125 kbps PHY Rx Sensitivity Not As Low As It Should Be

[GaryS]

Hi - 

In reading through the data sheets for the Nordic chips that support the low-rate (125 kbps) BT 5 PHY, I'm seeing Rx sensitivity numbers around -103 dBm. For the 1 Mbps PHY, it's around -96 dBm. I would expect the Rx sensitivity for 125 kbps to be 12 dB lower than 1 Mbps, i.e., -96 - 12 = -108 dBm. 9 of the 12 dB comes from reducing the bit rate by 8x, the other 3 dB comes from coding gain added to the low rate S=8 PHY. Why are you guys leaving 5 dB on the table? This is a big miss, in my opinion, since 5 dB amounts to almost a 2x range increase outdoors!

Do you have any plans in the future to improve the PHY performance to make up for the 5 dB shortfall?

Thanks,

Gary Sugar

[Dmitry]

Hi Gary,

I'm not an expert in signal processing, but I don't think you can make a direct assumption from bitrate factor. Coded PHY uses exactly the same physical channel bitrate as 1M PHY - 1 mbit/s, so you get the same BER/SNR curve after GFSK demodulation. First stage (500k) is a convolutional encoder that let us survive with higher level of error rates (for uncoded, any error in a single bit means packet is completely lost). I couldn't find a right method to compare sensitivity for these two cases. This simulation shows an 1dBm gain, but it considers only physical layer. Anyway it would be interesting to simulate a decoder with a noisy channel to see which BER of underlying channel will match, say, 10^-3 after decoding.
At second stage (125k), each symbol is encoded with 4 bits - here you can expect a direct increase of sensitivity up to 6 dBm as by lowering bitrate, also it helps to suppress interference from adjacent channels.

[GaryS]

One more thing ... there may be a bug in the Matlab simulation code mapping EbN0 to SNR. Something doesn't look quite right in there. And 14 dB sounds like it could be too high. I will have a look and get back to you.