Analytical Modeling of Ad Hoc Networks that Utilize Space-Time Coding

This paper presents the first analytical model for ad hoc networks equipped with multiple-input multiple-output (MIMO) radios using space-time coding (STC) that considers the impact of the underlying radio-based topology on network performance. In particular, we consider the space-time block coding (STBC) technique known as the "Alamouti scheme." We derive the effective signal-to-interference-plus-noise density ratio (SINR) of the Alamouti scheme under multiple access interference (MAI), and we propose the moment generating function (MGF) method to derive closed-form expressions for its symbol error probability under different modulation schemes when fading paths are independent but not necessarily identically distributed. The impact of the Alamouti scheme on IEEE 802.11 ad hoc networks is studied by introducing a new analytical model for the IEEE 802.11 DCF MAC. The model we introduce takes into account the impact of errors in both control and data frames, the carrier-sensing activity, and the finite-retry limit of frame retransmissions. Both PHY- and MAC-layer analytical models are incorporated into our previously-designed, general analytical model for ad hoc networks based on interference matrices. We apply the Alamouti scheme to different antenna system configurations and compare their performance with respect to the basic single-input-single-output (SISO) IEEE 802.11 DCF MAC. I

Throughput Analysis of Large Networks:Spatial Diversity, Beamforming Gain, and Transmission Modes

While wired infrastructure constitutes the backbone of most wireless networks, wireless systems appeal the most to the dynamic and rapidly evolving requirements of today's communication systems because of their ease of deployment and mobility, not to mention the high cost of building a wired infrastructure. This led to an increased interest in the so called wireless ad hoc networks formed of a group of users, known as nodes, capable of communicating with each other through a shared wireless channel. Needless to say, these nodes are asked to use the shared wireless medium in the most efficient fashion, which is not an easy task given the absence of wired backbone. This requires a profound understanding of the wireless medium to establish a decentralized cooperation scheme, if needed, that best utilizes the resources available in the wireless channel. A significant part of this thesis focuses on the properties of the shared wireless channel, whereby we are interested in studying the spatial diversity and the beamforming capabilities in large wireless networks which are crucial in analyzing the throughput of ad hoc networks. In this thesis, we mainly focus on the problem of broadcasting information in the most efficient manner in a large two-dimensional ad hoc wireless network at low SNR and under line-of-sight propagation. A new communication scheme, which we call multi-stage back-and-forth beamforming, is proposed, where source nodes first broadcast their data to the entire network, despite the lack of sufficient available power. The signal's power is then reinforced via successive back-and-forth beamforming transmissions between different groups of nodes in the network, so that all nodes are able to decode the transmitted information at the end. This scheme is shown to achieve asymptotically the broadcast capacity of the network, which is expressed in terms of the largest singular value of the matrix of fading coefficients between the nodes in the network. A detailed mathematical analysis is then presented to evaluate the asymptotic behavior of this largest singular value. We further characterize the maximum achievable broadcast rate under different sparsity regimes. Our result shows that this rate depends negatively on the sparsity of the network. This is to be put in contrast with the number of degrees of freedom available in the network, which have been shown previously to increase with the sparsity of the network. In this context, we further characterize the degrees of freedom versus beamforming gain tradeoff, which reveals that high beamforming gains can only be obtained at the expense of reduced spatial degrees of freedom. Another important factor that impacts the throughput in wireless networks is the transmit/receive capability of the transceiver at the nodes. Traditionally, wireless radios are half-duplex. However, building on self-interference cancellation techniques, full-duplex radios have emerged as a viable paradigm over the recent years. In the last part of this thesis, we ask the fundamental question: how much can full-duplex help? Intuitively, one may expect that full-duplex radios can at most double the capacity of wireless networks, since they enable nodes to transmit and receive at the same time. However, we show that the capacity gain can indeed be larger than a factor of 2; in particular, we construct a specific instance of a wireless network where the the full-duplex capacity is triple the half-duplex capacity

