  An interference management problem among multiple overlapped random access
networks (RANs) is investigated, each of which operates with slotted ALOHA
protocol. Assuming that access points and users have multiple antennas, a novel
opportunistic interference alignment~(OIA) is proposed to mitigate interference
among overlapped RANs. The proposed technique intelligently combines the
transmit beamforming technique at the physical layer and the opportunistic
packet transmission at the medium access control layer. The transmit
beamforming is based on interference alignment and the opportunistic packet
transmission is based on the generating interference of users to other RANs,
which can be regarded as a joint optimization of the physical layer and the
medium access control layer. It is shown that the proposed OIA protocol
significantly outperforms the conventional schemes such as multi-packet
reception and interference nulling.
