Wireless Relay System Research Articles

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.

Non-Orthogonal N-MSK Modulation for Wireless Relay D2D Communication

This paper presents a novel multilevel convolutional encoder (MCE) modulation method, which is non-binary NMSK based on Rings of Integers. We also give the architecture used for D2D Non-Orthogonal Multiplex Access (NOMA) relay wireless system, which can be decoded by genetic algorithm. Through simulation different environments, it shows such codes have higher spectrum efficiency and 3dB out performance over the traditional TCM.

Unified analysis of energy harvesting–based MIMO relay wireless systems over Nakagami‐m fading channels

AbstractIn this paper, we consider two‐hop multi‐input multi‐output relay wireless systems with energy harvesting. In each hop, transmit antenna selection/receive antenna selection, transmit antenna selection/maximum ratio combining, or maximum ratio transmission/receive antenna selection is used. Also, an energy‐constrained relay harvests energy from the source via either a time switching–based relaying protocol or a power splitting–based relaying protocol. We perform unified analysis of the systems over Nakagami‐m fading channels. Specifically, we derive exact closed form expressions for ergodic capacity and throughput (for delay‐tolerant transmission mode) and outage probability and throughput (for delay‐limited transmission). Optimal energy‐harvesting time (in time switching–based relaying protocol–based system) and optimal power‐splitting ratio (in power splitting–based relaying protocol–based system) to achieve maximum throughput at high signal‐to‐noise‐ratio are also determined. Moreover, we derive an exact closed form expression for the bit error rate, which facilitates the evaluation of the system performance. Impacts of various multi‐input multi‐output processing schemes, numbers of equipped antennas, energy‐harvesting mechanisms, and transmission modes on the system performance are also examined and discussed. All theoretical analyses are corroborated by simulations.

Channel Estimation of MIMO Relay Systems With Multiple Relay Nodes

In this paper, we consider the channel estimation problem for multiple-input multiple-output wireless relay communication systems with multiple relay nodes. In particular, all individual channel matrices of the first-hop and second-hop links are estimated at the destination node by applying the superimposed channel training algorithm, where training sequences are superimposed at the relay nodes to assist the estimation of the relays-destination channel matrices. To improve the performance of channel estimation, we consider the estimation error inherited from the second-hop channel estimation and develop a new minimal mean-squared error-based algorithm to estimate the first-hop channel matrices. Furthermore, we derive the optimal power allocation and training sequences at the source and relay nodes. Numerical examples demonstrate a better performance of the proposed superimposed channel training algorithm.

Secure Transmission of Wireless Relaying Systems With Jammer and Multiple-User Selection

In this paper, we investigate the secure transmission of wireless relaying systems with multi-destinations and an eavesdropper (Eve) in Nakagami- $m$ fading channels. To protect the confidential message from leaking to the Eve, an intentional interference is sent from a selected destination. We propose two jammer selection schemes under two typical communication scenarios: 1) the channel state information (CSI) of the Eve is unknown, where a jammer is selected based on the CSI between the selected legitimate receiver and the other destination users and 2) the CSI of the Eve is available, where a jammer is selected based on the CSI between the Eve and the other destination users. To assess the performance of the two proposed schemes, we first derive the exact closed-form expressions of intercept probability and outage probability to evaluate the security and the reliability of the system, respectively. In addition, to evaluate the security and reliability as a whole, we propose a new definition of the security–reliability tradeoff, which allows us to easily optimize the power allocation and improve the performance. Finally, simulation results are provided to verify the availability of the proposed schemes.

Robust power optimization scheme for cooperative wireless relay system in smart city

SummaryUltra dense deployment of base stations is one of most significant features in smart city communication networks. Aiming at the large‐scale wireless communication issue in smart city, we propose a distributed robust power allocation scheme with proportional fairness for cooperative orthogonal frequency‐division‐multiple‐access relay network. With the amplify‐and‐forward relay mode, all of the relays assist the information transmission simultaneously on orthogonal subcarriers. Considering the uncertainty of channel gains, first we aim at achieving the maximum utility subject to the constraints of outage probability threshold and power bound. Subsequently, the problem is transformed to a solvable convex optimization problem with determination constraints. The dual‐decomposition method is applied to solve the formulated optimization problem. To reduce the information exchange of the whole system, we propose a computationally efficient distributed iteration algorithm. Numerical results reveal the effectiveness of the proposed robust optimization algorithm. Copyright © 2016 John Wiley & Sons, Ltd.

