A radio telecommunications receiver operates to receive digital data symbols or bits by iterative determination of soft estimates of symbols or bits followed by a hard decision as to what symbol or bit was intended. In this way, a modulation condition is set according to a condition of a target to be measured when multi-path interference light caused by double Rayleigh scattering light or reflection at the end of a connector is measured with a pulse-OSA method, so that measurement accuracy is improved. The brackets may have more than one support face. Therefore, maintenance and repair costs of the apparatus can be reduced, thereby enhancing operation efficiency. If the data are not matched, an error flag is generated in the comparator, and it is used as an interrupt signal, thereby the CPU recognizes the error.