Intorduction

NB-IoT

Narrow Band-IoT, generally known under the abbreviation NB-IoT, or LTE Cat NB, loosely translated narrowband Internet of Things, is a technology based on LPWAN standards that enables a wide range of new devices and services for the Internet of Things. NB-IoT significantly improves energy consumption of user devices, system capacity, and spectral efficiency, especially in indoor spaces located deep within buildings. The specification was finalized in June 2016, and was first released as part of 3GPP Release 13, also known as LTE Advanced Pro. In addition to the first release in the 3GPP Release 13 specification, the standard was also released in the 3GPP Release 14, Release 15 specifications, and is also included in the new 5G specifications Release 16 and Release 17. The main advantage of NB-IoT is the fact that as a network it uses already existing mobile networks, so there is no need to build new infrastructure, which massively reduces costs.[V21][V22] Compared to other Internet of Things technologies, NB-IoT focuses on uses that require sending only small amounts of data over long distances. It is mainly used in cases where a high density of connected devices, a long battery life in the device, a low price, and good coverage even in indoor spaces are required.[V23]

Fig. V7 NB-IoT Architecture [V31]

The architecture consists of a user device, a base station, and a mobility management entity. The user equipment connects to the base station using the "Uu" interface, which connects the user equipment to the terrestrial radio access network. The base stations are connected to each other via the X2 interface, and they are connected to the network core via the S1 interface. The S1 interface carries either NB-IoT control packets or data packets. This architecture can be seen in Figure V7. [V31] This architecture does not require setting up a data radio bearer, data packets are sent on a signal radio bearer, so a solution based on this architecture is ideal for transmitting irregular and small data packets. [V31]

Low power consumption can be achieved due to the fact that NB-IoT uses long-period monitoring area updates that periodically notify the device of network availability (Long-Periodic TAU). In addition, it also uses other energy-saving methods, such as PSM or eDRX.

NB-IoT specification

NB-IoT, or LTE-NB, uses a bandwidth of only 200 kHz, which corresponds to the name itself. Within NB-IoT, two specifications are used, namely LTE Cat NB1 and LTE Cat NB2. LTE Cat NB1 was released in 3GPP Release 13, and LTE Cat NB2 in 3GPP Release 14. A comparison of the specifications can be seen in Table 2. [V25] In the table it can be seen that the response for LTE Cat NB2 is not defined. According to the available sources, this value is not defined in any way, therefore it is considered that either this value is the same as in the previous generation, or it is irrelevant for the standard. Neither standard supports voice transmission, and both use only one antenna. Normally, NB-IoT devices do not support SMS messages, however, sending SMS messages can be achieved under certain conditions, but then it is important that the device is not in sleep mode, i.e. it has high energy consumption.[V25]

Within Release 13, 14 frequency bands were defined for LTE Cat NB1, 4 more frequency bands were added in Release 14, and 7 more frequency bands were added in Release 15. [V26][V27][V28] For 5G network and Release 16, it depends on which radio technology is used, in case of E-UTRA 34 frequency bands are defined for LTE Cat NB1 and LTE Cat NB2, in case of 5G New Radio 19 frequency bands are used. In 3GPP Release 17, two new frequency bands were added for E-UTRA.[V29][V30]

	LTE Cat NB1	LTE Cat NB2
3GPP Release	Release 13	Release 14
Channel bandwidth	180 kHz	180 kHz
UE bandwidth	200 kHz	200 kHz
Duplex	Half duplex	Half duplex
Maximum TX power	20, 23 dBm	14, 20, 23 dBm
Maximum speed for downlink	~26 kbps	~127 kbps
Maximum speed for uplink	~62 kbps	~159 kbps
Response	< 10 seconds	-
Data encryption	EPS-AKA	EPS-AKA
Device authentication	SIM	SIM
Positioning	Cell ID	OTDOA, E-CID

Long-Periodic TAU

The Long-Periodic TAU function is defined in 3GPP TS 24.301. It serves to periodically notify the network of the availability of an IoT device. The advantage of this approach is that the device can remain idle for a longer period of time, which reduces energy consumption. The length of the period is determined by the device using a periodic TAU timer. This period can be between 1 hour and 310 hours. The principle of Long-Periodic TAU can be seen in Figure 4. At the beginning, the device connects to the network and informs the network about its availability, then it is connected to RRC, and for a period of usually 20-30 seconds, the device is ready to receive or send data . After this time, the device switches to an idle state, periodically reporting its status to the network until the period time expires, after which this cycle repeats.[V24]

Fig. V4 Principle of operation of updating the monitored area with a long period [V24].

PSM – Power-Saving Mode

If the device turns off its radio module, the device would be disconnected from the network, and when it is turned on again, it would have to be reconnected to the network, which requires high energy consumption. The PSM function prevents this in cooperation with periodic TAU, the principle can be seen in Figure 5. In addition to the timer for LongPeriodic TAU, a PSM timer is introduced, the value of which is entered by the user, and must be in the range of 0 seconds to 186 minutes. During this timer, the device behaves the same as when using Long-Periodic TAU, after the specified time, the device goes into deep sleep mode, during which the radio functions are turned off, which massively reduces power consumption. During deep sleep, the device is still registered in the network, the network has saved status information about the device, but during this time it is not possible to communicate with the device. The device wakes up from this state either after the Long-Periodic TAU timer expires, or anytime before if necessary, with no re-registration to the network required for an earlier wakeup [V24].

Fig. V5: PSM operating principle [V24].

eDRX – Extended Discontinuous Reception

eDRX is an extension of existing LTE functionality that enables IoT devices to further reduce power consumption. This function is intended for devices that receive more than send data, especially in cases where immediate access to the device is not required. eDRX allows you to extend the time interval during which the device is idle while still being available to the network. eDRX can be used either alone or together with PSM, where eDRX offers a good compromise between device availability and power consumption. The principle of operation together with PSM can be seen in Figure 6. The device using eDRX provides two timers – T eDRX and T PTW, while the first timer determines the duration of the eDRX function, and the second timer determines the time during which the device sends information about itself to the network. Between these timers is a window during which the receiving part of the radio communication is deactivated. The length of the t eDRX timer is between 20.48 seconds and 10,485.76 seconds, for T PTW it is 2.56 seconds to 40.96 seconds [V24].

Fig. V6: Working principle of eDRX[V24]