Degrees of Freedom of Cooperative MIMO in Cellular Networks

In cellular networks, relays and/or mobiles can cooperate to improve the overall system performance. For example, once the transmission from a base station with multiple antennas is received by multiple nodes (relays and/or mobiles), they can exchange information to enhance the quality of intended signals while suppressing interference. Such virtual multiple- input multiple-output (MIMO) transmission is a key ingredient in hierarchical cooperation by Ozgiir et al., which was shown to improve the throughput scaling of ad hoc networks greatly. In this paper, we analyze the achievable rate of such a cooperative MIMO between a base station with multiple antennas and a node group. An antenna array with a fixed area has a limited number of degrees of freedom. We use a realistic channel model that can capture the effect of geometry of antenna arrays on the number of degrees of freedom. Our achievable rate is limited by the product of the aperture of the antenna array in the base station and the angular spread, which is consistent with some known results on the limit of the degrees of antenna arrays.