Effect of radio reflections / multi path on received message

RE: Effect of radio reflections / multi path on received message

Øyvind Karlsen — Mon, 13 Jun 2016 07:51:48 GMT

Hi,

I agree that it is not in line with the 3 MHz statement, on closer inspection the measurements that state this are from an open office, which is very different from your scenario.

There are expensive ways to circumvent this, however it is likely better to cope with the possibility of reduced datarates, ie. switch to a lower datarate when you get large packet drops. The problem you are facing is related to a physical limitation that causes symbol interference. Traditionally Inter Symbol Interference (ISI) is addressed by using an equalizer or applying matched filtering. Finally you can attempt to obtain some antenna diversity.

Øyvind

RE: Effect of radio reflections / multi path on received message

Robert — Mon, 06 Jun 2016 15:15:22 GMT

Hello Øyvind

Thanks for your feedback. Some remarks and additional questions.

We are aware of the possible interfernce issue due to multipath. That is the main reason why we selected a mesh solution. The mesh solution should make the data distribution independent from the actual position of a single transmitter and a single receiver.

Therefore we did all the measurement initially at the same position. In the same position we did the tests at 2Mbit/s, 1Mbits/s and 250 kbits/s, with the results reported above. Thereafter we moved the setup several times in several directions with half a wavelenght, with similair results. This gave us the idea that we are not having peak or interference at the 2.4 Ghz wave, i.e that the demodulator could follow the 2.4 Ghz wave. (Is there a way to see if the NRF radio has difficulties to lock the demodulator to the 2.4 Ghz wave ?)

As stated, the improvement from 2Mbit/s to 250 kbit/s where done at exactly the same postion. This is imho not in line with the remark that 3MHz coherence bandwidht could be expected. The difference between 2Mbit/s and 250 kbit/s at the same position gave us the idea that "bit" demodulation errors occur. For a pathlenght difference of 15 meter (extra lenght when traveled once in the trailer ) the reflected wave would have a delay of 50 nano seconds. This is 10% of period at 2Mbit/s and only 1.25% at 250 kbit/s. Could this be the case ? If so, is there a way to make the 2Mbit/s demodulation in the radio less sensitive to this effect (by filtering in the antenne circuit ?)

RE: Effect of radio reflections / multi path on received message

Øyvind Karlsen — Fri, 03 Jun 2016 08:26:48 GMT

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