Research Challenges in Wireless Communications

1
Research Challenges in Wireless Communications
Networking
D. Raychaudhuri WINLAB, Rutgers
University Piscataway, NJ 08854 ray_at_winlab.rutgers
.edu
2
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 yrs
• Technical challenges
• self-organizing (ad-hoc) networks
• low-power/low-cost/multipurpose wireless sensors
• scalable network routing and content distribution
• distributed information processing in the network
• end-user interfaces applications
• Above topics involve wireless, but are also
  inherently cross-layer or interdisciplinary...

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Wireless Product Trends
MIMO/OFDM, ATM/IP,
Broadband Wireless Access (BWA)
Wireless local loop (WLL)
BWA/3G combo (local access providers)
3G or BWA??
WCDMA, 3G.PP, etc.
Integrated Cellular (3G)
Digital Cellular (2/2.5G)
OFDM/CDMA, MIMO, diversity, RRM,..
Public WLAN
4G WLAN/3G/2G (cellular operators)
3G/WLAN IWF, self-org 802.11
OFDM, mob IP, security, QoS,..
Wireless LAN (802.11b)
Wireless LAN (802.11x)
low- tier 802.11
potentially disruptive technology areas
Home LAN
Home network sensor nets, etc. (consumer
verticals)
WPAN (802.15.3.x)
802.15.3 WPAN, etc.
Short-range radio (Bluetooth)
UWB, ad-hoc nets
gt2005
convergence opportunities??
driver technologies
2002-03
2001
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Wireless Research Challenges Major Areas

• Wireless research topics can be organized into
  following major categories
• radio modems signal processing and hardware
• wireless systems design and optimization
• mobile 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 Moores 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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Radio Technology Moores Law applies to wireless!

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

802.11a, UWB,..,
Gbps Router
802.11bWLAN,
1000
1000
1000
1000
1000
DSL
Cable Modem
ATM
100
100
100
100
100
CPU
Local Access
3G Mobile
LAN/WAN Switching
Memory Size
Local Access
Wireless Access
CPU Speed
56K modem
Wireless
short-range radio speeds outpacing Moores law
over last 5 yrs!
Sw Ethernet
10
10
10
10
10
LAN/WAN
CDPD
Memory
Mhz
Kbps
Mbps
MB
Kbps
1
1
1
1
1
1990
1995
2000
Year
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Radio Technology Modem Evolution
Time/Frequency processing
Time/Frequency spatial processing
Multicarrier Modulation (OFDM, etc.)
Multiple antenna spatial processing (MIMO, etc.)
DVB, 802.11a, etc..
4G and next-gen WLL 10-100 Mbps depending on
cell size mobility 5-10 bps/Hz achievable with
QAM
QPSK/GMSK
Equalized QPSK/QAM/ GMSK,..
Pulsed communication
IS-136, etc.
US HDTV, WLL, 802.11b
UWB
Spread Spectrum (CDMA)
WPAN and WLAN 100-500 Mbps no allocated
spectrum no RF carrier short-range, high-data
rate
Wideband CDMA (w/ interference canc. multiuser
det)
IS-95
UMTS/IMT-2000 2 Mbps depending on cell size 0.5
bps/Hz typical for proposed systems (works at
vehicular mobility speeds)
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Short-range radio channels
Example opportunistic transmission scenario
vehicular user passes by an Infostation
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Short-range radio channel
Initial results show that channel is well-behaved
for distance 5m ? 100s of Mbps readily achieved
with various modem techniques
Data from Domazetovic Greenstein 2001
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Radio Technology UWB
Source J. Foerster, Intel Research, 2001
sweet spot for use as nx100 Mbps WPAN UWB
appropriate for energy-efficient radio links,
typically short-range Also has
potential hardware complexity advantages...
Pragmatic bit-rate comparison between UWB and
802.11x options
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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 MEMS
• mixed signal design testing
• silicon integration and packaging
• UWB radio architecture
• software-defined radio _at_ 10-100 Mbps
• integrated wireless sensors (low-power)

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Wireless Systems
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Wireless SystemsResearch 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 capacity
• scaling ad-hoc network capacity throughput per
  user
• radio resource management for 3G and ad-hoc nets
• interference avoidance
• spectrum sharing in unlicensed bands

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Wireless Systems Increasing the scale of networks

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

Growing proportion of all computing devices --gt
50 ?
Telecom Network
Mobile Comm Devices
Sensors/ low-tier data
Wireless Access Networks
Mobile PDA/PIA
Semi-mobile Laptop, etc.
Internet
Example 10,000 devices/sq-Km _at_1 Mbps peak and
0.1 Mbps avg implies system capacity Gbps/sq-Km
Fixed PC/WS
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Wireless Systems Increasing the scale of networks

• 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

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Wireless Systems Architecture Evolution
Gigabit Metro Area Network (w/ integrated
mobility support)
Location-aware information services, mcast,
cache, etc.
Standard IP, ATM, etc.
Standard IP M interface
Dynamic provisioning/ QoS
Mobile/Wired Network GW
Regulated spectrum, static freq co-ord
High-speed radio hot spot
Radio macrocell
WAP services. etc.
Static provisioning
Unregulated spectrum, dynamic freq coordination
Radio Microcell (0.5-1 Km radius)
Custom wireless protocol
AP/ mini-BTS
BTS
WPAN
Mbps/Km2
Gbps/Km2
2G/3G end-users
IP end-users
Cellular Macrocell (5-10 Km radius)
Faster radio PHYs with high interference
rejection bps/Hz efficiency
2G/2.5G/3G radio access (single standard)
WLAN or 4G or new radio access (multiple
standards)
WLAN Microcell (100m radius)
Current Wireless Network
Scalable Heterogeneous Pico/Micro/Macrocellular
Wireless Network Model
IP end-users
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Wireless Systems RRM Model for Cellular systems
Source Prof. R. Yates, Rutgers U

• 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

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Wireless Systems RRM in 3G adaptive
incremental redundancy example
Source Dr. L. Razoumov, Rutgers U
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Wireless Systems Efficient Spectrum Use

• 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?

