Tech Journal

Becoming a Better Listener: The SX1302 Gateway Reference Design

Posted by Devesh Katta & Tim Cooper on Jan 25, 2021 7:57:29 AM

An increasingly important aspect of operating a LoRaWAN network in a shared ISM band is avoiding interference from existing devices operating in the same band, and minimizing the interference caused to other users of the spectrum. To help network operators achieve this, Semtech is introducing two new features to the SX1302 LoRa Corecell gateway reference design: Listen-Before-Talk (LBT) and Spectral Scan.

These two new features use a separate SX1261 sub-GHz transceiver for listening and scanning, meaning that the gateway can continue to receive packets whilst performing scanning operations.

Below we see the schematic and board of the LoRa Corecell Gateway Reference Design for Listen Before Talk (LBT) and Spectral Scan Based on SX1302 and SX1250. In addition to the full gateway functionality, there is a dedicated SX1261 receiver that is used for the Listen-Before Talk and Spectral Scan features and an STMicroelectronics microcontroller unit to provide a USB device interface.

 Schematic: Corecell Gateway Reference Design for Listen Before Talk and Spectral Scan based on SX1302 and SX1250

Listen-Before-Talk

When transmitting from a gateway, LBT is used to reduce interference to other radio services and is mandated by some radio regulations. In the example shown below, when LBT is enabled, the gateway monitors the downlink channel for signal power above a predetermined threshold prior to each packet transmission. Following the reception of a LoRa uplink (UL) packet, the ensuing downlink transmission is prevented by the detection of an interferer during the LBT scan. The downlink is hence rescheduled for another end-device receive window following a clear channel assessment.

Diagram of how Listen-Before-Talk functions

The downlink channel frequencies, clear channel power threshold, the duration of the LBT scan, and the time in which the gateway may transmit following a clear channel assessment are all fully configurable. 

Spectral Scan

Spectral Scan allows the periodic surveillance of the ISM band at each LoRa Corecell Gateway Reference Design for Listen Before Talk (LBT) and Spectral Scan Based on SX1302 and SX1250 location to build a picture of the occupancy and usage of the band. Operation of the spectral scan is simple: the start frequency and number of points to be measured are specified, with additional options on how many times and how often the scan should be repeated. An example output from the spectral scan is shown below, here identifying a strong interferer at 868MHz.

Spectral Scan Output

The spectral scan information can be used to identify interference issues on specific sites, to inform the selection of frequency selection ahead of a gateway deployment or, in the case of smaller private networks, navigate an existing channel plan and deployed end nodes out of polluted portions of the spectrum.

Conclusion

The addition of LBT functionalities allows the LoRa Corecell Gateway Reference Design for Listen Before Talk (LBT) and Spectral Scan Based on SX1302 and SX1250 to function in new regions where its use is mandated and to reduce the possibility of interference to incumbent systems in the ISM band. Spectral scan empowers the network operator to see the radio environment their gateways are operating in. Crucially, both functionalities use the SX1261 devoted to this task, with no impact on packet reception and quality of service.

To take advantage of these features, we encourage LoRa developers to consider developing gateways using Semtech’s SX1302 LoRa Corecell reference design. 

 

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Topics: LoRa, LoRa Developers, Gateway