On power minimisation and SNR maximisation of distributed beamforming in cooperative communication systems

A comparison of relay power minimisation subject to received signal-to-noise ratio (SNR) at the receiver and SNR maximisation subject to the total transmitted power of relays for a typical wireless network with distributed beamforming is presented. It is desirable to maximise receiver quality-of-service (QoS) and also to minimise the cost of transmission in terms of power. Hence, these two optimisation problems are very common and have been addressed separately in the literature. It is shown that SNR maximisation subject to power constraint and power minimisation subject to SNR constraint yield the same results for a typical wireless network. It proves that either one of the optimisation approaches is sufficient.

Power allocation strategies across N orthogonal channels for single-relay networks at both source and relay

We consider a wireless relay network with one source, one relay and one destination, where communications between nodes are preformed over N orthogonal channels. This, for example, is the case when orthogonal frequency division multiplexing is employed for data communications. Since the power available at the source and relay is limited, we study optimal power allocation strategies at the source and relay in order to maximize the overall source-destination capacity. Depending on the availability of the channel state information at both the source and relay or only at the relay, power allocation is performed at both the source and relay or only at the relay. Considering different setups for the problem, various optimization problems are formulated and solved. Some properties of the optimal solution are also proved.

Decode-and-forward relay beamforming for secrecy with finite-alphabet input

In this letter, we compute the secrecy rate of decode-and-forward (DF) relay beamforming with finite input alphabet of size M. Source and relays operate under a total power constraint. First, we observe that the secrecy rate with finite-alphabet input can go to zero as the total power increases, when we use the source power and the relay weights obtained assuming Gaussian input. This is because the capacity of an eavesdropper can approach the finite-alphabet capacity of 1/2 log(2) M with increasing total power, due to the inability to completely null in the direction of the eavesdropper. We then propose a transmit power control scheme where the optimum source power and relay weights are obtained by carrying out transmit power (source power plus relay power) control on DF with Gaussian input using semi-definite programming, and then obtaining the corresponding source power and relay weights which maximize the secrecy rate for DF with finite-alphabet input. The proposed power control scheme is shown to achieve increasing secrecy rates with increasing total power with a saturation behavior at high total powers.

Channel estimation for two-way MIMO relay systems in frequency-selective fading environments

© 2002-2012 IEEE. In this paper, we investigate the channel estimation problem for two-way multiple-input multiple-output (MIMO) relay communication systems in frequency-selective fading environments. We apply the method of superimposed channel training to estimate the individual channel state information (CSI) of the first-hop and second-hop links for two-way MIMO relay systems with frequency-selective fading channels. In this algorithm, a relay training sequence is superimposed on the received signals at the relay node to assist the estimation of the second-hop channel matrices. The optimal structure of the source and relay training sequences is derived to minimize the mean-squared error (MSE) of channel estimation. Moreover, the optimal power allocation between the source and relay training sequences is derived to improve the performance of channel estimation. Numerical examples are shown to demonstrate the performance of the proposed superimposed channel training algorithm for two-way MIMO relay systems in frequency-selective fading environments.

Optimal designs of collaborative relay-assisted multiuser beamforming for cellular systems

With careful calculation of signal forwarding weights, relay nodes can be used to work collaboratively to enhance downlink transmission performance by forming a virtual multiple-input multiple-output beamforming system. Although collaborative relay beamforming schemes for single user have been widely investigated for cellular systems in previous literatures, there are few studies on the relay beamforming for multiusers. In this paper, we study the collaborative downlink signal transmission with multiple amplify-and-forward relay nodes for multiusers in cellular systems. We propose two new algorithms to determine the beamforming weights with the same objective of minimizing power consumption of the relay nodes. In the first algorithm, we aim to guarantee the received signal-to-noise ratio at multiusers for the relay beamforming with orthogonal channels. We prove that the solution obtained by a semidefinite relaxation technology is optimal. In the second algorithm, we propose an iterative algorithm that jointly selects the base station antennas and optimizes the relay beamforming weights to reach the target signal-to-interference-and-noise ratio at multiusers with nonorthogonal channels. Numerical results validate our theoretical analysis and demonstrate that the proposed optimal schemes can effectively reduce the relay power consumption compared with several other beamforming approaches. © 2012 John Wiley & Sons, Ltd.

Analysis of distributed beamforming in cooperative communications network with phase shifter based smart antenna nodes

This research study investigates the application of phase shifter-based smart antenna system in distributed beamforming. It examines the way to optimise the transmit power by jointly maximising the directivity of the array antennas and the weight vector for distributed beamforming. This research study concludes that maximising directivity can lead to better transmit power minimisation compared to maximising field intensity. This study also concludes that signal to noise power ratio maximisation subject to a power constraint and power minimisation subject to a signal to noise power ratio constraint yield the same results.

