CMOS MultiAntenna Systems at 60 GHz

1
CMOS Multi-Antenna Systems at 60 GHz

• Sayf Alalusi, Robert Brodersen
• U. C. Berkeley

2
Path Loss at 60GHz

• Path loss loss by traveling a distance through
  free space
• 2.4 GHz ?125mm, path loss 30 dB (4pR/?)2, R
  10m
• 5.0 GHz ?60mm, path loss 66 dB
• 60GHz ?5mm, path loss 88 dB
• Already pushing the circuits to their limit
• Only way make up this extra 22dB of loss is with
  a higher gain antenna, and architectural
  advances.
• More practical target 30 dB antenna and
  architecture gain.

Therefore, we need a high gain antenna e.g.
aperture, helix, array
3
Adaptive Beamforming for High Gain

• Gain ltgt Directivity ltgt Beamwidth-1
• Antenna needs high gain in an arbitrary direction

N number of antennas
Beam pattern controlled by antenna weights.

a0
a1
a2
aN-1
x(t)

• Adaptive Beamformer
• Can adapt to achieve high gain in any direction,
  regardless of physical orientation
• Added bonus attenuate interfering signals from
  other directions
• Requires digital control and computation for
  adaptation of weights.

4
Adaptive Beamforming Advantage 1 Directivity in
Any Direction

• Direct all energy along chosen path only.
• Preferentially receive energy from chosen path
  only.
• High gain in any direction, controlled
  electronically.

Can influence many channel parameters.
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Adaptive Beamforming Advantage 2 Subdivision
Limited performance at 60 GHz
Relaxed spec.s for individual components

• Use Circuit level parallelism to achieve our
  performance goals.
• Use N power amplifiers to get total transmit
  power
• Use N low noise amplifiers to receive N copies of
  the signal
• This is critical because of limited performance
  of CMOS circuits
• due to low voltage swing, operation close to fT,
  etc.

6
Digitally Weighted Architecture

• Optimal capacity for all channel conditions N
  data streams
• Very high hardware complexity N full
  transceivers
• Very high system power consumption

Overlay of N Independent Beams
s1(t)
r1(t)
s2(t)
r2(t)
s3(t)
r3(t)
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RF Phase Shifter Architecture

• 1 data stream, RF phase shifters only, digitally
  controlled
• Achieves high antenna gain in an arbitrary
  direction
• Low hardware complexity N RF phase shifters
• Low system power consumption

r(t)
s(t)
a0
a0
a1
a1
S
a2
a2
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Number of Antennas

• Directivity D0Umax/U0 AFmax2 N
• Half Power Beamwidth(HPBW) 2arccos(1-?/Nd)
• Nulling of interferers reduces main beam gain (a
  little).
• Physical size of antenna array is not an issue
• Circuit complexity grows as N

D0
HPBW
(Uniform Array)
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Array of Power Amplifiers

• At 60GHz, we are power limited, so let max. PA
  power Pt
• One factor of N in EIRP from directivity of array
  pattern
• Another factor of N in EIRP from combining power
  of N PAs
• As we add antennas total gain is Pt N2
• 6dB EIRP for each doubling of N, with constant
  individual PA power
• 24 dB EIRP for N 16 antennas, compared to base
  system.

Original PA
Constant Individual Transmit Power
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RF Phase Shifters

• Provides weighting of array coefficients at full
  RF.
• 3 major types
• Passive Tuned high-, low-, all-pass filter ok,
  but require tunable elements on-chip, also have
  limited tuning range.
• Switched Delay Lines Provides phase shift
  through actual time delays. Virtually guaranteed
  to work, but bulky in CMOS.
• Vector Modulator Just need variable attenuators
  on the I and Q signals (gives us full phase and
  magnitude control).

Vector Modulator
?
x(t) e(j?t ß)
x(t)
90
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Phase Shifter Accuracy

• Primary problem is directivity care about gain
  and direction of main beam.
• For N 16, discretising to 3 levels ( 1, 0, -1)
  on each of I and Q channels preserves main beam
  direction and angle

2º
Angle error
-8º
3
Directivity error
2
1
12
/- Vector Modulator

• Only need to select ,- or 0 for I and Q.
• Then add the 2 signals to get the desired phase
  shift.
• Very easy if use differential signaling, but not
  necessary.

SEL
SEL-

Vin (I or Q)
-

VO
-
SEL0
SEL0
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Signal Distribution and Combination

• Need to split transmit signal and distribute to
  phase shifter array, PAs
• Need to combine signals from LNAs, phase shifters
  into 1 signal for the mixer (really 2 for I and
  Q)
• Final architecture will depend on loss, matching
  req.s, area, etc.

Z0
Z0
Z1
Z2
Z3
Z4
Z1
Z2
Z3
Z4
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Digital Correction and Calibration
1
2
3
4

• Paths could have very different lengths,
  unintended phase shifts
• Needs to be corrected, can do it through
  digital control of phase
• shifters

15
Summary

• 2 ways that an adaptive array increases EIRP in
  any direction
• Antenna gain
• Combining performance of arrays of traditional RF
  circuit blocks
• Array of 16 antennas and very simple RF phase
  shifters will give the needed performance for a
  1Gbps wireless link at 60GHz in CMOS.
• For More Info http//bwrc.eecs.berkeley.edu/Peopl
  e/Grad_Students/sayf/
• MATLAB Antenna Array analysis scripts
• Expanded Slide Sets
• Scripts for simultaneous conjugate matching

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Acknowledgments

• STMicroelectronics
• DARPA
• Industrial Members of the BWRC