Title: Lecture 2: RF Issues for Software Radios RF Engineering for the DSP Engineer
1Lecture 2RF Issues for Software Radios RF
Engineering for the DSP Engineer
- TOPICS
- RF Receiver Chain
- RF Transmitter Chain
- A Quantitative perspective of noise and
distortion - Overcoming RF limitations with DSP
2What Youll Learn
- Role of RF in SDR
- SDR RF Structures
- Common Performance Metrics
- SDR Amplifiers and Overcoming RF Problems with
DSP - Impact of MEMs
3Role of RF in SDR
- Why are we focusing first on Hardware for a
Software radio? - Radio may be defined digitally, but the real
world is analog.
4Generic Transmitter
Antenna
Digital
Input
Digital to
Selection and
Conversion
Analog
Amplification
Conversion
transmitter section
5Generic Receiver
Antenna
Digital
Output
Analog
-
to
-
Selection
Conversion
Digital
Conversion
RF Front
End
6Purpose for the RF
- Extract a desired low level signal 10-16 to 10-3
Watts. - Reject out of band noise and interference
- Convert signals center frequency to a range
compatible with the A/D. - Modulate, amplify, and filter signal for
transmission. - Minimize additive noise and distortion
7Goals of RF in SDR (1/3)
- Support MBMMR Multi-band, multimode radio
- Operate over numerous bandwidths, frequencies,
waveforms - Implies wideband operation, but
- Produce highest quality signal for baseband
processing - Reject out-of-band noise and interference
- Implies narrowband operation
8Goals of RF in SDR (2/3)
- Facilitate recovery of weak signals in presence
of strong interferers - Wide Dynamic Range
- Implies High Quality Components (Means high cost)
- Practical Considerations
- Low Cost
- Low Power
- Small Form Factors
9Goals of RF in SDR (3/3)
- Have your cake and eat it too
- However, complementing hardware with software
changes the RF cake equation
10RF View of Radio Systems (1)
- Antenna Design
- Receiver Design
- Topology of Receivers
- Component Issues
- Special capabilities
- Transmitter Design
- Topology of Transmitters
- Component Issues
- Special capabilities
11RF View of Radio System (2)
- Noise and Distortion Characterization
- Noise
- Non-linear distortion
- Compensation for distortion
- Power Supply and Consumption Considerations
12Key Antenna Issues
- Most antennas support bandwidths on the order of
10 of the carrier frequency, - Multimode radios 900 MHz to 2 GHz are difficult
to support with the same antenna - Impendence, hence matching can vary with the
environment - Form factor is important for handsets
13Key Antenna Design Issues (2)
- Gain versus directionality trade-off
- Sensitivity to coverings (e.g., hand)
- Diversity design -- multiple antennas and smart
antenna algorithms - Radiating head
14Key Receiver Parameters
- Sensitivity
- Defines the weakest signal that a receiver can
detect and is usually determined by the various
noise sources in the receiving system. - Selectivity
- The ability of the receiver to detect the desired
signal and reject others. - Spurious Response
- The spurious response is a receivers freedom
from interference due to these internally
generated signals or their interaction with
external signals. - Stability
- Receiver gain frequency change with
temperature, time, voltage, etc.
