Abstract—A low-latency, parameter modulationestimation feedback protocol for wideband channels is introduced for both pure line-of-sight and more general fading channels with several degrees of freedom. One round of the protocol consists of a data phase and a control phase and uses non-coherent detection. The asymptotic optimality in energy efficiency of the protocol is analyzed and an upper bound on the distortion level is derived for two-rounds. The proposed scheme as well as known one-way schemes are compared with classical and very recent lower-bounds. Both the lower-bounds and performance evaluation of the feedback protocol are extended to a multi-channel fading model. The improvement of the feedback protocol over one-shot transmission is shown to be very significant on both line-of-sight and fading channels. 1