Carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. Doppler-RAKE receivers have been developed for collecting the resulting diversity gains only for spread-spectrum systems. Relying on a basis expansion model of time-selective channels, we find that the maximum achievable Doppler diversity is determined by the rank of the correlation matrix of the channel's expansion coefficients. We also prove that RAKE reception can not collect maximum diversity gains, unless the transmission is appropriately designed. Finally, we design such block precoded transmissions to ensure maximum diversity gains, and provide thorough simulations to corroborate our theoretical findings.