interference in radio communication

[sarahr202]

Hi every body!

i was reading about wlan.

wlan uses radio waves to transmit and receive data. These radio waves( EM waves) are prone to interfernce from external sources (Our focus is interference from EM waves).

Few of those external sources are EM waves being used by other devices in the same area.

CASE # 1

Two frequencies , f1 and f2 both are from different bands. f1 has power,p1 and f2 has power ,p2.

Assume p1>p2 and both fequencies are being used in same area.

will they interfere with each other?

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case 2;

Let say we have one frequency f1.

Two radio station,,st1 and st2, in same coverage area, use this frequency. st1 uses phase modulation while st2 uses AM modulation.

Will these two stations successfully transmit?

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thanks a lot and have a nice weekend !

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

there are whole university courses dedicated to RF interference.

Let me give only some modest hints:

First of all there is a carrier that is an high frequency signal.

Modulation tecniques are usually the result of a non linear operation like

s(t)= c(t) * m(t)

for AM modulation

s(t) = C(t+phi(m(t))

phase modulation

the modulated signal results in a spectral power density centered around the nominal carrier frequency but actually spread by

carrier variation in frequency

times signal speed (for AM)

times more the signal speed (for phase modulation and other like QAM)

the receiving action in a radio box uses the etherodina concept:

to report the information the modulating signal at low frequency to extract the bit stream the received signal is multiplied for a local carrier nominally of the same frequency of the original carrier.

Then the signal is demodulated further if necessary.

So case 2) if the two radio stations use the same frequency are close and use comparable power they disturb each other.

in case 1)

if f1 and f2 are in different bands like 802.11 B 2.4 GHz and 802.11 A 5 Ghz they cannot disturb each other because the act of tuning the local carrier will put only the signals nearby the local carrier on the receiver

There is a low pass filter.

Important:

if you multiply two carriers the maths say that you are generating two signals at the following frequencies

f2+f1 and ABS(f2-f1)

the low pass filter allows only

ABS(f2-f1)

in the 2.4 GHz there are more sources of interference including microwaves hoves

So for VoIP over wireless is usually recommended to use 802.11A if possible.

Hope to help

Giuseppe

[sarahr202]

thanks Giuseppe!

"So case 2) if the two radio stations use the same frequency are close and use comparable power they disturb each other."

How about if two station use same frequency( carrier) but different modulation, will they distrub each other transmission ?

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

I would say yes but don't ask me a math explanation:

think of Asin(ft) function that is ideally the carrier in a period T=1/f the signal moves between +A and -A a phase shift can be seen as a variation in amplitude caused by the modulating signal (in AM modulation)

so when the receiver tries to retrieve the modulating the signal is distorted

analog transmission tries to reproduce the original signal at the other end.

Digital numeric communication associates a symbol (that can be a single binary digit or a sequence of binary digits) with a period Ts.

Each Ts the receiver actually calculates a value (you can see it for example as an integral operation over time).

if the exstimed value in Rx is different from the original symbol you have an error.

Errors can be:

detectable and correctable

only detectable

not detectable

Hope to help

Giuseppe

[Joseph W. Doherty · Accepted solution]

For a different explanation approach to your questions, using something we're all more familar with, think how speech (sound) works.

For your first question, consider two speakers, but on different frequencies. If a man and women are speaking at the same time, their speech will interfere with each other if listener can hear both. If one speaker is shouting, and the second is only whispering, a listener, that hears both, is more likely, but not guaranteed, to be able to understand the louder of the two speakers. If a listener can not hear both frequencies, e.g. human listener, human speaker and bat (sound echoing), more powerful sound source speaker shouldn't impact the other.

For your second question, consider two men or two women (i.e. speech on same frequency), but speaking different languages. There will be physical (sound) interference although the different languages (modulations) might, or might not, allow a listener to sucessfully understand a particular speaker.

As with sound, the best way to guarantee two stations can communicate, is communication isolation where receivers can not "hear" other sources.