CRB-based Performance Analysis of Semi-Blind Channel Estimation for Massive MIMO-OFDM Systems with Pilot Contamination

Channel estimation, which is a key task in massive multiple-input multiple-output orthogonal-frequency division-multiplexing systems (MIMO-OFDM), is severely affected by the problem of pilot contamination during the uplink transmission. Thus, the aim of this study is to investigate, via the Cramér-Rao Bound tool, the effectiveness of semi-blind (SB) methods for pilot contamination mitigation. For synchronous cells, these analyses demonstrate the possibility to efficiently solve the pilot contamination problem, with SB approaches, when considering a finite-alphabet (non-Gaussian) communications signal. However, considering only the signal's Second Order Statistics is not enough for solving such an issue even if the SB approach is adopted. Moreover, the analyses show that it is possible to get close to the optimal performance with a SB approach even if the pilots are non-orthogonal as long as they are not fully coherent. For the asynchronous cells case, it has been demonstrated that the pilot contamination still occurs under small inter-cell delays, but can be strongly mitigated with large inter-cell delays.