15Characteristics that Determine Suitability of
Receiver Topologies (2)
- Dynamic Range The difference in power between
the weakest signal that the receiver can detect
and the strongest signal that can be supported
(either in band or out of band) by the receiver
without detrimental effects
16Dynamic Range Constraints
Spurious Signal Limited
Noise Limited
Bit Error Rate (BER)
Usable Dynamic Range
Received Signal Power
17Factors Impacting Dynamic Range
A/D Converter Constraints
Range
Selected Dynamic Range
AGC and Power Control
Analog Front End Constrains
18Various Types of Receivers- Tuned Radio
Frequency Receiver
Input signal level may span 100 dB in dynamic
range
To A/D Converter
LNA
BPF
AGC
RX filter
Not very practical
19Tuned Radio Frequency Receiver
- Disadvantages
- Not as practical
- Extreme demands on RF and A/D
- Tunable very narrowband filters
- Dynamic range of LNA
- Advantages
- Few analog components
- Isolation problems minimized
20Direct Conversion Receiver
I
Input signal level may span 100 dB in dynamic
range
LNA
BPF
BPF
AGC
LO
ADC
90o
RX filter
RX filter
Q
- Advantages
- Fewer parts
- Analog Images Eliminated
- Conceptually simple
- Possibly lower power consumption
- Disadvantages
- High isolation needed in mixer between LO and LNA
input - eg, signal -116 dBm, LO 5dBm, thus isolation
gtgt 120 dB - Phase noise of LO critical
- DC offset at A/D substantial and dynamic
- Balanced mixers needed
- Second order distortions occur in band
21Direct Conversion Transmitter Architecture
Spurs typically 60 dB down or more
I
Binary Data Source
Frequency Source (DDS or Programmable VCO)
Power Amplifier
BPF
BPF
Q
I/Q LO source
RF VCO
- Advantage
- Conceptually simple filter requirements
- Low complexity
- Circumvents image problems
- Disadvantage
- Not as practical because of isolation problems
- Balance in IQ
- Consistent performance over wide band
22Multiple Conversion Receiver
I
LNA
BPF
BPF
LO
AGC
BPF
BPF
90o
Image filter
Image filter
RX filter
Q
IF LO
IF LO
- Advantages
- More isolation than direct conversion or single
conversion (due to distributing gain into to
sections) - Better rejection of ACI
- Better gain possible through distributed
amplificaiton
- Disadvantages
- More parts
- More power consumption?
Multiple conversion possible, but not as common
23IF and Low -IF Conversion Receiver
I
ADC
LNA
BPF
LO
BPF
BPF
AGC
90o
Image filter
RX filter
Q
IF LO
- Advantages
- More isolation than direct conversion less DC
offset - Lower parts count than dual conversion
Analog
Digital
- Disadvantages
- Gain is limited
- More image rejection required over direct
conversion
24Review of the Mixing Process
Amplitude
Desired signal
Adjacent channel interference
?
-?
136MHz
136MHz
?d
?1
-?1
-?d
Amplitude
Desired signal downconverted
Adjacent channel interference upconverted by the
mixer
Desired signal corrupted by interference
?
-?
?168wd-68
-?1-68-wd68
?1-68
?d68
-?168
-?d-68
25Two Stage Transmitter
BPF
Power Amplifier
I
Binary Data Source
Frequency Source (DDS or programmable VCO)
BPF
BPF
Q
RF VCO
RF VCO
- Advantage
- Better isolation
- Disadvantages
- More parts and higher cost
- Higher power consumption
26Transmitter Component Issues (1/2)
- Transmit IF VCO
- Phase Noise Provides Significant Modulation to
Narrowband Signals - Up-Converter
- Linearity to Reduce Spurious Products
- Modulator
- Balance Between IQ Required to Keep Distortion
(Sidebands) Down - Variable Gain Amplifier
- Linearity and Fidelity
27Transmitter Component Issues (2/2)
- Transmit Filters
- Must Prevent PA Transmitter Noise Leakage
(supplement duplexer) - Low Loss required
- Power Amplifiers
- Cost - especially for base stations
- Spurious response (source of interference)
- Packing to handle heat
- Low distortion traded for power efficiency traded
for bandwidth
(in practice only about 25 of the battery is
effectively used during the talk time
28Key Receiver Component Issues (1/2)
- Duplexers
- full duplex, e.g., AMPS is difficult and
expensive - half duplex, e.g., GSM still some difficulty in
integration - duplexers that work both for TDMA and FDMA
- Low noise amplifier (LNA)
- trade off in gain, noise, power consumption, and
dynamic range (noise figure is ratio of output
SNR to input SNR) - low power consumption needed
29Key Receiver Component Issues (2/2)
- RX filter
- Initial BPF after antenna
- rejects out of band interference
- helps isolate of the tx and rx
- Image Reject BPF before mixer
- protects mixer from interference
- suppresses spurious signals generated by mixer
impacting LNA - RF Mixer
- Spurious response
- LO drive level
- too high -- power consumption issue
- too low -- more harmonic distortion
- Isolation between RF, IF, and LO ports limits
post mixing harmonics - Harmonics due to mixer and LNA non-linearities
may end up in the IF pass band after mixing
30Impact on Constellation Due to Imperfect Mixing
31Key Receiver Design Issues AGC (1)
- Intermediate Frequency (IF) filter sets noise
bandwidth of the Receiver - Implementation impacted by cost, signal loss, and
adjacent channel rejection
32Key Receiver Design Issues AGC (2)
- Automatic Gain Control (AGC)
- Placement for minimal noise (after IF for
constant noise figure) - Large dynamic range to match the A/D dynamic
range - Response time of AGC loop is critical for min.
