Unmanned aerial vehicle (UAV)-aided communications have emerged as a promising paradigm for providing capacity enhancement and coverage expansion in terrestrial mobile communication systems. However, due to the infancy in research and the stringent requirements of the state-of-the-art, spectral and energy-efficiency (EE) issues remain a prime challenge. Recently, cell-free massive multiple-input multiple-output (CF-mMIMO), consisting of arbitrarily allocated access points (APs) linked to a central processing unit (CPU), has been in the limelight, particularly because of its distinctive benefits toward achieving optimal EE and spectral gains. This chapter provides a comprehensive discussion on the feasibility as well as the effectiveness of CF-mMIMO in providing an improved spectral and energy-efficient UAV cellular network. First, an exposition into the evolution from the classical cell-based structure to the novel CF structure is presented. Next, elaborate discussions on the system model, propagation model, channel estimation, and communication process of UAV-aided CF-mMIMO networks are provided. Further, novel expressions for the achievable lower and upper spectral efficiency (SE) bounds for UL and DL data transmission phases are derived. Finally, analytical and numerical results derived through intensive computer simulations and open research issues are presented to inform future research directions toward achieving reliable UAV communications.


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