Namal Rajatheva Elvino S. Sousa Gram-Schmidt precoding for two-tier cellular networks with massive MIMO. 1-6 2016 WCNC https://doi.org/10.1109/WCNC.2016.7564949 conf/wcnc/2016 db/conf/wcnc/wcnc2016.html#RajathevaS16