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What is a noise scan?

During a noise scan, the Base Station measures the number of times the energy appears at a certain level, per frequency range. This frequency range depends of the Base Station hardware and HAL.

For Base Stations using Semtech HAL (Hardware Abstraction Layer):

  • For gateways designed and based on Semtech's v1.0 reference design; that is to say, no dedicated FPGA or SX1272 chipsets for noise scans (so noise scan cannot be run in parallel with downlink/uplink LoRaWAN® transmission/reception):

    • Binary "rssi_histo_tools"

    • HAL version ≤ v3.2

    • No FPGA firmware

    • Measurement bandwidth for each center frequency: 125kHz

  • For gateways designed and based on Semtech's v1.5 and v2 reference designs; that is to say, with a dedicated FPGA and SX1272 chipsets to run noise scans without interrupting radio service:

    • Binary "util_spectral_scan"

    • HAL version > v3.2

    • FPGA firmware version v27. Last version, v33 will allow to change the frequency range from 25 to 500kHz.

    • Measurement bandwidth for each center frequency: 125kHz by default.

    • More information about util_spectral_scan can be found at the GitHub project.

The energy measured corresponds to the RSSI (Received Signal Strength Indicator): Signal (from an end-device, if communicating at this moment) + Interferences + Thermal Noise (of the Base Station). The unit is dBm: dB mWatt.

dBm is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW).

To express an arbitrary power P in mW as x in dBm, or vice versa, the following equivalent expressions may be used:

Here are some values:

dBmmW
01
-30.5
-100.1

 

The noise scan is always launched with input parameters:

  • Start frequency

  • Stop frequency

  • Step

By default, the values set are based on the ISM band configured on the Base Station. But, the values can be defined manually when launching a noise scan.

For better analysis of the spectrum's cleanliness (including potential RF blockers), it is recommended to extend the noise scan range beyond the frequencies of interest. From example, for EU868 ISM Band, LoRaWAN™ frequencies typically range from 865.5 MHz to 869.525 MHz, but the preconized noise scan frequencies range is from 863 to 870MHz.

Verify that the chosen step is neither too wide nor too tight, and includes all the frequencies used by the ISM Band. The preconized step is 0.1MHz; small steps directly impact the duration of each noise scan.

Noise scan result CSV file example for a Base Station using a Semtech HAL:

863000000,0.0,0,-0.5,0,-1.0,0,-1.5,0,-2.0,0,-2.5,0,-3.0,0,-3.5,0,-4.0,0,-4.5,0,-5.0,0,-5.5,0,-6.0,0,-6.5,0,-7.0,0,-7.5,0,-8.0,0,-8.5,0,-9.0,0,
-9.5,0,-10.0,0,-10.5,0,-11.0,0,-11.5,0,-12.0,0,-12.5,0,-13.0,0,-13.5,0,-14.0,0,-14.5,0,-15.0,0,-15.5,0,-16.0,0,-16.5,0,-17.0,0,-17.5,0,-18.0,0,
-18.5,0,-19.0,0,-19.5,0,-20.0,0,-20.5,0,-21.0,0,-21.5,0,-22.0,0,-22.5,0,-23.0,0,-23.5,0,-24.0,0,-24.5,0,-25.0,0,-25.5,0,-26.0,0,-26.5,0,-27.0,0,
-27.5,0,-28.0,0,-28.5,0,-29.0,0,-29.5,0,-30.0,0,-30.5,0,-31.0,0,-31.5,0,-32.0,0,-32.5,0,-33.0,0,-33.5,0,-34.0,0,-34.5,0,-35.0,0,-35.5,0,-36.0,0,
-36.5,0,-37.0,0,-37.5,0,-38.0,0,-38.5,0,-39.0,0,-39.5,0,-40.0,0,-40.5,0,-41.0,0,-41.5,0,-42.0,0,-42.5,0,-43.0,0,-43.5,0,-44.0,0,-44.5,0,-45.0,0,
-45.5,0,-46.0,0,-46.5,0,-47.0,0,-47.5,0,-48.0,0,-48.5,0,-49.0,0,-49.5,0,-50.0,0,-50.5,0,-51.0,0,-51.5,0,-52.0,0,-52.5,0,-53.0,0,-53.5,0,-54.0,0,
-54.5,0,-55.0,0,-55.5,0,-56.0,0,-56.5,0,-57.0,0,-57.5,0,-58.0,0,-58.5,0,-59.0,0,-59.5,0,-60.0,0,-60.5,0,-61.0,0,-61.5,0,-62.0,0,-62.5,0,-63.0,0,
-63.5,0,-64.0,0,-64.5,0,-65.0,0,-65.5,0,-66.0,0,-66.5,0,-67.0,0,-67.5,0,-68.0,0,-68.5,0,-69.0,0,-69.5,0,-70.0,0,-70.5,0,-71.0,0,-71.5,0,-72.0,0,
-72.5,0,-73.0,0,-73.5,0,-74.0,0,-74.5,0,-75.0,0,-75.5,0,-76.0,0,-76.5,0,-77.0,0,-77.5,0,-78.0,0,-78.5,0,-79.0,0,-79.5,0,-80.0,0,-80.5,0,-81.0,0,
-81.5,0,-82.0,0,-82.5,0,-83.0,0,-83.5,0,-84.0,0,-84.5,0,-85.0,0,-85.5,0,-86.0,0,-86.5,0,-87.0,0,-87.5,0,-88.0,0,-88.5,0,-89.0,0,-89.5,0,-90.0,0,
-90.5,0,-91.0,0,-91.5,0,-92.0,0,-92.5,0,-93.0,0,-93.5,0,-94.0,0,-94.5,0,-95.0,0,-95.5,0,-96.0,0,-96.5,0,-97.0,0,-97.5,0,-98.0,0,-98.5,0,-99.0,0,
-99.5,0,-100.0,0,-100.5,0,-101.0,0,-101.5,0,-102.0,0,-102.5,0,-103.0,0,-103.5,0,-104.0,0,-104.5,17,-105.0,23,-105.5,417,-106.0,736,-106.5,1353,
-107.0,4729,-107.5,3219,-108.0,2770,-108.5,2833,-109.0,399,-109.5,115,-110.0,30,-110.5,0,-111.0,0,-111.5,0,-112.0,0,-112.5,0,-113.0,0,-113.5,0,
-114.0,0,-114.5,0,-115.0,0,-115.5,0,-116.0,0,-116.5,0,-117.0,0,-117.5,0,-118.0,0,-118.5,0,-119.0,0,-119.5,0,-120.0,0,-120.5,0,-121.0,0,-121.5,0,
-122.0,0,-122.5,0,-123.0,0,-123.5,0,-124.0,0,-124.5,0,-125.0,0,-125.5,0,-126.0,0,-126.5,0,-127.0,0,
-127.5,0 863050000,0.0,0,-0.5,0,…

