Third Generation 3G Systems

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Third Generation (3G) Systems
3G broadband wireless communication systems

• Universal cell phones
• Mobile multimedia
• - Net phones
• Satellite radio
• Wireless internet
• Wireless local loops
• - Local data links
• - Bluetooth
• - Last-mile applications
• Automotive multimedia

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Some Needs for 3G Wireless
Average Power(W) Frequency Now Needed
Backoff Application Cellular 0.8
GHz 100 600 MCPA cellular 1.9 GHz 40 200
8-10 dB IMT-2000 PCS 2.1 GHz 40 100-200
8-10 dB IMT-2000 Satellite 2.3
GHz 125 4000 0 Satellite Radio 12
GHz 125 200-400 0 DirecTV Mobile 2.3
GHz 200 650 6 dB SatRad
repeaters 2.6 GHz 20 200 10 dB MMDS
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More Power.why
Higher data rates - higher bit transfer
rates - increase symbol transfer rate with
complex encription (16QAM etc) - broadband
modulation schemes (CDMA OFDM) require high peak
power Improved amplifier linearity - lower
adjacent channel power - increased backoff off
from peak power capability (more linearity
and higher peak-to-average ratio for CDMA
OFDM) - feed forward linearization (make up for
increased losses) Improved availability and
reliability - ability to compensate for weather
(rain) - ability to handle partial component
failure (and still broadcast)
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Higher Data Rates
Bit Error Rate for several modulation types
For fixed error rate the energy per bit is
fixed Higher data rates (more bits per second)
require higher power Higher symbol rate
requires higher energy per bit which corresponds
to higher power
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Crest Factors for Spread-Spectrum Signals
Broadband spread-spectrum signals have high peak
to average ratios (high crest-factors)
AWGN waveform
Advanced modulation techniques cause higher
peak to average ratios due to phase add up
For a given average power these waveforms
require higher peak power
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Adjacent Channel Power Intermodulation Distortion
2-Tones
Multi-tone operation produces intermodulation
distortion (IMD) Intermodulation
products cause adjacent channel power problems
8-Tones
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Adjacent Channel Power Reduction
Backoff from non-linear region
Running amplifiers backed off from saturation for
linearity (lower adjacent channel power) requires
higher peak power
Improve IMD
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Adjacent Channel Power Reduction
Multi-Channel Power Amplifier (with feed-forward
circuit)
TWT
TWT with feedforward
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Solid State rf Devices
Solid state device frequency and power
New developments driven by communications
needs Single device power level still
insufficient (6 dB backoff from 50 W is only
about 10 W per transistor) How do we get more
power
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Power Combining
Solid state devices have limited gain and power
capability per device Use series and parallel
arrays to produce gain and power
Power combined arrays are required
(10 dB per device)
Broadband produces high peak electric fields
Many devices needed to avoid breakdown damage
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Solid-State Arrays - Issues
Combiner losses are significant for large
numbers of devices - ultimately adding more
devices doesnt give more power Reliability of
an array (many-components) - failures from
transients junction avalanche overdrive high
VSWR etc. Aging of solid state devices -
metal migration at high current density and high
junction temperature - corrosion of intermetal
contacts - thermal fatigue
Aging produces - transconductance decrease -
threshold voltage changes - resistance
changes - operating point changes (impedance
change) - power and gain degradation
Example two devices in a Wilkinson power
combiner power output decreases directly with
impedance change
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The Solution - VED
Vacuum Electronic Devices
Tubes work everywhere within this box
Traveling wave tubes and klystrons are used in
90 of the satellite communcation applications
with demonstrated life and reliability well in
excess of solid state amplifiers!
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Amplifier Efficiency
TWTs are much more efficient than solid state
amplifiers
All data points are for multi-channel PCS
amplifiers with feedforward linearization and -70
dBc IMD
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Amplifier Linearity
Highest Power LDMOS PCS Solid State Devices
Solid state devices and tubes have similar
linearity but tubes have significantly higher
power capabilities!
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Satellite Radio Systems
Satellite Transmitter
Estimated link budget
Power combined array of 48 TWTs produces 4 kW of
radiated power
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Power combining of TWTs
Power combining of two TWTs
P 0.5P1 P2 2(P1 P2 )1/2 cos Df
Depends on power and phase balance (10 deg of
phase or 2 dB in power exceeds Magic-T losses)
Amplitude
Phase
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Phase Variability of TWT array
Phase versus input drive measured for 35 TWTs

• The power loss in the array of TWTs is
  proportional to cos Df
• Using the phase deviation from the mean the
  total power loss at saturation is about 0.1
• Measured phase distribution creates negligible
  power loss

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Gain Variability of TWT array
Gain versus input drive measured for 35 TWTs
Gain distribution 0.5 dB at saturation Produces
very small power variation
Gain change with time for different types of TWTs
Gain is stable after sufficient burn-in time
D.M.Goebel Theory of Long Term Gain Growth in
Traveling Wave Tubes IEEE Transactions on
Electron Devices 42 (2000) p.1286.
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Power Combining Results
3G telecommunications applications require
operation 6 to 10 dB backed off from saturation
for linearity but spread spectrum signals still
sample saturation due to high crest factor
Phase and gain variations were measured for 35
Model 5525H TWTs operated 6 dB backed off from
saturation Arrays of these TWTs with 5
phase variation and 1 dB gain variation at
saturation produce negligible power combining
losses (0.2) Primary losses at low power
are in the combiners (Wilkenson hybrids) and
the primary cost at high power is in the
waveguide combiners
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Conclusion
Many 3G applications need higher transmit power
at higher frequency in addition to other
features like linearity high efficiency low
cost etc. The requirements for a high power
and higher frequency technology continue to point
obstinately in the direction of the vacuum
device. S.C. Cripps RF Power Amplifiers for
Wireless Communication Artech (1999)