RF Issues by ewghwehws

VIEWS: 10 PAGES: 118

									 RF Issues for Software Radios

RF Engineering for the DSP Engineer

                RF Receiver Chain
              RF Transmitter Chain
A Quantitative perspective of noise and distortion
      Overcoming RF limitations with DSP
 MPRG                                           1
        What You’ll Learn
 Role of RF in SDR
 SDR RF Structures
 Common Performance Metrics
 SDR Amplifiers and Overcoming RF
 Problems with DSP
 Impact of MEMs

MPRG                                2
         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.

MPRG                                       3
           Generic Transmitter

                                         Digital to   Input
  Selection and       Conversion
  Amplification                           Analog

                  transmitter section

MPRG                                                            4
             Generic Receiver
  Antenna                           Digital
       Selection   Conversion   Analog-to-
                    RF Front

MPRG                                         5
       Purpose for the RF
 Extract a desired low level signal: 10-16 to 10-3
 Reject out of band noise and interference
 Convert signal’s center frequency to a range
 compatible with the A/D.
 Modulate, amplify, and filter signal for
 Minimize additive noise and distortion

MPRG                                            6
       Goals of RF in SDR (1/3)
 Support MBMMR – Multi-band,
 multimode radio
   Operate over numerous bandwidths, frequencies,
   Implies wideband operation, but…
 Produce highest quality signal for
 baseband processing
   Reject out-of-band noise and interference
   Implies narrowband operation…
MPRG                                                7
       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
 Practical Considerations
   Low Cost
   Low Power
   Small Form Factors

MPRG                                             8
       Goals of RF in SDR (3/3)
 Have your cake and eat it too…

 However, complementing hardware
 with software changes the RF “cake”
MPRG                                   9
 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

MPRG                           10
RF View of Radio System (2)

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

MPRG                                 11
   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
  Tuning the antenna can help multi-band capability
  Form factor is important for handsets
  Antenna selectivity impacts subsequent RF
  performance requirements

MPRG                                               12
 Key Antenna Design Issues (2)
 Gain versus directionality trade-off
 Sensitivity to coverings (e.g., hand)
 Diversity design -- multiple antennas and
 smart antenna algorithms
 Trade-offs in bandwidth, efficiency, and
 Radiating head – signal loss and
 detuning impact

MPRG                                   13
     Key Receiver Parameters
    Defines the weakest signal that a receiver can detect and is
    usually determined by the various noise sources in the
    receiving system.
    The ability of the receiver to detect the desired signal and
    reject others.
 Spurious Response:
    The spurious response is a receiver’s freedom from
    interference due to these internally generated signals or their
    interaction with external signals.
    Receiver gain & frequency change with temperature, time,
    voltage, etc.
MPRG                                                             14
  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

 MPRG                                    15
                        Dynamic Range
Bit Error Rate (BER)

                       Noise Limited          Spurious Signal

                           Usable Dynamic Range
                                       Received Signal Power
  MPRG                                                    16
Factors Impacting Dynamic Range
        Range                   Converter


       Analog                     AGC
       Front End               and Power
       Constrains               Control

MPRG                                         17
    Various Types of Receivers
- Tuned Radio Frequency Receiver
Input signal level may span
                                           To A/D
100 dB in dynamic range
              BPF                    AGC
            RX filter
                Not very practical

MPRG                                              18
   Tuned Radio Frequency Receiver

Advantages            Disadvantages
•Few analog            Not as practical
components             Extreme demands on RF
•Isolation problems    and A/D
minimized                 Tunable very
                          narrowband filters
                          Dynamic range of LNA

MPRG                                      19
               Direct Conversion Receiver
Input signal level may span
100 dB in dynamic range

              BPF         LNA     BPF       AGC               LO ADC
          RX filter             RX filter              90

