Cellular IoT -- short-range radio solutions

Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs

June 19, 2018

Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim SachsJune 19, 2018

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). This series dives more deeply into options for IoT connectivity, compares the alternatives, and describes key selection criteria for choosing among the options. This article continues the examination of IoT connectivity introduced in part 1 and part 2.

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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 Short-Range Radio Solutions

In this section we provide an overview of the most promising unlicensed short-range radio communication technologies for the IoT, which are IEEE 802.15.4, Bluetooth Low Energy (BLE) and Wi-Fi HaLow. The choice to focus on these is made based on the following properties of those technologies: they address the communication requirements of IoT, they target an IP-based IoT solution according to Figure 9.5 and the right-hand side in Figure. 9.4, and they are based on open standards and are expected to reach a substantial economy of scale. We also address the capillary network architecture where any of these solutions may be used to provide the short-range connectivity within a larger network context. IEEE 802.15.4

One of the early standards for 6LoWPAN is IEEE 802.15.4 [10e12]. It was standardized in 2003 at a time of intense research on wireless sensor networking technologies [13,14], and it is applicable for a wide range of IoT use cases, ranging from office automation, connected homes to industrial use cases. Several application-specific protocol stacks have been developed, which build on parts of the IEEE 802.15.4 standard (mostly the physical layer and to some extend the medium access control (MAC)) [15], including ZigBee, WirelessHART, ISA-100, and Thread.

IEEE 802.15.4 has been specified for the three frequency bands of 868 MHz (for Europe), 915 MHz (for United States), and 2.4 GHz (global) [16e18]. In the 2.4 GHz band, IEEE 802.15.4 has 16 channels available of 2 MHz bandwidth. It uses offset quadrature phase shift keying with DSSS and a spreading factor of 8. A gross data rate of 250 kbps is achievable, see References [16,18]. In 868 MHz one channel of 600 kHz is available, which uses differential binary phase shift keying modulation and DSSS with a spreading factor of 15. The achievable data rate at 868 MHz is 20 kbps [16,18]. In the 915 MHz 10 channels are available with a gross data rate of 40 kbps, see Reference [18].

IEEE 802.15.4 uses carrier-sense multiple access with collision avoidance (CSMA-CA) for access to the radio channel; this can be complemented with optional Automatic Repeat Request (ARQ) retransmissions. Typical coverage ranges are in the order of 10e20 m [11]. Two different topologies are supported: star topology and mesh (or peer-to-peer) topology. Two types of devices are defined: full-function devices that provide all MAC functionality and can act as network coordinator of the local network, and reduced function devices that can only communicate with a full-function device and are intended for very simple types of devices. The network can operate in beaconed mode, which allows a set of devices to synchronize to a superframe structure that is defined by the beacon transmitted by a local coordinating device. The channel access in this case is slotted CSMA-CA. In non- beacon mode, unslotted CSMA-CA is applied. In case of direct data transmission, a device transmits data directly to another device. In indirect data transmission, data is transferred to a device, e.g., from a network coordinator. When beacon transmission is active, the network coordinator can indicate the availability of data in the beacon; the device can then request the pending data from the network coordinator. In nonbeaconed transmission, the network coordinator buffers data and it is up to the device to contact the network coordinator for pending data.

In Reference [18] the performance of IEEE 802.15.4 has been evaluated in an experiment with an ideal link and devices placed at 1 m distance. It has been found that for a configuration at 2.4 GHz with a theoretical gross data rate of 250 kbps, a net data rate of 153 kbps was measured for direct transmission (from the device), and a net data rate of 66 kbps for indirect transmission (towards the device). Furthermore, it has been shown that the effective data rate and delivery ratio decrease with an increasing number of devices.

A major step for broader relevance of IEEE 802.15.4 for the IoT has been to address end-to-end IP-based communication. To this end the IETF working group 6LoWPAN has been chartered in 2005 and it has developed IETF standards for header compression and data fragmentation. The maximum physical layer payload size of 802.15.4 is limited to 127 bytes, which is further reduced by various protocol headers and optional security overhead and can leave as little as 81 bytes available for application data within an IEEE 802.15.4 frame. IETF has developed standards that provide header compression and IP packet fragmentation that enable the transmission of IPv6 over 802.15.4 networks [17,19e22]. In addition, the RPL routing protocol has been developed to enable IP mesh routing over IEEE 802.15.4 [17,22,23]. In 2014 the Thread group was formed with the objective to harmonize the usage of IEEE 802.15.4 together with 6LoWPAN for home automation.

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