442 research outputs found
Asymptotic Behaviour of Random Vandermonde Matrices with Entries on the Unit Circle
Analytical methods for finding moments of random Vandermonde matrices with
entries on the unit circle are developed. Vandermonde Matrices play an
important role in signal processing and wireless applications such as direction
of arrival estimation, precoding, and sparse sampling theory, just to name a
few. Within this framework, we extend classical freeness results on random
matrices with independent, identically distributed (i.i.d.) entries and show
that Vandermonde structured matrices can be treated in the same vein with
different tools. We focus on various types of matrices, such as Vandermonde
matrices with and without uniform phase distributions, as well as generalized
Vandermonde matrices. In each case, we provide explicit expressions of the
moments of the associated Gram matrix, as well as more advanced models
involving the Vandermonde matrix. Comparisons with classical i.i.d. random
matrix theory are provided, and deconvolution results are discussed. We review
some applications of the results to the fields of signal processing and
wireless communications.Comment: 28 pages. To appear in IEEE Transactions on Information Theor
Many-to-Many Matching Games for Proactive Social-Caching in Wireless Small Cell Networks
In this paper, we address the caching problem in small cell networks from a
game theoretic point of view. In particular, we formulate the caching problem
as a many-to-many matching game between small base stations and service
providers' servers. The servers store a set of videos and aim to cache these
videos at the small base stations in order to reduce the experienced delay by
the end-users. On the other hand, small base stations cache the videos
according to their local popularity, so as to reduce the load on the backhaul
links. We propose a new matching algorithm for the many-to-many problem and
prove that it reaches a pairwise stable outcome. Simulation results show that
the number of satisfied requests by the small base stations in the proposed
caching algorithm can reach up to three times the satisfaction of a random
caching policy. Moreover, the expected download time of all the videos can be
reduced significantly
A Multi-Game Framework for Harmonized LTE-U and WiFi Coexistence over Unlicensed Bands
The introduction of LTE over unlicensed bands (LTE-U) will enable LTE base
stations (BSs) to boost their capacity and offload their traffic by exploiting
the underused unlicensed bands. However, to reap the benefits of LTE-U, it is
necessary to address various new challenges associated with LTE-U and WiFi
coexistence. In particular, new resource management techniques must be
developed to optimize the usage of the network resources while handling the
interdependence between WiFi and LTE users and ensuring that WiFi users are not
jeopardized. To this end, in this paper, a new game theoretic tool, dubbed as
\emph{multi-game} framework is proposed as a promising approach for modeling
resource allocation problems in LTE-U. In such a framework, multiple,
co-existing and coupled games across heterogeneous channels can be formulated
to capture the specific characteristics of LTE-U. Such games can be of
different properties and types but their outcomes are largely interdependent.
After introducing the basics of the multi-game framework, two classes of
algorithms are outlined to achieve the new solution concepts of multi-games.
Simulation results are then conducted to show how such a multi-game can
effectively capture the specific properties of LTE-U and make of it a
"friendly" neighbor to WiFi.Comment: Accepted for publication at IEEE Wireless Communications Magazine,
Special Issue on LTE in Unlicensed Spectru
Fractional Pilot Reuse in Massive MIMO Systems
Pilot contamination is known to be one of the main impairments for massive
MIMO multi-cell communications. Inspired by the concept of fractional frequency
reuse and by recent contributions on pilot reutilization among non-adjacent
cells, we propose a new pilot allocation scheme to mitigate this effect. The
key idea is to allow users in neighboring cells that are closest to their base
stations to reuse the same pilot sequences. Focusing on the uplink, we obtain
expressions for the overall spectral efficiency per cell for different linear
combining techniques at the base station and use them to obtain both the
optimal pilot reuse parameters and the optimal number of scheduled users.
