  Grant-free non-coherent index-modulation (NC-IM) has been recently considered
as an efficient massive access scheme for enabling cost- and energy-limited
Internet-of-Things (IoT) devices that transmit small data packets. This paper
investigates the grant-free NC-IM scheme combined with orthogonal frequency
division multiplexing for applicant to unmanned aerial vehicle (UAV)-based
massive IoT access. Specifically, each device is assigned a unique
non-orthogonal signature sequence codebook. Each active device transmits one of
its signature sequences in the given time-frequency resources, by modulating
the information in the index of the transmitted signature sequence. For
small-scale multiple-input multiple-output (MIMO) deployed at the UAV-based
aerial base station (BS), by jointly exploiting the space-time-frequency domain
device activity, we propose a computationally efficient space-time-frequency
joint activity and blind information detection (JABID) algorithm with
significantly improved detection performance. Furthermore, for large-scale MIMO
deployed at the aerial BS, by leveraging the sparsity of the virtual
angular-domain channels, we propose an angular-domain based JABID algorithm for
improving the system performance with reduced access latency. In addition, for
the case of high mobility IoT devices and/or UAVs, we introduce a
time-frequency spread transmission (TFST) strategy for the proposed JABID
algorithms to combat doubly-selective fading channels. Finally, extensive
simulation results are illustrated to verify the superiority of the proposed
algorithms and the TFST strategy over known state-of-the-art algorithms.
