device-to-device (D2D) communication in cellular networks can significantly improve the efficiency of spectrum utilization, which benefits local area cooperative services. On the other hand, network coding can realize more efficient cooperation among cellular users. Thus, it is natural to combine network coding with D2D communication in order to further increase the system sum rate. In this paper, a coalitional graph game framework is proposed to jointly accomplish resource allocation and relay selection, two challenging problems in network coding-aided D2D communication networks. It is shown that this framework can model D2D communication combined with various network coding schemes. Based on the coalitional graph game framework, a distributed algorithm with low computational complexity is proposed to solve the formulated problem for physical layer network coding-aided D2D communication. Finally, the performance of the proposed transmission scheme is evaluated through extensive simulations.