RSSI and PHY on nRF52840

[Chaabane]

Hello,

I did a quick test using 2 nRF52840 kits to measure the change of the RSSI value based on the distance and the used PHY layer.

For each test, I configured both modules to use only one PHY layer: CODED, 1M and finally 2M.

.phys.tx_phys             = BLE_GAP_PHY_1MBPS,
.phys.rx_phys             = BLE_GAP_PHY_1MBPS,

The RSSI was the same for all the tests. Ex. At 125m distance the RSSI was about -90 dBm.

From my understanding on how the physical layers on BLE work, at distance of 200m, the RSSI value will be the same no matter what the PHY layer is used ? the difference will be noticeable only when connecting to another device. Am I correct ?

If what's stated above is correct, for application where only the RSSI value is required and no connection needed, the PHY layer doesn't matter ! maybe using 2M will help reduce on air time for advertising messages thus power consumption !

In the other hand, at a relatively long distance, will the read of the RSSI value (advertise message) will be possible when using Coded PHY and not possible with 1M and 2M PHYs

Thank you Chaabane

[Jørn]

Hello Chaabane

BLE uses the Gaussian Frequency Shift Keying (GFSK) modulation scheme, in which the peak amplitude (and therefore power of) the signal is constant, regardless of whether you're transmitting a 1 or a 0. The 1Mbit/s PHY allows the receiver to use 1us to determine what a given transmitted symbol represents (1 or 0). 2Mbit/s PHY allows the receiver to use 500ns. So the transmitted signal is essentially the same, you just change symbol more often.

The coded PHY uses the 1Mbit/s symbol rate, but represents each bit with more than one symbol, in addition to adding forward error correction to the packet.

So the output power never changes based on what PHY you have selected, hence no change in RSSI between them, regardless of what range you are at. Strictly speaking, as the RSSI is measured by the receiver and not transmitted by the transmitter, the RSSI shouldn't be different for different PHYs at long ranges either. However if the receiver doesn't recognize the incoming signal as that of a BLE device then it would probably ignore it, so it wouldn't display it.

Best regards

Jørn Frøysa

[Jørn]

I see the 2Mbit/s PHY's sensitivity is worse than the other modes, so that will affect your overall range. In an environment with a lot of noise it can be favorable to have a lower bitrate, as it makes it easier for the receiver to separate a 1 from a 0. Also I do not think you can advertise with 2Mbit/s PHY. I believe you would typically connect using standard 1Mbit/s or Long range and then change PHY to 2 Mbit/s.