Physical Layer Network Research Articles

Joint Channel Estimation and Decoding for Trellis-Coded MIMO Two-Way Relay Networks

We present a method for joint decode-and-forward physical layer network coding in two-way relay networks. The two source nodes send their packets simultaneously over time-varying channels to a relay node, then the relay broadcasts the received superimposed packets to the source nodes using network coding. The nodes use trellis coding for the sake of error correction and multi-antenna equipments to combat multipath fading. A challenging multiple access problem occurs at the relay node, which performs joint channel estimation and decoding for the individual source packets. We design message passing algorithms based on factor-graphs to solve this problem. The relay has two separate modules that perform channel estimation and decoding for the packets received from each source node. The interference generated by the other source node is taken into account by exchanging messages between the two modules.

Joint LDPC and Physical-Layer Network Coding for Asynchronous Bi-Directional Relaying

For practical bi-directional relaying, symbols transmitted by two sources cannot arrive at the relay with perfect symbol and frame alignments and the asynchronous multiple-access channel (MAC) should be seriously considered. In this paper, we consider the Low-Density Parity-Check (LDPC)-coded BPSK signalling over the general asynchronous MAC with both frame and symbol misalignments. For the symbol-asynchronous MAC, we present a formal log-domain generalized sum-product-algorithm (Log-G-SPA) for efficient decoding. When the frame-asynchronism is encountered at the relay, we propose an original approach by employing the cyclic LDPC codes and the simple cyclic-redundancy-check (CRC) coding technique. Simulation results demonstrate the effectiveness of the proposed approach.

Synchronous Physical-Layer Network Coding: A Feasibility Study

Recently, physical-layer network coding (PNC) attracts much attention due to its ability to improve throughput in relay-aided communications. However, the implementation of PNC is still a work in progress, and synchronization is a significant and difficult issue. This paper investigates the feasibility of synchronous PNC with M-ary quadrature amplitude modulation (M-QAM). We first propose a synchronization scheme for PNC. Then, we analyze the synchronization errors and overhead of potential synchronization techniques, which includes phase-locked loop (PLL) and maximum likelihood estimation (MLE) based synchronization schemes. Their effects on the average symbol error rate and the goodput are subsequently discussed. Based on the analysis, we perform numerical evaluations and reveal that synchronous PNC can outperform conventional network coding (CNC) even when taking synchronization errors and overhead into account. The theoretical throughput gain of PNC over CNC can be approached when using the MLE based synchronization method with optimized training sequence length. The results in this paper provide some insights and benchmarks for the implementation of synchronous PNC.

ENHANCING TRAFFIC CAPACITY OF TWO-LAYER COMPLEX NETWORKS

As two-layer or multi-layer network model can more accurately reveal many real structures of complex systems such as peer-to-peer (P2P) networks on IP networks, to better understand the traffic dynamics and improve the network traffic capacity, we propose to efficiently construct the structure of upper logical layer network which can be possibly implemented. From the beginning, we assume that the logical layer network has the same structure as the lower physical layer network, and then we use link-removal strategy in which a fraction of links with maximal product (ki* kj) are removed from the logical layer, where ki and kj are the degrees of node i and node j, respectively. Traffic load is strongly redistributed from center nodes to noncenter nodes. The traffic capacity of whole complex system is enhanced several times at the expense of a little average path lengthening. In two-layer network model, the physical layer network structure is unchanged and the shortest path routing strategy is used. The structure of upper layer network can been constructed freely under our own methods. This mechanism can be employed in many real complex systems to improve the network traffic capacity.

Rethinking the physical layer of data center networks of the next decade

Not only do big data applications impose heavy bandwidth demands, they also have diverse communication patterns denoted as *-cast) that mix together unicast, multicast, incast, and all-to-all-cast. Effectively supporting such traffic demands remains an open problem in data center networking. We propose an unconventional approach that leverages physical layer photonic technologies to build custom communication devices for accelerating each *-cast pattern, and integrates such devices into an application-driven, dynamically configurable photonics accelerated data center network. We present preliminary results from a multicast case study to highlight the potential benefits of this approach.

