Cellular IoT — Comparison of CIoT technologies

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

9.3 CHOICE OF CIoT TECHNOLOGY

9.3.1 COMPARISON OF CIoT TECHNOLOGIES

The different CIoT technologies EC-GSM-IoT, NB-IoT, and LTE-M have been extensively analyzed in Chapters 3-8. Here we summarize and compare the performance and characteristics. For NB-IoT we consider in this summary only in-band and stand-alone deployment options for simplicity. The performance of guardband mode of operation is to a large extent similar to the in-band performance. The full NB-IoT performance analysis including guardband operation can be found in Chapter 8.

9.3.1.1 Coverage and Data Rate

The data rate in uplink and downlink for all CIoT technologies are summarized in Figures 9.7 and 9.8 for different coupling losses. All of those technologies have introduced extended coverage features, which enable an operation at a coupling loss of up to 164 dB. This is a significant extension of coverage range compared to what can be found in Global System for Mobile Communications (GSM), UMTS, or Long-Term Evolution (LTE) networks today. For EC-GSM-IoT the 164 dB coupling loss is based on a device with an output power of 33 dBm, as it is common in GSM networks. However, this means that a 10 dB higher device output power is needed for full extended range in EC-GSM-IoT compared to the device output power for NB-IoT and LTE-M for achieving the same uplink coverage. When looking more into details of the extended coverage results in Chapters 4, 6, and 8, it is seen that NB-IoT can operate at a lower control channel block error rate than EC-GSM-IoT and LTE-M at 164 dB MCL, making it more robust at extreme coverage. It can be noted that LTE-M and EC-GSM-IoT can apply frequency hopping, which provides some additional coverage robustness due to added frequency diversity.

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FIGURE 9.7 Coverage and physical layer data rate for uplink.

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FIGURE 9.8 Coverage and physical layer data rate for downlink.

Figures 9.7 and 9.8 also provide the physical layer data rates values for the different CIoT technologies. The instantaneous peak physical layer data rate specifies the achievable data rate of the data channels only. The other data rate values in the tables refer to the effective physical layer data rates for the transmission of a single message, where also the latencies for scheduling and control signaling is taken into account in the transmission time of the message. In this comparison it is assumed that half- duplex operation is used for all technologies but it should be noted that LTE-M devices can also be implemented with support for full-duplex operation which will achieve higher data rates (with peak rates close to the instantaneous peak physical layer data rates). These rates are provided for devices with different coupling loss to the base station: peak physical layer data rate corresponds to device with an ideal error free connection to a base station. Physical layer data rates at 144 dB coupling loss corresponds to the normal cell edge of the GSM or LTE radio cell, and 154 and 164 dB correspond to 10 and 20 dB of coverage extension compared to the cell edge of GSM.

What can be seen is that LTE-M can achieve significantly higher data rates in uplink and downlink compared to NB-IoT or EC-GSM-IoT. This is, in particular, the case for devices, which are within normal coverage of the radio cell. When devices are located in extended coverage areas, the uplink is limited by device output power, and all CIoT technologies make us of repetitions to achieve the required link quality. In extreme coverage situations like at 164 dB coupling loss, the achievable data rates for different technologies become quite similar when using the same output power. EC-GSM-IoT has at the 164 dB MCL a higher data rate than the other technologies due to the 10 dB higher output power of the device. Within the same LTE carrier, LTE-M has in general higher data rates than in-band NB-IoT.

All three technologies fulfill the 3GPP requirement on achieving 160 bps at the MCL of 164 dB.

9.3.1.2 Latency

The latency of the CIoT technologies has been evaluated with respect to an exception report , which is an infrequent high-important IoT message contained in an 85 byte IP packet, which is being transmitted from a device over the CIoT network. All technologies, LTE-M, NB-IoT, and EC-GSM-IoT, fulfill the 3GPP latency target of 10 s first defined in Release 13, as depicted in Figure 9.9. When a device is within normal coverage, LTE-M can achieve somewhat lower latencies due to the higher data rates provided by LTE-M. In extended coverage, EC-GSM-IoT can provide the lowest latency due to the higher device output power, which can provide higher data rates. Stand-alone NB-IoT has a lower latency compared with in-band NB-IoT due to the higher power used for downlink channels.

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FIGURE 9.9 Latency for exception report.

9.3.1.3 Battery Lifetime

The battery lifetime has been analyzed for all CIoT technologies, assuming two AA batteries with a joint capacity of 5 Wh. A power amplifier efficiency of 45%-50% has been assumed for all three IoT technologies.

