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This paper conceives a novel sparse code multiple access (SCMA) codebook design which is motivated by the strong need for providing ultra-low decoding complexity and good error performance in downlink Internet-of-things (IoT) networks, in which a massive number of low-end and low-cost IoT communication devices are served. By focusing on the typical Rician fading channels, we analyze the pair-wise probability of superimposed SCMA codewords and then deduce the design metrics for multi-dimensional constellation construction and sparse codebook optimization. For significant reduction of the decoding complexity, we advocate the key idea of projecting the multi-dimensional constellation elements to a few overlapped complex numbers in each dimension, called low projection (LP). An emerging modulation scheme, called golden angle modulation (GAM), is considered for multi-stage LP optimization, where the resultant multi-dimensional constellation is called LP-GAM. Our analysis and simulation results show the superiority of the proposed LP codebooks (LPCBs) including one-shot decoding convergence and excellent error rate performance. In particular, the proposed LPCBs lead to decoding complexity reduction by at least $97\%$ compared to that of the conventional codebooks, whilst owning large minimum Euclidean distance. Some examples of the proposed LPCBs are available at \url{https://github.com/ethanlq/SCMA-codebook}.

Non-orthogonal Multiple Access (NOMA) has been envisioned as one of the key enabling techniques to fulfill the requirements of future wireless networks. The primary benefit of NOMA is higher spectrum efficiency compared to Orthogonal Multiple Access (OMA). This paper presents an error rate comparison of two distinct NOMA schemes, i.e., power domain NOMA (PD-NOMA) and Sparse Code Multiple Access (SCMA). In a typical PD-NOMA system, successive interference cancellation (SIC) is utilized at the receiver, which however may lead to error propagation. In comparison, message passing decoding is employed in SCMA. To attain the best error rate performance of PD-NOMA, we optimize the power allocation with the aid of pairwise error probability and then carry out the decoding using generalized sphere decoder (GSD). Our extensive simulation results show that SCMA system with “ $5\times 10$ ” setting (i.e., ten users communicate over five subcarriers, each active over two subcarriers) achieves better uncoded BER and coded BER performance than both typical “ $1\times 2$ ” and “ $2\times 4$ ” PD-NOMA systems in uplink Rayleigh fading channel. Finally, the impacts of channel estimation error on SCMA, SIC and GSD based PD-NOMA and the complexity of multiuser detection schemes are also discussed.

In recent years, the various Internet of Things (IoT) communication technologies have been researched. The third generation partnership project (3GPP) is dedicated to building the IoT platforms, including NB-IoT, eMTC, and URLLC. However, these platforms cannot adequately meet all the needs of future IoT use cases which are usually broad and beyond cellular. In order to reduce the deployment cost, making the full use of the unlicensed spectrum is strongly appealing to the IoT scenarios. Transform domain communication system (TDCS) is a typical system with spectrum sensing and sharing, and it has unique advantages in supporting multiuser communication in a complex electromagnetic environment. Thus, TDCS is proposed for the IoT massive multiple access scenarios. Previous studies indicate that traditional TDCS has poor BER performance when encountering a large number of users, especially with the near-far effect. To address this, a hybrid clustering TDCS targeting at cognitive IoT applications is proposed to support massive multiple access with the near-far effect. The optimal construction method of the sequence set is also given for the hybrid clustering TDCS. Compared with traditional TDCS, the simulation results show that the proposed system achieves an improvement in multiple access ability, and the system performance can be improved by making a compromise on the number of clusters.

Sr0.99Ce0.01Ti1-xO3 (SCT, x = 0, ± 0.0025, ± 0.0050, 0.0075) ceramics were prepared by solid state reaction methods and sintered in air atmosphere at different temperatures, with a soaking time of 2 h. The dielectric properties of all samples presented excellent temperature independence over a broad temperature range from 25 to 330 ℃ and frequency independence between 10 kHz and 1 MHz. Sr0.99Ce0.01Ti0.9925O3 (SCT0.9925) ceramics sintered in air atmosphere exhibited a high permittivity (~5400) and a low dielectric loss (~0.01) measured at room temperature and 1 kHz. XPS and complex impedance spectroscopy analysis confirmed that the high permittivity and low dielectric loss were attributed to the fully ionized oxygen vacancies and giant defect-dipoles in Ti-deficient samples. However, a higher dielectric loss of Ti-rich samples is owing to the destruction of giant defect dipoles, in which highly localized electrons were transformed into hopping electrons.