Advantages                                                          Q
•Fewer parts                     High isolation needed in mixer between LO
•Analog Images Eliminated       and LNA input
•Conceptually simple               eg, signal = -116 dBm, LO = 5dBm, thus
•Possibly lower power              isolation >> 120 dB
                                 Phase noise of LO critical
consumption                      DC offset at A/D substantial and dynamic
                                 Balanced mixers needed – can be freq. dep.
        MPRG                     Second order distortions occur in band
      Direct Conversion Transmitter
                                            Spurs typically
                                            60 dB down or more
Binary I Frequency Source
Data                                  BPF      Power
         (DDS or                                            BPF
Source Q Programmable VCO)                    Amplifier
                             RF VCO     I/Q LO source
• Conceptually simple filter Disadvantage
  requirements               • Not as practical because of
• Low complexity               isolation problems
• Circumvents image problems • Balance in I&Q
                             • Consistent performance
                               over wide band         21
      Multiple Conversion Receiver                                     I

    BPF                BPF            BPF            BPF   AGC         LO
                   Image filter                                  90
  RX filter                         Image filter
                              IF LO          IF LO                     Q
• More isolation than direct conversion or single    Disadvantages
  conversion (due to distributing gain into to       • More parts
  sections)                                          • More power
• Better rejection of ACI
• Better gain possible through distributed

     MPRG                                                         22
    IF and Low -IF Conversion Receiver

    BPF       LNA      BPF             BPF   AGC      D              LO
                                                      C       90
  RX filter         Image filter

                               IF LO                                 Q
Advantages                                   Analog       Digital
• More isolation than direct
  conversion – less DC offset
                                       • Gain is limited
• Lower parts count than dual
                                       • More image rejection
                                         required over direct
     MPRG                                conversion
              Review of the Mixing Process          Amplitude

                                                                  Adjacent                        Desired
                                                                  channel                         signal

-                      136MHz                                                   136MHz                     
               -d                   -1                                1                  d

                                                                                           Desired signal
                                                                Adjacent channel
                                                                interference                     Desired signal
                                                                upconverted by                   corrupted by
                                                                the mixer                        interference

-                   -1-68=-wd+68                                           1+68=wd-68                     
     -d-68                                -1+68               1-68                               d+68
     MPRG                                                                                                   24
            Two Stage Transmitter

Binary I Frequency Source
Data     (DDS or                       BPF            BPF
Source Q programmable VCO)
                              RF VCO         RF VCO

  Advantage                  Disadvantages
  • Better isolation         • More parts and higher cost
                             • Higher power consumption
     MPRG                                                   25
Transmitter Component Issues (1/2)
Transmit IF VCO
  Phase Noise Provides Significant Modulation to
  Narrowband Signals
  Linearity to Reduce Spurious Products
  Balance Between I&Q Required to Keep
  Distortion (Sidebands) Down
Variable Gain Amplifier
  Linearity and Fidelity
 MPRG                                        26
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
           (in practice only about 25% of the battery
 MPRG      is effectively used during the talk time
Key Receiver Component Issues (1/2)
   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

 MPRG                                                     28
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 due to mixer and LNA non-linearities may end up
       in the IF pass band after mixing
  MPRG                                                      29
  Impact on Constellation Due to
         Imperfect Mixing
               Q                              Q


                                  I                          I

       a. Ideal QPSK                  b. DC bias due to self-mixing
       Constellation                                                  I

              Q                              Q

                              I                          I

                                      d. Phase mismatach
       c. Gain mismatch
MPRG                                                                      30
       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

MPRG                                 31
         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

MPRG                                           32
                       Digital AGC
                                                                To Software Receive
Input Signal                           A/D
               Amp                   Converter


     D/A         Gain Factor   Inactive
   Converter      Mapping                     Mode              +
                               Tracking      Selector   


   MPRG                                                                     33
                           AGC Modes
Slew                                                  Slew
Mode                                                  Mode
Low amplitude                                      High amplitude
level                                              level
                                                           Input Signal
                Tracking                  Tracking         Level
                Mode                      Mode

                           AGC Inactive
  MPRG                                                             34
Key Transmitter RF Design Issues
 Power Efficiency
 Modulation Accuracy and Linearity
 Spurious Signal Reduction
 SNR of Transmitted Signal
 Power Control Performance
 Output Power Level
MPRG                                 35
 Transmitter Component Issues:
       Ocsillator & Mixer
 Transmit IF VCO
  noise floor
  power consumption
  phase noise provides significant
  modulation to narrowband signals
  Linearity to reduce spurious products
  Noise floor
  Power consumption
MPRG                                      36
Key Transmitter Component Issues:
        balance between I&Q required to
        keep distortion (sidebands) down
       Noise figure
         Power consumption
       Variable Gain Amplifier
         Linearity and fidelity
MPRG     Noise figure                37
 Transmitter Component Issues:
        Transmit Filters
  Transmit Filters
    Isolation of transmitter
    noise from PA leaking into
    the receiver (supplement
    low loss required
MPRG                             38
 Transmitter Component Issues:
      Power Amplifier (1)