Physical layer network coding with channel and delay estimation

Decode-and-forward physical layer network coding (PLNC) is one of the promising high-performance techniques for wireless relay networks. This study presents a channel and delay estimation algorithm along with a detection scheme for time-domain two-way relay network in Rayleigh block fading channels. Preambles are attached for frame-based synchronisation and channel estimation. Moreover, to achieve low-complexity estimation, the preambles are designed in Alamouti code structure. The Cramer-Rao bound (CRB) of the channel estimation is given, and simulations show that the mean-square error of channel estimation of the proposed scheme could reach the CRB. Compared with the traditional approach, the newly proposed combined scheme can achieve almost the same performance, but with significant improvement in computational complexity. At last, bit-error-rate performances analysis is given. Simulation validates the authors' analysis, and shows the upper bound and lower bound are tight.

Achieving Full Diversity in Multi-Antenna Two-Way Relay Networks via Symbol-Based Physical-Layer Network Coding

This paper considers physical-layer network coding (PNC) with M-ary phase-shift keying (MPSK) modulation in two-way relay channel (TWRC). A low complexity detection technique, termed symbol-based PNC (SPNC), is proposed for the relay. In particular, attributing to the outer product operation imposed on the superposed MPSK signals at the relay, SPNC obtains the network-coded symbol (NCS) straightforwardly without having to detect individual symbols separately. Unlike the optimal multi-user detector (MUD) which searches over the combinations of all users' modulation constellations, SPNC searches over only one modulation constellation, thus simplifies the NCS detection. Despite the reduced complexity, SPNC achieves full diversity in multi-antenna relay as the optimal MUD does. Specifically, antenna selection based SPNC (AS-SPNC) scheme and signal combining based SPNC (SC-SPNC) scheme are proposed. Our analysis of these two schemes not only confirms their full diversity performance, but also implies when SPNC is applied in multi-antenna relay, TWRC can be viewed as an effective single-input multiple-output (SIMO) system, in which AS-PNC and SC-PNC are equivalent to the general AS scheme and the maximal-ratio combining (MRC) scheme. Moreover, an asymptotic analysis of symbol error rate (SER) is provided for SC-PNC considering the case that the number of relay antennas is sufficiently large.

Physical Layer Security Solutions for High Speed Data Networks

To improve the confidentiality and availability of the network, it is important to provide multilevel security in the physical layer of optical networks. Based on the orthogonal characterization of OFDM, we introduce a new electronic encryption method for the encryption of the subcarriers symbols after data mapping. Since all-optical networks are emerging as implementations for very high data rate communications, flexible switching and broadband application support, we present a novel all-optical security system technique using random sequence encryption. The noise function was applied via a pseudo-random noise generator seeded with an encryption key. We analyze the performance of both security systems under different application circumstances. Our results confirm the effectiveness of the security technique to make the system less vulnerable to attack.

Symbol and Bit Mapping Optimization for Physical-Layer Network Coding with Pulse Amplitude Modulation

In this paper, we consider a two-way relay network in which two users exchange messages through a single relay using a physical-layer network coding (PNC) based protocol. The protocol comprises two phases of communication. In the multiple access (MA) phase, two users transmit their modulated signals concurrently to the relay, and in the broadcast (BC) phase, the relay broadcasts a network-coded (denoised) signal to both users. Nonbinary and binary network codes are considered for uniform and nonuniform pulse amplitude modulation (PAM) adopted in the MA phase, respectively. We examine the effect of different choices of symbol mapping (i.e., mapping from the denoised signal to the modulation symbols at the relay) and bit mapping (i.e., mapping from the modulation symbols to the source bits at the user) on the system error-rate performance. A general optimization framework is proposed to determine the optimal symbol/bit mappings with joint consideration of noisy transmissions in both communication phases. Complexity-reduction techniques are developed for solving the optimization problems. It is shown that the optimal symbol/bit mappings depend on the signal-to-noise ratio (SNR) of the channel and the modulation scheme. A general strategy for choosing good symbol/bit mappings is also presented based on a high-SNR analysis, which suggests using a symbol mapping that aligns the error patterns in both communication phases and Gray and binary bit mappings for uniform and nonuniform PAM, respectively.

