MIMO mode adaptation in femtocellular systems

Hierarchical femtocell 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 macrocell system performance due to increased interference caused to macrocel-lular users. With MIMO precoding, the Least Interference (LI) technique can be employed at the femtocellular base stations (FBSs) to minimize the interference to the macrocellular user. This approach maximizes the macrocellular throughput, but also results in significant reduction in femtocellular throughput. As is well-known, a Precoding Matrix Index (PMI) approach to beamforming can maximize the signal power at a desired receiver, with minimal feedback. In this paper, we apply mode adaptation between LI and PMI at the FBSs in order to increase both the macrocellular and femtocellular throughputs. We show that allowing for mode adaptation at each FBS improves the system performance when compared with using the same mode across the system, and a simple binary choice at each FBS can nearly achieve the optimum mode-adaptation performance. Analysis and simulation results in a multicell environment are presented to illustrate the improvement in system performance with the proposed techniques.