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
Some Needs for 3G Wireless
Frequency
Average Power(W)
Now
Needed
0.8 GHz
1.9 GHz
2.1 GHz
100
40
40
600
≥200
100-200
2.3 GHz
12 GHz
125
125
4000
200-400
2.3 GHz
2.6 GHz
200
20
650
200
Backoff
Application
8-10 dB
8-10 dB
MCPA cellular
IMT-2000 PCS
IMT-2000
Cellular
Satellite
0
0
Satellite Radio
DirecTV
Mobile
6 dB
10 dB
SatRad repeaters
MMDS
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)
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
Crest Factors for Spread-Spectrum
Signals
Broadband, spread-spectrum signals have high
peak to average ratios (high “crest-factors”)
100
AWGN waveform
Time (%)
10
• Advanced modulation
techniques cause higher peak
to average ratios due to
“phase add up”
1
• For a given average power,
these waveforms require
0.1
higher peak power
0.01
-15
-10
-5
0
5
10
Output Power (dB relative to average)
15
Adjacent Channel Power
Intermodulation Distortion
Carriers
2-Tones
3rd-order
distortion
C/3IM (dBc)
• Multi-tone operation produces
intermodulation distortion (IMD)
(2f1 -f2)
f1
f2
(2f2 -f1)
8-Tones
• Intermodulation products cause
adjacent channel power problems
Adjacent Channel Power Reduction
Backoff from non-linear region
Output Power (dBm)
65
Saturation
60
6 dB backoff (DARS)
55
Linear
regime
9 dB backoff (PCS)
50
0
0
5
10
15
20
25
30
Input Drive Power (dBm)
35
50
Improve IMD
2-Tone C/3IM (dBc)
45
40
35
30
25
20
12
11
10
9
8
7
6
Backoff from Saturation (dB)
5
4
Running amplifiers backed off
from saturation for linearity
(lower adjacent channel power)
requires higher peak power
Adjacent Channel Power Reduction
Predistorter
Power
Amp
Delay
line
Input
Signal
Output
Multi-Channel Power Amplifier
(with feed-forward circuit)
-10 to -20 dBc
TWT
Delay
line
- -30 dBc
Correction
Amp
-30 dBc
TWT with feedforward
Solid State rf Devices
• Solid state device frequency and power
HF
3 MHz
VHF
30 MHz
UHF
300 MHz
µwave
3 GHz
mm-wave
30 GHz
300 GHz
Si MOSFETs, JFETs
Bipolar transistors
GaAs, GaN FETs
from "RF Power Design Techniques"
by I.M. Gottlieb
105
• New developments driven
by communications needs
from 50 W is only about 10 W per
transistor)
• How do we get more
power?
Power (W)
• Single device power level
still insufficient (6 dB backoff
104
103
LDMOS
(for PCS)
102
Single-device
transistors
(all types)
10
1
104
105
106
107
GaN
(goals)
108
Frequency (Hz)
109
1010
1011
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
Output
Input
(≈10 dB per device)
Gain
Power
• Broadband produces high
peak electric fields
• Many devices needed to
avoid breakdown damage
Peak RMS Electric Field
8
7
6
Coherent
Phase
5
4
3
Random
Phase
2
1
5
10
15
20
Number of Tones
25
30
35
Solid-State Arrays - Issues
• Combiner losses are significant for large numbers of devices
- ultimately adding more devices doesn’t 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
The Solution - VED
Vacuum Electronic Devices
105
Power (W)
104
Tubes
Tubes work
everywhere
within this box
103
102
Single-device
transistors
(all types)
10
1
104
105
106
107
108
109
1010
1011
Frequency (Hz)
• 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!
Amplifier Efficiency
TWTs are much more efficient than solid state amplifiers
MCPA Efficiency (%)
30
25
TWT-Amplifiers
20
15
10
Solid state
Amplifiers
5
0
0
50
100 150
200 250 300
350 400
Output Power (W)
All data points are for multi-channel PCS amplifiers with feedforward
linearization and -70 dBc IMD
Amplifier Linearity
Highest Power LDMOS
PCS Solid State Devices
Intermodulation Distortion (dBc)
60
1.9 GHz 2-Tone
55
50
45
40
35
TWT
Solid State
30
25
20
0
100
200 300 400 500
Output Power (W)
600
Solid state devices and tubes have similar linearity,
but tubes have significantly higher power capabilities!
700
Satellite Radio Systems
Satellite Transmitter
Estimated link budget
Parameters
frequency (GHz)
wavelength (m)
amplifier power (Watts)
power (dBm)
transmitter elliptic antenna
dimensions (m)
antenna efficiency (%)
transmitter antenna gain (dB)
EIRP
distance (km)
propagation loss (dB)
atmosperic loss
receiver elliptic antenna
dimensions (m)
antenna efficiency (%)
receiver antenna gain (dB)
receiver noise figure (dB)
background sky temperature (K)
equivalent temperature (K)
No, noise level (dBm/Hz)
received C/No (dB/Hz)
data rate (bps)
Eb/No (dB)
entered v alues c alculated v alues
2.34
0.1281
4000
66.00
3.9
4.8
70
38.96
104.99
35,784
2
0.05
0.05
55
-190.91
-2.00
Input
Output
x 48
TWTs
-0.83
13
25
5,521
-161.18
7,000,000
-68.45
3.99
Power combined
array of 48 TWTs
produces ≥4 kW of
radiated power
Power combining of TWTs
Power combining of two TWTs
P = 0.5[P1 + 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)
0.14
65
1-Tone
OFDM
0.08
Amplitude
0.06
0.04
0.02
Combining Loss (dB)
0
0
0.5
1
1.5
2
Power Imbalance (dB)
2.5
Output Power (dBm)
0.1
3
60
55
50
0
0.14
-5
0.12
-10
0.1
Phase
0.08
0.06
Phase (degrees)
Combining Loss (dB)
0.12
-15
-20
-25
0.04
-30
0.02
-35
0
2 different TWTs
-40
0
4
8
12
16
Phase Imbalance (degrees)
20
0
5
10
15
20
25
30
Input Drive Power (dBm)
35
Phase Variability of TWT array
5
(a)
10
(b)
-5
8
-10
-15
-20
-25
Count
Phase versus
input drive
measured for
35 TWTs
Phase change (degrees)
0
6
4
=2.6Þ
-30
-35
2
-40
-45
-35 -30 -25 -20 -15 -10 -5 0
Input Power (dBm relative to sat)
0
10
-10
-5
0
5
Phas e relative to the mean at sat (deg)
• 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
Gain versus
input drive
measured for
35 TWTs
Gain Change (dB relative to gain at Pave)
Gain Variability of TWT array
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
Gain distribution
±0.5 dB at saturation
Produces very small
power variation
0
Input power (dBm relative to saturation)
58
Ka-band (-15 dB)
Ku-band (-1 dB)
C-band
S-band (-1 dB)
theory
Gain change
with time for
different types
of TWTs
Saturated Gain (dB)
56
=2400 hrs
=1000 hrs
54
=400 hrs
52
Gain is stable after
sufficient burn-in time
=330 hrs
50
pre-burn
48
-500
D.M.Goebel, “Theory of Long Term Gain Growth in Traveling Wave
Tubes, IEEE Transactions on Electron Devices, 42 (2000) p.1286.
0
500
1000
1500
2000
2500
3000
Time (hours)
Gf =
3500


 Ib  h  f  Po   1- e-w/ 
P t 
1- e-t/  b + Go
2

2 e kT A w
Po  


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
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)