Nortel Networks Institute University of Waterloo
Nortel Networks InstituteUniversity of Waterloo
Dr. Slim Boumaiza
EmRG Research GroupElectrical and Computer Engineering,
University of Waterloo, Waterloo, ON, Canada
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Advanced Techniques in Power Efficiency
and Linearity Enhancement of 3G and Beyond Wireless Transmitters
• Wireless Communication Evolution & Challenges
• SDR Key Enabling Technologies
– Ultra Linear Front-End
• Linearization techniques
• Digital Predistortion
– High Efficiency Front End
• Doherty Amplifier
• Linear Amplification Using Nonlinear Components
• Conclusion
Outline
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Overview on Wireless Communication Evolution & Challenges
Wireless Communication Evolution
CDMA
GSM Edge
3GPP
TD-SCDMA
WiMAXLTE
PAR
LowFairHigh
New standards boost Spectral Efficiency bits/s/Hz improves thanks to more advanced modulation schemes: up to 100MB/s capability targeted by LTE
RF signal Peak to Average Ratios typically: 3.4dB Edge / 6.5dB W-CDMA / 10dB WiMAX
Infrastructure Deployment Phase OutStandardization
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EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
-100
-90
-80
-70
-60
-50
-40
-30
-20
2130 2135 2140 2145 2150
PA output
Original signal
PS
D (
dB
m)
Frequency (MHz)
The out-of-band noise increases and affects the adjacent channels
(ACPR)
The in-band noise increases and affects the transmission quality
(EVM)
Wireless Communication Challenges
Dollars in CapEx and OpEx:• utility costs (power for PA and
cooling)
• space (cost of rent)
• racks, heat sinks, fans, etc.
• number of PAs required
Wireless Communication Challenges
Very Stringent Linearity Requirements for RF front end
Operation in Large Power Backoff Region good linearity but poor Efficiency
Linearization to enhance the linearity vs. efficiency tradeoff is more critical than ever
EfficiencyTranslates
Better Reliability is inversely
proportional to heat-load
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Standards operate at diverse frequencies with a more and more higher median frequency as
available spectrum is scarce
GSM and CDMA already deployed
WiMAX in “design” stages
3G & 3G Evolution (HSDPA deployment, LTE on the
horizon, China (TD-SCDMA))
Wireless Communication Challenges
Median frequency is evolving: 800/900MHz 5 years ago, 2.1 GHz now , 3+ GHz in the future
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These different standards will go on coexisting
Broad spectrum of base stations and terminals has to be maintained to cover all operator’s needs
Market evolution calls for flexible infrastructure to enable dynamic networks management and infrastructure sharing
Solution: Software Defined Radio where one flexible architecture for both base stations and terminals is used to meet the different requirements
Wireless Communication Challenges
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An
ten
na
s
Multi-band RFTransmitter
Multi-band RF Receiver
Basebandanalogue
& digital proc.
Higher LayerFunctions
control
Software Defined Radios Generic Diagram
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Wireless Communication Challenges
SDR poses several challenging design factors that call for Key Enabling Technologies
New device technology
New Design Methodologies: RF and
Digital Mixed Design
Multi-band RF Sub-Blocks
(Antenna, Power
Amplifiers,Filters,
Combiners)
Ultra Linear RF
Front Ends: Advanced Linearizati
on Techniques
High Efficiency RF Front
Ends
Advanced Digital Signal
Processing Algorithms
Wireless Communication Challenges
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SDR Key Enabling Technologies: Ultra Linear RF Front-End
Linearization TechniquesOverview
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Feedback
Requires high gain and stable analog feedback at the carrier frequency
Close the feedback at baseband Cartesian and Polar
The non-linear inter-modulation products are reduced by the same ratio as the feedback gain.
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Feedback has a limited bandwidth capability.
Potential instability due to the propagation delay in the feedback loop.
Its design become challenging at high frequencies.
It is more suitable for an integrated handset transmitter design as the important transmitter delay makes it very difficult for base station
Feedforward
The output of the Power Amplifier is coupled and compared to original input. Finally added to the output of main amplifier.
The delay of two amplifiers have to be replicated precisely
This is an open-loop system which is vulnerable to environmental changes
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Feedforward
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Predistortion
Basically estimate the nonlinear behavior of PA at the output and tries to modulate the input signal with inverse function.
The complexity of the system might incur significant power overhead
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PredistortionThe Predistorter generates non-linearities that cancel amplifier distortions when combined.
It can be performed at RF or at base band using FPGA processor.
