This paper considers optimization of transmit power allocation in a two-Tier single input multiple output uplink network. The proposed system model is general enough to cover both emerging heterogeneous network (HetNet) architectures and cognitive radio (CR) networks. The first tier, which is assumed to have one user, represents a macrocell (or a primary user network) and the second tier represents a femtocell (or a secondary user network) in a HetNet (or a CR network). Second tier users share the same resources with the first tier user and they form a single input multiple output multiple access channel to their intended base station while causing interference to the first tier user. Near-optimum allocation of transmission powers maximizing the aggregate communication rate under individual transmission power constraints and a constraint on the total interference power at the first tier user is obtained by considering low-complexity signal decoders without interference cancelation at the second tier base station. It is shown that the power allocation is a modified version of the classical water-filling algorithm.