Co-channel and adjacent channel interference

Co-channel interference (CCI)

• Interference between signals from cells that use the same set of frequencies (co-channel cells) under frequency reuse.
• Cannot be removed by increasing transmitter power, because the interfering signal grows equally. It is reduced only by physically separating co-channel cells by a minimum distance D, i.e. a larger reuse ratio Q = D/R = √(3N), or by sectoring.
• Measured by S/I = (√(3N))ⁿ / i₀ for the first tier of i₀ interferers. AMPS needs S/I ≥ 18 dB, which gives N = 7 with n = 4.

Adjacent channel interference (ACI)

• Interference from signals at frequencies adjacent (next) to the wanted signal, caused by imperfect receiver filters letting nearby frequencies leak into the passband.
• Becomes serious through the near–far effect: a nearby transmitter on an adjacent channel overpowers a weak, distant wanted signal.
• Reduced by careful filtering and channel assignment – not assigning adjacent channels in the same cell, keeping a large frequency separation between channels in a cell (e.g. channels 1, 22, 43, … in AMPS), and power control.