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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].

All solutions have their pros and cons, as discussed in Section 1.2. Therefore, when choosing between licensed and unlicensed, it is important to pay attention to the application needs and the trade-offs between different technologies. In general, cellular IoT device’s cost and service fees tend to be higher than unlicensed alternatives. On the other hand, unlicensed options have limited capabilities concerning scalability, coverage, and security.