Transcript Resource Allocation in Wireless Networks

EE360: Lecture 16
Ad Hoc Networks
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Announcements
Introduction to ad-hoc networks
Applications
Design issues:
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“Towards self-organized mobile ad-hoc networks,” Mung Chiang
Link layer
Multiple access
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“Power controlled multiple access,” Xiangheng Liu
Routing and mobility management
Network issues
Power/energy management
Announcements
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Homework due today
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Progress reports due next Wednesday.
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Last class: next-generation standards debate
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Detailed course evaluations (hard copy/web/email)
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What will be the baseline technology
What data rates must be supported and how
What features must be supported and how (voice, data, video, etc.)
Economic issues: pricing, spectral auctions, market penetration, etc.
10 bonus points if turned in by June 8
tbp forms will be done in class June 6
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Can also pick up from Joice any time
10 bonus points if turned in by June 8
Ad-Hoc Networks
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Each node generates independent data.
Source-destination pairs are chosen at random.
Routing can be multihop.
Topology is dynamic (links change, nodes enter and leave)
Fully connected network but with different link SNRs
Can allocate resources dynamically (rate, power, BW, routes,…)
Applications
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Battlefield communications
Wireless LANs
Emergency infrastructures
Short-term networks (e.g. convention)
Sensor networks
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Medical applications (on-body)
Buildings
Wide area
Cellular phone evolution
Communication infrastructure for automated vehicles
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Automobiles, airplanes, UAVs, robots, etc.
Different channel characteristics, distances, mobility, and rate requirements.
Design Issues
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Link layer design
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Channel access and frequency reuse
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Reliability
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Routing
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Network issues
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Power/energy management
Must exploit synergies between design layers
Link Layer Design
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Modulation and Coding
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Robustness
Rate requirements
Performance
Adaptive techniques: rate, power, BER, code, framing, etc.
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Bandwidth requirements
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Power control
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Typically distributed
Antenna design
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Smart antennas
Multipath mitigation
Multiuser detection
Channel Access and Reuse
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ALOHA
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Collision detection or avoidance
Power control in multiple access (Xiangheng)
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ALOHA with DS/FH Spread Spectrum
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Frequency reuse
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Bandwidth efficient
Distributed allocation
Dynamic channel allocation hard for packet data
ALOHA
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Poor efficiency
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Poor capture
Busy Tone
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Hidden terminal problem
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Carrier sensing, collision detection/avoidance
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Hidden nodes degrade performance
Busy tone may interfere with transmission to other
nodes (exposed terminal).
Power control
DS Spread Spectrum:
Code Assignment
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Common spreading code for all nodes
 Collisions occur whenever receiver can “hear” two or
more transmissions.
 Near-far effect improves capture.
 Broadcasting easy
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Receiver-oriented
 Each receiver assigned a spreading sequence.
 All transmissions to that receiver use the sequence.
 Collisions occur if 2 signals destined for same receiver
arrive at same time (can randomize transmission time.)
 Little time needed to synchronize.
 Transmitters must know code of destination receiver
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Complicates route discovery.
Multiple transmissions for broadcasting.
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Transmitter-oriented
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Each transmitter uses a unique spreading sequence
No collisions
Receiver must determine sequence of incoming packet
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Complicates route discovery.
Good broadcasting properties
Poor acquisition performance
Preamble vs. Data assignment
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Preamble may use common code that contains
information about data code
Data may use specific code
Advantages of common and specific codes:
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Easy acquisition of preamble
Few collisions on short preamble
New transmissions don’t interfere with the data block
Reliability
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Packet acknowledgements needed
 May be lost on reverse link
 Should negative ACKs be used.
Combined ARQ and coding
 Retransmissions cause delay
 Coding may reduce data rate
 Balance may be adaptive
Hop-by-hop acknowledgements
 Explicit acknowledgements
 Echo acknowledgements
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Transmitter listens for forwarded packet
More likely to experience collisions than a short
acknowledgement.
Hop-by-hop or end-to-end or both.
Routing (1987)
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Flooding
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Broadcast packet to all neighbors
Inefficient
Robust for fast changing topologies.
Little explicit overhead
Point-to-point routing
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Routes follow a sequence of links
Connection-oriented
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Explicit end-to-end connection
Less overhead/less randomness
Hard to maintain under rapid dynamics.
Connectionless
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Packets forwarded towards destination
Local adaptation
Route dessemination
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Route computed at centralized node
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Distributed route computation
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Most efficient route computation.
Can’t adapt to fast topology changes.
BW required to collect and desseminate information
Nodes send connectivity information to local nodes.
Nodes determine routes based on this local information.
Adapts locally but not globally.
Nodes exchange local routing tables
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Node determines next hop based on some metric.
Deals well with connectivity dynamics.
Routing loops common.
Routing (1999*)
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Table-driven
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Destination-sequenced distance-vector
Clusterhead gateway switch routing
Wireless routing protocol
On-Demand Routing
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On-demand distance vector routing
Dynamic source routing
Temporally ordered routing
Associativity-based routing
Signal stability routing
*”A review of current routing protocols for ad hoc mobile wireless networks,”
Royer and Toh, IEEE Personal Communications Magzine, April 1999.
Other Network Issues
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Network Capacity
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Admission Control
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Interface with wired networks
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Security
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Upgrades
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Software changes
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Software radios
Energy Constraints
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Non-renewable batteries impose a hard energy
constraint on link and network design
Channel capacity must be redefined for energyconstrained nodes
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Not possible to send a finite number of bits with finite
energy and Pe arbitrarily small
Capacity per unit cost (Gallager’87, Verdu’90) defines the
number of bits transmitted per unit
Dynamic resource allocation must take into
account a finite battery life
Routing optimization must take into account
nodes dying away due to battery drainage
What has changed since 1985?
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Batteries are not much better
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DSPs are better, cheaper, and use less power.
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Better coding and modulation.
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Multiuser detection and smart antennas.
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Adaptive techniques available
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How would we leverage these developments to
make better ad-hoc networks?
Summary
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Ad-hoc networks provide a flexible network infrastructure
for many emerging applications
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Recent advances in communication techniques should be
incorporated into ad-hoc network design
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Design issues traverse all layers of the protocol stack, and
cross layer designs are needed
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Energy constraints impose an interesting challenge for
link design, resource allocation, and routing