1

Research Challenges in Wireless Communications

& Networking

D. Raychaudhuri

WINLAB, Rutgers UniversityPiscataway, NJ 08854

ray@winlab.rutgers.edu

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Introduction

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Wireless Research: Strategic Themes (near-

future)Several fundamental problems need to be solved before the “mobile Internet” can take off:Developing PHY/MAC for broadband radios

~Kbps Mbps Gbps, adaptive, robust, QoS,...

Scaling wireless system capacity widespread service implies ~Gbps/Sq-Km

Designing wireless system-on-chip (SOC)low-cost/low-power, integrated CMOS

Unifying wireless network architectures (WLAN/IP, 2.5G, 3G cellular) & protocols

multiple radio technologies, faster/simpler standards process

Creating “useful” mobile information services...beyond web browsing on hand-held devices

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Wireless Research: Strategic Themes (long-

term)Pervasive computing via large-scale sensor networks (connecting people with their physical environment) viable in 5-10 yrsTechnical challenges:

self-organizing (ad-hoc) networkslow-power/low-cost/multipurpose wireless sensorsscalable network routing and content distributiondistributed information processing in the networkend-user interfaces & applications

Above topics involve wireless, but are also inherently cross-layer or interdisciplinary...

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Wireless Product Trends

Short-rangeradio

(Bluetooth)

Wireless LAN(802.11b)

DigitalCellular (2/2.5G)

Wireless local loop(WLL)

802.15.3 WPAN, etc.

OFDM, mob IP, security, QoS,..

WCDMA, 3G.PP, etc.

MIMO/OFDM, ATM/IP,

Wireless LAN(802.11x)

Integrated Cellular (3G)

Public WLAN

BroadbandWireless Access

(BWA)

3G+ or BWA+??

WPAN(802.15.3.x)

Home LAN

BWA/3G combo(local access

providers)

4G: WLAN/3G/2G(cellular operators)

Home networksensor nets, etc.

(consumer & verticals)

convergenceopportunities??

driver technologies

potentially disruptive technology areas

2001 2002-03

>2005

OFDM/CDMA,MIMO, diversity, RRM,..

3G/WLAN IWF,self-org 802.11

low- tier802.11

UWB,ad-hoc nets

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Wireless Research Challenges: Major Areas

Wireless research topics can be organized into following major categories

radio modems: signal processing and hardwarewireless systems: design and optimizationmobile networks & protocols

Many wireless problems of current importance are cross-layer in nature, so that a holistic approach is essential ....

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Radio Technology

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Radio Technology: Research Topics

Selected research topics in the radio/modem area include:

putting radio modems on “Moore’s Law”signal processing innovations (MIMO, adaptive antennas)flexible software-defined radios (SDR)ultra wideband (UWB)integrated wireless system-on-chip (sensors, etc.)

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• As computing and communications converge, network BW must follow CPU & memory size….

1990 1995 2000Year

1

10

100

1000

Mhz

1

10

100

1000

Kbps

1

10

100

1000

Mbps

1

10

100

1000

MB

LA

N/W

AN

Swit

chin

g

Loc

al A

cces

s

CP

U S

peed

Mem

ory

Size CPU

LAN/WAN

LocalAccess

MemoryKbps

1

10

100

1000

Wir

eles

s A

cces

s

Wireless

CDPD

3G Mobile

802.11bWLAN,

CableModem

DSL

GbpsRouter

ATM

56K modem

Sw Ethernet

Radio Technology: Moore’s Law applies to wireless!

802.11a, UWB,..,

short-rangeradio speedsoutpacing Moore’slaw over last~5 yrs!

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QPSK/GMSKEqualized

QPSK/QAM/GMSK,..

MulticarrierModulation

(OFDM, etc.)

Spread Spectrum(CDMA)

Multiple antennaspatial processing

(MIMO, etc.)

Wideband CDMA(w/ interference canc.

& multiuser det)

IS-136, etc.

DVB, 802.11a, etc..

