Factor graph based detection approach for high-mobility OFDM systems with large FFT modes

In this article, a novel detector design is proposed for orthogonal frequency division multiplexing (OFDM) systems over frequency selective and time varying channels. Namely, we focus on systems with large OFDM symbol lengths where design and complexity constraints have to be taken into account and many of the existing ICI reduction techniques can not be applied. We propose a factor graph (FG) based approach for maximum a posteriori (MAP) symbol detection which exploits the frequency diversity introduced by the ICI in the OFDM symbol. The proposed algorithm provides high diversity orders allowing to outperform the free-ICI performance in high-mobility scenarios with an inherent parallel structure suitable for large OFDM block sizes. The performance of the mentioned near-optimal detection strategy is analyzed over a general bit-interleaved coded modulation (BICM) system applying low-density parity-check (LDPC) codes. The inclusion of pilot symbols is also considered in order to analyze how they assist the detection process

Compressed Sensing of Sparse Multipath MIMO Channels with Superimposed Training Sequence

Recent advances in multiple-input multiple-output (MIMO) systems have renewed the interests of researchers to further explore this area for addressing various dynamic challenges of emerging radio communication networks. Various measurement campaigns reported recently in the literature show that physical multipath MIMO channels exhibit sparse impulse response structure in various outdoor radio propagation environments. Therefore, a comprehensive physical description of sparse multipath MIMO channels is presented in first part of this paper. Superimposing a training sequence (low power, periodic) over the information sequence offers an improvement in the spectral efficiency by avoiding the use of dedicated time/frequency slots for the training sequence, which is unlike the traditional schemes. The main contribution of this paper includes three superimposed training (SiT) sequence based channel estimation techniques for sparse multipath MIMO channels. The proposed techniques exploit the compressed sensing theory and prior available knowledge of channel’s sparsity. The proposed sparse MIMO channel estimation techniques are named as, SiT based compressed channel sensing (SiT-CCS), SiT based hardlimit thresholding with CCS (SiT-ThCCS), and SiT training based match pursuit (SiT-MP). Bit error rate (BER) and normalized channel mean square error are used as metrics for the simulation analysis to gauge the performance of proposed techniques. A comparison of the proposed schemes with a notable first order statistics based SiT least squares (SiT-LS) estimation technique is presented to establish the improvements achieved by the proposed schemes. For sparse multipath time-invariant MIMO communication channels, it is observed that SiT-CCS, SiT-MP, and SiT-ThCCS can provide an improvement up to 2, 3.5, and 5.2 dB in the MSE at signal to noise ratio (SNR) of 12 dB when compared to SiT-LS, respectively. Moreover, for BER=10 −1.9 BER=10−1.9, the proposed SiT-CCS, SiT-MP, and SiT-ThCCS, compared to SiT-LS, can offer a gain of about 1, 2.5, and 3.5 dB in the SNR, respectively. The performance gain in MSE and BER is observed to improve with an increase in the channel sparsity

Effect of different adhesive application approaches on bond strength in over-etched dentin

PURPOSE To investigate the effect of over-etching and subsequent use of different adhesive application approaches including active, double, preheated, and ultrasonic application on dentin bond strength. METHODS Dentin specimens from sound human third molars were prepared and allocated into six groups (n= 10 per group). In Group 1 (negative control), the dentin specimens were over-etched with phosphoric acid for 30 seconds, and a two-step etch-and-rinse adhesive (Prime & Bond XP; Dentsply Sirona) was applied for 20 seconds according to the manufacturer's instructions. In test Groups 2-5, the application of the adhesive varied: In Group 2, the adhesive was actively rubbed into the dentin surface, whereas in Group 3, a second adhesive layer was applied after light-curing of the first one. In Group 4, the adhesive was heated to 68°C prior to application, and in Group 5, the adhesive was ultrasonically activated. Acid-etching for 15 seconds and adhesive application for 20 seconds was performed in the positive control group (Group 6) according to the manufacturer's instructions. After light-curing and composite build-up [Ceram.x Spectra ST (HV); Dentsply Sirona], microtensile bond strength and fracture types were determined. Data were statistically analyzed by Kruskal-Wallis rank test, followed by post-hoc pairwise comparisons using Conover tests and P-value adjustment according to Holm (α= 0.05). RESULTS Active, double, preheated, or ultrasonic adhesive application showed no significant effect on dentin bond strength when compared to the control groups. Predominantly adhesive failures were observed in all groups. CLINICAL SIGNIFICANCE A modification of adhesive application by using active, double, preheated, or ultrasonic application does not improve bonding to accidentally over-etched dentin

Influence of light-curing protocols on polymerization shrinkage and shrinkage force of a dual-cured core build-up resin composite

This study investigated the influence of time delay and duration of photo-activation on linear polymerization shrinkage, shrinkage force, and hardening of a dual-cured core build-up resin composite. The test material (Rebilda DC) was light-cured for 20 or 60 s either early (2 min) or late (7 min) after the start of mixing. Non-irradiated self-cured specimens served as controls. Linear shrinkage and shrinkage force were measured for 60 min using custom-made devices. Knoop hardness was determined at the end of the observation period. Self-cured controls, showing a linear shrinkage similar to that of specimens early light-cured for 20 s generated the lowest shrinkage force and hardness. A shorter light exposure time (20 s vs. 60 s) reduced linear shrinkage, shrinkage force, and hardness when early light-curing was performed, but did not affect the three properties in specimens light-cured late after the start of mixing. Late photo-activation increased linear shrinkage, irrespective of irradiation time, and resulted in a higher shrinkage force and hardness for short light exposure time. A moderate correlation was found between the two shrinkage properties studied (r(2) = 0.65). In conclusion, improvements in shrinkage behavior of the tested core build-up material were associated with inferior hardening, making it important to adapt curing protocols to the clinical situation