1. Slide 1 1 April 2006 Texas Instruments Linearization Fundamentals Driving Digital Pre-Distortion and the GC5322!

2. Slide 2 PRELIMINARY Subject to Change 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. Slide 3 PRELIMINARY Subject to Change 3 Introduction and Impact  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 1980198519901995200020052010 AMPS/ D-AMPS TDMA/ GSM EDGE/ CDMA 30kHz BW @ 800MHz CDMA 2000 WCDMA WiMAX.16a/d/e Super 3G & 4G 1G - Analog Cellular 2G - Digital Cellular 3G – Digital Wideband Cellular 4G Cellular & WiMAX 2015 <=200kHz BW @ 8-900MHz 200kHz BW @ 800MHz 1.25MHz BW @ 1.9GHz 5MHz BW @ 2.1GHz 10-40MHz BW @ 2.5, 3.5 & 5GHz 20MHz BW @ 2.1GHz Cellular Channel BW @ Band Increased signal bandwidth and complexity A big challenge for MCPA designers!

4. Slide 4 PRELIMINARY Subject to Change 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. PA TX Board If Gain = 30dB, RFout = 50dBm (100W) RFin == 20dBm @ >800MHz 3 to 4 gain stages typical DACIF->RFDUC From Baseband 50 Ohm Typical Input Antenna A Pre-Amp

5. Slide 5 PRELIMINARY Subject to Change 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 Pre-DistortionNo Yes CFRNoModerate Yes Tx Power10W PAR12dB9dB 6dB Backoff15dB12dB9dB6dB PA Power Rating320W160W80W40W Efficency5%9%18%30% Power Dissipation120W101W45W7W Theoretical Performance of Class AB PA

6. Slide 6 PRELIMINARY Subject to Change 6 Origin and History of the Problem  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 1. Linearization Fundamentals

7. Slide 7 PRELIMINARY Subject to Change 7 Origin and History of the Problem  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 2. Polynomial Power Amplifier Modeling

8. Slide 8 PRELIMINARY Subject to Change 8 Origin and History of the Problem  Two tone test is useful for measuring spectral regrowth in a nonlinear and memoryless system 2. Power Amplifier Characterization

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

10. Slide 10 PRELIMINARY Subject to Change 10 Origin and History of the Problem  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 2. Power Amplifier Characterization where

11. Slide 11 PRELIMINARY Subject to Change 11 Origin and History of the Problem  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 2. Power Amplifier Characterization and

12. Slide 12 PRELIMINARY Subject to Change 12 Origin and History of the Problem 2. Power Amplifier Characterization AM-AM CharacteristicAM-PM Characteristic

13. Slide 13 PRELIMINARY Subject to Change 13 Origin and History of the Problem  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 3. Crest-Factor Reduction

14. Slide 14 PRELIMINARY Subject to Change 14 Origin and History of the Problem  Preferred PA bias point for a typical modulated signal 3. Crest-Factor Reduction

15. Slide 15 PRELIMINARY Subject to Change 15 Origin and History of the Problem  Preferred PA bias point for a CFR signal 3. Crest-Factor Reduction

16. Slide 16 PRELIMINARY Subject to Change 16 Origin and History of the Problem  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 4. Digital Pre-Distortion

17. Slide 17 PRELIMINARY Subject to Change 17 Origin and History of the Problem  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 4. Digital Pre-Distortion where

18. Slide 18 PRELIMINARY Subject to Change 18 Origin and History of the Problem  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 4. Digital Pre-Distortion

19. Slide 19 PRELIMINARY Subject to Change 19 System Implementation  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 1. Crest-Factor Reduction and Digital Pre-Distortion

20. Slide 20 PRELIMINARY Subject to Change 20 System Implementation Complex Canceling Pulse 1. Crest-Factor Reduction and Digital Pre-Distortion

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

22. Slide 22 PRELIMINARY Subject to Change 22 System Implementation Typical Peak Detection and Cancellation through Pulse Injection Input SignalOutput Signal Cancellation Signal + - 1. Crest-Factor Reduction and Digital Pre-Distortion

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

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

25. Slide 25 PRELIMINARY Subject to Change 25 System Implementation Power and efficiency improvement  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 1. Crest-Factor Reduction and Digital Pre-Distortion

26. Slide 26 PRELIMINARY Subject to Change 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. Slide 27 PRELIMINARY Subject to Change 27 System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers  Memory Polynomial Pre-Distortion Implementation And (D=2) Where (K=7)

28. Slide 28 PRELIMINARY Subject to Change 28 System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers  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).  Simulated Performance of Wideband Pre-Distortion

29. Slide 29 PRELIMINARY Subject to Change 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. Slide 30 PRELIMINARY Subject to Change 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. Slide 31 PRELIMINARY Subject to Change 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. Slide 32 PRELIMINARY Subject to Change 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. Slide 33 PRELIMINARY Subject to Change 33 System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers Typical Doherty Amplifier configuration and Performance Results

34. Slide 34 PRELIMINARY Subject to Change 34 System Implementation 2. Adaptive Memory Pre-distortion of Power Amplifiers  Hardware Implementation of Wideband Pre-Distortion

35. Slide 35 PRELIMINARY Subject to Change 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 EVM Efficiency ACLR 3GPP Relative Tradeoffs EVM Efficiency ACLR OFDM Relative Tradeoffs

36. Slide 36 PRELIMINARY Subject to Change 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 Cost DPD Complexity DPD Relative Tradeoffs EVM Efficiency ACLR CFR+DPD

37. Slide 37 PRELIMINARY Subject to Change 37 Questions