Spectral efficiency in non-terrestrial heterogeneous networks with spectrum underlay access

Abstract

In this paper, the sum Spectral Efficiency (SE) maximization in a two-tier non-terrestrial heterogeneous network is investigated. This network consists of a High Altitude Aerial Base Station (HABS) and several Low Altitude Aerial Base Stations (LABSs) that share downlink spectrum with Orthogonal Frequency Division Multiple Access (OFDMA). The Aerial Base Stations (ABSs) are horizontally located based on spatial regularity maximization algorithm to maximize their coverage. Then, the weighted minimum distance criterion is selected for the users-ABS (UE-ABS) association. Due to the advantages of distributed optimization, problem formulation is performed in a distributed manner, indicating that the resource allocation is accomplished locally for each ABS. Two types of problems are formed for HABS and LABSs, respectively. The main objective of the problems is the sum SE under maximum power and inter-tier interference constraints. The defined problems are mixed continuous and combinational, where the tight upper bound solutions are derived by employing the Lagrangian Dual Decomposition (LDD) method. Finally, the effects of increasing the number of LABSs on achieving better SE performance in various cases for inter-tier interference and the effect of the weighting factor in UE-ABS association are evaluated