1、 Introduction to Lora wireless technology

Lora is a long-distance wireless transmission technology based on spread spectrum technology. In fact, it is also one of many lpwan communication technologies. It was first adopted and promoted by Semtech company of the United States. This scheme provides users with a simple means of long-distance and low-power wireless communication. At present, Lora mainly operates in ISM band, mainly including 433, 868, 915 MHz, etc.

Lora is a physical layer or wireless modulation used to establish a long-distance communication link. Many traditional wireless systems use frequency shift keying (FSK) modulation as the physical layer because it is a very effective modulation to achieve low power consumption. Lora is based on LFM spread spectrum modulation. It maintains the same low power consumption characteristics as FSK modulation, but significantly increases the communication distance. Linear spread spectrum has been used in the field of military and space communication for decades, because it can achieve long communication distance and robustness to interference, but Lora is the first low-cost implementation for commercial use.

Lora's advantage lies in its long-distance capability in technology. A single gateway or base station can cover the whole city or hundreds of square kilometers. At a given location, the distance depends largely on the environment or obstacles, but Lora and lorawan? There is a link budget better than any other standardized communication technology. Link budget, usually expressed in decibels (DB), is the main factor determining the distance in a given environment.

2、 Lora wireless technology network composition

Lora network is mainly composed of terminal (LORA module can be built in), gateway (or base station), server and cloud. Application data can be transmitted in both directions.

Lora alliance Lora alliance is an open and non-profit organization led and established by Semtech in March 2015. Its sponsors include French actity, Chinese augtek and Royal Dutch Telecom KPN. In less than a year, the alliance has developed more than 150 member companies, including IBM, Cisco, French Orange and other heavyweight manufacturers. There are a large number of enterprises in each link of the industrial chain (terminal hardware manufacturers, chip manufacturers, module gateway manufacturers, software manufacturers, system integrators and network operators). The openness of this technology and the sufficiency of competition and cooperation have promoted the rapid development and ecological prosperity of Lora.

3、 Lora network architecture and principle

In mesh networks, individual terminal nodes forward the information of other nodes to increase the communication distance and network area size. Although this increases the scope, it also increases the complexity, reduces the network capacity, and reduces the battery life, as nodes receive and forward information from other nodes that may not be related to them. When realizing long-distance connection, the most meaningful thing of long-distance star architecture is to protect the battery life.

In lorawan? In the network, the node is not associated with a private gateway. On the contrary, the data transmitted by a node is usually received by multiple gateways. Each gateway forwards the received data packets received from the terminal node to the network server based on cloud computing through some backhaul (cellular, Ethernet, etc.). Intelligence and complexity are put on the server. The server manages the network and filters redundant received data, performs security checks, performs scheduling confirmation through the optimal gateway, and performs adaptive data rate.

4、 Introduction to lorawan protocol

Lorawan is a low-power Wan communication protocol based on open source MAC layer protocol released by Lora alliance. It mainly provides local, national or global network communication protocols for battery powered wireless devices.

LoRaWAN? The communication protocol and system architecture of the network are defined, and the Lora physical layer can make the long-distance communication link possible. Lorawan is a bottom-up design that optimizes the lpwan (low power WAN) for battery life, capacity, distance and cost. For different regions, a lorawan? Specification summary and high-level comparison of different technologies competing in lpwan space.

5、 Lorawan network topology

Lorawan network is a typical mesh network topology. In this network architecture, Lora gateway is responsible for data aggregation and connecting terminal devices and back-end cloud data servers. The gateway connects to the TCP / IP network between servers. All nodes and gateways have two-way communication. Considering the situation of battery power supply, the terminal node generally sleeps. When there is data to be sent, wake up and then send data.

Therefore, using Lora technology, we can obtain a longer transmission distance with low transmission power. This low-power wide area technology is necessary for large-scale deployment of wireless sensor networks.

6、 Advantages and disadvantages of Lora Technology

Generally speaking, transmission rate, working frequency band and network topology are the three main parameters affecting the characteristics of sensor networks. The selection of transmission rate will affect the transmission distance and battery life of the system; The selection of working frequency band shall compromise the design objectives of frequency band and system; In FSK system, the choice of network topology is determined by the transmission distance requirements and the number of nodes required by the system. Lora combines digital spread spectrum, digital signal processing and forward error correction coding technology, and has unprecedented performance. Previously, only those high-level industrial radio communications would integrate these technologies. With the introduction of Lora, the situation in the field of embedded wireless communication has been completely changed.

Forward error correction coding technology adds some redundant information to the data sequence to be transmitted, so that the error symbols injected in the data transmission process will be corrected in time at the receiving end. This technology reduces the need to create "self-healing" packets for retransmission, and performs well in solving the sudden bit error caused by multipath fading. Once the data packets are established and injected into the forward error correction coding to ensure reliability, these data packets will be sent to the digital spread spectrum modulator. This modulator feeds each bit in the packet into a "spreader", and divides each bit time into many chips.

Even if the noise is very large, Lora can handle it calmly. After being configured, the Lora modem can be divided into 64-4096 chips / bit, and the highest spread spectrum factor (12) in 4096 chips / bit can be used at most. Relatively speaking, ZigBee can only be divided into 10-12 chips / bit.

By using high spreading factor, Lora technology can transmit small capacity data through a wide range of radio spectrum. In fact, when you measure through the spectrum analyzer, these data look like noise, but the difference is that the noise is uncorrelated, while the data is correlated. Based on this, the data can actually be extracted from the noise. The higher the spread spectrum factor, the more data can be extracted from the noise. At a well functioning GFSK receiver, the minimum signal-to-noise ratio (SNR) of 8dB needs to be reliably demodulated. By configuring angelblocks, Lora can demodulate a signal with a signal-to-noise ratio of - 20dB. The gap between GFSK and this result is 28dB, which is equivalent to a lot of expansion of range and distance. In the outdoor environment, the 6dB gap can be twice the original transmission distance.

Strong link budget makes the signal fly farther. In order to effectively compare the performance of transmission range between different technologies, we use a quantitative index called "link budget". The link budget includes each variable affecting the signal strength of the receiving end, and its simplified system includes the transmission power plus the sensitivity of the receiving end. The transmission power of angelblocks is 100MW (20dbm), the receiver sensitivity is - 129dbm, and the total link budget is 149db. In comparison, GFSK wireless technology with sensitivity of - 110dbm (which is already its excellent data) requires 5W power (37dbm) to reach the same link budget value. In practice, the receiver sensitivity of most GFSK wireless technologies can reach - 103dbm. In this case, the transmission frequency of the transmitter must be 46dbm or about 36W to reach the link budget value similar to Lora. Therefore, using Lora technology, we can obtain a wider transmission range and distance with low transmission power. This low-power wide area technology is exactly what we need.

7、 About lpwan

Low power Wan Low power wide area network (lpwan) is an indispensable part of the Internet of things. It has the characteristics of low power consumption, wide coverage and strong penetration. It is suitable for applications that send and receive a small amount of data every few minutes, such as water transportation positioning, street lamp monitoring, parking space monitoring, etc. Lora alliance, a related organization of lpwan, currently has 145 members all over the world, with its lush ecology The system makes the devices that follow the lorawan protocol have strong interoperability. A communication gateway that fully conforms to the lorawan standard can access tens of thousands of wireless sensor nodes within 5 to 10 kilometers. Its efficiency is much higher than the traditional point-to-point polling communication mode, and can also greatly reduce the node communication power consumption.