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RF and Microwave Challenges for Future

Radio Spectrum Access

Larry Larson

Dean – School of Engineering larry_larson@brown.edu

New Applications Create a Need for New

Allocations of Spectrum

But the Spectrum is Already Spoken For!!

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Different Models of Spectrum Allocation Have

Differing Economic and Technical Consequences

• Example: Traditional Licensed Spectrum – Based on Radio Act of 1912: – “First. Every station shall be required to designate a certain

definite wave length as the normal sending and receiving wave length of the station. This wave length shall not exceed six

hundred meters…”

• Economic/Social Consequences: – Low-Cost Consumer Nodes Enable Widespread Access.

– Assured Quality of Service for Public Safety.

– Inefficient Use of Spectrum

– Little Incentive for Delivery of New Services and Technology

• Technical Consequences – Mobile Terminals can be “dumb” – little network awareness

required.

– Frequency, modulation, and receiver standard is set at time of manufacture.

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What is the Problem with the Existing

Spectrum Allocation System?

Less than 2% of the spectrum is used from 700-800MHz in an urban (Dupont

Circle) environment. (Data from NAF study 2003)

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Worst Case Occupancy Summary

Roughly 80% of the spectrum between 30 MHz and 1000 MHz represents

“white space” – unused spectrum.

Spectrum Use in Practice

(Wan Vazer) • Some spectrum is

intensively used

• Other spectrum has

highly variable use in

when viewed in terms of

time and geography

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TV White Space Channel

Restrictions – 802.22 and 802.11af

Shellhammer, Qualcomm

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Channel Availability in Selected

Markets

Shellhammer, Qualcomm

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Channel Availability in Top Ten

Markets

Shellhammer, Qualcomm

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White Space Availability – Geo-Location

• Use FCC post-DTV transition database to

evaluate white space availability for 50

largest US cities.

Shellhammer, Qualcomm

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Geo-location Challenges

• Geo-location database access:

– Required for Fixed and Portable Services to +/- 50 meters

– Fixed Devices: Geo-Location known at installation

– Portable Devices: Geo-location must use GPS, but…

• Indoor GPS is not always reliable

• Physical layer ranging my be inaccurate due to shadowing and multi-path effect.

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• Challenging Characteristic of IEEE 802.22 CR System

– Since a IEEE 802.22 CR device must coexist with commercial TV systems,

the presence of a strong TV signal transmitted by a nearby TV station is a big problem

[9] E. Au, G. Chouinard, and Z. Lei, “IEEE P802.22 wireless RANs receiver performance evaluation criteria,” IEEE 802.22-

08/0326r2, http://www.ieee802.org/22/, Dec. 2008.

– Blocking TV signal: - 8 dBm [9]

– Sensitivity Level: -102 dBm [9] TV Broadcasting

Station

CR Device

TV Receiver

Freq.CR Device Ch.

TV

Signal

-8 dBm

-102 dBm

94 dB

Dynamic

Range

Presence of Strong TV Signals

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Presence of Strong TV Signals • Interaction of a CR device’s LO phase noise with an adjacent TV signal

– Resulting in-band interference should meet “SIR = 11.2 dB” [9]

[9] E. Au, G. Chouinard, and Z. Lei, “IEEE P802.22 wireless RANs receiver performance evaluation criteria,” IEEE 802.22-08/0326r2,

http://www.ieee802.org/22/, Dec. 2008.

(100 ) 133 /kHz dBc Hz L

Without notch filter

With 15 dB Attenuation

(100 ) 118 /kHz dBc Hz L

Notch Filter

Antenna

RF Switch

TX

Mixer

LO

Without

Notch Filter

BPF

ωLO

ω

Antenna

RF Switch

TX

Mixer

LO

BPF

ωLO

ω

With

Notch Filter

TVSignal

DesiredSignal

ω

TV Signal

ω

DesiredSignal

Significant in-band interference

TV Signal

ω

DesiredSignal

Negligiblein-band interference

Attenuation

ω

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TV White Space Receiver

Challenges

• Each transmitter could have -25 dBm at receiver input.

• IM3 noise floor is -109 dBm. Receiver IIP3 ~ +20 dBm!

• Tunable RF FE filter required for low-power operation.

Nguyen, Qualcomm

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TV White Space Receiver

Challenges

• Cross-Modulation of Transmitter and

Jammer in FDD White-Space Systems.

Nguyen, Qualcomm

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TV White-Space Transmit Mask

Challenges

Fixed Device Spectral Transmit Mask

Shellhammer, Qualcomm

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TV White-Space Transmit Mask

Challenges

Shellhammer, Qualcomm

RF Architecture Challenges

• Available Spectrum can Vary Widely in Space and Time

• Receiver and Transmitter must map to the available white space spectrum.

• Independent tuning of TX and RX frequencies

• Highly linear receiver over wide dynamic range to accommodate in-band power TV broadcasting

• Highly linear and wideband transmitter to minimize adjacent channel interference.

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Enabling RF Technologies for

TV White Space Devices

•The radio front-end of a TV White Space Device will

require.

•Tunable across a decade bandwidth (60 MHz – 700 MHz)

•Extremely High Linearity Receiver (IIP3 > 20 dBm)

•Extremely Linear Power Amplifiers (-55 dBc spectral

regrowth at maximum output power )

•Low-power and low-cost/high-level of integration for

consumer applications.

•Tunable passive filters (MEMS, varactors, etc.) and high

linearity wideband power amplifiers (DPD) are key

enabling technologies.

