In this letter, we propose a lattice reduction (LR) aided joint precoding design for MIMO-relay broadcast communication with the average bit error rate (BER) criterion. We jointly design the signal process flow at both the base station (BS), and the relay station (RS), using the reduced basis of two-stage channel matrices. We further modify the basic precoding design with a novel shift method and a modulo method to improve the power efficiency at the BS and the RS respectively. In addition, the MMSE-SIC algorithm is employed to improve the performance of precoding. Simulations show that, the proposed schemes achieve higher diversity order than the traditional precoding without LR, and the modified schemes significantly outperform the basic design, proving the effectiveness of the proposed methods.

In this paper, we investigate the effect of outdated channel state information (CSI) on decode-and-forward opportunistic mobile relaying networks with direct link (DL) between source node and destination node. Relay selection schemes with different levels of CSI are considered: 1) only outdated CSI is available during the relay selection procedure; 2) not only outdated CSI but also second-order statistics information are available in relay selection process. Three relay selection schemes are proposed based on the two levels of outdated CSI. Closed-form expressions of the outage probability are derived for the proposed relay selection schemes. Meanwhile, the asymptotic behavior and the achievable diversity of three relay selection schemes are analyzed. Finally, simulation results are presented to verify our analytical results.

This work presents the exact outage performance and throughput of two-way cognitive decode-and-forward relaying wireless sensor networks with realistic transceiver relay. The relay is a self-powered wireless node that harvests radio frequency energy from the transmitted signals. We consider four configurations of a network with formed by combining two bidirectional relaying protocols (multiple access broadcast protocol and time division broadcast protocol), and two power transfer policies (dual-source energy transfer and single-fixed-source energy transfer). Based on our analysis, we provide practical insights into the impact of transceiver hardware impairments on the network performance, such as the fundamental capacity ceiling of the network with various configurations that cannot be exceeded by increasing transmit power given a fixed transmission rate and the transceiver selection strategy for the network nodes that can optimize the implementation cost and performance tradeoff.

The new generation of telemedicine systems enables healthcare service providers to monitor patients not only in the hospital but also when they are at home. In order to efficiently exploit these systems, human information collected from end devices must be sent to the medical center through reliable data transmission. In this paper, we propose an adaptive relay transmission scheme to improve the reliability of data transmission for wireless body area networks. In our proposal, relay nodes that have successfully decoded a packet from the source node are selected as relay nodes in which the best relay with the highest channel gain is selected to forward the failed packet instead of the source node. The scheme addresses both the data collision problem and the inefficient relay selection in relay transmission. Our experimental results show that the proposed scheme provides a better performance than previous works in terms of the packet delivery ratio and end-to-end delay.

The acquisition of accurate channel state information at the transmitter (CSIT) is a difficult task in multiple-input multiple-output (MIMO) systems. Partial CSIT is a more realistic assumption, especially for high-mobility mobile users (MUs) whose channel varies very rapidly. In this letter, we propose a MIMO two-way relaying (MTWR) scheme, in which the communication between the BS and a high-mobility MU is assisted by other low-mobility MUs serving as relays. This produces a beamforming effect that can significantly improve the performance of the high-mobility MU, especially for a large number of MUs and unreliable CSIT.

This paper proposes an application of splitting source-node power for a two-phase cooperative relaying system where the transmit powers of the source and the relay node are individually constrained. In the proposed usage, the limited source power is divided into two parts that are used in the first and the second phase. Unlike conventional relaying methods, in the second phase the source retransmits its signal with the split power and, at the same time, the relay forwards the signal received at the first phase, which causes interference between the signals. In order to avoid the intervention, so-called a co-phasing weight for aligning the phases of the two signals is used at at the source before the second transmission. The forwarding operation at the relay however is exactly the same as the conventional techniques. Optimal power-splitting as well as the co-phasing weight is provided in this paper. With numerical investigation, the proposed power-splitting is shown to significantly improve the achievable throughput as well as reduce the outage probability compared with the conventional individual power allocation.

Network coding on the physical-layer has recently been widely discussed as a potentially promising solution to the wireless access problem in a relay network. However, the existing research on physical-layer network coding (PNC), usually assumes that the symbol timing of the nodes is fully synchronized and hardly investigates the unavoidable symbol timing errors. Similar to many telecommunication systems, symbol timing plays a critical role in PNC and precise alignment has to be provided for the encoding. In this work, we propose a novel symbol timing algorithm with a low oversampling factor (samples per symbol) based on the a priori knowledge of the transmitted pulse shape. The proposed algorithm has the dual advantages of the low oversampling rate and high precision. The mean square error (MSE) performance is verified by simulations to be at least one order of magnitude better than that of the conventional optimum phase (OP) algorithm for a signal noise ratio (SNR) greater than 5dB.

