Texas Instruments Linearization Fundamentals Driving Digital Pre-Distortion and the GC5322!

1. Texas InstrumentsLinearization Fundamentals Driving Digital Pre-Distortionand the GC5322! April 2006

2. Agenda • Introduction and Impact • Origin and History of the Problem • Linearization Fundamentals • Polynomial Power Amplifier Modeling • Crest Factor Reduction • Digital Pre-Distortion • System Implementation • Crest Factor Reduction and Digital Pre-Distortion • Adaptive Memory Pre-distortion of Power Amplifiers • Conclusions

3. Introduction and Impact 4G Cellular & WiMAX 3G – Digital Wideband Cellular 2G - Digital Cellular 1G - Analog Cellular 1980 1985 1990 1995 2000 2005 2010 2015 • The demands for spectrally efficient modulation schemes have increased; however these schemes are subject to severe intermodulation distortion (IMD) when the power amplifiers (PA) are operated near saturation • Unfortunately, PAs are most efficient when operated near saturation 20MHz BW@ 2.1GHz Super 3G & 4G 10-40MHz BW @ 2.5, 3.5 & 5GHz WiMAX .16a/d/e 5MHz BW@ 2.1GHz WCDMA Cellular Channel BW @ Band Increased signal bandwidth and complexity 1.25MHz BW @ 1.9GHz CDMA 2000 A big challenge forMCPA designers! 200kHz BW @ 800MHz EDGE/ CDMA <=200kHz BW @ 8-900MHz TDMA/ GSM 30kHz BW @ 800MHz AMPS/ D-AMPS

4. Introduction and Impact • High Power RF PA’s (>10W) use multiple driver stages to amplify an input signal. • Different PA architecture’s (Class A, AB, C, etc …) offer various degrees of linearity, cost and efficiency. • RF PA’s are notoriously inefficient – Air is a convenient but poor transmission medium. • RF PA’s are designed (tuned) for specific frequency range and bandwidth • MCPA ~= wideband RF PA, does not have to process multiple carriers • PA Gain is usually fixed – so pre-amps may be required to drive the PA input. TX Board Antenna RFout =50dBm(100W) RFin ==20dBm@ >800MHz FromBaseband PA DUC DAC IF->RF A 50 OhmTypicalInput 3 to 4 gain stages typical If Gain = 30dB, Pre-Amp

5. Introduction and Impact • Linearization techniques allow a PA to be operated at higher power with minimal IMD increases, thus greater efficiency • Recent technological advances have made digital pre-distortion the focus of research efforts • Crest factor reduction (CFR) further increases the efficiency of the PA by reducing the peak-to-average ratio (PAR) of the transmitted signal Theoretical Performance of Class AB PA

6. Origin and History of the Problem 1. Linearization Fundamentals • The trade-off between efficiency and linearity is the primary concern for PA designers • A PA operating at a high percentage of its power rating requires external linearization to maintain linearity • The linearization of the PA reduces back-off, thus increasing efficiency

7. Origin and History of the Problem 2. Polynomial Power Amplifier Modeling • Accurate representation of the nonlinear effects in PAs is achieved using a polynomial expression, as follows • The coefficients represent the linear gain, and the gain constants for the quadratic and cubic nonlinearities • A system with memory (phase) versus memory effects (non-linearities) • Envelope and frequency memory effects

8. Origin and History of the Problem 2. Power Amplifier Characterization • Two tone test is useful for measuring spectral regrowth in a nonlinear and memoryless system

9. Origin and History of the Problem 2. Power Amplifier Characterization • Theoretically, only odd-degree nonlinearities generate in-band distortion products • The simplified polynomial PA model is expressed as follows

10. Origin and History of the Problem 2. Power Amplifier Characterization • A PA is often characterized by its amplitude-amplitude and amplitude-phase transfer characteristics • The simple polynomial is unable to model AM-PM effects • Both AM-AM and AM-PM effects are represented by the complex baseband model where

11. Origin and History of the Problem 2. Power Amplifier Characterization • A simple case considering only 3rd degree nonlinearities in the AM-AM and AM-PM transfer characteristics is represented by the following • In the linear range, the PA can be characterized by the following and

12. Origin and History of the Problem 2. Power Amplifier Characterization AM-AM Characteristic AM-PM Characteristic

13. Origin and History of the Problem 3. Crest-Factor Reduction • The DPD optimal performance depends greatly on signal characteristics • Multi-carrier signals can have a PAR as high as 13dB increasing the level of back-off to maintain acceptable IMD levels • The application of CFR allows the PA to operate at higher input/output power levels while maintaining linearity at the output of the PA • Achieved through pulse generation and digital clipping

14. Origin and History of the Problem 3. Crest-Factor Reduction • Preferred PA bias point for a typical modulated signal

15. Origin and History of the Problem 3. Crest-Factor Reduction • Preferred PA bias point for a CFR signal

16. Origin and History of the Problem 4. Digital Pre-Distortion • Pre-distortion effectively performs a mathematical inversion of the Volterra PA model • The output of the pre-distortion processor is described by the following • The PA is linearized when

