In a cellular system, the same frequencies are reused in geographically separated cells. However, radio signals propagate beyond cell boundaries. When two cells using the same frequency (co-channel cells) transmit simultaneously, their signals interfere with each other. This co-channel interference degrades call quality and must be managed through adequate geographical separation.
Co-channel interference is the interference between signals from co-channel cells — cells that use the same frequency set. The key metric is the Carrier-to-Interference ratio (C/I): the ratio of desired signal power to interference power. A high C/I means a clear call; a low C/I means a noisy, unusable call.
- Two geographically separated cells use the same frequency (co-channel cells)
- Signals from the distant co-channel cell travel beyond its boundaries
- The desired signal and the interfering signal arrive at a receiver in the overlap zone
- C/I = (desired signal strength) / (interfering signal strength)
- For acceptable quality, C/I must exceed a minimum threshold (typically 9 dB for GSM)
- To maintain C/I, the co-channel reuse ratio Q = D/R must be large enough (D = co-channel distance, R = cell radius)
- Q is increased by: larger cluster size N (more cells per cluster) or sectoring (directional antennas)
- C/I ratio: primary quality metric; must exceed minimum threshold
- Directly constrains frequency reuse — determines minimum co-channel reuse distance D
- Q = √(3N) where N is the cluster size; N=7 gives Q=4.58
- Non-co-channel (adjacent channel) interference: reduced by guard bands between adjacent frequencies
- Sectoring reduces interference by focusing antenna energy away from co-channel cells
- Co-channel interference is a fundamental limit on cellular capacity
- Related: Frequency Reuse — co-channel interference is what constrains frequency reuse planning
- Related: Cellular Mobile System — interference management is fundamental to cellular design
- Related: Sectoring — sectored antennas reduce co-channel interference
- Related: Cell Splitting — splitting cells can increase interference if not planned carefully
- Contrasts with: Adjacent Channel Interference — interference from nearby frequencies, reduced by guard bands
- In urban areas, building reflections can create unexpected interference paths
- C/I requirements vary with modulation scheme; higher-order modulation needs higher C/I
- Power control helps maintain C/I by adjusting mobile transmit power
- Soft handoff in CDMA provides macro-diversity, which also reduces co-channel interference