Physical-layer Network Coding Scheme Research Articles

On Secure CDRT With NOMA and Physical-Layer Network Coding

This paper proposes a new scheme to enhance the secrecy performance of non-orthogonal multiple access (NOMA)-based coordinated direct relay transmission (CDRT) systems with an untrusted relay. The physical-layer network coding (PNC) and the NOMA schemes are combined to improve spectrum efficiency. Furthermore, inter-user interference and friendly jamming signals are utilized to suppress the eavesdropping ability of the untrusted relay without compromising the acceptance quality of legitimate users. Specifically, the far user in the first slot and the near user in the second slot act as jammers that generate jamming signals to ensure secure transmissions of confidential messages. We investigate the secrecy performance of the NOMA-based CDRT systems with the PNC scheme and derive the closed-form expression for the ergodic secrecy sum rate. The asymptotic analysis at a high signal-to-noise ratio is performed to obtain more insights. Finally, simulation results are presented to demonstrate the proposed scheme’s effectiveness and the theoretical analysis’s correctness.

DNN-Based Physical-Layer Network Coding for Visible Light Communications

The key difference between visible light communication (VLC) and radio frequency (RF) communication is the former’s line-of-sight (LOS) transmission nature, and hence a relay node has to be adopted for VLC to extend its coverage. Physical-layer network coding (PNC) has the advantage of doubling the throughput of a two-way relay network (TWRN), where two end nodes exchange information via the help of a relay, compared with the conventional store-and-forward routing strategy. Although PNC has been studied for VLC in the literature, the state-of-the-art schemes are highly inefficient, requiring tight phase synchronization between the two end nodes, and hence difficult to realize. This paper proposes the application of a deep neural network (DNN) to a PNC VLC system, named DP-VLC, that enables misaligned phases and can deal with the light channel gains and noises in a satisfactory manner without introducing additional computation complexities. We implement DP-VLC using the universal software radio peripheral (USRP) software radio platform and a self-developed VLC optical front-end using commercial off-the-shelf (COTS) light-emitting diodes (LEDs) and photo-diodes (PDs). We find that irregular constellations generated by DP-PNC can be transmitted and recovered in a 1.5 m VLC link effectively. Experimental results show that our DP-PNC prototype performs better than conventional PNC VLC system when the signal-interference-to-noise ratio (SINR) of received optical signals is larger than 13.63 dB and can achieve a throughput of up to 77.38 Mbps in a 20 MHz channel under PNC scheme when the SINR is 22.86 dB. More importantly, we find that DP-VLC performs even better than fixed-constellation PNC system in the saturated SINR regime (e.g., 20–25 dB) where non-linear effects may happen compared with moderate SINR regimes (e.g., 10–20 dB), showing its adaptability to unpredictable impairments in optical links. Our first attempt at realizing DNN-based optical PNC in a TWRN has paved the way for future PNC-enhanced VLC systems.

IRS-Assisted Physical Layer Network Coding Over Two-Way Relay Fading Channels

This article investigates the performance of intelligent reflective surfaces (IRS)-aided physical layer network coding (PNC) in two-way relaying channels (TWRC). Specifically, IRS is used to eliminate carrier phase offset (CPO) at the relay node. To this end, the IRS reflectors’ phase shifts are optimized to align the received signals from two source nodes at the relay. This facilitates using a simple mapping function at the relay to map the superimposed signal to a network-coded signal. Two scenarios are considered, the first of which assumes that each source node is served by a separate IRS panel, while the second scenario considers the more challenging case where only one IRS panel is available for the two source nodes. In the latter case, the IRS panel is seen by both source nodes and its phase shifts are optimized to mitigate the CPO problem while maximizing the received signal amplitude at the relay. This optimization problem is formulated and solved over the complex circle manifold. Finally, we extend the IRS-assisted PNC system to include channel coding and higher modulation orders, for which a repeat accumulate (RA) channel-coded IRS-aided PNC scheme is proposed for general quadrature amplitude modulation (QAM) signals. A belief propagation (BP) based algorithm is designed to decode the network-coded sequences over a q-Ring using modular arithmetic. Our simulation results validate the theoretical error expressions derived for the two-IRS scenario as well as the efficacy of the proposed manifold optimization approach for the one-IRS scenario. The results also confirm the efficacy of the designed channel-coded IRS-aided PNC using high QAM modulation orders.

