Amplifier Architectures for Wireless Communication Systems

Ever-increasing demand in modern wireless communication systems leads researchers to focus on design challenges on one of the main components of RF transmitters and receivers, namely amplifiers. On the transmitter side, enhanced efficiency and broader bandwidth over single and multiple bands on power amplifiers will help to have superior performance in communication systems. On the other hand, for the receiver side, having low noise and high gain will be necessary to ensure good quality transmission over such systems. In light of these considerations, a unique approach in design methodologies are studied with low noise amplifiers (LNAs) for RF receivers and the Doherty technique is analyzed for efficiency enhancement for power amplifiers (PA) on the transmitters. This work can be outlined in two parts. In the first part, Low Noise RF amplifier designs with Bipolar Junction Transistor (BJT) are studied to achieve better performing LNAs for receivers. The aim is to obtain a low noise figure while optimizing the bandwidth and achieving a maximum available gain. There are two designs that are operating at different center frequencies and utilizing different transistors. The first design is a wideband low-noise amplifier operating at 2 GHz with a high power BJT. The proposed design uses only distributed elements to realize the input and output matching networks. Additionally, a passive DC bias network is used instead of an active DC bias network to avoid possible complications due to the lumped elements parasitic effects. The matching networks are designed based on the reflection coefficients that are derived based on the transistor’s available regions. The second design is a low voltage standing wave ratio (VSWR) amplifier with a low noise figure operating at 3 GHz. This design is following the same method as in the first design. Both these amplifiers are designed to operate in broadband applications and can be good candidates for base stations. The second part of this work focuses on the transmitter side of communication systems. For this part, Doherty Power Amplifier (DPA) is analyzed as an efficiency enhancement technique for PAs. A modified architecture is proposed to have wider bandwidth and higher efficiency. In the proposed design, the quarter-wave impedance inverter was eliminated. The input and the output of the main and peak amplifiers are matched to the load directly. Additionally, the input and output matching networks are realized only using distributed elements. The selected transistor for this design is a 10 W Gallium Nitride (GaN). The fabricated amplifier operates at the center frequency of 2 GHz and provides 40% fractional bandwidth, 54% of maximum power-added efficiency, and 12.5 dB or better small-signal gain. The design is showing promising results to be a good candidate for better-performing transmitters over the L- and S- band

CFD-Based Prediction of Combustion Dynamics and Nonlinear Flame Transfer Functions for a Swirl-Stabilized High-Pressure Combustor

Thermoacoustic instabilities in gasturbine combustor systems can be predicted in the design phase with a thermoacoustic network model. In this model, the coupling between acoustic pressure fluctuations and the combustion rate is described by the Flame Transfer Function. The present paper introduces a new, efficient, and robust method for deriving the FTF from CFD predictions by means of a discrete multi-frequency sinusoidal fuel flow excitation method. The CFD-based FTF result compares well with experimental data for the time delay, but for the gain, only up to 400 Hz. Above 400 Hz, the CFD result reveals a smooth low-amplitude gain, which is not found in the measured data. A novel, accurate continuous correlation function for the FTF gain is computed based on the results for discrete frequencies. When this is implemented into a 1D acoustic network model, the stability map shows, below 600 Hz, two eigenfrequencies, by both the experiment and CFD-based FTF, that are identical. The CFD-based FTF correctly predicts marginal activity at the highest eigenfrequency, while the experimentally based FTF suggests an unstable operation. The unstable operation is not observed in the experiments. This suggests that the CFD-based FTF is also correct for high frequencies.</p

Measurement and Modeling of the Acoustic Response in a High Pressure Combustor

In this paper, a one dimensional acoustic network model is presented which can be used as a design tool to predict the limit cycle pressure oscillations in a gas turbine combustor. Analytically represented models are combined with measured flame transfer functions and well defined boundary conditions. Additionally acoustic damping due to turbulence, acoustic reflection at contractions, modification of the acoustic speed of sound due to a mean flow and effect of temperature gradient that play a role in the acoustic modeling of combustion systems have been included in this network model. The model is applied on a high-pressure laboratory combustor. Finally, the measured and predicted dynamic behaviour in the combustor is compared. The results indicate the network modelling approach is a promising design tool for gas turbine combustion application

Experimental Investigation of Thermoacoustics and High-Frequency Combustion Dynamics with Band Stop Characteristics in a Pressurized Combustor

In combustor systems, thermoacoustic instabilities may occur and must be avoided for reliable operation. An acoustic network model can be used to predict the eigenfrequencies of the instabilities and the growth rate by incorporating the combustion dynamics with a flame transfer function (FTF). The FTF defines the interconnection between burner aerodynamics and the rate of combustion. In the current study, the method to measure the FTF in a pressurized combustor is explored. A siren unit, mounted in the fuel line, induced a fuel flow excitation of variable amplitude and high maximum frequency. This was performed here for pressurized conditions at 1.5 bar and 3 bar and at a thermal power of 125 kW and 250 kW. In addition to the experimental investigation, a 1-D acoustic network model approach is used. In the model, thermoviscous damping effects and reflection coefficients are incorporated. The model results compare well with experimental data, indicating that the proposed method to determine the FTF is reliable. In the approach, a combination of an FTF with a band stop approach and a network modeling approach was applied. The method provides a good match between experimentally observed behavior and an analytical approach and can be used for instability analysis

