Third Generation (3G) Systems Universal cell phones Mobile multimedia - Net phones Satellite radio...

Third Generation (3G) Systems

• Universal cell phones

• Mobile multimedia- Net phones

• Satellite radio

• Wireless internet

• Wireless local loops - Local data links- Bluetooth- Last-mile applications

• Automotive multimedia

3G “broadband, wireless communication systems”

Some Needs for 3G Wireless

Average Power(W)Frequency Now Needed Backoff Application

Cellular0.8 GHz 100 600 MCPA cellular1.9 GHz 40 ≥200 8-10 dB IMT-2000 PCS2.1 GHz 40 100-200 8-10 dB IMT-2000

Satellite2.3 GHz 125 4000 0 Satellite Radio12 GHz 125 200-400 0 DirecTV

Mobile2.3 GHz 200 650 6 dB SatRad repeaters2.6 GHz 20 200 10 dB 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

6 8 10 12 14 16 18 20

Energy-per-bit/Noise-density (E /N in dB)

10-1

-2

-3

-4

-5

-6

10

10

10

10

10

10

10

10

10

-7

-8

-9

-10

b o

Bit Error Rate (BER)

Crest Factors for Spread-Spectrum Signals

Broadband, spread-spectrum signals have high peak to average ratios (high “crest-factors”)

• 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

0.01

0.1

1

10

100

-15 -10 -5 0 5 10 15

Time (%)

Output Power (dB relative to average)

AWGN waveform

Adjacent Channel Power Intermodulation Distortion

Carriers

3rd-orderdistortion

C/3IM (dBc)

(2f -f ) f f1 2 1 2 (2f -f )12

2 MHz/div

3rd-orderIMD

5th-orderIMD

Carriers

1.9 GHz

Video Ave.50 sweeps

8-Tones

2-Tones

• Multi-tone operation produces intermodulation distortion (IMD)

• Intermodulation products cause adjacent channel power problems

Adjacent Channel Power Reduction

Running amplifiers backed off from saturation for linearity (lower adjacent channel power) requires higher peak power

Backoff from non-linear region

20

25

30

35

40

45

50

456789101112

2-Tone C/3IM (dBc)

Backoff from Saturation (dB)

Improve IMD

Adjacent Channel Power Reduction

Multi-Channel Power Amplifier(with feed-forward circuit)

InputSignal

Output

PowerAmp

CorrectionAmp

Delayline

Delayline

Pre-distorter

-10 to -20 dBc ≈ -30 dBc -30 dBc

TWT TWT with feedforward

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?

3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz

HF VHF UHF µwave mm-wave

Si MOSFETs, JFETs

Bipolar transistors

GaAs, GaN FETs

from "RF Power Design Techniques"by I.M. Gottlieb

Power Combining

Gain Power

Input Output

Power combined arrays are required

• Solid state devices have limited gain and power capability per device

• Use series and parallel arrays to produce gain and power

1

2

3

4

5

6

7

8

5 10 15 20 25 30 35

Peak RMS Electric Field

Number of Tones

Coherent Phase

Random Phase

• Broadband produces high peak electric fields

• Many devices needed to avoid breakdown damage

(≈10 dB per device)

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 combinerpower output decreases directly with impedance change

The Solution - VED

• 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!

Tubes work everywhere within this box

Vacuum Electronic Devices

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

Highest Power LDMOS PCS Solid State Devices

Amplifier Linearity

Solid state devices and tubes have similar linearity, but tubes have significantly higher power capabilities!

Satellite Radio Systems

Power combined array of 48 TWTs produces ≥4 kW of radiated power

Estimated link budget

Input Output

x 48TWTs

Satellite Transmitter

entered values

frequency (GHz) 2.34wavelength (m) 0.1281amplifier power (Watts) 4000power (dBm) 66.00

3.94.8

antenna efficiency (%) 70transmitter antenna gain (dB) 38.96EIRP 104.99distance (km) 35,784propagation loss (dB) -190.91atmosperic loss 2 -2.00

0.050.05

antenna efficiency (%) 55receiver antenna gain (dB) -0.83receiver noise figure (dB) 13background sky temperature (K) 25equivalent temperature (K) 5,521No, noise level (dBm/Hz) -161.18

received C/No (dB/Hz)data rate (bps) 7,000,000 -68.45

Eb/No (dB) 3.99

transmitter elliptic antennadimensions (m)

receiver elliptic antennadimensions (m)

calculated valuesParameters

Power combining of TWTs

Power combining of two TWTs

Amplitude

Phase

P = 0.5[P1 + P2 + 2(P1 P2 )1/2 cos

Depends on power and phase balance(10 deg of phase or 2 dB in power exceeds Magic-T losses)

Phase versus input drive measured for 35 TWTs

Phase Variability of TWT array

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

Phase change (degrees)

Input Power (dBm relative to sat)-35 -30 -25 -20 -15 -10 -5 0

(a)

0

2

4

6

8

10

-10 -5 0 5 10

Count

Phase relative to the mean at sat (deg)

σ=2.6˚

( )b

• The power loss in the array of TWTs is proportional to cos

• 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 Variability of TWT array

Gain distribution ±0.5 dB at saturation

Produces very small power variation

Gain is stable after sufficient burn-in time

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

Input power (dBm relative to saturation)

Gain Change (dB relative to gain at P )

ave

48

50

52

54

56

58

-500 0 500 1000 1500 2000 2500 3000 3500

Ka-band (-15 dB)Ku-band (-1 dB)C-band

theoryS-band (-1 dB)

Saturated Gain (dB)

Time (hours)

τ=1000hrs

τ=400hrs

τ=330hrs

-pre burn

τ=2400hrs

G f = αIbσ hκ fτPoΓολ 1-e- /w λ

( )2 e kT A w2 1-e- /t τ +

PbtPoτ

⎡

⎣⎢⎤

⎦⎥+ Go

D.M.Goebel, “Theory of Long Term Gain Growth in Traveling Wave Tubes, IEEE Transactions on Electron Devices, 42 (2000) p.1286.

Gain change with time for different types of TWTs

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)