Energy Efficient Transmitters for Future Wireless Applications

Energy Efficient Transmitters for Future Wireless Applications Christian Fager christian.fager@chalmers.se C E N T R E Microwave Electronics Laboratory Department of Microtechnology and Nanoscience Chalmers University of Technology Microwave Road Seminar Series 2012 03 23

Outline Background and motivation Why is the energy efficiency of wireless transmitters of concern? Transmitter energy efficiency How does the transmitter efficiency relate to microwave power amplifiers? High efficiency power amplifiers How can the power amplifier efficiency be improved? Energy efficient transmitters How can efficient amplifiers be used to realize energy efficient transmitters for wireless applications? Summary & Future outlook 2

Background and Motivation Why is the energy efficiency of wireless transmitters of concern?

Data intense mobile applications emerge Data traffic in wireless networks is increasing dramatically Increased revenue for telecom operators Mobile data traffic forecast Source: Cisco High quality of service to users means broadband wireless access everywhere 4

Number of wireless users increase dramatically It was predicted that the number of mobile subscribers exceeded 4 billion in 2010 More users combined with increase of data intense applications -> More base stations will be installed! Tree Base station 5

Energy consumption in wireless networks Wireless communication networks consume energy Estimation for Sweden: 1-2 TWh/yr Industry (57 TWh), household (20 TWh), heating (20 TWh) It has been estimated that up to 1% of the global energy consumption relates to wireless infrastructure Environmental perspective Waste of energy and resulting CO 2 emissions Systems in remote areas sometimes operated with Diesel generators Economical perspective Operators waste money on electrical bills Reduction of energy consumption in wireless networks has high priority Source: Ericsson 7

The role of the base stations ~90% of the energy in wireless networks is consumed by the base stations The base stations provide the wireless access points between the users and the network infrastructure 8

Base station energy breakdown Source: "Energy logic for telecom operators," white paper, Emerson Network Power, Sept 2008 The radio transmitter dominates the energy consumption Only a fraction is delivered to the antenna; the rest is dissipated as heat Active cooling is needed which further multiplies the energy consumed Energy efficiency enhancement of radio transmitters is the key to sustainable wireless communications! 9

Transmitter Efficiency How does the transmitter efficiency relate to microwave power amplifiers?

Properties of the base station signal transmitted 3G base station: Transmitted WCDMA signal RF signal power [dbm] 0-20 -40-60 Frequency domain -80 2130 2135 2140 2145 2150 Frequency [MHz] Time domain (sample) 0 50 51 52 53 54 55 Time [µs] The modulation creates dynamic signal power variations 10 RF signal power 40 30 20 10 Probability [%] 8 6 4 2 Peak power: 40.3 dbm Average power: 30.0 dbm Peak-to-average: 10.3 db 0 0 10 20 30 40 Output power [dbm] 11

Transmitter efficiency versus output power Transmitters are implemented with microwave power amplifiers Microwave power amplifier Typical power amplifier characteristics RF in RF out Energy efficiency [%] Probability [%] DC in Transmitted power [dbm] Peak efficiency occurs at maximum output power But - highest probability occurs at much lower power levels Very low average efficiency when modern communication signals are used 12

Efficiency is even lower in future systems Emerging systems have even larger amplitude variations Related to increased spectral density = data rate per bandwidth GSM Energy efficiency [%] 3G LTE Probability [%] Transmitted power [dbm] The transmitter energy efficiency problem is getting worse! 13

How can the transmitter efficiency be improved? Transistor level Transistors with higher performance Circuit level Power amplifiers with higher efficiency Transmitter architecture level Modulation of high efficiency power amplifiers Digital signal processing Advancements at all levels must be combined to meet energy saving requirements of future systems 14

High Efficiency Power Amplifiers How can the power amplifier efficiency be improved?

