LoRa technology

In this series of articles, we want to introduce the hardware community to the experience of RTL-Service in the study of LoRa wireless technology. In our opinion, this technology has a number of features that make it very interesting for solving a certain range of tasks.
We consider the history of the emergence of technology, how it is positioned, the typical architecture of LoRaWAN networks. Next, we turn to a detailed examination of the physical features of its implementation. And finally, our experience with two implementations of transceivers from Semtech and Microchip.

The history of the emergence of technology LoRa.

In early 2015, Semtech Corporation and IBM Research Center presented a new open energy efficient network protocol LoRaWAN (Long Range Wide Area Networks), which provides significant advantages over Wi-Fi and cellular networks thanks to the possibility of deploying machine-to-machine (M2M) communications, diluting the lull in the wireless market. technologies.

LoRa technology was born under the auspices of the non-profit organization LoRa Alliance, founded by companies such as IBM, Semtech, Cisco, and others, with the goal of adopting and promoting the LoRaWAN protocol as a single standard for low-power global networks ( LPWAN ). Power Wide Area Network).
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Actually, the abbreviation LoRa combines the modulation method of LoRa in wireless networks LPWAN, developed by Semtech, and the open protocol LoRaWAN.

The developers of LoRa Alliance position LoRa as a technology that has significant advantages over cellular networks and WiFi due to the possibility of deploying machine-to-machine (M2M) communications at distances up to 20 km. and speeds up to 50 kbps., with a minimum consumption of electricity, providing several years of autonomous work on one AA battery.

The range of applications of this technology is huge: from home automation and the Internet of Things (Internet of Things, IoT) to industry and smart cities.

Architecture of LoRaWAN networks.

Consider the architecture of LoRaWAN networks. A typical LoRaWAN network consists of the following elements: end nodes, gateways, network server, and application server.
The end node (End Node) is designed to implement control or measurement functions. It contains a set of necessary sensors and control elements.
Gateway LoRa (Gateway / Concentrator) - a device that receives data from end devices using a radio channel and sends them to the transit network. Ethernet, WiFi, cellular networks and any other telecommunication channels can act as such a network. The gateway and end devices form a star network topology. Typically, this device contains multichannel transceivers for processing signals in several channels at the same time or even several signals in one channel. Accordingly, several such devices provide network coverage and transparent bidirectional data transfer between end nodes and the server.
The network server (Network Server) is designed to manage the network: setting the schedule, adapting the speed, storing and processing the received data.
The Application Server can remotely monitor the operation of the end nodes and collect the necessary data from them.


Fig. 1 Architecture of the LoRaWAN network.

Ultimately, the LoRaWAN network has a star topology of stars, has end nodes that communicate with the central server of the network through gateways that form transparent bridges. With this approach, it is usually assumed that the gateway and central server are owned by the network operator, and the end nodes are subscribers. Subscribers have the ability to transparently bidirectional and secure data transmission to the end nodes.
Since LoRaWAN form a global network, the developers paid special attention to the security and privacy of the transmitted data, which is provided by AES encryption at several levels:

• At the network level using a unique network key (Unique Network key, EUI64).
• End-to-end application-level security using a unique application key (Unique Application key, EUI64).
• And a special device key (Device specific key, EUI128).

There are three classes of devices for solving various problems and applications in the LoRaWAN network:


Fig. 2 Classes of devices in LoRaWAN networks.

1. Bidirectional “Class A” end devices (Bi-directional end-devices, Class A). Devices of this class are used when minimum power consumption is required when data transfer to the server prevails. The end node acts as the initiator of the communication session, sending a packet with the necessary data, and then selects two windows during which it waits for data from the server. Thus, the transfer of data from the server is possible only after the connection to the end device.
2. Bidirectional “B” class end devices (Bi-directional end-devices, Class B). The main difference from the “Class A” devices is the allocation of an additional reception window, which the device opens on a schedule. For scheduling, the end device synchronizes with a special signal from the gateway. Thanks to this additional window, the server has the ability to start data transfer at a known time.
3. Bidirectional "Class C" end devices with a maximum receiving window (Bi-directional end-devices, Class C). Devices of this class have an almost continuous data reception window and closes it only for the time of data transfer, which allows them to be used for solving problems that require obtaining a large amount of data.

Total, LoRaWAN allows you to build global distributed wireless networks with a large number of end nodes. According to Semtech, one LoRa gateway allows servicing up to five thousand end devices, which is achieved by:

• Network topology
• Adaptive data rate and adaptive output power of devices defined by the network server.
• Temporal separation of access to the environment.
• Frequency division channels.
• A feature of LoRa-modulation, which allows to simultaneously demodulate signals transmitted at different speeds in one frequency channel.

Conclusion

In the next article we will try to look under the hood of the LoRa technology, namely, we consider the type of modulation used in it and its basic parameters, data encoding methods. In general, everything that makes this technology unique and competitive.
Authors: Fedorov Alexander, Pushkarev Victor.