Recent Developments of Dual-Band Doherty Power Amplifiers for Upcoming Mobile Communications Systems

Power amplifiers in modern and future communications should be able to handle different modulation standards at different frequency bands, and in addition, to be compatible with the previous generations. This paper reviews the recent design techniques that have been used to operate dual-band amplifiers and in particular the Doherty amplifiers. Special attention is focused on the design methodologies used for power splitters, phase compensation networks, impedance inverter networks and impedance transformer networks of such power amplifier. The most important materials of the dual-band Doherty amplifier are highlighted and surveyed. The main problems and challenges covering dual-band design concepts are presented and discussed. In addition, improvement techniques to enhance such operations are also exploited. The study shows that the transistor parasitic has a great impact in the design of a dual-band amplifier, and reduction of the transforming ratio of the inverter simplifies the dual-band design. The offset line can be functionally replaced by a Π-network in dual-band design rather than T-network

Vitamin D serum level predicts stroke clinical severity, functional independence, and disability—A retrospective cohort study

BackgroundStroke is a leading cause of mortality and disability and one of the most common neurological conditions globally. Many studies focused on vitamin D as a stroke risk factor, but only a few focused on its serum level as a predictor of stroke initial clinical severity and recovery with inconsistent results. The purpose of this study was to assess the relationship between serum vitamin D levels and stroke clinical severity at admission and functional independence and disability at discharge in Saudi Arabia.MethodologyA retrospective cohort study of adult ischemic stroke patients who had their vitamin D tested and admitted within 7 days of exhibiting stroke symptoms at King Abdulaziz Medical City (KAMC) Jeddah, Saudi Arabia. Based on vitamin D level, the patients were categorized into normal [25(OH)D serum level ≥ 75 nmol/L], insufficient [25(OH)D serum level is 50–75 nmol/L], and deficient [25(OH)D serum level ≤ 50 nmol/L]. The primary outcome was to assess the vitamin D serum level of ischemic stroke patients’ clinical severity at admission and functional independence at discharge. The National Institute of Health Stroke Scale (NIHSS) was used to assess the clinical severity, whereas the modified Rankin scale (mRS) was used to assess functional independence and disability.ResultsThe study included 294 stroke patients, out of 774, who were selected based on the inclusion and exclusion criteria. The mean age of the participants was 68.2 ± 13.4 years, and 49.3% were male. The patients’ distribution among the three groups based on their vitamin D levels is: normal (n = 35, 11.9%), insufficient (n = 66, 22.5%), and deficient (n = 196, 65.6%). After adjusting for potential covariates, regression analysis found a significant inverse relationship of NIHSS based on 25(OH)D serum level (beta coefficient: −0.04, SE: 0.01, p = 0.003). Patients with deficient serum vitamin D level also had significantly higher odds of worse functional independence in mRS score [OR: 2.41, 95%CI: (1.13–5.16), p = 0.023] when compared to participants with normal vitamin D level.ConclusionLow vitamin D levels were associated with higher severity of stroke at admission and poor functional independence and disability at discharge in patients with acute ischemic stroke. Further randomized clinical and interventional studies are required to confirm our findings

Noise Cancellation for HIPERLAN/2 with Open Loop Transmit Diversity Technique

A new proposed method is presented, where multiple antennas have been applied into HIPERLAN/2 system in addition to employing space-time diversity technique, especially the Alamouti technique. The suggested approach is used to cancel or reduce the effect of the transmitted power using a feedback signal process within the transceiver unit, especially when the antennas are closely located and working in full-mode duplexing. Several parameters including the transmitted power, the received power, and the feedback accuracy have been considered for testing the performance of the system in term of the signal to noise ratio (SNR) versus bit error rate (BER). A software programme using MATLAB and Simulink is implemented to evaluate the proposed method. The results showed that the system performance is heavily dependent on the amount of the mismatch in the feedback, the received power, and the transmitted power. The performance of the system decreases as the feedback accuracy increases when the transmitted power and the received power are constant. At the same time, the performance of the system decreases as the transmitted power increases when the received power and the mismatch are constant. Finally, the increase in the received power enhances the system performance when the other parameters are constant

Doherty Power Amplifier for LTE-Advanced Systems

The design and implementation of an asymmetrical Doherty power amplifier are discussed, where two Cree GaN High Electron Mobility Transistors (HEMTs) devices are used for designing an asymmetrical Doherty power amplifier to achieve saturated power of 48 dBm and optimal back-off efficiency of 8 dB in the frequency band of 3.3–3.5 GHz. Rogers RO4350B material is used as a substrate material, a back-off of 8 dB was achieved with an average gain of 10 dB. Load-pull data are an important tool for determining the optimum load impedance that the transistor needs to see. Additionally, the measured efficiency was 50% when the designed amplifier was tested by a modulated signal of 8 dB peak-to-average-power ratio when the average output power was 40 dBm. At the same time, the linearity of the designed amplifier was measured and found 31.8 dB which can be improved using a digital pre-distorter. The gain phase measurement can be used as an indicator for compensating the phase difference between the two cells

Load‐modulation technique without using quarter‐wavelength transmission line

A proposed method for achieving active load-modulation technique without using a quarter-wavelength transmission line is discussed and evaluated. The theoretical analysis shows that the active load-modulation can be achieved without using a quarter-wavelength line, where the main amplifier sees a low impedance when the input signal level is low, and this impedance increases in proportion to the amount of current contributed from the peaking amplifier. The peaking amplifier sees an impedance decreasing from infinity to the normalized impedance. To validate the method, a circuit was designed, simulated and fabricated using two symmetrical gallium nitride (GaN) transistors (6 W) to achieve a peak power of 12 W and 6 dB output back-off efficiency. The design operates with 400 MHz bandwidth at 3.6 GHz and showed an average efficiency of 50% at 6 dB back-off and an efficiency of 75% at peak power. The designed circuit was tested with CW and modulated signals, the amplifier showed an Adjacent Channel Power Ratio (ACPR) of 31–35.5 dB when tested with a wideband code division multiple access signal of 6 dB peak-average-power ratio (PAPR) at 35.5 dBm average power. Additional 20 dB of linearity improvement was achieved after adding a lineariser