Envelope Tracking for Cellular RF Power Amplifiers

Mobile device functionality has evolved dramatically over the past decade and continues to expand with applications such as social media, music and video streaming, gaming, cloud storage, and connectivity with other devices.

The redefined user experience requires high data rates offered by long term evolution (LTE) technology. High data rates, achieved with complex RF modulation and higher average output power, reduce the efficiency of traditional RF power amplifiers (PA) to unacceptable levels, both for the thermal dissipation and the battery lifetime of existing smartphone designs. Envelope tracking (ET) is a new power management technology for RF PAs that increases efficiency, minimizing heat generation, and extending battery life. High data rates with longer battery lifetime increases the overall LTE user experience.

 

LTE impact on RF PA efficiency

LTE requires the RF PA to transmit at 9 dB to 15 dB higher power levels to maintain sufficient energy per bit. Despite increased LTE spectral efficiency, the larger amount of transmitted data also requires more power.

The existence of more than 20 distinct LTE bands increases RF frontend complexity for switch, filter, and tuner networks. PA-to-antenna losses are increased, which in turn requires more PA output power. Up-link carrier aggregation further increases complexity for future RF front-ends, continuing the upward PA output power trend. The large number of LTE bands requires multi-mode multi-band (MMMB) PAs, which are less efficient than single-band PAs.

LTE signals have a very high peak-to-average power ratio (PAR). LTE uses single-carrier frequency division multiple access (SC-FDMA) for uplink communication. SC-FDMA modulation PAR (6-7dB) is higher than that of W-CDMA (3-4 dB) and GSM (0dB). Note that HSPA can have high PAR in some cases, making for similar PA efficiency challenges.

RF PA transistors optimized for high PAR and high power are much less efficient at reduced power levels, for example, when the user is close to a base station or during low data-rate transmission. This inefficiency results in increased PA heating and reduced battery lifetime, compared to 3G and 2G legacy systems. Envelope tracking is an RF PA power management technology that can improve system efficiency at high power levels with high-PAR signals in any band or with any LTE bandwidth.

 

ETPS

Average power tracking (APT) is a widely-implemented approach to reduce unnecessary power consumption in RF PAs. An efficient DC/ DC converter connected between the battery and PA supply voltage (PAVCC) dynamically changes PAVCC based on the PA average output power. When the PA output power is below maximum the PA supply voltage is reduced and improves PA efficiency. Adjustments in PAVCC occur whenever average output power changes. This can be as frequently as once per 3G transmit time slot or LTE frame. A high-conversion-efficiency DC/DC converter is required in order to achieve the lowest system-level current consumption.

Unfortunately, APT does not address the key challenges of LTE transmission: high PAR and high average output power. This limitation exists because the average PAVCC at full output power cannot be reduced without sacrificing linearity. Envelope tracking uses a dynamic PAVCC, which tracks the RF modulation amplitude (the instantaneous output power level) instead of the average output power level. An envelope-tracking power supply (ETPS) is used as a dynamic power supply for the RF PA, adjusting PAVCC at the speed of 3G/LTE modulation and optimizing RF PA efficiency for every point of time. Thus, ET improves efficiency for high-PAR modulation at high average output power. An ETPS significantly reduces worst-case PA heating, recovers PA linearity (enhancing ACLR), and raises maximum average output power capability due to the enhanced PA efficiency and linearity.

Figure 1 shows the PAVCC for an RF PA at full power (left), and reduced output power (right). The reduction in PAVCC correlates with the PA efficiency in each case.

 

Figure 1: Power amplifier supply voltage for fixed voltage, APT and ET

 

Envelope-tracking benefits vs. average power tracking

Higher efficiency

The primary benefit of ET is an increase in PA efficiency. For example, system efficiency improvement of more than 23% (from 30% APT to 39% ET) is achieved at +28 dBm PA average output power (Figure 2). The efficiency benefit, or lift, extends to average output power levels as low as +20 dBm today. As system components continue to improve, ET lift will be realized at even lower average output power levels. Figure 2a shows a system efficiency measurement using a 3.8V battery voltage and a 25RB QPSK LTE signal with a prototype ET MMMB RF PA operating in LTE band 1. ET operation at +28 dBm reduces current from a 3.8V battery by more than 125 mA compared to APT operation.

