SX1278LORA Module FAQ

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SX1278LORA Module FAQ [Copy link]

Q: What is the LoRaWAN data rate?

A: For LoRa, the LoRaWAN data rate ranges from 0.3kbps to 11kbps, and the GFSK data rate in Europe is 50kbps. In North America, the minimum data rate is 0.9kbps due to FCC restrictions. In order to maximize the battery life of the terminal device and the overall network capacity, the LoRaWAN network server manages the data rate and RF output of each terminal device separately through the adaptive data rate (ADR) algorithm. ADR is critical for high-performance networks and has scalability. In terms of infrastructure, a network is deployed with minimal investment. When capacity needs to be increased, more gateways are deployed. ADR will make the data rate higher and expand the network capacity by 6 to 8 times.

Q: How does LoRa handle interference?

A: The LoRa modem can suppress co-channel GMSK interference by up to 19.5dB, or in other words, it can accept signals 19.5dB lower than the interference signal or the noise floor. Because of such strong anti-interference performance, the LoRaTM modulation system can be used not only in frequency bands with high spectrum utilization, but also in hybrid communication networks to expand coverage when the original modulation scheme in the network fails.

Q: What is the actual Tx power that can be achieved on the LoRa device antenna?

A: The power output at the chip pin is +20dBm, and after matching/filtering loss at the antenna, the power at the antenna is +19dBm +/-0.5dB. The maximum output power is specified differently in different regions, and the LoRaWAN specification defines different output powers in different regions to maximize the link budget.

Q: What is the process of LoRa Channel Activity Detection (CAD) mode?

A: CAD is used to detect the presence of LoRa signals, rather than using a received signal strength (RSSI) method to identify whether a signal exists. It can distinguish between noise and the required LoRa signal. The CAD process requires two symbols. If detected by CAD, the CAD_Detected interrupt becomes valid and the device is in RX mode to receive data payload.

Q: Why can't my LoRa device or module output power reach 20dBm?

A: The +20dBm specification refers to the chip pin output power. In any RF system, the bandpass filter and RF switch have the characteristics of insertion loss. The typical performance on the antenna after matching filtering can reach +19dBm. Our company's module is 18dbm

Q: Can I frequently change the mode between FSK and LoRa modulation?

A: Yes, no problem. LoRa devices can switch from FSK to LoRa (and vice versa) by simple SPI register writes. There is no impact on the performance and reliability of the device. LoRa devices can be configured or reconfigured to any parameters as specified in the data sheet.

Q: What is the relationship between signal bandwidth (BW), symbol rate (Rs), and data rate (DR)?

A: Theoretically, Rs=BW/(2^SF), DR= SF*(BW/2^SF)*CR, but we recommend that you use the Semtech LoRa Modem Calculator to evaluate data rates and transmission times according to different configurations.

Q: How to choose LoRa signal bandwidth (BW), spreading factor (SF) and coding rate (CR)?

A: LoRaWAN primarily uses a 125kHz signal bandwidth setting, but other proprietary protocols can utilize other signal bandwidth (BW) settings. Changing BW, SF, and CR also changes the link budget and transmission time, requiring a trade-off between battery life and distance. Please use the LoRa Modem Calculator to evaluate the trade-offs.

Q: What are the steps for troubleshooting when two SX127x modules from different manufacturers cannot communicate with each other?

A: First, check the frequency offset caused by the crystal between the two devices. Bandwidth (BW), center frequency and data rate are all derived from the crystal frequency. Second, check the software/firmware settings on both sides to ensure that the frequency, bandwidth, spreading factor, coding rate and packet structure are consistent.

Q: In LoRa mode, when cyclic redundancy check (CRC) is enabled, how is it possible to receive an erroneous packet?

A: In LoRa mode, the payload is added to the FIFO even if the CRC is wrong. The PayloadCrcError bit must be checked before getting the payload to know its integrity. In Explicit Header mode, there is a small possibility of a false detection generating a "cloned" packet.

Either the erroneous header turns on the CrcOn bit, then the payload will be erroneous and the modem will flag it as a PayloadCrcError condition, so the packet is easily filtered out; or the erroneous header disables the CrcOn bit, in which case the mode considers the packet to be good. These occasional bad packets will have a random length (extracted from the erroneous header information) and are easily filtered out by the host, for example by seeing an abnormal size.

Q: Can I send or receive a data packet with an unlimited length payload using a LoRa device?

A: No, the maximum packet length in LoRa mode is 256 bytes.

Q: How to use DIOx pins in LoRa mode? Do all DIOx pins need to be connected to MCU?

A: When you start designing, check the DIO mapping in both LoRa and FSK modes. You can find the DIO mapping information in the SX127x LoRa datasheet. DIO does not have the same functionality as the usual (typical) MCU GPIO. There are some special interrupt information (or clock output) to indicate events or chip status, which makes your firmware design easier to implement. In theory, you may not connect the DIO pins, and then poll the relevant registers to know the status results. Of course, we recommend connecting DIO as much as possible to use as external interrupt functions, saving the MCU's resource load and being able to work in very low power mode (MCU sleeps when sending or receiving packets).

Q: Why are there two RSSI registers in LoRa mode? What is the difference?

A: In LoRa mode, both RegPktRssiValue and RegRssiValue registers are useful. RegPktRssiValue refers to the packet RSSI level, and RegRssiValue is similar to RSSI in FSK mode (non-LoRa mode). As you know, LoRa can demodulate packets below the noise floor (PktRssi result), then CurrentRssi is equal to or greater than the noise floor. For more information on how to calculate these two RSSI values, please refer to the Semtech API or the latest LoRa data sheet.

Q: The payload length of LoRa mode can be configured to 256 bytes at any data rate

A: The SX127x LoRa devices have a 256-byte FIFO in LoRa mode. In theory, all 256 bytes can be used for TX or RX. However, with a low data rate configuration, the transmission time for a 256-byte payload will be very long (several seconds or more), which is not good for fading resistance and high interference environments. This is not a robust configuration in most environments, so it is recommended that if a long payload using a low data rate is desired, the packet can be broken into several shorter packets.