As seen in the propagation measurement, the signal at the receiver, caused by a transmitted sub-nanosecond pulse, can ring for a long time in an indoor environmnet, typically several hundred nanoseconds. In free space, we would expect the received waveform to look roughly like a few derivatives of the transmitted pulse.
The high-level view of multipath, either specular or diffuse, is that the received signal is composed of a sum of copies of the free-space equivalent signal, with appropriate delays and amplitude changes. We can estimate or attempt to measure that free space received signal and use it as the reference (template) signal in a correlator. When the correlator operates on a real multipath signal with a UWB template, it is possible to detect many "paths" and determine their associated amplitudes and delays.
While the multipath model for which motivates this measurement and template selection may be questioned, the signal strengths identified by this correlation process with a given template are real. These can be used to predict the performance of a simple binary pulse-position signaling scheme with the same transmitted pulse shape, running over the same measured channel, and using a correlation receiver with the same template.
Multiple correlators, collecting signals that arrive at resolvable time delays, can be used in a diversity-combining receiver (e.g., a selective Rake receiver) to improve performance over that achieved by a single correlation receiver. The curves below indicate the projected bit-error-rate performance of such a receiver as a function of the number L of correlation computations implemented (i.e., "paths" tracked). It is assumed that the receiver chooses the "path" delays and amplitudes to optimize performance.