Providing Unlimited Wireless Capacity

1
Providing Unlimited Wireless Capacity
Bob Brodersen Berkeley Wireless Research
Center Adaptrum, Inc and SiBEAM, Inc. Univ.
of California, Berkeley
2
More Capacity Do we really need it ??

• Major bandwidth driver is moving from voice to
  video and data a 1000-10,000 fold requirement
  increase
• User base is moving to be a significant fraction
  of the 6 Billion world population
• Devices (without an attached human) will
  communicate wirelessly
• Source to HD display with uncompressed video gt 4
  Gbit/sec
• A home will have a 1000 radios
• This is not going to be addressed by improving
  any radio system we have now

3
Looks like we need it .

• Claim There are technological solutions to
  providing this capacity which is based on
  abandoning the property rights model of owning
  frequency spectrum
• Assertions
• The concept of fixed frequency spectrum
  allocation has become fundamentally flawed
• We need to exploit wireless communication
  strategies that exploit the time, space and
  frequency degrees of freedom
• Exploiting these new approaches could allow
  essentially unlimited capacity

4
Why has Frequency Allocation become a Flawed
Concept?

• The applications are continually changing and
  allocation doesnt mean use
• Frequency is only one of the 3 degrees of freedom
  to use to avoid interference - time and space
  are actually more effective

5
UWB, 60 GHz and Cognitive radios individually
exploit the 3 DOF

• Frequency
• Separate users by using different frequency bands
  
• traditional method using analog filtering
• Exploit wide bandwidths and DSP
• Exploit higher frequencies
• present CMOS technology allows use up to 100GHz
• Space and Angle
• Reduce transmit power
• decreases radius of omni-directional cells
• Exploit the angular nature of the spatial channel
• Multiple antennas
• Time
• Impulse filtering
• Sense interference and avoid

6
UWB, 60 GHz and Cognitive radios individually
exploit the 3 DOF

• Frequency
• Separate users by using different frequency bands
  
• traditional method using analog filtering
• Exploit wide bandwidths and DSP (UWB)
• Exploit higher frequencies
• present CMOS technology allows use up to 100GHz
  (60 GHz)
• Space and Angle
• Reduce transmit power
• decreases radius of omni-directional cells (UWB)
• Exploit the angular nature of the spatial channel
• Multiple antennas (60 GHz)
• Time
• Impulse filtering (UWB)
• Sense interference and avoid (Cognitive Radio)

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Talk Organization

• A discussion of the emerging techniques that are
  exploiting the three degrees of freedom in new
  ways
• UWB
• Cognitive Radios
• 60 GHz
• How these can be combined to achieve unlimited
  capacity

8
Lets start with UWB

• Breakthrough! For the first time the regulators
  allowed sharing of the frequency spectrum
• Underlay Allow sharing of the spectrum if the
  interference is negligible
• Ultrawide bandwidths were allowed
• Fundamental choices remain on how to best to use
  the wide bandwidth
• Stay with conventional frequency domain thinking
• Or exploit the time degree of freedom

9
UWB Frequency and Time domain strategies
Noise
Traditional user (narrowband)
Power
Frequency
Time
Interference in frequency domain
Frequency UWB OFDM (multiple sine waves)
Power
Frequency
Time
Time domain interference
Power
Time UWB Impulse
Time
Frequency

• Time domain filters (block the impulse in time)
  can be easily made adaptive (unlike frequency
  domain)

10
UWB is allowed in over 11 GHz of spectrum
Comm
Vehicular
UWB
UWB
UWB
0
20
40
60
80
100
GHz

• Limited power allows a high level of spatial
  reuse
• Chip sets are available for both OFDM and Impulse
  approaches
• But .. If this is such a good idea why has it
  not been commercially successful

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What happened

• Two competing approaches were attempted which
  resulted in a standards battle which was waged
  without good technical input
• The (comfortable) frequency domain approach
  (OFDM) had too high a complexity and too low a
  performance
• The time domain (impulse) approach is too
  different and needs much more RD in theory and
  implementation

12
Cognitive Radios

• Basic idea
• Exploit the time degree of freedom by sensing if
  a signal is present
• Then take steps to assure there isnt
  interference
• This is quite restricted, others use a more
  expanded definition

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A Cognitive Radio using Time and Frequency
PU1
PU3
Power
PU2
PU4
Frequency

