Discrete spectral peak estimation in incoherent backscatter heterodyne lidar. I. Spectral accumulation and the Cramer-Rao lower bound

The theory of the Cramer-Rao lower bound (CRLB) and maximum-likelihood (ML) estimators is summarized in the context of a heterodyne lidar. Numerical experiments are described that indicate the scaling of this CRLB with parameters such as the signal bandwidth and the level of noise. This CRLB is also compared with the CRLB of a highly idealized noiseless direct detection system using photon counting. It is found that the asymptotic bounds developed in the radar literature for the heterodyne CRLB should not be used as approximations for the correct expression in lidar applications at intermediate signal levels. Moreover, the variance of the ML estimator may be greater or even less than the heterodyne CRLB, depending on the mechanism leading to the departure from the bound.>