Short note: Interference in mobile communication

Interference is the major factor limiting the capacity and quality of cellular systems. It causes cross-talk in voice calls, missed and dropped calls, and errors in data. Sources include other mobiles in the same cell, calls in neighbouring cells, other base stations on the same frequency, and non-cellular systems leaking into the band.

1. Co-channel interference (CCI)

• Caused by cells that use the same frequency set under frequency reuse.
• Cannot be reduced by increasing power; reduced by increasing the co-channel reuse ratio Q = D/R = √(3N).
• S/I = (√(3N))ⁿ / i₀ for i₀ first-tier interferers; e.g. N = 7, n = 4 gives about 18.7 dB.

2. Adjacent channel interference (ACI)

• Caused by signals on neighbouring frequencies leaking through imperfect receiver filters; worsened by the near–far effect.
• Reduced by good filtering, keeping channels in a cell widely separated in frequency, and power control.

3. Other types

• Intersymbol interference from multipath delay spread (digital systems).
• Multiple access interference in CDMA from other users' codes.
• Out-of-band and intermodulation interference from other radio systems.

Methods to reduce interference

• Proper cluster size and frequency planning
• Sectoring with directional antennas and down-tilt
• Power control (keep each mobile at the minimum needed power)
• Good filters and channel assignment
• Frequency hopping, equalisers and diversity

Interference-limited design trades capacity (small N) against quality (high S/I).