• Sense the spectral environment over a wide
  bandwidth
• Transmit in White Space
• Detect if primary user appears
• Move to new white space
• Adapt bandwidth and power levels to meet QOS
  requirements

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Cognitive Radio system level view
Network coordination
Network Link Layers
Resource Allocation
Medium Access
Sensing Signal Processing
Wideband signaling
Physical Layer
Sensing radio
Wideband radio
Spectrum sensing is the key enabling
functionality and must be very sensitive to limit
unwanted interference
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Sensing Weak Signals
Cyclostationary Feature Detector
Spectral correlation

• A new radio functionality requires new
  algorithms and understanding

16
Sensing Performance (Danijela Cabric)
Incoherent processing
Log Time
(Cyclostationary)
Cyclostationary
Coherent sensing

• Incoherent sensing time goes as 1/(SNR2)
• Coherent sensing time goes as 1/SNR

17
Coherent sensing - ATSC signal

• Correlation of fixed header is used by Adaptrum
  in FCC trials being held now highest performing
  results

18
Adaptrum TV-band CR prototype
Bowtie wideband UHF antenna
CR Transceiver
CR Baseband/MAC FPGA (Altera)
19
ATSC sensitivity measurement result
20
Planned Bay Area Cognitive Radio - 400 MHz
experimental testbed
21
Using the Space Degree of Freedom to improve
sensing

• Spatially separated sensing radios can make
  independent measurements
• Single radio sensitivity can be improved by the
  use of multiple antennas using beamforming

22
Spatially separated sensing radios
Exploit spatial diversity in Sensing SNRs
Primary System
Tx
Rx
Decoding SNR

• Expected result for independent measurements

Pd, network1-(1-Pd, radio)N
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Experimental Setup
Sensing PHY/MAC processors
Location (11,9)
Sensing radio
Central combining and processing
Location (16,3)
Sensing radio
controlled PHY and MAC integration
Fiber provides 1/3 mile separation between radios
and platform
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Network Spectrum Sensing Results
If spacing gtgt ?/2 a few cooperative radios give
big improvements
Prob. of detection
5 radios
1 radio
Prob. of false alarm
Danijela Cabric, Mubaraq Mishra and Anant Sahai
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Dynamic range problem in wideband sensing
PSD
AGC
LNA
A/D
Freq.
Fixed LO
Band of interest

• Wideband sensing is required to quickly sense the
  open bands
• Small signals need to be sensed in the presence
  of strong interference and then processed
  digitally
• Places difficult requirements on RF front end and
  A/D
• Multi-antenna spatial processing provides two
  solutions

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Multi-antenna spatial processing to improve
sensing
Primary user f1
Phased antenna array
Primary user f2

• Improvements with N antennas
• allow suppression of up to N-1 large signals
• provide up to an N times increase in sensitivity
• Well find more uses for these arrays

27
Time Domain Interference Cancellationto address
the dynamic range problem (Jing Yang)
One possible implementation
Yields NM equivalent bits of dynamic range
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Simulated interference attenuation
Strong FSK modulated interfering signal
Attenuated strong interfering signal
Moderate sinusoidal interfering signal
CR signal
After
Before

• Attenuate the strong interference and reduce the
  dynamic range to the Residue ADC by 35dB
• Extending the effective number of bits for this
  system by nearly 6 bits

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The opportunity of higher frequencies
7 GHz of unused and unlicensed spectra
0-3 GHz gtgt 99 of all wireless transmission
0
20
40
60
80
100
GHz

• Effectively no use above 5 GHz
• Antenna arrays require only a small area
• Absolutely necessary to get to gigabit/sec rates

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Use of Higher Frequencies (e.g. 60 GHz)

• Conventional wisdom is that lower frequencies are
  better
• Only line of sight operation is possible and
  cant penetrate materials
• The technology to process signals is expensive
• As you go up in frequency there is an inherent
  path loss that reduces range
• Not True!!!