US HDTV, WLL, 802.11b

IS-95

4G and next-gen WLL~10-100 Mbps depending on cell size & mobility~5-10 bps/Hz achievable with QAM

UMTS/IMT-2000~2 Mbps depending on cell size~0.5 bps/Hz typical for proposed systems(works at vehicular mobility speeds)

Time/Frequency processing

Time/Frequency + spatial processing

Radio Technology: Modem Evolution

UWB

WPAN and WLAN~100-500 Mbpsno allocated spectrumno RF carriershort-range, high-data rate

Pulsed communication

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W

z

d traj

ec

tory

Offset w

Example opportunistic transmission scenario:: vehicular user passes by an “Infostation”

Short-range radio channels

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Initial results show that channel is well-behaved for distance ~5m 100’s of Mbps readily achieved with various modem techniques

Data fromDomazetovic& Greenstein[2001]

Short-range radio channel

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Radio Technology: UWB

0

50

100

150

200

250

300

350

400

450

500

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

Distance (m)

Thro

ughp

ut (M

bps)

IEEE802.11a

UWB

IEEE802.11g

Source:J. Foerster,Intel Research,2001

Pragmatic bit-rate comparison between UWB and 802.11x options

“sweet spot”for use asnx100 MbpsWPAN

UWB appropriatefor energy-efficientradio links, typicallyshort-range

Also has potentialhardware complexityadvantages...

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Radio Technology: Hardware Innovations

As wireless modems become faster and more ubiquitous, key hardware innovations urgently needed:

compact RF components, including MEMSmixed signal design & testingsilicon integration and packagingUWB radio architecturesoftware-defined radio @ 10-100 Mbpsintegrated wireless sensors (low-power)

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Wireless Systems

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Wireless Systems:Research Topics

Designing and optimizing wireless systems via radio resource management (power control, interference avoidance, scheduling, etc.)Selected research topics in the wireless systems area include:

scaling cellular system capacityscaling ad-hoc network capacity & throughput per userradio resource management for 3G and ad-hoc netsinterference avoidancespectrum sharing in unlicensed bands

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• Rapidly increasing use of untethered data devices implies that wireless access network capacity (bps/sq-Km) will soon have to scale to “gigabit” levels...

Internet

WirelessAccess Networks

Mobile CommDevices

Fixed PC/WS

Mobile PDA/PIASemi-mobileLaptop, etc.

Growing proportion ofall computing devices --> 50% +?

TelecomNetwork

Sensors/low-tier data

Wireless Systems: Increasing the scale of

networks

Example:~10,000 devices/sq-Km@1 Mbps peak and 0.1 Mbpsavg implies system capacity~Gbps/sq-Km

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• Consider first the scaling limits of existing and emerging wireless network standards...• 2G cellular/PCS:

cell size ~ 3-5 Km, avail BW ~ 5 Mhz, spectral eff ~ 0.2-0.3 bps/Hz max capacity ~ 100 Kbps avg, 1 Mbps peak (with packet MAC) per sq-Km off by 3 orders-of-magnitude!

• 3G Cellular/PCS: cell size ~ 3-5 Km, avail BW ~ 25 Mhz, spectral eff ~ 0.3-0.5 bps/Hz max capacity ~ 1 Mbps avg, 10 Mbps peak (with packet MAC) per sq-Km still off by 2 orders-of-magnitude!

• Wireless LAN (802.11x, Hiperlan): cell size ~ 0.1-0.5 Km, avail BW ~ 100 Mhz, spectral eff ~ 0.2-0.3 bps/Hz max capacity ~ 100 Mbps avg, 1 Gbps peak per sq-Km correct order-of-magnitude, but too many access points & limited mobility

Wireless Systems: Increasing the scale of

networks

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Mobile/WiredNetwork GWMobile/WiredNetwork GW

Cellular Macrocell(~5-10 Km radius)

Customwirelessprotocol

Standard IP, ATM, etc.

2G/2.5G/3Gradio access(single standard)

Gigabit Metro Area Network(w/ integrated mobility support)

BTSAP/mini-BTS

WLAN+ or “4G”or new radio access(multiple standards)

Standard IP + Minterface

Regulated spectrum,static freq co-ord

Unregulated spectrum,dynamic freq coordination

Faster radio PHY’s with high interference rejection & bps/Hz efficiency

IP end-users

2G/3G end-usersMbps/Km2 Gbps/Km2

Current Wireless Network Scalable Heterogeneous Pico/Micro/Macrocellular Wireless Network Model

Staticprovisioning

Dynamic provisioning/ QoS

Radio Microcell(~0.5-1 Km radius)

Location-awareinformation services,mcast, cache, etc.

WAP services.etc.

High-speedradio hot spot

Radio macrocell

Wireless Systems: Architecture Evolution

WPAN

WLAN Microcell(~100m radius)

IP end-users

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Wireless Systems: RRM Model for Cellular

systems

• Multiple cell scenario with desired and interfering signals

• Algorithms for allocation of bit-rate, base station, channel, tx schedule, power

• Common theme: reduce interference, transmit when the channel is “good”

1kh BS k

2kh3kh

14hBS 1

15h16h

4kh

5kh

Source:Prof. R. Yates,Rutgers U

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Wireless Systems: RRM in 3G – adaptive incremental redundancy

example

Source:Dr. L. Razoumov,Rutgers U

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• Scaling of wireless services will need new spectrum (~Ghz) particularly for new high-speed data services

• Need to rethink traditional approach to spectrum regulation More unlicensed spectrum (e.g. 5 Ghz U-NII) Market mechanisms other than one-time spectrum auctions? Spectrum etiquette procedures for coexistence of QoS-based wireless services (beyond “LBT”) Incentives for efficient utilization of spectrum resources? Relationship to property rights?