 

Figure 2: Measurements of ET and APT system efficiency (a) and linearity (b)

 

Heat reduction

In the same measurement used for Figure 2, the PA operating temperature is reduced by 20°C (Figure 3). The reduction in PA heating significantly eases the thermal design of thin small form factor devices such as phones and tablets.

 

Figure 3. PCB temperature measured with thermal camera for APT and ET at POUT = +28 dBm

 

High output power levels

Some ET power supplies such as TI’s LM3290/91 provide optimized supply voltage to PA, even when PAVCC is above battery voltage. Figure 4 shows that ET operation enables 3 dB higher PA output power than APT. Therefore, relaxation of transmit power requirements, known as maximum power reduction (MPR), is no longer needed. By avoiding MPR the ET system can maintain maximum data rates in all situations, even with low battery voltage. This capability is becoming increasingly important with emerging low-voltage batteries and more complex (higher loss) RF front-ends.

Figure 4 shows a system efficiency measurement using 3V battery voltage and 25RB QPSK LTE signal. A prototype ET MMMB PA is used

 

Figure 4: ET and APT system efficiency compared

 

Reduced receive-band noise

In frequency division duplex (FDD) systems like FDD LTE transmit and receive paths operate simultaneously, making it critical to ensure that out-of-band noise generated by the transmitter does not degrade the receiver’s sensitivity. Although transmit and receive circuits are separated in frequency and isolated by a duplex filter, there is still coupling (often on the order of –50 dB) between transmit and receive paths, requiring limits on noise at the PA output. Typical receive-band noise (RxBN) at the PA output should be below –130 dBm/Hz.

The added complexity of envelope signal computation and noise from ETPS complicates achieving good RxBN. RxBN in ET operation can be higher or lower than in APT operation, depending on LTE operating conditions, given a sufficiently low-noise ETPS. Figure 5 shows a measurement with 25RBs Band1 for high power levels. ET noise level is only 2-3 dB higher than APT noise. The overall ET noise is below –130 dBm/Hz, meeting typical ET system requirements.

 

Increased linearity

An ET system with a high-fidelity, high-peak voltage ETPS can improve PA ACLR performance (Figure 2). If not needed, the excess ACLR performance can be easily traded for improved system efficiency, or it can be used to maintain acceptable ACLR if it is degraded, for example, due to antenna mismatch.

 

Figure 5: PA output noise with ET and APT

 

ET versus APT: system-level implementations

APT and ET systems are conceptually similar. Both consist of a chipset, PA power management component, and an RF PA (Figure 6). Both adjust RF PA supply voltage levels over time. But ET operates at the bandwidth of RF modulation causing different system-level requirements. This section presents the key blocks and their requirements in an ET system versus an APT implementation.

 

ET System

An ET system comprises a transceiver supporting ET, PAs optimized for ET, and an ETPS.

A transceiver supporting ET must generate 3G/LTE RF signals while simultaneously providing a corresponding envelope reference signal to the ETPS. The ETPS supplies the PA supply voltage PAVCC. Due to the high bandwidth of the envelope reference signal, it cannot be transmitted using the MIPI RFFE interface as in an APT system. Instead, a differential analog interface called eTrakTM is employed

(Figure 6). eTrak is a new MIPI® Alliance standard for connecting ET-capable transceivers to EMs, and is being adopted by major platform suppliers. The transceiver must maintain an optimum timing alignment between the envelope signal and RF signal paths to prevent degradation of PA linearity and output noise.

Envelope-tracking PAs are different from average power tracking PAs. While one can try ET with normal APT PAs, such attempts suffer from performance limitations because PAs were never designed to operate in ET mode. High-speed PAVCC modulation requires low PA supply network capacitance to prevent nonlinearity and reduced efficiency. Many ET PAs have fairly flat insertion phase and gain dependency on PAVCC. This minimizes signal distortion, which can impact EVM, ACLR and RxBN. Alternatively, phase and gain variation may be compensated with pre-distortion techniques. Third, the linear gain of an ET PA must be increased with respect to the APT-optimized PA, since the ET PA is operated in compression throughout most of the output power range to increase PA efficiency. Compression increases the susceptibility of PA output to PAVCC noise feedthrough. As a result, a very low-noise PA supply is required to keep PA output noise low. These properties do not exist in APT PAs.