   Power Amplifier (very critical)
     Cost - especially for base stations
     Noise floor
     Spurious response (source of
MPRG                                 39
 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)

MPRG                                     40
 General Performance Metrics
 Noise Characterization and Figure
 Spurious Free Dynamic Range
 Blocking Dynamic Range
 Power Consumption

MPRG                                 41
Noise Characterization (1)
Noise  is    introduced    into    resistive
components due to thermal actions.

V  4kTRB
   2                         V
                         P   n
  n                          4R .
where k is Boltzman’s 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.

MPRG                                     42
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

MPRG                                    43
             Noise Figure
Noise Figure (NF) measure the amount of
noise an element (or elements) adds to a
             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.
 MPRG                                    44
     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.
                          NF2  1 NF3  1 NF4  1
NFtotal  1  ( NF 11)                          
                           G1       G1G2    G1G2 G3
 (equation 1)
     Here NFi represents the noise figure at
     the ith stage and Gi represents the gain at
     the ith stage (units are linear).
 MPRG                                           45
       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
MPRG                                             46
       Example NF Calculations (1/2)
                                       NF3 =2 dB
                            NF2=2 dB
                                       G3=10 dB    NF4=6 dB
                            G2=-2dB                           To Next IF
           Cable, G1=-3dB                                     Chain
Anttenna                      BPF      LNA           X


                              NF2  1 NF3  1 NF4  1
 NFtotal     1  ( NF 11)                         
                               G1      G1G2    G1G2 G3

 The total noise figure equals 5.975 .

   MPRG                                                              47
  Example Noise Calculations (2/2)
Does ordering of the components yield optimal NF?
                      NF3 =2 dB
           NF2=2 dB
                      G3=10 dB                     NF4=6 dB
           G2=-2dB                                            To Next IF
                                  Cable, G1=-3dB              Chain
Anttenna     BPF      LNA                            X

 the total noise figure equals 3.6.       LO

 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
   MPRG                                                              48
      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
  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.

MPRG                                               49
   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.
MPRG                             50
    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.

MPRG                                     51
    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.

MPRG                                           52
                  RF Distortion - BDR
Output Power            Output Power = G  Input Power
   (dBm)                Output Power(dB) = Input Power(dB) + GdB
       P1dB,out   1dB             MDS Minimum Detectable Signal

                                  P1dB,in Input 1 dB compression point
                                  P1dB,out Output 1 dB compression point
                                  BDR Blocking Dynamic Range
             1                    BDR = P1dB,in - MDS
                                  Noise Floor

MDS                     P1dB,in     Input Power (dBm)
   MPRG                                                           53
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.

MPRG                                   54
 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
MPRG                                        55
  Spurious Free Dynamic
        Range (2)
 This test mimics real world situations
 where adjacent channel interference
 can cause significant intermodulation
 Typical dynamic range values extend
 from 60dB to 90 dB.

MPRG                                      56
  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
             RF Distortion - Intermod
Output Power                 Predicts Susceptibility to Adjacent
   (dBm)                     Channel / Nearby Interference
         OIP3             IIP3 3rd Order Input Intercept Point
                          OIP3 3rd Order Output Intercept Point
                          SFDR Spurious Free Dynamic Range
                                SFDR= 2/3 (IIP3 – MDS)
             1       3    IIM3 Intermod due to 3rd Order
                          IIM3 = 3PI - 2 IIP3 (dBm)
                 1         Noise Floor

MDS                      IIP3 Input Power (dBm)
   MPRG                                                      58
          System Level Distortion
   The effects of non-linear distortion are cumulative. An
   overall IIP (either IIP2 or IIP3), IIPtotal can be computed
   using the following approximation.
  1       1     G1 G1G2     G1G2 Gn 1
                     
