Physical Layer Network Research Articles

Least Squares Channel Estimation for Analog Network Coding Over Frequency Selective Fading Channels

Two-way relay networks can have their throughputs improved by adopting physical-layer network coding. To work properly, these systems need to know the channel impulse responses involved. Most previous works on channel estimation for physical-layer network coding systems consider flat fading or frequency-selective fading together with orthogonal frequency division multiplexing modulation. However, for single carrier systems under frequency-selective channels these estimators can not be applied directly. Therefore, a least squares channel estimator for these scenarios is proposed here. Simulations are performed to evaluate the performance of this channel estimator and the results show the effectiveness of the proposed technique.

A Communication Network Robustness Estimation Method Based on an Improved ELM

The communication network robustness is becoming increasingly important because of its wider applications on higher reliability requirement systems, such as UAV, CBTC, etc. But communication network robustness estimation is becoming more difficult due to related various factors such as attacks, EMI, errors, mobility, fault-tolerant strategies, etc. With the purpose of estimating the robustness of communication network, most existing network robustness estimation researches mainly focus on failures in network physical layer and take complex network methods to estimate network robustness; recently, others have used Monte Carlo method to consider nodes mobility and several researches have summed up whole factors with weights to estimate the network robustness by AHP methods. Though these methods have largely been exploited to estimate the robustness of networks approximately, there is still no proper method to estimate the robustness of real communication network with consideration of nonlinear relationships among multiple factors. In this study, firstly, a double-layer network robustness model has been constructed; secondly, in order to take more robustness related factors into consideration, we have constructed a simulation platform based on NS-3; finally, wehave proposed a novel robustness estimation method based on improved ELM (extreme learning machine), which is powerful to estimate the nonlinear relationships. The novel robustness estimation method has been formed by four steps: simulation and data collection, improved EML, model training, and estimation. In the end, contrast analyzes have been made to illustrate three facts: (1) fault-tolerant strategies can improve the robustness obviously;(2) our double-layer network robustness model is better than the previous model; (3) the improved ELM has a better cross-validation accuracy which is above 95%.

Experimental Validation of Time-Synchronized Operations for Software-Defined Elastic Optical Networks

Elastic optical networking (EON) has been proposed as a solution to efficiently exploit the spectrum resources in the physical layer of optical networks. Moreover, by centralizing legacy generalized multiprotocol label switching control-plane functionalities and providing a global network view, software-defined networking (SDN) enables advanced network programmability valuable to control and configure the technological breakthroughs of EON. In this paper, we review our recent proposal [Optical Fiber Communication Conf., 2017, paper W4J.6] of time-synchronized operations (TSO) to minimize disruption time during lightpath reassignment in EON. TSO has been recently standardized in SDN, and here we discuss its implementation using NETCONF and OpenFlow in optical networks. Subsequently, we update our analytical model considering an experimental characterization of the WSS operation time. Then, we extend our previous work with an experimental validation of TSO for lightpath reassignment in a five-node metropolitan optical network test-bed. Results validate the convenience of our TSO-based approach against a traditional asynchronous technique given its reduction of disruption time, while both techniques maintain a similar network performance in terms of optical signal-to-noise ratio and optical power budget.

Channel and delay estimation for asynchronous physical layer network coding

Relative Delay estimation is an important issue for asynchronous physical layer network coding (PLNC). In this paper, an fast Fourier transform (FFT) based channel and delay estimation method for asynchronous PLNC is proposed. The extended properties of discrete Fourier transform are exploited to design the pilots, which makes the pilots from different nodes orthogonal in frequency domain. Then channels between relay and different terminal nodes are estimated separately in frequency domain. Finally, after inverse FFT, the relative delay is extracted from the estimated time domain channel information. Compared with the existing approaches, the proposed method can achieve almost the same performance, but with significant improvement in computational complexity.

