Adjacent Channel interference (ACI)
Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak into the pass band. This type of interference occurs not because of mobile phones or cellular towers in different coâ€channel cells transmitting signals at the same frequency, but it occurs because of mobile phones in the same cell transmitting signals atdifferent but close (adjacent) frequencies that the cellular tower has difficulty in filtering out thedifferent channels from each other.
Adjacent Channel interference
Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference
Near And Far Effect:
- The problem can be particularly serious if an adjacent channel user is transmitting in very close range to a subscriber's receiver, while the receiver attempts to receive a base station on the desired channel.
- This is referred to as the near-far effect,where a nearby transmitter the receiver of the subscriberas shown in Fig.5.4
- If the frequency reuse factor is small, the separation between adjacent channels may not be sufficient to keep the adjacent channel interference level within tolerable limits
Figure 5.4 Near And Far Effect
Why ACI is a big problem?
There is two issues related to ACI which makes it a big problem
- Desensitization(also called Blocking) due to compressive nature of nonlinear elements .one large interferer together with a very small desired signal causes the small signal gain to go to zero, and therefore Blocks the desired small signal since it doesn’t experience any gain.
- Intermodulation Caused by 3rd order linearity in the Rx components .This is shown in above fig 5,where M2 and M3 cause intermodulation distortion which effects reception of M1
How the ACI can be reduced?
- 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 channel interference may be reduced considerably
- A channel separation greater than six is needed to bring the adjacent channel interference to an acceptable level, or tighter base station filters are needed when close-in and distant users share the same cell
- In practice, each base station receiver is proceeded by a high Q cavity filter in order to reject adjacent channel interference