Analog RF Predistortion is widely used for slow-slope non-linear amplifiers such as TWTA.
Analog RF Predistortion is a mature technology for mass production.
Good linearity improvement in TWTAs and less significant improvement for MESFET class A and AB.
Does not degrade much the power efficiency of the PA.
Needs an adaptation loop to track the drift of the PA.
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Predistortion
DPD is becoming technically mature and an increasing number of RF transmitters and PAs designers are bet on DPD for tackling linearization problems.
Number of published patents and scientific articles
per year on the subject "digital predistortion"
D. Rönnow, Measurement, analysis, modelling and digital predistortion of RF/Microwave power amplifiers, Racomna
Research AB,
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Comparaison Predistortion vs.Feedforward vs. Feedback
SSPAs for Base
StationsPAs for Mobile
Stations
TWTAs & SSPAs for
Satcom applications
Intrinsically
non adaptiveIntrinsically
adaptive
Intrinsically non
adaptiveAdaptation
LowHighHigh/HighPower
Efficiency
HighMediumMedium/MediumComplexity
UltraGoodMedium/UltraLinearity
WideNarrowUltra/ MediumFrequency
bandwidth
FeedforwardFeedbackRF Predistortion/
Digital Predistortion
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High>100 MHz25 - 35 dBFeedforward
Relative
Cost
Correction
Bandwidth
Correction
Capability*
Linearization
Technique
Medium<5MHz10 - 20 dBEnvelope
Feedback
Low>25MHz5 - 10 dBAnalog Pre-
Distortion
Medium>50MHz10 - 20 dBAdaptive Pre-
Distortion
High>100 MHz25 - 35 dBFeedforward
Relative
Cost
Correction
Bandwidth
Correction
Capability*
Linearization
Technique
Medium<5MHz10 - 20 dBEnvelope
Feedback
Low>25MHz5 - 10 dBAnalog Pre-
Distortion
Medium>50MHz10 - 20 dBAdaptive Pre-
Distortion
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Comparaison Predistortion vs.Feedforward vs. Feedback
Digital Predistortion Technique
( )inx t
Pin
G
Pin
Gn
Gn
Pin Pin
G G
Pin
Nonlinear PA/TRx Linearized PA/TRx
Pin
G
Psatin
Psatin
Predistortion Function
( )inx t ( )outy t( )dx t ( )outy t
Predistortion function adjusts the input signal envelope so that it brings in distortions out of phase with those generated by the PA/TRx nonlinearities.
Digital Predistortion Concept
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The response of nonlinear circuits can be assessed using different types of excitation signals
• Continuous Wave signal (trace gain in power sweep mode)
• Two tones signal (Inter-modulation products)
• Multi-sine signal (in-phase or randomly distributed phases)
• Modulated signals synthesized according to the targeted standard
Nonlinear Circuit Characterization
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46
47
48
49
50
51
52
-25 -20 -15 -10 -5 0 5
CW
WCDMA
8-Tones
Gain
(d
B)
Pin (dBm)
-80
-75
-70
-65
-60
-55
-25 -20 -15 -10 -5 0 5
CW
WCDMA
8-TonesPh
ase
(d
egree
)Pin (dBm)
Measured transmitter’s AM/AM and AM/PM characteristics for various excitation signals
Nonlinear Circuit Characterization
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The power amplifier nonlinearity characterization is assessed under realistic signal and not continuous wave
Time-Domain Nonlinear Circuits Characterization Approach
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Gain vs. input power measurements with VNA under CW signal, peak power analyzer and the Time domain test bed with CDMA 2000 signal.
Phase compression vs input power measurements with VNA with CW signal
and the Time domain test bed with CDMA 2000 signal.
