BLE Coded PHY Preamble and Range

[David]

I'm interested in BLE coded phy and am wondering how the improved range is realized given that the preamble is not coded.  In other words, how does the receiver have improved sensitivity when the preamble itself is not actually coded?  Related -- is the RSSI value provided by the softdevice calculated based on the preamble or a packet?  Put differently: should the RSSI value of a coded vs non-coded packet be similar at fixed range holding all other variables constant?

[Simonr]

Hi

Please check out this blog post for a thorough walkthrough of Long range BLE in the Coded PHY. Basically, the raw data is still transmitted at the rate of 1Mbps, but the data includes redundancy in the user data which brings down the rate to either 500 or 125 kbps, depending on the configuration used (the nRF52 series only support 125 kbps, or S=8). Redundancy allows the receiver to recover the original data from the errors that occur in the transmission using FEC (Forward Error Correction) algorithms rather than increasing transmit power, and the higher the redundancy, the higher the probability of recovering data.

Best regards,

Simon

[David]

Thanks -- I am familiar with this blog post and redundancy/FEC.  I think my questions still remains. The blog post states:

"The Preamble is never coded, which allows the packet to be detected in any mode (Coded PHY, 1M PHY, or 2M PHY) before determining which mode was used for the remaining packet’s data."

My question is given that the preamble is not coded, how does the receiver detect this preamble at increased range/lower sensitivity?  Additionally, is the RSSI value calculated based on the preamble of a packet, the entire packet, something else?