Enhancing throughput of star topology sensor network by group acknowledgement method and MCR SS-CSMA/CA

Sensor networks always face bottleneck problem at sink nodes where communication traffic concentrates. In order to achieve highly efficient communication in such networks, a multi-code reception spread-spectrum (MCR-SS-) CSMA/CA protocol has been proposed and shown to be effective to solve the uplink bottleneck when gathering information from sensor nodes. However, actual throughput for the network is limited not only by the airtime usage efficiency of uplink but also by that of downlink acknowledgement (ACK) transmission. This issue is remarkable when the data packet size is not much larger than that of ACK packet. This paper proposes a new practical group ACK method that can improve the network performance as a result of downlink throughput enhancement. Instead of sending individual ACK packets, a multiplexed group ACK packet is broadcasted to the data sender nodes that transmit data packets in the preceding communication frame. Network throughput and delay performance of the proposed method are evaluated by calculations and simulations based on the IEEE 802.15.4 PHY specification and the enhanced MCR-SS-CSMA/CA MAC. The results show that the proposed group ACK method exploits full potential of the MCR-SS-CSMA/CA and improves total network throughput significantly.