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Material Penetration actually not so bad
60 GHz 2.5 GHz
Pine board ¾ 8 db 1.5dB
Clay Brick 9 dB 2 dB
Glass with wire mesh 10.2 dB 7.7 dB
Asphalt Shingle 1.7 dB 1.5 dB
Plywood ¾ 7 dB 1.5 dB
Clear Glass 6.4 dB 3.6 dB
What about Oxygen absorption?
Atmosphere per 100m 1.5 dB 0 dB
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Millimeter wave radios

• Misconception Implementing millimeter wave
  radios requires exotic materials
• Conventional state-of-the-art digital CMOS can be
  used to implement integrated radios up to 100 GHz
  
• Future technology scaling will allow even higher
  frequency operation (research is beginning into
  Terahertz operation)

33
60-GHz CMOS operation (130 nm)
11-dB Gain _at_ 60 GHz

• Use of transmission line interconnect allows
  control of electrical and magnetic fields
• Better control than at lower frequencies!

34
60-GHz CMOS Receiver front-end

• CMOS integration means even a 60 GHz receiver
  will eventually cost about the same as a WiFi

35
Millimeter wave radios

• Misconception As you go up in frequency there is
  an inherent path loss that reduces range
• This comes from only considering omnidirectional
  antennas which have a size that is inverse with
  carrier frequency
• Solution is to keep area constant using
  directional antennas - then the received signal
  increases with frequency

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Antenna fundamentals Receive Antenna Energy
Captured energy
Low Gain Tx Antenna
Area of sphere 4p r2
Ar Area of receive antenna
Tx
Rx

• Fraction of power received from an
  omnidirectional transmission

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Effective transmit antenna power
Pt Gt
At Area of transmitter
Tx
Rx

• Maximum increase in power in direction of beam
• (l wavelength of carrier)
• Effective maximum power as if it came from an
  omnidirectinal antenna Pt Gt

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The link budget with directional antennas
Wasted energy
Captured energy In Area Ar
Tx
Rx
Receive power improves with frequency!!
22 dB more gain at 60 GHz over 5 GHz if antenna
size is kept constant (Compare to Friis path
loss formula )
39
Millimeter wave radios

• Misconception Only line of sight transmission is
  possible
• millimeter waves reflect like lower frequency
  waves, so adaptive directional antenna arrays can
  choose strongest signal
• millimeter waves have higher material penetration
  loss, but this can be compensated for with the
  higher power and antenna gain

40
Non line of sight transmission

• Phased Array Antennas
• Power used more efficiently for better
  reception, longer distance, higher bandwidth
• Dynamically steers beam to specific receiving
  station

SiBEAM Module
41
Phased array circuitry

• N antennas allow N power amps to transmit in
  parallel
• Phase accuracy requirement is very low as the
  number of antennas go up (2 bits for 16 antennas)

42
Algorithms for adaptive beamforming

• Separate the multipath into separate channels
  through an SVD
• Choose the strongest one and put all the energy
  into it

Blind tracking
s1
s2
s3
s4
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As beams get narrow the capacity increases (Ada
Poon)
The old (Shannon) Formula
Capacity ? W T log SNR
Transmission interval Bandwidth
The time frequency degrees of freedom
With spatial processing
Capacity ? 2 A W l-2W T log SNR
New spatial degrees of freedom provide
multiplicative increases
Carrier frequency Cumulative scattering solid
angle Transmit Array area Polarization
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How do we get to unlimited capacity?

• Two ways probably more
• Angular isolation of beamformed signals requires
  not only co-location but the same angle
• This essentially eliminates interference
• Add in UWB-like spectrum sharing and cognitive
  techniques to achieve essentially unlimited
• Increasing the number of users with multiple
  antennas provides an unlimited increase if
  frequency or antenna area is increased

45
Angular isolation
C
A
B

• A transmits towards B with phased array
• B only receives in direction of A
• Transmitter C doesnt interfere with B

46
Angular isolation
C
A
B

• Interference between beamformed signals has to
  not only be in the same space, but also have the
  same angle

47
Another solution to aligned receivers
D
B
C
A

• Start with link AB
• Add aligned link CD
• What do we do now.

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Use a reflection

• Usually at least 2-3 reflections
• This is requiring more resolution in the phase
  shifters to control sidelobes

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Capacity increases with number of RXTXN
Scheme No.ofAntennas CapacityIncrease
Frequency/Time Subdivision N 1
Packet Multihop N N1/2
Cooperative Diversity (Tse) N N
MIMO at each user (Chung) N2 N2
MIMO with reflections N WA/l2 (N WA/l2)2
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Back to our Frequency Allocation Chart

• If we use all 3 degrees of freedom then a chart
  like this really is meaningless

51
A future allocation chart

• Now how do we get to this!!