As a long-distance communication technology of low-power WAN (LPWAN), LoRa has attracted more and more attention in recent years.
LoRa is a kind of LPWAN communication technology. It is a kind of ultra long distance wireless transmission scheme based on spread spectrum technology adopted and promoted by Semtech company of the United States.Physical layer or wireless modulation is used to establish long-distance communication links. 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 linear frequency modulation and spread spectrum modulation. It keeps the same low power consumption characteristics as FSK modulation, but increases communication distance obviously.
LoRa technology itself has ultra-high receiving sensitivity (RSSI) and super strong signal-to-noise ratio (SNR). In addition, frequency hopping technology is used to carry out frequency shift keying through pseudo-random code sequence, which makes the carrier frequency jump continuously and spread the spectrum to prevent fixed frequency interference.
At present, LoRa mainly operates in the global free frequency band, including 433, 868, 915 MHz, etc.
LoRa transmitter is characterized by:
Long transmission distance
Low power consumption
Large quantity networking nodes
In mesh networks, individual terminal nodes forward information from other nodes to increase the communication distance and the size of network area. While this increases range, it also increases complexity, reduces network capacity, and reduces battery life as nodes receive and forward information from other nodes that may not be relevant to them. When long-distance connection is realized, the most significant thing of long-distance star architecture is to protect battery life.
If the gateway is installed in the location of the existing mobile communication base station, and the transmission power is 20dbm (100MW), then the coverage can be about 2km in the city with dense buildings, and 10km in the suburbs with low density. The gateway / concentrator also contains MAC layer protocol, which is transparent to the high layer.
LoRa network is mainly composed of terminal (LoRa module can be built in), gateway (or base station), network server and application server. Application data can be transmitted in two directions.
LoRaWAN network architecture is a typical star topology. In this network architecture, LoRa gateway is a transparent transmission relay, connecting terminal equipment and back-end central server. The terminal equipment uses single hop to communicate with one or more gateways. All nodes and gateways are two-way communication.
The terminal node of LoRa may be various devices, such as water meter, gas meter, smoke alarm, pet tracker, etc. These nodes connect with LoRa gateway through LoRa wireless communication, and then connect to network server through 3G network or Ethernet network. The gateway communicates with the network server through TCP / IP protocol.
LoRaWAN is a low power wide area network (LPWAN) standard which is based on open source MAC layer protocol. This technology can provide a local, national or global network for battery powered wireless devices. LoRaWAN aims at some core requirements of the Internet of things, such as secure two-way communication, mobile communication and static location identification services. This technology can realize seamless docking and interoperability between intelligent devices without local complex configuration, and give users, developers and enterprises in the field of Internet of things free operation rights.
LoRa VS NB IOT
We have introduced NB IOT in the previous article. At the same time, as the wireless communication technology of the Internet of things, NB IOT and LoRa have different technical and commercial characteristics, so they have different application scenarios.
At present, from the perspective of industrial development, a complete industrial chain has been formed, which is composed of chips, modules, terminals, communication equipment, platforms, operators and applications:
Frequency band, quality of service and cost
LoRa works in the unlicensed frequency band below 1GHz, so there is no extra charge for the application. NB IOT and cellular communications use authorized frequency bands below 1GHz. The frequency band between 500MHz and 1GHz is the best choice for long-distance communication, because the actual size and efficiency of the antenna are quite advantageous.
LoRaWAN uses free unlicensed frequency band and is an asynchronous communication protocol, which is the best choice for battery power and low cost. LoRa and LoRaWAN protocols have unique characteristics in dealing with interference, network overlap and scalability, but they can not provide the same quality of service (QoS) as cellular protocols.
It is reported that the bid for the authorized sub GHz band will cost more than US $500 million per MHz. Cellular networks and NB IOT can not provide the same battery life as LoRa for the sake of quality of service (QoS). Due to QoS and high frequency band usage fee, it is necessary to ensure that cellular network and NB IOT are recommended for QoS application scenarios, and LoRa transmitter is a good choice if low cost and large number of connections are preferred.
The concept of cellular network design is to optimize the frequency band utilization, which will sacrifice the node cost and battery life. On the contrary, LoRaWAN node is designed for low cost and long battery life, and has a certain lack of band utilization.
There are two important factors to consider in terms of battery life: the current consumption of the node (peak current and average current) and the content of the protocol. LoRaWAN is an asynchronous Aloha based protocol, that is to say, nodes can sleep long or short according to the requirements of specific application scenarios, while nodes of synchronous protocols such as cellular must be networked regularly.
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