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Enabling high efficiencies Digital signal conditioning in modern RF transmitters

Enabling high efficiencies Digital signal conditioning in modern RF transmitters. Thomas BOHN June 2008. Agenda. 1. Overview 2. Peak-to-Average-Power-Ratio (PAPR) Reduction 3. Power amplifier linearisation 4. Image suppression and DC-offset compensation 5. Conclusions. 1. Overview

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Enabling high efficiencies Digital signal conditioning in modern RF transmitters

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  1. Enabling high efficienciesDigital signal conditioning in modern RF transmitters Thomas BOHN June 2008

  2. Agenda • 1. Overview • 2. Peak-to-Average-Power-Ratio (PAPR) Reduction • 3. Power amplifier linearisation • 4. Image suppression and DC-offset compensation • 5. Conclusions

  3. 1 Overview Architecture of a modern RF transmitter

  4. Conversion-Module PA-Module Digital-Module Image- & Carrier- Suppr. Modulation & Wide bandUp-conversion D/A- conv. Clipping Predistortion PA chain Wide band Down- conversion Time Delay Compens. A/D- conv. Transmitter architecture • Digital-in analogue-out RF amplifier • Signal conditioning right in front of the analogue PA chain • Easiest (cheapest) possible analogue design • Highest possible power efficiency

  5. 2 PAPR Reduction

  6. PAPR ReductionWhat is PAPR reduction ? Clipping ! • Reducing signal power peaks to a certain threshold • increasing the EVM Power @ Soft Clipping Power @ Hard Clipping

  7. PAPR Reduction Why do we perform PAPR reduction ? • PA has to be designedfor the signal peak • driving into saturationproduces intermodulation • PA delivers an averageoutput power • Black: • PAPR: 10.9 dB • 43dBm (12% efficiency) • Blue: • PAPR: 5.5 dB • 45.2 dBm (16% efficiency) Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes)

  8. 3 PA linearisation Digital predistortion for higher efficiencies

  9. PA linearisationHow to linearise a PA ? With (digital) predistortion ! • Principle of PA linearisation: • Amplitude distortions can be compensated up to the PA’s saturation point • Phase distortion can be compensated for arbitrary input amplitudes

  10. PA linearisationDigital predistortion - the PA • Complex baseband model for the PA: • Quasi-static nonlinear model (static in the complex baseband) • AM-AM and AM-PM distortions • Complex gain model: • x, y: Complex envelope of RF input and output signal • GPA(|x|) Complex gain depending on the magnitude of the input signal • G(|x|) Magnitude of the gain • F(|x|) Phase shift • ai Complex coefficients

  11. PA linearisationDigital predistortion - the linearisation • PA linearisation with inverse PA characteristic: • Linear PA transfer characteristics up to saturation point: • x, y: Complex envelope of RF input and output signal • M Constant real gain • Linearised PA transfer characteristics up to saturation point: • G(|x|) Magnitude of the gain • F(|x|) Phase shift • ai Complex coefficients

  12. PA linearisationDigital predistortion - the non-linearity (1) • The intervals over the input signal power: • Pout • PA transfer characteristic just for illustration • The “r” depicts the power of the current input sample in the 2nd interval • r1 r r2 r3 (max.) PIN

  13. PA linearisationDigital predistortion - the non-linearity (2) • In the interval :

  14. PA linearisationDigital predistortion - the non-linearity (3) • Block diagram of the PIPOP: • GPD calculation foreach input sample • Predistortion ofeach input sample • Issues to ease theimplementation: • f(x) (= 1 ?) • Base functionsgranularity • Interval widths(equal ?)

  15. PA linearisationDigital predistortion - the adaptation • Coefficient update formula: • For each of the four ( 0 k 3) coefficients within one interval • c - coefficient • d - step width of the algorithm •  - hermite base function • y’* - complex conjugated PD output signal (y’ is the delayed y) • e - error (y’ - w) •  - PD gain

  16. PA linearisationDigital predistortion - the system • Schematic view:

  17. PA linearisationEfficiency and spectral distortions in case of 1 carrier UMTS (1) • Blue: 46.0 dBm • 18.2% efficiency • out of spec • Black: 43.0 dBm • 11.9% efficiency • in spec • Green: 40.6 dBm • 8.35% efficiency • in spec Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR)

  18. PA linearisation Efficiency and spectral distortions in case of 1 carrier UMTS (2) • Green: 46.0 dBm • 18.2% efficiency • out of spec • Blue: 46.0 dBm • memory-less PD • in spec with 5dB margin Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR)

  19. PA linearisation Efficiency and spectral distortions in case of 1 carrier UMTS (3) • Green: 47.0 dBm • 20.4% efficiency • out of spec • Blue: 47.0 dBm • memory-less PD • in spec Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR)

  20. PA linearisation Efficiency and spectral distortions in case of 4 carrier UMTS (1) • Blue: 46.0 dBm • 17.8% efficiency • out of spec • Black: 31.7 dBm • 1.2% efficiency • out of spec • Green: 25.5 dBm • 0.3% efficiency • out of spec Set-up: PA rated for 46.0 dBm; 4 x 3GPP test model 1 (64 codes, 5.5 dB PAPR)

  21. PA linearisation Efficiency and spectral distortions in case of 4 carrier UMTS (2) • Green: 46.0 dBm • 17.8% efficiency • out of spec • Black • memory-less PD • out of spec • Green • PD with memory effectcompensation • in spec Set-up: PA rated for 46.0 dBm; 4 x 3GPP test model 1 (64 codes, 5.5 dB PAPR)

  22. 4 Image suppressionand DC-offset compensation

  23. LO(t) QM Modulator v DAC I r(t) k DCI DCQ 90°+p z(t) Q DAC Image suppression and DC-offset compensation Zero-IF transmitter and efficiency • Simple architecture with small amount of analogue components: • 1 analogue quadrature modulator (QM, also known as IQ modulator) • 1 local oscillator (LO) • Half of the signal bandwith @ DACs, Performance of DACs is best around 0 Hz • No frequency selective filters • But: QM has imperfections resulting in DC feed trough and image

  24. Image suppression and DC-offset compensation Spectral distortions in case of 1 carrier UMTS (1) • Green: 46.0 dBm • 18.2% efficiency • out of spec • ACP both • ALT low • Blue: • memory-less PD • out of spec • DC @ ACP low • Image @ ALT low Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR) @ 5MHz

  25. Image suppression and DC-offset compensation Spectral distortions in case of 1 carrier UMTS (2) • Green : 46.0 dBm • 18.2% efficiency • out of spec • Black: • memory-less PD • out of spec • Blue: • memory-less PD • Image suppression andDC-offset compensation • in spec Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR) @5MHz

  26. 5 Conclusion

  27. Conclusion • We are working hard on increasing the energy efficiency of our products ! • What about the energy efficiency of our workshop ?

  28. www.alcatel-lucent.com

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