Wireless Systems: Efficient Spectrum Use

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• Spectrum etiquette procedure a key issue for U-NII scenario

• “CSCC” approach proposed as possible solution... Coordination channel using simple standard protocol at edge of band Semantics of higher layer coordination protocol TBD... Support arbitrary spectrum policies based on user priority, cost bids, etc.

Common SpectrumCoord Channel (CSCC)

Channel: #1

#2#3

#4

#5

#6

#N

.....

....

....

Packet service

Streaming service A

Streaming service B

Periodic announcements incl..:Service type, User #,Channel #, service params,Priority, Cost/Price Bids, etc.

Wireless Systems: Efficient Spectrum Use

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Example of CSCC etiquette used for “dynamic pricing” based spectrum allocation:

channel

UserID

CSCC

channel

Price Bid$.07/hr

fn

fn

fnfn

Price Bid$.09/hr

ServiceType

Price Bid$.05/hr

A

B

Bcontends for

fn

Araises

bid on fnA winscontention( B records& reportstransaction!)

…e-cash exchange?

A AB

Wireless Systems: Efficient Spectrum Use

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Mobile Networks

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Mobile Networks: Some Research Topics

Selected research topics in the mobile networks area include:

new MAC protocols: 802.11x, 803.15.x, sensor nets“4G” network architecturesmobility protocols: beyond mobile IPnew architectures (WLAN hot-spots, Infostations, ..)self-organizing wireless networks (sensors, etc.)ad-hoc network routingmulticasting and mobile content deliverywireless network security

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Mobile Networks: “4G” Protocol Evolution

WPAN radio

Today’s Wireless Systems The Future

Low-tier services

IP

802.11 Radio

Ethernet

Mobile ServiceMiddleware

IP

WLAN Services

3G/4GRadio

WLANradio

WPAN/low-tier radio

2.5G/3G Radio

GSM/GPRS

2.5G/3G Services

3G AccessNetwork

PSTN IP

WPAN networklayer (e.g. Bluetooth)

Generic Radio Access Network

Radio-specific vertically integrated systems withcomplex intetworking gateways

Security QoS VPNContentDelivery

4G Services

Radio Independent modular system architecturefor heterogeneous networks

uniformradio API’s

genericnetwork API

uniform serviceAPI (Internet+)

Unified IP-based mobile network

incl supportfor multihop,mcast, etc,

servicefeaturemodules

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Mobile Networks: Protocols beyond mobile IP

Radio Access Network 1

Global Internet

Mobile IP overlay networkMobile IP overlay network

radio bridge/router(forwardingnode)

accesspoint

Mobile IP provides a permanent IP address for users moving between wireless AP’s

Desired RAN features for ad-hoc WLAN, sensor nets, 4G:- handoff support (micro-mobility)- discovery and self-organization- ad-hoc routing, integrated with MAC- peer-to-peer modes- multicast, QoS, security, etc.

closer layer 2/3 coupling neededIP extensions or generalized L2 MAC??

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Techniques for seamless service:

- Authentication, global roaming

- Security issues

- Dynamic handoff

- End-to-end QoS control

- Network management

- Service level agreements

Bluetooth<-> 3G IWF

WLAN<->3G IWF

BluetoothUWB,

Bluetooth<->WLAN IWF

WLAN,HiperLAN,UWB,

Cellular/2.5G,3G

3G/WLAN interworking

Unified Mgmt Layer

Protocol stacks

PHY

link

net

IWF1 IWF2BT WLAN 3G

Mobile Networks: 3G/WLAN interworking

Multiple devices with various radio interfaces

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• Mobile user passes through hot-spot (Infostation) in sec during which ~MB files are downloaded/uploaded– Requires modifications to conventional WLAN MAC, incl fast synch, pre-authentication, etc. – Motivates 2-tier arch with ~10m service zone (for high-speed data transfer) and ~50m access control zone

Transit time ~sec

Infostationsaccess point

Data cache

~100 MB/sFast transfer

Low-speed control channel(for synch & service setup)

ServiceZone

Access ControlZone

Total transit time ~10sec

Mobile Networks: Hot-Spot MAC

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• 802.11a MAC can be used for opportunistic service– Pre-authenticate user in low-bit rate mode (~50m range)– Mobile terminal waits for modem to reach max 54 Mbps (~10m range)– High priority access mode used for Infostations access

Mobile Networks: Hot-Spot MAC

APBeacon

IS Controlpacket

Terminalenters WLANcoverage area

Mobile requestsadvance authentication

........

