Massive MIMO and inter-tier interference coordination

We study the performance of a two-tier system where a large number of small cells is deployed under a macro-cellular “umbrella”. The macro-cellular tier provides coverage and handles mobile users, while the small cell tier provides high rate locally to nomadic users. While the standard approach consists of operating the two tiers in different frequency bands, for various reasons (e.g., lack of licensed spectrum), it may be useful to operate both tiers in the whole available spectrum. Hence, we consider schemes for inter-tier interference coordination that do not assume any explicit data or channel state information sharing between tiers. In particular, we consider co-channel TDD and reverse TDD schemes, when the macro (tier-1) base station has a very large number of antennas and the tier-2 base stations have a moderately large number of antennas. We show that by exploiting the spatial directionality of the channel vectors, very efficient inter-tier interference management can be obtained with relatively low complexity. Our approach consists of a sort of “spatial blanking” of certain angle-of-departure of the tier-1 base station at given scheduled time-frequency slots, in order to create transmission opportunities for the corresponding tier-2 small cells. In particular, such “spatial blanking” is significantly more efficient than isotropic slot blanking (enhanced Inter-Cell Interference Coordination, eICIC) currently proposed in LTE standardization.