Frequently Asked Questions about SX1278 LoRa Module

Q: What is the LoRaWAN data rate?

A: For LoRa, the LoRaWAN data rate ranges from 0.3 Kbps to 11 kbps, and the European GFSK data rate is 50 kbps. In North America, the minimum data rate is 0.9 Kbps due to FCC restrictions. To maximize the battery life and overall network capacity of the device, the LoRaWAN network server manages each device's data rate and RF output separately through an adaptive data rate (ADR) algorithm. ADR is critical for high performance networks and scalable. As for infrastructure investment, we can deploy a network with minimal investment first, and then deploy more gateways when capacity needs to be increased, while the ADR will result in higher data rates, expanding network capacity by six to eight times.

Q: How does LoRa handle signal interference?

A: The LoRa modem can suppress co-channel GMSK interference up to 19.5dB, or in other words, it can accept a signal 19.5dB lower than the interference signal or background noise. Base on this strong anti-jamming capability, LoRa modulation system can be used not only for bands with high frequency spectrum utilization, but also for 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 chip output power is +20dBm, after matching/filtering loss, the power is +19dBm +/-0.5dB on the antenna. Maximum output power is regulated differently in different regions, and the LoRaWAN specification defines different output power for 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 a method of accepting signal strength (RSSI) to identify if a signal exists. It distinguishes between noise and the desired LoRa signal. The CAD process requires two symbols, if detected by the CAD, CAD_ Detected interrupts become valid, and the device is in RX mode to accept data payloads.

Q: Why is the output power of my LoRa device or LoRa module less than 20dBm?

A: +20dBm is mainly for the chip output power. In any RF system, bandpass filters and RF switches have insertion loss characteristics, and typical antenna performance can reach +19dBm after matched filtering.

Q: Can I switch frequently between FSK and LoRa modulation modes?

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

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

A: In theory, Rs=BW/(2^SF), DR= SF*(BW/2^SF) *CR, but we recommend that you use Semtech LoRa modem calculator to evaluate data rate and transmission time according to different configuration selections.

Q: How do I select LoRa signal bandwidth (BW), spread spectrum factor (SF), and coding rate (CR)?

A: LoRaWAN mainly uses 125kHz signal bandwidth settings, but other private protocols can use other signal bandwidth (BW) settings. BW, SF, and CR changing will also change link budget and transmission time, battery life and distance must be taken into consideration. Please use the LoRa modem calculator to do the evaluation.

Q: In LoRa mode, how can an incorrect packet be received when the cyclic redundancy check (CRC) is enabled?

A: In LoRa mode, payloads are added to FIFO even if the CRC is wrong. Bit PayloadCrcError must be checked for its integrity before a payload can be obtained. In Explicit Header mode, there is a small possibility that a false detection will result in a "cloned" packet.

If the wrong header opens the CrcOn bit, then the payload will be wrong, and the modem will mark it as a PayloadCrcError condition, so the packet will be easily filtered out; while if the wrong header prohibits CrcOn bits, in which case the pattern assumes that the packet is good. These incidental bad packages have a random length (can be extracted from the error header information) that can be easily filtered out by the host, which can be identified by abnormal length

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

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

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

A: When you start designing, check the DIO mapping in both LoRa and FSK modes. You can find DIO mapping information in the SX127x LoRa data sheet. DIO does not have the same functionality as the usual (typical) MCU GPIO. There are special interrupt messages (or clock outputs) that indicate events or the state of the chip, which making your software design easier to implement.

In theory, you may not connect the DIO pins to poll the associated registers for status results. However, we recommend connecting DIOs to use as many external interrupt functions as possible, thereby saving the resource load of the MCU, and working in a very low power mode (the MCU will sleep when packaged to send or receive packets).

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

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

Q: Can the LoRa mode payload length be configured to 256 bytes at any data rate?

A: The SX127x LoRa device has a 256 byte FIFO in LoRa mode. Theoretically, all 256 bytes can be used for TX or RX. However, with a low data rate configuration, the 256-byte payload will take a long time to transmit (several seconds or longer), which is not good for anti-signal attenuation and high-interference environments. This is not a robust configuration in most environments, so it is recommended that if you want a payload that uses a long, low data rate, then the packets can be divided into several short packets.