Frequency-time selective fading degrades the performance of communication systems, but it also provides an opportunity to collect multipath diversity and Doppler diversity. In this paper, an oversampled grouped-linear-constellation-precoding (GLCP) scheme is proposed for orthogonal frequency division multiplexing (OFDM) systems to extract the diversity inherent in doubly selective fading channels. In the proposed scheme, GLCP is employed to precode the information symbols of OFDM at the transmitter, and the precoded OFDM symbols are oversampled in the time domain at the receiver. A full-groups maximum likelihood (ML) decoder and a successive group decoder to decode the oversampled OFDM symbols with diversity gains are developed. The successive group decoder combines the group ML decoding algorithm with a block-decision-feedback-equalization (BDFE) algorithm. The proposed scheme can take advantage of both the GLCP scheme and the oversampling scheme to achieve multipath diversity gains and Doppler diversity gains in doubly selective fading channels. Simulation results show that the proposed scheme can significantly improve the performance of OFDM systems in the frequency selective fading channel and the doubly selective fading channel in terms of bit error ratio, indicating that the diversity gains can be greatly enhanced by the proposed scheme in doubly selective fading channels.
There is a critical value of quenching concentration or optimal doping concentration in the laser glass due to the concentration quenching effect. It is particularly important to quickly and effectively determine the lumi• nescent quenching concentration. In this paper, a semi-quantitative method is established for predicting the quench• ing concentration of laser glass by studying the relationship between the population of excited states of rare earth ions in glass and the doping concentration. It is found that the linear correlation between the transition rate of spontaneous emission and doping concentration is greater than its squared correlation with a concentration in Nd 3+ -doped phos• phate glass. This indicates that OHaffects more to the transition probability of spontaneous emission than the energy transfer process between rare earth ions. The fluorescence lifetime at low concentration is selected instead of the spontaneous radiative lifetime to predict the quenching concentration, which effectively reduced the influence of multi-phonon relaxation and OH -, and the absolute error between the predicted luminescence quenching concentra• tion and the experimental value was reduced from 0. 82% to 0. 16%. The proposed prediction calculation method has high accuracy and strong universality. This work guides determining the quenching concentration of laser glass and is beneficial to the research and exploration of a new type of laser glass.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.