Design Optimization of Envelope Tracking Power Amplifiers (ETPAs) for 5G LTE Using NI PXI, VST, LabVIEW, and NI AWR Microwave Office™

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"Similar test benches have been built in the past and have taken around one year or more to complete. With NI’s excellent integration and the use of LabVIEW, we were able to complete the NI ET test bench in less than two months."

- Jonmei Johana Yan, Ph. D, MaXentric Technologies, LLC

The Challenge:
Developing a flexible envelope tracking power amplifier (ETPA) test bench capable of real-time efficiency and linearity measurements to optimize design and accommodate different 5G signals.

The Solution:
Using NI LabVIEW software to design and optimize the ETPA with the NI Vector Signal Transceiver (VST) for RF signal generation, using NI arbitrary waveform generator (AWG) technology for envelope signal generation, and using NI AWR Microwave Office for onboard retuning and final optimization.

Author(s):
Jonmei Johana Yan, Ph. D - MaXentric Technologies, LLC
Paul Theilmann, Ph. D - MaXentric Technologies, LLC
Donald F. Kimball - MaXentric Technologies, LLC

About MaXentric Technologies

MaXentric Technologies, LLC is a specialty R&D firm that provides product design, development, and manufacturing services for the military defense and telecommunications/broadcast commercial markets.

The company’s products range from simple, low-cost millimeter broadband wireless transceivers to passive RFID readers to a high-efficiency ETPA. Typical applications include intelligence, surveillance and reconnaissance (ISR) components; high-bandwidth wireless communications; electronic warfare, and broadband, high-efficiency PAs.

Challenges With PAs and 5G

The need to address the crowded frequency spectrum allocation and increasing demand for higher data rates and larger signal bandwidth, while maintaining signal integrity has led to the use of signals with nonconstant envelope and high peak to average power ratio (PAPR). High linearity is required in these communication systems to minimize signal distortion, reduce bit error rate, improve spectral efficiency, and reduce adjacent channel interference. The PA efficiency determines the radio’s power consumption, size, and battery lifetime. Various techniques such as envelope tracking (ET) have been explored to increase the efficiency of PAs for high PAPR signals such as orthogonal frequency division multiplexing.

Simply put, ET technology allows the operator to use only as much power as necessary to provide the amplified output. With this technology, energy consumption can be reduced, which lowers operating costs significantly while providing environmental sustainability. In addition, from the hardware system perspective, this means a smaller form factor, high reliability due to lower junction temperatures, and much lower weight due to the reduced battery and energy requirement.

Advantages of NI PXI, VST, and LabVIEW

Traditional test benches are made to test PAs with a constant supply voltage.  In ET, a modulator is used as a dynamic power supply that varies as a function of the signal’s envelope. This technique is deliberately designed for signals with high PAPR; traditional PA tests cannot be used to optimize performance.  The average performance for a modulated signal is what needs to be evaluated in an ETPA (as opposed to the continuous wave performance at the peak supply voltage). The problem is further complicated by the lack of accurate models over a wide range of supply voltages.  Most device models are valid at the nominal constant supply voltage ±10 percent; but in ET, the supply voltage can be 90 percent lower than the peak supply voltage.  Hence, real-time performance measurements are highly desired for optimizing the ETPA.  

Using NI PXI and AWG hardware along with the VST for ET offers many benefits. Similar test benches have been built in the past and have taken around one year or more to complete. With NI’s excellent integration and the use of LabVIEW, we were able to clabomplete the NI ET test bench in less than two months.  An important feature that PXI offers is the ease of synchronization between the equipment modules. Due to the nature of ET, the supply envelope signal must arrive at a specific time with respect to the RF signal. Additionally, 5G PAs need to support various types of modulations, which the VST can easily generate. Another advantage to the VST is the wide, tunable RF frequency it offers (65 MHz to 6 GHz).  This covers most of the (non-mmWave) LTE bands of interest with 200 MHz of instantaneous bandwidth, which allows the system to be flexible for various applications in addition to LTE.  Because ET is inherently wideband in terms of tunable RF bandwidth, a wideband ETPA with this NI ET test bench can be used to test various LTE bands and GPS and military applications all on the same day with simply a click to change the RF frequency. 

