Interference occurs when signals from one wireless channel leak into another, degrading performance. Adjacent channel interference specifically refers to interference between neighboring frequency bands. It results from imperfect receiver filters allowing nearby frequencies to blend together. This causes distorted transmissions, dropped calls, and reduced throughput. Interference can be minimized by carefully assigning non-adjacent channels to different cells and using adequate filtering to separate signals.

2. Key Points of discussion:
 What is Interference and it’s types
 Adjacent-Channel Interference (ACI)
 A Simple Example
 Sources of Interference
 Effects of Interference
 Causes of Adjacent Channel Interference
 Avoidance procedure

3. What is interference?
Interference is the major limiting factor when
evaluating the performance of cellular radio
systems.
Types of Interference :
Co-channel interference
Adjacent channel interference

4. Adjacent - Channel Interference (ACI) :
Adjacent channel interference (ACI) is interference between links
that communicate geographically close to each other using
neighbouring frequency bands. Adjacent channel interference
results also from imperfect receiver filters
This type of interference occurs when the information on the
adjacent channel seeps into pass band of the channel being
transmitted, due to which performance of the main channel is
degraded.

5. Adjacent - Channel Interference (ACI) :
P Adjacent - Channel Interference
f
Channel 1 Channel 2

6. Adjacent - Channel Interference (ACI) :
Channel 1 is adjacent to channel 2, which is adjacent to channel 3,
and so on. These adjacent channels overlap each other because
each channel is 22 MHz wide and their centre frequencies are only 5
MHz apart. Adjacent channel interference happens when two or
more access points using overlapping channels are located near
enough to each other that their coverage cells physically overlap.
Adjacent channel interference can severely degrade throughput in a
wireless LAN.

7. A Simple Example:
Adjacent channel Interference is the worst type of Wi-Fi interference.
To illustrate, Let’s think about being at a concert, where a band
playing really loud and Thousands of people are listening to it. ,
Each person are with their own group of friends. With this much
going on, it's difficult to talk to your friends, and when you start to
talk louder, the person next to you has to raise his/her voice to talk
to their group. You're hearing multiple conversations happening, as
well as music from the band, and it seems impossible to
communicate. This is exactly what happens to wireless devices
trying to communicate in a Interference environment.

8. A Simple Example: (Cont.)
The diagram below shows a model of how the above conversation
scenario looks when access points on channel 4 (red), channel 6
(green) and channel 5 (blue) are all active at once. As one of these
APs tries to talk to its clients, its transmissions become distorted
because of the transmissions of the other two. This harms the
performance of all of the networks.

9. Sources of Interference :
 Another mobile in the same cell
 A call in progress in the neighbouring cell
 Other BS’s operating in the same frequency band
 Any non-cellular system which inadvertently leaks energy into
the cellular frequency band.
 Interference on voice channels causes cross talk due to an
undesired transmission.

10. Causes of Adjacent Channel Interference:
 Poor frequency control. It occurs when one or both of the adjacent
channels are broadcast with too much or too little power behind
them.
 Inadequate filtering. It occurs when the receiving channel does not
have the proper modulation to filter out the interfering signal.

11. Effects of Interference:
 Interference has been recognized as a major bottleneck when
looking for increasing capacity and is often responsible for dropped
calls.
 On control channels it leads missed and blocked calls due to errors
in the digital signalling.
 Interfering signals are often generated within the cellular system,
they are difficult to control in practice (due to random propagation
effects).

12. Effects of Interference: (Cont.)
 On voice channel causes crosstalk.
 Even more difficult to control this interference caused out-of-band
users, which arises without warning due to front end overload of
subscriber equipment or intermittent inter-modulation products.

13. How It Happens:
 Interference resulting from signals which are adjacent in frequency
is called as adjacent channel interference.
 It results from imperfect receiver filters which allow nearby
frequencies to leak into the pass band.
 This issue is very serious if an adjacent channel user is transmitting
in very close range to subscriber's receiver while the receiver
attempts to receive a base station on the desired channel.
 This is referred to as near far effect.

14. How It Happens: (Cont.)
 Near far effect also occurs in a case when a mobile close to a base
station transmits on a channel close to one used by a weak mobile.
 The base station may have difficulty in discriminating the desired
mobile user from the “bleedover” caused by the close adjacent
channel mobile.
 This can be minimized through careful filtering and channel
assignments

15. How It Happens: (Cont.)
 By keeping the frequency separation between each channel in a
given cell as large as possible, this interference can be reduced
considerably.
 By sequentially assigning successive channels in the frequency
band to different sells, many channel allocation schemes also
prevent a secondary source of adjacent channel interference by
avoiding the use of adjacent channels in neighbouring cell sites.

16. Avoidance procedure:
 Broadcast regulators frequently manage the broadcast spectrum in
order to minimize adjacent-channel interference.
For example, in North America, FM radio stations in a single region
cannot be licensed on adjacent frequencies — that is, if a station is
licensed on 99.5 MHz in a city, the first-adjacent frequencies of
99.3 MHz and 99.7 MHz cannot be used anywhere within a certain
distance of that station's transmitter, and the second-adjacent
frequencies of 99.1 MHz and 99.9 MHz are restricted to specialized
usages such as low-power stations. Similar restrictions formerly
applied to third-adjacent frequencies as well (i.e. 98.9 MHz and
100.1 MHz in the example above), but these are no longer
observed.

17. Avoidance procedure: (Cont.)
 Adjacent channel interference can be minimized through
careful filtering and channel assignments.
 By keeping the frequency separation between each channel
in a given cell as large as possible, the adjacent interference
may be reduced considerably.

18. Thank You
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