Physical Layer Network Research Articles

Hierarchical Network Transfer Function and Doubly-Greedy Half-Duplex Stage Scheduling for WPNC Networks

Wireless Physical Layer Network Coding (WPNC) based communication networks deliver the information from sources to destinations through the complex relay network. To properly design the Network Coded Modulation transmitted by network nodes and the relay processing including the Hierarchical Network Code (HNC) maps, we need a formal description of the global network processing function. Any practical implementation also implies the half-duplex constrained relays which imposes the network to work in multiple stages. We present a polynomial based formalism defining the Hierarchical Network Transfer Function (H-NTF). It captures all phenomena related to the stage dependent transmit and receive activity over the network, including potential buffering, and mainly the hierarchy of forming the local HNC maps into the global hierarchical flow description. Using the polynomial formalism of H-NTF, we develop the half-duplex constrained stage scheduling algorithm. It starts with finding a causal minimal latency (or close to minimal) critical sequence with subsequent doubly (first Rx then Tx) greedy mapping of the node activity compliant with the half-duplex constraint.

Breakthroughs in Photonics 2014: Optical Physical-Layer Network Coding, Recent Developments, and Challenges

Network coding is a revolutionary technique that can enhance network throughput and protection. This paper introduces optical physical-layer network coding (OPNC) and the recent development of OPNC, with focused discussion on “common-channel” OPNC that can efficiently utilize network resources. It also describes the challenges ahead of OPNC, including incorporating more than two signals for OPNC; circumventing the higher signal processing complexity when higher order modulations are adopted; and exploiting OPNC in more sophisticated multichannel systems.

Building Blocks of Physical-Layer Network Coding

This paper investigates the fundamental building blocks of physical-layer network coding (PNC). Most prior work on PNC focused on its application in a simple two-way-relay channel (TWRC) consisting of three nodes only. Studies of the application of PNC in general networks are relatively few. This paper is an attempt to fill this gap. We put forth two ideas: a general network can be decomposed into small building blocks of PNC, referred to as the PNC atoms, for scheduling of PNC transmissions; and we identify nine PNC atoms, with TWRC being one of them. Three major results are as follows. First, using the decomposition framework, the throughput performance of PNC is shown to be significantly better than those of the traditional multi-hop scheme and the conventional network coding scheme. For example, under heavy traffic volume, PNC can achieve 100% throughput gain relative to the traditional multi-hop scheme. Second, PNC decomposition based on a variety of different PNC atoms can yield much better performance than PNC decomposition based on the TWRC atom alone. Third, three out of the nine atoms are most important to good performance. Specifically, the decomposition based on these three atoms is good enough most of the time, and it is not necessary to use the other six atoms.

Symbol Timing Estimation for Physical-Layer Network Coding

Recently, physical-layer network coding (PNC) has attracted much attention due to its ability to double the throughput in relay-aided communications. However, most research in PNC assumes that the symbol timings of the end nodes are ideally aligned. In fact, they have to be recovered accurately at the relay. In this letter, we present a novel symbol timing error estimator for PNC. It is shown that accurate error estimates can be obtained by the interpolation algorithm based on a small-size Discrete Fourier Transformation (DFT). The Mean Square Error (MSE) performance is one order of magnitude better than that of the conventional maximum likelihood based optimum sample (OS) algorithm for signal-to-noise ratio (SNR) greater than 10 dB.

Physical-layer network coding in coherent optical OFDM systems.

We present the first experimental demonstration and characterization of the application of optical physical-layer network coding in coherent optical OFDM systems. It combines two optical OFDM frames to share the same link so as to enhance system throughput, while individual OFDM frames can be recovered with digital signal processing at the destined node.

Asynchronous Convolutional-Coded Physical-Layer Network Coding

This paper investigates the decoding process of asynchronous convolutional-coded physical-layer network coding (PNC) systems. Specifically, we put forth a layered decoding framework for convolutional-coded PNC consisting of three layers: symbol realignment layer, codeword realignment layer, and joint channel-decoding network coding (Jt-CNC) decoding layer. Our framework can deal with phase asynchrony (phase offset) and symbol arrival-time asynchrony (symbol misalignment) between the signals simultaneously transmitted by multiple sources. A salient feature of this framework is that it can handle both fractional and integral symbol misalignments. For the decoding layer, instead of Jt-CNC, previously proposed PNC decoding algorithms (e.g., XOR-CD and reduced-state Viterbi algorithms) can also be used with our framework to deal with general symbol misalignments. Our Jt-CNC algorithm, based on belief propagation, is BER-optimal for synchronous PNC and near optimal for asynchronous PNC. Extending beyond convolutional codes, we further generalize the Jt-CNC decoding algorithm for all cyclic codes. Our simulation shows that Jt-CNC outperforms the previously proposed XOR-CD algorithm and reduced-state Viterbi algorithm by 2 dB for synchronous PNC. For both phase-asynchronous and symbol-asynchronous PNC, Jt-CNC performs better than the other two algorithms. Importantly, for real wireless network experimentation, we implemented our decoding algorithm in a PNC prototype built on the USRP software radio platform. Our experiment shows that the proposed Jt-CNC decoder works well in practice.

