Envelope Tracking basics tutorial | Amplifier, Power supply, Advantages, Disadvantages
This envelope tracking tutorial covers basics of envelope tracking. The concept is explained with envelope Tracking Power supply used for Envelope Tracking Power Amplifier to provide power supply according to the amplitude of the time domain input waveform. Advantages and disadvantages of envelope tracking and applications are also mentioned.
About Envelope Tracking: The term envelope tracking is a power supply technology. The system trackes input of the amplifier and adjusts the supply voltage to be fed to the amplifier to obtain required output power at any instant with optimum peak efficiency.
Constant DC supply voltage is used in conventional RF amplifiers and amplifiers usually operate in compression region to achive higher efficiency. This is suitable when amplitude of the amplifier signal input is not varying which is possible in frequency and phase modulated signals. In FM and PM modulation types, amplitude of the modulated signal is constant.
In the modern wireless communication systems such as 4G/LTE and higher versions of WLAN such as 802.11ac and 802.11ad, complex modulation schemes such as QAM is employed along with multi-carrier OFDM techniques to achieve higher data rate. In this situation, it is not desirable to operate amplifier in compression region as it leads to clipping of waveform amplitudes. This clipping results into loss of information which is carried by amplitude part of time domain envelope.
Hence in such condition running amplifier in compression region option is ruled out. In this case, supply voltage is fed as per time domain input waveform to PA (Power Amplifier), i.e. high supply is used during maxima points and low supply during minima points. This results into peak efficiency of the Power Amplifier as desired. Let us understand basic concept of envelope tracking with its system block diagram.
4G LTE Envelope Tracking Power Amplifier | Envelope Tracking Power Supply
The figure-1 depicts envelope tracking system block diagram. As shown it consists of baseband IQ transmitter, envelope detector, shaping table as per LUT, RF upconverter, PA (Power amplifier) and envelope tracking power supply.
The figure-2 depicts waveform without envelope tracking. As shown DC supply voltage do not follow RF power ampplifier input and hence there will be gap between supply and amplifier signal input. This results into unnecessary voltage being pumped into amplifier for its operation. This leads to wastage of power and consecutively leads to lower efficiency of the PA (Power Amplifier). This is because, efficiency is expressed as the ratio of output power of the amplifier with DC power input.
To avoid this situation, circuit as shown in the figure-1 is used. In envelope tracking, power supply input is constantly varied according to the envelope of the input power goint to PA. Due to envelope tracking, only appropriate power supply is fed at the input as required, it makes amplifier operate at peak efficiency for the desired instantaneous power output specifications.
Both DC supply voltage and amplifier input should be time synchronized in order to obtain alignment otherwise there will be mismatch between supply needed and amplifier input. This is achieved by providing trigger input to the RF VSG from the same AWG used to generate variable supply voltage for the power amplifier.
As we know that due to AM-AM and AM-PM distortion, PA do not have linear response always. This results into system not meeting EVM and ACPR specifications as desired. Due to this, pre-distortion compensation is essential. This is acheived using LUT based approach. The figure-3 depicts the DPD (Digital Pre Distortion) LUT concept. As shown, LUT coefficients are derived based on estimated PA nonlinear response. As LUT output response negates the nonlinear response of the PA, linear response can be achieved.
Keysight provides test equipments which can be used for testing the envelop tracking concept. The models 33500 or 33600 series are used as AWG. AWG is baseband IQ generator. X-series generator or PXIe-M9381A is used as VSG (Vector Signal Generator). The output of the ETPA (Envelope Tracking Power Amplifier) can be measured using M9391A PXIe VSA hardware and software application (89600 VSA).
let us go through advantages and disadvantages of this technique as part of the envelope tracking tutorial.
Benefits or Advantages of Envelope Tracking
Following are the advantages of envelope tracking:
• Improves Efficiency of amplifier
• Increases battery life as battery drains slowly
• Reduces running costs of wireless system infrastructure due to improvement in efficiency.
• Meets ACPR and EVM requirements of complex systems such as 4G LTE, LTE Advanced 5G having complex OFDM/OFDMA multi-carrier system along with QAM modulation types such as 1024-QAM, 2048-QAM etc.
Disadvantages of Envelope Tracking
Power Supply used in the envelope tracking should meet following requirements.
• Bandwidth (BW) of the supply should be 2 to 3 times than modulation bandwidth of RF signal. Due to this 4G LTE system needs supply BW of about 50 MHz.
• Supply output should have very low noise as this directly affects the adjacent channel noise. This causes noise in the receive band which masks the wanted signal.
• It should have higher efficiency otherwise gains in using envelope tracking is lost.
• Other than the above power supply requirements, Supply control signal and RF input envelope should be time aligned to achive perfect envelope tracking. To achieve time synchronization, delay balancer is introduced in the chain.
• Above mentioned rigorous envelope tracking power supply requirements, are essential in envelope tracking power amplifier based wireless system.
It is used in wide variety of applications.
• 4G LTE low power Mobile phone handsets
• High power cellular Base Stations or eNodeBs or eNBs
• RF power Amplifiers
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