Numerical results show a remarkable improvement in terms of spectral efficiency
with respect to the existing techniques.Comment: Paper presented at the IEEE ICC 2015 Workshop on 5G & Beyond -
Enabling Technologies and Application
Distributed Binary Detection with Lossy Data Compression
Consider the problem where a statistician in a two-node system receives
rate-limited information from a transmitter about marginal observations of a
memoryless process generated from two possible distributions. Using its own
observations, this receiver is required to first identify the legitimacy of its
sender by declaring the joint distribution of the process, and then depending
on such authentication it generates the adequate reconstruction of the
observations satisfying an average per-letter distortion. The performance of
this setup is investigated through the corresponding rate-error-distortion
region describing the trade-off between: the communication rate, the error
exponent induced by the detection and the distortion incurred by the source
reconstruction. In the special case of testing against independence, where the
alternative hypothesis implies that the sources are independent, the optimal
rate-error-distortion region is characterized. An application example to binary
symmetric sources is given subsequently and the explicit expression for the
rate-error-distortion region is provided as well. The case of "general
hypotheses" is also investigated. A new achievable rate-error-distortion region
is derived based on the use of non-asymptotic binning, improving the quality of
communicated descriptions. Further improvement of performance in the general
case is shown to be possible when the requirement of source reconstruction is
relaxed, which stands in contrast to the case of general hypotheses.Comment: to appear on IEEE Trans. Information Theor
Circuit-Aware Design of Energy-Efficient Massive MIMO Systems
Densification is a key to greater throughput in cellular networks. The full
potential of coordinated multipoint (CoMP) can be realized by massive
multiple-input multiple-output (MIMO) systems, where each base station (BS) has
very many antennas. However, the improved throughput comes at the price of more
infrastructure; hardware cost and circuit power consumption scale
linearly/affinely with the number of antennas. In this paper, we show that one
can make the circuit power increase with only the square root of the number of
antennas by circuit-aware system design. To this end, we derive achievable user
rates for a system model with hardware imperfections and show how the level of
imperfections can be gradually increased while maintaining high throughput. The
connection between this scaling law and the circuit power consumption is
established for different circuits at the BS.Comment: Published at International Symposium on Communications, Control, and
Signal Processing (ISCCSP 2014), 4 pages, 3 figures. This version corrects an
error related to Lemma
A Multi-Service Oriented Multiple-Access Scheme for Next-Generation Mobile Networks
One of the key requirements for fifth-generation (5G) cellular networks is
their ability to handle densely connected devices with different quality of
service (QoS) requirements. In this article, we present multi-service oriented
multiple access (MOMA), an integrated access scheme for massive connections
with diverse QoS profiles and/or traffic patterns originating from both
handheld devices and machine-to-machine (M2M) transmissions. MOMA is based on
a) stablishing separate classes of users based on relevant criteria that go
beyond the simple handheld/M2M split, b) class dependent hierarchical spreading
of the data signal and c) a mix of multiuser and single-user detection schemes
at the receiver. Practical implementations of the MOMA principle are provided
for base stations (BSs) that are equipped with a large number of antenna
elements. Finally, it is shown that such a
massive-multiple-input-multiple-output (MIMO) scenario enables the achievement
of all the benefits of MOMA even with a simple receiver structure that allows
to concentrate the receiver complexity where effectively needed.Comment: 6 pages, 3 figures, accepted to the European Conference on Networks
and Communications (EuCNC 2016
Massive MIMO and Small Cells: Improving Energy Efficiency by Optimal Soft-Cell Coordination
To improve the cellular energy efficiency, without sacrificing
quality-of-service (QoS) at the users, the network topology must be densified
to enable higher spatial reuse. We analyze a combination of two densification
approaches, namely "massive" multiple-input multiple-output (MIMO) base
stations and small-cell access points. If the latter are operator-deployed, a
spatial soft-cell approach can be taken where the multiple transmitters serve
the users by joint non-coherent multiflow beamforming. We minimize the total
power consumption (both dynamic emitted power and static hardware power) while
satisfying QoS constraints. This problem is proved to have a hidden convexity
that enables efficient solution algorithms. Interestingly, the optimal solution
promotes exclusive assignment of users to transmitters. Furthermore, we provide
promising simulation results showing how the total power consumption can be
greatly improved by combining massive MIMO and small cells; this is possible
with both optimal and low-complexity beamforming.Comment: Published at International Conference on Telecommunications (ICT
2013), 6-8 May 2013, Casablanca, Morocco, 5 pages, 4 figures, 2 tables. This
version includes the Matlab code necessary to reproduce the simulations; see
the ancillary files. This version also corrects errors in Table 1 and in the
simulations, which affected Figs. 3-
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