## 2 Related Work  
LEACH [2] and LMAC [7] are built on the concept of TDMA and allocate single independent transmission slots to each sensor node in a cluster. The TDMA-based schedule provides collision free slot allocations to sensor nodes, leading to reduced energy waste; however it is faced with a challenge of idle listening on nodes that do not have data to send during their allocated slots. ETDMA [8] extends the operation of TDMA to further reduce energy consumption due to idle listening, by turning off the radio modules of the sensor nodes that have no data to send during their assigned slots. Though energy is conserved by avoiding idle listening, the protocol incurs a lot of delay due to the allocated unutilized slots. BMA [9] is an intra-cluster communication protocol for large-scale cluster-based WSNs intended for event-driven applications. It reduces energy waste by avoiding idle listening by scheduling only nodes that have data to transmit. The protocol allocates a single transmission slot to each node that has data to transmit. The enhancement of the protocol, BMA-RR [10] provides variable data slots to sensor depending on their load. The scheduler is based only on the available packets on the sensor node, without considering the remnant energy of the sensor nodes, which could enhance the schedule performance. The data transmission of this protocol also does not feature cooperative transmission to address wireless channel reliability issues in WSNs.  
The cooperative communication research presented in [11] introduces both proactive and reactive protocols of cooperative communication; the protocols are both efficient, but due to the vast deployment of sensor nodes, there is high possibility of overhead incurred by the system due to the receipt of control signals from individual nodes by neighbors. Work done in [12] presents a threshold-based adaptive relay selection scheme, which uses a threshold to select the relays for cooperation. The drawback of this model is its use of multiple relays while sometimes only one relay might have enough channel gain to make a successful transmission, leading to unnecessary energy consumption due to transmission of multiple relay terminals. CPS-MAC [13] selects the relay nodes based on the buffer state and the channel state information (CSI) obtained through the exchange of control messages. The protocol however does not consider the remaining energy of the relay nodes, which can cause early network partitioning if relay nodes run out of energy. A close relation to our cooperative model is WcoopMAC [14], a cooperative MAC protocol adopting decode and forward technique for relaying of data to improve the energy performance of WSNs. The protocol selects the relay nodes based on the residual energy information and CSI, however it does not consider the relay node distance. The protocol is reactive, making an inefficient use of the channel as the direct link is always used regardless of the channel conditions, which leads excessive energy waste due to retransmissions.  
Although the effects of the techniques mentioned in this work and applied to the communication protocols have been independently investigated, not many protocols have applied cooperative diversity in clustered WSNs. This work makes the following contributions: first a cooperative MAC protocol featuring CQI, energy and distance for relay selection combined with cross-layer based scheduling is presented. Secondly, an analytical model for the energy consumption and delay for the proposed protocol is developed. This work differs from other works as it incorporates the effects of clustering and cooperative diversity in a scheduled MAC for WSNs, and does not examine the two techniques independently. This work also proposes a new scheduling algorithm at cluster level with the aim toimprove energy performance among nodes with varying loads within the cluster such that all nodes are given a fair chance of transmission.