Design and Analysis of Hierarchical Physical Layer Network Coding

This paper proposes a new relaying technique, hierarchical physical layer network coding (H-PNC), aimed at increasing the spectrum and energy efficiency in multi-hop wireless networks by supporting two bi-directional traffic flows simultaneously. H-PNC is applicable to the scenario in which two source nodes exchange data with the help of a relay, and the relay also needs to exchange data with one source node in an asymmetric two-way relay channel network, where the channel conditions of two source-relay links are asymmetric. H-PNC arranges transmissions in two stages, the multiple access (MA) stage and the broadcast (BC) stage. In the MA stage, the source node with the better channel to the relay can superimpose the symbol targeting to the relay on the symbol targeting to the other source node. In the BC stage, the relay can superimpose the symbol targeting to the source node with the better channel on the broadcast symbol. Thus, one more bidirectional information exchange is achieved beyond traditional PNC. Designs and optimizations of three H-PNC schemes are presented, and the error performance of QPSK–BPSK H-PNC is derived. Extensive simulations have been conducted to evaluate the system performance. Both theoretical analysis and simulation results demonstrated that H-PNC achieves a substantial performance gain.

Bi-Directional Multi-Hop Wireless Pipeline Using Physical-Layer Network Coding

In this paper, the design of multi-hop physical layer network coding (PNC) is investigated. In the existing multi-hop PNC designs, the effects of error propagation and mutual-interference are not well addressed. Error propagation refers to that the estimation error at any node may propagate to the neighboring nodes, which may result in serious end-to-end bit errors. The impact of the mutual-interference from other transmitting nodes to a receiver determines the upper bound SINR of two neighboring nodes given end-to-end SNR. By carefully addressing these issues, we propose two multi-hop PNC designs, the direct multi-hop PNC (D-MPNC) and the stored multi-hop PNC (S-MPNC), where both designs achieve the throughput upper bound of one symbol per symbol duration, which is the same as that of the traditional PNC with a single relay. There is a tradeoff between the applications of D-MPNC and S-MPNC, which targets for simple-implementation and optimal end-to-end bit error rate (BER), respectively. We provide the detailed designs of D-MPNC and S-MPNC and obtain the end-to-end BER bounds theoretically. Extensive simulation results demonstrate the performance gain of the proposed multi-hop PNC compared with the traditional PNC in terms of end-to-end BER and end-to-end throughout.

Joint detection and decoding for physical‐layer network coding in power line channel

In this study, a joint detection and generalised sum-product decoding algorithm for a two-way relay physical-layer network coding (PLNC) system in power line communication is proposed. Distinct from the existing works which mainly focused on separate or iterative PLNC detection and decoding, the proposed algorithm is a belief propagation-based joint PLNC detection and decoding algorithm, which integrates the PLNC, symbol detector and channel decoder together. The proposed algorithm is described by a joint factor graph, in which the detection nodes are connected to the factor graph of a generalised sum-product decoder. The PLNC demodulation is redesigned to make sure that the degree of each detection node in the joint factor graph is more than one. A low computational complexity solution of the proposed joint decoding algorithm based on the majority-logic algorithm is also proposed. The simulation results reveal the validity of the proposed algorithms.

Successful transmission probability of cognitive device-to-device communications underlaying cellular networks in the presence of hardware impairments

This paper studies the cognitive wireless powered device-to-device (D2D) communication underlaying a cellular network, where there are two types of communications, including peer-to-peer (P2P) communication and a multi-hop D2D communication, which are assumed to be affected by hardware impairments (HWIs). We investigate the one-hop P2P communication and the multi-hop D2D communication, where the relay node helps other two user equipments (UEs) exchange information with a two-time-slot physical-layer network coding scheme. To examine the system performance, we derive closed-form expressions for the average energy efficiency (EE) and spectral efficiency (SE). Besides that, we also obtain the successful transmission probability (STP) for the two considered communications, and the optimal values of time switching (TS) and power splitting (PS) ratios are achieved with the help of a genetic algorithm (GA)-based optimization algorithm in our proposed energy harvesting (EH) protocol so-called Hybrid TS-PS (HTPS) relaying protocol. Comparisons between amplify-and-forward (AF) and decode-and-forward (DF) transmission schemes are provided. The simulation results give evidence that the STP is significantly improved regardless of the presence of HWIs thanks to the derived parameters, i.e., average EE, SE, and the optimal TS and PS ratios.

