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Cognitive Ultra Wideband Radio

Trial Lecture

Greg Harald Håkonsen29. June 2007

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Outline

● Ultra wideband (UWB)● Cognitive radio● Cognitive ultra wideband radio● Summarize

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Outline

● Ultra wideband (UWB)● Cognitive radio● Cognitive ultra wideband radio● Summarize

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What is UWB?

● Transmission of very short pulses. Typical less than 1 ns.

● Use very large bandwidth. Often more than 1 GHz.

● First wireless transmission was ultra wideband. Heinrich Hertz (1893) Guglielmo Marconi

Early spark gap transmission.

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Definitions of UWB● Common Definitions

UWB: Fractional bandwidth = (fH - fL)/fc > 25% or total BW > 1.5 GHz.

Narrowband: (fH - fL)/fc < 1%.

● FCC1 Definition of UWB Fractional bandwidth (measured at the -10dB points), (fH - fL)/fc, > 20% or total BW > 500 MHz.

1FCC: Federal communications commision, USA

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Why ultra wideband(UWB)?

● High data rate. Shannon:

C: channel capacity, B: channel bandwidthS: transmitted power, N: noise power.

Rates over 100 Mb/s has been shown.

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Why UWB cont’d

● Low equipment cost● Multipath immunity● Ranging, (object location)

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UWB Multipath

● UWB signals have as many as 30 resolvable multipath components.

● Can use Rake receiver to improve performance.● Each path has very small energy, difficult to

perform accurate channel estimation for each path. Each path could have experienced different distortion. Complexity to estimate 30 different paths can be high.

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Interference

● Low power, at noise floor.● Interference problem since large bandwidth is

used. Especially for shared bandwidth. Problem for both parties.

● Does UWB increase noise floor for other services? Huge debate

● Low power makes it best for data transmission over short distances.

● Possibility for very high data rates, up to 100 Mb/s.

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Emission mask● FCC released emission masks for coexistence with

traditional radio services. (2002) Started new interest in UWB.

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Example of interference issues

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Basic design approaches● Single band UWB

Short pulses without carrier Short pulses with carrier. Time hopping or direct sequence spread spectrum for

multiple access.

● Multi-band UWB Spectrum divided into several sub-bands > 500 MHz. Can for example use OFDM in each sub-band. Frequency

and time hopping for multiple access. Increased flexibility, and complexity.

Chung, August and Ha “Signaling and Multiple Access Techniques for Ultra Wideband 4G Wireless Communication Systems,” Wireless communications, April 2005

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Pulse shaping, example

● Much research on finding optimal pulses that follow emission mask.

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Outline

● Ultra wideband (UWB)● Cognitive radio● Cognitive ultra wideband radio● Summarize

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First, a few words about the radio spectrum

● Access is granted through regulatory bodies.● Exclusive access granted in licensed bands.● Access for others in unlicensed bands, e.g.

802.11x, Bluetooth.● Seem to be running out.

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Are there no bands left?

● “In many bands, spectrum access is a more significant problem than physical scarcity of spectrum, in large part due to legacy command-and-control regulation that limits the ability of potential spectrum users to obtain such access.”

– FCC Spectrum Policy Task Force Report.

● Apparent spectrum scarcity● Actual measurements show that > 70% of spectrum is

unused.● Spectrum holes.

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Spectrum usage, example

At Berkley wireless research center,S.M. Mishra, A. Sahai, R. W. Brodersen, “Cooperative sensing among cognitive radios,”ICC 2006. Istanbul,

June 11-14, 2006

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What is Cognitive Radio?

● “The point in which wireless personal digital assistants and the related networks are sufficiently computationally intelligent about radio resources and related computer-to-computer communications to detect user communications needs as a function of use context, and to provide radio resources and wire less services most appropriate to those needs”

– Joseph Mitola III, Cognitive Radio: An Integrated Agent Architecture for Software Defined Radio, PhD dissertation, Royal Institute of Technology (KTH) Stockholm, Sweden, 8 May, 2000

● Claimed to be “The next big thing”.

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What is cognitive radio, cont’d● Extension of software-defined radio (SDR).● SDR: radio with the capability of reconfiguring the

entire radio interface with software – modulation, coding, carrier frequency, bandwidth, etc

● Replace analog and hardwired digital signal processing circuits with programmable devices

● Device-centric vs. policy-based spectrum management.

● Full cognitive radio (Mitola) is many years away. ● Spectrum sensing radio.

Sense spectrum. Use if available. Also licensed bands.

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Other definition

● “Cognitive radios should be able to ascertain their location and adapt to real time conditions of their operating wireless environment, including the ability to sense spectrum usage by neighboring devices, change operating frequency, adjust output power and modify transmission parameters”.

● In other words: Manage time, frequency, space, power and coding. Spectrum sensing CR

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Spectrum pooling

● Allow secondary users to use already licensed bands as long as it does not disturb the primary users.

● Extra income for licensed band owners.

● Extra access for secondary users.● Need detection guarantees.● Need sophisticated protocols to

manage.

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Spectrum sensing

● Cognitive radio depends on the ability of sensing spectrum usage. Passive listening. Geolocation with spectrum usage databases. Dedicated beacon transmitters.

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Work around existing signals

Adaptive WaveformsDifferent Bandwidth-- Same Information

f0 f0

Discontiguos WaveformsWork Around Existing Signals and Interference

f0 f0

Signals in Band Adapting Waveform

Waterfilling WaveformsWork Around Existing Signals and Interference

f0 f0

Signals in Band Adapting Waveform

Threshold

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Outline

● Ultra wideband (UWB)● Cognitive radio● Cognitive ultra wideband radio● Summarize

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UWB and cognitive radio

● Why combine UWB and cognitive radio? Combine high data rate and ability to coexist for UWB

with awareness of cognitive radio. Cognitive radio require wideband sensing, UWB uses

wideband signals.

● Can use licensed bands without disturbing primary user. Would you let someone use your house if you would not

notice?

● Needs efficient protocols. Low power is hard to detect. Reduce overhead

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Adapting to environment

● Can adapt UWB transmission to minimize interference. Adaptive notch filters at

desired frequencies. Bandstop, bandpass

filters. Soft spectrum

adaptation. Carefully designed waveforms on to mitigate interfering signals.

Suppression of interfering sub-carriers.

0 5 10 15 200

1

2

3

4

5

6x 10

-5

Frequency, (GHz)M

ag

nitu

de

Sub-carrier suppression

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Adapting to environment, cont’d

● Compensate front-end distortion. Predistortion for lack of linearity in antennas and power

amplifiers.● Power conservation.

Turn of interfering bands, or deep-fading bands. Power allocation important at low SNR values.

● Mitigate fading Adaptive sub-carrier loading. Pathloss can vary over bands in UWB. Increased received SNR. Low risk of fast fading, due to short symbols.

● Multiple Antenna Spatial direction, reduce interference.

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Spectrum holes•Use spectrum when there is room (spectrum hole)

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Problems to be solved

● Many open issues.● UWB still new field.● Cognitive radio very new field.● Large bandwidth, high

frequency, everything done in software. Require higher processing

power.● To large implementation

complexity.● Need new protocols● Need to address performance

gain vs. overhead cost.

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Summary

● Ultra wideband New interest for short distance high rate

communication. Pulse-shaping needed to efficiently use spectrum Coexistence

● Cognitive radio Intelligent radio with ability to adapt and learn. Opportunistic

● Cognitive ultra wideband radio Opportunistic, with high level of adaptation.

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Current status?