Background - Smartphone-Based Human Activity Recognition - page 16

approaches directly from the users. This characteristic, known as opportunistic sens-
ing (Sect. 2.4.3 ), along with the high availability of these mass-marketed devices
which nearly cover the entire population, seems an interesting area to be exploited
for its application in HAR.

2.4.1 Characteristics and Selection Criteria

Various essential aspects are required for the selection of an appropriate smartphone
for HAR applications. These include:

• Hardware specifications, regarding memory capacity, processor speed, and power

consumption able to cope with the algorithm requirements. Other aspects, such
as size and weight, need to be considered as they could contribute to the device's
obtrusiveness during use.

• Mobile Operating System (OS), which is in charge of the device's resource man-

agement and controls the operation of apps. Moreover, other characteristics such
as open source model availability, user interface and developing interface should
be taken into account.

• Sensors available, this aspect examines the type of sensors embedded on the device

(and possible connection with others), their specifications and access possibilities
through the OS.

First mobile phones used embedded systems to control their operation. In con-
trast, smartphones now employ mobile OSs for this purpose. Their systems have
similar characteristics to OSs in PCs such as being responsible for providing com-
mon services to apps and programs, and the management of resources and hardware.
However, OSs for smartphones are also highly oriented towards energy efficiency
due to the device's mobile nature and battery limitations.
Furthermore, the development of a newgeneration of portable devices (e.g. tablets,
laptops, etc.) is helping to vanish the line from fixed to mobile computing. This
extends the possibility of transferring smartphone tools easily, not only to differ-
ent OSs but also devices. As an example, Microsoft Windows latest OS's platform
(Windows 8) has hybrid characteristics that allows its operation either in personal
computers and tablets (Honeycutt 2012 ).
The smartphone operation is not limited only to its OS, it should also be accom-
panied with an appropriate hardware. The computational power of high-end smart-
phones is nowadays not far from the one in standard personal computers regarding
processor speeds and communication services. Nevertheless, substantial improve-
ments are needed in some aspects. Particularly and most importantly in battery life.
It is, in general, very short, in the order of hours rather than days as it used to be with
first mobile phones. Memory management and shared resources are other aspects
that affect the device performance and also need special attention. Meanwhile, we
overcome these issues by developing light and energy efficient applications. These
aspects are then taken into consideration for the design of our HAR systems.