This paper from 2011 provides an excellent treatment of the difference between discrete return airborne lidar and full waveform digital signal processing. Each has its strengths and weaknesses.
- Discrete return (DR) airborne lidar systems can record 4 returns
- Full waveform (FW) can record an unlimited number in theory
- Some systems provide both sensors
- DR is catching up to FW
Excerpts from the paper.
Lidar sensors currently used for 3D vegetation mapping and forestry research can be categorized as either discrete return (DR) or full waveform (FW) systems. They differ from one another mainly with respect to the range measurement method, which implies significant differences in sensor hardware design.