Wireless Information and Energy Transfer in Nonregenerative OFDM AF Relay Systems

Energy harvesting (EH) is a promising strategy to prolong the operation of energy-constrained wireless systems. Simultaneous wireless information and energy transfer (SWIET) is a potential EH technique which has recently drawn significant attention. By employing SWIET at relay nodes in wireless relay systems, the relay nodes can harvest energy and receive information from their source nodes simultaneously as radio signals can carry energy as well as information at the same time, which solves the energy scarcity problem for wireless relay nodes. In this paper, we study SWIET for nonregenerative orthogonal-frequency-division multiplexing (OFDM) amplify-and-forward systems in order to maximize the end-to-end achievable rate by optimizing resource allocation. Firstly, we propose an optimal energy-transfer power allocation policy which utilizes the diversity provided by OFDM modulation. We then validate that the ordered-signal-to-noise ratio (SNR) subcarrier pairing (SP) is the optimal SP scheme. After that, we investigate the information-transfer power allocation (IPA) and EH time optimization problem which is formulated as a non-convex optimization problem. By making the approximation at high SNR regime, we convert this non-convex optimization problem into a quasi-convex programming problem, where an algorithm is derived to jointly optimize the IPA and EH time. By analytical analysis, we validate that the proposed resource allocation scheme has much lower computational complexity than peer studies in the literature. Finally, simulation results verify the optimality of our proposed resource allocation scheme.

Discrete-time Gaussian interfere-relay channel

In practical wireless relay communication systems, nondestination nodes are assumed to be idle not receiving signals while the relay sends messages to a particular destination node, which results in reduced bandwidth efficiency. To improve the bandwidth efficiency, we relax the idle assumption of non-destination nodes and assume that non-destination nodes may receive signals from sources. We note that the message relayed to a particular node in such a system gives rise to interference to other nodes. To study such a more general relay system, we consider, in this paper, a relay system in which the relay first listens to the source, then routes the source message to the destination, and finally produces interference to the destination in sending messages for other systems. We obtain capacity upper and lower bounds and study the optimal method to deal with the interference as well as the optimal routing schemes. From analytic results obtained, we find the conditions on which the direct transmission provides higher transmission rate. Next, we find the conditions, by numerical evaluation of the theoretical results, on which it is better for the destination to cancel and decode the interference. Also we find the optimal source power allocation scheme that achieves the lower bound depending on various channel conditions. We believe that the results provided in this paper will provide useful insights to system designers in strategically choosing the optimal routing algorithms depending on the channel conditions.

Proposal of channel estimation method for wireless two-way relay system of using SFBC MIMO-OFDM technique

In a two-way relay communication system, two user terminals (UTs) can communicate and exchange their information data through a relay station by using two timeslots. To realize the two-way relay communication system, each UT is required to estimate channel frequency responses (CFRs) both for up and down links in two timeslots efficiently which be employed in the demodulation of the other user's information data with frequency domain equalization. To satisfy this requirement, this paper proposes a novel CFR estimation method for the wireless two-way relay communication system of using SFBC MIMO-OFDM technique. The salient feature of proposed CFR estimation method is to employ the maximum likelihood estimation method for the proposed scattered pilot subcarriers assignment which can achieve higher CFR estimation accuracy even in higher time-varying fading channel and when the transmission OFDM signal is sampled by the non-Nyquist rate. In the proposed system, the SFBC technique is also employed for the data subcarriers both for the pilot symbols including data subcarriers and data symbols consisting of all data subcarriers in the frequency axis to improve the bit error rate (BER) performance in the two-way relay system. From the computer simulation results, this paper demonstrates the effectiveness of proposed two-way relay communication system of using SFBC MIMO-OFDM technique. To demonstrate the effectiveness of proposed two-way relay communication system of using SFBC MIMO-OFDM technique, this paper conducts various computer simulations as comparing with the conventional methods in higher time-varying fading channel. From the computer simulation results, this paper confirms that the proposed method even at the non-Nyquist rate can achieve higher channel estimation accuracy evaluated by the normalized mean square error (NMSE) and better BER performances by 5 and 22 times, respectively as comparing with the conventional methods when the normalized Doppler frequency ($$f_dT_S$$fdTS) is $$10^{-2}$$10-2 and the carrier to noise power ratio (C/N) is 25 dB.

Exact Closed-Form Expression for End-to-End Average SNR of Dual-Hop Amplify-and-Forward in Wireless Relaying Systems

In this paper, we investigate the end-to-end average signal-to-noise ratio (SNR) of dual-hop amplify-and-forward wireless communication systems operating over independent but not necessarily identically distributed (i.n.i.d.) $$\eta {-}\mu $$?-μ fading channels. Specifically, we derive a novel exact closed-form expression for the generalized end-to-end average SNR for arbitrary fading parameters $$\eta $$? and $$\mu $$μ. Using the obtained closed-form expression for the generalized end-to-end average SNR, some important performance metrics such as the average end-to-end SNR, the amount of fading and the channel capacity can be easily obtained. The effect of the fading parameters and the power imbalance on the overall system performance is investigated. The analytical results are validated by means of Monte-Carlo simulations.

