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Research Challenges in Wireless Communications
& Networking
D. Raychaudhuri
WINLAB, Rutgers UniversityPiscataway, NJ 08854
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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)