Performance Evaluation of MU-MIMO Systems with Multi-Antenna Users for Different Precoding Strategies

Radio resource management of the downlink of multi-user multiple-input multiple-output systems with multi-antenna users is considered to evaluate the performance of three zero-forcing precoding strategies: 1) Block Diagonalization (BD) where all streams are used for each selected user; 2) Coordinated-Transmit-Receive-1 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{CTR}_{\mathbf{1}})$</tex> where only the strongest stream is used for each scheduled user; 3) Coordinated-Transmit-Receive-Flexible <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{CTR}_{\mathbf{F}})$</tex> that allows a flexible stream allocation per selected user. Although the more complex <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{CTR}_{\mathbf{F}}$</tex> has the potential for better performance due to its flexibility, it might be difficult to implement it in practice. Hence, it is crucial to comprehend when the simpler CTR<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> or BD can be used instead of CT <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{R}_{\mathbf{F}}$</tex>. Our analysis compares the precoding strategies within <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{3GPP}$</tex> -compliant scenarios using realistic modulation and coding schemes in systems featuring large numbers of users and Base-Station (BS) antennas. We compare the performance of these precoding strategies under Sum-Rate (SR) maximization and Proportional Fairness (PF) and show that previous research conclusions carried out for SR maximization only hold for a small number of BS antennas. Indeed, for SR maximization, BD matches the performance of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{CTR}_{\mathbf{F}}$</tex> when BS antenna arrays are sufficiently large. For PF, the results are more nuanced, depending significantly on system parameters.