Performance Analysis of Physical Layer Network Coding in Massive MIMO Systems With M-QAM Modulations

In this paper, we develop a practical approach for deploying Physical Layer Network Coding (PNC) in multi-user M-Ary Quadrature Amplitude Modulation (M-QAM) Massive Multiple-Input Multiple-Output (MIMO) systems. We formulate a PNC mapping scheme as a function of clusters of estimated summation and difference (SD) of the transmitted symbols from user pairs. Utilizing existing linear detection schemes, such as Zero Forcing (ZF) and Minimum Mean Square Error (MMSE), a cluster of SD symbols are detected using an SD linearly transformed channel matrix. Furthermore, utilizing Maximum a Posteriori (MAP) soft decoding, the SD symbols are mapped to the PNC symbols, leveraging on the PNC symbol that maximizes the likelihood function. For each variant of M-QAM, we derive and simplify a specialization of the generalized PNC mapping function. The error performance results, through simulation, reveal that the proposed PNC scheme achieves twice the spectral efficiency in Massive MIMO, without changing the latter's underlying framework and without any degradation in the bit-error-rate (BER). In fact, our investigation has proved that the BER of the proposed Massive MIMO and PNC is slightly better than that of the conventional Massive MIMO. The feasibility of deploying our proposed PNC scheme in Massive MIMO systems paves way for NC applications to be realized in cellular systems.

High-Order Modulation Based on Deep Neural Network for Physical-Layer Network Coding

Physical-layer network coding (PNC) is an effective technique for enhancing wireless network throughput. Recently, it has been demonstrated that convolutional autoencoders effectively works in point-to-point communication systems, but their application to wireless relay networks is scarcely explored. In this letter, we propose a convolutional autoencoder for PNC in a two-way relay channel. The constellation mapping and demapping of symbols at each node are determined adaptively through a deep learning technique, such that the bit error rate performance is improved for high-order modulation. Simulation results verify the advantages of the proposed scheme over the conventional PNC scheme for various modulation types.

A Framework for Evaluating Physical-Layer Network Coding Gains in Multi-Hop Wireless Networks

Physical-Layer Network Coding (PLNC) was first introduced as a solution to increase the throughput of a two-way relay channel communication. Unlike most wireless communication techniques which try to avoid collisions, PLNC allows two simultaneous transmissions to a common receiver. In basic topologies, this technique has been shown to significantly enhance the throughput performance compared to classical interference-free communications. However, quantifying the impact of PLNC in large multi-hop networks remains an open question. In this work, we introduce the first theoretical framework that, given a particular network topology and traffic matrix, can evaluate the optimal network throughput subject to a fairness constraint relative to the initial demand of each flow, when PLNC is adopted. Based on linear programming, our solution ensures to respect the particularities imposed by PLNC. We use this framework to evaluate three state-of-the-art PLNC schemes across a variety of topologies and traffic matrices. Our numerical analysis reveals that while in simple toy topologies PLNC can significantly increase the network throughput, in large topologies the verdict is mixed. For certain topologies and traffic patterns adopting PLNC can double the throughput while in others, depending on the scheme used for implementing PLNC, the gain can be as high as 60 percent or as low as 0 percent when compared to traditional interference-free transmissions.

Application of block Markov superposition transmission to physical-layer network coding for maritime communications

Physical-layer network coding (PNC) is a promising technology that can improve the throughput of two-way relay networks. This paper focus on designing physical-layer network coding schemes for maritime communication systems. Unlike terrestrial environment, radio propagation at sea is seriously affected by wave movement, ship shaking and other complicated environmental factors. In order to improve the transmission reliability, we apply block Markov superposition transmission (BMST) to PNC schemes. Simulation results show that the proposed schemes outperform the existing convolutional coded PNC schemes over Rician channels, suggesting that the proposed schemes are more appropriate for marine applications.

When analog meets digital: Source-Encoded Physical-Layer Network Coding

We revisit Physical-Layer Network Coding (PLNC) in order to integrate this interference management technique in multi-hop wireless networks. We identify its strong coupling to the Two-Way Relay Channel (TWRC) as a main reason preventing it from becoming a staple in large networks. We introduce SE-PLNC (Source-Encoded PLNC), a new PLNC scheme that is traffic pattern independent and involves coordination only among one-hop neighbors, making PLNC significantly more practical to adopt in multi-hop wireless networks. To accomplish this, SE-PLNC introduces three innovations: it combines bit-level with physical-level network coding, it shifts most of the coding burden from the relay to the source of the PLNC scheme, and it leverages multi-path relaying opportunities available to a particular traffic flow. We evaluate SE-PLNC using proof-of-concept implementation on a Universal Software Radio Peripherals (USRP) testbed, theoretical analysis, and simulations. Testbed experiments show its real-life feasibility over a wide range of different channel conditions. The theoretical analysis highlight the scalability of SE-PLNC and its efficiency in large ad-hoc networks while large-scale simulations show that SE-PLNC improves network throughput by over 30% compared to state-of-the-art PLNC schemes.