distortion and maximum dynamic range
33Digital AGC
To Software Receiver
A/D Converter
Input Signal
Amp
Gain Control
Energy Detector
Slew
Gain Factor Mapping
D/A Converter
Inactive
Mode Selector
?
Tracking
-
Reference Level
34AGC Modes
Reference Level
Slew Mode
Slew Mode
High amplitude level
Low amplitude level
Input Signal Level
Tracking Mode
Tracking Mode
AGC Inactive Zone
35Key Transmitter RF Design Issues
- Power Efficiency
- Modulation Accuracy and Linearity
- Spurious Signal Reduction
- SNR of Transmitted Signal
- Power Control Performance
- Output Power Level
36Transmitter Component Issues Ocsillator Mixer
- Transmit IF VCO
- noise floor
- power consumption
- phase noise provides significant modulation to
narrowband signals - Up-Converter
- Linearity to reduce spurious products
- Noise floor
- Power consumption
37Key Transmitter Component Issues Modulation
- Modulator
- balance between IQ required to keep distortion
(sidebands) down - Noise figure
- Power consumption
- Variable Gain Amplifier
- Linearity and fidelity
- Noise figure
38Transmitter Component Issues Transmit Filters
- Transmit Filters
- Isolation of transmitter noise from PA leaking
into the receiver (supplement duplexer) - low loss required
39Transmitter Component Issues Power Amplifier
(1)
- Power Amplifier (very critical)
- Cost - especially for base stations
- Noise floor
- Spurious response (source of interference)
40Transmitter Component Issues Power Amplifier
(2)
- Packing to handle heat
- Low distortion traded for power efficiency traded
for bandwidth - (in practice only about 25 of the battery is
effectively used during the talk time)
41General Performance Metrics
- Noise Characterization and Figure
- Spurious Free Dynamic Range
- Blocking Dynamic Range
- Intermod
- Power Consumption
42Noise Characterization (1)
- Noise is introduced into resistive components due
to thermal actions. - where k is Boltzmans constant (1.38.10-23 J/K),
T is the temperature in Kelvin, R is component
resistance (in ohms), and B is the bandwidth in
Hz.
43Noise Characterization (2)
- Antenna is the first and the base line source of
noise for which other noise sources are compared. - Thermal noise and quantization noise introduced
by the A/D
44Noise Figure
- Noise Figure (NF) measure the amount of noise an
element (or elements) adds to a signal. - NF SNRin/SNRout
- where SNRin is the input SNRout and is the device
output SNR. - Active Components The manufacturer of a device
usually supplies a noise figures for equals the
loss of the passive components.
45Using the Noise Figure (1/2)
- It is possible to provide an equivalent system
wide noise figure NFtotal that relates the noise
back to the antenna. - (equation 1)
- Here NFi represents the noise figure at the ith
stage and Gi represents the gain at the ith stage
(units are linear).
