42. design challenges

8/12/2019 42. Design Challenges

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Design Challenges For Energy-Constrained Ad Hoc Wireless Networks

Andrea J. Goldsmith, Stephen B. Wicker

IEEE Wireless Communications, August 2002.

2006. 11. 20

Summarized by Lee Chulki, IDS Lab., Seoul National UniversityPresented by Lee Chulki, IDS Lab., Seoul National University


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Contents

Introduction

Applications

Cross-Layer Design

Conclusions


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Introduction

Ad hoc wireless network

Peer-to-peer communication

Distributed networking and control functions among all nodes

Multihop routing

Dont think that it must be completely flat The distinguishing emphasis in the ad hoc approach lies in the

design requirements

Energy constraints are not inherent to all ad hoc wireless

network But some of the most exciting applications are in energy-

constrained category


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Applications

Data Networks

Data exchange between laptops, PDA

Ex) LAN

Home Networks

Using PDA in the bedroom to scan music in PC

Device Networks Replace inconvenient cabled connections with wireless connections

Ex) Bluetooth

Sensor Networks

With Non-rechargeable battery / minimize human intervention

Distributed Control Systems Remote plants, sensors and actuators linked together via wireless

communication channels

Ex) Automated Highway System


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Cross-Layer Design

Layered approach

Simplified network design

Led to the robust scalable protocols in the Internet

Problem

Inflexibility and suboptimality A wide range of network requirements / energy constraints

Poor performance for ad hoc wireless networks

With Energy constraint, high bandwidth needs, delay constraints

Need Cross-Layer Design

Supports adaptivity and optimization across multiple layers


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Cross-Layer Design

Link Layer

Adapt rate, power and coding to meet the requirements of the application

MAC Layer

Adapt based on link and interference conditions, delay constraints, bit

priorities

Network Layer

Use adaptive routing protocols based on current link, network and traffic

conditions

Application Layer

Utilize a notion of soft QoS Adapts to the network conditions to deliver the highest possible application

quality


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Cross-Layer Design


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Cross-Layer Design

Two fundamental questions

What information should be exchanged across protocol layers and

how should that information be adapted to?

How should global system constraints and characteristics be

factored into the protocol designs at each layer?

Discuss the design of the different layers

Link

MAC

Network

Application


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Link Design Issues

Goal

Achieve rates close to the fundamental capacity limits of the

channel while overcoming channel impairments using relatively little

energy

Contents

Fundamental Capacity Limits

Coding

Multiple Antennas

Power Control

Adaptive Resource Allocation


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Fundamental Capacity Limits

The maximum data rate that can be transmitted over the channel with

arbitrarily small probability of error

Researches

The capacity of an AWGN (Additive White Gaussian Noise) Channel

with B (Bandwidth), SNR (Signal-to-Noise power Ratio)

Recent works: for models that better reflect underlying current wireless

system

More concepts

Capacity per unit energy

Capacity in bits

With finite energy -> Can transmit finite bit

Information transmission

Exchange of routing information

Forwarding bits for other nodes


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Coding

Reduce the power required to achieve a given Bit Error Rate

Researches

Family of codes on graphs with iterative decoding algorithms

Ex) Turbo code

Require more signal processing power


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Multiple Antennas

Improve the performance

Reduce transmit power

Categories

Diversity

Beamsteering

MIMO (Multiple Input Multiple Output)

Trade-off

Save transmission power

Often require significant power for signal processing (complexity)


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Power Control

Potent mechanism for improving wireless network performance

Strategies

Maintain SINR on the link above a required threshold by increasing

power relative to fading and interference

Works well for continuous stream traffic with a delay constraint

Not power-efficient

Dynamic programming to minimize the transmit power required to

meet a hard delay constraint

Significant impact on protocols above the link layer

The level of transmitter power defines the local neighborhood


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Adaptive Resource Allocation

Provides robust link performance with high throughput while

meeting application-specific constraints.

A relatively new technique

Researches

Combinations of power, rate, code, and BER adaptation Variation of the link layer retransmission strategy as well as its

frame size

Diversity combining of retransmitted packets or retransmitting

additional redundant code bits instead of the entire packet


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Medium Access Control Design Issues

Goal: How different users share the available spectrum?

Divide the spectrum into different channels

Assign these different channels to different users

Contents

Channelization

Random Access

Scheduling

Power Control


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Channelization

Frequency division

The system bandwidth is divided into non-overlapping channels

Simple but inflexible

Time division

Time is divided into orthogonal time slots

More flexible than frequency division

Code division

Time and bandwidth are used simultaneously by different users,

modulated by orthogonal or semi-orthogonal spreading codes

Hybrid combinations

Combinations of above methods


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Random Access

Assign channels to active users (not to idle users)

Most systems have many more total users than channels

Collision can be reduced by CSMA (Carrier Sense Multiple Access)

Make hidden / exposed terminal problem

Solutions: 4-way handshake / busy tone transmission / hybrid techniques

Researches Sleep: more energy-efficient

Dynamic programming approach to decisions about transmissions

More flexible and more energy aware


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Scheduling

Random access protocols

Good to bursty traffic

Poor to long strings of packet or continuous stream data

Solution: not easy

Distributed scheduled access in ad hoc wireless networks in

general is an NP-hard problem.

Researches

PRMA (Packet Reservation Multiple Access)

Combines the benefits of random access with scheduling

Optimal scheduling algorithms to minimize transmit energy


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Power Control

Researches

Maintaining the SINR of each user sharing the channel above a

given threshold

Performed in a distributed manner

Strategy for multiple access that takes into account delay

constraints

For cellular systems

Centralized / distributed power control

This issue remains Active area of research


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Network Design Issues

Contents

Neighbor Discovery And Network Connectivity

Routing

Scalability and Distributed Protocols

Network Capacity


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Neighbor Discovery And Network Connectivity

Neighbor discovery

Higher transmit power, more neighbors

Require larger neighborhoods for high mobility

Connectivity

Influenced by the ability to adapt parameters at the link layer Such as rate, power, coding

Sleep decisions are important

Network connectivity

Neighbor discovery


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Routing

Multihop routing protocols

Flooding, centralized/distributed proactive routing, reactive routing

Combination of reactive and proactive routing

Mobility

Flooding is effective under high mobility

Multipath routing: modification of flooding

A packet is duplicated on only a few paths with a high likelihood of

reaching its final destination

Energy constraints

Reactive routing is effective

With listening mode, proactive and reactive routing have roughly

the same energy consumption


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Scalability and Distributed Protocols

Scalability

Important in the design of self-configuring ad hoc wireless networks

Most work on scalability has focused on small networks (


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Network Capacity

Fundamental capacity limit of an ad hoc wireless network

Researches

The per-node rate in a large ad hoc wireless network goes to zero

Even with optimal routing and scheduling

So, All nodes should not communicate with all other nodes

Node mobility actually increases the per-node rate to a constant

Determine achievable rate regions using adaptive transmission

strategies

Information theoretic analysis on achievable rates between nodes


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Application Design Issues

Adaptive QoS

Unrealistic: low-capacity, mobile users, dynamic topology

Applications must adapt to time-varying QoS parameters offered by

the network

Ex) Rate-Delay trade-off curve: Decide point to operate

Application Adaptation

Ex) Video: Change compression rate

Demanding applications can deliver good overall performance under

poor network conditions if the application is given the flexibility to

adapt


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Conclusions

Cross-layer design is particularly important under energy

constraints

Energy across the entire protocol stack must be minimized

Out-of-box thinking is required The box of layered protocol designs

The box of wireline protocols

The box of guaranteed QoS for demanding applications

42. design challenges

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