11 research outputs found
Towards Scalable Design of Future Wireless Networks
Wireless operators face an ever-growing challenge to meet the throughput and processing requirements of billions of devices that are getting connected. In current wireless networks, such as LTE and WiFi, these requirements are addressed by provisioning more resources: spectrum, transmitters, and baseband processors. However, this simple add-on approach to scale system performance is expensive and often results in resource underutilization. What are, then, the ways to efficiently scale the throughput and operational efficiency of these wireless networks? To answer this question, this thesis explores several potential designs: utilizing unlicensed spectrum to augment the bandwidth of a licensed network; coordinating transmitters to increase system throughput; and finally, centralizing wireless processing to reduce computing costs.
First, we propose a solution that allows LTE, a licensed wireless standard, to co-exist with WiFi in the unlicensed spectrum. The proposed solution bridges the incompatibility between the fixed access of LTE, and the random access of WiFi, through channel reservation. It achieves a fair LTE-WiFi co-existence despite the transmission gaps and unequal frame durations. Second, we consider a system where different MIMO transmitters coordinate to transmit data of multiple users.
We present an adaptive design of the channel feedback protocol that mitigates interference resulting from the imperfect channel information. Finally, we consider a Cloud-RAN architecture where a datacenter or a cloud resource processes wireless frames. We introduce a tree-based design for real-time transport of baseband samples and provide its end-to-end schedulability
and capacity analysis. We also present a processing framework that combines real-time scheduling with fine-grained parallelism. The framework reduces processing times by migrating parallelizable tasks to idle compute resources, and thus, decreases the processing deadline-misses at no additional cost.
We implement and evaluate the above solutions using software-radio platforms and off-the-shelf radios, and confirm their applicability in real-world settings.PhDElectrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133358/1/gkchai_1.pd
A Critical Review of Physical Layer Security in Wireless Networking
Wireless networking has kept evolving with additional features and increasing capacity. Meanwhile, inherent characteristics of wireless networking make it more vulnerable than wired networks. In this thesis we present an extensive and comprehensive review of physical layer security in wireless networking. Different from cryptography, physical layer security, emerging from the information theoretic assessment of secrecy, could leverage the properties of wireless channel for security purpose, by either enabling secret communication without the need of keys, or facilitating the key agreement process. Hence we categorize existing literature into two main branches, namely keyless security and key-based security. We elaborate the evolution of this area from the early theoretic works on the wiretap channel, to its generalizations to more complicated scenarios including multiple-user, multiple-access and multiple-antenna systems, and introduce not only theoretical results but practical implementations. We critically and systematically examine the existing knowledge by analyzing the fundamental mechanics for each approach. Hence we are able to highlight advantages and limitations of proposed techniques, as well their interrelations, and bring insights into future developments of this area
Rethinking Wireless: Building Next-Generation Networks
We face a growing challenge to the design, deployment and management of wireless networks that largely stems from the need to operate in an increasingly spectrum-sparse environment, the need for greater concurrency among devices and the need for greater coordination between heterogeneous wireless protocols. Unfortunately, our current wireless networks lack interoperability, are deployed with fixed functions, and omit easy programmability and extensibility from their key design requirements.
In this dissertation, we study the design of next-generation wireless networks and analyze the individual components required to build such an infrastructure. Re-designing a wireless architecture must be undertaken carefully to balance new and coordinated multipoint (CoMP) techniques with the backward compatibility necessary to support the large number of existing devices. These next-generation wireless networks will be predominantly software-defined and will have three components: (a) a wireless component that consists of software-defined radio resource units (RRUs) or access points (APs); (b) a software-defined backhaul control plane that manages the transfer of RF data between the RRUs and the centralized processing resource; and (c) a centralized datacenter/cloud compute resource that processes RF signal data from all attached RRUs. The dissertation addresses the following four key problems in next-generation networks: (1) Making Existing Wireless Devices Spectrum-Agile,
(2) Cooperative Compression of the Wireless Backhaul, (3) Spectrum Coordination and (4) Spectrum Coordination.PhDComputer Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102341/1/zontar_1.pd
Information Theoretic Limits for Wireless Information Transfer Between Finite Spatial Regions
Since the first multiple-input multiple-output (MIMO) experiments
performed at Bell Laboratories in the late 1990’s, it was clear
that wireless communication systems can achieve improved
performances using multiple antennas simultaneously during
transmission and reception. Theoretically, the capacity of MIMO
systems scales linearly with the number of antennas in favorable
propagation conditions. However, the capacity is significantly
reduced when the antennas are collocated.
A generalized paradigm for MIMO systems, spatially distributed
MIMO systems, is proposed as a solution. Spatially distributed
MIMO systems transmit information from a spatial region to
another with each region occupying a large number of antennas.
Hence, for a given constraint on the size of the spatial regions,
evaluating the information theoretic performance limits for
information transfer between regions has been a central topic of
research in wireless communications. This thesis addresses this
problem from a theoretical point of view.
Our approach is to utilize the modal decomposition of the
classical wave equation to represent the spatially distributed
MIMO systems. This modal analysis is particularly useful as it
advocates a shift of the “large wireless networks” research
agenda from seeking “universal” performance limits to seeking
a multi-parameter family of performance limits, where the key
parameters, space, time and frequency are interrelated. However,
traditional performance bounds on spatially distributed MIMO
systems fail to depict the interrelation among space, time and
frequency.
Several outcomes resulting from this thesis are: i) estimation of
an upper bound to degrees of freedom of broadband signals
observed over finite spatial and temporal windows, ii) derivation
of the amount of information that can be captured by a finite
spatial region over a finite bandwidth, iii) a new framework to
illustrate the relationship between Shannon’s capacity and the
spatial channels, iv) a tractable model to determine the
information capacity between spatial regions for narrowband
transmissions. Hence, our proposed approach provides a
generalized theoretical framework to characterize realistic MIMO
and spatially distributed MIMO systems at different frequency
bands in both narrowband and broadband conditions