Lecture 2: RF Issues for Software Radios RF Engineering for the DSP Engineer

1
Lecture 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

2
What Youll Learn

• Role of RF in SDR
• SDR RF Structures
• Common Performance Metrics
• SDR Amplifiers and Overcoming RF Problems with
  DSP
• Impact of MEMs

3
Role 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.

4
Generic Transmitter

Antenna

Digital
Input

Digital to
Selection and
Conversion


Analog
Amplification


Conversion



transmitter section


5
Generic Receiver
Antenna
Digital
Output


Analog
-
to
-
Selection

Conversion

Digital


Conversion

RF Front
End

6
Purpose 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

7
Goals 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

8
Goals 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

9
Goals of RF in SDR (3/3)

• Have your cake and eat it too
• However, complementing hardware with software
  changes the RF cake equation

10
RF 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

11
RF View of Radio System (2)

• Noise and Distortion Characterization
• Noise
• Non-linear distortion
• Compensation for distortion
• Power Supply and Consumption Considerations

12
Key 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

13
Key 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

14
Key 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.

15
Characteristics 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

16
Dynamic Range Constraints
Spurious Signal Limited
Noise Limited
Bit Error Rate (BER)
Usable Dynamic Range
Received Signal Power
17
Factors Impacting Dynamic Range
A/D Converter Constraints
Range
Selected Dynamic Range
AGC and Power Control
Analog Front End Constrains
18
Various 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
19
Tuned 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

20
Direct 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

21
Direct 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

22
Multiple 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
23
IF 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

24
Review 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
25
Two 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

26
Transmitter 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

27
Transmitter 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
28
Key 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

29
Key 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

30
Impact on Constellation Due to Imperfect Mixing
31
Key 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

32
Key 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

33
Digital 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
34
AGC Modes
Reference Level
Slew Mode
Slew Mode
High amplitude level
Low amplitude level
Input Signal Level
Tracking Mode
Tracking Mode
AGC Inactive Zone
35
Key Transmitter RF Design Issues

• Power Efficiency
• Modulation Accuracy and Linearity
• Spurious Signal Reduction
• SNR of Transmitted Signal
• Power Control Performance
• Output Power Level

36
Transmitter 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

37
Key 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

38
Transmitter Component Issues Transmit Filters

• Transmit Filters
• Isolation of transmitter noise from PA leaking
  into the receiver (supplement duplexer)
• low loss required

39
Transmitter Component Issues Power Amplifier
(1)

• Power Amplifier (very critical)
• Cost - especially for base stations
• Noise floor
• Spurious response (source of interference)

40
Transmitter 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)

41
General Performance Metrics

• Noise Characterization and Figure
• Spurious Free Dynamic Range
• Blocking Dynamic Range
• Intermod
• Power Consumption

42
Noise 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.

43
Noise 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

44
Noise 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.

45
Using 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).

46
Using 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).

47
Example 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 .
48
Example 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
49
Calculating 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.

50
Calculating 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.

51
Distortion 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.

52
Distortion 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.

53
RF 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
54
Spurious 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.

55
SFDR 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.

56
Spurious 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.

57
3rd 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

58
RF 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)
59
System 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.

60
RF 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
61
A/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.

62
Example 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.

63
Example 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
64
RF Distortion Cascaded SFDR
Cascaded SFDR Recipe

1. Determine Input Noise Power
2. Calculate System Gain
3. Calculate NFTotal
4. Calculate Output Noise Power
5. Calculate MDS
6. Calculate IIPTotal
7. Calculate SFDR

65
Using 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

66
Significance 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

67
Transmitter Component Issues Power Amplifier
(1)

• Power Amplifier (very critical)
• Cost - especially for base stations
• Noise floor
• Spurious response (source of interference)

68
Transmitter 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)

69
Handling 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
70
Realizing 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
71
Summary 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.
72
TX 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
73
Non-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

74
Simple Power Amplifier Model(no memory effect)
75
Memory 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

76
Predistortion with Memory
Tapped Delay Line PD (TDL PD)
Complex gain polynomial
Hammerstein PD
Filter
77
Memory Effects in PA (2/2)
hysteresis loops

• 4-carrier W-CDMA input, PAPR 13.7 dB
• Class B power amplifier (30W approx.)

78
Distortion Effects
Non-Linearity gtgtgt Spectral Regrowth
Low PA Efficiency reduces battery life
79
Why 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

80
Why 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

81
Non-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
82
Techniques 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

83
Backoff

• 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
84
Backoff Pros and Cons

• Benefits
• Simple
• Drawbacks
• Wastes amplifier capacity/efficiency
• Requires amplifier with power limit significantly
  higher than operating point
• Very expensive solution

85
Predistortion

• 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

86
Predistortion 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
87
Benefits 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.

88
Conceptual 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
89
Conceptual 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
90
Example Predistortion Technique

• AM/AM and AM/PM distortion compensated
• Large calibration table needed and must be
  updated
• Linearity of feedback loop is an issue.

91
Digital Predistortion

• W-CDMA input, PAPR 7.8 dB
• 3rd order polynomial Predistorter (PD)

92
Implementation 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

93
Flexibility Tradeoff

• SDR Necessitates a Flexible Front-end in terms of
  Center Frequency and Bandwidth
• Modern Signaling Requires High Performance
  Components, Indicates Specific Component Design

94
New 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

95
New 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

96
New 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

97
LINC (1)

• First proposed by Chireix in 1935

S1(t)
S(t)
S2(t)
98
LINC (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.

99
EER (1)

• First introduced by Kahn in 1952

Video Power Conditioner
Envelope Detector
May be a digital input
Splitter
Limiter
Power Amplifier
100
EER (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

101
Why 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

102
Power Supply Issues Battery Life
Battery technology energy density doubles every
35 years.
Powers, Proc. of IEEE, April 95
103
Other 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)

104
Example 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
105
Micro ElectroMechanical (MEM) Systems

• RF MEMS is a unique technology that offers a
  significant impact on RF flexibility, performance
  and cost

A Near Perfect Switch
106
HRL MEMS Circuit
DC Bias
RF Signal
107
Example of MEMS in SDR
MEMS Reconfigurable Antenna
MEMS Switchable Impedance Matching Circuit
  ADC
LO
  Software Control
108
MEMS 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)

109
MEMS 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)

110
MEMS 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

111
MEMS 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

112
MEMS Designs for RF Front Ends
E-tennas Reconfigurable Antenna

• Tunable antenna with narrow fixed bandwidth
• Patch antenna connected by RF switches

113
Role 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.
114
Role 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

115
Role of Superconductors In Software Radios (3/3)

• High Purity Clock Sources to Reduce Jitter
• Extremely Fast Decimation and Matched Filters

116
Transmitter 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
117
Receiver 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
118
Summary

• 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