Introducción a LoRa
LoRa is an ultra-long-distance, low-power IoT solution provided by Semtech. Semtech and many industry-leading companies, such as Cisco, IBM and Microchip, initiated the LoRa (Long Range, Wide Range) Alliance and are committed to promoting its alliance standard LoRaWAN technology to meet various needs for wide area coverage and low power consumption. M2M device application requirements. LoRaWAN currently has more than 150 members, and several Chinese companies are involved. It has been commercially deployed in several European countries, and has also begun to be used domestically.
Características técnicas de LoRa
The design of LoRa’s physical layer and MAC layer fully reflects its consideration of IoT business needs. The LoRa physical layer uses spread spectrum technology to improve receiver sensitivity, and the terminal can work in different working modes to meet the power saving needs of different applications.
The network architecture and protocol stack of LoRa are shown in Figure 1. The network architecture includes terminals, gateways, network servers and business servers. The terminal node includes the implementation of the physical layer, MAC layer and application layer; the gateway completes the processing of the air interface physical layer; and the network server is responsible for MAC layer processing, including adaptive rate selection, gateway management and selection, MAC layer mode loading, etc. The application server obtains application data from the network server and performs application status display, real-time alarms, etc. The MAC layer can follow the alliance’s standard LoRaWAN protocol or the MAC protocol developed by each manufacturer.
(1) LoRa physical layer and MAC layer design LoRa is a half-duplex system, and the uplink and downlink work in the same frequency band. The current LoRa system bandwidth supported by a domestic single chip is 2Mbit/s, including 8 channels with a fixed bandwidth of 125kbit/s. Each fixed-bandwidth channel requires a 125kHz guard band, which requires at least 2Mbit/s system bandwidth. Each channel supports 6 spreading factors SF7~12. Adding 1 to the spreading factor increases the receiver sensitivity by 2.5dB.
The terminal uses random channel selection to avoid interference. Each time the terminal transmits uplink data or retransmits data, it will randomly select a channel among the 8 channels for access. The communication between the terminal and the gateway can use different rates, that is, different SFs. The selection of the rate needs to weigh factors such as communication distance or signal strength, message sending time, etc., so that the terminal can obtain the maximum battery life and maximize the gateway capacity. When the link environment is good, a lower spreading factor can be used, which means a higher data rate. When the terminal is far away from the gateway and the link environment is poor, the spreading factor can be increased to obtain higher sensitivity. However, At the same time the data rate will be reduced. For a 125kbit/s fixed bandwidth channel, the data rate can be selected within a considerable range from 250bit/s to 5kbit/s.
(2) Terminal working mode
The LoRa design terminal has three different modes, namely Class A, B and C, but the terminal can only work in one mode at a time, and each mode can be loaded by software. Different modes are suitable for different business models and power-saving modes. Currently, the Class A working mode is widely used to adapt to the power-saving needs of IoT applications.
Class A (two-way terminal equipment): Class A terminal equipment provides two-way communication, but cannot perform active downlink transmission. The sending process of each terminal will be followed by two short downlink receiving windows, as shown in Figure 2. The downlink transmission time slot is determined based on the needs of the terminal and a small random amount, so Class A terminals save the most power.
Class B (two-way terminal that supports downlink time slot scheduling): Class B terminals are compatible with Class A terminals and support receiving downlink Beacon signals to maintain synchronization with the network in order to monitor information at the downlink scheduled time, so the power consumption will be greater than A Class terminal.
Class C (bidirectional terminal with maximum receiving time slot): Class C terminal only stops the downlink receiving window at the moment of transmitting data, and is suitable for applications with large amounts of downlink data. Compared with Type A and Type B terminals, Type C terminals consume the most power, but for server-to-terminal services, Type C mode has the smallest delay.
