We consider a multipacket reception channel shared by several communication applications. This is the case, for example, in a single radio mesh network where neighboring cells use the same radio channel. In such scenarios, unlike the common multiple access model, several transmissions may succeed simultaneously, depending on the actual locations of the sending and receiving stations, and thus channel utilization may be greater than 1. Our goal is to derive a decentralized access control mechanism that maximizes the channel utilization, while taking into account fairness among the different users. We focus on a simple case where each user can adjust a single parameter that determines its transmission probability in any time slot, and develop such a protocol for the general problem, where users are distributed arbitrarily, based on strong motivation which is derived from analytical bounds for homogeneous interferences. We further show, using extensive simulations, that this protocol achieves a high utilization of radio resources compared to any other protocol (not necessarily based on a simple parameter), while maintaining fairness between all users.