This article develops a new Mac layer scheme (WSC-MAC) for Wireless Sensor Network (WSN) wish improves the network reliability by using cooperative communication. Our work is focused on a way to define a relay node among the neighborhood of a node, efficiently and with only few signaling messages. We develop a solution based on an automatic forwarder selection and a link state evaluation in order to define the relay node. This automatic selection uses a group identifier uniformly spread in the network and ensures that only few nodes at the time will be chosen as relay. As the sensor nodes switch from active mode to the sleep one, we based our solution on the Long Preamble Emulation (LPE) Mac layer Algorithm which emulates the asynchronous MAC protocol proposed in BMAC. Our simulations results show that WSC-MAC increases the overall reliability of the sensor network and adjusts to large variety of node density