**6. Summary**

In this chapter, we have introduced the OBF/RBF and the current developments in the literature. First of all, we have given an overview for the single-cell case, summarizing some of the most important contributions so far. Furthermore, we have reviewed the recent investigations on the rate performance of multi-cell RBF systems in both finite- and high-SNR regimes. These results are useful for the optimal design of multi-cell RBF in practical cellular systems. In particular, it is revealed that collaboration among the BSs in assigning their respective numbers of data beams based on different per-cell user densities is essential to achieve the optimal throughput tradeoffs among different cells. Moreover, the results show that spatial receive diversity is also a significant factor to be considered, noting that there exists, however, a tradeoff between the rate/DoF performance and the complexity/delay time of RBF systems with different receivers. The preference of multi-cell RBF is justified by the scheme's optimality albeit requiring only partial CSI at transmitters as compared to other full-CSI transmission schemes, when the numbers of users in all cells are sufficiently large.

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