NI AWR Design Environment
MaXentric Technologies, LLC is a specialty R&D firm that provides product design, development, and manufacturing services for the military defense market, as well as telecommunications/broadcast commercial markets. MaXentric products include simple low-cost millimeterwave broadband wireless transceivers, passive radio-frequency identification (RFID) readers, and high-efficiency envelope-tracking (ET) power amplifiers (PAs). Typical applications include intelligence, surveillance, and reconnaissance (ISR) components, high-bandwidth wireless communications, electronic warfare (EW), broadband high-efficiency PAs, and more.
PAs are an integral part of all cellular phones, base stations, and radio systems and represent one of the most expensive component sub-assemblies in modern wireless infrastructure equipment. Both performance and cost are important drivers in system design and efficiency, making physical size, linearity, and reliability among the principal challenges. As systems and their waveforms get more complicated, new and innovative techniques and materials must be used to provide the required performance.
The increasing demand for higher data rates and larger signal bandwidth, while maintaining signal integrity, have led to the use of signals with non-constant envelope and high peak-to-average power ratio (PAPR). In conventional fixed-bias PAs, the maximum efficiency occurs at saturation, with a single-tone signal. As the PA operates at power levels away from saturation, the efficiency degrades. This creates an issue in the presence of high PAPR signals (such as orthogonal frequency-division multiplexing [OFDM]), where the average power is well below saturation.
Various techniques have been explored to increase the efficiency of PAs for high PAPR signals. One area of research is the ET technique. The drain voltage of the RFPA is varied dynamically to track the envelope of the signal, providing the appropriate DC supply signal and keeping the RF transistor operating continuously in its saturation region. Since the DC supply power is changing with the input envelope signal, the overall transmitter will not consume excessive DC power in a low-output power region. This results in a dramatic increase in PA efficiency.
The platform development of the ETPA is very important because the ETPA transmitter architecture is different from the conventional transmitter.
The bandwidth of the RF signal from the up-converter is the same as that of the signal at the antenna. The bandwidths of both the RF and envelope signals are five times wider than that of the conventional transmitter. Many of the available evaluation platform/testbeds today are limited in bandwidth to ~200 MHz. This limits the bandwidth of the supported signal to ~40 MHz. In addition, the feedback path is needed for the digital pre-distortion (DPD) linearization. The transmitter for the conventional PA is simple, but the efficiency of the PA is relatively low. On the other hand, the efficiency of the ETPA is quite high thanks to the dynamic modulator design. To simplify ETPA development, it is important to establish the ETPA design platform.
The challenge for the MaXentric design team was to develop an ETPA using real-time efficiency and linearity measurements for optimizing ETPA design, with flexibility to accommodate different 5G signals. As talk of future 5G LTE systems calls for signal bandwidths of greater than 100 MHz, the need for an evaluation platform with 500 MHz was crucial.
MaXentric designers chose NI AWR Design Environment™, specifically Microwave Office circuit design software, to design and optimize their ETPA, as well as the NI vector signal transceiver (VST) for RF signal generation and the NI arbitrary waveform generator (AWG) for envelope signal generation. On-board tuning and final optimization of the PA impedance matching network was performed with Microwave Office software, which provided nonlinear harmonic balance circuit simulation in conjunction with design features such as load-pull analysis, transient analysis, circuit envelope, and verification through circuit/electromagnetic (EM) co-simulation via NI AWR Design Environment’s AXIEM 3D planar EM simulator.
Results of Envelope Tracking using PXI and VST
The VST and PXI were used to optimize the LTE Band 1 (2.14 GHz) access point (AP) using MaXentric’s MaXEA 1.0 modulator. The MaXEA 1.0 is a 30 V integrated envelope modulator with greater than 70 percent modulator efficiency, capable of outputting up to 7 W of average envelope power. It is designed to support signals with high PAPRs, such as those used in 5G LTE. It is compatible with various semiconductor technologies, such as laterally diffused metal oxide semiconductor (LDMOS), gallium nitride (GaN), gallium arsenide (GaAS), and more. In this application, a GaN device was used for the PA design. The PA was tuned and optimized for envelope tracking operation using the NI PXI system.
Initially, output and input external tuners were used to optimize the efficiency, gain, and output power of the ETPA. The desired input and output impedances were measured using a vector network analyzer (VNA) and the impedance tuning measurements were de-embedded via simulations performed using Microwave Office software in order to derive the required matching structures at the input/output of the power transistor. The retuned ETPA was then measured again using the PXI, VST, and LabVIEW ET setup to confirm its performance. Time alignment between the RF (VST) and the envelope (AWG) paths was performed digitally in LabVIEW for best efficiency and linearity.
The use of Microwave Office software enabled the designers to significantly reduce PA optimization time without sacrificing measurement accuracy. The close correlation between simulation and measurement enabled them to perform most of the optimizing in software before physically implementing it on the board, thus reducing the number of iterations that had to be performed.
MaXentric designers addressed their engineering challenges with help from the technical support team for NI AWR software, which helped to accelerate product training and assisted with software customization for the unique challenges of developing a 5G ETPA evaluation platform. As an added benefit, the learning curve for using Microwave Office is relatively low, which enabled the designers to easily train new interns and employees on the software. Along with the intuitive user interface and powerful design automation features, NI AWR Design Environment delivered faster simulation speed, particularly the AXIEM 3D planar EM simulator, which provided excellent correlation between the simulations and measurements. The MaXentric team also noted that compared to competitive products, the NI AWR Design Environment demonstrated better convergence and faster simulation times, enabling designers to perform most of their optimization in the software. As a result, design time was cut in half over that of previous methods.