 

Figure 6: Simplified APT (left) and ET (right) block diagrams

 

The relationship between PA supply voltage and input RF power impacts many system-level performance metrics. For example, a higher ratio of PA supply voltage to RF input power leads to increased PA gain, reduced PA supply noise sensitivity, and impacts ET linearity. Therefore, ET system performance is not just a function of the PA. Instead, it is a function of all system components and the signal processing method used to generate the PAVCC waveform.

The ETPS adjusts the PA supply voltage constantly. This is unlike an APT DC/DC, which adjusts the PA supply voltage only when the average output power level changes. This causes significant differences and design challenges in modulator bandwidth, output noise, and efficiency.

 

Bandwidth

To accurately track the amplitude of RF signal modulation without introducing distortion requires a power supply with one to two times the bandwidth of the RF modulation. The envelope bandwidth depends on the actual transmitted signal. For example, the required 20 MHz to 40 MHz LTE signal bandwidth is more than 200 times the bandwidth of APT DC/DC converters.

 

ETPS output noise

The ET PA operates in compression, has very small supply bypass capacitance, and is more sensitive to supply noise than an APT PA. To meet RxBN system requirements of –130 dBm/Hz, ETPS output noise needs to be below –135 dBm/Hz (referenced to a 50-Ohm system). This is a very stringent, but feasible design challenge for the ETPS. In an APT system, the RF DC/DC converter noise requirements are more relaxed as a large bypass capacitance on APT PA supply attenuates PA supply noise.

 

Efficiency

The high-bandwidth and low-noise characteristics of an ETPS must be combined with high efficiency. While an APT DC/DC can reach 95% efficiency levels, ETPSs operate with reduced conversion efficiency to achieve higher bandwidth and lower output noise performance requirements. Reduced ETPS efficiency (compared to the APT DC/DC converter) is compensated on the system level by increased PA efficiency in ET operation (versus the PA in APT operation). ETPS efficiency in the range of 80%-90% is required.

 

Envelope-tracking power supply supports average power tracking

ET is an efficient method of powering RF PAs when the transmitted signal has high average output power and high PAR. For lower average PA output power levels APT becomes a more efficient option. Therefore, an ETPS must support two operating modes: ET and APT, as shown in Figure 7.

 

Figure 7: ET and APT operating modes of an EM

 

In a dual-mode ETPS the ability to switch between APT and ET operation seamlessly is critical; otherwise, the RF output signal will be distorted when changing operating modes. Figure 8 shows a seamless transition from APT to ET and back to APT mode using Texas Instruments’ LM3290/91 ETPS. The chipset controls the transitions via the MIPI RFFE interface to ensure system synchronization to RF signal and transmission frames or slots.

 

Figure 8: EM mode changes APT-ET-APT

 

Conclusion

Envelope tracking is a new power management technology that significantly improves the PA efficiency of LTE transmitters. ET provides longer battery life and significantly reduced PA operating temperature. Additional benefits include improved linearity (ACLR), increased output power capability and – with boost capability – elimination of MPR at low battery voltages. At low PA output power levels high-performance average power tracking is a necessary feature for best overall system efficiency. Therefore, a dual-mode ET/APT RF PA power management solution is needed.

Implementation of ET requires a complete ecosystem: An ET-capable chipset, an ET-optimized PA and an efficient, low-noise and high-bandwidth ETPS that supports both APT and ET modes. The system components are already available from leading LTE component vendors such as the LM3290/91 ETPS from Texas Instruments.

 

About the Author

Juha Pennanen received his M.Sc. (EE) with honors Field Of StudyTelecommunications, Analog & RF IC design at the University of Oulu. He worked as a System Engineering Manager at Texas Instrument.

 

References

• Download for the datasheets: LM3290, and LM3291.

 

This article originally appeared in the Bodo’s Power Systems magazine.