Narrowband IoT (NB-IoT) is an LPWAN technology that operates in the licensed spectrum, as an alternative to LoRaWAN and similar technologies using the unlicensed spectrum. Therefore, NB-IoT can only be deployed by mobile telecom operators that own the licensed spectrum in a given country for cellular communications. Just like 3G, 4G, and 5G, it is standardized by the 3rd Generation Partnership Project (3GPP). It builds on the LTE architecture and reuses the already deployed infrastructure for mobile broadband. NB-IoT can generally be deployed by network operators simply as a software update on their existing systems. This leads to fast development, scalability, and global roaming, and strong security features, which are already well established in cellular networks.
The typical network characteristics of NB-IoT is given in Table 4.1 in terms of the spectrum, peak data rate, transmission power, transmission range, and energy efficiency. We will explain those specific mechanisms for increasing the energy efficiency later in this section, as power saving is an important characteristic for massive IoT deployments.
Table 4.1: Typical NB-IoT specifications
|
Spectrum |
Licensed spectrum (Same frequency bands as 3G, 4G, 5G) |
|
Modulation and Access |
Modulation: BPSK/QPSK |
|
Peak Data Rate |
Downlink: 250 kbps |
|
UE Max Tx Power |
23 dBm (200 mW) |
|
Range |
~ 1 km (urban), ~ 10 km (rural) |
|
Energy Efficiency |
Power Saving Mode (PSM) and |
¶ 4.1 NB-IoT Architecture
The network architecture for NB-IoT is no different from the typical cellular network architecture for mobile broadband connectivity, as depicted in Figure 4.1. Here the IoT devices act as the “end user” devices in the cellular network architecture. These devices should have a cellular modem and a SIM card (physical SIM or electronic SIM / e-SIM), just like mobile phones. IoT devices are connected via their Modem + SIM to the cellular base stations through the air interface using licensed frequencies. The connected is then directed via the cellular core network to the Internet and then to the IoT application servers.
¶ 4.2 NB-IoT Energy Efficiency
Cellular network use a procedure called Tracking Area Update (TAU), which is a periodic messaging mechanism to inform the network about the availability and location of the mobile devices. This way, the network has always a complete picture of the topology and availability of the devices in the network.
NB-IoT utilizes this periodic TAU mechanism to implement a form of duty-cycle or sleep cycle, as we introduced in the previous section, so that IoT devices do not continuously consume power by keeping their radio transceivers on all the time. This Power Saving Mode (PSM) mechanism is shown in Figure 4.2 and then explained below.
TAU: Tracking Area Update, RRC: Radio Resource Control, DRX: Discontinuous Reception
An IoT device “wakes up” and transmits only during the TAU state, and then goes into an “idle” state for listening the channel and receiving any messages from the base station. After the idle state, the devices goes into the Power Saving Mode (PSM) until the next TAU slot. In PSM mode, the IoT device is virtually attached to the network, but is not reachable and cannot be contacted. As illustrated in Figure 4.2, the power consumption is much less in the PSM mode compared to the sending and receiving states, and the IoT devices spends most of its time in this state. The two timers shown in the figure, T3324 and T3324, allow the network operators to configure the timing of each mode and thus adjust the tradeoff between energy savings and the communication latency, depending on the IoT application requirements.
The Discontinuous Reception (DRX) cycle further allows the IoT device to minimize the time that is keeps the radio receiver on. As depicted in Figure 4.2, the idle state include bursts of “reception windows”, where the power consumption increases, and then low-power states in between. This mechanism of “discontinuous reception” provides further energy savings, while keeping a long enough duration for the network to contact the IoT device after each TAU state.
¶ 4.3 Celluar-IoT Evolution in 5G
As mentioned earlier, NB-IoT is based on the LTE standards by 3GPP organization, and it keeps evolving with new releases of the 3GPP specifications, together with other features of the cellular networks, as depicted in Figure 4.3.
The three lanes of the figure represents the three service types that we introduced at the beginning of this lecture, i.e. Enhanced Mobile Broadband (eMBB), Massive Machine-Type Communications (mMTC) and Ultra-Reliable and Low-Latency Communications (URLLC). The standardization of cellular networks by 3GPP is done in a continuous and iterative manner. Just like “software releases” in programming / software engineering, 3GPP “releases”1 provide an updated set of features for the network architecture and network functions. The releases are enumerated consecutively as R13, R14, R15, etc. as shown at the top of the figure. This results in continuous and smooth evolution of the network, which gave its name to the 4G technology “Long-Term Evolution” (LTE). As the figure illustrates, there has been a gradual shift from 4G to 5G, as the standard moved from releases R13/R14 to R15.
The NB-IoT technology is similarly evolving with new improvements, functionalities, and application support, under the mMTC standardization track in 3GPP. Another cellular-IoT technology that is shown here is “LTE-M”, which is an alternative form of the IoT connectivity offering by mobile network operators. Typically, LTE-M technology can provide higher data rates at the expense of higher power consumption and higher service fees, compared to NB-IoT. Therefore, it provides a different option for IoT deployments that require better quality of service (QoS). However, as we have covered in Section 2.1, NB-IoT better matches the characteristics of Massive-IoT use cases and thus it is much more commonly adopted as the LPWAN technology for IoT connectivity, when compared with LTE-M.