Age of Process Information of Mobile Edge Computing Assisted IoT Status Update System Based on Layered Non-Orthogonal Multiple Access and HARQ

This work focuses on a mobile edge computing (MEC) assisted IoT status update network with multi-objective sensing, which consists of a wireless sensing network, MEC network, and an information receiver (IR). To simultaneously guarantee information freshness and system throughput, a layer-superposed non-orthogonal multiple access (NOMA) HARQ (LS-NOMA-HARQ) scheme is proposed. In the proposed LS-NOMA-HARQ scheme, an entire status update delivery circle consists of multiple rounds for NOMA symbol feedforward transmission. In each round, the source constructs and transmits a NOMA symbol to the AP that first performs feedforward decoding (FD). Each NOMA symbol includes the newly generated packet, termed the primary packet, and the part of the currently failed packet, termed the secondary one. If the received primary packet can not be correctly recovered by the AP, the NOMA symbol is offloaded to the edge server and stored in the buffer of the edge server. The source continuously generates and sends new NOMA symbols to AP until a successful FD occurs. On the contrary, the decoded result is directly delivered to IR, and backtrack decoding (BD) is triggered at the edge server. Then, the edge server successively decodes the previously stored NOMA symbols by using sophisticated successive interference cancellation (SIC), and delivers the recovered packet to IR. Once SIC-based BD fails, it is declared that a circle of LS-NOMA-HARQ status update delivery completes. Because the primary and secondary packets are independently modulated and superposed in the MAC layer, the proposed LS-NOMA-HARQ outperforms the layer-coded HARQ scheme that is executed in the physical layer. Moreover, this work also considers the two cases of finite and infinite buffer size at the edge server, and truncated HARQ is used for the retransmission of a single NOMA symbol. Under the finite buffer size case, the circle-shift preemption is used at the buffer edge server. The edge service follows exponentially distributed processes due to the user schedule and can be interrupted by one In-Out process due to the energy computation at the edge server, which results in a huge data processing delay at the edge server. To characterize this specific issue, this work investigates the age of process information (AoPI). In addition, considering the joint impact of both FD and BD, two modified AoPI metrics, that is, FD-based AoPI (FD-AoPI) and BD based AoPI (BD-AoPI), are proposed. The FD-AoPI is defined as the elapsed time since the generation of the last successfully feedforward decoded update, but the BD-AoPI is based on the classical definition of AoI. While the FD-AoPI simultaneously captures both throughput and information freshness, the BD-AoPI results in a loss in information timeliness. With the statistical characterization of related statistics, the closed-form expressions of both FD-AoPI and BD-AoPI are derived. The simulated and numerical results give insight into the impact of system parameters on both AoPI and throughput.