Cellular IoT -- unlicensed spectrum usage
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).
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Adapted from Cellular Internet of Things, by Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs.
Chapter 9. The competitive Internet of Things technology landscape (Cont.)
By Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs
Cellular IoT -- unlicensed spectrum usage
The most common form of unlicensed spectrum usage is for short-range communication. One reason is that short-range communication provides a certain level of interference robustness. The amount of interference that a receiver is exposed to depends very much on the location of the interferer. Assuming that different transmitters in unlicensed spectrum use similar output powers, e.g., the maximum that is allowed by regulation, an interferer can be considered as a strong interferer if it is located closer to the receiver than the intended transmitter. Devices that jointly form a local network typically have some coordination or coexistence functionality provided by the wireless communication standard they are using, which avoids or reduces interference within the local network. However, other unlicensed radio technologies are typically not part of this interference coordination. In Figure 9.2, it is depicted how different groups of devices use various unlicensed communication technologies. If these devices are separated in space, the interference is limited because it is typically significantly below the power levels of the communication within the group. However, if different unlicensed radio technologies operate at the same location, significant interference can occur.
FIGURE 9.2 Coexistence among different groups of unlicensed devices. Intersystem interference is most severe if different groups are overlapping in space.
Figure 9.3 shows the challenge of long-range communication in unlicensed spectrum. With long-range communication it becomes more likely that an interferer is located closer to the receiver than the intended transmitter. The example of the figure shows a long-range system that is designed to cover a large path loss of e.g., 150 dB for transmission over several kilometers. There may exist several other local unlicensed networks using the same spectrum in vicinity of the long-range receiver. If the long-range receiver is, e.g., placed on the roof of a building, there may be some local unlicensed networks used in the same or neighboring buildings, e.g., for home automation. Because these devices are significantly closer to the long-range receiver, they may cause interference at the location of the long-range receiver, which is significantly higher, by e.g., several 10’s of dB, than the strongly attenuated signal of the long-range transmitter, which is coming from far away.
FIGURE 9.3 Coexistence between long-range and short-range devices.
Furthermore, if we assume that the devices in the local network are adaptive devices, which e.g., use LBT to avoid interfering with other devices, this operation is likely to fail to adapt to long-range transmitters that are far away because the long-range signal is so strongly attenuated that it is below a sensitivity threshold used for CCA. As long as unlicensed spectrum is barely used, such interference situations may be unlikely. If it is anticipated that unlicensed IoT use cases (and other use cases) will drive the deployment of various local area networks using unlicensed spectrum the inference in unlicensed spectrum will increasingly play a role; long-range unlicensed radio tech nologies are more exposed to this interference.
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