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RF MEMS Series Switch for DC-40 GHz

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0m

m

DC Contact

RF current

Pull down electrode

DC bias pad

RF in RF out

RF MEMS Device 1mm

Very low loss packaging up to 50 GHz

SEM pictures of metal-contact switch

*Rebeiz: UCSD

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6-10 GHz Differential Tunable Filter: Measured Results

2-pole, 5% BW, Qu=50, 4 dB IL*

CR is a 4-bit capacitor. CM is a 3-bit capacitor.

*1 dB of loss is due to bias lines, 1 KW/sq. 5x4 mm on a Corning Glass substrate

Prof. G. Rebeiz

Time - Frequency - Space

Each Domain has Opportunities for Spectral Reuse (Fette)

T1

R1,R4

T2

T3

R3

R3,T4

Interfering

Signal Placed

In Null

Transmitter forms Beam Toward Intended Recipient

Receiver forms Null Toward Interference Sources

“Smart Antenna” Applications

• Smart Antennas offer

– Array Gain – Increasing the C/N at the receiver

– Diversity Gain – to Mitigate Against Fading

– Angle Reuse and Co-channel Interference Reduction

– Spatial Multiplexing for Increased Spectral Efficiency

Smart Antenna Example – Adaptive Array Steered by Local Phase

Shifters*

The complete beamforming network was realized

in a single BiCMOS IC.

*Obayashi, Toshiba, APMC 2002

RF MEMS in Phased Array Topologies

4x better than Si or GaAs !

Prof. G. Rebeiz

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Interference Cancelling Receivers

[1] [2]

Active Tx Canceller Vladimir Aparin [1,2] Active devices adding Noise.

Feedback circuit needs to be very linear.

Stability issue.

Tx signal routing is required.

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Interference Cancelling Receivers

Himanshu Khatri High noise figure penalty due to aux. path TIA noise folding.

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• Tunable Impedance with Balanced Passive Mixer

– Translating baseband impedance to tunable RF impedance

– Image rejection technique is required

Tunable Impedance

Iin

Zin

ZL

SW

SW

SW

SW

+

+

RS

Frequency (MHz)550 560 570 580 590 600 610 620 630 640 650

10-8

10-6

10-4

10-2

100

Sp

ectu

m o

f F

ilte

r O

utp

ut

10-10

10-12

JammerImage of Jammer

Image of Desired Signal

Desired Signal

Switching Frequency

(fSW)

Downconversion

Upconversion

Upconversion

Upco

nve

rsion

ωωSωSWωJωSW - ωJ

ωωSωSWωJωSW - ωJ

Downconve

rsion

Downconversion

Desired

SignalJammer

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RF Tunable Notch Filter • RF Tunable Filter with Parallel LC Tank

• Parallel LC Tank Load

– Low impedance at DC & 2fSW

• Resulting Low Input Impedance at fSW

• 25% Duty-Cycle LO Signals

– Isolation bet. I and Q paths

• Tuning Step and Range:

– Controlled by fSW

LNARFIN

RLNA RMixerMixer

Baseband

SWI(25%)+

Mixer

SWQ(25%)+

Tunable RF Notch Filter

CL

LL

CL

LL

TV Signal

Desired

Signal

Antenna

RF Switch

TX

LNANotch Filter Mixer

LO

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• High Linearity: Switching Action of Transistors

• Low Noise: Low Losses of CMOS switches

Design Parameter Simulation Performance

NMOS 300 µm / 0.1 µm Attenuation 17 dB

CL 2 µH Insertion Loss 1.6 dB

LL 10 pF Quality Factor 32

RS 50 Ω Noise Figure 1.7 dB

Vdd 1 V IIP3 (Notch) 16 dBm

IIP3 (Signal) 22 dBm

450 500 550 600 650 700 750-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Frequency (MHz)

Imp

ed

an

ce

Ra

tio

, Z

in_IQ

/RS (

dB

)

fSW = 605 MHz

fSW = 695 MHz

fSW = 515 MHz

Channel 51Channel 36Channel 21

: Simulation : Calculation

Variation of Peak

Attenuation as fSW varies

Zin-IQ

SWI(25%)+

SWQ(25%)+

Iin RS

CL LL

ZL

CL LL

ZL

RF Tunable Notch Filter

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• Possible Application is Tunable Bandpass Filter

• Tunable BPF using Capacitive Load

– High impedance at DC

RF Tunable Bandpass Filter

Design Parameter Simulation Performance

NMOS 300 µm / 0.1 µm Attenuation 18 dB

CL 0.53 nF Insertion Loss 1.8 dB

RS 50 Ω 3 dB Bandwidth 5.5 MHz

Vdd 1 V Noise Figure 1.7 dB

IIP3 (Notch) 18 dBm

IIP3 (Signal) 23 dBm

Zin-IQ

SWI(25%)+

SWQ(25%)+

Iin RS

CL

ZL

CL

ZL

: Simulation : Calculation

Imp

ed

an

ce

Ra

tio

, Z

in_IQ

/RS (

dB

)

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

450 500 550 600 650 700 750

Frequency (MHz)

Channel 21 Channel 36 Channel 51

fSW = 515 MHz

fSW = 605 MHzfSW = 695 MHz

Variation of Insertion Loss

as fSW varies

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Conclusions

•Future Dynamic RF front-ends will require

•Tunability across a decade bandwidth (60 MHz – 700

MHz)

•Extremely High Linearity Receiver (IIP3 > 20 dBm)

•Extremely Linear Power Amplifiers (-55 dBc spectral

regrowth at maximum output power )

•Low-power and low-cost/high-level of integration for

consumer applications.

Thank You!

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