Traditional power amplifier operation The transistor is used as time-varying current source RF in Simplified power amplifier schematic RF out Ids 0.20 0.15 Transistor current 0.10 0.05 Transistor operation Measured vs. modelled Ids(Vds) Loadline 60 50 40 30 20 10 Voltage and current waveforms Voltage Current Dissipated power 0.00 0 5 10 15 20 25 30 Transistor voltage Vds 0 0 0.2 0.4 0.6 0.8 1 Time Simultaneous voltage current across transistor Excessive power dissipation in the transistor Typically limits practical peak efficiency to < 50% How can the "voltage current" overlap be minimized? 16

Switch mode power amplifiers (SMPAs) Designed to use the transistor as a switch Only operated in its "on" and "off" regions Switched transistor operation Voltage and current waveforms Transistor current Loadline off region Transistor voltage Time Clearly "voltage current" overlap is minimized Theoretically 100% efficiency is enabled Switch operation achieved by careful design of the transistor load network... 17

Examples of switch mode power amplifier circuits... Class E power amplifier circuit Class F power amplifier circuit Inverse class D power amplifier circuit 18

High performance switch transistors Switch operation requires high performance transistors Low switch resistance High efficiency High breakdown voltage High power Low transistor parasitics Fast switching Gallium-Nitride transistors emerge as replacement for traditional silicon transistors in high power switch applications High performance GaN transistors are processed in MC2 cleanroom Clean room activity at MC2... Processed GaN devices Chalmers GaN transistor ca 1mm ca 3mm 19

Example of a high efficiency GaN switched mode power amplifier World record at IMS2011 PA Student Competition! Peak efficiency: 78% Frequency: 3.5 GHz Output power: 11W Circuit designs by GHz centre students: Hossein Nemati & Paul Saad Measurements at the competition! The winning amplifier GaN transistor Proud Students and Supervisor! 20

Energy Efficient Transmitters How can efficient amplifiers be used to realize energy efficient transmitters for wireless applications?

Efficient modulation of power amplifiers Efficiency at peak power can be improved by switch mode PAs How can the average efficiency be increased? Energy efficiency [%]?? Probability [%] Transmitted power [dbm] Efficiency at average power levels must improve! Different efficiency enhancement architectures are investigated Dynamic supply voltage modulation Dynamic load modulation Pulsed/digital architectures... 22

Dynamic supply voltage modulation a.k.a. Envelope tracking Exemplified with traditional power amplifier 0.20 Traditional transistor operation 0.20 Supply modulation transistor operation Ids 0.15 Transistor current 0.10 0.05 Transistor voltage unnecessarily high! High power loadline -> OK efficiency Ids 0.15 Transistor current 0.10 0.05 High power loadline -> OK efficiency Reduction of supply voltage 0.00 0 5 10 15 20 25 30 Transistor voltage Vds Low power loadline -> Poor efficiency 0.00 0 5 10 15 20 25 30 Transistor voltage Vds Low power loadline -> High efficiency maintained Supply modulation means to dynamically reduce the supply voltage at low power levels to avoid unnecessary dc energy consumption 23

Supply modulation characterization Solid contours = constant RF output power Efficiency = RF output power RF input power DC input power RF input power Grayscale contours = Efficiency DC supply voltage An efficiency optimized input power / dc input voltage combination can be found at each output power level Careful co-control of both input power and dc voltage needed 24

Dynamic supply modulation transmitter With dynamic supply modulation Transmitter implementation Energy Efficiency [%] Fixed supply Transmitted power [dbm] Large efficiency improvement at average output power Main disadvantage Additional amplifier needed to provide the dynamic supply voltage Degrades total efficiency State-of-the art ~55% efficiency for 3G base station transmitter 25

Dynamic load modulation Exemplified with traditional power amplifier 0.20 Traditional transistor operation 0.20 Load modulation transistor operation Ids 0.15 Transistor current 0.10 0.05 Supply voltage too high! High power loadline -> OK efficiency Ids 0.15 Transistor current 0.10 0.05 Load resistance is increased High power loadline -> OK efficiency 0.00 0 5 10 15 20 25 30 Transistor voltage Vds Low power loadline -> Poor efficiency 0.00 0 5 10 15 20 25 30 Transistor voltage Vds Low power loadline -> High efficiency maintained Load modulation means to increase the transistor load impedance at low power levels Full transistor voltage swing is maintained 26

