MIMO Precoding and Mode Adaptation in Femtocellular Systems
Hierarchical femtocellular architectures have become popular recently because of their potential to provide increased coverage and capacity in cellular systems. However, introduction of femtocells might reduce the overlay macrocellular system performance due to increased interference caused to macrocellular users. In this paper, two MIMO precoding techniques are considered at the Femtocellular Base Stations (FBSs) to control the interference to the macrocellular users: Precoding Matrix Index (PMI), and Least Interference (LI). With MIMO precoding, the limited CSI at the transmitter is the index of the precoder chosen from the codebook fed back by the receiver. The LI technique can be employed at the FBSs to maximize the macrocellular throughput, but it also results in significant reduction in femtocellular throughput.