An incremental relaying protocol is a promising scheme for preventing the inefficient use of resources in half-duplex cooperative relay networks. In particular, the incremental selection amplify-and-forward (ISAF) relaying scheme is a well-designed protocol under the condition that the source-to-destination (SD) link is static during the two transmission phases. However, from a practical viewpoint, the SD link is not static but varies with time, and thus the ISAF relaying scheme may not work well in the field. In this work, we first show that the outage performance of the ISAF relaying scheme may decrease when the SD link is not static during the two transmission phases. We then propose a modified version of the ISAF relaying scheme which overcomes such a limitation of the ISAF relaying scheme under time-varying environments. Finally, numerical and simulation results are provided to support our findings.

In this paper, we analyze a cooperative communication network with multi energy-harvesting and decode-and-forward relays in which the best relay is selected based on criteria such as Maximizing First-Hop Signal to Noise Ratios (SNRs) (MFHS protocol), Maximizing Second-Hop SNRs (MSHS protocol), and Maximizing End-to-End SNRs (MEES protocol). In these protocols, the relays apply power-splitting receivers to harvest energy from radio frequency signals emitted from a source. Thus, each received SNR in the second hop is a function of a direct relay-destination gain and an indirect source-relay gain. The system performance of the proposed protocols is evaluated via exact outage probability analyses and Monte Carlo simulations. For further comparisons, an energy-harvesting decode-and-forward scheme with randomly relay selection (RRS protocol) and an energy-harvesting amplify-and-forward scheme (BAF protocol) are investigated and discussed. The simulation results show that 1) the MEES protocol outperforms the MFHS and MSHS protocols, and the MFHS protocol is more efficient than the MSHS protocol in the low SNR regions; 2) the proposed protocols achieve the best performance at the specific optimal power splitting ratios for which the MEES protocol has a balanced ratio for energy harvesting and decoding capacity; and 3) the theoretical analyses agree well with the simulation results.

Amplify-and-forward (AF) relay multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems can achieve high data rate and high quality communications. On the other hand, it has to estimate all channels between the source-relay and relay-destination nodes in the destination node. In MIMO/OFDM systems, high time resolution carrier interferometry (HTRCI) has been proposed to achieve an accurate channel estimation (CE) with a small number of pilot signals. However, since it has many interferences, an accurate CE is not obtained and the system performance is degraded in AF relay MIMO/OFDM systems. Therefore, in this paper, we propose the complex HTRCI (C-HTRCI) pilot signal and the enhanced minimum mean square error (E-MMSE) equalization to achieve an accurate CE and to improve the system performance for AF relay MIMO/OFDM systems.

In this paper, we study a distributed time-reversal space-time block coded single-carrier (D-TR-STBC-SC) system for amplify-and-forward (AF) half-duplex relaying in frequency-selective Rayleigh fading channels. Under the imperfect channel estimation condition, we analyze the mean-square-error (MSE) performance of the optimal and channel-mismatched frequency domain minimum MSE (FD-MMSE) and least square (LS) equalization. Our analysis results show that, unlike the point-to-point communications, the channel-mismatched FD-MMSE equalization of D-TR-STBC-SC relaying network leads to the ceiling effect that the MSE increases as the signal-to-noise ratio (SNR) of relay-to-destination link increases. Decomposing the MSE, it is found that the primary cause of the ceiling effect is the source-to-destination link in the first time-slot, which makes the covariance matrix of noise vector ill-conditioned. In order to resolve the channel-mismatching problems in the equalization process, we develop optimum relay power control strategies by considering practical channel estimations, i.e., training-based LS and linear minimum MSE (LMMSE) channel estimations. It is shown that the optimum power control resolves the trade-off between MSE performance and relay power consumption, and improves the robustness against the channel-mismatching. Finally, we introduce a performance evaluation to demonstrate the performance of channel equalization combined with the proposed power controls in D-TR-STBC-SC relaying network.

Copper arc runners are fixed on silver electrical contacts. Break arcs are generated between the contacts in a DC resistive circuit. Circuit current when contacts are closed is 10A. Supply voltage is changed from 200V to 450V. The following results are shown. Cathode spots stay on the cathode surface but anode spots run on the runner when the supply voltage is 250V and over. In cases of the supply voltage is greater than 250V, the break arcs run on the runner when the arcs are successfully extinguished, and stays on the runner in cases of the failure of arc extinction. The arc lengths just before arc extinction with or without the runners are also investigated. The arc lengths are the same with or without the runners for each supply voltage.

This paper presents the design of an ad hoc two-way two-hop relay network using physical-layer network coding (PNC) in which multiple antennas are used at all nodes. In the considered network, the Alamouti's space-time block code (STBC) is used for transmission while linear detection is used for signal recovery. In order to facilitate linear estimation, we develop an equivalent multiuser STBC model for the proposed network and design the sum-and-difference matrix which allows convenient combination of the transmitted symbols from the end nodes. In addition, a simple relay selection method based on minimum mean square error (MSE) is proposed for performance improvement. Simulation results show that the proposed network achieves diversity order 2 while requiring only polynomial complexity. Moreover, it is possible to achieve significant bit error rate (BER) performance improvement when the proposed relay selection algorithm is used.