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Wireless Systems Efficient Spectrum Use

• 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 Spectrum Coord Channel (CSCC)
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Wireless Systems Efficient Spectrum Use
Example of CSCC etiquette used for dynamic
pricing based spectrum allocation
A
fn
B
fn
B contends for fn
A raises bid on fn
channel
channel
fn
fn
A
A
B
CSCC
Price Bid .07/hr
Price Bid .09/hr
e-cash exchange ?
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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 architectures
• mobility protocols beyond mobile IP
• new architectures (WLAN hot-spots, Infostations,
  ..)
• self-organizing wireless networks (sensors, etc.)
• ad-hoc network routing
• multicasting and mobile content delivery
• wireless network security

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Mobile Networks 4G Protocol Evolution
2.5G/3G Services
4G Services
PSTN
IP
uniform service API (Internet)
GSM/ GPRS
3G Access Network
service feature modules
Security
QoS
VPN
Content Delivery
2.5G/3G Radio
generic network API
WLAN Services
Unified IP-based mobile network
Low-tier services
Mobile Service Middleware
incl support for multihop, mcast, etc,
Generic Radio Access Network
IP
IP
uniform radio APIs
Ethernet
WPAN network layer (e.g. Bluetooth)
3G/4G Radio
WLAN radio
WPAN/low- tier radio
WPAN radio
802.11 Radio
Radio-specific vertically integrated systems
with complex intetworking gateways
Radio Independent modular system architecture for
heterogeneous networks
The Future
Todays Wireless Systems
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Mobile Networks Protocols beyond mobile IP
Global Internet
Mobile IP overlay network

• Mobile IP provides a permanent IP address
• for users moving between wireless APs
• 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 needed

access point
radio bridge/ router (forwarding node)
Radio Access Network 1
IP extensions or generalized L2 MAC??
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Mobile Networks 3G/WLAN interworking
Cellular/2.5G,3G
Unified Mgmt Layer
Bluetooth UWB,
Bluetoothlt-gt 3G IWF
net
link
PHY
IWF1
IWF2
WLAN
3G
BT
Protocol stacks
Bluetoothlt-gtWLAN IWF

• Techniques for seamless service
• - Authentication, global roaming
• - Security issues
• - Dynamic handoff
• - End-to-end QoS control
• - Network management
• - Service level agreements

WLAN, HiperLAN, UWB,
WLANlt-gt3G IWF
3G/WLAN interworking
Multiple devices with various radio interfaces
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Mobile Networks Hot-Spot MAC

• 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

Infostations access point
Data cache
Low-speed control channel (for synch service
setup)
100 MB/s Fast transfer
Service Zone
Access Control Zone
Transit time sec
Total transit time 10sec
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Mobile Networks Hot-Spot MAC

• 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

normal channel activity
AP Beacon
Infostations file transfer
..
........
A1
A3
PIFS
IS Control packet
A2
PIFS
IS transfer request
ACK
Mobile requests advance authentication
Authentication message exchange
Terminal enters max PHY speed zone
Terminal enters WLAN coverage area
Priority Access initiated
RTS/CTS msgs not shown
time
32
Mobile Networks UWB Sensors

• UWB potentially well-suited for sensor networks
• Bit-rate readily traded off against range
• Energy efficient modulation
• Robust to interference
• Multiple radio links supported by single UWB RF
• Low cost silicon for integrated sensor device

UWB (R23, code 23)
UWB (R12, code 12)
S2
S1
UWB (R13, code 13)
S3
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Mobile Networks UWB Sensor MAC

• 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

Beacon S1
Beacon S2
S1
S2
Link establishment signal (S1,S2, C12)
Link ACK (S1,S2, C12)
S1
S2
Rate adaptation, ARQ
Common code
Control
Code B
Code A
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Mobile Networks Ad-hoc Networks

• 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

• 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.

SN
radio links for multi-hop routing
MAC cluster (optional)
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Mobile Networks Hierarchical Ad-Hoc Net

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

Internet
BTS
Access Point
AP
WLAN micro-cell
Forwarding node
FN
3G cell
personal-area pico-cell
low-tier (e.g. sensor) user nodes
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Mobile Networks Higher Layers

• 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

37
Mobile Networks Content Multicast

• 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.

Interest profile
SX
Semantic
Router
B
SX
Mobile interest profile contains (user,
location, terminal capability,..)
Semantic
content multicast
Router
A
Content Provider
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Mobile Networks Experimental Research

• A flexible, open-architecture mobile/ad-hoc
  sensor network testbed recently established at
  WINLAB
• open-source Linux routers and APs (commercial
  hardware)
• Linux and embedded OS forwarding and sensor nodes
  (custom)
• radio link and global network monitoring/visualiza
  tion tools

802.11b PDA
Management stations
Radio Monitor
802.11b Linux PC
Forwarding Node/AP (custom)
AP
Commercial 802.11
Router network with arbirtrary topology
Compute storage servers
Sensor Node (custom)
PC-based Linux router
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Wireless Research Multidisciplinary Research
Topics

• In 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)