54 research outputs found
Beam-searching and Transmission Scheduling in Millimeter Wave Communications
Millimeter wave (mmW) wireless networks are capable to support multi-gigabit
data rates, by using directional communications with narrow beams. However,
existing mmW communications standards are hindered by two problems: deafness
and single link scheduling. The deafness problem, that is, a misalignment
between transmitter and receiver beams, demands a time consuming beam-searching
operation, which leads to an alignment-throughput tradeoff. Moreover, the
existing mmW standards schedule a single link in each time slot and hence do
not fully exploit the potential of mmW communications, where directional
communications allow multiple concurrent transmissions. These two problems are
addressed in this paper, where a joint beamwidth selection and power allocation
problem is formulated by an optimization problem for short range mmW networks
with the objective of maximizing effective network throughput. This
optimization problem allows establishing the fundamental alignment-throughput
tradeoff, however it is computationally complex and requires exact knowledge of
network topology, which may not be available in practice. Therefore, two
standard-compliant approximation solution algorithms are developed, which rely
on underestimation and overestimation of interference. The first one exploits
directionality to maximize the reuse of available spectrum and thereby
increases the network throughput, while imposing almost no computational
complexity. The second one is a more conservative approach that protects all
active links from harmful interference, yet enhances the network throughput by
100% compared to the existing standards. Extensive performance analysis
provides useful insights on the directionality level and the number of
concurrent transmissions that should be pursued. Interestingly, extremely
narrow beams are in general not optimal.Comment: 5 figures, 7 pages, accepted in ICC 201
Medium Access Control and Routing Protocols Design for 5G
In future wireless systems, such as 5G and beyond, the current dominating human-centric communication systems will be complemented by a tremendous increase in the number of smart devices, equipped with radio devices, possibly sensors, and uniquely addressable. This will result in explosion of wireless traffic volume, and consequently exponential growth in demand of radio spectrum. There are different engineering techniques for resolving the cost and scarcity of radio spectrum such as coexistence of diverse devices on the same pool of radio resources, spectrum aggregations, adoption of mmWave bands with huge spectrum, etc.
The aim of this thesis is to investigate Medium Access Control (MAC) and routing protocols for 5G and beyond radio networks. Two scenarios are addressed: heterogeneous scenario where scheduled and uncoordinated users coexist, and a scenario where drones are used for monitoring a given area. In the heterogeneous scenario scheduled users are synchronised with the Base Station (BS) and rely on centralised resource scheduler for assignment of time slots, while the uncoordinated users are asynchronous with each other and the BS and rely unslotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) for channel access. First, we address a single-hop network with advanced scheduling algorithm design and packet length adaptation schemes design. Second, we address a multi-hop network with novel routing protocol for enhancing performance of the scheduled users in terms of throughput, and coexistence of all network users.
In the drone-based scenario, new routing protocols are designed to address the problems of Wireless Mesh Networks with monitoring drones. In particular, a novel optimised Hybrid Wireless Mesh Protocol (O-HWMP) for a quick and efficient discovery of paths is designed, and a capacity achieving routing and scheduling algorithm, called backpressure, investigated. To improve on the long-end-to-end delays of classical backpressure, a modified backpressure algorithm is proposed and evaluated
Distributed SIR-Aware Opportunistic Access Control for D2D Underlaid Cellular Networks
In this paper, we propose a distributed interference and channel-aware
opportunistic access control technique for D2D underlaid cellular networks, in
which each potential D2D link is active whenever its estimated
signal-to-interference ratio (SIR) is above a predetermined threshold so as to
maximize the D2D area spectral efficiency. The objective of our SIR-aware
opportunistic access scheme is to provide sufficient coverage probability and
to increase the aggregate rate of D2D links by harnessing interference caused
by dense underlaid D2D users using an adaptive decision activation threshold.
We determine the optimum D2D activation probability and threshold, building on
analytical expressions for the coverage probabilities and area spectral
efficiency of D2D links derived using stochastic geometry. Specifically, we
provide two expressions for the optimal SIR threshold, which can be applied in
a decentralized way on each D2D link, so as to maximize the D2D area spectral
efficiency derived using the unconditional and conditional D2D success
probability respectively. Simulation results in different network settings show
the performance gains of both SIR-aware threshold scheduling methods in terms
of D2D link coverage probability, area spectral efficiency, and average sum
rate compared to existing channel-aware access schemes.Comment: 6 pages, 6 figures, to be presented at IEEE GLOBECOM 201
Analytical characterization of inband and outband D2D Communications for network access
Mención Internacional en el tÃtulo de doctorCooperative short-range communication schemes provide powerful tools to solve interference
and resource shortage problems in wireless access networks. With such schemes, a mobile node
with excellent cellular connectivity can momentarily accept to relay traffic for its neighbors experiencing
poor radio conditions and use Device-to-Device (D2D) communications to accomplish
the task. This thesis provides a novel and comprehensive analytical framework that allows evaluating
the effects of D2D communications in access networks in terms of spectrum and energy
efficiency. The analysis covers the cases in which D2D communications use the same bandwidth
of legacy cellular users (in-band D2D) or a different one (out-band D2D) and leverages on the
characterization of underlying queueing systems and protocols to capture the complex intertwining
of short-range and legacy WiFi and cellular communications.
