Research Challenges in
Wireless Communications &
Networking
D. Raychaudhuri
WINLAB, Rutgers University
Piscataway, NJ 08854
ray@winlab.rutgers.edu
1
Introduction
2
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
3
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...
4
Wireless Product Trends
Wireless local loop
(WLL)
MIMO/OFDM, ATM/IP,
Broadband
Wireless Access
(BWA)
3G+ or BWA+??
WCDMA, 3G.PP, etc.
Integrated
Cellular (3G)
Digital
Cellular (2/2.5G)
Wireless LAN
(802.11b)
OFDM, mob IP,
security, QoS,..
2001
802.15.3 WPAN, etc.
driver technologies
OFDM/CDMA,
MIMO, diversity, RRM,..
Public WLAN
3G/WLAN IWF, 4G:
self-org 802.11
Wireless LAN
(802.11x)
potentially disruptive
technology areas
Short-range
radio
(Bluetooth)
BWA/3G combo
(local access
providers)
Home LAN
WPAN
(802.15.3.x)
2002-03
WLAN/3G/2G
(cellular operators)
low- tier
802.11
Home network
sensor nets, etc.
(consumer & verticals)
UWB,
ad-hoc nets
>2005
convergence
opportunities??5
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 ....
6
Radio Technology
7
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.)
8
Radio Technology: Moore’s Law
applies to wireless!
• As computing and communications converge,
network BW must follow CPU & memory size….
802.11a, UWB,..,
1000
1000
1000
1000
1000
Cable
Modem
Gbps
Router
802.11bWLAN,
DSL
ATM
10
10
100
Memory Size
10
100
CPU Speed
100
LAN/WAN
Switching
10
100
Local Access
Wireless Access
100
CPU
Kbps
Kbps
Mbps
1
1
1
1
3G Mobile
56K modem
Wireless
Sw Ethernet
10
Mhz
Local
Access
LAN/WAN
CDPD
Memory
MB
short-range
radio speeds
outpacing Moore’s
law over last
~5 yrs!
1
1990
Year
1995
2000
9
Radio Technology: Modem
Evolution
Time/Frequency processing
Time/Frequency + spatial processing
Multicarrier
Modulation
(OFDM, etc.)
DVB, 802.11a, etc..
QPSK/GMSK
Equalized
QPSK/QAM/
GMSK,..
Multiple antenna
spatial processing
(MIMO, etc.)
4G and next-gen WLL
~10-100 Mbps depending on cell size & mobility
~5-10 bps/Hz achievable with QAM
Pulsed communication
IS-136, etc.
US HDTV, WLL, 802.11b
UWB
Spread Spectrum
(CDMA)
IS-95
Wideband CDMA
(w/ interference canc.
& multiuser det)
WPAN and WLAN
~100-500 Mbps
no allocated spectrum
no RF carrier
short-range, high-data rate
UMTS/IMT-2000
~2 Mbps depending on cell size
~0.5 bps/Hz typical for proposed systems
(works at vehicular mobility speeds)
10
Short-range radio channels
Example opportunistic transmission scenario:
: vehicular user passes by an “Infostation”
z
Offset
w
W
d
11
Short-range radio channel
Initial results show that channel is well-behaved for distance ~5m 
100’s of Mbps readily achieved with various modem techniques
Data from
Domazetovic
& Greenstein
[2001]
12
Source:
J. Foerster,
Intel Research,
2001
Throughput (Mbps)
Radio Technology: UWB
500
450
400
IEEE802.11a
UWB
IEEE802.11g
350
300
250
200
150
100
50
0
0.00
“sweet spot”
for use as
nx100 Mbps
WPAN
10.00
20.00
30.00
40.00
50.00
60.00
70.00
Distance (m)
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
13
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 @ 10-100 Mbps
integrated wireless sensors (low-power)
14
Wireless Systems
15
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 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
16
Wireless Systems: Increasing
the scale of networks
• Rapidly increasing use of untethered data devices
implies that wireless access network capacity (bps/sqKm) will soon have to scale to “gigabit” levels...
Growing proportion of
all computing devices --> 50% +?
Telecom
Network
Internet
Mobile Comm
Devices
Wireless
Access Networks
Sensors/
low-tier data
Mobile PDA/PIA
Semi-mobile
Laptop, etc.
Fixed PC/WS
Example:
~10,000 devices/sq-Km
@1 Mbps peak and 0.1 Mbps
avg implies system capacity
~Gbps/sq-Km
17
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
18
Wireless Systems:
Architecture Evolution
Standard IP, ATM, etc.
Standard IP + M
interface
Dynamic provisioning/ QoS
Mobile/Wired
Network GW
WAP services.
etc.
Static
Regulated spectrum,
static freq co-ord
High-speed
radio hot spot
Radio macrocell
Unregulated spectrum,
dynamic freq coordination
Radio Microcell
(~0.5-1 Km radius)
provisioning
Custom
wireless
protocol
Location-aware
information services,
mcast, cache, etc.