An Adaptive Optimal Mapping Selection Algorithm for PNC Using Variable QAM Modulation

Fifth generation wireless networks will need to serve much higher user densities than existing 4G networks, and will therefore require an enhanced radio access network (RAN) infrastructure. Physical layer network coding (PNC) has been shown to enable such high densities with much lower backhaul load than approaches, such as Cloud-RAN and coordinated multipoint. In this letter, we present an engineering applicable PNC scheme which allows different cooperating users to use different modulation schemes, according to the relative strength of their channels to a given access point. This is in contrast with compute-and-forward and previous PNC schemes which are designed for the two-way relay channel. A two-stage search algorithm to identify the optimum PNC mappings for given channel state information and modulation is proposed in this letter. Numerical results show that the proposed scheme achieves low bit error rate with reduced backhaul load.

Implementation of uplink network‐coded modulation for two‐hop networks

AbstractWith the fast growing number of wireless devices and demand of user data, the backhaul load becomes a bottleneck in wireless networks. Physical‐layer network coding (PNC) allows access points to relay‐compressed, network‐coded user data, therefore reducing the backhaul traffic. In this paper, an implementation of uplink network‐coded modulation (NetCoM) with PNC is presented. A five‐node prototype NetCoM system is established using Universal Software Radio Peripherals, and a practical PNC scheme designed for binary systems is utilized. An orthogonal frequency‐division multiplexing waveform implementation and the practical challenges (eg, device synchronization and clock drift) of applying orthogonal frequency‐division multiplexing to NetCoM are discussed. To the best of our knowledge, this is the first PNC implementation in an uplink scenario in radio access networks, and our prototype provides an industrially applicable implementation of the proposed NetCoM with PNC approach.

On Medium Chemical Reaction in Diffusion-Based Molecular Communication: A Two-Way Relaying Example

Chemical reactions are a prominent feature of molecular communication systems, with no direct parallels in wireless communications. While chemical reactions may be used inside the transmitter nodes, receiver nodes, or the communication medium, we focus on its utility in the medium in this paper. Such chemical reactions can be used to perform computation over the medium as molecules diffuse and react with each other (physical-layer computation). We propose the use of chemical reactions for the following purposes: 1) to reduce signal-dependent observation noise of receivers by reducing the signal density; 2) to realize molecular physical-layer network coding (PNC) by performing the natural XOR operation inside the medium; and 3) to reduce the inter-symbol interference (ISI) of other transmitters by canceling out the remaining molecules from previous transmissions. To make the ideas formal, we consider an explicit two-way relaying example with a transparent receiver (which has a signal-dependent noise). The proposed ideas are used to define a modulation scheme (which we call the PNC scheme). We compare the PNC with a previously proposed scheme for this problem, where the XOR operation is performed at the relay node (using a molecular logic gate). We call the latter, the straightforward network coding (SNC). It is observed that in addition to the simplicity of the proposed PNC scheme, it outperforms the SNC scheme especially when we consider ISI.

Construction of Rate-Compatible Physical Layer Network Coding in Two-Way Relay Systems

The existing physical-layer network coding (PNC) only considers channel codes of a constant coding rate. In this paper, we propose a rate-compatible (RC) channel coding scheme for PNC in two-way relay systems. Our contributions are as follows. First, we propose a two-edge-type-low-density-paritycheck-aided PNC (JTNC) scheme due to its simple implementation and excellent performance. Second, we design an RC scheme, i.e., choice of locations and symbols, for the proposed JTNC in the relay. The simulation results show that the proposed RC-JTNC has good error correction performance and achieves large gains over the existing puncturing algorithms aided PNC over a wide range of rates.

Optimal Hybrid Network Coding Scheme Over Two-Way Relaying

The paper introduces a novel hybrid network coding scheme in a three-node two-way relaying network in a wireless fading environment. The proposed hybrid scheme is featured by an opportunistic mechanism which employs one of the two network coding schemes, namely physical-layer network coding (PNC) and digital network coding (DNC), according to the instant rate requirement and dynamic channel condition. We first develop a close-form expression for the power consumption of the proposed scheme, which is further used to formulate a series of optimization problems under both symmetric and asymmetric traffic scenarios. Solving the optimization problems yields important system parameters that determine the optimal power allocations, time split between uplink and downlink transmissions as well as between PNC and DNC, respectively. We will discuss the possible overhead in the real implementation of the proposed scheme. Extensive numerical experiments are conducted to compare the performance of the proposed hybrid scheme with the conventional PNC and DNC scheme, respectively.

Buffer-Aided Physical-Layer Network Coding With Optimal Linear Code Designs for Cooperative Networks

In this paper, we propose buffer-aided physical-layer network coding (PLNC) techniques for improving data transmission over cooperative networks. In particular, we develop buffer-aided PLNC schemes and relay pair selection algorithms for direct-sequence code-division multiple access (DS-CDMA) systems. We devise PLNC techniques based on optimal linear network coding matrices according to the maximum likelihood and minimum mean-square error design criteria in order to generate the network coded symbols that are sent to the destination. In the proposed buffer-aided PLNC schemes, relay pair selection algorithms are developed to obtain the relay pair and the packets in the buffer entries with the best performance and the associated link combinations are used for the data transmission. An analysis of the computational complexity of the proposed techniques along with their sum-rate analysis is carried out. Simulation results show that the proposed techniques significantly outperform previously reported approaches.