56.5
57
57.5
58
58.5
-24 -22 -20 -18 -16 -14 -12 -10 -8
VNA mesurements
Peak power analyser
Proposed test bed
Power input (dBm)
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Time-Domain Nonlinear Circuits Characterization Approach
To achieve accurate characterization the device under test should be excited as close as possible to the real operation condition
This approach demonstrated and solved the inaccuracy of VNA based Characterization of PA nonlinearity
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Time-Domain Nonlinear Circuits Characterization Approach
Amplifier : cascade of MRF21045 and MRF21085 (Freescale semiconductor)
Frequency : 2140 MHz
Output power : 90W Peak
Signal : WCDMA 1-carrier
25 dB ACPR decrease from -35dBc to -60dBc
-100
-90
-80
-70
-60
-50
-40
-30
-20
2130 2135 2140 2145 2150
memoryless DPD
without DPD
Original signal
PS
D (
dB
m)
Frequency (MHz)
The good accuracy of the characterization is demonstrated through the Digital Predistorter performance
Digital Predistortion Technique
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-100
-90
-80
-70
-60
-50
-40
-30
-20
2120 2125 2130 2135 2140 2145 2150 2155 2160
sans mémoiresans prédistorsionSignal original
PS
D (
dB
)
Frequence (MHz)Frequency (MHz)
-100
-90
-80
-70
-60
-50
-40
-30
-20
2120 2130 2140 2150 2160
sans mémoiresans prédistorsionSignal original
PS
D (
dB
)
Frequence (MHz)Frequency (MHz)
New phenomenon takes place and neglected before
As the signal bandwidth signal increases the predistortion
capability is reduced
Digital Predistortion Technique
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The residual nonlinearity shown by the linearized transmitter can
be explained•Nonlinearity characterization inaccuracy
•Memory effects which is defined as
Frequency domain: *IMD3 products magnitude and phase
changes as a function of the frequency spacing.
Time domain: PA output depends not only on the current input but also on its previous values
)(),(),...,()( Mtxtxtxfty
f1 f2 f1 f22f1-f2 2f2-f1 *IMD3=Amp(2f2-f1)
Memory Effects Investigation
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EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Hypothesis: PA nonlinearity depends only the current input signal amplitude
AM/AM Curve AM/PM Curve
Memory Effects Investigation
Tx
C
Vgs
C
Vds
Power amplifier schematic
The time domain characterization approach is used once again
to investigate the memory effects
RF Transmitters Block Diagram
PAD/A RF_out
Up
converterDigital
Mod
I
Q
Memory Effects Investigation
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Asymmetry between the upper and lower IMD3 components
Dependency of the IMD3 magnitude and phase on tone spacing
-65
-60
-55
-50
-45
0 5 10 15 20 25
IMD3_L
IMD3_R
IMD
3 (
dB
m)
Frequency spacing (MHz)
What to measure: IMD3 distortion at the transmitters output vs.
tone spacing
IMD3_L=Amplitude(2f1-f2)IMD3_R=Amplitude(2f2-f1)
Two regions are considered:
- Low freq spacing long term memory effects
- Medium to high freq spacing short term memory effects
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Memory Effects Investigation
thR
thC
)(tPdissip
tTjcT
thR
thC
)(tPdissip
tTjcT thC
)(tPdissip
tTjcT
Long-term (electro-thermal) memory effects
attributed to the dynamic changes of the junction
temperature as a function of the input signal strength
variation of transistor electrical parameters
Heat Flux
Equivalent thermal circuit
ChipPackage
Heat sink Tamb
Tj
Tc
1
tt
j th dissip cT t e e R P t T dt K
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Memory Effects Investigation
Long-term memory effects
• IMD3 distortion attributed to thermal effects dependent on
the frequency spacing.
• IMD3 attributed to the PA electrical non-linearity remains
constant when varying the frequency spacing.
tx ty
NL Electrical
PA Behaviour
NL Thermal
Effects
lev ,
lthv ,
rev ,
rthv ,
lv
rv
rerthr
lelthl
vvv
vvv
,,
,,
rerthr
lelthl
vvv
vvv
,,
,,
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Memory Effects Investigation
Short-term (electrical) memory effects
-Attributed to terminals impedance variation of the biasing and matching circuits
as function of frequency
Im(IM3L)
Re(IM3L)
2sd ordre(envelop)
2sd ordre(harmonic)
w1-
w2
w2-
w1
0
2w1-
w2
2w2-
w1
w1
2w1
2w2
w1+
w2
w2 freq
A
Spectrum composition under two tones excitation
Tr POUTPINMatching
circuit
at the carrier frequency and its harmonics
Biasing circuit
at the envelop frequency
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Memory Effects Investigation
• Narrow-band signals
Electro-thermal memory effects are attributed to the dynamic
changes of the junction temperature as a function of the input
signal strength which leads to the transistor electrical parameters
variation (e.g. gain magnitude and phase)
• Wideband signals
Electrical memory effects are due to the variation of terminal
impedances of the biasing and matching circuits impedances over
the input signal bandwidth around- the carrier frequency
- its harmonics
- base band frequencies
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Memory Effects Investigation
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
• Several approaches were used to develop a DPD that can compensate for the static nonlinearity of the PAs as well as the dynamic distortion due to the memory effects:
– Volterra series: for mild nonlinear case
– Hammerstein and Wiener with less success
– Neural Networks
– Augmented Wiener and Hammerstein
– Memory polynomial which constitute a good approximation of the Volterra series
Memory Effects Investigation
Memoryless static nonlinear subsystem
( )u n
FIR2
Memory effect subsystem
FIR1
( ) ( )x n x n
( )u n ( )x nAM/AM and
AM/PM LUT
( )y nx +
( ) ( ) ( ) ( ) ( ) ( )i qx n G u n jG u n u n G u n u n
1 21 1
0 0
( ) ( ) ( ) ( )M M
i i
i i
y n a x n i b x n i x n i
Augmented Hammerstein Model
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Augmented Weiner/Hammerstein Nonlinear Behavioral Models
-100.0
-90.0
-80.0
-70.0
-60.0
-50.0
-40.0
1.92 1.93 1.94 1.95 1.96 1.97 1.98 1.99 2
Frequency (GHz)
PS
D (
dB
/Hz)
(a)
(b)
(c)
(d)
(a) Without predistorter. (b) With memoryless predistorter. (c) Hammerstein predistorter with a 128-tap FIR filter. (d) Augmented Hammerstein predistorter with two 20-tap filters.
