The Cellular Internet of Things — Low power wide area networks

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 part one, the authors described the evolving landscape for cellular and its role in the IoT, while part two reviewed massive machine-type communications (mMTC) and ultra reliable low latency communications (URLLC). This installment discusses technologies and market factors related to low power wide area networks. 

Elsevier is offering this and other engineering books at a 30% discount. To use this discount, click here and use code ENGIN318 during checkout.

Adapted from Cellular Internet of Things, by Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs.

1.3 LOW POWER WIDE AREA NETWORKS
By Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs

1.3.1 AN INTRODUCTION TO TECHNOLOGIES FOR UNLICENSED OPERATION

As mentioned in Section 1.1, the 3GPP cellular technologies are not the only solutions competing for IoT traffic. Also well-known technologies such as Bluetooth and Wi-Fi can serve as bearers for MTC traffic. A distinction between the group of cellular technologies and Bluetooth and Wi-Fi is that the former is intended for operation in licensed spectrum while the latter two belong to the group of systems operating in unlicensed spectrum, in so-called license exempt frequency bands.

Licensed spectrum corresponds to a part of the public frequency space that has been licensed by national or regional authorities to a private company, typically a mobile network operator, under the condition of providing a certain service to the public such as cellular connectivity. At its best, a licensed frequency band is globally available, which is of considerable importance for technologies aiming for worldwide presence. The huge success of GSM is, for example, to a significant extent built around the availability of the GSM 900 MHz band in large parts of the world. Licensed spectrum is, however, commonly associated with high costs, and the media frequently give reports of spectrum auctions bringing in significant incomes to national authorities all across the world.

Unlicensed spectrum, on the other hand, corresponds to portions of the public frequency space that can be said to remain public and therefore free of licensing costs. Equipment manufacturers using this public spectrum must, however, meet a set of national or regional technical regulations for technologies deployed within that spectrum. Among of the most popular license exempt frequency bands are the so-called industrial, scientific and medical (ISM) bands identified in article 5.150 of the ITU Radio Regulations [9]. Regional variations for some of these bands exist, for example, in the frequency range around 900 MHz while other bands such as the range around 2.4 GHz can be said to be globally available. In general, the regulations associated with license exempt bands aim at limiting harmful interference to other technologies operating within as well as outside of the unlicensed band.

As the name implies, the ISM bands were originally intended for ISM applications. Later, it was made available for devices providing more general types of services such as short-range devices benefiting from operation in a set of harmonized frequency bands [10]. Bluetooth and Wi-Fi, and thereto related technologies such as Bluetooth Low Energy, ZigBee, and Wi-Fi Halow, commonly use the ISM bands to provide relatively short-range communication, at least in relation to the cellular technologies. Bluetooth can be said to be part of a Wireless Personal Area Network while Wi-Fi provides connectivity in a Wireless Local Area Network (WLAN). In recent years, a new set of technologies have emerged in the category of LPWANs. These are designed to meet the regulatory requirements associated with the ISM bands, but in contrast to WPAN and WLAN technologies they provide long-range connectivity, which is an enabler for supporting wireless devices in locations where WPAN and WLAN systems cannot provide sufficient coverage.

1.3.2 LICENSED AND LICENSE EXEMPT BAND REGULATIONS

To understand the potential of LPWAN solutions operating in license exempt bands in relation to those designed for licensed bands, it is important to understand the regulations setting the ultimate boundary for the design of the systems. Systems deployed within both licensed and unlicensed bands operate in accordance with regulations that are determined on a national or regional basis. For the cellular systems these regulations as a general rule follow the requirements set by the 3GPP technical specifications. For license exempt bands, the regulations are not coordinated across regions to the same extent, and the local variations are higher. For example, in the United States it is the Federal Communications Commission (FCC) that publishes the Electronic Code of Federal Regulations, which defines the regulations for operation in the license exempt bands 902-928 MHz and 2400-2483.5 MHz [11], while in Europe it is the European Telecom Standards Institute (ETSI) that publishes the Harmonized standards for the license exempt bands 863-870 MHz [12] and 2400-2483.5 MHz [13].

The 3GPP requirements are, on one hand, defined to secure coexistence between 3GPP systems operating adjacent to each other within the same band as well as between systems operating in different bands. On the other hand, they define in-band requirements that guarantee a minimum level of performance of the system as a whole as well as the performance on a per link basis. In case the regulatory bodies add or modify requirements compared with the ones set by 3GPP, the impact is typically limited to emission levels outside of the 3GPP bands, which have limited impact on the fundamental aspects of the system design.