46Using Noise Figure (2/2)
- Given a component with a noisy input having noise
power Pi-1 (dBm), gain Gi (dB) and noise figure
NFi (dB) the output noise power Pi (dBm) is given
by - Pi (dBm) Pi-1 (dbm) NFi (dB) G (dB)
-
- Units are linear unless proceeded by (dB) or
(dBm).
47Example NF Calculations (1/2)
NF3 2 dB G310 dB
NF22 dB G2-2dB
NF46 dB
To Next IF Chain
Cable, G1-3dB
From Anttenna
BPF
X
LNA
LO
The total noise figure equals 5.975 .
48Example Noise Calculations (2/2)
Does ordering of the components yield optimal NF?
NF3 2 dB G310 dB
NF22 dB G2-2dB
NF46 dB
To Next IF Chain
Cable, G1-3dB
From Anttenna
BPF
X
LNA
- the total noise figure equals 3.6.
- In the system, the LNA has the biggest impact on
the noise figure (because of its high gain) - In general, it best to have higher gain
components (like the LNA) located as early as
possible in the RF chain.
LO
49Calculating Sensitivity (1)
- Sensitivity of the receiver to achieve a minimal
signal-to-noise ratio SNRmin is defined as - S dBm Noise floor dBm SNRmin dB
- where
- Noise floor dBm 10 log (kTB) NFtotal dB
- 10 log(kT) dB NFtotal dB 10 log(B) dB
- and B is the end of system bandwidth and NF
is the overall system noise figure.
50Calculating Sensitivity (2)
- For room temperature, the sensitivity becomes
- S dBm -174 dBm/Hz NF dB 10 log(B) SNRmin
- A good conservative practice keeps the noise
floor due to analog components lower than the
noise introduced by the A/D converter.
51Distortion Characterization 1 dB Compression
Point (1)
- Devices that exhibit cubic characteristic, the
third order distortion power grows at a rate of
3x the rate of the desired signal. - Eventually the device begins to saturate and when
the actual output power level differs by 1 dB
with the ideal output value, the 1 dB compression
point P1dB is reached.
52Distortion Characterization 1 dB Compression
Point (2)
- Amplitude compression tends to block the
detection of lower level signals in the presence
of stronger signals and the blocking dynamic
range (BDR)quantifies this effect. - BDR P1dB - MDS
- The MDS level occurs when the input -signal is
equal to the noise floor.
53RF Distortion - BDR
Output Power G ? Input Power
Output Power(dB) Input Power(dB) GdB
MDS Minimum Detectable Signal
P1dB,in Input 1 dB compression point P1dB,out
Output 1 dB compression point
BDR Blocking Dynamic Range BDR P1dB,in - MDS
54Spurious Free Dynamic Range (SFDR) Definition
- The difference between the input levels for the
MDS and the onset of third-order distortion (when
the third order distortion equals the noise
floor) defines SFDR.
55SFDR Measurement (1)
- The on-set of third-order distortion can be
determined using the two-tone test, where two
closely spaced tones of equal amplitude form the
input to the system and the amplitude is
increased until the third-order cross-product
produces a signal equal to the noise floor.
56Spurious Free Dynamic Range (2)
- This test mimics real world situations where
adjacent channel interference can cause
significant intermodulation distortion. - Typical dynamic range values extend from 60dB to
90 dB.
573rd Order Intercept (IIP3)
- IIP3 is found by extrapolating the fundamental
and third-order intermod. product lines until
they intersect. - The output power at this point is called the
third-order intercept point (OIP3). - SFDR can be found from the two linear equations
for the harmonic and third-order intermodulation
product - SFDR 2/3 IIP3 MDS
58RF Distortion - Intermod
Predicts Susceptibility to Adjacent Channel /
Nearby Interference
IIP3 3rd Order Input Intercept Point OIP3 3rd
Order Output Intercept Point SFDR Spurious Free
Dynamic Range SFDR 2/3 (IIP3 MDS)
IIM3 Intermod due to 3rd Order IIM3 3?PI - 2
?IIP3 (dBm)
59System Level DistortionCharacterization
- The effects of non-linear distortion are
cumulative. An overall IIP (either IIP2 or
IIP3), IIPtotal can be computed using the
following approximation. - where IIPi represents, in mW the Intermod
Intercept Point (IIP) for stage i. - Like parallel resistors, the overall total is
limited by the lowest value and the non-linearity
at the later stages becomes more critical since
its impact is magnified by the gain of all of the
previous stages.