Low‐complexity iteration‐based interference cancellation in asynchronous physical‐layer network coding

When two terminals exchange information through an intermediate relay, physical-layer network coding (PLNC) improves the spectrum efficiency by allowing cocurrent packet transmission. Synchronisation is one of the most important issues in distributed wireless communications systems. In time-domain (TD)-based PLNC, signals transmitted from user terminals may arrive the relay at different time. The fractional symbol level delay will introduces inter-symbol interference because of the use of practical pulse-shaping and matched-filter. Orthogonal-frequency division-multiplexing (OFDM) can be used to deal with time asynchrony. However, OFDM systems are very sensitive to carrier frequency offsets, which will introduce inter-carrier interference. In this study, a novel low-complexity symbol-based decoding iterative interference cancellation schemes are proposed for TD-based and OFDM-based PLNC. Signals from two sources are separately decoded, and interference is reconstructed and eliminated. Monte Carlo simulations show that the proposed scheme with over just one iteration can significantly improve the bit error rate performance.

Single and Dual Relay Selection in Two-Way Network-coded Relay Networks

In this paper, the relay selection problem for two-way relaying networks in single and dual relay selection schemes is addressed. In the single relay selection scheme, the problem is to find the best relay node which leads to the minimum bit error rate (BER) between the source nodes. We then derive the upper and lower bounds for the signal to noise ratio and the end-to-end BER of this scheme. In the dual relay selection scheme, the problem is to find the two best relay nodes. In this scheme, Alamouti space-time coding and physical layer network coding are utilized in order to achieve the higher performance. For improving the system performance, optimal power allocation between the sources and the relays is considered based on decreasing the BER. Finally, simulation results are provided to verify the correctness of analytical results.

Cloud Compute-and-Forward With Relay Cooperation

-We study a cloud network with M distributed receiving antennas and L users, which transmit their messages towards a centralized decoder (CD), where M ≥ L. We consider that the cloud network applies the Compute-and-Forward (C&F) protocol, where L antennas/relays are selected to decode integer equations of the transmitted messages. In this work, we focus on the best relay selection and the optimization of the Physical-Layer Network Coding (PNC) at the relays, aiming at the throughput maximization of the network. Existing literature optimizes PNC with respect to the maximization of the minimum rate among users. The proposed strategy maximizes the sum rate of the users allowing non-symmetric rates, while the optimal solution is explored with the aid of the Pareto frontier. The problem of relay selection is matched to a coalition formation game, where the relays and the CD cooperate to maximize their profit. Efficient coalition formation algorithms are proposed, which perform joint relay selection and PNC optimization. Simulation results show that a considerable improvement is achieved compared to existing results, both in terms of the network sum rate and the players' profits.

Service Enhancement for User Equipments in LTE-A Downlink Physical Layer Network

In this paper, we propose a real LTE-A downlink physical layer network with multiple relay stations (RSs). RSs improve the weak received signal at user equipments (UEs). LTE-A with RSs can be used to increase the coverage area of BSs and increase the system capacity. It also enhances the service of the user at cell edge and cover shadowed areas. The performance of the proposed LTE-A network is illustrated and modeled. We also take the effect of different environments scenarios of the WINNER Phase II channel model. The received signals of UEs scenarios are mathematically derived and analyzed in this paper. The weighted least square method for signal combining is mathematically analyzed. The performance of LTE-A with RSs is evaluated by calculating the SER and the capacity for a certain range of SNRs. The SER and capacity are calculated for different UEs scenarios to investigate the effect of RSs in LTE-A network. Simulation results reveal that UEs which receive multiple signals from RSs have low SER values for a certain range of SNRs and enhances the service performance of the UEs at cell edge.