The first value of the line is the frequency, the second value is the RSSI level, and the third value is the number of times the Base Station measured this RSSI level for this frequency (that is to say, the RSSI count).

The lower the RSSI measured by the Base Station, the better.

Following are some reference values for channels using 125 kHz bandwidth:

  • < -130dBm: impossible result. The RSSI measured by the Base Station should not be even lower than the thermal noise floor (around -120dBm).

  • -130 to -120 dBm: wrong result. The RSSI measured by the Base Station should not be even lower than the thermal noise floor (around -120dBm). Probably a measurement issue on the Base Station.

  • -120 to -110dBm: excellent result. No signal or interferences around the Base Station.

  • -110 to -100dBm: very good result. Very few interferences around the Base Station.

  • -100 to -95dBm: good result. Little interferences around the Base Station, but not disturbing the Base Station.

  • > -95dBm: bad result. Signal coming from an end-device or strong interferences around the Base Station. Do not use the corresponding channel.

  • ≥ 0dBm: impossible result. RSSI cannot be null or positive.

Following are some reference values for channels using 500 kHz bandwidth:

  • < -125dBm: impossible result. The RSSI measured by the Base Station should not be even lower than the thermal noise floor (around -114dBm).

  • -125 to -114 dBm: wrong result. The RSSI measured by the Base Station should not be even lower than the thermal noise floor (around -114dBm). Probably a measurement issue on the Base Station.

  • -114 to -105 dBm: excellent result. No signal or interferences around the Base Station.

  • -105 to -95 dBm: very good result. Very few interferences around the Base Station.

  • -95 to -90 dBm: good result. Little interferences around the Base Station, but not disturbing the Base Station.

  • > -90 dBm: bad result. Signal coming from an end-device or strong interferences around the Base Station. Do not use the corresponding channel.

  • ≥ 0 dBm: impossible result. RSSI cannot be null or positive.

The duration of the noise scan can depend on the version of the Semtech HAL, and the input parameters set.

The noise scan is organized in captures: each capture is 4096 samples long. By default, each frequency has 90 captures, which means around 3s for 125 kHz sample rate:

  • Samples * captures = 4096 * 90 = 368640 consecutive points.

  • The sample rate is rate = 125 kHz.

  • Thus, the analysis of one frequency point, that is, the duration of one step is:

  • Duration = samples * captures / rate = 2.95 seconds ~ 3 seconds

 

Example 1: How long would a scan over the [915...928] MHz range last, provided that the resolution (measurement step) is 25 kHz?

Range = 928 - 915 = 13 MHz

Resolution = 25 kHz

Steps = range / resolution = 13 / 0.025 = 520

Duration_total = steps * duration = 520 * 3 = 1560 seconds = 26 minutes

 

Example 2: How long would a scan over the [902...927.6] MHz range last, provided that the resolution is 100 kHz?

Range = 927.6 - 902 = 25.6 MHz

Resolution = 100 kHz

Steps = range / resolution = 25.6 / 0.1 = 256

Duration_total = steps * duration = 256 * 3 = 768 seconds = 12.8 minutes

 

For a better analysis, it is preconized to run several noise scans per base station at different hours of the day, to identify the energy coming from end-devices communicating, and punctual interferences.

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