60RF Distortion Intermod for Cascaded Devices
IIP3 2 dB G310 dB
IIP22 dB G2-2dB
IIP46 dB
Cable, G1-3dB
From Antenna
To Next IF Chain
LNA
BPF
X
LO
Dominated by Worst IIPi
61A/D Distortion Characterization
- Composite RF and A/D noise and distortion is
needed to quantify the overall receiver
performance. - A conservative design approach is to choose an
A/D converter that introduces insignificant noise
contribution compared to the overall RF chain.
62Example of A/D Impact
- For instance, given an input noise at the antenna
of 99 dBm, and a conversion gain of 25 dB and a
noise figure of 10 dB, the input noise to the A/D
is Ptotal (-99dBm 25dB 10dB) -64 dBm. - The percentage of noise power actually delivered
to the A/D load from the RF front end can then be
calculated. This noise voltage due to the analog
components can be compared to the noise figure of
the A/D converter. - A more precise analysis can determine the
overall noise voltage by summing the effective
voltage due to quantization with the voltage due
to the analog components, VA/D,total Vquant
VA/D,analog where Vquant iA/D RA/D.
63Example of A/D Impact (2)
Ranalog
iA/D Pquant / RA/D
V A/D,total
RA/D
i analog P analog,total / (R analog RA/D )
-
i analog effective current from analog noise
(RMS) iA/D effective current due to A/D
quantization noise P analog,total noise power
presented by the analog front end Pquant
quantization noise power Ranalog equivalent
analog resistance in series with A/D
converter RA/D resistance of the A/D converter
64RF Distortion Cascaded SFDR
Cascaded SFDR Recipe
- Determine Input Noise Power
- Calculate System Gain
- Calculate NFTotal
- Calculate Output Noise Power
- Calculate MDS
- Calculate IIPTotal
- Calculate SFDR
65Using SDR to Change the Cake Equation
- Software Radios have the added benefit of using
both software and hardware which changes
traditional tradeoffs - Examine two problems addressable by software
radio - PA nonlinearity vs efficiency
- RF flexibility vs performance
66Significance of the PA
- Quality determines capacity
- Output power defines coverage
- Impacts size of BTS
- Dominates infrastructure costs
- Major contributor to BTS operating costs
- Dominates power consumption
67Transmitter Component Issues Power Amplifier
(1)
- Power Amplifier (very critical)
- Cost - especially for base stations
- Noise floor
- Spurious response (source of interference)
68Transmitter Component Issues Power Amplifier
(2)
- Packing to handle heat
- Low distortion traded for power efficiency traded
for bandwidth - (in practice only about 25 of the battery is
effectively used during the talk time)
69Handling Multiple Channels Todays Realizations
SCPA Single Carrier Power Amplifier MCPA
Multi Carrier Power Amplifier
Antenna
Radio
Low power combiner
Radio
Band pass filter diplexer
MCPA
Radio
MCPA based BTS GSM, GPRS EDGE
Radio
70Realizing Multiple Channels with SDR and a Single
PA
Antenna
Wideband digital radio
Band pass filter diplexer
MCPA
Advantages over SCPA and Multiple Radio MCPA Most
cost effective with multiple carriers More
flexible More efficient Saves space Disadvantage
No redundancy and demanding PA specs
71Summary of Cost Drivers in TX Design
- Signal Peak-to-Average Power Ratio (PAR)
- Signal Peak-to-Minimum Power Ratio (PMR)
- Transmitter Power Control Dynamic Range (PCDR)
- Signal Bandwidth
- Transmitter Duplex Mode half or full
- Bandwidth Confinement Requirements (transmit
mask) - Adjacent Channel Power (ACP, ACPR, ACLR)
Earl McCuner, SDR Radio Subsystems using Polar
Modulation, SDR Technical Conference, Nov. 11,
2002, pp. 23-27.