The analysis also unveils how D2D affects the use and scope of other optimization techniques
used for, e.g., interference coordination and fairness in resource distribution. Indeed, characterizing
the performance of D2D-enabled wireless access networks plays an essential role in the optimization
of system operation and, as a consequence, permits to assess the general applicability of
D2D solutions. With such characterization, we were able to design several mechanisms that improve
system capabilities. Specifically, we propose bandwidth resource management techniques
for controlling interference when cellular users and D2D pairs share the same spectrum, we design
advanced and energy-aware access selection mechanisms, we show how to adopt D2D communications
in conjunction with interference coordination schemes to achieve high and fair throughputs,
and we discuss on end-to-end fairness—beyond the use of access network resources—when
D2D communications is adopted in C-RAN. The results reported in this thesis show that identifying
performance bottlenecks is key to properly control network operation, and, interestingly,
bottlenecks may not be represented just by wireless resources when end-to-end fairness is of
concern.Programa Oficial de Doctorado en IngenierÃa TelemáticaPresidente: Marco Ajmone Marsan.- Secretario: Miquel Payaró Llisterri.- Vocal: Omer Gurewit
Practical interference management strategies in Gaussian networks
Increasing demand for bandwidth intensive activities on high-penetration wireless hand-held
personal devices, combined with their processing power and advanced radio features, has
necessitated a new look at the problems of resource provisioning and distributed management
of coexistence in wireless networks. Information theory, as the science of studying
the ultimate limits of communication e ciency, plays an important role in outlining guiding
principles in the design and analysis of such communication schemes. Network information
theory, the branch of information theory that investigates problems of multiuser and
distributed nature in information transmission is ideally poised to answer questions about
the design and analysis of multiuser communication systems. In the past few years, there
have been major advances in network information theory, in particular in the generalized
degrees of freedom framework for asymptotic analysis and interference alignment which have
led to constant gap to capacity results for Gaussian interference channels. Unfortunately,
practical adoption of these results has been slowed by their reliance on unrealistic assumptions
like perfect channel state information at the transmitter and intricate constructions
based on alignment over transcendental dimensions of real numbers. It is therefore necessary
to devise transmission methods and coexistence schemes that fall under the umbrella of
existing interference management and cognitive radio toolbox and deliver close to optimal
performance.
In this thesis we work on the theme of designing and characterizing the performance of
conceptually simple transmission schemes that are robust and achieve performance that is
close to optimal. In particular, our work is broadly divided into two parts. In the rst part,
looking at cognitive radio networks, we seek to relax the assumption of non-causal knowledge
of primary user's message at the secondary user's transmitter. We study a cognitive channel
model based on Gaussian interference channel that does not assume anything about users
other than primary user's priority over secondary user in reaching its desired quality of
service. We characterize this quality of service requirement as a minimum rate that the
primary user should be able to achieve. Studying the achievable performance of simple
encoding and decoding schemes in this scenario, we propose a few di erent simple encoding
schemes and explore di erent decoder designs. We show that surprisingly, all these schemes
achieve the same rate region. Next, we study the problem of rate maximization faced by
the secondary user subject to primary's QoS constraint. We show that this problem is not
convex or smooth in general. We then use the symmetry properties of the problem to reduce
its solution to a feasibly implementable line search. We also provide numerical results to
demonstrate the performance of the scheme.
Continuing on the theme of simple yet well-performing schemes for wireless networks, in
the second part of the thesis, we direct our attention from two-user cognitive networks to
the problem of smart interference management in large wireless networks. Here, we study
the problem of interference-aware wireless link scheduling. Link scheduling is the problem of
allocating a set of transmission requests into as small a set of time slots as possible such that
all transmissions satisfy some condition of feasibility. The feasibility criterion has traditionally
been lack of pair of links that interfere too much. This makes the problem amenable to
solution using graph theoretical tools. Inspired by the recent results that the simple approach
of treating interference as noise achieves maximal Generalized Degrees of Freedom (which is
a measure that roughly captures how many equivalent single-user channels are contained in
a given multi-user channel) and the generalization that it can attain rates within a constant
gap of the capacity for a large class of Gaussian interference networks, we study the problem
of scheduling links under a set Signal to Interference plus Noise Ratio (SINR) constraint.
We show that for nodes distributed in a metric space and obeying path loss channel model, a
re ned framework based on combining geometric and graph theoretic results can be devised
to analyze the problem of nding the feasible sets of transmissions for a given level of desired
SINR. We use this general framework to give a link scheduling algorithm that is provably
within a logarithmic factor of the best possible schedule. Numerical simulations con rm
that this approach outperforms other recently proposed SINR-based approaches. Finally, we
conclude by identifying open problems and possible directions for extending these results
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