Simulation environment of distance protection with ATP and foreign models

This paper presents an interactive simulation environment for distance protection, developed with ATP and foreign models based on ANSI C. Files in COMTRADE format are possible to generate after ATP simulation. These files can be used to calibrate real relays. Also, the performance of relay algorithms with real oscillography events is possible to assess by using the ATP option for POSTPROCESS PLOT FILE (PPF). The main purpose of the work is to develop a tool to allow the analysis of diverse fault cases and to perform coordination studies, as well as, to allow the analysis of the relay's performance in the face of a real event. © 2011 IEEE.

Ajuste e ensaio de sistemas de proteção de geradores síncronos.

Os sistemas de proteção dos elementos da rede elétrica desempenham um papel de fundamental importância na segurança e confiabilidade dos sistemas de potência. A não atuação ou a atuação incorreta dos relés de proteção durante uma falta localizada em um componente da rede pode transformar-se em um evento sistêmico de grandes proporções (blecaute). Esses eventos trazem riscos e elevados prejuízos econômicos à sociedade. A proteção dos geradores síncronos, apesar do alto custo e complexidade deste tipo de equipamento, não recebe a mesma atenção na literatura que a dedicada à proteção de outros elementos da rede, como, por exemplo, a das linhas de transmissão. Isso decorre do menor número de geradores existentes na rede e também da ideia que as faltas neste tipo de equipamento são menos frequentes. Este trabalho aborda os principais aspectos envolvidos com o projeto de um sistema de proteção para geradores síncronos de grande porte. Incialmente, discutese os principais conceitos associados com os geradores, de interesse para a tarefa de proteção. Particular atenção é dedicada às formas de aterramento e aos critérios adotados para projeto do resistor de aterramento utilizado nesse equipamento. Em seguida, apresentam-se as principais funções de proteção aplicáveis aos geradores, particularmente aquelas voltadas para a detecção de faltas nos enrolamentos do estator. Discute-se também os critérios de ajustes dos parâmetros dessas funções. Descreve-se o uso de uma plataforma laboratorial, baseada em simulador de tempo real (RTDS), para ensaio e análise do sistema de proteção visando validar seu correto desempenho frente às possíveis condições operativas que podem ser encontradas em campo. Finalmente, utilizando os conceitos desenvolvidos ao longo do trabalho, desenvolve-se um estudo de caso, onde é realizado o projeto e implementação do sistema de proteção dos geradores de uma usina hidrelétrica hipotética. Para avaliar e analisar o desempenho do sistema de proteção dessa rede exemplo, parametrizou-se o IED G60 (GE) e realizou-se inúmeras simulações na plataforma de testes proposta.

Distance Relay Co-ordination Using Support Vector Machines in Power Transmission System

In this paper, a new approach to enhance the transmission system distance relay co-ordination is presented. The approach depends on the apparent impedance loci seen by the distance relay during all possible disturbances. In a distance relay, the impedance loci seen at the relay location is obtained by extensive transient stability studies. Support vector machines (SVMs), a class of patterns classifiers are used in discriminating zone settings (zone-1, zone-2 and zone-3) using the signals to be used by the relay. Studies on a sample 9-bus are presented for illustrating the proposed scheme.

Dead-time minimisation using active voltage control gate drive for high-power IGBT converters

High-power converters usually need longer dead-times than their lower-power counterparts and a lower switching frequency. Also due to the complicated assembly layout and severe variations in parasitics, in practice the conventional dead-time specific adjustment or compensation for high-power converters is less effective, and usually this process is time-consuming and bespoke. For general applications, minimising or eliminating dead-time in the gate drive technology is a desirable solution. With the growing acceptance of power electronics building blocks (PEBB) and intelligent power modules (IPM), gate drives with intelligent functions are in demand. Smart functions including dead time elimination/minimisation can improve modularity, flexibility and reliability. In this paper, a dead-time minimisation using Active Voltage Control (AVC) gate drive is presented. © 2012 IEEE.

Power Allocation Strategies across N Orthogonal Channels at Both Source and Relay

A wireless relay network with one source, one relay and one destination is considered, where nodes communicate via N orthogonal channels. We develop optimal power allocation strategies at both the source and relay for maximizing the overall source-destination capacity under individual power constraints at the source and relay. Some properties of the optimal solution are studied. © 2012 IEEE.

Secure Transmission with Optimal Power Allocation in Untrusted Relay Networks

We consider the problem of secure transmission in two-hop amplify-and-forward untrusted relay networks. We analyze the ergodic secrecy capacity (ESC) and present compact expressions for the ESC in the high signal-to-noise ratio regime. We also examine the impact of large scale antenna arrays at either the source or the destination. For large antenna arrays at the source, we confirm that the ESC is solely determined by the channel between the relay and the destination. For very large antenna arrays at the destination, we confirm that the ESC is solely determined by the channel between the source and the relay.