A Multiband MAC Protocol for Impulse-based UWB Ad Hoc Networks

Ioannis Broustis, Srikanth V. Krishnamurthy, Michalis Faloutsos,

Mart Molle and Jeffrey R. Foerster

{broustis, krish, michalis, mart} @ cs.ucr.edu

jeffrey.r.foerster @ intel.com

Ioannis Broustis, Srikanth V. Krishnamurthy, Michalis Faloutsos,

Mart Molle and Jeffrey R. Foerster

{broustis, krish, michalis, mart} @ cs.ucr.edu

jeffrey.r.foerster @ intel.com

2

The context

• Wireless needs• High Speed networking• Low cost, low power transport

• Home, enterprise environments

• Current wireless solutions• Low data rates, high power consumption

• UWB pros• High data rates• Low-power operation and low

cost• Low probability of detection• Low interference levels

Picture from http://kom.aau.dk/group/03gr1096/thesis.pdf

3

Motivation & contribution

• A lot of work has been done in the PHY layer of UWB

• Only a few MAC proposals for UWB• Most of them for master-slave deployments• Many assumptions - some of them cannot be implemented in the real

world• Some do not take into account the PHY characteristics

• We design and evaluate a novel multiband MAC protocol for UWB ad hoc networks• Utilizes efficiently the available bandwidth• Achieves much better performance than other MAC protocols for Ad

Hoc UWB• Conforms with FCC regulations

4

Roadmap

UWB Overview The problem Our MAC protocol

Simulation ResultsConclusions

5

UWB definitions

• Any signal that occupies: • At least 500 MHz of bandwidth, or• More than 25% of a fractional bandwidth:

• Available bandwidth: 7500 MHz • FCC has allocated the band from 3.1 GHz to 10.6 GHz for UWB

communications• Emission levels must fall under max limits (average -41.25 dBm/MHz)• Traditionally: pulse transmissions• Range: 0 to 15m

CLH

LH

f

BW

ff

ff=

+−

=)(2η

UWB Spectrum (7.5 GHz)

3.1 10.65.725 - 5.825

802.11a (0.1 GHz)

Frequency (GHz)

EIR

P

FCC limit:- 41.25 dBm/MHz

PSD

6

Bandwidth utilization

• Single-Band Implementation• One transmission occupies the whole BW at a time

• Multi-Band Implementation• The 7.5 GHz are divided into multiple bands• FCC regulations must be obeyed

• Benefits from multiband approach • Low interference from/to systems that share a portion of the BW• Parallel data transmissions in the different bands• Similar H/W cost with single-band implementations

3.1 10.6

802.11a (5.725-5.825 GHz)

EIR

P

7

• Time Hopping, as per Time Hopping Sequences (THS)• Binary Pulse Position Modulation• Many pulses per bit, to increase reliability

• THS overlap Pulse collisions

• Tx, Rx based THS• PAM also possible

Impulse-based UWB

Tf

Tc

0 1 2 3 4 5 6 7

THS1: 0, 3, 2, 6

THS2: 4, 6, 3, 3

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

time

Tc frame

0 1

8

Roadmap

UWB Overview The problem Our MAC protocol

Simulation ResultsConclusions

9

What is the problem?

• UWB pulses are subject to Multipath Delay Spread• Multiple time-shifted pulse copies appear at the receiver

• Intersymbol Interference (ISI)• Tens of nanoseconds (~ 25 to 30nsec for indoor environments)• Collisions at the receiver, with subsequent pulse transmissions

– From the same or different transmitter

A

B

Power Delay Profile

timeobstacle

obstacle

A

10

Potential solutions

• Equalizers, CDMA + Rake receiver• Add overhead and Hardware cost

• Pulse spacing at least equal to the delay spread duration• The adoption of a multi-band mechanism does not reduce the data

rate• A set of carriers modulate the pulse in each band and determine the

pulse shape

Single-band

Multi-band

time

Tc frame

Pulse width

Delay Spread

~0.3nsec

~3nsec (10 bands)

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Roadmap

UWB Overview The problem Our MAC protocol

Simulation ResultsConclusions

12

MAC overview

• We divide the available BW into B bands• One band for requests and band information. The rest for data

transmissions and ACKs

• Map of Band availability• Superframes: Transmission of all control and data packets• Availability frames: Declare intention to keep using a band

time

frequency

Control (REQ)

Data 1

Data 2

Data 3

Data 4

Data B-1

Superframe SuperframeAvailability framek1 k

3

kB-1…..k2

..…

..

..…

..

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• Bandwidth: each of our bands is 500 MHz wide• Emission limits : -41.25 dBm/MHz

• For the received SNR we have:

• Attenuation for each band• PT: Transmitter PSD (-41.25 dBm/MHz) • N0: PSD of the thermal noise (-114 dBm/MHz)• d: Tx-Rx distance• SNRR = 3 dB• fc for the upper band

• For the last band: fc = 10.35 GHz distance ~ 7 meters• We set this distance as the maximum distance for all bands• We use lower transmission powers for the other (lower) bands• We conform with the average power and the pulse frequency is 1

MHz. We conform with the peak power constraint as well :)

Conformance with FCC regulations

contribution

dc

fNPSNR c

TR log204

log200 −⎟⎠

⎞⎜⎝

⎛−−=π

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• Nodes that intend to keep occupying a data band, transmit a short beacon during the availability frame

