46 research outputs found
On Certain Large Random Hermitian Jacobi Matrices with Applications to Wireless Communications
In this paper we study the spectrum of certain large random Hermitian Jacobi
matrices. These matrices are known to describe certain communication setups. In
particular we are interested in an uplink cellular channel which models mobile
users experiencing a soft-handoff situation under joint multicell decoding.
Considering rather general fading statistics we provide a closed form
expression for the per-cell sum-rate of this channel in high-SNR, when an
intra-cell TDMA protocol is employed. Since the matrices of interest are
tridiagonal, their eigenvectors can be considered as sequences with second
order linear recurrence. Therefore, the problem is reduced to the study of the
exponential growth of products of two by two matrices. For the case where
users are simultaneously active in each cell, we obtain a series of lower and
upper bound on the high-SNR power offset of the per-cell sum-rate, which are
considerably tighter than previously known bounds
Cellular Systems with Full-Duplex Compress-and-Forward Relaying and Cooperative Base Stations
In this paper the advantages provided by multicell processing of signals
transmitted by mobile terminals (MTs) which are received via dedicated relay
terminals (RTs) are studied. It is assumed that each RT is capable of
full-duplex operation and receives the transmission of adjacent relay
terminals. Focusing on intra-cell TDMA and non-fading channels, a simplified
relay-aided uplink cellular model based on a model introduced by Wyner is
considered. Assuming a nomadic application in which the RTs are oblivious to
the MTs' codebooks, a form of distributed compress-and-forward (CF) scheme with
decoder side information is employed. The per-cell sum-rate of the CF scheme is
derived and is given as a solution of a simple fixed point equation. This
achievable rate reveals that the CF scheme is able to completely eliminate the
inter-relay interference, and it approaches a ``cut-set-like'' upper bound for
strong RTs transmission power. The CF rate is also shown to surpass the rate of
an amplify-and-forward scheme via numerical calculations for a wide range of
the system parameters.Comment: Proceedings of the 2008 IEEE International Symposium on Information
Theory, Toronto, ON, Canada, July 6 - 11, 200
Distributed MIMO Systems with Oblivious Antennas
A scenario in which a single source communicates with a single destination
via a distributed MIMO transceiver is considered. The source operates each of
the transmit antennas via finite-capacity links, and likewise the destination
is connected to the receiving antennas through capacity-constrained channels.
Targeting a nomadic communication scenario, in which the distributed MIMO
transceiver is designed to serve different standards or services, transmitters
and receivers are assumed to be oblivious to the encoding functions shared by
source and destination. Adopting a Gaussian symmetric interference network as
the channel model (as for regularly placed transmitters and receivers),
achievable rates are investigated and compared with an upper bound. It is
concluded that in certain asymptotic and non-asymptotic regimes obliviousness
of transmitters and receivers does not cause any loss of optimality.Comment: In Proc. of the 2008 IEEE International Symposium on Information
Theory (ISIT 2008), Toronto, Ontario, Canad
Cellular Systems with Full-Duplex Amplify-and-Forward Relaying and Cooperative Base-Stations
In this paper the benefits provided by multi-cell processing of signals
transmitted by mobile terminals which are received via dedicated relay
terminals (RTs) are assessed. Unlike previous works, each RT is assumed here to
be capable of full-duplex operation and receives the transmission of adjacent
relay terminals. Focusing on intra-cell TDMA and non-fading channels, a
simplified uplink cellular model introduced by Wyner is considered. This
framework facilitates analytical derivation of the per-cell sum-rate of
multi-cell and conventional single-cell receivers. In particular, the analysis
is based on the observation that the signal received at the base stations can
be interpreted as the outcome of a two-dimensional linear time invariant
system. Numerical results are provided as well in order to provide further
insight into the performance benefits of multi-cell processing with relaying.Comment: To appear in the Proc. of the 2007 IEEE International Symposium on
Information Theor
Opportunistic Relaying in Wireless Networks
Relay networks having source-to-destination pairs and half-duplex
relays, all operating in the same frequency band in the presence of block
fading, are analyzed. This setup has attracted significant attention and
several relaying protocols have been reported in the literature. However, most
of the proposed solutions require either centrally coordinated scheduling or
detailed channel state information (CSI) at the transmitter side. Here, an
opportunistic relaying scheme is proposed, which alleviates these limitations.
The scheme entails a two-hop communication protocol, in which sources
communicate with destinations only through half-duplex relays. The key idea is
to schedule at each hop only a subset of nodes that can benefit from
\emph{multiuser diversity}. To select the source and destination nodes for each
hop, it requires only CSI at receivers (relays for the first hop, and
destination nodes for the second hop) and an integer-value CSI feedback to the
transmitters. For the case when is large and is fixed, it is shown that
the proposed scheme achieves a system throughput of bits/s/Hz. In
contrast, the information-theoretic upper bound of bits/s/Hz
is achievable only with more demanding CSI assumptions and cooperation between
the relays. Furthermore, it is shown that, under the condition that the product
of block duration and system bandwidth scales faster than , the
achievable throughput of the proposed scheme scales as .
Notably, this is proven to be the optimal throughput scaling even if
centralized scheduling is allowed, thus proving the optimality of the proposed
scheme in the scaling law sense.Comment: 17 pages, 8 figures, To appear in IEEE Transactions on Information
Theor
Budget-Constrained Item Cold-Start Handling in Collaborative Filtering Recommenders via Optimal Design
It is well known that collaborative filtering (CF) based recommender systems
provide better modeling of users and items associated with considerable rating
history. The lack of historical ratings results in the user and the item
cold-start problems. The latter is the main focus of this work. Most of the
current literature addresses this problem by integrating content-based
recommendation techniques to model the new item. However, in many cases such
content is not available, and the question arises is whether this problem can
be mitigated using CF techniques only. We formalize this problem as an
optimization problem: given a new item, a pool of available users, and a budget
constraint, select which users to assign with the task of rating the new item
in order to minimize the prediction error of our model. We show that the
objective function is monotone-supermodular, and propose efficient optimal
design based algorithms that attain an approximation to its optimum. Our
findings are verified by an empirical study using the Netflix dataset, where
the proposed algorithms outperform several baselines for the problem at hand.Comment: 11 pages, 2 figure
Distributed Exploration in Multi-Armed Bandits
We study exploration in Multi-Armed Bandits in a setting where players
collaborate in order to identify an -optimal arm. Our motivation
comes from recent employment of bandit algorithms in computationally intensive,
large-scale applications. Our results demonstrate a non-trivial tradeoff
between the number of arm pulls required by each of the players, and the amount
of communication between them. In particular, our main result shows that by
allowing the players to communicate only once, they are able to learn
times faster than a single player. That is, distributing learning to
players gives rise to a factor parallel speed-up. We complement
this result with a lower bound showing this is in general the best possible. On
the other extreme, we present an algorithm that achieves the ideal factor
speed-up in learning performance, with communication only logarithmic in