(NB-IoT). NB-IoT inherits many of its features from LTE, and as its name suggests, it focuses on narrowband signals that occupy 180 kHz bandwidth, which corresponds to one resource block in the legacy LTE system. NB-IoT design is leaner than its counterpart, LTE-M, to match the requirements of many battery-constrained IoT applications. Therefore, it offers good indoor coverage, support to massive connectivity, low power consumption and optimized network architectures (also a characteristic in LTE-M), while operating under licensed spectrum [9, 28]. These two cellular technologies differ vastly by their target.
Both, NB-IoT and LTE-M, are optimized to reduce power consumption, extending devices battery life, though NB-IoT has even lower device power consumption and lower chipset cost. One advantage of these technologies is that both support in-band deployment with the legacy LTE (including LTE guard-bands in NB-IoT), or as standalone with dedicated spectrum.
A recent comparison of these LPWA technologies is available in [9, 27, 29]3. Figure 1.4 compares the LPWA technologies with respect to cell coverage and data rates. As discussed above, unlicensed technologies have limited data rates and cell coverage, usually ranging from few kilometers in urban environments up to tens of kilometers in rural areas, when compared to licensed technologies. On the other hand, device cost and network deployment are often advantageous. However, cellular technologies provide quality-of-service (QoS) guarantees via long-term service level agreements. At the same time, operation under licensed spectrum comes with the advantage of predictable and controllable interference through efficient mechanisms that enable a massive number of connections.
Figure 1.4 Comparison of LPWA technologies in terms of cell coverage and data rate.
Source: Illustrative numbers based on [29].
Massive MTC (mMTC). Massive IoT flourishing markets impose increasingly challenging connectivity demands. Third generation partnership project (3GPP) has taken revolutionary steps in Release 12 when introducing MTC, and evolutionary ones in posterior releases by enhancing the performance of LTE-M and introducing new services (NB-IoT and EC-GSM-IoT introduced in Release 13).
MTC revolutionized the mobile communications industry by shifting the focus from broadband services towards the IoT. Clear evidence comes with the fifth generation (5G) that inherently dedicates two-thirds of the service modes to the IoT. In 5G, MTC branched out to massive MTC (mMTC) and critical MTC (cMTC), also known as ultra-reliable low-latency communications (URLLC)4.
mMTC stands for massive referring to large number of users connected to the network, widely expected in many IoT applications. Therefore, in the communications community, mMTC and massive IoT are used as synonyms and interchangeably.
Even though mMTC is encrusted 5G jargon, it is an umbrella term that specifies the ensemble of solutions toward massive IoT. Thus, it comprises the cellular IoT technologies discussed so far, namely LTE-M and NB-IoT, and solutions under development toward current and next generations, as well as non-cellular technologies, such as LoRa and SigFox. This is because 1. LTE-M and NB-IoT are compliant with the evolution of mobile communications, thus both solutions operate in-band with 5G, and will evolve (so-called future proof) complying with 5G mMTC requirements [9, 25]; 2. mMTC encloses LPWANs; therefore, non-cellular LPWA technologies fall within this definition. Another point is that even with the advantages of cellular IoT, it is unlikely that a single technology will be ubiquitous in a fragmented market. Therefore, future generations beyond 5G are likely to coexist and complement unlicensed solutions as foreseen in [33].
mMTC key challenges are:
Energy efficiency In many applications, devices rely solely on batteries, and very often, replacement is costly, dangerous, or simply not possible (see Chapter 2). Though battery lifetime may be extended with smart sleeping mode techniques, it may not be sufficient.
Scalability Support to a massive number of connections. The network capacity should also scale to accommodate the demands of such large number of devices. Figure 1.1 illustrates the projected growth for cellular IoT solution.
Coverage Deep indoor coverage is a crucial requirement for many applications, requiring regional, national, or even global coverage.
Heterogeneity Different applications impose different requirements, e.g., in terms of data rates, latency, reliability, energy efficiency, coverage. Flexible connectivity is imperative to handle heterogeneous requirements.
Device costCost is a critical factor in meeting the economies of scale and many use-cases. For instance, cellular IoT solutions have reduced peak rate and device complexity, and half-duplex operation and narrow bandwidths help address this challenge.
These are the most representative challenges of mMTC, although other capabilities exist, as initially identified by the ITU [34], e.g., peak and experienced data rates, spectrum efficiency, mobility, latency, reliability, and security. In this context, cellular IoT becomes advantageous by coping with the LPWAN requirements and key performance indicators (KPIs) while pushing some of these to their limits, whereas handling security and QoS requirements [4].
1.3 Requirements and KPIs
mMTC challenges arise from related massive IoT challenging requirements. The essential requirements and KPIs for massive IoT are:
Energy efficiency Battery life becomes an aggregated performance indicator for energy efficiency. Current technologies target 10 to 20 years of lifetime.
Scalability Support to a massive number of connections. A target for mMTC is one million MTDs per square kilometer.
Coverage Deep indoor coverage is a crucial requirement for many applications. Overall, the maximum coupling loss (MCL) of 164 dB is the target for current solutions.
Device costThe cost impacts scalability and market penetration. Most of the current chipsets available in the market average around 10 dollars.
Data rates Many applications demand few bytes of information to report, thus a data rate of a few hundreds of bps is sufficient. However, other applications upload photo or video to a remote server, or cloud, which require a few kpbs or even Mbps. Minimal throughput is set to 160 bps in NB-IoT.
Security It comprises security, privacy, authentication, encryption, integrity protection of user data, and denial of service attacks, and the demands vary for each application.
In 2015, the International Telecommunication Union (ITU) identified KPIs for massive connectivity, which has served as a guide for the technology evolution [34].
The identified KPIs are: peak and user experienced data rate, spectrum efficiency, mobility, latency, connection density, network energy efficiency, and area traffic capacity. Out of these, connection density received the highest importance among ITU ranking, followed by network energy efficiency. Current technologies have targeted these KPIs and offered many solutions to enhance connection density, battery life, and network energy efficiency.
Figure 1.5 captures the most relevant requirements and KPIs for our discussion. For this figure we split the technologies into two major groups: unlicensed and licensed (also known as cellular IoT). The latter comprises technologies based on mobile communications and addresses most of these requirements as illustrated in the figure; while the former comprises solutions build upon ISM frequency bands, which are unlicensed.
Figure 1.5 KPIs for massive IoT.
Source: Illustrative numbers based on [29].
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