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Cellular IoT -- long-range radio solutions

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

September 16, 2018

Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim SachsSeptember 16, 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).

 

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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

9.1.2.2 Long-Range Radio Solutions

For unlicensed spectrum usage, short-range radio systems are most common. However, for IoT applications that require very low data rates, it is possible to trade lower data rate for a longer transmission range. Many technology concepts have been developed in recent years for unlicensed LPWAN. Many different variants of unlicensed LPWAN exist; some of which are more often referred to are as follows:

  • LoRa: developed for deployment in unlicensed spectrum below 1 GHz,

  • Sigfox Ultra-Narrow Band (UNB): developed for deployment in unlicensed spectrum below 1 GHz,

  • Ingenu Random Phase Multiple Access (RPMA): developed for deployment in unlicensed spectrum at 2.4 GHz. 

All of those have in common that they target wireless M2M/IoT communication over a long range of multiple kilometers, where devices transmit only infrequently very low amounts of data. Message sizes are small and there is often a focus on uplink transmission. Devices are desired to be simple and battery-powered operation should be possible over extended time periods. All these technologies are proprietary and not standardized in standards developing organizations. 

In the following we provide a briefly overview of the LoRa, Sigfox, and RPMA technologies. 

9.1.2.2.1 LoRa 

LoRa is a network technology designed to provide long-range connectivity to battery operated devices; it is specified within an industry alliance. The LoRa Alliance claims to provide a Maximum Coupling Loss (MCL) of 155 dB in the European 867-869 MHz band, and 154 dB in the United States 902-928 MHz band [45]. LoRa has the target to provide secure bidirectional communication. LoRa operates in the sub-GHz unlicensed frequency bands. The physical layer is based on Chirp spread-spectrum modulation technology, and it can use one or more channels. The channel bandwidth is mainly 125 kHz for European spectrum bands, and 125 or 500 kHz for US spectrum bands. Different data rates are supported and are reported to lie in the range of 300 bpse50 kbps. The selection of data rate is a trade-off between transmission duration, i.e., the time during which the message is transmitted over the air and range. LoRa does not deploy LBT but instead uses the duty cycle restrictions required by regulation and the maximum dwell time (i.e., the maximum time that a device may continuously occupy a channel). The system architecture comprises LoRa end devices, LoRa gateways, and a network server. LoRa gateways correspond to base stations in a cellular network. Communication is between the end device and the network server. The communication between the network server and the gateway is based on IP communication; the communication between the end device and a gateway is based on LoRa specific protocols without IP. IP communication is terminated at the LoRa gateway. When a device transmits in uplink, the message can be received by one or more gateways.

For bidirectional communication a downlink transmission opportunity is provided after an uplink transmission. If downlink data arrives for a device in between uplink transmission, the data needs to be buffered in the network and can only be transmitted during the devices downlink receive window, which follows on an uplink transmission by the device.

For more information on LoRa see Reference [46].

9.1.2.2.2 Sigfox

Sigfox is a proprietary technology and of which no specifications are publicly available. Some indicative properties of the so-called UNB communication scheme of Sigfox is provided in Third Generation Partnership Project (3GPP) contribution [47] and technical reports of an industry specification group in ETSI [48,49].

UNB is targeted for operation in sub-GHz unlicensed spectrum bands. The channel bandwidths are 600 Hz in the United States and 100 Hz elsewhere [49]. Data rates are limited in the order of some few hundreds of bits per second. The maximum payload size for uplink data is 12 bytes. UNB does not use LBT but applies duty cycle limitations per transmitter. The channel access scheme is based on ALOHA, which starts to deteriorate at higher loads when the channel utilization exceeds around 15% [47].

The Sigfox network architecture comprises devices, which communicate with Sigfox servers. The radio communication is between the devices and Sigfox access points or base stations. Devices can transmit at any time without prior synchronization to the network. Typically, messages are transmitted on three different uplink channels, which are randomly selected. The base stations observe the entire system bandwidth to detect and decode uplink data. Messages can be received by different base stations, which provide selection diversity.

Downlink transmission is “piggybacked” onto uplink transmissions. After an uplink transmission a device maintains an open receiver window to receive downlink data for a certain time. The server sends buffered data to the device after receiving an uplink message. If the server has received uplink data via multiple base stations, it selects one of the base stations for downlink transmission.

9.1.2.2.3 RPMA

RPMA is a proprietary LPWAN technology provided by the company Ingenu. In contrast to Sigfox and LoRa it operates in the unlicensed 2.4 GHz band, which is globally available. No technical specification of the technology is publicly available. RPMA uses DSSS modulation and applies a pseudo- random time of arrival that helps separating users that are multiplexed on the same radio resource [50]. Transmission consists of two slots, a downlink slot and an uplink slot. Variable packet sizes can be transmitted within a slot. RPMA uses closed loop power control and a large range of spreading factors can be selected. The transmission is adapted to the estimated link performance. It is claimed that RPMA supports an extreme maximum path loss of around 170 dB.

The next installment in this series will discuss the benefits of cellular IoT.

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

 

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