            1                  IIPn
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.

MPRG                                                              59
          RF Distortion – Intermod for
               Cascaded Devices
                                       IIP3 =2 dB
                           IIP2=2 dB
                                       G3=10 dB     IIP4=6 dB
          Cable, G1=-3dB
From                                                            To Next IF
Antenna                      BPF       LNA            X         Chain


      1       1     G1 G1G2     G1G2 Gn 1
                         
    IIPtotal IIP IIP2 IIP3
                1                  IIPn

              “Dominated” by Worst IIPi
    MPRG                                                                60
        A/D Distortion
 Composite RF and A/D noise and distortion
 is needed to quantify the overall receiver
 A conservative design approach is to
 choose an A/D converter that introduces
 insignificant noise contribution compared to
 the overall RF chain.

MPRG                                            61
       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

MPRG                                                        62
           Example of A/D Impact (2)
                                                 Ranalog                           2
                                                                +              iA/D = Pquant / RA/D
  2                                                    RA/D     V A/D,total
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 =
       MPRG resistance of the A/D converter                                               63
      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

     MPRG                             64
Using SDR to Change the “Cake
 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

MPRG                                  65
       Significance of the PA
 Quality determines capacity
 Output power defines coverage
 Impacts size of BTS
 Dominates infrastructure costs
 Major contributor to BTS operating
 Dominates power consumption
MPRG                              66
  Transmitter Component Issues:
       Power Amplifier (1)

Power Amplifier (very critical)
  Cost - especially for base stations
  Noise floor
  Spurious response (source of
MPRG                                67
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)

MPRG                                     68
Handling Multiple Channels– Today’s
          8dB LOSS
                                 SCPA – Single Carrier Power Amplifier
                                 MCPA – Multi Carrier Power Amplifier

                       4 x 20W


SCPAs       Combiners
                     Radio        power
                                                       Band pass filter
                                 combiner    MCPA         diplexer

                                              MCPA based BTS
                                             GSM, GPRS & EDGE
Realizing Multiple Channels with SDR
           and a Single PA     Antenna

      digital                      Band pass filter
       radio                          diplexer

      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

  MPRG                                                     70
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
 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.
MPRG                                                                   71
TX Requirements for Common Standards
  System                       PAR           PMR             Duplex       PCDR
                               (dB)          (dB)                         (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-             Full         73
  CDMA2000-1xRTT               4-9           Infinite        Full         80
  TD-SCDMA                     2.5-7         Infinite        Half         80
   MPRG    Earl McCuner, “SDR Radio Subsystems using Polar Modulation,”
           SDR Technical Conference, Nov. 11, 2002
Non-Linearity and Power Amps

         Class A – Best
         Class AB, B – Mid-range
         Class C – Worst
         Class A – Worst
         Class AB, B – Mid-range
         Class C – Best
MPRG                               73
 Simple Power Amplifier Model
      (no memory effect)

MPRG                            74
    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

MPRG                                         75
Predistortion with Memory
Tapped Delay Line PD

 Complex gain

Hammerstein PD
                x(n)            y(n)

MPRG                              76
   Memory Effects in PA (2/2)

                                 hysteresis loops

       4-carrier W-CDMA input, PAPR = 13.7 dB
MPRG   Class B power amplifier (30W approx.)
                                                     Distortion Effects
Non-Linearity >>> Spectral Regrowth
                                            Input to a Nonlinear Amplifier                                                           Output from a Nonlinear Amplifier
                      0                                                                                         0


   Magnitude in dB


                                                                                             Magnitude in dB


                     -30                                                                                       -25

                     -35                                                                                       -30

                     -40                                                                                       -35

                           0   0.1   0.2   0.3     0.4 0.5 0.6 0.7           0.8   0.9   1                     -40
                                                 Normalized Frequency                                                0   0.1   0.2   0.3     0.4 0.5 0.6 0.7             0.8   0.9   1
                                                                                                                                           Normalized Frequency

   Low PA Efficiency reduces battery life
 MPRG                                                                                                                                                                                    78
  Why is this a Problem? (1/2)
 Modern comm. systems use non-
 constant envelope modulation
 Non-Constant envelope signals
 require linear amplifiers
   Changes in amplitude cause
   spurious emissions
MPRG                             79
  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

