The co-channel interference (CCI) is the principal factor of signal degradation in cellular networks, even more when a better frequency planning is required, because it results in a high-power CCI. On the other hand, the cooperative communication allows to create a virtual antenna array in order to obtain a diversity gain and combat the fading effects. In the present paper, the impact of a high-power CCI in a decode-and-forward (DF) wireless cooperative network using optimum combining (OC), K relays and N antennas at destination is analysed. Analytical expressions of average bit-error rate (BER) and upper bound for BPSK modulation are derived. It is proved that only one link is used to eliminate the high-power CCI, obtaining a diversity gain of N(K+1)-1.