IEEE 802.15.4 protocol is intended to accomplish the characteristics of low power and low rate Wireless Sensor Network. It employs carrier sense multiple access with collision avoidance (CSMA/CA) protocol to access the network using either unslotted or slotted mode. This paper concentrates on the unslotted CSMA/CA where the node tends to wait for a very limited backoff exponent value, and cannot perform clear channel assessment until backoff procedure is completed. Therefore, the collision probability and the average delay are high.

To enhance the unslotted CSMA/CA performance, this paper proposes a modified unslotted CSMA/CA that divides the backoff delay into Main Backoff and Secondary Backoff. The Markov model for the modified CSMA/CA is presented to assess a single hop IEEE 802.15.4 behavior, and derive expressions for average delay, energy consumption and reliability. An optimization problem is also presented to minimize the delay while guaranteeing energy consumption and reliability constraints for the modified CSMA/CA. OPNET simulation tool is applied to compare the modified to the standard unslotted CSMA, and to validate the Markov model proposed. The results demonstrate that the modified CSMA improves the reliability, while reducing the average delay significantly.

IEEE 802.15.4协议旨在实现低功耗和低速率无线传感器网络的特性。它采用具有冲突避免功能的载波侦听多路访问（CSMA / CA）协议，以无时隙或时隙模式访问网络。本文着重于未分配时隙的CSMA / CA，其中节点倾向于等待非常有限的退避指数值，并且在退避过程完成之前无法执行清晰的信道评估。因此，碰撞概率和平均延迟高。

为了增强无时隙CSMA / CA的性能，本文提出了一种改进的无时隙CSMA / CA，它将退避延迟分为主退避和辅助退避。提出了用于修改后的CSMA / CA的马尔可夫模型，以评估单跳IEEE 802.15.4行为，并得出平均延迟，能耗和可靠性的表达式。还提出了一个优化问题，以最大程度地减少延迟，同时保证修改后的CSMA / CA的能耗和可靠性约束。应用OPNET仿真工具将修改后的内容与标准无槽CSMA进行比较，并验证提出的Markov模型。结果表明，改进后的CSMA提高了可靠性，同时显着降低了平均延迟。