— We consider the application of spatial multiplexing to ad hoc networks where nodes have multiple antennas. At the physical level, we suppose that layered space–time multiuser detection (LAST–MUD) is applied to separate multiple streams arriving at the receiver simultaneously. Our contributions here consist first in the reproduction of the multiuser detection process performance by a simple analysis, where we also specify when the analytical results are expected to be accurate and why. Second, we use this approximation to perform a cross–layer design of a MIMO ad hoc network where physical layer and Medium Access Control strategies are integrated to maximize the network throughput. We finally corroborate our conclusions, by comparing analysis with simulation results both at both the link and the network level.