Development of Wireless Systems for Disaster Recovery Operations

This paper presents wireless systems for use in disaster recovery operations. The Great East Japan Earthquake of March 11, 2011 reinforced the importance of communications in, to, and between disaster areas as lifelines. It also revealed that conventional wireless systems used for disaster recovery need to be renovated to cope with technological changes and to provide their services with easier operations. To address this need we have developed new systems, which include a relay wireless system, subscriber wireless systems, business radio systems, and satellite communication systems. They will be chosen and used depending on the situations in disaster areas as well as on the required services. key words: disaster recovery operation, business radio, subscriber wireless system, satellite communications, the Great East Japan Earthquake

Throughput Maximization for Wireless Relay Systems with AMC and HARQ

Throughput Maximization for Wireless Relay Systems with AMC and HARQ

On the Diversity of Noncoherent Distributed Space–Frequency Coded Relay Systems With Relay Censoring

This paper considers a noncoherent distributed space-frequency coded (SFC) wireless relay system with multi- ple relays. Each relay adopts a censoring scheme to determine whether the relay will decode and forward the source's informa- tion toward the destination. We analytically obtain the achievable diversity for both cases of perfect and imperfect relay censoring. With perfect censoring, we show that the same diversity of a conventional noncoherent SFC MIMO-OFDM system is achiev- able in the considered noncoherent distributed SFC system with maximum-likelihood (ML) decoding, regardless of whether partial information of channel statistics and relay decoding status is avail- able at the destination. With imperfect censoring, we analytically investigate how censoring errors affect the achievability of the system's diversity. We show that the two types of censoring errors, which correspond to useless and harmful relays, respectively, can decrease the achievable diversity significantly. Our analytical insights and numerical simulations demonstrate that the nonco- herent distributed system can offer a comparable diversity as the conventional MIMO-OFDM system if relay censoring is carefully implemented.

Superposition Coding Based User Combining Schemes for Non-Orthogonal Scheduling in a Wireless Relay System

We consider a wireless system where multiple users are served in Downlink (DL) by one Base Station (BS) and one Relay Station (RS). Previous research has shown that, combining two users' messages via Superposition Coding (SC) could enhance their achievable rate and fairness, in both two-user and multi-user systems. In this work, we propose two novel SC schemes referred as Relayed/Direct (RD-SC) and Relayed/Relayed SC (RR-SC) schemes, that combine the messages to a pair of Relayed/Direct users or Relayed/Relayed users into three or four SC layers, where the number of SC layers is equal to the number of available links. For each scheme, power allocation under sum-rate maximization is analyzed, enabling to derive the optimal power ratios to each SC layer numerically. Then, we design the non-orthogonal Max Rate-Two User-SC (MR-TU-SC) Scheduler and Proportional Fair-Two User-SC (PF-TU-SC) Scheduler where a pair of selected users are allocated simultaneously based on RD-SC and RR-SC schemes. The simulation results show that the proposed schedulers significantly outperform the conventional orthogonal schedulers where a single user is allocated on a unit resource block, in terms of system throughput, fairness and outage, while approaching the upper bound performance.

Analysis of Wireless Relay System under Interference Environment by SNR Enhancement Method

This paper emphasizes the wireless relaying performance analysis in interference environment. We will analyze the main performance metrics such as outage and error probability of the communication system in interference environment, and will make some simulation to demonstrate the result of theory analysis. In relay transmission system, either two relaying or multiple relaying, its basic principles are based on the two relaying system. In this paper, the wireless relay system performance in the interference environment will be analyzed and simulated on the basis of previous studies.

Performance Analysis of Multi-Branch Multi-Hop Wireless Relay Systems over Log-Normal Channels

In this paper, analytical models are proposed to study the performance of amplify-and-forward (AF) multi-branch multi-hop (MBMH) wireless relay systems (WRS) over log-normal fading channels. The presented analytical models are used to estimate the end-to-end performance of the MBMH WRS with maximal ratio combining (MRC) and selection combining (SC) schemes. In particular, we derive closed-form expressions for the parameters of the end-to-end signal-to-noise ratio (SNR) of an AF multi-hop chain relay link. Further, we characterize the distribution of the end-to-end SNR and derive closed-form expressions for the mean and standard deviation of the end-to-end SNR's natural logarithm for AF MBMH WRS with MRC scheme, and the probability density function (pdf) for the end-to-end SNR of AF MBMH WRS with SC scheme. Then, under MRC and SC schemes, we analyze the diversity gain by adopting asymptotical relative diversity order (ARDO) and develop accurate analytical expressions for average bit error probability, outage probability, and ergodic capacity, respectively. Our analysis reveals that the computational complexity of the proposed models is acceptably low for practical applications. Numerical and Monte Carlo simulation results are presented to substantiate the accuracy of the proposed analytical models.

