A preceding technique applied to symbols transmitted in an ultra-wideband (UWB) system was proposed in , which can concentrate the signal power at the receiver to facilitate data detection. The design of the optimal codeword for preceding is investigated in this work. After the problem is formulated, we examine its solution in the full-rank space as well as the reduced-rank subspace to get different tradeoff between detection performance and the amount of feedback messages. Furthermore, we propose two subspace selection schemes that are suitable for different channel conditions. The codeword obtained from this work improves the performance of the result in  significantly at the price of a larger amount of feedback messages.