Providing Unlimited Wireless Capacity Bob Brodersen Berkeley Wireless

Providing Unlimited Wireless Capacity Bob Brodersen Berkeley Wireless Research Center Adaptrum, Inc and Si. BEAM, Inc. Univ. of California, Berkeley

More Capacity – Do we really need it ? ? l l l 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 => 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

Looks like we need it …. l l 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”

Why has Frequency Allocation become a Flawed Concept? l l The applications are continually changing and allocation doesn’t mean use Frequency is only one of the 3 degrees of freedom to use to avoid interference - time and space are actually more effective

UWB, 60 GHz and Cognitive radios individually exploit the 3 DOF l 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 100 GHz l Space and Angle » Reduce transmit power – decreases radius of omni-directional cells » Exploit the angular nature of the spatial channel – Multiple antennas l Time » Impulse filtering » Sense interference and avoid

UWB, 60 GHz and Cognitive radios individually exploit the 3 DOF l 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 100 GHz (60 GHz) l 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) l Time » Impulse filtering (UWB) » Sense interference and avoid (Cognitive Radio)

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

Lets start with UWB… l 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 l 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

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

UWB is allowed in over 11 GHz of spectrum Comm UWB 0 l l Vehicular UWB 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 l But …. . If this is such a good idea why has it not been commercially successful

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

Cognitive Radios l Basic idea » Exploit the time degree of freedom by sensing if a signal is present » Then take steps to assure there isn’t interference l This is quite restricted, others use a more expanded definition

A Cognitive Radio using Time and Frequency Power PU 1 PU 3 PU 2 PU 4 Frequency l l l 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

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

Sensing Weak Signals Energy Detector Cyclostationary Feature Detector High SNR Low SNR Spectrum density Spectral correlation A new radio functionality – requires new algorithms and understanding

Log Time Sensing Performance (Danijela Cabric) Incoherent processing (Cyclostationary) Cyclostationary Coherent sensing l Incoherent sensing time goes as 1/(SNR 2) l Coherent sensing time goes as 1/SNR

Coherent sensing - ATSC signal l Correlation of fixed header is used by Adaptrum in FCC trials being held now – highest performing results

Adaptrum TV-band CR prototype Bowtie wideband UHF antenna CR Transceiver CR Baseband/MAC FPGA (Altera)

ATSC sensitivity measurement result

Planned Bay Area Cognitive Radio - 400 MHz experimental testbed

Using the Space Degree of Freedom to improve sensing Spatially separated sensing radios can make independent measurements l Single radio sensitivity can be improved by the use of multiple antennas using beamforming l

Spatially separated sensing radios Exploit spatial diversity in Sensing SNRs Primary System Tx Rx Decoding SNR Expected result for independent measurements: Pd, network=1 -(1 -Pd, radio)N

Experimental Setup Location (11, 9) Sensing PHY/MAC processors 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

Network Spectrum Sensing Results Prob. of detection If spacing >> λ/2 a few cooperative radios give big improvements 5 radios 1 radio Prob. of false alarm Danijela Cabric, Mubaraq Mishra and Anant Sahai

PSD Dynamic range problem in wideband sensing Freq. Band of interest l AGC LNA A/D Fixed LO 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 l Multi-antenna spatial processing provides two solutions

Multi-antenna spatial processing to improve sensing Primary user f 1 Phased antenna array Primary user f 2 l l Improvements with N antennas » allow suppression of up to N-1 large signals » provide up to an N times increase in sensitivity We’ll find more uses for these arrays

Time Domain Interference Cancellation to address the dynamic range problem (Jing Yang) One possible implementation Yields N+M equivalent bits of dynamic range

Simulated interference attenuation Strong FSK modulated interfering signal Moderate sinusoidal interfering signal CR signal Before Attenuated strong interfering signal After l Attenuate the strong interference and reduce the dynamic range to the Residue ADC by 35 d. B l Extending the effective number of bits for this system by nearly 6 bits

The opportunity of higher frequencies 7 GHz of unused and unlicensed spectra 0 -3 GHz >> 99% of all wireless transmission 0 20 40 60 80 100 GHz Effectively no use above 5 GHz l Antenna arrays require only a small area l Absolutely necessary to get to gigabit/sec rates l

Use of Higher Frequencies (e. g. 60 GHz) Conventional wisdom is that lower frequencies are better » Only line of sight operation is possible and can’t 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!!!

Material Penetration – actually not so bad 60 GHz 2. 5 GHz Pine board – ¾ ” 8 db 1. 5 d. B Clay Brick 9 d. B 2 d. B Glass with wire mesh 10. 2 d. B 7. 7 d. B Asphalt Shingle 1. 7 d. B 1. 5 d. B 6. 4 d. B 3. 6 d. B Plywood – ¾ ” Clear Glass What about Oxygen absorption? Atmosphere per 1. 5 d. B 0 d. B 100 m

Millimeter wave radios l 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)

60 -GHz CMOS operation (130 nm) 1 mm 11 -d. B Gain @ 60 GHz 1. 3 mm l l Use of transmission line interconnect allows control of electrical and magnetic fields Better control than at lower frequencies!

60 -GHz CMOS Receiver front-end CMOS integration means even a 60 GHz receiver will eventually cost about the same as a Wi. Fi

Millimeter wave radios l 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

Antenna fundamentals: Receive Antenna Energy Captured energy Low Gain Tx Antenna Area of sphere = 4 p r 2 Ar = Area of receive antenna Tx Rx Fraction of power received from an omnidirectional transmission =

Effective transmit antenna power Pt G t At = Area of transmitter Tx Rx l Maximum increase in power in direction of beam = l (l = wavelength of carrier) Effective maximum power as if it came from an omnidirectinal antenna = Pt Gt

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

Millimeter wave radios l 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

Non line of sight transmission l Phased Array Antennas » Power used more efficiently for better reception, longer distance, higher bandwidth » Dynamically steers beam to specific receiving station Transmitter in Source Si. BEAM Module

Phased array circuitry l l 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)

Algorithms for adaptive beamforming l Separate the multipath into separate channels through an SVD l Choose the strongest one and put all the energy into it Blind tracking σ1 1 st path, σ1=10 σ2 Array Processing 2 nd path, σ2=6 σ3 σ4

As beams get narrow the capacity increases (Ada Poon) The old (Shannon) Formula: Capacity W T log SNR Transmission interval Bandwidth With spatial processing: Capacity 2 A W l-2 W T log SNR Carrier frequency The time frequency degrees of freedom New “spatial” degrees of freedom provide multiplicative increases Cumulative scattering solid angle Transmit Array area Polarization

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

Angular isolation C A l l l B A transmits towards B with phased array B only receives in direction of A Transmitter C doesn’t interfere with B

Angular isolation C A B Interference between beamformed signals has to not only be in the same space, but also have the same angle

Another solution to aligned receivers C A Start with link AB l Add aligned link CD l What do we do now…. l B D

Use a reflection Usually at least 2 -3 reflections l This is requiring more resolution in the phase shifters to control sidelobes l

Capacity increases with number of RX&TX=N No. of Antennas N Capacity Increase 1 Packet Multihop N N 1/2 Cooperative Diversity (Tse) N N MIMO at each user (Chung) N 2 Scheme Frequency/Time Subdivision MIMO with reflections N WA/l 2 (N WA/l 2)2