# Noise Interference multipath propagation environment in wireless communications

#### JimmyCho

Joined Aug 1, 2020
109
Hi guys,
I'm trying to understand something which is really confusing me and I made my best to understand it but still confused.

If I have transmitted and receiver. Assume I want to transmit S(t) data and there's two multipath ways L1 , L2 in order to arrive the receiver.
Why when I have constructive interference then it's good for me ? in the paths/multipath there's might be destructive interference or constructive interference...my question why constructive interference is good for me..assume I get 2S(t) -constructive interference- in the received signal on the receiver ..why it's good for me ? what does it mean that I have 2S(t) and not S(t) ?

Im confused on the term constructive interference and how it's related to my received signal ..?

thanks alot

#### ronsimpson

Joined Oct 7, 2019
2,261
• JimmyCho

#### JimmyCho

Joined Aug 1, 2020
109
In the picture we have red path and green path.
If green is longer so the signal is delayed 180 degrees then the two signals substract. (Think of 2 sines wave 180 degrees apart, one is at the + peak while the other is at the - peak)
If the green is longer by 360 degrees the two signals will add to make 2X. (could be 2x360 and will add or 3x360)
View attachment 232598

This is exactly I understand it and all fine .. what's confusion me is that at the receiver is 2X , what does that mean in the side of the receiver .. is that good signal ? if yes then why ? ..my confusion comes from that I send X from the transmitter ..so expected to get X on the receiver and not 2X ..because 2X isn't equal to X so 2X isn't the data that I transmit from the transmitter.

May my understandings wrong and not correctly .. that's why I asked that question and why constructive interference like 2X equal to what I transmitted X , 2X isn't equal to X so it should be considered as wrong received data ..no ? ..Im trying to understands how the meaning of constructive interference related to the receiver's reception and why it's considered like 2X equal to X and it shouldn't be equal because I transmitted X and I received 2X which it's not the same transmitted data X , really weird.

#### LesJones

Joined Jan 8, 2017
3,773
My understanding is the 2X is the fact that the strength of the signals from each path add. I think this is assuming the the path loss on both paths is the same. If the signal is not a simple carrier but contains information it will have sidebands at slightly different frequencies. So as the sidebands are at a different frequency to the carrier they could cancel out but the carrier could add. Even though the time difference of the signals via the two paths is the same the phase difference would not be the same. You seem to be considering the "X" as data not signal strength.

Les.

#### JimmyCho

Joined Aug 1, 2020
109
My understanding is the 2X is the fact that the strength of the signals from each path add. I think this is assuming the the path loss on both paths is the same. If the signal is not a simple carrier but contains information it will have sidebands at slightly different frequencies. So as the sidebands are at a different frequency to the carrier they could cancel out but the carrier could add. Even though the time difference of the signals via the two paths is the same the phase difference would not be the same.

Les.
I assumed it's constructive .. so all good and already know when it's constructive and when it's destructive .
ONCE again my confusion as I explained in the comment above .. I have no problem when the constructive happens or destructive happens .. my problem is how we say that 2X is good and I transmitted X ..so 2X isnt the same X that I transmitted and it should be considered as distortion and something not good ..because 2X isn't the same as X and then it should be considered distortion ..

#### Ya’akov

Joined Jan 27, 2019
6,286
With multipath, the constructive interference is only “good” if the phase shift isn’t enough to interfere with the modulation scheme. Otherwise you just have a stronger, useless signal instead of a weaker, useless one.

• Delta Prime and LesJones