Implementação e simulação de redes AD HOC de comunicações sem fio com múltiplas antenas

Monografia (graduação)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Elétrica, 2012.Este trabalho estuda o desempenho de redes sem o ad hoc com nós habilitados com múltiplas antenas (MIMO) utilizando tecnologias-chave para permitir altas taxas de transmissão e/ou maior robustez à transmissão. Para tanto, busca-se avaliar as técnicas de transmissão do sistema Vertical Bell Labs Layered Space-Time (V-BLAST) (o qual permite a multiplexagem espacial) e do esquema de Alamouti (que explora a diversidade de transmissão). Para realizar este estudo, propomos uma implementação de um sistema MIMO que utiliza estas técnicas no simulador de rede NS-3. Esta implementa ção é baseada em modelos analíticos e expressões matemáticas resultantes das abstrações da decodi cação do sinal na recepção na camada física e consiste em simples modi cações na norma IEEE 802.11b que incluem não só os aspectos do cálculo da SINR e da BER, mas também como o mecanismo de detecção de portadora é realizado e como a negociação de 4 vias é con gurada. Os resultados de simulação para redes ad hoc IEEE 802.11 habilitadas com o esquema de Alamouti, sob canais com desvanecimento Rice, indicam ganhos signi cativos na vazão média da rede sobre sistemas de única antena, especialmente sob forte desvanecimento de múltiplos percursos. Além disso, os resultados para redes habilitadas com o sistema V-BLAST, sob canais com desvanecimento Rayleigh, mostram que, dependendo da con guração de antenas, ganhos signi cativos de vazão quase lineares podem ser alcançados, se os ganhos de diversidade forem explorados. Com base nestes resultados, é proposto um sistema MIMO híbrido com protocolo MAC adaptativo que comuta a técnica de transmissão de acordo com as condições do canal, a m de prover melhores ganhos de desempenho explorando as vantagens da diversidade e da multiplexagem. Este protocolo segue o paradigma do projeto com camadas interrelacionadas ( cross-layer design ) para realizar a comutação da técnica segundo informações trocadas pelas camadas física e MAC. Estudos são realizados para avaliar os melhores limiares de comutação a m de otimizar o desempenho. Os resultados de simulação mostram ganhos de vazão de rede maiores que o desempenho de redes utilizando cada técnica individualmente em toda a rede.This work studies the performance of wireless ad hoc networks when nodes are equipped with multiple antennas (MIMO) utilizing key technologies that allow higher data rates and/or robustness to transmission. To do so, we seek to evaluate the transmission techniques known as Vertical Bell Labs Layered Space-Time (V-BLAST) system (which allows spatial-division multiplexing) and the Alamouti scheme (which exploits transmit diversity). To accomplish this task, we propose an implementation of these MIMO techniques in the network simulator NS-3. This implementation is based on analytical models and expressions obtained from signal decoding abstractions from the physical layer reception, and consists in simple modi cations to the IEEE 802.11b standard. These modi cations include not only SINR and BER computation aspects, but also how clear channel assessment is performed and how the four-way handshake mechanism is set. Simulation results for IEEE 802.11 ad hoc networks enabled with the Alamouti scheme, under Rice fading channel, indicate signi cant gains in network throughput over single antenna systems, especially under strong multipath fading. Furthermore, simulation results for networks enabled with V-BLAST show that, depending on the antenna con guration, signi cant quasi-linear network throughput gains can be achieved, especially if the diversity gains are also exploited. Based on these results, we propose a MIMO hybrid system with adaptive MAC protocol that selects the transmission technique depending on channel conditions, in order to provide better performance gains exploiting diversity and multiplexing gains. This protocol follows the cross-layer design paradigm to choose the best technique according to information exchanged by the physical and MAC layers. Studies are carried out to evaluate the best selection thresholds in order to optimize throughput. Simulation results demonstrate higher network throughput gains compared to the performance of networks that utilize each technique individually in all network nodes (or elements)