Cross Layer Design in Wireless Networks

Andrea GoldsmithStanford University

Crosslayer Design PanelICC

May 14, 2003

Future Wireless Networks

Wireless Internet accessNth generation CellularWireless Ad Hoc NetworksSensor Networks Wireless EntertainmentSmart Homes/SpacesAutomated HighwaysAll this and more…

Ubiquitous Communication Among People and Devices

•Hard Delay Constraints•Hard Energy Constraints

Challenges Wireless channels are a difficult and capacity-

limited broadcast communications medium

Traffic patterns, user locations, and network conditions are constantly changing

Applications are heterogeneous with hard constraints that must be met by the network

Energy and delay constraints change design principles across all layers of the protocol stack

These challenges apply to all wireless networks, but are amplified in ad hoc/sensor networks

Evolution of Current Systems

Wireless systems today2G Cellular: ~30-70 Kbps.WLANs: ~10 Mbps.

Next Generation3G Cellular: ~300 Kbps.WLANs: ~70 Mbps.

Technology Enhancements Hardware: Better batteries. Better

circuits/processors.Link: Antennas, modulation, coding, adaptivity, DSP,

BW.Network: Dynamic resource allocation. Mobility

support.Application: Soft and adaptive QoS.

“Current Systems on Steroids”

Future Generations

Rate

Mobility

2G

3G

4G802.11b WLAN

2G Cellular

Other Tradeoffs: Rate vs. Coverage Rate vs. Delay Rate vs. Cost Rate vs. Energy

Fundamental Design Breakthroughs Needed

Design objective

Want to provide end-to-end “QoS”

The challenge for this QoS is the system dynamicsScheduling can help shape these dynamics

Adaptivity can compensate for or exploit these dynamics

Diversity provides robustness to unknown dynamics

Scheduling, adaptivity, and diversity are most powerful in the context of a crosslayer design

Energy must be allocated across all protocol layers

Crosslayer Design

Hardware

Link

Access

Network

Application

Delay ConstraintsRate Constraints

Energy Constraints

Adapt across design layersReduce uncertainty through scheduling

Provide robustness via diversity

Crosslayer Techniques Adaptive techniques

Link, MAC, network, and application adaptation Resource management and allocation (power

control) Synergies with diversity and scheduling

Diversity techniques Link diversity (antennas, channels, etc.) Access diversity Route diversity Application diversity Content location/server diversity

Scheduling Application scheduling/data prioritization Resource reservation Access scheduling

Key QuestionsWhat is the right framework for crosslayer

design? What are the key crosslayer design synergies? How to manage its complexity? What information should be exchanged across layers,

and how should this information be used?

How do the different timescales affect adaptivity?

What are the diversity versus throughput tradeoffs?

What criterion should be used for scheduling?

How to balance the needs of all users/applications?

Single User Systems

TrafficGenerator

DataBuffer

SourceCoder

ChannelCoder

Modulator(Power)

Receiver

Stringent QoS constraints require that thefull dynamics of the system be represented

Channel

Cross-Layer System

Average performance metrics are misleading

The “Myth” of Averages:

Minimizing Average Delay

What if we constrain delayacross all channel gains?

Average Delay

Channel Gain in dB

Dela

y in

mill

iseco

nd

s

Hard Delay Constraints (50 ms)

Pow

er

in m

illiw

att

s

Source data rate in bits per second

50ms constraint on delayacross all channel gains

with power adaptation only

Joint source-channel coding power saving

200mW

Crosslayer design in multiuser systems

• Users in the system interact (interference, congestion)

• Resources in the network are shared

• Adaptation becomes a “chicken and egg” problem

• Protocols must be distributed

Energy-Constrained Nodes

Each node can only send a finite number of bits.Transmit energy minimized by maximizing bit timeCircuit energy consumption increases with bit time Introduces a delay versus energy tradeoff for each bit

Short-range networks must consider transmit, circuit, and processing energy.Sophisticated techniques not necessarily energy-

efficient. Sleep modes save energy but complicate networking.

Changes everything about the network design:Bit allocation must be optimized across all protocols.Delay vs. throughput vs. node/network lifetime tradeoffs.Optimization of node cooperation.

Distributed Control over

Wireless Links

Network design must meet control requirements. Automated highway controllers unstable with any delay

Controller design should be robust to network faults. Need joint application and communication network design.

Automated highways,factories, and homes

Design Approach

Theory Optimizati

on Simulatio

n Testbeds

NetworkDesign

Opt

imiz

atio

n

Sim

ulat

ion

LinkDesign

DeviceDesign

ApplicationDesign

Summary Crosslayer design needed to meet requirements

and constraints of future wireless networks

Key synergies in crosslayer design must be identified

The design must be tailored to the application

Crosslayer design should include adaptivity, scheduling and diversity across protocol layers

Energy can be a precious resource that must be shared by different protocol layers

Lots of fun and challenging research problems