Adaptive hierarchical modulation and power allocation for superposition-coded relaying

Abstract We propose a relaying scheme based on superposition coding (SC) with discrete adaptive modulation and coding (AMC) for a three-node wireless relay system, based on half duplex transmission, where each node decodes messages by successive interference cancelation (SIC). Unlike the previous works where the transmission rate of each link is assumed to achieve Gaussian channel capacity, we design a practical superposition-coded relaying scheme with discrete AMC while taking into account the effect of decoding errors at each stage of the SIC process at each node. In our scheme, hierarchical modulation (HM) is used to create an SC message composed of one basic and one superposed message with optimized power allocation. We firstly introduce the proposed scheme without forward error correction (FEC) for high signal-to-noise ratio (SNR) region and provide the optimal power allocation between the superimposed messages. Next, we extend the uncoded scheme to incorporate FEC to overcome bad channel conditions. The power allocation in this case is based on an approximated expression of the bit error rate (BER). Numerical results show the performance gains of the proposed SC relaying scheme with HM compared to conventional schemes, over a large range of SNRs.

Outage Probability of Two-Hop Relay Networks with Related Interference

We consider a specific interference-limited wireless relay system that comprises several cooperation units (CUs) which are defined as a source and destination node pair with an associated relay node. In the wireless relay system, all source nodes simultaneously transmit their own signals and the relay node in each CU then forwards the received signal to the destination node, causing co-channel interference at both the relay node and the destination node in each CU. The co-channel interference at the relay node is closely related to that at the destination node in each CU. We first derive the end-to-end outage probability in a CU over Rayleigh slow-fading channels with interference for the decode-and-forward (DF) relaying strategy. Then, on the assumption that each CU is allocated with equal power we design an optimal power allocation between the source node and the relay node in each CU to minimize the outage probability of the investigated CU. At last, in the case that each CU is not allocated with equal power and the sum of their power is constrained, we present an optimal power allocation between CUs to minimize the sum of the outage probability of all CUs. The analytical results are verified by simulations.

Cross-Layer QoS-Constrained Optimization of Adaptive Multi-Rate Wireless Systems using Infrastructure-Based Cooperative ARQ

This research work aims at analyzing the effects of using cooperative-ARQ (CARQ) in wireless relay systems employing AMC at the physical layer and an infinitely persistent type-I Hybrid FEC/CARQ protocol at the DLC layer. To that end, a cross-layer multidimensional discrete-time Markov chain (DTMC)-based queuing model is developed to jointly exploit the capabilities of CARQ and AMC. Based on the stationary state probability distribution of the proposed multidimensional DTMC, closed-form analytical expressions for performance metrics such as average throughput, queue length, packet delay and packet loss rate are derived, and the impact of cooperative relaying on these performance parameters is analyzed. Numerical results confirm the validity of the presented AMC/CARQ cross-layer analytical framework and reveal that the proposed CARQ protocol consistently outperforms the classical Type-I Hybrid FEC/ARQ schemes and provides a noticeable improvement when facing correlated fading channel scenarios.

On the Energy Efficiency of a Relaying Protocol with HARQ-IR and Distributed Cooperative Beamforming

Hybrid automatic repeat request (HARQ) protocols can be employed for reliable transmissions of delay tolerant traffics over fading channels. Relay-aided transmissions can offer various advantages over direct transmissions in wireless communications including transmission power saving. In this paper, we consider the energy efficiency of a relaying protocol that is based on the HARQ protocol with incremental redundancy (HARQ-IR) when distributed cooperative beamforming (DCB) is employed in a wireless relay system. It is assumed that multiple relays between a source node (SN) and a destination node (DN) are available for relaying and some of them are selected for DCB. In the proposed relaying protocol, the signal transmission consists of two phases and each phase can have an independent HARQ-IR protocol. It is shown in this paper that although the performance can be improved when the number of selected relays for DCB increases as the diversity gain increases, it is not true in terms of the energy efficiency. For Rayleigh fading channels, closed-form expressions are derived for the average numbers of transmissions for each phase. From them, under a certain energy efficiency criterion, we can find the optimal number of multiple selected relays for DCB to forward signals from relays to DN.