The requirements for license exempt bands are defined to support coexistence toward other systems outside the band as well as inside the band. Inside the band the different systems are not as in licensed operation separated in the frequency domain but are overlapping in the sense that they may use the same frequency resource at any point in time. To limit the interference between the unlicensed systems, regulations are commonly defined to limit the used output power. In addition, requirements on the duty cycle as well as the dwell time on a specific frequency resource may be defined. The duty cycle defines the ratio by which a transmitting device may use a radio resource. The dwell time, on the other hand, sets the maximum contiguous time by which a transmitting device may use a radio resource. These requirements establish strict design boundaries that make it challenging for any systems operating in the ISM bands to provide high and robust coverage for a multitude of devices while meeting service requirements on, e.g., latency and throughput.

Chapter 9 will further review the regulations for unlicensed operation including the herein introduced requirements.

1.3.3 LOW POWER VERSUS WIDE AREA

A LPWAN system is characterized by its ability to provide high coverage while using low device output power. The cellular IoT systems EC-GSM-IoT, NB-IoT, and LTE-M can, for example, all be said to belong to the category of LPWAN systems. Coverage of a wireless device is, in general, limited by the device-intrinsic thermal noise. The power of the noise is linearly increasing with increasing temperature and system bandwidth. The steepness of the linear increase is determined by the device’s Noise Factor, which resembles the difference in noise power measured in an ideal device and a real device implementation.

As the simplest approach to combat thermal noise power and improve the Signal to Noise power Ratio (SNR) is to increase a system’s useful signal power, it may seem like a contradiction to associate low power with wide area coverage as done for LPWANs. The use of low power is, however, highly motivated because it facilitates low device cost, enables compact device design, and supports flexible usage of a device for diverse applications. As explained in Section 1.3.2, low output power is also a typical requirement in systems operating in licensed exempt bands to limit the harmful interference within and between the systems using these bands.

Fortunately, there are means besides increased signal power to cope with challenging coverage conditions. Commonly, it is the choice of system bandwidth that becomes the decisive factor impacting the absolute noise level measured in a wireless device. This implies that a system designed to use a low signal bandwidth enjoys the benefit of operating at a lower absolute noise level, which for a given useful power level improves the SNR.

Figure 1.3 illustrates the impact on SNR when increasing the useful signal bandwidth from 1 kHz to 1 MHz for a system operating at a transmission power of 20 dBm, i.e., 100 mW, and at a constant ambient temperature of 20 C. The SNR is calculated for a coupling loss between the transmitter and receiver of 164 dB and assumes a 3dB receiver Noise Figure (NF), which is the logarithmic representation of the Noise Factor. 164 dB coupling loss corresponds to very challenging coverage conditions as elaborated in Section 1.2 while an NF of 3 dB is a value that can be considered realistic for a base station implementation.

In their regulations for the ISM bands the FCC and ETSI explicitly mention spread spectrum technologies as a means of conveying information. Spread spectrum is a family of technologies where a transmitter transforms, or spreads, a low bit rate information signal to a high bit rate and wideband carrier of low power spectral density. The intended recipient of the carrier is able to regenerate the useful signal to extract the payload information therein, while other users ideally experience the wideband carrier as a weak interferer with Gaussian noise-like characteristics. One of the most popular spread spectrum techniques is the Direct Sequence Spread Spectrum (DSSS) modulation where the transmitter spreading operation is defined by the multiplication of the useful signal with a known pseudo random spreading code of high bit rate. The ratio between the bit rate of the spreading signal, i.e., the chip rate, and the useful signal bit rate defines the processing gain by which the receiver can improve the SNR of the useful signal compared with the SNR over the received wideband carrier. In fact, with the spread spectrum technique, the achieved processing gain compensates for the lower SNR as a result of using a wider bandwidth signal shown in Figure 1.3, and eventually the SNR penalty of using a wider bandwidth signal is fully compensated for.

click for larger image

FIGURE 1.3 SNR versus system bandwidth at 164 dB MCL, a NF of 3 dB, and a signal power of 20 dBm.

When reading about the 3GPP systems in Chapters 3-8, it will become apparent that narrowband and spread spectrum modulations are merely two alternatives among a multitude of available methods that can be adopted to improve the coverage of a system. Improving the code rate of a technology by trading useful throughput for increased redundancy is one of the most established methods that is commonly used to increase the robustness of a wireless system. Another frequently used method is to introduce repetition-based transmissions to increase the overall transmission time of a radio block, which allows a system to improve the receiver processing gain. Just as in the case of the spread spectrum technique, through the achieved receiver processing gain, the repetition-based approach will eventually equalize the SNR penalty that can be associated with a wideband transmission, compared with transmission of relatively smaller bandwidth not using repetitions.

As explained in later chapters, these and other methods are combined in the design of EC-GSM-IoT, NB-IoT, and LTE-M to make sure that the systems using these technologies meet the ambitious MCL that 3GPP considers to be sufficient to provide the services required by a LPWAN solution.

