Some wireless sensor networks (WSN), like intra-vehicular WSNs, battle field WSNs etc consist of a large number of wireless sensor nodes that are usually densely and randomly deployed for unattended operation. For these networks, the sensors are batterypowered, and hence an important design issue is the amount of the energy available at each node, requiring WSNs to have energy-efficient routing schemes and transmission algorithms. This thesis presents an energy-efficient routing approach that is based on a physical layer that uses cooperative transmission (CT). CT is the strategy wherein one user helps another user transmit multiple copies or versions of the same message through independently faded channels, to ultimately be received by a destination node [1, 2]. By sharing information this way, the users can create a “virtual array”[3] and achieve spatial array and diversity gain. Because of the diversity gain, all users can reduce their fade margins (ie, their transmit power) by as much as 12-15 dB, thereby reducing the energy consumed by each transmitter [3]. Because of the array gain (the simple summing of average powers from each antenna), the required transmission power for a link can be divided across multiple radios; this provides a convenient mechanism for applications in which each node has extreme transmit power constraints or heat restrictions.