Browsing by Subject "capacity"
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Item Capacity and scale-free dynamics of evolving wireless networks(Texas A&M University, 2005-02-17) Iyer, Bharat VishwanathanMany large-scale random graphs (e.g., the Internet) exhibit complex topology, nonhomogeneous spatial node distribution, and preferential attachment of new nodes. Current topology models for ad-hoc networks mostly consider a uniform spatial distribution of nodes and do not capture the dynamics of evolving, real-world graphs, in which nodes "gravitate" toward popular locations and self-organize into non-uniform clusters. In this thesis, we first investigate two constraints on scalability of ad-hoc networks ? network reliability and node capacity. Unlike other studies, we analyze network resilience to node and link failure with an emphasis on the growth (i.e., evolution) dynamics of the entire system. Along the way, we also study important graph-theoretic properties of ad-hoc networks (including the clustering coefficient and the expected path length) and strengthen our generic understanding of these systems. Finally, recognizing that under existing uniform models future ad-hoc networks cannot scale beyond trivial sizes, we argue that ad-hoc networks should be modeled from an evolution standpoint, which takes into account the well-known "clustering" phenomena observed in all real-world graphs. This model is likely to describe how future ad-hoc networks will self-organize since it is well documented that information content distribution among end-users (as well as among spatial locations) is non-uniform (often heavy-tailed). Results show that node capacity in the proposed evolution model scales to larger network sizes than in traditional approaches, which suggest that non-uniformly clustered, self-organizing, very large-scale ad-hoc networks may become feasible in the future.Item Capacity dynamics of feed-forward, flow-matching networks exposed to random disruptions(Texas A&M University, 2006-10-30) Savachkin, AliakseiWhile lean manufacturing has greatly improved the efficiency of production operations, it has left US enterprises in an increasingly risky environment. Causes of manufacturing disruptions continue to multiply, and today, seemingly minor disruptions can cause cascading sequences of capacity losses. Historically, enterprises have lacked viable tools for addressing operational volatility. As a result, each year US companies forfeit billions of dollars to unpredictable capacity disruptions and insurance premiums. In this dissertation we develop a number of stochastic models that capture the dynamics of capacity disruptions in complex multi-tier flow-matching feed-forward networks (FFN). In particular, we relax basic structural assumptions of FFN, introduce random propagation times, study the impact of inventory buffers on propagation times, and make initial efforts to model random network topology. These stochastic models are central to future methodologies supporting strategic risk management and enterprise network design.Item Capacity Results for Wireless Cooperative Communications with Relay Conferencing(2012-10-19) Huang, ChuanIn this dissertation we consider cooperative communication systems with relay conferencing, where the relays own the capabilities to talk to their counterparts via either wired or wireless out-of-band links. In particular, we focus on the design of conferencing protocols incorporating the half-duplex relaying operations, and study the corresponding capacity upper and lower bounds for some typical channels and networks models, including the diamond relay channels (one source-destination pairs and two relays), large relay networks (one source-destination pairs and N relays), and interference relay channels (two source-destination pairs and two relays). First, for the diamond relay channels, we consider two different relaying schemes, i.e., simultaneous relaying (for which the two relays transmit and receive in the same time slot) and alternative relaying (for which the two relays exchange their transmit and receive modes alternatively over time), for which we obtain the respective achievable rates by using the decode-and-forward (DF), compress-and-forward (CF), and amplify-and-forward (AF) relaying schemes with DF and AF adopted the conferencing schemes. Moreover, we prove some capacity results under some special conditions. Second, we consider the large relay networks, and propose a "p-portion" conferencing scheme, where each relay can talk to the other "p-portion" of the relays. We obtain the DF and AF achievable rates by using the AF conferencing scheme. It is proved that relay conferencing increases the throughput scaling order of the DF relaying scheme from O(log(log(N ))) for the case without conferencing to O(log(N )); for the AF relaying scheme, it achieves the capacity upper bound under some conditions. Finally, we consider the two-hop interference relay channels, and obtain the AF achievable rates by adopting the AF conferencing scheme and two different decoding schemes at the destination, i.e., single-user decoding and joint decoding. For the derived joint source power allocation and relay combining problem, we develop some efficient iterative algorithms to compute the AF achievable rate regions. Moreover, we compare the achievable degree-of-freedom (DoF) performance of these two decoding schemes, and show that single-user decoding with interference cancellation at the relays is optimal.Item Concatenated codes for the multiple-input multiple-output quasi-static fading channel(Texas A&M University, 2005-02-17) Gulati, VivekThe use of multiple antennas at the transmitter and/or the receiver promises greatly increased capacity. This can be useful to meet the ever growing demand of wireless connectivity, provided we can find techniques to efficiently exploit the advantages of the Multiple-Input Multiple-Output (MIMO) system. This work explores the MIMO system in a flat quasi-static fading scenario. Such a channel occurs, for example, in packet data systems, where the channel fade is constant for the duration of a codeword and changes independently from one transmission to another. We first show why it is hard to compute the true constrained modulation outage capacity. As an alternative, we present achievable lower bounds to this capacity