Aligning Guard Zones of Massive MIMO in Cognitive Femtocell Networks

Femtocells may coexist with macrocells in the same band and significantly improve the spatial reuse (SR) of spectrum. To avoid high interference to the scheduled macrocell users (MUs), femtocell base stations (BSs) sense and access the channel opportunistically. When massive MIMO is deployed at macrocell BS, we find that this cognitive behavior forms too many guard zones scattered over macrocell´s coverage area and seriously degrades spatial reuse efficiency. In this work, we consider not only the inter-user channel orthogonality, but also geographic locations for massive MIMO user scheduling in cognitive two-tier networks. We propose to choose MUs not far from each other as receivers of the macrocell´s downlink spatial multiplexing streams, such that guard zones can be aligned spatially and more SR opportunities can be provided to femtocells. Simulations show that our scheme can significantly improve femto-tier throughput while only slightly sacrificing macro-tier throughput.