109 research outputs found
Multicast Multigroup Beamforming under Per-antenna Power Constraints
Linear precoding exploits the spatial degrees of freedom offered by
multi-antenna transmitters to serve multiple users over the same frequency
resources. The present work focuses on simultaneously serving multiple groups
of users, each with its own channel, by transmitting a stream of common symbols
to each group. This scenario is known as physical layer multicasting to
multiple co-channel groups. Extending the current state of the art in
multigroup multicasting, the practical constraint of a maximum permitted power
level radiated by each antenna is tackled herein. The considered per antenna
power constrained system is optimized in a maximum fairness sense. In other
words, the optimization aims at favoring the worst user by maximizing the
minimum rate. This Max-Min Fair criterion is imperative in multicast systems,
where the performance of all the receivers listening to the same multicast is
dictated by the worst rate in the group. An analytic framework to tackle the
Max-Min Fair multigroup multicasting scenario under per antenna power
constraints is therefore derived. Numerical results display the accuracy of the
proposed solution and provide insights to the performance of a per antenna
power constrained system.Comment: Presented in IEEE ICC 2014, Sydney, AUS. arXiv admin note:
substantial text overlap with arXiv:1406.755
Cellular-Broadcast Service Convergence through Caching for CoMP Cloud RANs
Cellular and Broadcast services have been traditionally treated independently
due to the different market requirements, thus resulting in different business
models and orthogonal frequency allocations. However, with the advent of cheap
memory and smart caching, this traditional paradigm can converge into a single
system which can provide both services in an efficient manner. This paper
focuses on multimedia delivery through an integrated network, including both a
cellular (also known as unicast or broadband) and a broadcast last mile
operating over shared spectrum. The subscribers of the network are equipped
with a cache which can effectively create zero perceived latency for multimedia
delivery, assuming that the content has been proactively and intelligently
cached. The main objective of this work is to establish analytically the
optimal content popularity threshold, based on a intuitive cost function. In
other words, the aim is to derive which content should be broadcasted and which
content should be unicasted. To facilitate this, Cooperative Multi- Point
(CoMP) joint processing algorithms are employed for the uni and broad-cast PHY
transmissions. To practically implement this, the integrated network controller
is assumed to have access to traffic statistics in terms of content popularity.
Simulation results are provided to assess the gain in terms of total spectral
efficiency. A conventional system, where the two networks operate
independently, is used as benchmark.Comment: Submitted to IEEE PIMRC 201
Multicast Multigroup Beamforming for Per-antenna Power Constrained Large-scale Arrays
Large in the number of transmit elements, multi-antenna arrays with
per-element limitations are in the focus of the present work. In this context,
physical layer multigroup multicasting under per-antenna power constrains, is
investigated herein. To address this complex optimization problem
low-complexity alternatives to semi-definite relaxation are proposed. The goal
is to optimize the per-antenna power constrained transmitter in a maximum
fairness sense, which is formulated as a non-convex quadratically constrained
quadratic problem. Therefore, the recently developed tool of feasible point
pursuit and successive convex approximation is extended to account for
practical per-antenna power constraints. Interestingly, the novel iterative
method exhibits not only superior performance in terms of approaching the
relaxed upper bound but also a significant complexity reduction, as the
dimensions of the optimization variables increase. Consequently, multicast
multigroup beamforming for large-scale array transmitters with per-antenna
dedicated amplifiers is rendered computationally efficient and accurate. A
preliminary performance evaluation in large-scale systems for which the
semi-definite relaxation constantly yields non rank-1 solutions is presented.Comment: submitted to IEEE SPAWC 2015. arXiv admin note: substantial text
overlap with arXiv:1406.755
Multicast Multigroup Precoding and User Scheduling for Frame-Based Satellite Communications
The present work focuses on the forward link of a broadband multibeam
satellite system that aggressively reuses the user link frequency resources.
Two fundamental practical challenges, namely the need to frame multiple users
per transmission and the per-antenna transmit power limitations, are addressed.
To this end, the so-called frame-based precoding problem is optimally solved
using the principles of physical layer multicasting to multiple co-channel
groups under per-antenna constraints. In this context, a novel optimization
problem that aims at maximizing the system sum rate under individual power
constraints is proposed. Added to that, the formulation is further extended to
include availability constraints. As a result, the high gains of the sum rate
optimal design are traded off to satisfy the stringent availability
requirements of satellite systems. Moreover, the throughput maximization with a
granular spectral efficiency versus SINR function, is formulated and solved.