Dielectrics for energy storage devices should commonly achieve the requirement of high permittivity, large breakdown strength, and large polarization when used in harsh working environments. A question is gradually raised about permittivity/polarization/breakdown strength in a dielectric for high voltage applications which one is more conducive to energy storage performances. Here, an attempt has been made to investigate the effect of high permittivity on the energy performances of dielectric materials in order to provide a viable option for high voltage devices. High values of dielectric permittivity (1300–7400) combined with low dielectric loss (

Evolving from cognitive radio networks (CRNs), the concept has developed into a new paradigm of cloud and big data-based next-generation CRNs due to huge amount of data processing, complicated spectrum resource scheduling, and real-time information exchange. In cloud-based CRNs, control channels are needed for CR nodes to perform certain handshakings for network self-organization, spectrum sensing, network coordination, and flexible data connections. This paper investigates a transmission scheme for control channel (CC) in cloud-based CRNs, which is over several noncontiguous spectral holes. Transform domain communication system (TDCS)-based transmission scheme with spectrally-constrained sequence design is presented for CC. A practical testbed design for TDCS-based CC with multiple National Instruments PXIe devices and six universal software defined radio reconfigurable input/output devices is presented. Details of system design as well as main implementation challenges are described. Bit-error rate of the system is validated through both theoretical analysis and simulation results under realistic channel conditions.

Effect of titanium deficiency on the structure and electrical properties of lead-free (Ba0.838Ca0.162)(Ti0.908-xZr0.092)O3 (0.46BZT-0.54BCTx, x = 0.000, 0.0004, 0.0008, 0.0012, 0.0016, 0.0020) compositions was investigated. The Ti-deficient compositions exhibited typically tetragonal-structured characteristics confirmed by XRD and Raman spectra regardless of trace amount of Ti deficiency. Ti-deficiency could result in double doping mechanism, namely soft characteristic for low Ti deficiency and hard characteristic for high Ti deficiency. Thermal stability of piezoelectricity could be effectively improved by Ti deficiency compared to pure 0.46BZT-0.54BCT, but inferior to Mn-doping composition. This defect engineering can be an effective and promising way to tailor the thermal stability of piezoelectric materials suitable for high temperature applications.

We investigated the structure, dielectric properties and energy density performances of cubic perovskite-structured Mg-doped SrTiO3 ceramics that were prepared by the solid-state reaction method. SrTiO3 ceramic exhibited a relatively stable permittivity about 265–290 and enhanced dielectric breakdown strength (DBS) by Mg isovalent doping. Doping effects on the energy-storage properties in SrTiO3 ceramics was performed by complex impedance analysis and polarization–electric field hysteresis loops. The energy storage density was dependent on DBS while energy efficiency was closely related to the remnant polarization. The possible physical mechanisms, including grain, gain boundary and interfacial polarization effects, were discussed to explain the improvement of dielectric breakdown strength. The bulk Mg-doped SrTiO3 materials have shown interesting energy densities (1.86 J/cm3) with good energy storage efficiency (about 89.3%) indicating that they can be a promising candidate for high energy density capacitor applications.

In this paper, the transform-domain communication system based multi-user access system has been implemented and demonstrated via software-defined radio. A practical testbed with two National Instruments PXIe devices and four universal software defined radio reconfigurable input/output (USRP RIO) devices is presented for system implementation. A successive interference cancellation (SIC) receiver is designed to improve its multiuser performance. The testbed is capable of detecting interference and transmitting in real-time. Additionally, the centralized layout and distributed layout of system devices are both evaluated. We have demonstrated and demonstrated real-time seamless data transmission among six users, while avoiding environment interference. Details of system design including testbed synchronization, SIC receiver, and bit error rate are also presented.