Co-channel Interference:

• Frequency reuse implies that in a given coverage area there are several calls that use the same set of frequencies.
• These cells are called co-channel cells and the interference between signals from these cells is called Co-channel interference.
• Co-channel Interference can‘t be combated by simply increasing the carrier power of transmitter. This is because an increase in carrier transmit power increases the interference to neighbouring Co-channel cells must be physically reported by a minimum distance to provide sufficient isolation due to propagation.
• When the size if each cell is approximately same and the base station transmit the same power, the co-channel interference ratio is independent of the transmitted power and becomes a function of the radius of the cell (R) and distance between centres of the nearest Co-channel Cells (D).
• By increasing the ratio of D/R. the spatial separation between Co-channel. Cells relative to the coverage distance of a cell is increased.
• Thus, interference is reduced from improved isolation of RF energy from the co-channel cell. The parameter Q called the Co-channel reuse ratio, is related to the cluster size .
• For hexagonal geometry Q = $\frac{D}{R}$ = $\sqrt{3N}$
• Small Q provides large capacity since cluster size N is small. Whereas large value of Q improves the transmission quality, due to smaller level of Co-channel interference.