(3) LoRa network security
The terminal device must complete the network security key acquisition during a joining process before interacting with the network server data. The terminal needs to have the following security information when accessing and using it: including terminal equipment identification (DevEUI), application identification (AppEUI) and AES-128 application key (AppKey). Among them, DevEUI is the global terminal device ID that uniquely identifies the terminal device. AppEUI is a global application ID stored in the terminal device, which uniquely identifies the application provider (i.e. user) of the terminal device. AppKey is an AES-128 application key defined on the terminal device. It is assigned to the terminal device by the owner of the application. It is derived from the independent root key of each application. The root key is known by the application provider and is in the application. under the control of the program provider. Whenever a terminal device joins the network through the joining process, AppKey is used to deduce the session keys NwkSKey and AppSKey defined for the terminal device. The session key is used to ensure the security of network communications, and the application key is used to ensure the security of the application. End-to-end security.
(4) Performance testing and evaluation
As an LPWA technology, LoRa focuses on its key performance indicators such as coverage, power consumption and cost.
①Cover
Since it supports spread spectrum technology, different spreading factors can achieve different sensitivity requirements. When the transmit power reaches 23dBm, LoRa supports an MCL (maximum coupling loss) of approximately 160dB, which almost meets the MCL requirements of narrowband IoT technology with new air interface designs such as NB-IoT, and can achieve deep indoor coverage goals. The corresponding sensitivities of different spreading factors of the LoRa system are shown in Table 1. The interference in the frequency band where the LoRa system is located will directly affect its coverage performance. Judging from the current test results in the 470MHz and 915MHz frequency bands of LoRa field in Shanghai, the noise floor of 470MHz is about -110dBm, and the minimum SINR in the case of SF12 is 15dB, so the actual minimum reception level of 470MHz is about -125dBm, so the actual coverage is affected by external interference. If there is a loss of nearly 10dB, the coverage advantage over GPRS is not outstanding. The 915MHz frequency band has large interference, with the lowest RSSI reaching -100dBm, which cannot reflect the coverage advantage, so it is not recommended to choose this frequency band.
②Capacity
At present, the LoRa system mainly uses Class A mode, which triggers uplink data transmission and cannot perform resource scheduling. It mainly relies on frequency hopping on different channels to avoid interference. Therefore, random channel selection and collision avoidance mechanisms have an impact on system capacity. According to the business model of 50B/2h reporting, it is estimated that the number of reports successfully sent per hour is that each LoRa gateway supports about 50,000 reporting messages, which exceeds the current industry’s capacity requirements for LPWA technology. Simulate the scenario of 4 gateways and 4000 users with a distance of 1km. Users report 120B data packets every half hour. The simulation results are shown in Figure 3. Due to different channel conditions of the terminal, the terminal will adaptively select the appropriate SF, that is, different rates for communication. The total channel occupancy of different SFs is statistically calculated. The total channel occupancy rate does not exceed 10%.
③Power consumption
The receiving state current of LoRa is 12mA. When the transmitting power is 14dBm, the current is approximately 32mA; and when entering the Sleep state, the current consumption is less than 1μA. The rate-adaptive ADR mechanism can transmit at a higher rate when wireless conditions permit, thereby reducing the Tx state duration and reducing total battery power consumption. Table 2 shows the estimated battery life (in years) of LoRa applications with built-in 5Wh batteries under different coverage conditions and different business models. Judging from the estimation results, the power consumption of the LoRa system has considerable advantages over current cellular communication systems and narrowband IoT systems.
④Delay
Currently, Class A terminals are widely used, that is, they can only support uplink-triggered downlink transmission, but cannot support active downlink services. Therefore, for services with downlink active transmission, LoRa cannot support the corresponding business requirements. At the same time, for uplink data transmission, if data confirmation is required, the downlink ACK must be sent in the fixed downlink time slot triggered by the uplink. At present, the interval between uplink and downlink time slots is generally set to 1s, which means that the delay is at least more than 1s. Since the system itself is not designed with a complete QoS mechanism to ensure reliable reception, its delay characteristics do not have any advantages compared with scheduling-based cellular systems.
⑤Cost
LoRa’s current chip cost is about US$1, and the module cost is about US$5, which basically meets the industry’s requirements for LPWA technology.
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