Research
Contributions
Learning a Content from Unstructured Corpus of Text Data! Chasing
Exemptions: Social Media, Big Data, and the Measles Crisis E. Ebrahimzadeh
M. Falahi T. Tangherlini
V. Roychowdhury A. Wadia The 20142015 measles outbreak in California caught many
observers by surprise. Health officials attributed the crisis to the
increasing number of children whose parents had secured exemptions from
inoculation for various vaccinepreventable diseases (VPDs), without
providing a quantifiable analysis of what was driving these exemptions.
Focusing on two social media sites popular among mothers, we show that largescale
text analysis tools can be used both to identify vaccinerelated
conversations and to discover the topic of these conversations. We uncover a
strong, persistent signal focusing on securing exemptions for childhood
vaccinations. We consider this type of signal to indicate that the
conversations about exemptions are ÒendemicÓ to these sites. If people take
realworld actions based on the suggestions in these conversations, the
cumulative effect would contribute to a diminution of herd immunity in their
communities. This effect has been borne out by real world events. Our method
based on analyzing social media chatter at scale can provide an alert
mechanism to health officials and to others charged with early childhood
health decisions, helping to discover ÒhiddenÓ health crises such as the
current one surrounding childhood vaccination. Journal Version:
Submitted to Proceedings of the National Academy of Sciences 

Finding a
Needle in Haystack Efficiently! Quickest
Transient Change Point Detection with Sampling Constraint E. Ebrahimzadeh
A. Tchamkerten Consider the problem of sequential detection of a transient
change in distribution of a random sequence, where there is a constraint on
the fraction of samples observed. We cast the problem into a Minimax setting, where the change time is completely
unknown, all random variables are independent and both the distributions
along the change and pre/post change are known. The objective is to design a
statistical test with large average time to false alarm to minimize a measure
of worst case delay. Leveraging the results in the
nontransient setting, we show that there exists an asymptotic threshold on
the minimum duration of a change that can be detected reliably with such
false alarm constrained statistical tests. Next, given a transient change
with duration above this asymptotic threshold, we characterize the smallest sampling rate needed
so that under sparse sampling the change can be detected as efficiently as
under full sampling. Conference Version: proceedings of ISIT
2015 

Let
Signals Mix up! Signalling in FrequencySelective Interference Channels: Outage Analysis E. Ebrahimzadeh
K. Moshksar A. Khandani We study outage probability in a twouser parallel Gaussian
interference channel consisting of two parallel subchannels. Each subchannel is modeled by a twouser Gaussian interference
channel with quasistatic and flat fading. Each user employs a
singlelayer Gaussian codebook, splits its average transmission power equally
and maintains a statistical correlation between the signals transmitted over
the underlying subchannels. Under mild nondegeneracy conditions on the
fading statistics, it is shown that the value of correlation coefficient that
minimizes the outage probability approaches 1 as the signaltonoise ratio
(SNR) approaches infinity. This holds whether the receivers treat
interference as Gaussian noise (TIN) or cancel interference (CI). We compute
the outage probability for sufficiently large SNR under the assumptions that
the correlation coefficient is 1 and the direct and crossover channel
coefficients are independent zeromean complex Gaussian random variables with
possibly different variances. In particular, it is shown that if the
receivers have the freedom of choice to perform TIN or CI, a smaller outage
probability is achieved in contrast to a scenario where the users are
orthogonal, i.e., interference is completely avoided by assigning disjoint
subchannels to the users. Journal version: Revised for IEEE
Transactions on Information Theory Conference Version: Proceedings of ISIT
2015 

ItÕs a
Small world in Random Apollonian Networks! On the Longest
Path and Diameter of Random Apollonian Networks E. Ebrahimzadeh
L. Farczadi P. Gao A. Mehrabian C. Sato  N. Wormald
J. Zung We consider the following iterative construction of a random
planar triangulation. Start with a triangle embedded in the plane. In each
step, choose a bounded face uniformly at random, add a vertex inside that
face and join it to the vertices of the face. After n − 3 steps, we
obtain a random triangulated plane graph with n vertices, which is called a
Random Apollonian Network (RAN). We show that asymptotically almost surely (a.a.s.) every path in a RAN has length o(n),
refuting a conjecture of Frieze and Tsourakakis. We
also prove that a.a.s. the
diameter of a RAN is asymptotic to c log n, where is the solution of an explicit
equation. Journal version: In Random Structures and Algorithms(RSA)December 2014 Short version published in Electronic Notes
in Discrete Mathematics 

There is
Room for Opportunism in Random Access! Random
Access in Wireless XNetworks S. MahboubiE.
EbrahimzadehA. Khandani We studied a 2user symmetric X channel with random access
capability for transmitters. Transmitters can work in two modes of operation
independent of each other, either active or inactive. There is
an independent message from each transmitter to each receiver. It is aimed in
this study at maximizing the throughput of this network over the set
of rates which can be reliably communicated. To
characterize this rate region, an information theoretic outrebound is
derived. Achievability is based on opportunistic signal transmission along
with a linear precoding scheme. Once the rate region is exactly determined,
throughput maximization can be solved as a linear programming problem. In this work, we followed the footsteps of Minero
et al. for modeling random access in an information theoretic
framework, The coding scheme we used for tackling this problem is based on
the new deterministic model proposed by Niesen and MaddahAli, where capacity region of the Xchannel is
approximated within constant bit. Conference version: Proceedings of ISIT
2012 

ItÕs
Better to Look at Things from both Sides! Sufficient
Conditions for the Existence of Fixfree codes E. EbrahimzadehM.
KhosravifardA. AghajanA.
Gulliver A code is said to be fixfree if any codeword
is neither a prefix nor a suffix of any other. Using Þxfree codes, the
decoding speed can be doubled and the robustness to transmission errors is
increased. It is conjectured by Ahlswede et al.
in [1] that there exists a fixfree code for a length vector, if the Kraft
sum of a length vector is less than 3/4. We showed that this conjecture is
true for all length vectors of size less than or equal to 32 and improved the
known upper bounds for a certain class of length vectors. This problem is
tackled as a counting problem using the principle of inclusionexclusion
combining with a novel idea introduced by Yekhanin
to prove the existence of a fixfree code for length vectors with Kraftsum
less than 5/8. Journal version: Accepted for publication
in IEICE Trans. on fundamentals of Communications Weakly
Symmetric Fixfree codes A.Aghajan M. Khosravifar (I have collaborated
in this work) A fixfree code is said to be symmetric if each codeword is a palindrome. These codes provide more error
resilience and are proved to be more suitable for joint sourcechannel coding
than the asymmetric ones. Moreover, the same codebook can be used for the
forward and backward decoding. Maintaining these advantages, we defined a new
class of fixfree codes, so called Quasisymmetric fixfree codes, with
certain symmetry properties and provided sufficient conditions for the
existence of these codes. In fact, we imposed the symmetric condition over
the whole codebook rather than each single codeword.
This means that if a codeword is not a palindrome,
then the reverse codeword should be in the codebook. We first showed that
the set of Quasisymmetric fixfree codes is a strict subset of Asymmetric
fixfree codes, then provided evidence that the ¾ conjecture still
holds for this class of fixfree codes. In particular, we showed that the 5/8
upperbound is also a sufficient condition for
Quasisymmetric codes. 

Journal version: Published in IEEE
Transactions on Information Theory