Overview of cellular LPWA and communication technology that achieves low power consumption

What is LPWA?

LPWA is a wireless communication technology for IoT that achieves low power consumption and long-distance communication by reducing the amount of data that communication devices can send at one time.By using LPWA, IoT devices can be connected to the network even in suburban areas where communication infrastructure is not well developed.

Five features of LPWA

Below are the five main features of LPWA.

1) Low power consumption
Using narrowband communication reduces the amount of power required for transmission and reception, and low power consumption is achieved by putting communication devices into sleep mode when not communicating. Battery-powered IoT devices can operate for long periods of time by appropriately controlling the amount of communication data and frequency.

2) Long-distance communication
LPWA communication distances range from several tens of meters to over 10 km, allowing it to cover a wide area.
LPWA has the following features to enable long-distance communication:

• It uses narrowband to transmit and receive radio waves, minimizing interference from other communications.
• It uses a simplified communication protocol to reduce the number of messages sent.

3) Low-speed communication
LPWA uses relatively slow communication speeds, ranging from around 100 bps to several hundred kbps.
It is suitable for applications that periodically transmit small amounts of data, such as sensor data measured by IoT devices, or for monitoring remote locations. On the other hand, it is not suitable for applications that require high-speed real-time data communication or large-volume data transmission.

4) Low cost
LPWA eliminates unnecessary functions, which reduces the initial cost of communication equipment. Furthermore, because the amount of data transmitted is small, communication costs are also lower than with conventional communication equipment.

5) Simultaneous connection of multiple devices
LPWA allows multiple IoT devices to be connected stably in the same area.
When Access is concentrated in the same area at the same time, network congestion occurs, causing communication equipment to wait for processing and packet loss, resulting in a decrease in communication speed. LPWA reduces processing waits by limiting communication frequency and reduces packet loss by using narrow bandwidth.

Types of LPWA and major standards

LPWA communication standards are broadly divided into licensed bands, which require a license and registration for radio stations, and unlicensed bands, which do not, and are classified according to frequency band, communication speed, and communication range as shown in Table 1 below.
There are two types of licensed bands, which are frequency bands that require a license: Cat. M1 and NB-IoT.

Figure 1 shows an image of the communication flow of IoT devices using licensed bands.

Figure 1. IoT devices using licensed bands (cellular LPWA)

What is LTE Cat. M1?

Cat. M1, one of the licensed bands in the LPWA communications standard, is commonly known as eMTC (enhanced Machine Type Communication) in the 3GPP standard, and is a standard designed for communication between IoT devices. Cat. M1 can use existing LTE communications networks, enabling communication over a wide area.
The carriers offering Cat. M1 services in Japan are NTT Docomo, KDDI, SoftBank, and Rakuten Mobile.
Cat. M1 communication speeds range from 300 kbps to 1 Mbps. For example, the maximum communication speed for NTT Docomo's LTE-M is 300 kbps down and 375 kbps up.

Supports voice communication/SMS and handover

Cat. M1 supports VoLTE voice communications and SMS. It also supports handover, which switches communication devices to base stations with stronger signal strength while maintaining a connection, making it suitable for use as a mobile IoT device.

Coverage Extension

By repeatedly transmitting the same signal, coverage is extended, especially in environments with poor signal-to-noise ratios. Rather than using the same frequency, frequency hopping is used to retransmit on multiple frequencies within a single LTE transmit/receive band, improving coverage.

<eDRX (extended Discontinuous Reception)>

In cellular communications, power consumption is reduced by suspending wireless communication functions when the device is in discontinuous reception (DRX) mode, when it is not receiving data.
While the DRX reception interval for conventional LTE communications was 1.28 seconds or 2.56 seconds, the eDRX reception interval (eDRX cycle) for LPWA can be extended to a maximum of 43 minutes. The commonly used eDRX reception interval is 81.92 seconds, but this still reduces standby power consumption by up to 90%. This eDRX reception interval is requested by the communications device from the base station, and the base station defines the time. Furthermore, because communications devices receive information from the base station at regular intervals, data communication is possible via external calls and activation from IoT devices.
Figure 2 shows the current consumption for each operation during eDRX operation, and Figure 3 shows the operation when PTW is set to 4 (times).