Gigabit Metro Area Network
(w/ integrated mobility support)
BTS
AP/
mini-BTS
WPAN
Mbps/Km2
Gbps/Km2
2G/3G end-users
IP end-users
2G/2.5G/3G
radio access
(single standard)
Cellular Macrocell
(~5-10 Km radius)
WLAN+ or “4G”
or new radio access
(multiple standards)
Faster radio PHY’s with
high interference rejection
& bps/Hz efficiency
WLAN Microcell
(~100m radius)
Current Wireless Network
IP end-users
Scalable Heterogeneous Pico/Micro/Macrocellular
19
Wireless Network Model
Wireless Systems: RRM Model
for Cellular systems
hk 1
BS k
hk 4
h14
hk 5
hk 2 hk 3
h15
BS 1
Source:
Prof. R. Yates,
Rutgers U
h16
• 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”
20
Wireless Systems: RRM in 3G – adaptive
incremental redundancy example
Source:
Dr. L. Razoumov,
Rutgers U
21
Wireless Systems: Efficient
Spectrum Use
• Scaling of wireless services will need new
spectrum (~Ghz) particularly for new highspeed 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?
22
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.
Channel: #1
#2
#3
#4
#5
#6
#N
Common Spectrum
Coord Channel (CSCC)
Packet service
.....
Streaming service A
Streaming service B
....
....
Periodic announcements incl..:
Service type, User #,
Channel #, service params,
Priority, Cost/Price Bids, etc.
23
Wireless Systems: Efficient
Spectrum Use
Example of CSCC etiquette used for “dynamic pricing” based
spectrum allocation:
A
fn
B
fn
B
contends for
fn
channel
fn
A
CSCC
User
ID
B
Service
Type
Price Bid
$.05/hr
A
raises
bid on fn
channel
fn
A
Price Bid
$.07/hr
Price Bid
$.09/hr
A wins
contention
(
B records
& reports
transaction!)
…e-cash exchange
?
24
Mobile Networks
25
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
26
Mobile Networks: “4G” Protocol
Evolution
2.5G/3G Services
4G Services
uniform service
API (Internet+)
PSTN
IP
GSM/
GPRS
3G Access
Network
service
feature
modules
2.5G/3G Radio
Security
QoS
VPN
Content
Delivery
generic
network API
WLAN Services
Low-tier services
IP
Mobile Service
Middleware
IP
WPAN network
layer (e.g. Bluetooth)
Ethernet
WPAN radio
802.11 Radio
Radio-specific vertically integrated systems with
complex intetworking gateways
Today’s Wireless Systems
Unified IP-based mobile network
Generic Radio Access Network
3G/4G
Radio
WLAN
radio
incl support
for multihop,
mcast, etc,
uniform
radio API’s
WPAN/lowtier radio
Radio Independent modular system architecture
for heterogeneous networks
The Future
27
Mobile Networks: Protocols
beyond mobile IP
Global Internet
Mobile IP overlay network
access
point
radio bridge/
router
(forwarding
node)
Radio Access Network 1
IP extensions or generalized L2 MAC??
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 needed
28
Mobile Networks: 3G/WLAN
interworking
Cellular/2.5G,3G
Bluetooth
UWB,
Bluetooth<-> 3G IWF
Unified Mgmt Layer
net
link
PHY
Bluetooth<->WLAN IWF
WLAN<->3G IWF
WLAN,
HiperLAN,
UWB,
3G/WLAN interworking
BT
IWF1
WLAN
IWF2
3G
Protocol stacks
Techniques for seamless service:
- Authentication, global roaming
- Security issues
- Dynamic handoff
- End-to-end QoS control
Multiple devices with
various radio interfaces
- Network management
- Service level agreements29
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)
Service
Zone
~100 MB/s
Fast transfer
Access Control
Zone
Transit time ~sec
Total transit time ~10sec
30
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
A1
IS Control
packet
Terminal
enters WLAN
coverage area
Mobile requests
advance
authentication
........
A3
..
A2
Authentication
message
exchange
PIFS
PIFS
Terminal
enters max
PHY speed
zone
IS transfer
request*
Priority
Access
initiated
Infostations
file transfer*
ACK
*RTS/CTS msgs
not shown
time
31
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
(R12, code 12)
S1
UWB
(R23, code 23)
S2
UWB
(R13, code 13)
S3
32
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)
S1
Common code
Control
Code A
Rate adaptation,
ARQ
Link ACK (S1,S2, C12)
S2
Code B
33
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
SN
MAC cluster
(optional)
radio links for
multi-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.
34
Mobile Networks: Hierarchical Ad-Hoc Net
• 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
Internet
BTS
AP
WLAN
micro-cell
Access Point
Forwarding node
FN
3G cell
low-tier
(e.g. sensor)
user nodes
personal-area
pico-cell
35
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
36
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
User
XML
Descriptor
SX
SX
Semantic
Router
A
Semantic
Router
B
content
multicast
Mobile interest
profile contains:
(user, location,
terminal capability,..)
Content Provider
37
Mobile Networks: Experimental Research
• 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
802.11b
PDA
Management
stations
Radio Monitor
AP
Forwarding Node/AP
(custom)
802.11b
Linux PC
Commercial
Router network
802.11
with arbirtrary topology
Compute
& storage
servers
Sensor Node
(custom)
PC
PC-based
Linux router
38
Wireless Research:
Multidisciplinary Research Topics
In conclusion, we mention some wirelessrelated 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)
39