Effect of radio reflections / multi path on received message

[Robert]

Hi

We use the NRF51 in a mesh sensor network. We noticed in a trailer suited for cold chain transport (double metal walls with thermal isolation between) that the radio has problems with receiving message. Even at short distances (< 5 meter) we missed a lot of message at 2Mbit/s mode (90% of send message either missed or CRC erros). This only occurs in the trailer. The RSSI of the received message was around -40 dBM, so in general it is not a distance problem. We expect some effect of reflected message. However, distance (compared to speed of light) are to short to expect pathlengt delay's in the order of one bit time. Reducing the data rate to 1Mbit/s improves (less missed messages), but still yield very variable results. However at 250kbit/s we had 99 to 100% succes rate, no missed message nor CRC erros. The RSSI receive strenght of the message is still around -40 dBM.

We don't understand the effect we see completly. Can somebody explain what is happening with the FSK modulation and relfections and why message are received at 250 kbit/s oke ? Is there a way to achieve the same performance at 2Mbit/s by a certain radio setting ?

[Øyvind Karlsen]

Hi,

In an environment like this you are likely to have issues with destructive interference from multipath. This effect is highly dependent on the position of transmitter and receiver.

When an electromagnetic wave is reflected at a slant angle its phase will be inverted, i.e. a phase shift of 180 degrees is applied. When this reflected component arrives it will interact with the main component depending on the difference in distance. The phase changes 360 degrees over one wavelength, in the given scenario if the distance is 0 or nλ there will be total cancellation of the signal, constructive interference occurs at nλ/2. This means that you need to make sure that you do the measurements for the same positions in order for them to make sense. If you get very poor signals in one spot try moving one of the devices on the order of 13cm(one wavelength).

It is interesting that you see changes from 2 Mbit/s to 250 kbit/s, were these for the exact same positions? The corresponding bandwidth from 2Mbit/s is within the coherence bandwidth for this type of channel (indoor 2.4GHz usually has a minimum of 3 MHz coherence bandwidth according to this study).

Best regards,

Øyvind