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 l “Power controlled multiple access,” Xiangheng Liu Routing and mobility management Network issues Power/energy management

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/ ) 10 bonus points if turned in by June 8  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

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, … )

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.

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

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

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

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

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 l Complicates route discovery. l Multiple transmissions for broadcasting.

Transmitter-oriented Each transmitter uses a unique spreading sequence No collisions Receiver must determine sequence of incoming packet l Complicates route discovery. l 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: l Easy acquisition of preamble l Few collisions on short preamble l New transmissions don’t interfere with the data block

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

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 l Explicit end-to-end connection l Less overhead/less randomness l Hard to maintain under rapid dynamics. Connectionless l Packets forwarded towards destination l Local adaptation

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.

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.

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

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 P e 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? 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?

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