Residual BER – error floor is the ultimate metrics for the end-to-end transmission performance, but in practice can be assessed only by out-of-service tests, which network operators in particular try to avoid best they can, preferring in-service practical test methods. In this regard, constellation analysis is an excellent tool to not only identify and visualize various error generating analog impairments such as power amplifier compression, carrier lock angle and quadrature errors, but also enable in-service estimation of BER with OFDM transmission over multipath channel. In this paper, after reviewing concepts of constellation analysis, we relate the crucial constellation analysis parameter – closure to the delay dispersion standard describing parameter - mean delay spread, so modeling the multipath delay dispersion with the equivalent additive white Gaussian noise source that would produce the same degradation of the accordingly modified residual BER expression. For the exemplar test situation in the LTE downlink environment, very good matching was found between the in-service constellation - based and the predicted error floor values.