This paper proposes a hybrid medium access protocol named orthogonal coded medium access control (OrMAC), which extends the principle of distributed queuing collision avoidance protocol (DQCA) of wireless local area network (WLAN) to delay-sensitive machine-to-machine (M2M) networks. OrMAC pre-assigns orthogonal codes, which serve as the channel contention signals, to the nodes entering the network. The "pre-assignment" eliminates contention collisions since it guarantees that no two nodes share the same contention code. Moreover, OrMAC employs a prioritized channel access by allowing nodes to control the transmission power of the contention signal depending on the delay sensitivity of the data. The power at which a contention signal arrives at the access point reflects the urgency of the packets waiting for transmission in the buffer. A contention signal with a high received power is assigned a high priority and vice versa for a contention signal with a low received power. Numerical experiments are carried out to compare the performance of OrMAC to that of DQCA in terms of the packet delivery ratio, latency, discarded packet ratio, and throughput. The results show that OrMAC can outperform DQCA in all the aforementioned performance metrics.
Keywords: hybrid; latency; machine-to-machine; medium access control; orthogonal code.