A novel design of physical layer network coding in strong asymmetric two-way relay channels

The quality of two-way links is always asymmetric in practical two-way relay channels (TWRC) and therefore, the capacity of TWRC will be limited by the weaker link. An asymmetric modulation scheme, with physical layer network coding, was presented in order to improve the transmission reliability in TWRC. This makes full use of the stronger link to improve the overall transmission rate and also ensures the reliability of the weaker link. The simulation results show that the proposed asymmetric modulation scheme in the case of strongly asymmetric channels, compared to the symmetric transmission, enhances the system capacity significantly and also guarantees the system reliability.

Energy Efficient Cross Layer STDMA Design in Solar-Powered Wireless Mesh Network

Energy efficiency is a major concern for mesh nodes with limited power supply. So STDMA(spatial time division multiple access) is applied to reduce node’s power consumption in mesh networks. However, the energy cost was still high for solar panel applying with STDMA, due to the fixed slots and numerous start times of nodes. A VF-STDMA(variable frame in STDMA) is proposed based on STDMA protocol. The VF-STDMA dynamically changes the frame size and assigns timeslots to each link according to the network data flows. The start times are reduced by recombining slots. Finally, the optimum routing is obtained through a cross-layer mechanism covering physical layer, MAC layer and network layer. The results show that the algorithms improve energy efficiency as well as decrease energy consumption of solar panel.

Signal Alignment: Enabling Physical Layer Network Coding for MIMO Networking

We apply signal alignment (SA), a wireless communication technique that enables physical layer network coding (PNC) in multi-input multi-output (MIMO) wireless networks. Through calculated precoding, SA contracts the perceived signal space at a node to match its receive capability, and hence facilitates the demodulation of linearly combined data packets. PNC coupled with SA (PNC-SA) has the potential of fully exploiting the precoding space at the senders, and can better utilize the spatial diversity of a MIMO network for higher system degrees-of-freedom (DoF). PNC-SA adopts the idea of `demodulating a linear combination' from PNC. The design of PNC-SA is also inspired by recent advances in IA, though SA aligns signals not interferences. We study the optimal precoding and power allocation problem of PNC-SA, for SNR (singal-to-noise-ratio) maximization at the receiver. The mapping from SNR to BER is then analyzed, revealing that the DoF gain of PNC-SA does not come with a sacrifice in BER. We then design a general PNC-SA algorithm in larger systems, and demonstrate general applications of PNC-SA, and show via network level simulations that it can substantially increase the throughput of unicast and multicast sessions, by opening previously unexplored solution spaces in multi-hop MIMO routing.

Physical Layer Network Coding for the K-User Multiple Access Relay Channel

We propose a Physical layer Network Coding (PNC) scheme for the K-user wireless Multiple Access Relay Channel, in which K source nodes want to transmit messages to a destination node D with the help of a relay node R. The proposed scheme involves (i) Phase 1 during which the source nodes alone transmit and (ii) Phase 2 during which the source nodes and the relay node transmit. At the end of Phase 1, the relay node decodes the messages of the source nodes and during Phase 2 transmits a many-to-one function of the decoded messages. To counter the error propagation from the relay node, we propose a novel decoder which takes into account the possibility of error events at R. It is shown that if certain parameters are chosen properly and if the network coding map used at R forms a Latin Hypercube, the proposed decoder offers the maximum diversity order of two. Also, it is shown that for a proper choice of the parameters, the proposed decoder admits fast decoding, with the same decoding complexity order as that of the reference scheme based on Complex Field Network Coding (CFNC). Simulation results indicate that the proposed PNC scheme offers a large gain over the CFNC scheme.