Selective physical layer network coding in bidirectional relay channel

In this paper, the authors show that using physical layer network coding (PNC) in wireless relay systems does not always increase throughput performance, and to achieve maximum throughput, PNC should be exploited intelligently regarding to the channel conditions. In this study, they propose a selective PNC (SPNC) scheme in bidirectional relay systems for binary phase shift keying. In the proposed scheme, the relay chooses either to detect network-coded data from superimposed signal by PNC scheme or single source data by a so-called single node detection (SND) scheme, based on which offers more end-to-end throughput for current channel realisation. They analytically determine the region of channel state plane in which SPNC selects SND scheme. Also, they obtain expressions for instantaneous bit error rates (BERs) of PNC and SND protocols. Moreover, they derive upper bounds in closed-form for average BERs of PNC and SND schemes during multiple access phase at the relay. Then, they investigate SPNC with high-order modulations. Simulations and analytical results confirm that SPNC achieves considerable throughput gain over PNC in Rayleigh fading channels.

Multi-channel Tracking with Different Fading Rates in an OFDM-PLNC Network

In this paper, we address the problem of tracking the time varying/frequency selective fading channels with different fading rates in a decode-and-forward physical layer network coding (PLNC) structure. Since, in a PLNC system multi users send their information to the relay, simultaneously, the relay needs to estimate the uplink multi-channels, simultaneously, to be able to extract the network coded signal from superimposed signals. Different fading rates of channels due to different velocities of mobile users in a PLNC system make the channel tracking problem more complex. Providing an algorithm to simultaneous track multi-channels in such a system is the main object of this paper. To this end, a time domain channel tracking algorithm based on sum–product rules is derived, where, both frequency and time selectivity characteristics of the channels are taken into account. In order to stabilize against the frequency selectivity property of the channels, orthogonal frequency division multiplexing is also considered in this system. High accuracy, no phase ambiguity, and stability in fast fading conditions are some advantages of the proposed approach. The performance of the algorithm in the existence of channel coding has been also considered and compared with the non-coded channel system.

Exploiting the overhearing capabilities of transmitting nodes to increase the energy efficiency in dense networks

The 1000-fold capacity increase envisioned by dense 5G networks results also in a tremendous increase in the energy consumption of the whole network. Utilizing relays in combination with physical-layer network coding (PNC) has been proposed as an energy-efficient solution to this problem by creating several short-distance low-power transmissions and by reducing the transmission time. However, deploying relay nodes can be very costly for dense networks. On the other hand, the proximity of transmitting nodes in dense networks allows the transmitting nodes to serve as relays and retransmit the signals overheard from other transmitting nodes using PNC. This approach has been shown to increase the spectral efficiency, but the impact on energy efficiency has not been studied yet. Therefore, in this paper, we analyze two approaches that exploit the overhearing capabilities of the transmitting nodes in terms of spectral efficiency, energy efficiency, and success rate. We then provide a low-complexity power control strategy that achieves a performance close to the optimal for each approach. We show that when at least one indirect link is stronger than the direct links, exploiting the overhearing capabilities of the transmitting nodes provides the highest performance in both the transmit power-dominated and circuit power-dominated regimes.

Design and Analysis of BICM-ID for Two-Way Relay Channels With Physical-Layer Network Coding

This paper proposes a general iterative demapping- decoding approach for two-way relay (TWR) communications with physical-layer network coding, where the relay node performs decode-and-forward. The bit-interleaved coded modulation (BICM) with high-order quadrature amplitude modulation is considered. The iterative decoding, namely the BICM-ID for TWR channels, is introduced. Based on a joint trellis, BICM-ID estimates the end nodes' coded bits, leading to an estimation of the transmitted symbol pair probability, which is crucial for achieving iterative decoding gains in bit error rate (BER). The end nodes' constellation design criterion is proposed to optimize the BICM-ID performance over the TWR fading channel. The EXtrinsic Information Transfer analysis is also performed, showing the iterative system's convergence behavior and providing information on code design. Our simulation results show the proposed BICM-ID scheme can achieve significant performance gains, yielding an asymptotic tendency of approaching the cut-set BER performance limit.

