Age of Error Information and Throughput Tradeoff for IoT Status Update Systems With Truncated and Layer-Coded HARQ

Abstract

A novel status update system is designed by combining adaptive modulation and coding, and truncated and layer-coded hybrid automatic repeat request (T-L-HARQ). With the channel state information (CSI) at transmitter (CSIT), which is obtained from the receiver by using pilot-assisted estimation, the source adaptively selects its transmission rate. Due to the delay, the CSI at receiver (CSIR) is the outdated version of the CSIT, and the decoding error is inevitable. Firstly, since the backtrack decoding used in the T-L-HARQ, the classical age of information is insufficient to quantify the status update freshness, we propose the age of error information (AoEI), which is defined as the elapsed time since the generation of the lastly successfully backtrack-decoded status update. Secondly, we characterize the interdeparture time duration of two consecutively successfully feedforward transmissions and the backtrack decoding depth of a successful T-L-HARQ transmission cycle. As a result, the closed-form expression of average AoEI is derived. The simulations show that a tradeoff design between throughput and AoEI is essential by controlling backtrack decoding depth.