• The rest of the nodes “listen” to the whole availability frame• Information about which bands will be occupied during the

upcoming superframe

MAC details: band selection

Availability frame SuperframeSuperframe

Data band k

slot kREQ band

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MAC details: request (REQ) initiation

• The REQ packet is transmitted in the Req-band• It includes the selected band of the Tx• The receiver’s THS is used• Nodes are allowed to initiate a REQ transmission only at the beginning

of a superframe

Availability frame Superframe

REQ (Receiver’s THS) REQ band

Data band

Data band

FreeFreeFree

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• 4 possible cases• 1. Everything goes fine

• The receiver decodes the request• Both nodes switch to the selected band• The receiver sends the RACK packet (consecutive pulses)• The Data and DACK packet transmissions follow

MAC details: REQ acknowledgment

Availability frame Superframe

REQ REQ band

Data band

Data band RACK DATA DACK

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• 4 possible cases• 2. Two or more requests towards the same receiver collide

• The receiver cannot decode the request• The transmitters switch to their selected bands, waiting for the RACK• After a specific time interval they will assume that their request did not

reach the receiver• Backoff timers are initiated (decreased by one per superframe)• When backoff=0 the node retransmits the request

MAC details: REQ acknowledgment

REQ (same THS)

Superframe … … … Superframe

REQ (same THS)REQ band

Data band

Data band

Response not received

Availability

frame

REQ (same THS)

Back

-off

countd

ow

n

Availability frame

18

• 4 possible cases• 3. The intended receiver is currently busy

• The receiver will not hear the request• The transmitter however will switch to its selected band• The transmitter initiates a backoff timer and retransmits the request as

soon as this timer becomes zero

MAC details: REQ acknowledgment

Availability frame Superframe … … … Superframe

REQ towards node CREQ band

Data band

Data-band DATA chunk from C to D DACK

REQ

Back-off countdownResponse not received

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MAC details: REQ acknowledgment

• 4 possible cases• 4. Two or more RACKs collide

• If two or more transmitters select the same band, a RACK collision is likely to occur in that data band

• Further actions are temporarily aborted, until the upcoming availability frame

• The requests are retransmitted after the end of the upcoming availability frame

• With our policy, Data packet collisions are avoided

Availability frame

REQ

Superframe

REQ REQ band

Data band

Data bandRACK

RACK

Abort

Tem

pora

rily

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MAC details: DATA and DACK

• The RACK, DATA and DACK packets are transmitted with consecutive pulses

• After the end of the session, transmitter and receiver switch to the REQ band

• If they don’t have packets to send, they stay idle listening to their own THSs

Availability

frame

Superframe

REQ REQ band

Data band

Data band RACK DATA DACK

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Roadmap

UWB Overview The problem Our MAC protocol

Simulation ResultsConclusions

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Comparisons

• We compare our scheme with a single-band approach, in which:• THSs are used for all kinds of packets.• Each pair of nodes has a predetermined common - unique THS

Steps:

REQ

RACK

DATA

DACK

READY

A B

The Tx sends a request to the Rx as per the Rx’s THSBoth Tx and Rx switch to their common THS. The Rx sends a reply

back The Tx further transmits the data packetThe Rx sends an ACK as soon as it receives the data packet

Both Tx and Rx switch to their own THSs. They further transmit a short beacon to indicate their availability

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Simulation set-up

• Simulator in C++ Nodes 6 to 30

Bands 15

Region 30x30 m2 square, multi-hop

Range 7 meters

Node degree 3, Brownian motion

Ratio Tf / Tc 6

Bit repetition 2, with 1/3 conv. encoder

Tc chip 60 nsec, (2 x delay spread)

Superframe 11200 chips

Availability frame 14 slots, 33 chips each

Light traffic CBR, arrival every 40 msec

Heavy traffic CBR, arrival every 1.4 msec

Poisson, (lambda = 5.028)

Data packet 250 bytes

Control packets 15 bytes

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Simulations: pulse collisions

• Decreased by an order of magnitude• Data packets in our case are collision-free

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• The bit error rate is decreased by more than 4 times in our case

• The bit error rate is decreased by more than 4 times in our case

Simulations: BER

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• Time from: packet arrival in the queue until completion of its transmission • Decreased by a factor of 6 for low densities

• Time from: packet arrival in the queue until completion of its transmission • Decreased by a factor of 6 for low densities

Simulations: average packet delay

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• Higher as much as 16.7% in our case• Light traffic beneficial for the single-band case• Would observe larger difference with heavier traffic

• Higher as much as 16.7% in our case• Light traffic beneficial for the single-band case• Would observe larger difference with heavier traffic

Simulations: average network throughput

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• High CBR arrival rate• More than an order of magnitude better throughput in our case

• High CBR arrival rate• More than an order of magnitude better throughput in our case

Simulations: average throughput for high loads

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Roadmap

UWB Overview The problem Our MAC protocol

Simulation ResultsConclusions

30

Conclusions

• We propose a novel multiband MAC protocol for UWB ad hoc networks• Better network performance than previous impulse-UWB MAC• No equalizer or CDMA required to address the delay spread

effects• Utilizes efficiently the 7.5 GHz bandwidth• Adopts all the advantages of a multiband UWB approach• Respects the FCC regulations

• Our ongoing work with UWB:• 1. New multiband MAC that employs binary conflict resolution

• Applicable for home, office and wearable ad hoc networks• Demonstrates much better performance in terms of throughput and

delay

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Questions? (References available upon request)