Transmit Power Optimization for Two-Way Relay Channels With Physical-Layer Network Coding

In this letter, we consider a two-way relay channel where two source nodes exchange their packets via a half-duplex relay node, which adopts physical-layer network coding (PNC) for exchanging packets in two time slots. Convolutional codes (CCs) are assumed to be applied as a channel code for each packet. The relay node directly decodes the XORed version of packets of two source nodes in the multiple access (MA) phase. We first mathematically analyze a bit error rate (BER) of the MA phase in the PNC with CCs in Rayleigh fading channels. Then, we propose a power allocation (PA) strategy for minimizing the derived BER expression at the relay node. It is shown that the proposed transmit power solution satisfies the following relationship: ${P_{1}^{\ast}\over P_{2}^{\ast}}=\sqrt{\Omega_{2}\over\Omega_{1}} $ , where $P_{i}^{\ast}$ and $\Omega_{i}$ denote the optimal power of the $i$ th source node and the variance of the channel gains between the $i$ th source node and the relay node. The proposed PA strategy significantly outperforms conventional PA schemes in terms of the BER.

Asynchronous Physical-Layer Network Coding With Quasi-Cyclic Codes

Communication in the presence of bounded timing asynchronism, which is known to the receiver but cannot be easily compensated, is studied. Examples of such situations include point-to-point communication over intersymbol interference (ISI) channels and asynchronous wireless networks. In these scenarios, although the receiver may know all the delays, it is often not an easy task for the receiver to compensate the delays as the signals are mixed together. A novel framework, which is called interleave/deinterleave transform (IDT), is proposed to deal with this problem. It is shown that the IDT allows one to design the delays so that quasi-cyclic (QC) codes with a proper shifting constraint can be used accordingly. When used in conjunction with QC codes, IDT provides significantly better performance than existing schemes relying solely on cyclic codes. Two instances of asynchronous physical-layer network coding, namely, the integer-forcing equalization for ISI channels and asynchronous compute-and-forward, are then studied. For integer-forcing equalization, the proposed scheme provides improved performance over using cyclic codes. For asynchronous compute-and-forward, the proposed scheme shows that there is no loss in the achievable information rates due to delays that are integer multiples of the symbol duration. Furthermore, the proposed approach shows that delays introduced by the channel can sometimes be exploited to obtain higher information rates than those obtainable in the synchronous case. The proposed IDT can be thought of as a generalization of the interleaving/deinterleaving idea proposed by Wang et al. , which allows the use of QC codes, thereby substantially increasing the design space.

Space–Time Physical-Layer Network Coding

A space–time physical-layer network coding (ST-PNC) method is presented for information exchange among multiple users over fully connected multiway relay networks. The method involves two steps: 1) side-information learning and 2) space–time relay transmission. In the first step, different sets of users are scheduled to send signals over networks, and the remaining users and relays overhear the transmitted signals, thereby learning the interference patterns. In the second step, multiple relays cooperatively send out linear combinations of signals received in the previous phase using space–time precoding so that all users efficiently exploit their side information in the form of 1) what they sent and 2) what they overheard in decoding. This coding concept is illustrated through two simple network examples. It is shown that ST-PNC improves the sum of degrees of freedom (sum-DoF) of the network compared to existing interference management methods. With ST-PNC, the sum-DoF of a general multiway relay network without channel knowledge at the users is characterized in terms of relevant system parameters, chiefly the number of users, the number of relays, and the number of antennas at relays. A major implication of the derived results is that efficiently harnessing both transmitted and overheard signals as side information brings significant performance improvements to fully connected multiway relay networks.

Compressed Relaying for Two-Way Relay Networks With Correlated Sources

In this letter, a compressed relaying scheme via Huffman and physical-layer network coding (HPNC) is proposed for two-way relay networks with correlated sources (TWRN-CS). In the HPNC scheme, both sources first transmit the correlated raw source messages to the relay simultaneously. The relay performs physical-layer network coding (PNC) on the received symbols, compresses the PNC-coded symbols using Huffman coding, and broadcasts the compressed symbols to both source nodes. Then, each source decodes the other source's messages by using its own messages as side information. Compression rate and block error rate (BLER) of the proposed scheme are analyzed. Simulation results demonstrate that the HPNC scheme can effectively improve the network throughput, and meanwhile, achieve the superior BLER performance compared with the conventional non-compressed relaying scheme in TWRN-CS.

On the Perspective Transformation for Efficient Relay Placement in Wireless Multicast Networks

This letter investigates the relay placement problem in wireless multicast networks consisting of multiple sources, relays, and destinations. The data transmission from the sources to the destinations is carried out via the relays employing physical-layer network coding technique. Hybrid automatic repeat request protocol with incremental redundancy is applied for reliable communication. In particular, considering a general setting of nodes in irregularly shaped network, an efficient relay placement algorithm is proposed based on perspective transformation technique to find optimal relay positions for minimizing either the total energy consumption or the total delay in the whole network. The proposed algorithm not only helps reduce the relay searching complexity but also facilitates the relay placement for optimizing networks of any shape.

