LoRa technology overview #

Until recently, telco mobile communication infrastructure (e.g. GSM/GPRS, 3G/4G) were the only choice for long-range connectivity of remote devices. However, these technologies are expensive and definitely not energy efficient for autonomous devices that must run on battery for months. While short-range radio, such as IEEE 802.15.4 radio, can overcome their limited transmission range with multi-hop transmission, they can actually only be realized in the context of developed countries smart cities infrastructures, where high node density with powering facility can be achieved. They can hardly be considered in isolated or rural environments.

Recent so-called Low-Power Wide Area Networks (LPWAN) such as those based on Sigfox or Semtech’s LoRa technology definitely provide a better connectivity answer for IoT as several kilometers can be achieved without relay nodes to reach a central gateway or base station which is the single interface to Internet servers through cellular/ADSL/WiFi technologies depending on what is available locally. It is therefore a star topology similar to cellular networks. Most of long-range technologies can achieve 20km or higher range in LOS condition and about 2km in urban NLOS.

LoRa belongs to the spread spectrum approaches where data can be “spreaded” in both frequencies and time to increase robustness and range by increasing the receiver’s sensitivity, which can be as low as -137dBm in 868MHz band or -148dBm in the 433MHz band. Throughput and range depend on the 3 main LoRa parameters: BW, CR and SF. BW is the physical bandwidth for RF modulation (i.e. 125kHz, 250kHz or 500kHz). Larger signal bandwidth allows for higher effective data rate, thus reducing transmission time at the expense of reduced sensitivity. CR, the coding rate for forward error detection and correction. Finally SF, the spreading factor, which can be set from 5 to 12. The lower the SF, the higher the data rate transmission but the lower the immunity to interference thus the smaller is the range. More information on LoRa are available on the Semtech’s LoRa Developer Portal Library (Tech Papers and Guides section). See for instance the LoRa® and LoRaWAN®: A Technical Overview and The Immune System | Avoiding RF Interference with LoRa.

LoRa chips #

Initially developed for the ISM sub-GHz band, LoRa modulation is now also available in the ISM 2.4GHz band to reduce frequency plan complexity and remove many duty-cycle regulation issues. Semtech LoRa chip are classified into 3 family: SX126X, SX127X and SX128X. SX1272/76/77/78/79 were the first and most used LoRa chip operating in the sub-GHz band. SX1261/62/68 are the next generation LoRa also for the sub-GHz band. It has lower power consumption (especially in receive mode with almost 50% less power consumption) and a +22dBm capability for the SX1262. It is currently under great interest and tested for low-orbit satellite LoRa. SX1280/81/82 are the LoRa chip operating in the ISM 2.4GHz band. In addition, it has ranging capability for the SX1280/82 models.

LoRaWAN #

LoRa is the physical layer. On top of LoRa, the networks can be managed in different ways. The LoRa Alliance defined the LoRaWAN specifications to manage large scale LoRa networks. More information on LoRaWAN are additionally available on the LoRa Alliance pages. It is also interesting to read the Developing LoRaWAN-based Devices: Things to Know document. From the IoT courses the dedicated course A-IOT-1 provides a comprehensive tutorial on essentials for understanding LoRa and LoRaWAN.

Enjoy!

2018 - Congduc Pham