72TX Requirements for Common Standards
System PAR (dB) PMR (dB) Duplex PCDR (dB)
1G 0 0 Full 25
IS-136 3.5 19 Half 35
GSM 0 0 Half 30
GPRS 0 0 Half/Full 30
EDGE 3.2 17 Half/Full 30
UMTS 3.5-7 Infinite Full 30
IS-95x 5.5-12 26-Infinite Full 73
CDMA2000-1xRTT 4-9 Infinite Full 80
TD-SCDMA 2.5-7 Infinite Half 80
Earl McCuner, SDR Radio Subsystems using Polar
Modulation, SDR Technical Conference, Nov. 11,
2002
73Non-Linearity and Power Amps
- Linearity
- Class A Best
- Class AB, B Mid-range
- Class C Worst
- Efficiency
- Class A Worst
- Class AB, B Mid-range
- Class C Best
74Simple Power Amplifier Model(no memory effect)
75Memory Effects In PA (1/2)
- High power, wideband amplifier characteristics
exhibit hysteresis-like effects - Frequency-dependent electrical memory effects at
high frequencies - Thermal memory effects at low frequencies
- Linearization scheme must cancel the dynamic
behavior of the PA
76Predistortion with Memory
Tapped Delay Line PD (TDL PD)
Complex gain polynomial
Hammerstein PD
Filter
77Memory Effects in PA (2/2)
hysteresis loops
- 4-carrier W-CDMA input, PAPR 13.7 dB
- Class B power amplifier (30W approx.)
78Distortion Effects
Non-Linearity gtgtgt Spectral Regrowth
Low PA Efficiency reduces battery life
79Why is this a Problem? (1/2)
- Modern comm. systems use non-constant envelope
modulation - QAM
- Non-Constant envelope signals require linear
amplifiers - Changes in amplitude cause spurious emissions
80Why is this a Problem ? (2/2)
- Linear amplifiers are power hungry
- Can lead to short battery life
- Amplifiers are the most expensive part of the
base station system
81Non-Linearity and Power Amps
Tradeoff
- Linearity
- Class A Best
- Class AB, B Mid-range
- Class C Worst
- Efficiency
- Class A Worst
- Class AB, B Mid-range
- Class C Best
Linearity for Efficiency
82Techniques to Change the Amplifier Nonlinearity
Tradeoff
- Backoff
- Cartesian Feedback
- Feedforward
- Analog Predistortion
- Digital Predistortion
- Linear amplification using Nonlinear Components
(LINC) - Envelope Elimination Restoration (EER) (also
known as polar amplifier) - Coding
83Backoff
- Non-linear region is at higher power levels
- Operate amplifier at a fraction of its rated
maximum power - Backoff level depends on amplifier
Non-linear Region
Linear Region
84Backoff Pros and Cons
- Benefits
- Simple
- Drawbacks
- Wastes amplifier capacity/efficiency
- Requires amplifier with power limit significantly
higher than operating point - Very expensive solution
85Predistortion
- Wideband power linear amplifiers are the most
costly part of a base station. - Predistortion can alleviate power amplifier
distortion - especially for non constant envelop
signals
86Predistortion Concept
- Input is distorted before feeding to the Power
Amplifier (PA) - The predistortion function generates anti-phase
Inter-Modulation Distortion components to those
generated in PA
z is the complex input to the predistorter and G0
is the linear gain of the overall system
87Benefits of Predistortion
- A trade-off exists between power efficiency and
linearity of the amplifier -- more favorable
trade - Reduced sidelobe regeneration
- 20dB possible
- Secondary benefits include compensation of
carrier leak and linearity problems of the mixers
and I/Q mismatch in the RF chain, possibly due to
the mixers.
