Evaluating the performance of convolutional codes over block fadingchannels

This correspondence considers union upper bound techniques for error control codes with limited interleaving over block fading Rician channels. A modified bounding technique is presented that relies on limiting the conditional union bound before averaging over the fading process. This technique, although analytically not very attractive, provides tight and hence useful numerical results     

Performance of truncated type-II hybrid ARQ schemes with noisyfeedback over block fading channels

This paper considers truncated type-II hybrid automatic repeat-request (ARQ) schemes with noisy feedback over block fading channels. With these ARQ techniques, the number of retransmissions is limited, and, similar to forward error correction (FEC), error-free delivery of data packets cannot be guaranteed. Bounds on the average number of transmissions, the average coding rate as well as the reliability of the schemes are derived using random coding techniques, and the performance is compared with FEC. The random coding bounds reveal the achievable performance with block codes and maximum-likelihood soft-decision decoding. Union upper bounds and simulation results show that over block fading channels, these bounds can be closely approached with simple terminated convolutional codes and soft-decision Viterbi decoding. Truncated type-II hybrid ARQ and the corresponding FEC schemes have the same probability of packet erasure; however, the truncated ARQ schemes offer a trade-off between the average coding rate and the probability of undetected error. Truncated ARQ schemes have significantly higher average coding rates than FEC at high and medium signal-to-noise ratio even with noisy feedback. Truncated ARQ can be viewed as adaptive FEC that adapts to the instantaneous channel conditions     

Synchrotron radiation investigations of the Sr2MgSi2OT：Eu^2＋,R^3＋ persistent luminescence materials

The electronic and defect energy level structure of polycrystalline Sr₂MgSi₂O₇:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and different synchrotron radiation spectroscopies (UV-VUV emission and excitation, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)). Special attention was paid on the effect of the R³⁺ co-dopants on the persistent luminescence properties of the materials. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of ca. 7.1 eV agreed very well with the DFT value of 6.7 eV. The variation of the E_g value was attempted to relate with the trap structure as well as with the different properties of the R³⁺ co-dopants. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining practically the same, however. The different processes in the mechanism of persistent luminescence from Sr₂MgSi₂O₇:Eu²⁺,R³⁺ were assembled and their contributions discussed.