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EE360: Lecture 16 Ad Hoc Networks

EE360: Lecture 16 Ad Hoc Networks. Announcements Introduction to ad-hoc networks Applications Design issues: “Towards self-organized mobile ad-hoc networks,” Mung Chiang Link layer Multiple access “Power controlled multiple access,” Xiangheng Liu Routing and mobility management

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EE360: Lecture 16 Ad Hoc Networks

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  1. EE360: Lecture 16Ad Hoc Networks • Announcements • Introduction to ad-hoc networks • Applications • Design issues: • “Towards self-organized mobile ad-hoc networks,” Mung Chiang • Link layer • Multiple access • “Power controlled multiple access,” Xiangheng Liu • Routing and mobility management • Network issues • Power/energy management

  2. Announcements • Homework due today • Progress reports due next Wednesday. • Last class: next-generation standards debate • 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. • Detailed course evaluations (hard copy/web/email) • 10 bonus points if turned in by June 8 • tbp forms will be done in class June 6 • Can also pick up from Joice any time • 10 bonus points if turned in by June 8

  3. Ad-Hoc Networks • 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,…)

  4. Applications • Battlefield communications • Wireless LANs • Emergency infrastructures • Short-term networks (e.g. convention) • Sensor networks • Medical applications (on-body) • Buildings • Wide area • Cellular phone evolution • Communication infrastructure for automated vehicles • Automobiles, airplanes, UAVs, robots, etc. Different channel characteristics, distances, mobility, and rate requirements.

  5. Design Issues • Link layer design • Channel access and frequency reuse • Reliability • Routing • Network issues • Power/energy management Must exploit synergies between design layers

  6. Link Layer Design • Modulation and Coding • Robustness • Rate requirements • Performance • Adaptive techniques: rate, power, BER, code, framing, etc. • Bandwidth requirements • Power control • Typically distributed • Antenna design • Smart antennas • Multipath mitigation • Multiuser detection

  7. Channel Access and Reuse • ALOHA • Collision detection or avoidance • Power control in multiple access (Xiangheng) • ALOHA with DS/FH Spread Spectrum • Frequency reuse • Bandwidth efficient • Distributed allocation • Dynamic channel allocation hard for packet data

  8. ALOHA • Poor efficiency • Poor capture • Hidden terminal problem • Carrier sensing, collision detection/avoidance • Hidden nodes degrade performance • Busy tone may interfere with transmission to other nodes (exposed terminal). • Power control Busy Tone

  9. DS Spread Spectrum:Code Assignment • Common spreading code for all nodes • Collisions occur whenever receiver can “hear” two or more transmissions. • Near-far effect improves capture. • Broadcasting easy • 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 • Complicates route discovery. • Multiple transmissions for broadcasting.

  10. Transmitter-oriented • Each transmitter uses a unique spreading sequence • No collisions • Receiver must determine sequence of incoming packet • Complicates route discovery. • Good broadcasting properties • Poor acquisition performance • Preamble vs. Data assignment • Preamble may use common code that contains information about data code • Data may use specific code • Advantages of common and specific codes: • Easy acquisition of preamble • Few collisions on short preamble • New transmissions don’t interfere with the data block

  11. Reliability • 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 • Transmitter listens for forwarded packet • More likely to experience collisions than a short acknowledgement. • Hop-by-hop or end-to-end or both.

  12. Routing (1987) • Flooding • Broadcast packet to all neighbors • Inefficient • Robust for fast changing topologies. • Little explicit overhead • Point-to-point routing • Routes follow a sequence of links • Connection-oriented • Explicit end-to-end connection • Less overhead/less randomness • Hard to maintain under rapid dynamics. • Connectionless • Packets forwarded towards destination • Local adaptation

  13. Route dessemination • Route computed at centralized node • Most efficient route computation. • Can’t adapt to fast topology changes. • BW required to collect and desseminate information • Distributed route computation • 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 • Node determines next hop based on some metric. • Deals well with connectivity dynamics. • Routing loops common.

  14. Routing (1999*) • Table-driven • Destination-sequenced distance-vector • Clusterhead gateway switch routing • Wireless routing protocol • On-Demand Routing • 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.

  15. Other Network Issues • Network Capacity • Admission Control • Interface with wired networks • Security • Upgrades • Software changes • Software radios

  16. Energy Constraints • Non-renewable batteries impose a hard energy constraint on link and network design • Channel capacity must be redefined for energy-constrained nodes • 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

  17. What has changed since 1985? • Batteries are not much better • DSPs are better, cheaper, and use less power. • Better coding and modulation. • Multiuser detection and smart antennas. • Adaptive techniques available • How would we leverage these developments to make better ad-hoc networks?

  18. Summary • Ad-hoc networks provide a flexible network infrastructure for many emerging applications • Recent advances in communication techniques should be incorporated into ad-hoc network design • Design issues traverse all layers of the protocol stack, and cross layer designs are needed • Energy constraints impose an interesting challenge for link design, resource allocation, and routing

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