Finally, a multicast-aware user scheduling policy, based on the channel state
information, is developed. Thus, substantial multiuser diversity gains are
gleaned. Numerical results over a realistic simulation environment exhibit as
much as 30% gains over conventional systems, even for 7 users per frame,
without modifying the framing structure of legacy communication standards.Comment: Accepted for publication to the IEEE Transactions on Wireless
Communications, 201
Sum Rate Maximizing Multigroup Multicast Beamforming under Per-antenna Power Constraints
A multi-antenna transmitter that conveys independent sets of common data to
distinct groups of users is herein considered, a model known as physical layer
multicasting to multiple co-channel groups. In the recently proposed context of
per-antenna power constrained multigroup multicasting, the present work focuses
on a novel system design that aims at maximizing the total achievable
throughput. Towards increasing the system sum rate, the available power
resources need to be allocated to well conditioned groups of users. A detailed
solution to tackle the elaborate sum rate maximization multigroup multicast
problem under per-antenna power constraints is therefore derived. Numerical
results are presented to quantify the gains of the proposed algorithm over
heuristic solutions. Besides Rayleigh faded channels, the solution is also
applied to uniform linear array transmitters operating in the far field, where
line-ofsight conditions are realized. In this setting, a sensitivity analysis
with respect to the angular separation of co-group users is included. Finally,
a simple scenario providing important intuitions for the sum rate maximizing
multigroup multicast solutions is elaborated.Comment: Submitted to IEEE GlobeCom 2014, Austin, TX. arXiv admin note:
substantial text overlap with arXiv:1406.7699, arXiv:1406.755
Frame Based Precoding in Satellite Communications: A Multicast Approach
In the present work, a multibeam satellite that employs aggressive frequency
reuse towards increasing the offered throughput is considered. Focusing on the
forward link, the goal is to employ multi-antenna signal processing techniques,
namely linear precoding, to manage the inter-beam interferences. In this
context, fundamental practical limitations, namely the rigid framing structure
of satellite communication standards and the on-board per-antenna power
constraints, are herein considered. Therefore, the concept of optimal frame
based precoding under per-antenna constraints, is discussed. This consists in
precoding the transmit signals without changing the underlying framing
structure of the communication standard. In the present work, the connection of
the frame based precoding problem with the generic signal processing problem of
conveying independent sets of common data to distinct groups of users is
established. This model is known as physical layer multicasting to multiple
co-channel groups. Building on recent results, the weighted fair per-antenna
power constrained multigroup multicast precoders are employed for frame based
precoding. The throughput performance of these solutions is compared to
multicast aware heuristic precoding methods over a realistic multibeam
satellite scenario. Consequently, the gains of the proposed approach are
quantified via extensive numerical results.Comment: Accepted for presentation at the IEEE ASMS 201
Weighted Fair Multicast Multigroup Beamforming under Per-antenna Power Constraints
A multi-antenna transmitter that conveys independent sets of common data to
distinct groups of users is considered. This model is known as physical layer
multicasting to multiple co-channel groups. In this context, the practical
constraint of a maximum permitted power level radiated by each antenna is
addressed. The per-antenna power constrained system is optimized in a maximum
fairness sense with respect to predetermined quality of service weights. In
other words, the worst scaled user is boosted by maximizing its weighted
signal-to-interference plus noise ratio. A detailed solution to tackle the
weighted max-min fair multigroup multicast problem under per-antenna power
constraints is therefore derived. The implications of the novel constraints are
investigated via prominent applications and paradigms. What is more, robust
per-antenna constrained multigroup multicast beamforming solutions are
proposed. Finally, an extensive performance evaluation quantifies the gains of
the proposed algorithm over existing solutions and exhibits its accuracy over
per-antenna power constrained systems.Comment: Under review in IEEE Transactions in Signal Processin
The impact of social networks on leadership behaviour
Dominant streams in leadership literature conceptualise it either as a role within sociopolitical structure or as a behavioural predisposition of agents. Leadership roles are determined by decisional power, most typically related to the hierarchical and structural position of agents within sociopolitical networks. Limitations in attaining meaningful predictions of leader potential can be related to the separation of leadership as an agency attribute from leaders as structurally embedded agents. Social network analysis allows for the contingent examination of both. In this article, a number of hypotheses are tested via an empirical case study where interaction and affiliation networks across multiple decision experiments are coupled with attribute and psychometric data of the actors. In this quasi-experimental setting, leadership emergence is studied among four groups of undergraduate students faced with a decision choice in an iterative political simulation game. Findings suggest that in egalitarian political systems, centrality in social networks is directly associated with political success, while in political systems imbued with power inequalities successful actors are idiocentric brokers. Methodologically, this study frames role simulation games as quasi-experimental tests. Group interactions can be controlled, but vitally also incorporated in studies of perceptions, behaviours and group outcomes. The use of attitudinal micro-surveys, psychometric tests, observation and relational surveys is combined for a comprehensive mapping of group dynamics suited to questions of agency. In conclusion, there are a number of insights offered on deploying these methods in tandem and the challenges inherent in such a research design. </jats:p
Is it important to know Jack? Using social network analysis to assess regional business connectivity in Bristol
AbstractThis article attempts to measure the effect of ‘public sector brokerage’ in facilitating the interaction between enterprises in Bristol. In particular we examine the effectiveness of a public sector project through formal social network analysis. The data originates from a publicly funded consulting project measuring interaction between actors as a result of the Science City Bristol Initiative 2008–12. The remit of the project has been to maximise investment, increase public engagement and boost connectivity among key businesses in the region. Bristol is identified as an important cluster for aerospace, IT, finance and creative technologies. To monitor and evaluate connections a network analysis was commissioned at the early stage of the initiative. A core-periphery structure has been identified with key brokerage roles held by public sector agencies - facilitating private gain and presumably an increase of GVA. A number of suggestions in developing this research platform are offered in conclusion. These focus on capturing the evolution of social networks, their effect on the interaction between private and public actors, their impact on entrepreneurship and their effect on the dissemination of innovation
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