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3
sensors and applying fixed sensor duty cycles so that the
proposed framework could recognize user states through
smartphone sensors while improving device battery life-
times. Unfortunately, sensors have fixed duty cycles, and
also they are not adjustable to respond differently to variant
user behaviors. In addition, energy consumption is reduced
by shutting down unnecessary sensors at any particular
time. On the other hand, classification of sensory data is
based on pre-defined test classification algorithms. Apart
from these studies, many other works have emphasized
to use deterministic sampling period schemes [10], and
to maximize power efficiency by solely applying less
complexity in computations or by changing transferring
methods of inferred contextual data packets [9]. The other
popular method is to fuse multiple sensory information to
decide future employment of a specific sensor, especially
in localization applications [11], [12].
This paper differs from other studies in the following
ways. First, this paper consider physical world as inho-
mogeneous. Therefore, the inhomogeneity is characterized
by time-variant system parameters. Second, adaptability
challenge in response to variant and rapid user activities is
integrated as well using the convergence of entropy rate in
conjunction with the inhomogeneity. Accordingly, entropy
rate is used to make an assumption on accurate working
of system parameters regulated by an ongoing stochastic
process. Third, power saving considerations are taken at the
low-level sensory operations. Fourth and most importantly,
a machine learning structure regulates sensor management
by estimating the trend of user preferences, and oppor-
tunistically finding out stable moments in user activity.
Thereby, sensor management could apply optimal sensing
policies, and change sensor sampling settings to respond
the defined tradeoffs in context-aware application services.
Finally, missing contextual inferences are estimated while
energy saving strategies are being applied.
III. PROPOSED FRAMEWORK
Context aware sensing systems have been put forward to
provide a required model for recognition of daily occurring
human activities via observations acquired by various sen-
sors built in mobile devices. These activities are inferred
as outcomes of a wide range of sensor applications utilized
in such areas of environmental surveillance, assisting tech-
nologies for medical diagnosis/treatments, and creation of
smart spaces for individual behavior model. Key challenges
that are faced in this concept is to infer relevant activity
in such a system that takes raw sensor readings initially
and processes them until obtaining a semantic outcome
under some constrictions. These constrictions mostly stem
from difficulty of shaping exact topological structure and
from modeling uncertainties in the observed data due to
saving energy wasted while physical sensor operations
and processing of data are being undergone. Finally, there
is not a common framework system which covers all
types of application settings, provides an adaptation toward
changing context, and acquires a collection of asynchronous
heterogeneous context to create different abstract entities.
Even, none of current frameworks succeeds to have a
full transparency, which eliminates a direct involvement of
applications into context modeling process, by imposing
less computational workload on resource-limited mobile de-
vices. In this direction, gathering diverse and asynchronous
information, and presenting it to the application would
be the future work in context-aware framework research,
which this paper intends to enlighten. By this means, this
paper could help the exciting vision of “Internet-of-Things”
[13] while creating a knowledge network which capable
of making autonomous logical decision to actuate environ-
mental objects and also to assist individuals, especially in a
resource-constrained smart device. In addition, this research
could give a solution to effective manage fusion of data
gathered from multiple sensor applications.
To this end, this paper proposes an inhomogeneous (time-
variant) Hidden Markov Model (HMM) based framework in
order to represent HAR based user states by defining them
as an outcome of either recognition or estimation model.
A statistical tool-based classification, mostly using Hidden
Markov Models (HMMs) [14], [15] or using AutoRegres-
sive (AR) [16] models, is one of the foremost methods to
infer context obtained via wearable or built-in smart device
sensors in HAR based applications. However, these stud-
ies mostly allow predefined and user-manipulated system
parameter settings, such as arbitrary formation of context
transition matrix in HMMs, or building filtering coefficients
in ARs, which is not suitable for online processing due
to increasing computational workload while enlarging the
data size. Therefore, a statistical model is added into our
approach to track time-variant user activity profiles in order
to predict the best likely user state that fits into instant
user behavior. The inhomogeneity is characterized by time-
variant system parameters, and the user profile adaptability
challenge is modeled using the convergence of entropy
rate. Accordingly, an implemented smartphone application
is provided to demonstrate how entropy rate converges
in response to distinctive time-variant user profiles under
different sensory sampling operations. The proposed frame-
work is designed to be based on a statistical machine to
obtain a better realization in context-awareness in order to
create adaptability to time-variant user preferences and be-
haviors, estimate missing context inferences in presence of
idle sensory operations, and also preserve the functionality
against aperiodically received sensory observations.
Most importantly, which is the key of this study, a
machine learning structure regulates sensor management
opportunistically to figure optimal sensing policies, and
change sensor sampling settings such as varying sensory
sampling and duty cycling so as to power efficiency could
be achieved while satisfying the accuracy of context-aware
application services.
The following two sections give further information
about two inter-operated core modules that our proposed
framework has: context inference module and sensor man-
agement system.