Overall, all CIoT technologies apply mechanisms to save battery lifetime for infrequent transmission of messages, as it is common for many IoT services. The main principles are that devices only become active for the transfer of data, and otherwise are put into a battery-saving sleep state. Efficient procedures have been defined, which minimize the signaling overhead associated with the data transfer. This is particularly important for small messages because any signaling overhead can then account for a significant part of the energy consumption.

For a daily report of a 200 byte message, the battery lifetimes for the different CIoT technologies is depicted in Figure 9.10. The results for different message sizes and periodicities of IoT data transfers are summarized in Table 9.5. Overall, all technologies enable battery lifetimes of 10 years, and for some cases even significantly longer. The biggest challenge for long battery lifetime is when a device is located in a very bad coverage position. In the extended coverage mode, very low data rates are used and many repetitions are applied for the data transfer. In this situation a device requires an extended effort for data transmissions, which reduces the opportunity for resting in a battery-saving sleep state. Accordingly, the battery lifetime is significantly reduced at the MCL of 164 dB for all CIoT technologies. With such a large coupling loss, a battery lifetime of 10 years can only be achieved, if data transfer events of a device occur rarely, like once per day. For more frequent data transfer events, like one message every 2 h, battery lifetimes of 1-3 years are achievable at a MCL of 164 dB.

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FIGURE 9.10 Battery lifetime for a device with a daily report of a 200 byte message.

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Table 9.5 Battery lifetime

All three technologies fulfill, or indicate a potential of fulfilling, the 3GPP requirement on achieving 10 years battery life at the MCL of 164 dB.

9.3.1.4 Device Complexity

All CIoT technologies have introduced similar features to reduce device complexity, and thereby enabling low-cost CIoT devices. The following design objectives have been pursued for low device complexity:

  • The frequency bandwidth used by the device for transmitting and receiving has been limited to avoid the high costs of wide-band front ends. For LTE-M the bandwidth that needs to be supported by a device is 1.4 MHz, which is significantly less than the maximum LTE channel bandwidth of 20 MHz. For NB-IoT and EC-GSM-IoT, the bandwidth that needs to be supported by the device is 200 kHz.

  • The peak data rate has been limited to reduce processing and memory requirements for a device. For LTE-M the peak rate has been reduced to 1 Mbps; for NB-IoT the peak rate has been limited to below 300 kbps and for EC-GSM-IoT it is below 500 kbps.

  • All CIoT technologies are specified so that CIoT device are not required to use more than one antenna.

  • All CIoT technologies have been specified to support half-duplex operation in frequency-division duplex (FDD) bands. This avoids the needs for a device to integrate one or more costly duplex filters. LTE-M devices can be implemented with support for half-duplex frequency-division duplex (HD-FDD), full-duplex frequency-division duplex (FD-FDD) or time-division duplex (TDD) operation.

  • All CIoT technologies have defined User Equipment (UE) categories with lower power classes. This enables a device to use cheaper power amplifiers. It can become an option to implement the power amplifier on the modem chip, and thereby avoiding the costs of a separate component. For EC-GSM-IoT a new 23 dBm device power class has been introduced, in addition to the 33 dBm device power class that is typically used for GSM. For LTE-M two device power classes are defined with 23 and 20 dBm output power. NB-IoT supports 23, 20, and 14 dBm. 

The features above enable to reduce the device costs for CIoT devices. However, it must be noted, that the device cost is not entirely depending on the communication standard. The cost of the device depends also on what peripherals are added to the device, such as power supply, CPU, or the real-time clock. 

In the end, the costs of the device depend on the market success and the market volume of the devices. A large economy of scale will help to reduce the production costs. 

In summary, all CIoT options have introduced low device cost features, and for all technologies low complexity and low cost devices can be expected to appear on the market. 

