8 research outputs found
Approximate Ergodic Capacity of a Class of Fading Two-User Two-Hop Networks
The fading AWGN two-user two-hop network is considered where the channel coefficients are independent and identically distributed (i.i.d.) according to a continuous distribution and vary over time. For a broad class of channel distributions, the ergodic sum capacity is characterized to within a constant number of bits/second/hertz, independent of the signal-to-noise ratio. The achievability follows from the analysis of an interference neutralization scheme where the relays are partitioned into M pairs, and interference is neutralized separately by each pair of relays. When M = 1, the proposed ergodic interference neutralization characterizes the ergodic sum capacity to within 4 bits/sec/Hz for i.i.d. uniform phase fading and approximately 4.7 bits/sec/Hz for i.i.d. Rayleigh fading. It is further shown that this gap can be tightened to 4 log pi-4 bits/sec/Hz (approximately 2.6) for i.i.d. uniform phase fading and 4-4 log(3 pi/8) bits/sec/Hz (approximately 3.1) for i.i.d. Rayleigh fading in the limit of large M.(1
Degrees of Freedom of Full-Duplex Multiantenna Cellular Networks
We study the degrees of freedom (DoF) of cellular networks in which a full
duplex (FD) base station (BS) equipped with multiple transmit and receive
antennas communicates with multiple mobile users. We consider two different
scenarios. In the first scenario, we study the case when half duplex (HD)
users, partitioned to either the uplink (UL) set or the downlink (DL) set,
simultaneously communicate with the FD BS. In the second scenario, we study the
case when FD users simultaneously communicate UL and DL data with the FD BS.
Unlike conventional HD only systems, inter-user interference (within the cell)
may severely limit the DoF, and must be carefully taken into account. With the
goal of providing theoretical guidelines for designing such FD systems, we
completely characterize the sum DoF of each of the two different FD cellular
networks by developing an achievable scheme and obtaining a matching upper
bound. The key idea of the proposed scheme is to carefully allocate UL and DL
information streams using interference alignment and beamforming techniques. By
comparing the DoFs of the considered FD systems with those of the conventional
HD systems, we establish the DoF gain by enabling FD operation in various
configurations. As a consequence of the result, we show that the DoF can
approach the two-fold gain over the HD systems when the number of users becomes
large enough as compared to the number of antennas at the BS.Comment: 21 pages, 16 figures, a shorter version of this paper has been
submitted to the IEEE International Symposium on Information Theory (ISIT)
201
Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks
An uplink-downlink two-cell cellular network is studied in which the first
base station (BS) with antennas receives independent messages from its
serving users, while the second BS with antennas transmits
independent messages to its serving users. That is, the first and second
cells operate as uplink and downlink, respectively. Each user is assumed to
have a single antenna. Under this uplink-downlink setting, the sum degrees of
freedom (DoF) is completely characterized as the minimum of
,
, , and , where denotes
. The result demonstrates that, for a broad class of network
configurations, operating one of the two cells as uplink and the other cell as
downlink can strictly improve the sum DoF compared to the conventional uplink
or downlink operation, in which both cells operate as either uplink or
downlink. The DoF gain from such uplink-downlink operation is further shown to
be achievable for heterogeneous cellular networks having hotspots and with
delayed channel state information.Comment: 22 pages, 11 figures, in revision for IEEE Transactions on
Information Theor
Function Computation over Networks:Efficient Information Processing for Cache and Sensor Applications
This thesis looks at efficient information processing for two network applications: content delivery with caching and collecting summary statistics in wireless sensor networks. Both applications are studied under the same paradigm: function computation over networks, where distributed source nodes cooperatively communicate some functions of individual observations to one or multiple destinations. One approach that always works is to convey all observations and then let the destinations compute the desired functions by themselves. However, if the available communication resources are limited, then revealing less unwanted information becomes critical. Centered on this goal, this thesis develops new coding schemes using information-theoretic tools.
The first part of this thesis focuses on content delivery with caching. Caching is a technique that facilitates reallocation of communication resources in order to avoid network congestion during peak-traffic times. An information-theoretic model, termed sequential coding for computing, is proposed to analyze the potential gains offered by the caching technique. For the single-user case, the proposed framework succeeds in verifying the optimality of some simple caching strategies and in providing guidance towards optimal caching strategies. For the two-user case, five representative subproblems are considered, which draw connections with classic source coding problems including the Gray-Wyner system, successive refinement, and the Kaspi/Heegard-Berger problem. Afterwards, the problem of distributed computing with successive refinement is considered. It is shown that if full data recovery is required in the second stage of successive refinement, then any information acquired in the first stage will be useful later in the second stage.
The second part of this thesis looks at the collection of summary statistics in wireless sensor networks. Summary statistics include arithmetic mean, median, standard deviation, etc, and they belong to the class of symmetric functions. This thesis develops arithmetic computation coding in order to efficiently perform in-network computation for weighted arithmetic sums and symmetric functions. The developed arithmetic computation coding increases the achievable computation rate from to , where is the number of sensors. Finally, this thesis demonstrates that interaction among sensors is beneficial for computation of type-threshold functions, e.g., the maximum and the indicator function, and that a non-vanishing computation rate is achievable
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