Social Network Aware Device-to-Device Communication in Wireless Networks

Device-to-device (D2D) communication is seen as a major technology to overcome the imminent wireless capacity crunch and to enable new application services. In this paper, a novel social-aware approach for optimizing D2D communication by exploiting two layers, namely the social network layer and the physical wireless network layer, is proposed. In particular, the physical layer D2D network is captured via the users' encounter histories. Subsequently, an approach, based on the so-called Indian Buffet Process, is proposed to model the distribution of contents in the users' online social networks. Given the social relations collected by the base station, a new algorithm for optimizing the traffic offloading process in D2D communications is developed. In addition, the Chernoff bound and approximated cumulative distribution function (cdf) of the offloaded traffic are derived and the validity of the bound and cdf is proven. Simulation results based on real traces demonstrate the effectiveness of our model and show that the proposed approach can offload the network's traffic successfully.

Space–Time Coded Spatial Modulated Physical Layer Network Coding for Two-Way Relaying

Using the spatial modulation approach, where only one transmit antenna is active at a time, we propose two transmission schemes for two-way relay channel using physical layer network coding with space time coding using coordinate interleaved orthogonal designs (CIODs). It is shown that using two uncorrelated transmit antennas at the nodes, but using only one RF transmit chain and space–time coding across these antennas can give a better performance without using any extra resources and without increasing the hardware implementation cost and complexity. In the first transmission scheme, two antennas are used only at the relay, adaptive network coding (ANC) is employed at the relay and the relay transmits a CIOD space time block code (STBC). This gives a better performance compared to an existing ANC scheme for two-way relay channel which uses one antenna each at all the three nodes. It is shown that for this scheme at high SNR the average end-to-end symbol error probability (SEP) is upper bounded by twice the SEP of a point-to-point fading channel. In the second transmission scheme, two transmit antennas are used at all the three nodes, CIOD STBCs are transmitted in multiple access and broadcast phases. This scheme provides a diversity order of two for the average end-to-end SEP with an increased decoding complexity of ${\cal O}(M^{3})$ for an arbitrary signal set and ${\cal O}(M^{2}\sqrt{M})$ for square QAM signal set. Simulation results show that the proposed schemes performs better than the existing ANC schemes under perfect and imperfect channel state information.

Application of Coded Modulation for Physical-Layer Network Coding

It has been proved that Physical-layer Network Coding (PNC) can boost the system throughput obviously and decrease the error rate by reducing the time slots of inter-communication,which makes signal processing efficient and reliable.Coded Modulation (CM) considers coding and modulation as a whole,which makes coding and modulation effective without decreasing system transmission rate ,to guarantee bandwith efficiency.In this paper,the basic idea of the CM and its implementation are introduced.We analyze the effect of the combination of CM and PNC for communication system performance,which provide the foundation for future research.

On the Energy–Delay Tradeoff and Relay Positioning of Wireless Butterfly Networks

This paper considers energy–delay tradeoff (EDT) of data transmission in wireless-network-coded butterfly networks (WNCBNs) where two sources convey their data to two destinations with the assistance of a relay employing either physical-layer network coding (PNC) or analog network coding (ANC). Hybrid automatic repeat request with incremental redundancy (HARQ-IR) is applied for reliable communication. In particular, we first investigate the EDT of both PNC and ANC schemes in WNCBNs to evaluate their energy efficiency. It is found that there is no advantage to using a relay in a high-power regime. However, in a low-power regime, the PNC scheme is shown to be more energy efficient than both the ANC and direct transmission (DT) schemes if the relay is located far from the sources, whereas both the PNC and ANC schemes are less energy efficient than the DT scheme when the relay is located near the sources. Additionally, algorithms that optimize relay positioning (RP) are developed based on two criteria: minimizing total transmission delays and minimizing total energy consumption subject to node location and power allocation constraints. This optimization can be considered a benchmark for RP in either a low-latency or low-energy-consumption WNCBN.

Cross-Layer Design of Joint Beamforming and Random Network Coding in Wireless Multicast Networks

In this letter, we consider a wireless network where the transmitter, such as a base station, multicasts hard-deadline-constrained prioritized data to users over fading channels. We propose a new cross-layer transmission scheme that involves simultaneous smart antenna beamforming at physical layer and random network coding at network layer to maximize the overall network throughput. In this joint design, beamforming is used for dynamic transmitting power allocation to achieve power gain; while the random network coding is adopted to achieve coding gain. The simulation results show the effectiveness of the proposed algorithm.