Abstract: To take full advantages of multiple antennas equipped by nodes in ad hoc networks,based on spatial multiplexing, an MAC protocol named multi-antenna dual backoff(MADB-MAC) is presented.Nodes can exchange hand-shake signals in contention slot of transmission frame to implement simultaneous transmission based on spatial multiplexing.Taking to a detailed consideration of detection error propagation in physical layer,a cross-layer analytical model is presented to analyze this protocol.Results of theoretical analysis and numerical computation show that MADB-MAC can improve network throughput remarkably.