88Conceptual Approach to Predistortion -- Analog
Domain
Analog Domain
Digital Domain
Gain F(Sr)
Transmitted signal So KSrCos(?ct)
Sr ideal modulated signal
Sp
D/A
Transmitter IF
Gain G(Sp2)
Note F(Sr) G(Sp) K, K constant gain
Predistortion function
Non-linear power amplifier stage
89Conceptual Approach to Predistortion -- Digital
Domain
Transmitted signal So KSrCos(?ct)
Digital Domain
Analog Domain
Gain F(Vm)
Gain G(Sp)
Vm ideal modulated signal
Sp
D/A
Transmitter IF
Predistortion function
Non-linear power amplifier stage
90Example Predistortion Technique
- AM/AM and AM/PM distortion compensated
- Large calibration table needed and must be
updated - Linearity of feedback loop is an issue.
91Digital Predistortion
- W-CDMA input, PAPR 7.8 dB
- 3rd order polynomial Predistorter (PD)
92Implementation Issues
- Linearity and fidelity of the correction loop
- Demodulator distortion in down-converters affects
performance - Capabilities of the A/D stage
- Must over-sample to capture harmonics
- Convergence vs. Stability
- Large time constants for aging and thermal
effects of the PA - Slow convergence is acceptable
93Flexibility Tradeoff
- SDR Necessitates a Flexible Front-end in terms of
Center Frequency and Bandwidth - Modern Signaling Requires High Performance
Components, Indicates Specific Component Design
94New Amplifier Topologies (1)
- Not really new (Re-emerging)
- See Chapter 8 of RF Power Amplifiers for
Wireless Communications by Steve C. Cripps - Linear Amplification with Nonlinear Components
- LINC for short
- Uses two nonlinear amplifiers and combines power
95New Amplifier Topologies (2)
- Envelope Elimination and Restoration (EER)
- The signal is split amplitude and phase
- Use nonlinear amplifier for phase and restore
envelope by modulating supply voltage
96New Amplifier Topologies (3)
- Both EER and LINC show improved efficiency and
potential for improved linearity - These amplifiers made practical by digital radio
design - Commercial products based on both methods have
been introduced
97LINC (1)
- First proposed by Chireix in 1935
S1(t)
S(t)
S2(t)
98LINC (2)
- S(t) is decomposed into 2 constant envelope
signals S1(t) and S2(t) - Two nonlinear power amplifiers are used
- Design of combiner is interesting
- At low envelope levels, this is inefficient
- Four port combiner has been used, but this
wastes half the output power. However this
still approached 50 efficiency and improves
linearity.
99EER (1)
- First introduced by Kahn in 1952
Video Power Conditioner
Envelope Detector
May be a digital input
Splitter
Limiter
Power Amplifier
100EER (2)
- Signal is split in to envelope and phase
components - Envelope component is used to modulate PA supply
voltage - Constant envelope phase component is used to
drive the nonlinear power amplifier - Ideally 100 efficient
101Why is this Related to SW Radio?
- Both LINC and EER require standard I/Q signal
translated into different format - LINC needs two phase modulated signals
- EER requires polar form signal
- PA can use direct baseband digital signal rather
than RF analog signal - The dividing line between the radio and the PA is
blurred
102Power Supply Issues Battery Life
Battery technology energy density doubles every
35 years.