MPRG                              80
 Non-Linearity and Power Amps
  Linearity                 Tradeoff
    Class A – Best    Linearity for Efficiency
    Class AB, B –
    Class C – Worst
    Class A – Worst
    Class AB, B –
    Class C – Best
MPRG                                        81
Techniques to Change the Amplifier
 Backoff Nonlinearity Tradeoff
 Cartesian Feedback
 Analog Predistortion
 Digital Predistortion
 Linear amplification using Nonlinear Components
 Envelope Elimination & Restoration (EER) (also
 known as polar amplifier)

MPRG                                        82
 Non-linear region
is at higher power
levels                        Non-linear Region

 Operate amplifier at a   Linear
 fraction of it’s rated   Region
 maximum power
 Backoff level
 depends on amplifier

MPRG                                        83
       Backoff Pros and Cons
 Wastes amplifier capacity/efficiency
 Requires amplifier with power limit
 significantly higher than operating point
  • Very expensive solution
MPRG                                     84
  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

MPRG                                  85
          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
               fPA  fPRE ( z)  G0 z

        z is the complex input to the predistorter and G0 is the
MPRG linear gain of the overall system                             86
    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.
MPRG                                      87
               Conceptual Approach to
            Predistortion -- Analog Domain
       Digital               Analog
       Domain                Domain

                         Gain = F(|Sr|)
Sr ideal                                                                                 signal
                                          Transmitter   Sp
modulated signal                                                                         So = KSrCos(ct)
                   D/A                        IF                Gain =
                                                                               Note F(|Sr|) G(|Sp|) = K,
                          Predistortion                                        K= constant gain
                          function                           Non-linear
                                                             power amplifier

          MPRG                                                                                      88
            Conceptual Approach to
         Predistortion -- Digital Domain                                 Transmitted signal
                                                                         So = KSrCos(ct)
                      Digital              Analog
                      Domain               Domain
                   Gain = F(|Vm|)                                  Gain =

Vm ideal
                                          Transmitter   Sp
modulated signal
                                    D/A       IF

                   function                                  Non-linear
                                                             power amplifier

     MPRG                                                                              89
Example Predistortion Technique
   Source     Digital                                          Amplifier
   Signal   Predistorter              DAC
                                      Iout &      Quadrature
                                       Qout       Modulator

                           Calibration Signal

                              Iin &             Quadrature
            Adaptation                          Demodulator

     • AM/AM and AM/PM distortion compensated
     • Large calibration table needed and must be updated
 MPRG Linearity of feedback loop is an issue.       90
        Digital Predistortion

     W-CDMA input, PAPR = 7.8 dB
MPRG 3rd order polynomial Predistorter (PD)   91
     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

MPRG                                           92
        Flexibility Tradeoff
 SDR Necessitates a Flexible Front-end
 in terms of Center Frequency and
  Modern Signaling Requires High
 Performance Components, Indicates
 Specific Component Design

MPRG                                     93
 New Amplifier Topologies (1)
Not really new (Re-emerging)
  See Chapter 8 of “RF Power Amplifiers for
  Wireless Communications” by Steve C.
Linear Amplification with Nonlinear
  LINC for short
  Uses two nonlinear amplifiers and combines
MPRG                                    94
New Amplifier Topologies (2)

 Envelope Elimination and Restoration
   The signal is split amplitude and
   Use nonlinear amplifier for phase and
   restore envelope by modulating
   supply voltage

MPRG                                   95
New Amplifier Topologies (3)

  Both EER and LINC show improved
  efficiency and potential for improved
  These amplifiers made practical by
  digital radio design
  Commercial products based on both
  methods have been introduced

MPRG                                      96
                         LINC (1)
        First proposed by Chireix in 1935           S1(t)

                          Power                     S(t)

S(t)                                                     S2(t)
          AM-PM                      +

                 S2(t)   Amplifier

       MPRG                                         97
              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.
MPRG                                         98
                  EER (1)
  First introduced by Kahn in 1952

