A PHY and MAC protocol for MIMO concurrent transmissions, called Contrabass, is presented. Concurrent transmissions, also referred to as multi-user MIMO, are simultaneous transmissions by multiple interfering nodes over the same carrier frequency. Concurrent transmissions technique has the potential of mitigating the overhead of MAC protocols by amortizing protocol overhead among multiple packets. However, existing proposals for concurrent transmissions could not achieve this as MIMO channel training and collision avoidance typically involve an expensive process of coordination and control message exchanges. This overhead has made MIMO concurrent transmission Impractical and thus unused in real applications. Contrabass implements simultaneous channel training and optimal transmission control without any coordination. As a result, Contrabass achieves very high aggregate throughput, low delays and scalability even under dynamic environments and outperforms the existing MIMO protocols. T...