Network-Coded Multiple Access with High-Order Modulations

This paper presents the first network-coded multiple access (NCMA) system prototype operated on high-order modulations up to 16-QAM. NCMA jointly exploits physical-layer network coding (PNC) and multiuser decoding (MUD) to boost throughput of multipacket reception systems. Direct generalization of the existing NCMA decoding algorithm, originally designed for BPSK, to high-order modulations, will lead to huge performance degradation. The throughput degradation is caused by the relative phase offset between received signals from different nodes. To circumvent the phase offset problem, this paper investigates an NCMA system with multiple receive antennas at the access point, referred to as MIMO-NCMA . We put forth a low-complexity symbol-level NCMA decoder that, together with MIMO, can substantially alleviate the performance degradation induced by relative phase offset. To demonstrate the feasibility and advantage of MIMO-NCMA for high-order modulations, we implemented our designs on software-defined radio. Our experimental results show that the throughput of QPSK MIMO-NCMA is double that of both BPSK NCMA and QPSK MUD at SNR = 10 dB. For higher signal-to-noise ratios at which 16-QAM can be supported, the throughput of MIMO-NCMA can be as high as 3.5 times that of BPSK NCMA. Overall, this paper provides an implementable framework for high-order modulated NCMA.

Digital all-optical Physical-layer Network Coding for 2Gbaud DQPSK signals in mm-wave radio-over-fiber networks

We propose the first digital All-Optical Physical-layer Network Coding (AOPNC) scheme for up to 2Gbaud Differential Quadrature Phase Shift Keyed – Sub-Carrier Modulated (DQPSK-SCM) signals in future Radio-over-Fiber (RoF) networks. The proposed encoding unit employs Delay Interferometers (DIs) for the DQPSK-to-OOK conversion, a first stage of Semiconductor Optical Amplifier-Mach Zehnder Interferometers (SOA-MZIs) as optical OOK-XOR and OOK-NXOR logic gates, and a second stage of SOA-MZIs as phase regenerators of the final four-level phase-formatted encoded signal. The proposed encoding operation is performed on-the-fly at the Central Office (CO) and the resulting packet is broadcasted at the end-users, where the electrical decoding takes place. The proof of concept scheme is demonstrated for 4Gb/s Non-Return-to-Zero (NRZ) DQPSK-SCM signals modulated on a 60GHz Sub-Carrier (SC). The encoding and decoding operation are investigated and evaluated, achieving error free operation for both synchronous and asynchronous data.

The Performance of Physical-Layer Network Coding in Asymmetric Rayleigh Fading Two-Way Relay Channels

Up to now, most of the researches about physical-layer network coding (PNC) are based on symmetric two-way relay channels. In this paper, we mainly study PNC in asymmetric two-way relay Rayleigh fading channels and classify the systems into five asymmetric cases. So as to describe the asymmetric cases, we introduce the asymmetric factors. We simulate the bit error rate (BER) performance of PNC in both symmetric and asymmetric cases with asymmetric factors fixed. After that we analyze the BER performance of PNC with one of the asymmetric factors is variable when two channels are relatively unreliable. Finally, we give the conditions that one kind of asymmetric scenario transforming to other kind. Also, in the matter of anti-noise performance, uplink asymmetry has worst performance and the phase asymmetry-downlink is the best case.

Joint Encoding and Grouping Multiple Node Pairs for Physical-Layer Network Coding With Low-Complexity Algorithm

How to put fifth-generation (5G) into the commercial service by 2020 is still challenging. Since physical-layer network coding (PNC) does not bring unnecessary redundancy and offers satisfactory end-to-end services, it is promising to provide network services in 5G networks. Existing works on PNC mainly focus on the case that two packets are simultaneously sent by a node pair for information exchange, and the transmitted signals are superposed and encoded at a relay. To further exploit the potential performance improvement of PNC, this study aims at encoding multiple packets through jointly processing multiple superposed signals sent by diverse node pairs. Particularly, we formulate the problem of joint-encoding method design as a binary integer programming model and propose a low-complexity solution based on the idea of filling a Latin rectangle. We find that network throughput enhancement does not accompany the increasing number of coded node pairs, and some node pairs cannot be encoded together since they fail to decode the resulting encoded packet. Therefore, the decodability-based node pair grouping scheme is further investigated. Simulation results in various scenarios show that the proposed low-complexity scheme performs neck to neck to the optimum one with high computational complexity.