9.3.1.5 CIoT Capacity 

The capacity of EC-GSM, LTE-M, and NB-IoT has been analyzed in Sections 4.6, 6.6, and 8.6, respectively. The traffic model for the capacity analysis is based on autonomous device reports sent by devices; the traffic assumptions are described in detail in Section 4.6.1.1 and on average a device is transmitting an autonomous report every ~128.5 min. The initial capacity requirement for CIoT in Release 13 has been to be able to serve 60,680 devices/km2 , which corresponds to 40 devices per household in a city like London. Assuming an intersite distance of 1732 m, the size of a radio cell is 0.87 km2 , and the number of devices that need to be supported per radio cell becomes 52,547. With the above traffic model this corresponds to ~6.8 message arrivals per second per cell. This traffic load can be provided by EC-GSM-IoT at an uplink radio resource utilization of 27%, where the percentage of failed access attempts is below 0.1%. A similar analysis has been performed for LTE-M and NB-IoT. This can be seen in Figure 9.11; for a detailed analysis of the performance depicted see Sections 4.6.2, 6.6, and 8.6.2. The arrival rate of messages has been increased up to the level where the failed access attempts remained below 1%; the results are listed in Table 9.6. For LTE-M, the system bandwidth spans a number of nonoverlapping LTE-M narrowbands of size 1.08 MHz (6 PRBs). The smallest available LTE system bandwidth (1.4 MHz) contains a single LTE-M narrowband. An LTE-M narrowband can support up to 40.3 message arrivals per second at an outage probability of 1%, which corresponds to a 361,000 devices/km2 or 314,070 devices/cell. For NB-IoT the 1% outage limit supports up to 7.5 arrivals per second per cell on an anchor carrier and 12.3 arrivals per second per cell on a nonanchor carrier. This corresponds to 67,000 devices/km2 or 58,290 devices/cell as capacity limit for an NB-IoT anchor carrier; on a nonanchor carrier 110,000 devices/km2 or 95,700 devices/cell can be supported (Table 9.6). The radio resource utilization depending on the traffic load for all CIoT technologies is shown in Figure 9.11.

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Table 9.6 LTE-M and NB-IoT per carrier capacity

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FIGURE 9.11 Capacity of NB-IoT anchor, LTE-M, and EC-GSM-IoT.

For 5G, a capacity requirement has been defined to be able to serve 1,000,000 devices/km2 , as summarized in Table 10.1 in Section 10.1. With LTE-M this capacity can be provided with three LTE-M narrowbands, i.e., deployed within a 5 MHz LTE carrier. For NB-IoT one anchor carrier and nine nonanchor carriers are needed to be able to serve 1,000,000 devices/km2 ; this corresponds to 10 x 180 kHz of spectrum.

9.3.1.6 CIoT Deployments

CIoT standards have been specified for deployments in different spectrum ranges as shown in Table 9.7. All CIoT technologies, LTE-M, NB-IoT, and EC-GSM-IoT, can be deployed in the cellular bands just below 1 GHz and those below and around 2 GHz. In addition, LTE-M and NB-IoT can be deployed around 450 MHz and around 1500 MHz. LTE-M can further be deployed in the range around 2500-2700 MHz.

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Table 9.7 Spectrum ranges for CIoT

NB-IoT is configured for HD-FDD operation, which is also the common configuration for EC-GSM-IoT. LTE-M devices can be implemented with HD-FDD or FD-FDD support. For LTE-M, TDD bands around 1.9 GHz and 2.5 GHz are also specified.

All CIoT standards can in principle be deployed for stand-alone CIoT operation. The minimum spectrum required for such a deployment is listed in Table 9.8. For LTE-M, a whole LTE carrier of at least the minimum system bandwidth of 1.4 MHz needs to be deployed, which corresponds to 2 x 1.4 MHz for FDD (because of paired spectrum) or one 1.4 MHz carrier for TDD. EC-GSM-IoT could be deployed in 2 x 600 kHz of FDD spectrum, assuming three GSM carriers being used with a reuse factor of 3, see Section 4.8 for performance evaluation of this scenario. For NB-IoT a minimum of 2 x 200 kHz are needed for a stand-alone FDD deployment. However, it is most likely that CIoT systems are embedded in a mobile broadband system, which means that an LTE or GSM carrier is used for both mobile broadband and IoT traffic, and the radio resources are dynamically shared between the two types of traffic. Still for NB-IoT and EC-GSM-IoT a stand-alone operation is imaginable, e.g., to enable a migration of GSM spectrum resources to a future cellular technology, such as LTE or 5G, while maintaining a minimum allocation to continue serving existing IoT customers. For NB-IoT it can also be considered that it can be deployed in any spectrum that remains available for an operator, for example, when an allocated band cannot be fully exploited with the carrier bandwidths that are defined for LTE. Then an NB-IoT carrier can be configured adjacent to the LTE carrier within the band of an operator. The full list of bands that are specified for CIoT for 3GPP Releases 13 and 14 are defined in References [52-54], and are listed in Tables 9.9 and 9.10.

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Table 9.8 Minimum spectrum allocation for deploying a CIoT network

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Table 9.9 Spectrum bands defined for LTE-M and NB-IoT in Release 13 and 14

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Table 9.10 Spectrum bands defined for EC-GSM-IoT

The next and final installment in this series will discuss decision factors for the choice of CIoT technology and include references for this excerpt.

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

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