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RSSI switching values

Srikant

Hi, (I'm a computer scientist, please be kind)

In the below attached image, I have collected the data outdoors (so no signal reflection or signal fading) from a single BLE beacon at two different pre-set power levels. I see that RSSI values fluctuate in a systematic way over time between two modes. Can someone please explain this phenomenon? Why this happens? How can we reduce / remove this?

RSSI vs time

image description

  • 0

    Btw. half a meter apart you are still almost in the near field, behavior of these things with wide spectrum of radiating things can be very far from our linear intuition...

  • 0

    Hi,

    Could it simply be that you are scanning advertising packets with long scan windows and that the three distinct "levels" correspond to the three different advertising channels. I have seen a very similar pattern before where this was the case. Have a look at the picture below:

    sadfi

    If you scan with relatively long scan windows compared to advertising intervals you will pick up several packets from channel 37, then from channel 38, and finally from channel 39. Now remember that even though they are numbered in sequence, the three advertising channels are spaced out in the entire spectrum:

    adf

    Since it is impossible to design a radio with perfect frequency response across the entire BLE spectrum you will always have slightly different performance on the three different channels. In the image below the red line illustrates how the performance might differ over the 40 BLE channels (This is just an illustration. The curve might be very different). It shows how the antenna is able to output more power on advertising channel 38 and data channels 11-16 than e.g. on data channel 36 and advertising channel 39. More power on the air shows up in your plot as higher dBm.

    asdf

    The RSSI on each channel can also vary depending on your surroundings. A signal on a given frequency might e.g. travel more easily through a wall than a different frequency.

    The bottom line: You have no guarantee that all three advertising channels will perform equally well. In fact, that is why there are three of them; To increase the chances of reaching your receiver. If one channel is useless due to interference or just poor antenna design, then hopefully you can still reach your receiver through one of the other two channels.

    In the figure below I have reproduced a similar pattern just by playing around with scan windows and advertising interval:

    asdg

  • 0 in reply to MartinBL

    How did you read the advertisements? using a smartphone? What do you mean by different performance? does it mean slightly different powers in the three channels?

  • 0 in reply to MartinBL

    It has been a while a go since I did it, but I'm pretty sure I:

    1. Used a Central example from the SDK.
    2. Scanned for packets from a peripheral with a certain UUID in the advertising packet.
    3. Read the RSSI values (p_gap_evt->params.adv_report.rssi) on BLE_GAP_EVT_ADV_REPORT events.
    4. Printed the values via UART to a computer and stored the values in a .CSV file and opened it in excel.
  • 0 in reply to MartinBL

    and What do you mean by different performance? does it mean slightly different powers in the three channels?

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