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Highly dispersive nature of ultra-wideband (UWB) channels makes time of arrival (TOA) estimation extremely challenging, where the leading-edge path is not necessarily the strongest path. Since the bandwidth of a received UWB signal is very large, the Nyquist rate sampling becomes impractical, hence motivating lower complexity and yet accurate ranging techniques at feasible sampling rates. In this paper, we consider TOA estimation based on symbol rate samples that are obtained after a square-law device. A normalized threshold selection approach based on the minimum and maximum values of the energy samples is introduced, and optimal values of the thresholds for different signal to noise ratios (SNRs) are investigated via simulations. Also, the effect of window size when searching the leading edge prior to maximum energy block is analyzed. Theoretical closed form expressions are derived for mean absolute TOA estimation error for a fixed threshold case. Performances of different algorithms are then compared via simulations using IEEE 802.15.4a channel models and by choosing appropriate normalized threshold and search-back window parameters.