Augmented Weiner/Hammerstein Nonlinear Behavioral Models
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Memory Polynomial DPD
Typical Memory Polynomial (MP):
Each branch is a Polynomial Poly:
The General Model Equation is:
Where i is polynomial branch,
P is the polynomial order.
Poly 1
Z-1
Z-1
Z-1
+X y
Poly 2
Poly K)()()(
1
0 1
, inxinxhny
pK
i
P
p
ip
12
321 ...
P
P xxhxxhxxhxhPoly
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MEASUREMENT RESULTS
Memory Polynomial
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• DPD 2 achieves similar results to DPD 1.
• Number of coefficients required is reduced from 7x10=70 to 7+9x4=43.
Polynomial Order
DPD# 1 2 3 4
Po
lyn
om
ial B
ran
ch
P1 7 7 7 7
P2 7 4 4 2
P3 7 4 4 2
P4 7 4 4 2
P5 7 4 4 2
P6 7 4 2 2
P7 7 4 2 2
P8 7 4 2 2
P9 7 4 2 2
P10 7 4 2 2
4-Carrier WCDMA
PAPR = 7.4dB
Output Spectrum of Linearized 400W PEP Power Amplifier
SDR Key Enabling Technologies:
High Efficiency RF Front-End
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Doherty Amplifier
Conventional power amplifiers used in the previous experiments
have limited efficiency since the load impedance is kept constant
as the input power changes.
This load was chosen so that the efficiency is maximized for the
max input power level
Hence, for lower input power the efficiency will drop.
It is advantageous if the output load impedance can be
modulated as function of the input power so that the efficiency is
increased for low power levels
Tr POUTPINMatching
circuitZl
Doherty Amplifier
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Doherty Amplifier • Doherty amplifier is based on the inherent load modulation
vs. input power level• For low powers peak PA is off
The carrier PA see a 2Ro load
• For higher powers (6 dB before
saturation) the peak turns on
The actual load decreases up to Ro
Comprehensive design approach suitable for designing a Doherty amplifier is needed.
• The design of the carrier and peak amplifiers relies on an accurate – determination of the optimum loads vs. input power level– large signal transistor model. This can be built based on load
pull measurement data• The model provides the RF output voltage (magnitude and
phase components) and the drain current as a function of input power and load reflection coefficient.
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Doherty Amplifier
Load-Pull Test Bench
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Doherty Amplifier
The appropriate matching circuits at the output of the carrier and peak amplifiers can be designed to produce the required load impedance displacement at the output of the two transistors
Load impedance displacement of the main amplifier
14W Doherty amplifier prototype
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Doherty Amplifier
At 10dB Back off Doherty amplifier allows 10% higher PAE than
the class AB. Max PAE=57%
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Doherty Amplifier
Doherty Amplifier Linearization
DPD linearization of 2C-WCDMA signal
ACPR enhancement = 27 dB (final ACPR = -47dBc)
-100
-90
-80
-70
-60
-50
-40
1.92 1.94 1.96 1.98
Frequency (GHz)
PS
D (
dB
m)
W/o Pred.
With Pred.
