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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One aspect that is worth to mention for unlicensed spectrum, is about the maximum transmit power that may be emitted by the device. The maximum transmit power provided by spectrum regulation is typically given with respect to a certain reference configuration of emission. The reason is that different antenna configurations have different power emission patterns, which leads to that the antenna has different gains in different directions. Often maximum power levels are defined as either effective isotropically radiated power or as ERP, as shown above for Tables 9.1-9.4. For EIRP an ideal isotropic antenna is assumed, whereas for ERP a half-wave dipole antenna is assumed, which has a 2.15 dB antenna gained compared with an isotropic antenna in direction of the highest antenna gain as specified in Eq. (9.1). When the maximum transmit power is specified as EIRP or ERP, a transmitter with any antenna configuration should not emit power in any direction that would exceed the maximum power that could be emitted in this direction with an isotropic or dipole antenna, respectively. A practical result of this is that if a real antenna has an antenna gain of X dB with a maximum allowed radiated power of Y dBm EIRP, then the transmitter must limit its conducted power at the antenna port to Y-X dBm. One consequence is that downlink performance can be substantially limited compared with uplink, if a base station antenna with significant antenna gain is used. In uplink direction, the base station can use the antenna gain at the receiver side to improve the link budget. In the reverse downlink direction, the base station has to compensate the antenna gain by a reduced transmit power resulting in that the base station antenna gain does not improve the actual link budget, such as maximum path loss. This is one significant difference for a communication system operating in unlicensed spectrum compared with licensed spectrum, where the antenna gain could also be used in downlink direction and where higher maximum radiated powers are permitted in downlink.

Another cause for asymmetry of uplink and downlink in unlicensed spectrum can be found regarding capacity for nonadaptive devices that are limited by a duty-cycle. As an example, we assume a large number of N devices connected to a single base station, each device transmitting at a rate Ru and being limited by a maximum duty cycle of Du . The maximum achievable uplink capacity is then limited by the maximum data rate and the duty cycle limitation for each device. Assuming an ideal situation of all transmissions being successful by neglecting all possible collisions and interference situations, the upper bound of maximum achievable uplink capacity Cu becomes in this case

In downlink, the capacity of the base station is limited by its own maximum duty cycle. This duty cycle has to be used for the transmission to all N devices, in contrast to uplink where a duty cycle is valid per device. The upper bound of maximum achievable downlink capacity Cd under ideal error free transmissions becomes then

for a downlink data rate Rd and a maximum duty cycle of Dd . Assuming the same uplink and downlink parameters, the downlink capacity is thus by a factor of N smaller compared with the uplink capacity, which leads to a significantly lower achievable effective downlink data rate per device. To some extent this can be compensated by configuring downlink transmissions to certain subbands that allow higher duty cycles (increasing Dd ), see e.g., Table 9.1.

9.1.2 RADIO TECHNOLOGIES FOR UNLICENSED SPECTRUM

Unlicensed spectrum enables immediate market access for any new radio technology with minimal regulatory requirements. As a result, a very large number of different Machine-to-Machine (M2M) connectivity solutions have been developed and brought to the market making use of this spectrum. Most of them are designed to satisfy a very particular application and communication needs. Examples are connectivity for remote-controlled lighting, baby monitors, electric appliances, etc. For many of those systems the entire communication stack has been designed for a single purpose. Even if it enables, in a wider sense, an environment with a wide range of connected devices and objects, it is based on M2M technology solution silos usually without end-to-end IP connectivity and instead via proprietary networking protocols as depicted on the left hand side in Figure 9.4. This is quite different from the vision of the IoT (depicted on the right hand side in Figure 9.4), which is based on a common IP-based connectivity framework for connecting devices and smart objects, which enables the IoT at full scale.


FIGURE 9.4 From M2M technology silos to the IoT.


FIGURE 9.5 IoT protocol stack.

Different industrial alliances and standardization organizations, such as IPSO Alliance, IETF, are promoting an IP-based framework for connecting smart objects and devices, are defining corresponding components, and are developing according open standards. Figure 9.5 provides an overview of such a harmonized protocol stack, which has at its center the common IP connectivity protocols, such as IPv6, transmission control protocol (TCP), user datagram protocol (UDP), and the security protocols transport layer security (TLS) and datagram transport layer security (DTLS). Several additional protocols have been developed to simplify the communication procedures for small and constrained devices. Besides the transaction protocol HTTP, which is already widely used for Internet services like WWW, a simplified IoT-focused version for constrained devices has been developed as the Constrained Application Protocol (CoAP). Also the Message Queue Telemetry Transport (MQTT) protocol is widely used as publish/subscribe messaging protocol for IoT services. Lightweight M2M, specified by the Open Mobile Alliance (OMA), is a device management protocol for IoT. With these IoT application and communication protocols, IP-based IoT services can be provided via a variety of communication technologies, which enable IP-based transmission. Some simple wireless communication technologies have difficulties to cope with the overhead provided by IP-based communication.

For this reason, the adaptation framework IPv6 over low power wireless personal area networks (6LoWPAN) has been specified in IETF to enable communication over very constrained wireless communication technologies; this work is continued in IETF under the label “6lo” in the working group IPv6 over Networks of Resource-constrained Nodes.  

The next installment in this series discusses short-range radio solutions.

Reprinted with permission from Elsevier/Academic Press, Copyright © 2017

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