Acoustic Design Parameter Change of a Pressurized Combustor Leading to Limit Cycle Oscillations

When aiming to cut down on the emission of nitric oxides by gas turbine engines, it is advantageous to have them operate at low combustion temperatures. This is achieved by lean premixed combustion. Although lean premixed combustion is a proven and promising technology, it is also very sensitive to thermoacoustic instabilities. These instabilities occur due to a coupling between the unsteady heat release rate of the flame and the acoustic field inside the combustion chamber. In this paper, this coupling is investigated in detail. Two acoustic design parameters of a swirl-stabilized pressurized preheated air (300 °C)/natural gas combustor are varied, and the occurrence of thermoacoustic limit cycle oscillations is explored. The sensitivity of the acoustic field as a function of combustion chamber length (0.9 m to 1.8 m) and reflection coefficient (0.7 and 0.9) at the exit of the combustor is investigated first using a hybrid numerical and analytical approach. ANSYS CFX is used for Unsteady Reynolds Averaged Navier-Stokes (URANS) numerical simulations, and a one-dimensional acoustic network model is used for the analytical investigation. Subsequently, the effects of a change in the reflection coefficient are validated on a pressurized combustor test rig at 125 kW and 1.5 bar. With the change in reflection coefficient, the combustor switched to limit cycle oscillation as predicted, and reached a sound pressure level of 150 dB

Evaluation of epicardial fat tissue thickness in patients with multiple sclerosis

Aim: Multiple sclerosis (MS), which is inflammatory in its pathogenesis, damages the myelin sheath in the central nervous system (CNS) and causes axonal loss. Epicardial fat tissue (EFT), located between the myocardium and the visceral layer of the pericardium, surrounds the heart and several inflammatory cytokines is secreted from this tissue. In this study, we aimed to investigate EFT thickness in MS patients and compared with that of volunteer non-MS subjects. Methods: A total of 154 subjects comprising 61 MS patients and 93 volunteers matched for gender and age were included in our study.&nbsp; Epicardial fat tissue thickness was measured by echocardiography. All values were compared between groups. Results: Echocardiographic parameters were similar in both groups. However, the mean EFT thickness was significantly higher in the MS group than in the control group (p&lt;0.001). Epicardial fat tissue thickness was also significantly correlated with the presence of MS (r=0.33, p&lt;0.001). Conclusion: The results of our study suggest that the increase in epicardial adipose tissue thickness in MS patients may be a predictive factor for cardiovascular disease. However, the clinical significance of this finding and its relevance to MS pathogenesis should be investigated in further studies

Cold War in Asia: China's Involvement in the Korean and Vietnam War

As essential components of the Cold War, the Korean War and the Vietnam War have played significant roles in global policy among the Western forces under the leadership of the United States and the Eastern bloc led by the Soviet Union in the 20th century. Communist China, founded in 1949, was also part of the countries behind the Iron Curtain and provided substantial support to North Korea and North Vietnam in their fight against their ideological enemies. Despite the extensive research by scholars on the Korean War and the Vietnam War, as well as China’s role in both of these wars, it is interesting to know whether China’s role and attitude had changed from one war to the other. This thesis examines and compares China’s military interference in the Korean War in the beginning of the 1950s with its involvement more than ten years later in the Vietnam War through the investigation of China’s motives to enter the wars and their ways of support in connection with the development of foreign relations. While contrasting China’s role and involvement in Korea and Vietnam, similarities, but also major differences become distinguishable. This thesis argues that these major differences indicate a Chinese rethinking about a possible involvement in Vietnam because of China’s domestic problems and its foreign policy developments at that time

Burden of disease attributable to high body mass index: an analysis of data from the Global Burden of Disease Study 2021

Background Obesity represents a major global health challenge with important clinical implications. Despite its recognized importance, the global disease burden attributable to high body mass index (BMI) remains less well understood. Methods We systematically analyzed global deaths and disability-adjusted life years (DALYs) attributable to high BMI using the methodology and analytical approaches of the Global Burden of Disease Study (GBD) 2021. High BMI was defined as a BMI over 25 kg/m2 for individuals aged ≥20 years. The Socio-Demographic Index (SDI) was used as a composite measure to assess the level of socio-economic development across different regions. Subgroup analyses considered age, sex, year, geographical location, and SDI. Findings From 1990 to 2021, the global deaths and DALYs attributable to high BMI increased more than 2.5-fold for females and males. However, the age-standardized death rates remained stable for females and increased by 15.0% for males. Similarly, the age-standardized DALY rates increased by 21.7% for females and 31.2% for males. In 2021, the six leading causes of high BMI-attributable DALYs were diabetes mellitus, ischemic heart disease, hypertensive heart disease, chronic kidney disease, low back pain and stroke. From 1990 to 2021, low-middle SDI countries exhibited the highest annual percentage changes in age-standardized DALY rates, whereas high SDI countries showed the lowest. Interpretation The worldwide health burden attributable to high BMI has grown significantly between 1990 and 2021. The increasing global rates of high BMI and the associated disease burden highlight the urgent need for regular surveillance and monitoring of BMI