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Linear Amplification with Nonlinear Components (LINC)
LINC Scheme
The idea is to transform the signal into two constant envelop signals and make use of high efficient nonlinear amplifiers
Outphasing concept: Decompose the amplitude and phase varying signal into two constant envelope signals before amplification
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
• Use of nonlinear power amplifiers (PAs) with very high efficiency
• The PAs are operated in the saturation region but the signal quality is conserved
Ensure linear amplification with high efficiency PAs
• The output signals of the two PAs are combined to recover the amplitude and phase modulated signal
• LINC efficiency is given by the product of three efficiency types:
• The modulation or the combining efficiency is defined by:
1 2
cosoutM
P
P P
LINC PA C M
LINC Scheme
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Wilkinson
Combiner
Chireix
Combiner
R
LINC Scheme
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LINC Scheme
Wilkinson
(Lossy)
Chireix
(Lossless)
Chireix + jβ
(Lossless)
Isolation
Linearity
efficiency
At variable
back-off
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Combining Techniques
LINC Efficiency Degradation
Amplification of high signals that will be combined destructively
High DC power consumption for to generate power samples
Signal peaks’ probability is generally low
significant degradation of power efficiency.
0
20
40
60
80
0 5 10 15 20 25 30
LIN
C e
ffic
ien
cy (
%)
output power (dBm)
LINC efficiency < 7%
for signals with PAPR = 10 dB
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0
10
20
30
40
50
60
-20 -15 -10 -5 0 5
LINC using Wilkinson
Class B
Eff
icie
ncy (
%)
Pin
(dBm)
combining efficiency versus power back-off (measurement)
LINC Efficiency Degradation
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MM-LINC concept
• MM-LINC stands for mode-multiplexing LINC
• Since the instantaneous efficiency decreases rapidly as input power decreases, this approach proposes to:
– Use the 2 parallel PAs based architecture in balanced mode for high power levels to keep efficiency high
– Use the 2 parallel PAs based architecture in LINC mode for low power levels to keep gain constant in that region
Better linearity than class B or class C PAs
Better efficiency than LINC architecture
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MM-LINC Signal Decomposition
0
1 2
0
1
2
11
2
/
in
in
s t r r
s t
s t j e t r r
2
0
2
0
0
1
max
is the threshold to switch from one mode to another
is the normalized threshold value
re t
r t
r
r
r
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MM-LINC efficiency performance
Efficiency performance of the MM-LINC technique
compared to LINC and class B (measurement)
0
10
20
30
40
50
60
-20 -15 -10 -5 0 5
LINC
Part-time LINC
Class B
Pow
er a
dd
ed e
ffic
ien
cy
(%
)
Pin (dBm)
MM LINC
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MM-LINC Linearity performance
Linearity versus efficiency performance of the MM-LINC
technique compared to LINC and class B (measurement)
0
5
10
15
20
25
30
30
31
32
33
34
35
36
0 0.2 0.4 0.6 0.8 1
EVM
PAE
Pout
EV
M (
%),
PA
E (
%)
Pou
t (dB
m)
LINCClass B
0
5
10
15
20
25
30
30
31
32
33
34
35
36
0 0.2 0.4 0.6 0.8 1
EVM
PAE
Pout
EV
M (
%),
PA
E (
%)
Pou
t (dB
m)
LINCClass B
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MM-LINC performance
• The MM-LINC is suitable for mobile applications (low complexity)
• It improves a trade-off between efficiency and linearity depending on the linearity requirements
• MM-LINC has better efficiency and better dynamic range than the traditional LINC
• Limitations :The distortions introduced by the PAs saturation is not
corrected for in this architecture
The linearity of MM-LINC is lower than the linearity of LINC technique when using Wilkinson combiner
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
• Software Designed Radio Base Station is foreseen to be vital for future wireless networks.
• It causes many challenges and calls for many enabling technologies
• Digital Predistortion Technique using Time Domain Characterization method is needed to achieve Ultra Linear Front Ends
• As signals’ bandwidth is more and more wider, memory effects is a critical design factor.
Conclusion
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
• The mitigation of their effects can be addressed using DPD with memory.
• Augmented Hammerstein/Weiner and Polynomial behavior models showed good capability in predicting and linearizing the dynamic nonlinear behavior of wideband wireless transmitter
• However, it is advantageous to minimize them during the PA design
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Conclusion
• Doherty Amplifier showed substantial power efficiency enhancement
• Further improvement could be achieved using a Digital Asymmetrical Doherty
• Mode Multiplexing LINC achieved very interesting linearity vs. efficiency tradeoff. It could be a good candidate for handset Pas.
• RF/DSP mixed co-design approach will be required to tackle the SDR challenges and meet their stringent requirements
EmRG Research Group, Electrical and Computer Engineering www.ece.uwaterloo.ca\~sboumaiz
Conclusion