  In this work, we study optimal transmit strategies for minimizing the
positioning error bound in a line-of-sight scenario, under different levels of
prior knowledge of the channel parameters. For the case of perfect prior
knowledge, we prove that two beams are optimal, and determine their beam
directions and optimal power allocation. For the imperfect prior knowledge
case, we compute the optimal power allocation among the beams of a codebook for
two different robustness-related objectives, namely average or maximum squared
position error bound minimization. Our numerical results show that our
low-complexity approach can outperform existing methods that entail higher
signaling and computational overhead.