With the development of wireless technologies, wireless relay systems have become a popular topic. To design practical wireless relay systems, link adaptation is an important technique. Because there are both broadcast and multiple access channels in wireless relay systems, link adaptation is difficult to design and hence the optimal throughput is hard to achieve. In this study, a novel method is proposed to maximize the system throughput of wireless relay systems by utilizing the most popular link adaptation methods, adaptive modulation and coding (AMC) and hybrid automatic repeat request (HARQ). The proposed method utilizes the characteristics and operations of AMC and HARQ to adaptively adjust the thresholds for selecting modulation and coding scheme (MCS) to be used. Thus the system can keep tracking the optimal values of the thresholds. Therefore, the system throughput can be maximized. We set up simulations for different relay environment settings, such as different relay HARQ protocols, placements, and multiplexing schemes, to verify the capability of the proposed method. The simulation results show that, compared to the existing method, the proposed method indeed improves system throughput under a variety of relay settings and can be easily applied to different system platforms.

In this Letter, we investigate the outage performance of MIMO amplify-and-forward (AF) multihop relay networks with maximum ratio transmission/receiver antenna selection (MRT/RAS) over Nakagami-m fading channels in the presence of co-channel interference (CCI) or not. In particular, the lower bounds for the outage probability of MIMO AF multihop relay networks with/without CCI are derived, which provides an efficient means to evaluate the joint effects of key system parameters, such as the number of antennas, the interfering power, and the severity of channel fading. In addition, the asymptotic behavior of the outage probability is investigated, and the results reveal that the full diversity order can be achieved regardless of CCI. In addition, simulation results are provided to show the correctness of our derived analytical results.

This paper analyzes the performance of a mobile multihop relay (MMR) system which uses intermediate mobile relay stations (RSs) to increase service coverage area and capacity of a communication system. An analytical framework for an MMR system is introduced, and a scheme for allocating the optimum radio resources to an MMR system is presented. It is very challenging to develop an analytical framework for an MMR system because more than two wireless links should be considered in analyzing the performance of such a system. Here, the joint effect of a finite queue length and an adaptive modulation and coding (AMC) scheme in both a base station (BS) and an RS are considered. The traffic characteristics from BS to RS are analyzed, and a three-dimensional finite-state Markov chain (FSMC) is built for the RS which considers incoming traffic from the BS as well. The RS packet loss rate and the RS average throughput are also derived. Moreover, maximum throughput is achieved by optimizing the amount of radio resources to be allocated to the wireless link between a BS and an RS.

Generally, in order to extend the cell coverage of a mobile station, relay stations are used at a cell edge in a cellular system. But, received signals in a relay station of a cell edge have a large error because a neighboring cell transmits the signals for other users. Since the transmitted signals for other users are interference for received signals in the relay station of the cell edge, the relay station has a negative effect on the bit error ratio performance. The cell coverage can not be extended stably. In order to expand the cell coverage stably, the inter-cell interference has to cancel. Thus, in this paper, the technique that the inter-cell interference (ICI) is canceled by cooperative relays is proposed. Also, diversity gain is obtained by cooperative relays.

Information-theoretic limits of a multi-way relay channel with direct links (MWRC-DL), where multiple users exchange their messages through a relay terminal and direct links, are discussed in this paper. Under the assumption that a restricted encoder is employed at each user, an outer bound on the capacity region is derived first. Then, a decode-and-forward (DF) strategy is proposed and the corresponding rate region is characterized. The explicit outer bound and the achievable rate region for the Gaussian MWRC-DL are also derived. Numerical examples are provided to demonstrate the performance of the proposed DF strategy.

Multi-hop cooperative communication has been investigated in order to overcome disadvantages such as fading, obstruction and low power. In addition, with the goal of increasing access capacity, the orthogonal frequency division multiplexing (OFDM) modulation is being advanced as a solution. In this paper, we propose the approach of relay ordering in a Decode-and-Forward OFDM scheme. Combining techniques such as maximal ratio combining and selection combining are employed at receivers and approximate outage capacity probabilities are derived for evaluating system performance over frequency selective Rayleigh fading channels. Final, the expressions are validated by Monte-Carlo simulations, and are used to compare with the same scheme based relay selection.

To improve the outage performance of a wireless body area network (BAN), exploitation of the diversity in the channel obtained by letting different nodes cooperate and relay signals for each other is an attractive solution. We carry out multi-link channel measurements and modeling for all realistic locations of the on-body sensor nodes and for three different motion scenarios in a typical office environment to develop equivalent channel model for simple and practical cooperative transmission schemes. Using the developed model the performance of the transmission schemes are evaluated and compared. Incremental decode and forward relaying is found to be consistently better than the other schemes with gains of up to 16dB at 10% outage probability, and an average gain of more than 5.9dB for any location of the coordinator node. The best location of the coordinator node based on the performance is also determined. Such insights will be very useful in designing BANs.