Title: A 60GHz Antenna Array FrontEnd in CMOS for Gigabitpersecond Indoor Mobile Applications
1A 60GHz Antenna Array Front-End in CMOS for
Gigabit-per-second Indoor Mobile Applications
- BWRC Lunch Seminar
- Sayf H. Alalusi
- March 21, 2003
2CMOS for Consumer 60GHz
fT (speed)
100 G
0.13u
0.18u
30 G
0.25u
0.35u
0.5u
10 G
0.6u
Lmin
0.8u
1u
3 G
1.5u
1 G
2u
(Hz)
3u
75
77
79
81
83
85
87
89
91
93
95
97
99
01
03
Year
- Potential for very high integration
- Getting any gain out of CMOS circuits will be
difficult. - fT gt 60GHz, not fT gtgt 60 GHz
- Interesting design problem.
3Detailed Link Budget Calcs
- a extra EIRP needed antenna gain and/or xmit.
power - Shift 802.11a to 60GHz, 1Gbps Pt14dBm, NF5dB,
64 QAM - These optimistic numbers give a 39 dB !
- More realistic Pt10dBm, NF10dB, BPSK a 26
dB
e.g., for 802.11a, at 10m need a -6dB!
4Presentation Outline
- Antenna array advantages and tradeoffs
- Architecture exploration
- Canonical array formulation
- Antenna array design space exploration,
- effects on standard RF blocks
- Antenna weighting circuits
5Adaptive 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.
6Adaptive 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 change nearly all channel parameters.
7Adaptive 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.
8Architecture Exploration
9Digitally 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)
r2(t)
s2(t)
r3(t)
s3(t)
10RF 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
11Canonical Array Formulation
12Antenna Array Patterns
- ET F(?,I0) (a0a1ejkdcos?a2ej2kdcos? )
EFAF - Element Factor (EF) is the field of a lone
element. - Only Array Factor (AF) can be controlled
electronically, by changing the magnitude and
phase of ai. - For a beam to look at direction ?,
- set progressive phase
- ?ai1 - ?ai -kd cos(?)
- ai 1
k2p/?
r1
r2
T2
T1
kd cosT Phase shift due to physical separation.
d
13Element Factors
- Common array elements Dipole, Patch
- Other arrays 2-dim. or even 3-dim. arrays
- Element is chosen to be isotropic over region
of interest - Element can be chosen for its properties in other
dimensions
Parallel
AF
Collinear
AF
14Examples 16-Antenna Array
- Uniform Array mag. 1, progressive phase ß,
uniform spacing - We only need phase shifters!
N 16 kd p
Broadside ß 0
Isotropic radiators
D12 dB
Phased ß-kd cos(60)
D12 dB
15Antenna array Design space exploration, effects
on standard RF blocks
16Antenna Elements
- An efficient antenna needs to be ?/2 long in
some linear dimension ?/2 2.5mm in free space - Need ?/2 lateral spacing between element origins
- best angular resolution without grating lobes.
- Antenna unit cell is approx. 2.5mm x 2.5mm
- Typ. laptop is 200mm x 300mm 80 x 120 antennas
-- N 9600! - Typ. PDA is 70mm x 110mm 28 x 44 antennas -- N
1200! - Typ. PC Card antenna is 20mm x 50mm 8 x 20
antennas -- N 160!
10 mm
2.5mm
2.5mm
Dipole or Patch
2.5mm
Unit cell can be tiled to form array.
17Number 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)
18Array 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
19Array of Low Noise Amplifiers
- Each added antenna receives another copy of the
signal for free that is perfectly correlated, so - (S1 S2)2 S12 2S1S2 S22 4S2 gt
N2S2 - And another noise source, totally uncorrelated,
so - (n1 n2)2 n12 2n1n2 n22 2n2 gt
NE2n - NF (NF of lone LNA)/N
- Since CMOS is performance limited here also
- Also effective voltage gain of N(gain of one LNA)
a0
a1
20Antenna Weighting Circuits
21RF 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
22Phase 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
23/- 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
24IQ Generation at Receiver
- Must generate IQ before mixer, at full carrier
freq. - This is needed for the vector modulator
- There are 2 common ways of doing this
- Microwave, e.g. 90 degree Hybrid (a.k.a. 3-dB
Hybrid) - Lumped, e.g. 45/-45, using high-pass and
low-pass filter sections
Microwave
Lumped
in
I
in
I
isolated
Q
Q
25Target Prototype
One CMOS Chip, 0.13um
Each block PA, LNA, Phase Shifters
Each block 3 b. for phase 1 b for TX/RX
I
60 GHz
Q
n
- Antennas need to be off-chip, in the package.
(LTCC? PCB?) - Path length is not critical array is adaptive
and weights are relative.
26Summary
- 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. - Future Work
- Good transistor models for CMOS at 60GHz, also
models for passives. - Determine extent of performance space of circuits
- Evaluate specific architecture and circuit
tradeoffs - Testing environment, where partition, Packaging
technology - Antenna selection and design
- Digital control and adaptation algorithms
27Baseband Analog Architecture
- 1 data stream, antenna weights applied at
baseband - Achieves high antenna gain in an arbitrary
direction - Intermediate hardware complexity N RF mixers
- Intermediate power consumption
s(t)
a0
a0
r(t)
a1
a1
a2
a2