       Envelope        Video Power
       Detector        Conditioner

                                         May be a
                                         digital input

       Splitter    Limiter

MPRG                                                99
             EER (2)
 Signal is split in to envelope and phase
 Envelope component is used to
 modulate PA supply voltage
 Constant envelope phase component is
 used to drive the nonlinear power
 Ideally 100% efficient
MPRG                                   100
 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
MPRG                                   101
Power Supply Issues: Battery Life
Non               Watt-
Rechargable       hrs/kg        Rechargable         Watt-
Lithium Vinyl        684                            hrs/kg
Chloride                        Lithium Metal          140
LiSO2                350        Lihium Polymer         125
LiCFx                330        Lithium Ion            110
Lithium              305        Zinc Air                80
Manganese                       Nickel Metal            60
Dioxide                         Hydride
Li/LiCoO2            160        Nickel Cadmium          45
Alkaline              80        Lead Acid               35
                Battery technology energy density doubles
 MPRG           every 35 years. Powers, Proc. of IEEE, April 95
  Other Considerations
     with Batteries
  Environment (NiCa is terrible)
  Shelf life (length of time a
  charge is maintained)
  Cell Voltage
  Cycle Life (how often can it be
MPRG                            103
Example Power Budget for a Typical
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   104
Micro ElectroMechanical (MEM) Systems
     RF MEMS is a unique technology that offers a
    significant impact on RF flexibility, performance
    and cost
   Example MEMS Switch (Cantilevered Beam)
           Switch Up (Off State)
                                   A Near Perfect
  MPRG                                             105
       HRL MEMS Circuit

       DC Bias   RF Signal
MPRG                         106
          Example of MEMS in SDR
e             Switchable
Antenna       Impedance
            Matching Circuit
                                            MEMS Filter
                                             for Band 1
                                             MEMS Filter
                                              for Band 2        ADC

                                             MEMS Filter
                                             for Band N

                               Software Control
 MPRG                                                                 107
MEMS Applications and Future (1)
  Switches should be the first widespread
 - Very low insertion loss ~0.1 to 0.2 dB
 - Good isolation that depends on switch
 - Good RF power-handling (> 1 W)

MPRG                                        108
MEMS Applications and Future (2)
 Switches could enable a new class of
 - Integrated RF systems (e.g., multiband
 - New system architecture (e.g.,
 reconfigurable apertures)
 - New RF functionality (e.g., quasi-optical
 beam steering)

MPRG                                           109
MEMS Designs for RF Front Ends
Design flexible filters using
two-value switchable
Tunable capacitors
  Two distinct capacitor values Con
  and Coff

  Two value capacitors arranged in parallel to form digitally
  tunable capacitors

MPRG                                                            110
MEMS Designs for RF Front Ends
   Fixed or variable
   High Q inductors for filters
 Tunable filters
   Use MEMs filter banks to create tunable
   RF filters

MPRG                                         111
MEMS Designs for RF Front Ends

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

MPRG                                          112
    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.

MPRG      Superconductor Digital-RF Transceiver Components, SDR Technical Conference, Nov. 2002, pp227-232.

    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

MPRG                                     114
    Role of Superconductors In
      Software Radios (3/3)
 High Purity Clock Sources to Reduce
 Extremely Fast Decimation and Matched

MPRG                                115
          Transmitter Design for SDR
 PREDISTORTER                                      DAC
 I & Q can be                           Software can be used to
 predistorted by                        control sampling rate and
 software to compensate                  resolution for different
 for nonlinearity of                       signaling standards
 power amplifier

   Biasing can be            Flexibility in gain    Software based power
   dynamically adjusted      and bandwidth          management is possible
   by software to reduce     needed for             (i.e., periodically turn the
   distortion                multimode operation    PA off, or adjust bias to
                                                    lower power consumption
Mixer    Tuning controlled
         by software
                                 BPF                          PA
              LO                                                               116
                Receiver Design for SDR
        BPF                       LNA                         AGC
     Flexibility in         Software based
                            power management is       Receiver noise and
     gain and
                            possible (i.e.,           distortion can be
                            periodically turn the     minimized by
     needed for
                            LNA off, or adjust        software controlled
                            bias to lower power       gain and attenuation

  Biasing can be            Bandwidth and       Sampling rate,             IQ
  dynamically adjusted      center frequency    resolution, interference   Imbalance
  by software to reduce     controlled by       rejection controlled by    Digital
  distortion                software            Software                   Filtering
Mixer   Tuning controlled        LPF                     ADC
        by software
   MPRG LO                                                                      117
RF design for multimode radios can be very
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
MPRG                                   118

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