Optimizing With NI PXI and VST

To develop a test bench for optimizing ET, we used the various modules described in Figure 1. We used power SMUs to allow for real-time DC power-consumption measurements, and we used RF power meters to monitor the input and output power. In LabVIEW, we put these measurements together and made calculations to allow for instantaneous monitoring of the efficiency, gain, and output. The envelope signal was generated using the AWG. The VST served as the RF signal generator as well as the RF feedback analyzer. Using the feedback signal, digital predistortion was used to improve the linearity of the ETPA using LabVIEW MathScript. The envelope-shaping relationship between the true envelope of the signal and the supply voltage was optimized easily by simply loading a different equation. We could implement impedance tuning for best performance by load pulling and source pulling with external tuners. NI AWR Microwave Office was used to apply the optimized tuning to the onboard ETPA. The reduced characterization time and ability to optimized the ETPA in real time are significant game changer for PA designers.  

Figure 1. Block Diagram of the NI PXI ETPA Test Bench

Figure 2. Time Alignment Between RF Signal and Envelope Supply

The time alignment of the amplitude signal and RF signal at the RF transistor is critical for optimizing ETPA performance. A time misalignment in these two signals produces signal distortion, degrades ACPR performance, and reduces efficiency.

The characterization of the time-delay difference between these two signal paths allows for time alignment. This delay difference may vary with temperature or aging; therefore, the system needs to compensate for this variation to ensure optimum performance. Using NI’s PXI, VST, and AWG hardware and LabVIEW software, we were able to visually see the improvement/degradation in linearity and efficiency as the alignment between the RF signal and the envelope supply was altered in real time.  

Results of ET Using PXI and VST

We used the VST and PXI hardware to optimize our LTE Band 1 (2.14 GHz) PA 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. It can output 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 a variety of PA technologies such as LDMOS, GaN, GaAS, and so on.

In this example, a GaN device was used for the PA design. The PA was tuned and optimized for ET operation using the NI PXI system. Initially, output and input external tuners were used to optimized the efficiency, gain, and output power of the ETPA. The desired input and output impedances were measured using a vector network analyzer, and the tuning was folded back onboard via simulations performed with the NI AWR Microwave Office. 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.  

Figure 3. Retuning of the ETPA Using the NI AWR Microwave Office

Without linearization, the ETPA achieved 6 W of output power with 11.5 dB of gain and 53 percent power added efficiency (PAE) after applying MaXentric’s linearization (implemented in LabVIEW and LabVIEW MathScript). The output power and gain remained the same with a slight improvement in PAE to 54.6 percent with better than -45 dBc ACPR. To give some perspective of the improvement, in constant drain with the same GaN device, the PA would need to be operated in backoff to achieve the same linearity performance, resulting in low efficiency (<10 percent at 0.5 W output power without digital predistortion and about 25 percent at 2 W of output power with digital predistortion.

Figure 4. (a) MaXentric ETPA With MaXEA; (b) Spectra Before and After MaXPAL Linearization; (c) AMAM and AMPM Before MaXPAL Linearization; (d) AMAM and AMPM After MaXPAL Linearization

Conclusion

As a result of using the NI PXI and VST hardware and LabVIEW, we significantly reduced PA optimization time without sacrificing measurement accuracy. The flexibility and wide-tunable RF frequency offered by the VST allowed us to design, optimize, and demonstrate our wideband ETPAs across different bands and applications all on the same day, which has impressed our customers.  We have seen much interest in this NI ET test bench.

 

Author Information:
Jonmei Johana Yan, Ph. D
MaXentric Technologies, LLC

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