The Cellular Internet of Things -- Low power wide area networks
Editor's Note: Growing requirements for increased availability of IoT devices coincide with the emergence of cellular technologies well suited for the IoT. For developers, the need has never been more acute for more detailed information about cellular technologies and their application to the IoT.
Excerpted from the book, Cellular Internet of Things, this series introduces key concepts and technologies in this arena. In part one, the authors described the evolving landscape for cellular and its role in the IoT, while part two reviewed massive machine-type communications (mMTC) and ultra reliable low latency communications (URLLC). This installment discusses technologies and market factors related to low power wide area networks.
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Adapted from Cellular Internet of Things, by Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs.
1.3 LOW POWER WIDE AREA NETWORKS
By Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs
1.3.1 AN INTRODUCTION TO TECHNOLOGIES FOR UNLICENSED OPERATION
As mentioned in Section 1.1, the 3GPP cellular technologies are not the only solutions competing for IoT traffic. Also well-known technologies such as Bluetooth and Wi-Fi can serve as bearers for MTC traffic. A distinction between the group of cellular technologies and Bluetooth and Wi-Fi is that the former is intended for operation in licensed spectrum while the latter two belong to the group of systems operating in unlicensed spectrum, in so-called license exempt frequency bands.
Licensed spectrum corresponds to a part of the public frequency space that has been licensed by national or regional authorities to a private company, typically a mobile network operator, under the condition of providing a certain service to the public such as cellular connectivity. At its best, a licensed frequency band is globally available, which is of considerable importance for technologies aiming for worldwide presence. The huge success of GSM is, for example, to a significant extent built around the availability of the GSM 900 MHz band in large parts of the world. Licensed spectrum is, however, commonly associated with high costs, and the media frequently give reports of spectrum auctions bringing in significant incomes to national authorities all across the world.
Unlicensed spectrum, on the other hand, corresponds to portions of the public frequency space that can be said to remain public and therefore free of licensing costs. Equipment manufacturers using this public spectrum must, however, meet a set of national or regional technical regulations for technologies deployed within that spectrum. Among of the most popular license exempt frequency bands are the so-called industrial, scientific and medical (ISM) bands identified in article 5.150 of the ITU Radio Regulations . Regional variations for some of these bands exist, for example, in the frequency range around 900 MHz while other bands such as the range around 2.4 GHz can be said to be globally available. In general, the regulations associated with license exempt bands aim at limiting harmful interference to other technologies operating within as well as outside of the unlicensed band.
As the name implies, the ISM bands were originally intended for ISM applications. Later, it was made available for devices providing more general types of services such as short-range devices benefiting from operation in a set of harmonized frequency bands . Bluetooth and Wi-Fi, and thereto related technologies such as Bluetooth Low Energy, ZigBee, and Wi-Fi Halow, commonly use the ISM bands to provide relatively short-range communication, at least in relation to the cellular technologies. Bluetooth can be said to be part of a Wireless Personal Area Network while Wi-Fi provides connectivity in a Wireless Local Area Network (WLAN). In recent years, a new set of technologies have emerged in the category of LPWANs. These are designed to meet the regulatory requirements associated with the ISM bands, but in contrast to WPAN and WLAN technologies they provide long-range connectivity, which is an enabler for supporting wireless devices in locations where WPAN and WLAN systems cannot provide sufficient coverage.
1.3.2 LICENSED AND LICENSE EXEMPT BAND REGULATIONS
To understand the potential of LPWAN solutions operating in license exempt bands in relation to those designed for licensed bands, it is important to understand the regulations setting the ultimate boundary for the design of the systems. Systems deployed within both licensed and unlicensed bands operate in accordance with regulations that are determined on a national or regional basis. For the cellular systems these regulations as a general rule follow the requirements set by the 3GPP technical specifications. For license exempt bands, the regulations are not coordinated across regions to the same extent, and the local variations are higher. For example, in the United States it is the Federal Communications Commission (FCC) that publishes the Electronic Code of Federal Regulations, which defines the regulations for operation in the license exempt bands 902-928 MHz and 2400-2483.5 MHz , while in Europe it is the European Telecom Standards Institute (ETSI) that publishes the Harmonized standards for the license exempt bands 863-870 MHz  and 2400-2483.5 MHz .
The 3GPP requirements are, on one hand, defined to secure coexistence between 3GPP systems operating adjacent to each other within the same band as well as between systems operating in different bands. On the other hand, they define in-band requirements that guarantee a minimum level of performance of the system as a whole as well as the performance on a per link basis. In case the regulatory bodies add or modify requirements compared with the ones set by 3GPP, the impact is typically limited to emission levels outside of the 3GPP bands, which have limited impact on the fundamental aspects of the system design.
The requirements for license exempt bands are defined to support coexistence toward other systems outside the band as well as inside the band. Inside the band the different systems are not as in licensed operation separated in the frequency domain but are overlapping in the sense that they may use the same frequency resource at any point in time. To limit the interference between the unlicensed systems, regulations are commonly defined to limit the used output power. In addition, requirements on the duty cycle as well as the dwell time on a specific frequency resource may be defined. The duty cycle defines the ratio by which a transmitting device may use a radio resource. The dwell time, on the other hand, sets the maximum contiguous time by which a transmitting device may use a radio resource. These requirements establish strict design boundaries that make it challenging for any systems operating in the ISM bands to provide high and robust coverage for a multitude of devices while meeting service requirements on, e.g., latency and throughput.
Chapter 9 will further review the regulations for unlicensed operation including the herein introduced requirements.