17. Origin and History of the Problem 4. Digital Pre-Distortion • Digital pre-distortion (DPD) has become an effective linearization technique due to the renewed possibilities offered by DSP • Adaptive PD designs use feedback to compensate for PA variations • Look-up tables are updated to achieve optimal pre-distortion by comparing PD input to PA output • The PD function is expressed as a complex polynomial where

18. Origin and History of the Problem 4. Digital Pre-Distortion • Digital pre-distortion (DPD) requires feedback for sample-by-sample adaptation 5 times that of the signal bandwidth • Multi-carrier systems use signal bandwidths of up to 20MHz today, thus the feedback bandwidth must be 100MHz to compensate 3rd and 5th order IMD • Least-mean-square (LMS) is a popular gradient based optimization algorithm that requires wideband feedback

19. System Implementation 1. Crest-Factor Reduction and Digital Pre-Distortion • The combination of CFR and digital pre-distortion were investigated • In this case, linearization was achieved with a traditional wideband feedback LMS algorithm • The CFR technique used was proposed by Texas Instruments using the GC1115 signal pre-processor • Four stages ensure that the output PAR is reduced to values from 5 to 8dB, as specified by the user • Performance results were compared using a Cree Microdevices 30W PA operating at 1.96GHz and a signal bandwidth of 1.25MHz • The PAR of the IS-95 signal was reduced from 9.6dB to 5dB

20. System Implementation 1. Crest-Factor Reduction and Digital Pre-Distortion Complex Canceling Pulse

21. System Implementation 1. Crest-Factor Reduction and Digital Pre-Distortion Corrected and uncorrected signal with canceling peaks and detection threshold

22. System Implementation 1. Crest-Factor Reduction and Digital Pre-Distortion Typical Peak Detection and Cancellation through Pulse Injection Cancellation Signal Input Signal Output Signal - +

23. System Implementation X PA PC 1. Crest-Factor Reduction and Digital Pre-Distortion • Hardware Implementation of Wideband Pre-Distortion Waveform Generator Attenuator Down-Converter Analog RF Agilent 4432B ~20dB DUT Pre-Distorted Input Signal LO Tektronics TDS224 Oscilloscope Analog IF

24. System Implementation 1. Crest-Factor Reduction and Digital Pre-Distortion ACPR improvement with respect to output power

25. System Implementation 1. Crest-Factor Reduction and Digital Pre-Distortion • The ACPR measurements were recorded according to specifications with a 30kHz marker at and offset of 885kHz • Results were limited by the performance limitations of the test bed Power and efficiency improvement

26. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • The term memory effects refer to the bandwidth-dependant nonlinear effects often present in PAs. • These encompass envelope memory effects and frequency response memory effects. • Envelope memory effects are primarily a result of thermal hysteresis and electrical properties inherent to PAs. • Frequency memory effects are due to the variations in the frequency spacing of the transmitted signal and are characterized by shorter time constants.

27. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Memory Polynomial Pre-Distortion Implementation Where (K=7) And (D=2)

28. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Simulated Performance of Wideband Pre-Distortion • This traditional approach uses and LMS algorithm to adapt the PD coefficients on a sample-by-sample basis. • The memory PA model has D=1 (delay) and K=5 (order).

29. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Simulated Performance of Wideband Pre-Distortion • The memory PA model is characterized by the following AM-AM and AM-PM curves

30. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Simulated Performance of Wideband Pre-Distortion • DPD = 0: the LMS algorithm indicates an ACPL improvement of -3dB and an ACPH improvement of 3dB. • DPD = 1: the LMS algorithm indicates an ACPL improvement of -15dB and an ACPH improvement of -11dB.

31. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Simulated Performance of Wideband Pre-Distortion • DPD = 2: the LMS algorithm indicates an ACPL improvement of -24dB and an ACPH improvement of -23dB. • DPD = 3: the LMS algorithm indicates an ACPL improvement of -24dB and an ACPH improvement of -20dB.

32. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Hardware Implementation of Wideband Pre-Distortion • TI offers the complete high-performance signal chain including: DAC5687, CDCM7005, TRF3761, ADS5444, and TRF3703.

33. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers Typical Doherty Amplifier configuration and Performance Results

34. System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers • Hardware Implementation of Wideband Pre-Distortion

35. Conclusions • CFR improves DPD performance • CFR uses EVM and ACLR to tradeoff for added efficiency • Depending on modulation schemes the relative percentages may vary • OFDM modulations are sensitive to EVM • 3GPP modulations are sensitive to ACLR 3GPP Relative Tradeoffs OFDM Relative Tradeoffs EVM ACLR EVM ACLR Efficiency Efficiency

36. Conclusions • Relative to a PA that operates normally under backoff, DPD adds additional hardware (cost) and system complexity to tradeoff for added efficiency • DPD can effectively remove the negative effects of CFR enabling even greater levels of efficiency DPD Relative Tradeoffs Cost Complexity DPD EVM ACLR CFR+DPD CFR+DPD Efficiency

37. Questions