Global burden of metabolic diseases, 1990-2021

BACKGROUND: Common metabolic diseases, such as type 2 diabetes mellitus (T2DM), hypertension, obesity, hypercholesterolemia, and metabolic dysfunction-associated steatotic liver disease (MASLD), have become a global health burden in the last three decades. The Global Burden of Disease, Injuries, and Risk Factors Study (GBD) data enables the first insights into the trends and burdens of these metabolic diseases from 1990 to 2021, highlighting regional, temporal and differences by sex.METHODS: Global estimates of disability-adjusted life years (DALYs) and deaths from GBD 2021 were analyzed for common metabolic diseases (T2DM, hypertension, obesity, hypercholesterolemia, and MASLD). Age-standardized DALYs (mortality) per 100,000 population and annual percentage change (APC) between 1990 and 2021 were estimated for trend analyses. Estimates are reported with uncertainty intervals (UI).RESULTS: In 2021, among five common metabolic diseases, hypertension had the greatest burden (226 million [95 % UI: 190-259] DALYs), whilst T2DM (75 million [95 % UI: 63-90] DALYs) conferred much greater disability than MASLD (3.67 million [95 % UI: 2.90-4.61]). The highest absolute burden continues to be found in the most populous countries of the world, particularly India, China, and the United States, whilst the highest relative burden was mostly concentrated in Oceania Island states. The burden of these metabolic diseases has continued to increase over the past three decades but has varied in the rate of increase (1.6-fold to 3-fold increase). The burden of T2DM (0.42 % [95 % UI: 0.34-0.51]) and obesity (0.26 % [95 % UI: 0.17-0.34]) has increased at an accelerated rate, while the rate of increase for the burden of hypertension (-0.30 % [95 % UI: -0.34 to -0.25]) and hypercholesterolemia (-0.33 % [95 % UI: -0.37 to -0.30]) is slowing. There is no significant change in MASLD over time (0.05 % [95 % UI: -0.06 to 0.17]).CONCLUSION: In the 21st century, common metabolic diseases are presenting a significant global health challenge. There is a concerning surge in DALYs and mortality associated with these conditions, underscoring the necessity for a coordinated global health initiative to stem the tide of these debilitating diseases and improve population health outcomes worldwide.</p

Correction to: Two years later: Is the SARS-CoV-2 pandemic still having an impact on emergency surgery? An international cross-sectional survey among WSES members

Background: The SARS-CoV-2 pandemic is still ongoing and a major challenge for health care services worldwide. In the first WSES COVID-19 emergency surgery survey, a strong negative impact on emergency surgery (ES) had been described already early in the pandemic situation. However, the knowledge is limited about current effects of the pandemic on patient flow through emergency rooms, daily routine and decision making in ES as well as their changes over time during the last two pandemic years. This second WSES COVID-19 emergency surgery survey investigates the impact of the SARS-CoV-2 pandemic on ES during the course of the pandemic. Methods: A web survey had been distributed to medical specialists in ES during a four-week period from January 2022, investigating the impact of the pandemic on patients and septic diseases both requiring ES, structural problems due to the pandemic and time-to-intervention in ES routine. Results: 367 collaborators from 59 countries responded to the survey. The majority indicated that the pandemic still significantly impacts on treatment and outcome of surgical emergency patients (83.1% and 78.5%, respectively). As reasons, the collaborators reported decreased case load in ES (44.7%), but patients presenting with more prolonged and severe diseases, especially concerning perforated appendicitis (62.1%) and diverticulitis (57.5%). Otherwise, approximately 50% of the participants still observe a delay in time-to-intervention in ES compared with the situation before the pandemic. Relevant causes leading to enlarged time-to-intervention in ES during the pandemic are persistent problems with in-hospital logistics, lacks in medical staff as well as operating room and intensive care capacities during the pandemic. This leads not only to the need for triage or transferring of ES patients to other hospitals, reported by 64.0% and 48.8% of the collaborators, respectively, but also to paradigm shifts in treatment modalities to non-operative approaches reported by 67.3% of the participants, especially in uncomplicated appendicitis, cholecystitis and multiple-recurrent diverticulitis. Conclusions: The SARS-CoV-2 pandemic still significantly impacts on care and outcome of patients in ES. Well-known problems with in-hospital logistics are not sufficiently resolved by now; however, medical staff shortages and reduced capacities have been dramatically aggravated over last two pandemic years