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This letter presents a new method to directly analyze and optimize symbol error rate (SER) performance of minimum decoding complexity (MDC) ABBA space-time block codes based on a tight union bound on SER. Additionally, a new signal transformation for rectangular quadrature amplitude modulation is proposed to provide better performance than the existing ones with lower encoding/decoding complexities. It is also shown that MDC-ABBA codes achieve full-diversity with antenna selection and limited feedback. Index Terms--Quasi-orthogonal space-time block codes, ABBA codes, performance analysis.

Space-frequency (SF) codes that exploit both spatial and frequency diversity can be designed using orthogonal frequency division multiplexing (OFDM). However, OFDM is sensitive to frequency offset (FO), which generates intercarrier interference (ICI) among subcarriers. We investigate the pair-wise error probability (PEP) performance of SF codes over quasistatic, frequency selective Rayleigh fading channels with FO. We prove that the conventional SF code design criteria remain valid. The negligible performance loss for small FOs (less than 1%), however, increases with FO and with signal to noise ratio (SNR). While diversity can be used to mitigate ICI, as FO increases, the PEP does not rapidly decay with SNR. Therefore, we propose a new class of SF codes called ICI self-cancellation SF (ISC-SF) codes to combat ICI effectively even with high FO (10%). ISC-SF codes are constructed from existing full diversity space-time codes. Importantly, our code design provide a satisfactory tradeoff among error correction ability, ICI reduction and spectral efficiency. Furthermore, we demonstrate that ISC-SF codes can also mitigate the ICI caused by phase noise and time varying channels. Simulation results affirm the theoretical analysis. Index Terms--Inter-carrier interference (ICI), multiple input multiple output (MIMO), orthogonal frequency division multiplexing (OFDM), space-frequency (SF) codes.

Recently, Z-scheme systems have gained immense attention for photocatalytic hydrogen evolution (PHE). Herein, LaFeO3 perovskite (as photosystem II) was shelled by nitrogen-doped graphene (NGr as photosystem I) functionalized with ultralow Pt loading to produce an impactful and stable ternary Z-scheme platform: LaFeO3@NGr–Pt. Under visible-light irradiation, the LaFeO3@NGr3–Pt0.5 entity (a NGr shell thickness of 3 nm and Pt loading of 0.5 wt%) delivered a PHE performance of 3.52 μmol mgcat−1 h−1, superior to those of binary LaFeO3@NGr3 (1.86 μmol mgcat−1 h−1), NGr–Pt0.5 (0.88 μmol mgcat−1 h−1), and free-standing NGr (0.34 μmol mgcat−1 h−1) catalysts. It also showed outstanding PHE performance compared to previous advanced Z-scheme species. In addition, the ternary Z-scheme system exhibited a high maximum apparent quantum yield of 18.25% at 450 nm. This excellent performance of the Z-scheme platform can be attributed to the following. (i) The interfacial contact between LaFeO3 and NGr facilitated charge carrier transfer under light. (ii) The promising direct Z-scheme platform not only preserved the strong redox potentials of the two photosystems but also inhibited charge recombination significantly. (iii) Finally, the Pt cocatalyst attracted the electrons migrating from NGr and assisted the adsorption of hydrogen atoms, thus accelerating the overall catalytic reaction. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs), transition metal carbides (TMCs), and transition metal oxides (TMOs) have been widely investigated for electrocatalytic applications owing to their abundant active sites, high stability, good conductivity, and various other fascinating properties. Therefore, the synthesis of composites of TMDs, TMCs, and TMOs is a new avenue for the preparation of efficient electrocatalysts. Herein, we propose a novel low-cost and facile method to prepare TMD–TMC–TMO nano-hollow spheres (WS2–WC–WO3 NH) as an efficient catalyst for the hydrogen evolution reaction (HER). The crystallinity, morphology, chemical bonding, and composition of the composite material were comprehensively investigated using X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The results confirmed the successful synthesis of the WS2–WC–WO3 NH spheres. Interestingly, the presence of nitrogen significantly enhanced the electrical conductivity of the hybrid material, facilitating electron transfer during the catalytic process. As a result, the WS2–WC–WO3 NH hybrid exhibited better HER performance than the pure WS2 nanoflowers, which can be attributed to the synergistic effect of the W–S, W–C, and W–O bonding in the composite. Remarkably, the Tafel slope of the WS2–WC–WO3 NH spheres was 59 mV dec−1, which is significantly lower than that of the pure WS2 NFs (82 mV dec−1). The results also confirmed the unprecedented stability and superior electrocatalytic performance of the WS2–WC–WO3 NH spheres toward the HER, which opens new avenues for the preparation of low-cost and highly effective materials for energy conversion and storage applications. [ABSTRACT FROM AUTHOR]