based on existing space-time codes. The bounds we compute are the fundamental limits to the performance of these space-time codes under maximum-likelihood decoding, optimal outer codes and asymptotically long lengths. These bounds also indicate that MIMO systems have different behavior under Gaussian signaling (unconstrained input) and under the finite alphabet setting. Our results naturally suggest the use of concatenated codes to approach near-capacity performance. However, we show that a system utilizing an iterative decoder has a fundamental limit ? it cannot be universal and therefore it cannot perform arbitrarily close to its outage limit. Next, we propose two different transceiver structures that have good performance. The first structure is based on a novel BCJR-decision feedback decoder which results in performance within a dB of the outage limit. The second structure is based on recursive realizations of space-time trellis codes and uses iterative decoding at the receiver. This recursive structure has impressive performance even when the channel has time diversity. Thus, it forms the basis of a very flexible and robust MIMO transceiver structure.Item Examining Local Jurisdictions' Capacity and Commitment For Hazard Mitigation Policies and Strategies along the Texas Coast(2012-07-16) Husein, RahmawatiThere have been studies on the role of land use planning and development regulations on hazard mitigation and the importance of including these in effective mitigation planning initiatives. However, little empirical research has examined how the local capacity and commitment affect the adoption and implementation of land use and development regulations to mitigate any type of hazards in the coastal areas. This study investigates hazard mitigation policies and practices at municipal and county level in the Texas coastal area and examines the influence of capacity and commitment for the adoption and implementation of these hazard mitigation strategies and actions. The data utilized in this survey were collected as part of a web-based survey. Responses were solicited from 267 local jurisdictions that consist of 226 cities and 41 counties. The survey was targeted to leading planner, or mayor/city manager and county judges. In total 124 responses were obtained, yielding an overall response rate of 46%. Study results show that local jurisdictions are employing a very limited a set of land use and development regulations that the literature has identified as important for hazard mitigation. There are considerable differences between municipalities and counties in the implementation of those policies. Municipalities tend to put more effort in employing building standards and development regulations, whereas counties more extensively employ information dissemination and private-public sector initiatives. In addition, statistical models are developed to assess the influence of local capacity and commitment on the adoption and implementation of hazard mitigation policies and strategies. Other factors such as jurisdiction type and location, hazard experience and exposure as well as population characteristics, are also examined in multivariate models. Results suggest that capacity and commitment of local jurisdictions have significant effects on the adoption and implementation hazard mitigation policies and strategies. Additionally, factor such as floodplain area, jurisdiction type municipality, and hazard experience have strong associated with implementation of hazard mitigation policies and strategies.Item LDPC code-based bandwidth efficient coding schemes for wireless communications(2009-06-02) Sankar, HariThis dissertation deals with the design of bandwidth-efficient coding schemes with Low-Density Parity-Check (LDPC) for reliable wireless communications. Code design for wireless channels roughly falls into three categories: (1) when channel state information (CSI) is known only to the receiver (2) more practical case of partial CSI at the receiver when the channel has to be estimated (3) when CSI is known to the receiver as well as the transmitter. We consider coding schemes for all the above categories. For the first scenario, we describe a bandwidth efficient scheme which uses highorder constellations such as QAM over both AWGN as well as fading channels. We propose a simple design with LDPC codes which combines the good properties of Multi-level Coding (MLC) and bit-interleaved coded-modulation (BICM) schemes. Through simulations, we show that the proposed scheme performs better than MLC for short-medium lengths on AWGN and block-fading channels. For the first case, we also characterize the rate-diversity tradeoff of MIMO-OFDM and SISO-OFDM systems. We design optimal coding schemes which achieve this tradeoff when transmission is from a constrained constellation. Through simulations, we show that with a sub-optimal iterative decoder, the performance of this coding scheme is very close to the optimal limit for MIMO (flat quasi-static fading), MIMO-OFDM and SISO OFDM systems. For the second case, we design non-systematic Irregular Repeat Accumulate (IRA) codes, which are a special class of LDPC codes, for Inter-Symbol Interference (ISI) fading channels when CSI is estimated at the receiver. We use Orthogonal Frequency Division Multiplexing (OFDM) to convert the ISI fading channel into parallel flat fading subchannels. We use a simple receiver structure that performs iterative channel estimation and decoding and use non-systematic IRA codes that are optimized for this receiver. This combination is shown to perform very close to a receiver with perfect CSI and is also shown to be robust to change in the number of channel taps and Doppler. For the third case, we look at bandwidth efficient schemes for fading channels that perform close to capacity when the channel state information is known at the transmitter as well as the receiver. Schemes that achieve capacity with a Gaussian codebook for the above system are already known but not for constrained constellations. We derive the near-optimum scheme to achieve capacity with constrained constellations and then propose coding schemes which perform close to capacity. Through linear transformations, a MIMO system can be converted into non-interfering parallel subchannels and we further extend the proposed coding schemes to the MIMO case too.