Redes cell-free massive mimo auxiliadas por noma: estudo da não-reciprocidade e da coexistência com oma

Abstract

According to Ericsson, 5G mobile access technology is expected to reach 6.3 billion subscriptions by 2030, accounting for 80% of total mobile traffic. This growth is driven by the increasing demand for higher data traffic capacity, projected to meet the requirements of the next generation of mobile networks. In this context, system capacity can be increased either by using higher carrier frequencies or by employing the available bandwidth more efficiently through techniques such as cell-free mas sive multiple-input multiple-output (MIMO) and non-orthogonal multiple access (NOMA). Cell-free massive MIMO employs geographically distributed antennas to serve users in a joint and coordinated manner, eliminating the concept of cells and improving spectral efficiency (SE). Operating under time-division duplexing (TDD), it requires orthogonal pilot sequences for channel estimation to avoid interference. However, in practical scenarios, the number of users often exceeds the available pilot sequences, resulting in a phenomenon known as pilot contamination, which degrades system performance. NOMA can help mitigate this issue by allowing multiple users to share the same resources. In this context, this thesis focuses on two main lines of investigation aimed at contributing to the advancement of 6G and future wireless networks. The first examines effective channel non-reciprocities caused by hardware mismatches in cell-free massive MIMO systems operating with NOMA, which lead to channels being perceived as non-reciprocal. Accordingly, phase and amplitude non reciprocities at both the transmitter and the receiver are evaluated under slow-fading channel conditions. By employing maximum ratio transmission (MRT) precoders normalized by channel statistics for both single-antenna and multi-antenna access points (APs), the study reveals how practical non-reciprocities affect the perfor mance of cell-free massive MIMO–NOMA systems.The second line of investigation introduces an adaptive hybrid method that combines orthogonal multiple access (OMA) and NOMA in cell-free massive MIMO systems, enabling mode switching between uplink channel estimation and downlink transmission phases. This hybrid and simultaneous access approach mitigates pilot contamination, optimizes resource allocation, and balances the number of served users with acceptable achievable data rates. A real-time decision policy is proposed to alternate between OMA and NOMA based on the number of users and the number of available pilot sequences.Overall, this thesis contributes closed-form expressions for lower bounds on the signal-to interference-plus-noise ratio (SINR) and spectral efficiency, highlighting that phase non-reciprocity at the AP significantly impacts system performance. An effective adaptive hybrid model is developed to improve resource allocation and sum-rate performance, providing practical implementation guidelines for future applications in cell-free massive MIMO systems.