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Ultra Wide Band Wireless Communications

Ioannis Broustis

March 23rd, 2004

Department of Computer Science & EngineeringDepartment of Computer Science & EngineeringUniversity of California, RiversideUniversity of California, Riverside

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Motivation

Current wireless solutions (IEEE 802.11, Bluetooth…) face some of the following problems:

Limited channel capacity

High power consumption

Multipath fading

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Why UWB?

Advantages: Low-power operation. Low cost. Low probability of detection and low probability of

jamming capabilities. Low interference levels to existing services. Higher immunity to multi-path fading effects. Ability to penetrate walls, etc. Availability of precise location information.

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What are we trying to do…

There are no sufficient solutions for the MAC layer.

Limited previous work exists for UWB based ad hoc networks.

We will address the problem of a suitable MAC protocol for UWB ad hoc wireless networks.

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Roadmap

UWB definition and applications

Possible PHY layer implementations

MAC principles

Previous Work on MAC layer

Conclusions

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UWB: What is it?

Any signal that: Occupies at least 500MHz of BW, or More than 25% of a center frequency:

LH

LH

ff

ff

)(2

Wireless Personal Area Networks (WPANs) FCC allocates 7,500 MHz in the 3.1 to 10.6 GHz band.

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UWB Applications Stream DVD content to HDTVs

simultaneously.

Wirelessly synchronize appliance clocks.

Connect high-data rate peripherals.

Move huge files between digital cameras, camcorders, and computers.

Military applications (radars, penetrate walls, etc.)

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What makes UWB so interesting?

Manufacturers are still jumping on the UWB band wagon even with the frequency and power restrictions.

Why? Let an old friend explain:

Shannon’s theorem:

2 2log 1 7.5GHz log 1 5.62kW

93.4Gb/s

SC B C

N

C

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PHY: Single-Band and Multi-Band

Single-Band Implementation One pulse occupies the whole BW.

Multi-Band Implementation The 7.5GHz are divided into multiple bands. Information is independently encoded in the different

bands. The lower limit of 500MHz must be maintained.

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Single-Band and Multi-Band

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Single-Band and Multi-Band

Multi-band signals transmitted at different discrete times. The sequence repeats at each symbol. Center frequencies are shown in the vertical axis.

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Why prefer Multi-Band ?

Adaptive band selection Avoids interference.

Low complexity Smaller transceiver cost.

Low circuit frequency Power conservation.

Sacrifice one band for co-existence

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IEEE 802.15.3a Requirements

Parameter Value

Bit rate 110 and 200 Mb/s

Range 30 and 12 ft

Power Consumption 100 and 250 mW

Bit error rate 1e-5

Co-located piconets 4

Interference capability Robust to IEEE systems

Co-existence capability Reduced interference to IEEE systems

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MAC Principles

Two main aspects: Intra-WPAN interference

Polling schemes (master/slave) CSMA CDMA TDMA A new idea...

Inter-WPAN interference Time Hopping Spread Spectrum techniques.

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UWB ad-hoc MAC - Previous Work [1] Jin Ding, Li Zhao, Sirisha Medidi and K. M. Sivalingam, "MAC Protocols for Ultra-Wide-Band

Wireless Networks: Impact of Channel Acquisition Time", in SPIE ITCOM Conf. 4869, Boston, July 2002.

The channel acquisition time is large enough and prohibits CSMA or TDMA approaches:

DATA

TDMA Sl ot

GuardBand

Acqui si t i onTi me

GuardBand

AcqTi me

AcqTi me

DATA

DI FS

RTS CTS Contenti onWi ndow Sl ots

I dl e Next RTS packetCSMA-CA

TDMA

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UWB ad-hoc MAC - Previous Work [2] H. Yomo, P. Popovski, C. Wijting, I. Z. Kovacs, N. Deblauwe, A. F. Baena, and R. Prasad,

"Medium Access Techniques in Ultra-wideband Ad Hoc Networks", the 6th national conference of ETAI, 2003 September, 2003, Ohrid, Skopia.

They examine the Inter-WPAN interference approach of the MAC layer.

“The only way to control this kind of interference is to determine appropriate values for some Time-Hopping pattern parameters”.

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UWB ad-hoc MAC - Previous Work [3] Jean-Yves Le Boudec, Ruben Merz, Bozidar Radunovic, Joerg Widmer, “A MAC protocol for UWB

very low power mobile ad-hoc networks based on dynamic channel coding with interference mitigation”, EPFL Technical Report ID: IC/2004/02

It is the 1st ad hoc MAC with Dynamic Channel Coding. Time-Hopping for Inter-WPAN interference. Schemes for:

Interference mitigation. Synchronization between transmitter and receiver. Dynamic channel coding with incremental redundancy.

“Private MAC”: Enforce that several senders cannot communicate simultaneously with one destination.

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UWB ad-hoc MAC - Previous Work

PRIVATE MAC:

Request Response Data Ack.

The request is sent to the receiver’s THS (Time-Hopping sequence).

Response, Data and Ack are transmitted using a common Sender-Receiver THS.

If no feedback is received, the sender will re-transmit after a random backoff.

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UWB ad-hoc MAC - Previous Work

PRIVATE MAC (contd.) Assume that B is transmitting to A.

An interfering node C will send a request to A and will wait listening to A’s THS.

After the end of a successful transmission, A & B issue a beacon to their THS to inform other nodes that they are idle.

If multiple nodes wait for A, they start counting a backoff timer as soon as they hear the beacon.

With a proper synchronization scheme, collisions are avoided.

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Conclusions

UWB is an excellent solution for high-speed WPANs Many times the maximum required data rate Power efficiency and no multi-path fading. The Multi-Band approach provides with even more

advantages.

A lot of work has been done in PHY. Upper layers must be examined in detail. Above MAC, nothing has been proposed.

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Questions? (References available upon request)

General Atomics Multi-Band Transceiver Prototype