Authenticationmessage exchange

PIFS

normal channelactivity

..

IS transferrequest*

PIFS

Infostationsfile transfer*

Terminalenters maxPHY speed zone

ACK

A1

A2

A3

PriorityAccessinitiated

*RTS/CTS msgsnot shown

time

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• UWB potentially well-suited for sensor networksBit-rate readily traded off against rangeEnergy efficient modulationRobust to interference Multiple radio links supported by single UWB RFLow cost silicon for integrated sensor device

S1

S2

S3UWB(R13, code 13)

UWB(R12, code 12)

UWB(R23, code 23)

Mobile Networks: UWB Sensors

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• Potential MAC/link layer based on DS/CDMA UWB PHY: Continuous beacon for synchronization & sensor ID broadcast Low bit-rate, high-spreading gain common link establishment channel with a single

code used in random access mode Handshake protocol for setting achievable link bit-rate with dedicated code

S1 S2

Beacon S1Beacon S2

S1S2

Link establishment signal (S1,S2, C12)Link ACK (S1,S2, C12)

Control

Code A

Code B

Common codeRate adaptation,ARQ

Mobile Networks: UWB Sensor MAC

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• Ad-hoc network ideas proposed for tactical and sensor scenarios, with potential applications to WLAN/4G: flat network model with multi-hop routing radios on-demand routing protocols (DSR, AODV, etc.) designed for high node mobility (...fairly mature topic) enhancements via MAC clustering, energy-efficient routing, .. application-level data aggregation (diffusion routing, XML,..) geographically constrained routing

Mobile Networks: Ad-hoc Networks

SN

MAC cluster(optional)

radio links formulti-hop routing

Active problem areas:-Scaling of capacity- Dynamic behavior- Energy efficiency- MAC/routing interactions- QoS routing- Geo routing- Security of ad-hoc nodes- Integration with WLAN, etc.

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• Hierarchical, self-organizing network currently under consideration, based on: 3 service tiers (cellular, WLAN, personal area) BS’s, AP’s, FN’s (forwarding radio nodes), user devices automatic discovery and power mgmt protocols hierarchical, ad-hoc multihop routing and spatial MAC

Mobile Networks: Hierarchical Ad-Hoc Net

InternetInternet

Forwarding node

low-tier(e.g. sensor)user nodes

Access Point

FN

APBTS

3G cell

personal-areapico-cell

WLANmicro-cell

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• Research issues which arise in connection with information delivery over wireless nets: Qos with heterogeneous & time-varying radios transport layer problems (TCP timeouts, etc.) need for services such as reliable multicast information “pull” model vs. multicasting model opportunistic services (hot-spots, caching,..) delivery of the “right information” at the “right time and

place” (location/content aware) media scaling to match radio and terminal capabilities sensor network & pervasive computing software models

Mobile Networks: Higher Layers

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• New real-time, context- and location-aware information delivery paradigms under consideration ...

• Content multicasting based on XML investigated as possible option for delivering relevant info to mobiles.

SX

SX

User

Content Provider

SemanticRouter

A

SemanticRouter

B

XMLDescriptor

Interest profile

Mobile Networks: Content Multicast

Mobile interestprofile contains:(user, location, terminal capability,..)

contentmulticast

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• A flexible, open-architecture mobile/ad-hoc sensor network testbed recently established at WINLAB– open-source Linux routers and AP’s (commercial hardware)– Linux and embedded OS forwarding and sensor nodes

(custom)– radio link and global network monitoring/visualization tools

PC-basedLinux router

Mobile Networks: Experimental Research

PC

Router networkwith arbirtrary topology

AP

Compute& storageservers

Managementstations

Radio Monitor

Forwarding Node/AP(custom)

Sensor Node(custom)

802.11bPDA

802.11bLinux PC

Commercial 802.11

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Wireless Research: Multidisciplinary Research

TopicsIn conclusion, we mention some wireless-related multidisciplinary research topics:

spectrum regulation principles (...economics, policy)integrated wireless sensors (...materials, semiconductor)software models for pervasive computing (..CE, CS)dynamics of large-scale ad-hoc sensor nets (...math, control)security in ad-hoc sensor networks (...CS)new applications of sensors: environmental, medical, public safety, etc. (..CS, domain experts from various disciplines)robotics (..mechanical, controls)