In this paper we address the problem of the design of a transmitter in a Multi-Input-Multi-Output (MIMO) space diversity system. We consider a design which is robust to errors in the channel estimate available at the transmitter. The design is based on the minimization of the necessary transmit power while guaranteeing a minimum and specified Quality of Service (QoS) in terms of Signal to Noise Ratio (SNR) with a certain probability. According to this objective, the transmitter exploits all the eigenmodes of the MIMO channel estimate, distributing the transmit power in a robust way among the channel modes by using orthonormal temporal signatures. In order to guarantee a full rate transmission, the orthonormal temporal signatures designed for real constellations in Orthogonal Space Time Block Codes (OSTBC) are used, leading to a system structure based on the combination of OSTBC and orthogonal weighted beamforming.