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Abstract
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Large-scale deployments of the Internet of Things (IoT) are adopted for performance
improvement and cost reduction in several application domains. The four main IoT application
domains covered throughout this article are smart cities, smart transportation, smart healthcare, and
smart manufacturing. To increase IoT applicability, data generated by the IoT devices need to be
time-stamped and spatially contextualized. LPWANs have become an attractive solution for outdoor
localization and received significant attention from the research community due to low-power,
low-cost, and long-range communication. In addition, its signals can be used for communication
and localization simultaneously. There are different proposed localization methods to obtain the
IoT relative location. Each category of these proposed methods has pros and cons that make them
useful for specific IoT systems. Nevertheless, there are some limitations in proposed localization
methods that need to be eliminated to meet the IoT ecosystem needs completely. This has motivated
this work and provided the following contributions: (1) definition of the main requirements and
limitations of outdoor localization techniques for the IoT ecosystem, (2) description of the most
relevant GNSS-free outdoor localization methods with a focus on LPWAN technologies, (3) survey
the most relevant methods used within the IoT ecosystem for improving GNSS-free localization
accuracy, and (4) discussion covering the open challenges and future directions within the field.
Some of the important open issues that have different requirements in different IoT systems include
energy consumption, security and privacy, accuracy, and scalability. This paper provides an overview
of research works that have been published between 2018 to July 2021 and made available through
the Google Scholar database.
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