1.3.4 A COMPETITIVE MARKET

The linear relationship between bandwidth and SNR depicted in Figure 1.3 is a fact utilized, e.g., by French LPWAN vendor Sigfox with their Ultra Narrowband Modulation. It uses a narrow bandwidth carrier to support a claimed MPL of 162 dB at the European 868 MHz and US 902 MHz ISM frequencies [14]. Sigfox is among the most successful LPWAN actors and supports coverage in considerable parts of Europe including nationwide coverage in Portugal, Spain, and France [15].

LoRa Alliance [16] is a first example of a successful player in the LWPAN market using spread spectrum technology to meet the ISM band regulations. They are using Chirp Spread Spectrum modulation [17], which is a technique using frequency modulation to spread the signal. A radio bearer is modulated with up and down chirps, where an up chirp corresponds to a pulse of finite length with increasing frequency, while a down chirp is a pulse of decreasing frequency. The LoRa Alliance claims to provide a MCL of 155dB in the European 867-869MHz band, and 154dB in the US 902-928 MHz band [17].

A second example of an LPWAN vendor using spread spectrum is Ingenu with their Random Phase Multiple Access (RPMA) technology. RPMA is a DSSS-based modulation complemented by a pseudo random time of arrival that helps distinguishing users multiplexed on the same radio resource. Ingenu claims to achieve an MPL of 172 dB in the United States and 168 dB in Europe [18]. While Sigfox and LoRa Alliance are using the US 902-928 MHz and European 867-869 MHz ISM bands to gain the coverage advantage associated with low-frequency bands, Ingenu is focusing on the 2.4 GHz license exempt band. They claim that the higher coupling loss associated with 2.4 GHz is compensated by the benefits of this band in terms of higher allowed output power and as an enabler for compact antenna design facilitating receive diversity without compromising the device form factor. They furthermore bring forward that the 2.4 GHz band is globally available and provides a bandwidth of up to 80 MHz [19].

These three and other LPWAN vendors have attracted considerable market interest and media attention because of their operation in license exempt bands in combination with claims of support for high link budgets, long device battery life, low device complexity, and high system capacity. As explained in Section 1.2, these are all important capabilities for a technology aiming to serve applications in the mMTC market segment. This did ultimately bring the attention to the shortcomings of the traditional cellular technologies regarding their inability to provide full support for IoT type of services without further evolution. As a consequence, 3GPP triggered a massive effort in its Release 13 to start the development of the three Cellular IoT technologies described in Chapters 3e8. This is to empower traditional mobile network operators with a path to remain competitive with Sigfox, LoRa, and Ingenu and their likes while avoiding the deployment of a parallel wireless infrastructure targeting support for IoT services.

Chapter 9 will, as mentioned earlier, review the license exempt regulations. It will in addition discuss the pros and cons of the licensed LPWAN versus unlicensed LPWAN operation. During this discussion further details on the technologies developed by Sigfox, LoRa, and Ingenu will be presented. Chapter 9 will finally introduce a few of the most prominent technologies for short-range communication in unlicensed spectrum.

REFERENCES

[1]  Cisco, The Zettabyte Era: Trends and Analysis, Cisco White Paper, 2016. 


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[8]  Third Generation Partnership Project, Technical Report 45.820 v13.0.0, Cellular System Support for Ultra- 
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[9]  ITU, Radio Regulations, Articles, 2012. 


[10]  ITU-R, Recommendation ITU-R SM.1896, Frequency Ranges for Global or Regional Harmonization of Short-range Devices, November 2011. 


[11]  U.S. Government Publishing Office, Electronic Code of Federal Regulations, Article 15.247, July 7, 2016. 


[12]  European Telecom Standards Institute, EN 300 220-1 V2.4.1 Electromagnetic Compatibility and Radio Spectrum Matters; Short Range Devices; Radio Equipment to Be Used in the 25 MHz to 1 000 MHz Frequency Range with Power Levels Ranging up to 500 MW; Part 1: Technical Characteristics and Test Methods, January 2012. 


[13]  European Telecom Standards Institute, EN 300 328 V2.0.20 Wideband Transmission Systems; Data 
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[14]  Sigfox, About Sigfox, 2016. Available: http://makers.sigfox.com/. 


[15]  Sigfox, Sigfox Coverage, 2016. Available: http://www.sigfox.com/coverage. 


[16]  LoRa Alliance, LoRa Alliance Wide Area Networks for IoT, 2016. Available: https://www.lora-alliance.org/. 


[17]  LoRa Alliance Technical Marketing Workgroup 1.0, LoRaWAN™, What Is it?, A Technical Overview of 
LoRa® and LoRaWAN™, White Paper, November 2015. 


[18]  Ingenu, RPMA Technology, For the Internet of Things, Connecting Like Never Before, White Paper. 


[19]  Ingenu, 2.4 GHz & 900 MHz, Unlicensed Spectrum Comparison, White Paper. 


This series continues with an extended discussion of the IoT technology landscape.

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

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