orthogonal frequencies

[mujee]

hi
can anyone explain why dont orthognal frequencies interfere
they say in ofdm frequencies are orthognal so they dont interfere but i dont get it how does it happen

 

[Naveed Alam]

Some properties of orthogonal frequencies integral multiple of one frequency.

In OFDM, they are generated using IFFT (and are called subcarriers), and at the receiver FFT is taken. Thus the data is recovered, because both processes are inverse of each other.

If you find definition of ORTHOGONALITY in B.P LATHI, it will help you more.


Naveed

 

[saubhik]

if two frequencies r orthogonal means they r mutually perpendicular to each other then how they interfare as they already had the phase difference of a right angle between themmm , means one is leading and other is lagging with 90degree phase shift with first reference frequency /signal ...

 

[mujee]

so you mean to say they wont add up or subtract with each other like the other signals do?

 

[Naveed Alam]

if two frequencies r orthogonal means they r mutually perpendicular to each other then how they interfare as they already had the phase difference of a right angle between themmm , means one is leading and other is lagging with 90degree phase shift with first reference frequency /signal ...

What if there are 1024 subcarriers??...what will be the angular shifts???

They definitely add-up. I mean in time-domain u will get one signal. Real signal doesn't show how many frequencies have been added to make this signal. u will have to convert it to frequency domain (take fft) and u will get separate signals (subcarrier frequencies).

Naveed

 

[saubhik]

so you mean to say they wont add up or subtract with each other like the other signals do?

Yaah .... A good question atlast ....

yes they can be added or subtracted only when two signals are perfectly +_ 360 degree out of phase ... means that those signals can be added or subtracted who have same starting point .....because here the other modulated signals can overlap but can not interfere ......


in support check the attachment .....

 

[pufarina]

Orthogonality is also related to the correlation function, correlation is 0 between two orthogonal signals, so their interference will be 0 and also their separation easier (the easiest of all situations normally)

 

[inquisitive1]

but how would you define orthogonlity in ofdm

ok agreed as you said that they dont interfere as they have 90 degrees phase shift but what if there are 1024 signals(subcarriers in ofdm)-like someone said- how would all of them have 90 degrees phase difference relative to each other

 

[saubhik]

but how would you define orthogonlity in ofdm

ok agreed as you said that they dont interfere as they have 90 degrees phase shift but what if there are 1024 signals(subcarriers in ofdm)-like someone said- how would all of them have 90 degrees phase difference relative to each other

nice question but there comes another prcess named carrier thinning process
for the case 1024 and its above carriers ....

Check As i got some facts in the net ,,,,,

 

[tzushky]

I think that the word" interfere" shouldn't be used. There was another topic in this forum about this.

Interference is always present, but that does not affect the waves but the medium of propagation.

What you are trying to ask I think is why the 90 degree phase difference allows the recovery of the signal components in OFDM...and that is due to the receiver which knows how to capture the 90 degree phase shifted signals...

So, interference always happens, the idea is that the receiver knows how to separate the frequencies because they have the 90 degree phase shift...

Do you agree?

Added after 3 minutes:

And I'd like to make it clear just to be sure: the carriers are orthogonal (they have a 90 degree phase shift...) not the frequencies... Such a thing as "orthogonal frequencies" does not actually exist, it is just used in the abreviation for shortening the idea...

 

[engineer_mmamm]

hi all

I have an opinion about this issue..we all know that all signals are represented in mathematical equations..lets see for example if we have 2 signals the 1st one is (sin )function and the 2nd one is (cosine) function sent on the same frequency carrier ,they have a 90 degrees phase shift..when we demodulate them we multiply each one with the original carries sin*sin and cos*cos and we all know the sin*(sin+cos) will give an op only for the sin*sin and I cant remember the relation exactly but the op will only be for the same signals(0 phase shift) expand this principle with many carriers with multiple phase shift it will be true

I hope this true and will help u
Thanks
Mohammad