Design of Channel Coded Heterogeneous Modulation Physical Layer Network Coding

In a two-way relay channel network (TWRC), the integration of channel coding into symmetric physical layer network coding (PNC) has been well studied, where both sources use exactly the same channel coding and modulation schemes and the relay decodes and reencodes the codewords obtained from the superimposed signals. How to integrate the channel coding into heterogeneous modulation PNC (HePNC), where the sources apply different modulations, is an open issue. In this paper, we propose a channel coded HePNC (CoHePNC) scheme under asymmetric TWRC. For repeat-accumulate (RA) codes applied at the sources, a full-state sum-product decoding algorithm is proposed which enables the relay to decode the superimposed signals from the sources to the raw decoding results firstly, and then re-encode and obtain the network-coded codewords by mapping the raw decoding results according to the proposed bit-level mapping functions. We further optimized the bit-level mapping functions according to the two source-relay channel conditions. Extensive simulation results demonstrated that the proposed CoHePNC outperforms the existing channel coded PNC schemes in terms of the relay decoding error rate and the end-to-end bit error rate under asymmetric TWRC scenarios.

DNF-SC-PNC: a new physical-layer network coding scheme for two-way relay channels with asymmetric data length

To improve the bit error rate (BER) performance of physical-layer network coding (PNC) in data length asymmetric two-way relay channels (TWRC), a new PNC scheme named combined denoise-and-forward and superposition coded physical-layer network coding (DNF-SC-PNC) is proposed, and the decoding algorithm of the scheme is improved. In the new scheme, the mixed information is denoised and superposed at the relay node, which will be broadcasted to the destination nodes. The destination nodes can use successive interference cancellation (SIC) or likelihood rate (LLR) algorithm for decoding. Theoretical analysis and simulation results show that DNF-SC-PNC can provide better BER performance and better throughput rate performance when the data length is asymmetric. Furthermore, we also proved that LLR algorithm can provide better performance than SIC algorithm in data length asymmetric TWRC.

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.

Joint LDPC and physical‐layer network coding with incremental phase pre‐rotation for two‐way relaying

To obtain a robust physical-layer network coding (PLNC) for two-way relaying system, the authors propose a low-density parity-check (LDPC) coded PLNC scheme with phase pre-rotation to combat the carrier phase asynchronous in the multiple access stage. At two users, each modulation symbol is rotated according to a predefined incremental phase without the knowledge of carrier phase difference. Decoding simulation and numerical analysis are carried out to evaluate this joint LDPC and PLNC scheme with phase pre-rotation. Simulation results show that this strategy can achieve a significant improvement and a good trade-off between performance and complexity. Protograph-based extrinsic information transfer analysis demonstrates the proposed scheme is robust, which is consistent with the simulation results.

Physical-Layer Network Coding Scheme over Asymmetric Rayleigh Fading Two-Way Relay Channels

To improve the bit error rate (BER) performance of physical-layer network coding (PNC) in asymmetric two-way relay channels (TWRC), in this paper, we study a new PNC scheme named bi-quadrature physical-layer network coding (BQ-PNC) for Rayleigh flat fading TWRC. In BQ-PNC scheme, the two users employ quadrature carriers and the relay node use quadrature combining rather than XOR which is very common in other PNC schemes. We give the BER analysis of BQ-PNC and simulate the performance. Theoretical and simulation results show that the proposed scheme can significantly enhanced the BER performance, either in symmetric or asymmetric cases. Especially for uplink asymmetric TWRC, BQ-PNC can provide more than 4 dB gain compared with PNC scheme.

On robustness of physical layer network coding to pollution attack

Link layer network coding (LLNC) promises to provide high throughput in relay networks through combining packets at the relays and trading communication for computation. The emerging area of physical layer network coding (PLNC) exploits the electromagnetic nature of signals and eliminates the need for addition at the packet level, while making signal design and coding schemes adaptable to the channel conditions. Although network coding has been extensively studied recently, physical layer network coding has not received the attention it deserves. Several recent works introduced the pollution attack at the network layer; however, the network performance at the physical layer with pollution attacks has not been evaluated before. The main challenge with the pollution attack involves propagation of the corrupted packets in an epidemic manner, which degrades performance of the network. As PLNC schemes boost up the network throughput, a thorough study evaluating this superiority to the LLNC scheme in presence of an intruder is necessary. The robustness of both schemes towards an attack have been studied in this article.