TAU Tracking area update is a process to update the location registration to the base station when the terminal moves from the current tracking area (TA) to another TA.

Data transmission and reception: Devices send and receive data

calibration After sending and receiving data, the communication device will receive paging every 30 seconds for 10 minutes and then transition to eDRX/Hibernate.

eDRX /Hibernate: In power-saving standby mode (Sleep state), current consumption is a few μA

PTW: Attempts to receive paging from the base station according to the eDRX reception interval in the Paging Time Window. If an external wake-up occurs, the device is notified that a message is available at this time, and the device wakes up and begins sending and receiving data.
In Figure 2, it appears that PTW is performed once, but in reality, it is performed multiple times as shown in Figure 3.

eDRX cycle: eDRX reception interval. The communication device requests the reception interval from the base station using the AT+CEDRXS command, an AT command defined in 3GPP, and determines the eDRX reception interval based on the response from the base station.

<PSM (Power Save Mode)>

Cellular communications attempt to receive paging from the base station according to the DRX/eDRX reception interval, but PSM does not receive paging from the base station and completely suspends wireless communication functions, thereby significantly reducing power consumption.
Therefore, it cannot be awakened from the outside.
Figure 4 shows the current consumption during each operation in PSM.

Figure 4. PSM operation

TAU: Tracking area update is a process that updates the location registration to the base station when the terminal moves from the current tracking area (TA) to another TA.

Data transmission: The device transmits data.

Active time (T3324): Time to wait for data reception from the network after sending data. When the time expires, the device transitions to PSM again.

Periodic Tracking Area Update (T3412): Periodic update time for performing TAU

PSM/Hibernate: In power-saving standby mode (Sleep state), the current consumption is a few μA. Since the device does not receive paging from the base station, it cannot be woken up from outside.

What is LTE NB-IoT?

NB-IoT, which is one of the licensed bands in the LPWA communication standard, is an abbreviation for Narrow Band Internet of Things, and is a technology that enables network connections between multiple devices with low power consumption.
The carriers offering NB-IoT in Japan are SoftBank and Rakuten Mobile. NTT Docomo ended its service in March 2020.
The NB-IoT communication speed is 63kbps up and 27kbps down.

Voice communication/SMS and handover are not supported

NB-IoT does not support voice calls or SMS and is suitable for simple applications that require low power consumption and low bandwidth.
NB-IoT is best suited for fixed IoT devices because it does not support handover, which allows communication devices to switch to a base station with stronger signal strength while maintaining a connection.

Coverage Extension

Like Cat. M1, it extends coverage even in poor signal-to-noise ratio environments by repeatedly transmitting, and by using a narrow frequency band, it enables even longer distance communication.

Two low power consumption modes

Like the Cat. Cat. M1, there are two power saving modes: eDRX and PSM.

Frequency band usage (resource blocks)

LTE communications use bandwidths of 5/10/15/20MHz, but OFDMA (Orthogonal Frequency Division Multiple Access) divides that bandwidth into 180kHz increments, allowing multiple signals to be transmitted in parallel. These are called resource blocks, and the number of resource blocks used by each communications standard determines communication speed and power efficiency.

• Regular LTE uses all resource blocks to communicate at high speeds.
• Cat. M1 uses six resource blocks for communication.
• NB-IoT uses one resource block for communication.
  Therefore, it is a slow communication speed, but it also consumes the least amount of power.
  Figure 5 shows how resource blocks are used in the LTE communications standard.

Figure 5. Using resource blocks

NB-IoT guard band usage

Guard bands are frequency bands that are set aside at both ends of a band in use and are not used for communication in order to avoid affecting bands other than the band in use. Guard bands are normally not usable for data communication, but can be used with NB-IoT, as shown in Figure 6.
This means that communication can occur without conflict with other LTE communication standards.

Figure 6. Guard band usage example