1
\$\begingroup\$

I have a simulation to make where I have an array of transmitters that transmit the same signal. At a random point, which I have to consider as a receiver I have to measure the phase shift of the signals and also measure the total signal strength. The requirement is that the transmitter emit the same signal but with different phase offsets.

  • As far I understood because the transmitters are separated apart, the received signal is already is offset by some phase because they travel distances. Is my assumption right?
  • I know the formula to find the phase offset given the distance travelled of two signals - how do I calculate it for multiple signals? Or am I understanding the question wrong?
  • How do I calculate the total signal strength for all signals combined? I know for one signal, assuming a free space, line of sight connection I can use friis equation. But how do I use it for multiple signals?

Any help is very much appreciated. I am Software Engineer who have recently taken up Wireless communications course, and I'm not sure if I understand the motivation and meaning behind the simulation right. Thanks!

\$\endgroup\$
6
  • \$\begingroup\$ Is this one single transmitter with multiple antennae? What distances from the transmitter to each of the individual transmit antennae? \$\endgroup\$ May 2 '20 at 23:50
  • \$\begingroup\$ Yeah the requirement is multiple antennae, nothing about the transmitter. The distance can be assumed - there is no requirement. Thanks for helping. \$\endgroup\$
    – tamizhgeek
    May 2 '20 at 23:59
  • \$\begingroup\$ Is more than one antenna transmitting at once? or just one at a time? \$\endgroup\$
    – user4574
    May 3 '20 at 0:23
  • \$\begingroup\$ @user4574 That is not mentioned. But I guess if they transfer one at a time, that also automatically adds a phase offset isn't it? \$\endgroup\$
    – tamizhgeek
    May 3 '20 at 12:45
  • \$\begingroup\$ Useful search terms : multipath reception, fading. Simple answers : frankly, I don't think there are any. Also it gets more complex if the transmitters don't have a common reference signal. \$\endgroup\$ May 3 '20 at 15:09
3
\$\begingroup\$

Generally with this type of problem (1 receiver & multiple transmitter antenna elements, or 1 transmitter and multiple receive antenna elements), you take one of the paths and call that your reference. The total phase shift through that path is many multiples of 360 deg, because the path is longer than one wavelength. But by making that path your reference you're saying "I don't care what the total phase shift is, I'm going to assume it's zero". We'll call that path Path 0.

Now you only have to be concerned with the relative phase differences between Path 0 and the other paths, Path 1, Path 2, ... Path n. All you have to do is do is compute the complex sum of the signals through all the paths. Need to keep in mind the effects of modulo 360 phase differences because, for example, a phase shift of 360 deg looks exactly the same as a phase shift of 0 deg.

To get the total signal strength, you need to know the radiated power level from each transmitter (antenna element), the shape of the radiated beam and/or is directivity, the distance (since the power, measured in dBm, falls off with the square of the distance), any path attenuation, and the effective area of the receiving antenna element. Using the Friis equation you mentioned for each path, you can calculate the power at each element, which is summed in a complex fashion as discussed above.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks @SteveSh I understood the phase shift part of the answer. I don't understand the second part about Friis equation. You are saying, I need to do a complex sum, but the output from Friis equation is a scalar value isn't it? \$\endgroup\$
    – tamizhgeek
    May 3 '20 at 13:06
  • \$\begingroup\$ The Friis equation just gives you the amplitude of the signal at the receiving point. It doesn't say anything about the phase shift (absolute or relative). You have to determine that separately for each path, given the total path length or path length difference from the reference path. \$\endgroup\$
    – SteveSh
    May 3 '20 at 13:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.