# Two signals with the same frequency in the same area can interfere?

If one signal is transmitting form transmitter $TX_1$ with some frequency $X$ and some other transmitter $TX_2$ is sending some other signal with same frequency $X$ will there be any clash or interference between these two signals since these two transmitters $TX_1$ and $TX_2$ are located in the same area?

I would say yes almost certainly - it would depend on transmit power but if they are simultaneously broadcasting on the same frequency then there will be interference as they are in the same area. This is the reason valuable parts of the spectrum must be licensed by government agencies, to prevent multiple people using them at the same time without multiplexing etc.

• My pleasure :-) – PhilPotter1987 Jan 11 '16 at 11:00
• Hi @Philplotter i need one more clarification,suppose 4 persons are there in same area, if p1 person want to call person p2, and p3 person want to call person p4 will they be using same frequency to establish the calls? – rajee Jan 12 '16 at 8:01
• It depends on how they 'call' each other. I am no expert on exactly what techniques various telephony technologies use, but I can say that if all phones were transmitting on exactly the same frequency at exactly the same time, there would be problems. The way cell phones/mobiles get around this for example, is to each use different frequencies in the same general 'band' in order to talk to the local cell tower. This is how you can have four mobiles all working in the same room at the same time, on the same provider. – PhilPotter1987 Jan 12 '16 at 8:43
• Hi @PhilPotter Thanks for the explanation it helpful me a lot – rajee Jan 12 '16 at 16:29

The signals won't necessarily interfere with each other. It depends on the properties of the signals and what you're trying to achieve.

• Time-division multiplexing can be used to transmit multiple signals over the same frequency. In TDM each transmitter uses the channel for just a fraction of the time.

• Quadrature modulation essentially means transmitting two signals over the same frequency at the same time.

• Code-division multiple access allows multiple transmitters to share the same frequency; each transmitter multiplies its signal with a sequence that is orthogonal to those used by the remaining transmitters.

• Space-time coding: by using multiple antennas in the receiver, it is possible to recover signals transmitted over the same frequency and at the same time, while also fighting the effects of multipath fading.

Much recent research on efficient spectrum utilization is based on ways to "pack" signals both in time and frequency, to achieve the highest data rate over the smallest possible bandwidth.

Note also that, unless the transmitter, receiver and signaling are designed with this purpose, the signals will interfere as @PhilPotter1987 suggests. For example, if a TV broadcaster experiences a malfunction and starts transmitting over another TV channel's bandwidth, there is nothing your TV set can do to fix it.

• Hi @MBaz Thanks for the explanation it helpful me a lot and also can u please suggest me any book that give the detail explanation that tells about space time coding,code division multiple access,QAM techniques – rajee Jan 12 '16 at 16:32
• @rajee, Almost any book on digital and wireless systems will help you. In particular, Haykin's books on analog and digital communications are beginner-friendly. The books by Sklar and Couch are good introductions too. – MBaz Jan 12 '16 at 18:13
• why i was asking bcoz i have read some books which they explained QAM,DPSK etc by formulas i understand the formulas but i am facing difficulty in relating that to the Real world applications.Any way thanks u for Ur suggestion i will try these books .THAnk u so much – rajee Jan 13 '16 at 6:27
• @rajee, in that case, a couple more suggestions: Telecommunications Breakdown, a pre-print of which is linked at the bottom of this page: sethares.engr.wisc.edu/telebreak.html. There is a second edition of this book which is very good too. Also, RF and Baseband Techniques for Software Defined Radio, by Peter Kenington, Artech House. Also, Communication System Design Using DSP Algorithms, by S. Tretter, Springer. These books are more practical -- you still need to understand the equations, though :) – MBaz Jan 13 '16 at 15:23

If the two signals are just sine waves generated from point sources, with no modulation scheme, then yes they will interfere with each other.

To visualize this, suppose there are two such sources, one at $(10,10)$ and the other at $(90,90)$. The images below show 1) The waves just from the first source, 2) the waves from the second source, and 3) the overall effect when both sources are active.

As you can see, there is quite a bit of interference. R Code Below

#28211

x1 <- 10
y1 <- 10

x2 <- 90
y2 <- 90

z1 <- matrix(0,100,100)
z2 <- z1
ztot <- z1

for (x in 1:100)
{
for (y in 1:100)
{
z1[x,y] <- sin(sqrt(((x-x1)^2 + (y-y1)^2)))
z2[x,y] <- sin(sqrt(((x-x2)^2 + (y-y2)^2)))
ztot[x,y] <- z1[x,y] + z2[x,y]
}
}

par(pty="s", mfrow=c(3,1))
image(1:100,1:100,z1)
points(x1,y1,lwd=10,col="green")
points(x2,y2,lwd=10,col="blue")
image(1:100,1:100,z2)
points(x1,y1,lwd=10,col="green")
points(x2,y2,lwd=10,col="blue")
image(1:100,1:100,ztot)
points(x1,y1,lwd=10,col="green")
points(x2,y2,lwd=10,col="blue")

• Hi @Peter K Thanks for the explanation it helpful me a lot – rajee Jan 12 '16 at 16:33