Asynchronous Physical-Layer Network Coding: Symbol Misalignment Estimation and Its Effect on Decoding

In asynchronous physical-layer network coding (APNC) systems, the symbols from multiple transmitters to a common receiver may be misaligned. Knowledge of the amount of symbol misalignment, hence its estimation, is important to PNC decoding. This paper addresses the problems of symbol-misalignment estimation and optimal PNC decoding given the misalignment estimate, assuming the APNC system uses the root-raised-cosine pulse to carry signals (RRC-APNC). Our contributions are as follows. First, we put forth an optimal symbol-misalignment estimator that makes use of double baud-rate samples. Second, we devise optimal RRC-APNC decoders in the presence of non-exact symbol-misalignment estimates. In particular, we show how to whiten the colored noise in the double baud-rate samples to simplify the design of optimal decoders. Third, we investigate the decoding performance of various estimation-and-decoding schemes for RRC-APNC. Extensive simulations show that: 1) our double baud-rate estimator yields substantially more accurate symbol-misalignment estimates than the baud-rate estimator does; the mean square error gains are up to 8 dB and 2) an overall estimation-and-decoding scheme in which both estimation and decoding are based on double baud-rate samples yields much better performance than other schemes. Compared with a scheme in which both estimation and decoding are based on baud-rate samples, the double baud-rate sampling scheme yields 4.5 dB gains on symbol error rate performance in an additive white Gaussian noise channel, and 2 dB gains on packet error rate performance in a Rayleigh fading channel.

Practical Power-Balanced Non-Orthogonal Multiple Access

This paper is a theoretical-plus-experimental investigation of practical 5G strategies for power-balanced non-orthogonal multiple access (NOMA). By allowing multiple users to share the same time and frequency, NOMA can scale up the number of served users and increase spectral efficiency compared with existing OMA. Conventional NOMA schemes with successive interference cancellation (SIC) do not work well when users with comparable received powers transmit together. To allow power-balanced NOMA (more exactly, near power-balanced NOMA), this paper investigates a new NOMA architecture, named network-coded multiple access (NCMA). A distinguishing feature of NCMA is the joint use of physical-layer network coding (PNC) and multiuser decoding to boost NOMA throughputs. We first show that a simple NCMA architecture in which all users use the same modulation, referred to as rate-homogeneous NCMA , can achieve substantial throughput improvement over SIC-based NOMA under near power-balanced scenarios. Then, we put forth a new NCMA architecture, referred to as rate-diverse NCMA , in which different users may adopt different modulations commensurate with their relative SNRs. A challenge for rate-diverse NCMA is the design of a channel-coded PNC system. This paper is the first attempt to design channel-coded rate-diverse PNC. Experimental results on our software-defined radio prototype show that the throughput of rate-diverse NCMA can outperform the state-of-the-art rate-homogeneous NCMA by 80%. Overall, rate-diverse NCMA is a practical solution for near power-balanced NOMA.

Channel quantization-based physical-layer network coding for two-way relay STBC system

Wireless multiple-input multiple-output (MIMO) two-way relay systems have received increasing attention recently. In a simple two-hop relay system, physical-layer network coding (PNC) is an effective scheme for relay to map the transmitted symbols from two terminal nodes into the network coded symbol. This paper considers a two-hop two-way relay system which uses the Alamouti space-time code for signaling between all nodes and channel quantization for PNC mapping at the relay. Specifically, based on the concept of CQ-PNC we derive a PNC mapping rule for two pairs of network coded symbols. An adaptive mapping scheme based on minimum residual interference power is introduced to reduce the effect of quantization error. Finally, a relay selection method based on minimum residual interference power is proposed as a means to increase the system performance.

Design of adaptive constellations and error protection coding for wireless network coding in 5-node butterfly networks

In this paper, we focus on the design of a reliable communication scheme for generic 5-node wireless butterfly networks (WBN). Since the WBN represents a natural extension of the basic two-way relay communication channel, we follow the paradigm of Wireless Physical Layer Network Coding and we show that the design of novel adaptive constellations is desirable to exploit the promising performance of wireless network coding in WBN. We introduce a systematic constellation design algorithm, and we show that the proposed constellations are capable of outperforming conventional point-to-point modulations in WBN over the whole range of channel signal to noise ratios. To further support the applicability of the proposed constellations in a practical system, we integrate a simple binary channel coding scheme, providing a robust adaptive modulation and coding scheme for relaying in symmetric WBNs with arbitrary channel conditions.