Cellular IoT -- unlicensed spectrum usage
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