What Is Cellular IoT?

Detailed Explanation

Cellular IoT emerged as a response to a specific gap: existing cellular standards (standard LTE, 3G) consumed too much power for battery-operated sensors, while unlicensed-band protocols (LoRa, Sigfox) required gateway infrastructure and offered no guaranteed coverage. LTE-M and NB-IoT were standardised by 3GPP in Release 13 (2016) as LPWAN (Low Power Wide Area Network) technologies running on existing licensed cellular infrastructure, combining wide-area coverage with meaningful power savings.

LTE-M (Cat-M1)

LTE-M operates within the existing LTE frequency bands using a 1.4 MHz channel bandwidth (a subset of the 20 MHz LTE channel). Key characteristics:

• Peak downlink: ~1 Mbit/s; peak uplink: ~1 Mbit/s
• Device mobility: handover between cells supported (suitable for tracking and vehicle applications)
• VoLTE: voice calls supported
• PSM and eDRX: both supported for low-power operation
• In Australia: deployed on Telstra's 4G LTE network nationwide

LTE-M is suitable for mobile assets (fleet tracking, stolen equipment recovery), devices requiring moderate data throughput (OTA firmware updates, JPEG image transmission), or devices that also need voice capability (personal alarms, elderly care devices).

NB-IoT (Narrowband IoT)

NB-IoT uses a 200 kHz channel that can be deployed in-band (within an LTE carrier), guard-band (in the guard band between LTE carriers), or standalone (on a dedicated 200 kHz allocation). Key characteristics:

• Peak downlink: ~26 kbit/s; peak uplink: ~62 kbit/s
• No mobility support: devices must remain stationary or near-stationary
• Deeper building penetration than LTE-M (typically +20 dB coverage improvement)
• PSM and eDRX: both supported with very long PSM periods (up to approximately 413 days specified by 3GPP)
• Lower module cost than LTE-M typically

NB-IoT is optimal for stationary sensors in locations with poor radio coverage: underground utility meters, basement sensors, and rural assets where the deeper coverage gain compared to LTE-M is beneficial.

Module integration

Cellular IoT modules provide a complete radio and modem stack, typically controlled by the host MCU via AT commands over a UART interface. Common modules:

The nRF9160 and nRF9161 are notable for integrating the application processor — the user firmware runs directly on the module rather than on a separate host MCU, reducing BOM cost and board space.

AT command interface

Cellular modules are configured and operated via a UART AT command interface derived from the Hayes modem command set:

AT+COPS=1,2,"50501"     ; connect to Telstra AU (MCC=505, MNC=01)
AT+CFUN=1               ; set full functionality
AT+CEREG?               ; check network registration status
AT+CPSMS=1,...          ; configure PSM timer values
AT+UDNSRN=0,"api.server.com" ; DNS resolve hostname

Most modules also support TLS sockets, MQTT clients, and HTTP clients as built-in firmware features, reducing the firmware complexity required on the host MCU.

Practical Examples

A remote water meter in regional New South Wales uses NB-IoT with an nRF9160 module. It wakes from PSM every 15 minutes, reads the meter pulse counter, transmits the reading over MQTT to an AWS IoT Core endpoint, and returns to PSM. Average current over the 15-minute cycle is approximately 8 µA — providing estimated battery life of over 5 years on a 3600 mAh D-cell battery.

A vehicle GPS tracker uses LTE-M for its combination of mobility support and nationwide Telstra coverage. The tracker sends position updates every 30 seconds while moving and enters PSM when stationary for more than 5 minutes. LTE-M's cell handover support allows continuous tracking across multiple base stations without the GPS coordinates jumping between towers.

Design Considerations

• Band selection: LTE-M and NB-IoT operate on specific LTE frequency bands. In Australia, Telstra's LTE-M operates primarily on Band 28 (700 MHz APT) and Band 3 (1800 MHz). Select a module that supports these bands and confirm with your carrier before designing the hardware.
• Antenna design: cellular antennas for Band 28 (700 MHz) require physically larger antennas than 2.4 GHz Wi-Fi or BLE — a λ/4 monopole at 700 MHz is approximately 107 mm. Use a certified flexible printed antenna or an active ceramic patch designed for Band 28.
• SIM and eSIM selection: for products deployed across multiple countries, an eSIM with a multi-carrier profile avoids the logistics of physical SIM card management. For Australian-only deployments, a physical IoT SIM from Telstra or Optus is straightforward.
• Cellular vs LoRaWAN: LoRaWAN has no per-device data cost and lower module cost, but requires gateway infrastructure and has lower data throughput. Cellular is appropriate when coverage outside existing LoRaWAN gateway range is required, or when the application needs guaranteed nationwide coverage without gateway deployment.
• Cellular vs satellite: for truly remote locations (outback Australia, offshore) beyond cellular coverage, satellite IoT is the only option. Satellite has higher latency, lower throughput, and higher per-message cost than cellular, but provides global coverage.

Common Mistakes

• Assuming LTE coverage equals LTE-M/NB-IoT coverage: not all LTE infrastructure is updated for LTE-M and NB-IoT. In Australia, Telstra's NB-IoT and LTE-M network covers major populated areas, but rural coverage gaps exist. Verify actual coverage for the deployment area before committing to cellular.
• Ignoring PSM timer negotiation: PSM timer values are requested by the device but granted by the network. The carrier may not grant the exact PSM period requested. Firmware must read the granted values back and adjust wake scheduling accordingly.
• Not handling module boot time in power budget: a cellular module takes 5–15 seconds to boot, register on the network, and establish a connection after power-on. This is a significant power spike that must be included in the battery life calculation.
• Omitting the RF antenna ground plane: cellular antennas require a proper PCB ground plane for correct impedance matching and radiation efficiency. A small or poorly shaped ground plane significantly reduces the effective antenna gain.