Powers, Proc. of IEEE, April 95
103Other Considerationswith Batteries
- Environment (NiCa is terrible)
- Shelf life (length of time a charge is
maintained) - Cell Voltage
- Cycle Life (how often can it be recharged)
104Example Power Budget for a Typical Handset
- An Actual DECT Receiver Chip
- LNA power consumption 40 mW
- Mixer/downconverter section 50 mW
- A/D converter and BPFs 100 mW
- Total 200 mW
Rudell, et al., Proc. 1997 ISSCC
105Micro ElectroMechanical (MEM) Systems
- RF MEMS is a unique technology that offers a
significant impact on RF flexibility, performance
and cost
A Near Perfect Switch
106HRL MEMS Circuit
DC Bias
RF Signal
107Example of MEMS in SDR
MEMS Reconfigurable Antenna
MEMS Switchable Impedance Matching Circuit
ADC
LO
Software Control
108MEMS Applications and Future (1)
- Switches should be the first widespread
application - - Very low insertion loss 0.1 to 0.2 dB
- - Good isolation that depends on switch
configuration - - Good RF power-handling (gt 1 W)
109MEMS Applications and Future (2)
- Switches could enable a new class of RFICs
- - Integrated RF systems (e.g., multiband
radios-on-a-chip) - - New system architecture (e.g., reconfigurable
apertures) - - New RF functionality (e.g., quasi-optical beam
steering)
110MEMS Designs for RF Front Ends
- Design flexible filters using two-value
switchable capacitors - Tunable capacitors
- Two distinct capacitor values Con and Coff
- Two value capacitors arranged in parallel to form
digitally tunable capacitors
111MEMS Designs for RF Front Ends
- Inductors
- Fixed or variable
- High Q inductors for filters
- Tunable filters
- Use MEMs filter banks to create tunable RF filters
112MEMS Designs for RF Front Ends
E-tennas Reconfigurable Antenna
- Tunable antenna with narrow fixed bandwidth
- Patch antenna connected by RF switches
113Role of Superconductors In Software Radios (1/3)
- Extremely fast ADCs and DACs
- Based on superconducting quantum interference
device (SQUID) - Enable Digital-RF processing (TRF)
- Sampling rates now 20-40 GHz and 15 bits
- Enable more precise predistortion with DAC to RF
low feedback time for predistorter
Deepnarayan Gupta,etl, Benefits of Superconductor
Digital-RF Transceiver Technology to Future
Wireless Systems, SDR Technical Conference, Nov.
2002, pp 221-226. Superconductor Digital-RF
Transceiver Components, SDR Technical Conference,
Nov. 2002, pp227-232.
114Role of Superconductors In Software Radios (2/3)
- Sensitive enough to eliminate the LNA and lower
noise floor resulting in - Lower power
- Lower interference
- Greater range
- Greater capacity
115Role of Superconductors In Software Radios (3/3)
- High Purity Clock Sources to Reduce Jitter
- Extremely Fast Decimation and Matched Filters
116Transmitter Design for SDR
DIGITAL PREDISTORTER
DAC
Software can be used to control sampling rate
and resolution for different signaling standards
I Q can be predistorted by software to
compensate for nonlinearity of power amplifier
Software based power management is possible
(i.e., periodically turn the PA off, or adjust
bias to lower power consumption
Biasing can be dynamically adjusted by software
to reduce distortion
Flexibility in gain and bandwidth needed for
multimode operation
Mixer
Tuning controlled by software
BPF
PA
LO
117Receiver Design for SDR
LNA
BPF
AGC
Software based power management is possible
(i.e., periodically turn the LNA off, or adjust
bias to lower power consumption
Flexibility in gain and bandwidth needed for
multimode operation
Receiver noise and distortion can be minimized by
software controlled gain and attenuation
Sampling rate, resolution, interference rejection
controlled by Software
Biasing can be dynamically adjusted by software
to reduce distortion
Bandwidth and center frequency controlled by
software
Mixer
ADC
LPF
Tuning controlled by software
LO
118Summary
- RF design for multimode radios can be very tricky
- Best design must balance the performance of the
RF, A/D, and back-end DSP - Numerous tradeoffs must be made
- Software radio techniques can be used to
compensate for imperfection in RF components and
change the nature of the tradeoffs