Robust precoder-decoder design for physical layer network coding-based MIMO two-way relaying system

In this paper, we investigate a robust joint precoder-decoder design scheme for a multiple-input multiple-output physical layer network coding (PNC)-based two-way relay system. An orthogonal training sequence is used to estimate the channels. The estimate is imperfect, and a robust design is proposed to find precoders at the source nodes and decoder at the relay node to facilitate PNC operations during multiple-access stage. Both channel estimation error and antenna correlations are used to formulate the optimization problem to minimize the weighted mean square error (WMSE) under a total power constraint. The problem becomes non-convex, and we propose an algorithm to solve it optimally. During the broadcast stage, an algorithm is proposed to find a precoder at the relay node and decoders at source nodes. The system performance is evaluated with estimation error and antenna correlation parameters. The effect of weighting parameters, relay location, and number of antennas at nodes are also considered in the numerical analysis. Numerical results confirm that our joint precoder-decoder algorithms provide the optimal solution to the minimization of WMSE with the total available power.

Analysis of communication signals based on physical-layer network coding

The Physical-layer Network Coding(PNC) is difficult to be wiretapped.In order to wiretap and recover the signals from the PNC system,a blind signal processing algorithm based on decoding was proposed to recover the information sequences from the senders in the paper.Firstly,the algorithm analyzed the mapping signal from the second slot in PNC scheme in order to get the parameters of modulation and encoding.Secondly,the algorithm processed the mixed signal from the first slot in the PNC scheme,and clustered the sampling data to judge the component of bit information.At last,the paper used the decoding algorithm to correct the information and took the mixed signal apart successfully.Extensive simulation studies show that the new algorithm based on constellation analysis and decoding can recover the signals blindly,and the Bit Error Rate(BER) reaches 10-3 when the Signal-to-Noise Ratio(SNR) is 11 dB.

Distributed MAC Protocol Supporting Physical-Layer Network Coding

Physical-layer network coding (PNC) is a promising approach for wireless networks. It allows nodes to transmit simultaneously. Due to the difficulties of scheduling simultaneous transmissions, existing works on PNC are based on simplified medium access control (MAC) protocols, which are not applicable to general multihop wireless networks, to the best of our knowledge. In this paper, we propose a distributed MAC protocol that supports PNC in multihop wireless networks. The proposed MAC protocol is based on the carrier sense multiple access (CSMA) strategy and can be regarded as an extension to the IEEE 802.11 MAC protocol. In the proposed protocol, each node collects information on the queue status of its neighboring nodes. When a node finds that there is an opportunity for some of its neighbors to perform PNC, it notifies its corresponding neighboring nodes and initiates the process of packet exchange using PNC, with the node itself as a relay. During the packet exchange process, the relay also works as a coordinator which coordinates the transmission of source nodes. Meanwhile, the proposed protocol is compatible with conventional network coding and conventional transmission schemes. Simulation results show that the proposed protocol is advantageous in various scenarios of wireless applications.

Performance of Physical-layer Network Coding in Asymmetric Rayleigh Fading Two-way Relay Communication Systems

Performance of Physical-layer Network Coding in Asymmetric Rayleigh Fading Two-way Relay Communication Systems

SER-Based Relay Selection for Two-Way Relaying with Physical Layer Network Coding

SER-Based Relay Selection for Two-Way Relaying with Physical Layer Network Coding

SER-Based Relay Selection for Two-Way Relaying with Physical Layer Network Coding

To enhance the symbol error rate (SER) performance of the two-way relay channels with physical layer network coding, this letter proposes a relay selection scheme, in which the relay with the maximal minimum distance between different points in its constellation among all relays is selected to assist two-way transmissions. We give the closed-form expression of minimum distance for binary phase-shift keying and quadrature phaseshift keying. Additionally, we design a low-complexity method for higher-order modulations based on look-up tables. Simulation results show that the proposed scheme improves the SER performance for two-way relay networks.

Design of Physical Layer Network Coded LDPC Code for a Multiple-Access Relaying System

In this letter we propose a novel physical layer network coded LDPC code structure, named PNC-LDPC code, for a non-orthogonal multiple access relay channel. An iterative detection-and-decoding receiver is designed to tackle the multi-user interference at the destination. Based on the code structure and the iterative receiver, we optimize the PNC-LDPC code profile to approach the system achievable rate by utilizing the extrinsic mutual information transfer (EXIT) chart. Simulations show that the performance of our PNC-LDPC code outperforms other conventional network coding based LDPC codes.