Load modulation characterization Optimum load impedance trajectory Load impedance points tested Peak power Low power = Traditional, fixed load impedance An efficiency optimized input power / load impedance combination is identified at each output power level Used for the design of a tunable load network Careful co-control of both input power and varactor network needed 27

Dynamic load modulation transmitter Transmitter implementation Energy Efficiency [%] With dynamic load modulation Fixed load Transmitted power [dbm] Large efficiency improvement also here Electrically controlled load network is designed Varactor diodes are used High performance varactors needed - MC2 specialty! Simpler drive electronics than supply modulation 28

Mixed signal transmitter interface Traditional transmitter architecture Efficiency enhanced transmitter architecture The digital signal processing and RF circuit design is getting more integrated Multiple parallel input signals need to be co-controlled More degrees of freedom are added Transmitter input signals may be designed to benefit from this freedom Increased energy efficiency Compensation of non-idealities in the transmitter circuits Wider signal bandwidth, etc. Advanced signal processing methods are adopted... 29

Load modulation transmitter demonstrator Integration of technologies at several levels Circuit: High efficiency power amplifier design Transmitter: Load modulation for average efficiency enhancement System: Signal design for compensation of non-idealities and energy efficiency maximization 30

Transmitter demonstrator measurements Transmitted power [db] The transmitter exhibits non-idealities that need to be compensated for Spectral regulations are broken Output frequency [MHz] 31

Transmitter demonstrator measurements Transmitter type Energy efficiency Traditional 38% Load modulation 53% First successful demonstration of a dynamic load modulation transmitter! Transmitted power [db] The transmitter exhibits non-idealities that need to be compensated for Spectral regulations are broken Nonlinear signal processing methods are employed High signal quality achieved Energy efficiency significantly enhanced Output frequency [MHz] 32

Digital RF Pulse Width Modulation Transmitters Operating the transistor as a true digital switch Ideally suited for Switched Mode Power Amplifiers Amplitude control: Pulse duty-cycle Phase control: Timing of RF pulses Integration with digital driver circuits (software control) Challenges Electrical interface between digital/analog circuits (matching?) Reconstruction filtering and 33

RF Pulse-Width modulated Class-E amplifier ordinary SMPAs are only efficient at fixed 50% duty-cycle! Traditional Class E PA RF-pulse width modulation Class E PA Solution: The RF-PWM class E power amplifier Electrical tuning of class E load network High efficiency for variable output power 34

Class-E RF PWM demonstrator circuit 10W, 2 GHz prototype CW efficiency versus output power Cree GaN transistor NXP CMOS PWM chip Chalmers SiC varactors CMOS and GAN HEMT interconnect Chalmers SiC varactors First demonstration of efficient RF-PWM Excellent high efficiency Drain efficiency >70% over 6.5 db dynamic range Significant CMOS driver consumption 35

RF-PWM modulated measurements Measurements with 5 MHz WCDMA signal Average total efficiency: 55% while meeting spectrum regulations 36

Summary and Future Outlook

Summary Increased data traffic and more users Energy consumption in wireless networks is of growing concern Cross-disciplinary research needed to realize energy efficient base station transmitters High efficiency switch mode power amplifiers Drives advanced transistor technology development New transmitter architectures investigated to maintain high efficiency with realistic signals Multiple inputs need to be controlled Integration of signal processing and hardware design Efficiency improvement Compensation of transmitter non-idealities Requirements of future transmitters will push the research needed - from transistors to system level 38

Future Outlook Increased signal bandwidth and more frequency bands Wideband, flexible transmitters with reconfigurable center frequency Complement with smaller basestations (pico/micro-cells) Compact multi-antenna transmitters Reduced overall network energy consumption Energy efficiency in each transmitter yet equally important Functionality in future transmitters must increase further Further integration of digital signal processing and circuit design techniques Energy consumption may not be compromised! Our research will continue..! Solar driven base-stations? 39

Acknowledgments C E N T R E Past and present members of power amplifier research group Thomas Rik Hossein Ulf Mustafa Sepideh Ali and Haiying Paul David Industrial collaborators in the GHz centre VINNOVA, VR, and companies for funding the research 40