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User cooperation is an efficient approach to obtain diversity in both centralized and distributed wireless networks. In this paper, we consider a coded cooperative system under quasi-static Rayleigh fading and investigate the partner-choice problem. We find conditions on the interuser and user-to-destination channel qualities for cooperation to be beneficial. Using frame-error rate as a metric, we define the user cooperation gain (G) for evaluating the relative performance improvement of cooperative over direct transmission when a particular channel code is used. We introduce the cooperation decision parameter (CDP), which is a function of user-to-destination average received signal-to-noise ratios (SNRs), and demonstrate that whether cooperation is useful or not (G > 1 or G < 1) depends only on the CDP, not the interuser link quality. We use an analytical formulation of the CDP to investigate user cooperation gain and provide insights on how a user can choose among possible partners to maximize cooperation gain. We first consider the asymptotic performance when one or both partners have high average received SNR at the destination. We then provide conditions on user and destination locations for cooperation to be beneficial for arbitrary SNRs. We illustrate these cooperative regions, and study geometric conditions for the best partner choice. We also define the system cooperation gain and illustrate cooperation benefits for both users. All of our theoretical results are verified through numerical examples. Index Terms--Cooperative diversity, diversity methods, error-correction coding, fading channels, frame-error rate (FER), wireless networks.

The use of orthogonal spreading codes has attracted much attention due to their ability to suppress interference from other users, compared with the nonorthogonal sequences in the synchronous case. In this paper, new sets of orthogonal sequences derived from the unified complex Hadamard transforms (UCHTs) are investigated. Various correlation properties of the sequences are mathematically derived and analyzed. It is shown that some UCHT sequences provide better autocorrelation properties than orthogonal Walsh-Hadamard sequences. Performance comparisons between UCHT sequences, Gold, small set of Kasami, and m-sequences show that some UCHT sequences outperform these well-known spreading sequences under simulation of systems in the presence of multiple access interference and additive white Gaussian noise. Index Terms--Code-division multiple access (CDMA), orthogonal codes, orthogonal transform, spreading sequences, unified complex Hadamard transform (UCHT).

Developing effective bifunctional catalysts for the oxygen reduction and evolution reaction (ORR/OER) is essential for accelerating the cathode efficiency of Zn–air batteries. Herein, atomically dispersed Ni single atoms are supported by sub-10 nm Pd nanocrystals embedded in N-doped carbon frameworks (Ni SAs-Pd@NC), in an effort to achieve superior bifunctional activity in both the ORR and OER. The key synthetic point depends on the protection mechanism of 1-naphthylamine, which could provide a carbon source for Ni SAs and restrict the Pd size under sub-10 nm during 600 °C pyrolysis, simultaneously. The synergistic effect of sub-10 nm Pd with superior ORR activity and Ni–N4 SAs with favourable OER activity leads to bifunctional catalytic performance, meanwhile the rod-like carbon frameworks with ultrathin, porous and N-doped features contribute to accelerated electron transfer and structural robustness. As a proof-of-concept application, Ni SAs-Pd@NC demonstrates ultrahigh ORR activity with a positive half-wave potential of 0.84 V and a low OER overpotential of 380 mV at 10 mA cm−2, in an alkaline medium. For a rechargeable Zn–air battery, the Ni SAs-Pd@NC cathode delivers a low charge–discharge voltage gap of 0.87 V, a high energy density of 884.6 W h kgZn−1, a high power density of 134.2 mW cm−2 and remarkable long-term cyclability for operation over 700 cycles, outperforming commercial Pt/C + RuO2 benchmarks. This work successfully integrates single atom sites with small-sized noble metals to break out their incompatibility in synthesis and to improve their thermostability, which offers a versatile approach to develop single atom-based bifunctional catalysts for energy devices. [ABSTRACT FROM AUTHOR]

Both the monodispersed Pd/C (2–5 nm) and Fe‐NC single atoms (SAs) are promising non‐Pt catalysts for oxygen reduction reaction (ORR), which belongs to precious metal and nonprecious metal camps, respectively. However, the poles apart of sub‐5 nm Pd/C and Fe‐NC SAs in synthesis and thermostability leave the challenge to integrate them together in one system. Herein, a 1‐naphthylamine protected pyrolysis mechanism is devised to couple the atomically dispersed Fe sites with sub‐5 nm Pd nanocrystals embedded in N‐doped carbon nanobelts (FeN3‐Pd@NC NBs). The FeN3 SAs represent the minimal surface blockage to tune the electronic structure of Pd, while the carbon frameworks are born with ultrathin, porous, and N‐doped feature's. As inspired, the FeN3‐Pd@NC NBs exhibit outstanding activity (E1/2 = 0.926 V) and durability (2 mV decay in E1/2 after 2000 cycles) for ORR, as well as achieving a maximum power density of 831.2 mW cm−2 in a microbial fuel cell operated for over 100 d. Density functional theory calculation reveals that the FeN3 SAs can shift the density of states of Pd toward the Fermi level, and their coupling can decrease the limiting reaction barrier with a value of −0.62 eV, thus greatly accelerating the ORR kinetics. [ABSTRACT FROM AUTHOR]