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 an earlier series, the authors described the evolving landscape for cellular, its role in the IoT, and technologies for massive machine-type communications (mMTC) and ultra reliable low latency communications (URLLC).
The 2.15 dB offset stems from the different reference antennas, i.e., dipole and isotropic, assumed for EIRP and ERP. It can be noticed that the permitted radiated power is higher for the unlicensed spectrum in the United States than in Europe. The European recommendations rely on duty cycle limitations, i.e., the percentage of time a transmitter may be active within a defined time span, while the US regulations defines maximum dwell times, i.e., the maximum continuous time by which a transmitting device may use a specific radio resource (e.g., a specific hopping channel). Regardless whether the limitations are on duty cycle or dwell time, these limits aim to avoid persistent interference. A short allowed dwell time can be understood to limit the coverage of a system, whereas a strict duty cycle requirement may limit coverage and system capacity. The duty cycle also limits the availability of downlink and uplink transmission opportunities which has a negative impact on service latency.
European recommendation allows to deviate from duty cycle limitations in case listen-before-talk (LBT) is used, i.e., when a transmitter first observes if a channel is not used by other devices before starting to transmit. Sometimes LBT needs to be combined with adaptive frequency agility (AFA), which is a method for avoiding transmission on already occupied channels (see e.g., Reference ).
The different rows in Tables 9.1 and 9.2 can be understood to cater for different types of applications and different types of equipment. For more details please see References [2,4].
The spectrum usage recommendation for unlicensed spectrum at 2400-2483.5 MHz in Europe is given in Table 9.3 and the rules for the United States in Table 9.4. Also in this band the maximum allowed radiated power is higher in United States than in Europe. The European power limitation is given as EIRP. Higher radiated power is allowed for wide band transmission such as DSSS modulation, FHSS, and OFDM under the condition of using a mitigation method such as LBT.
Table 9.3 European unlicensed spectrum at 2400-2483.5 MHz, for more details see Reference 
Table 9.4 US unlicensed spectrum at 2400-2483.5 MHz, for more details see Reference 
22.214.171.124 Spectrum Coexistence in Unlicensed Spectrum
Different types of radio equipment and technologies can transmit on any frequency of the unlicensed spectrum. As depicted in Figure 9.1, the simultaneous transmissions of different devices interfere with each other. As a result, this interference may lead to that one or both of the transmissions depicted in the figure fail. Spectrum coexistence mechanisms are mechanisms that limit the interference that a transmitter may cause on other nearby devices.
FIGURE 9.1 Transmission of two different unlicensed devices to their corresponding receivers.
The simultaneous usage of unlicensed spectrum can occur between homogenous or heterogeneous types of devices, which means devices that use the same or different wireless communication technologies. A wireless communication technology designed for unlicensed spectrum, often, has some mechanism, which specifies how the spectrum is shared among different devices of that communication technology, so that each device has good transmission opportunities, while at the same time minimizing the interference to other devices. The coordination scheme typically follows the spectrum regulation introduced earlier but may also have additional technology specific features. While such a coordination scheme can be applied within one wireless communication technology, it can typically not provide the same level of interference mitigation in-between different wireless communication technologies.
The spectrum regulation for unlicensed spectrum provides requirements on devices, which shall provide technology neutral coexistence, i.e., independent of a particular wireless communication technology being used. Spectrum regulation has thereby mainly two types of communication devices in mind: adaptive devices and nonadaptive devices.
Nonadaptive devices are considered to transmit in unlicensed spectrum, while staying unaware of the other types of devices. To limit the amount of interference that can be generated, the devices are limited as follows:
- In the radiated power they may use, see Tables 9.1-9.4, sometimes further limited by an allowed power spectral density,
- By a duty cycle or dwell time, see Tables 9.1 and 9.2, which limit what fraction of time a device may transmit on a channel.
By these means the total amount of interference a device may cause on others is restricted.
In contrast, an adaptive device is aware of other devices in its vicinity, which also make use of the same channel. As a result, it can adapt its transmission, reducing the interference to other devices. This provides a device with the opportunity to transmit for a longer amount of time if there are few devices in the surrounding. In contrast, if many devices in the vicinity want to use the unlicensed spectrum, the adaptive device reduces the frequency of its transmissions and provides less interference. The common way to provide this type of adaptive device is LBT, where a device before a transmission listens on the radio channel and observes if other devices are communicating, and after a clear channel assessment (CCA) it transmits for a limited time. In addition, technical standards consider a fair use of spectrum among devices, and thus consider that at a high utilization of the spectrum an adaptive device transmits less than in case of a low utilization.
The next installment in this series examines unlicensed spectrum usage for short-range communication and discusses the challenges of its use for long-range communications.
Reprinted with permission from Elsevier/Academic Press, Copyright © 2017