VIEWS: 6 PAGES: 22 POSTED ON: 6/8/2012
Announcements Project progress reports due today. Homework 2 ready later today – due 6/2 (next Friday) Graded HW 1 and solutions ready shortly. Third paper summary on ad-hoc networks due next Wednesday. Ad-Hoc Wireless Networks Main Characteristics Each node generates independent data Any node can communicate with any other. No centralized controller (self-configuring) Data transmitted in (short) packets Links typically symmetric. Nodes may be mobile and/or power constrained. Typically a large number of nodes 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 Widely different channel characteristics, distances, mobility, and rate requirements. Design Issues Link Layer design Channel sharing (MAC/reuse) Reliability/QOS Routing Network topology Network management/control Must exploit synergies between design layers Link Layer Issues Modulation and Coding Robustness Rate requirements Performance Adaptive techniques Rate, power, BER, code, framing. Bandwidth requirements Control and communication requirements Power control Typically distributed Antenna design Smart antennas Multipath mitigation Multiuser detection Connectivity Binary or adaptive. Channel Access Frequency-Division Time-Division DS Spread Spectrum FH Spread Spectrum Frequency reuse Bandwidth efficient Distributed allocation Dynamic channel allocation hard for packet data Frequency Division Fixed allocation inefficient Hard to implement when node locations dynamically change Distributed dynamic channel allocation hard to do FD typically only used to create hierarchical networks Time-Division Fixed allocation inefficient and impractical (as in FD) Aloha Inefficient No capture Carrier sensing Hidden nodes degrade performance Busy tone may interfere with transmission to other nodes. Busy Tone 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. 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 Data link control 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 Not possible with directive antennas. Large delays in FIFO queues. More likely to experience collisions than a short acknowledgement. Hop-by-hop or end-to-end or both. Connectivity Determining connectivity SNR measurements Bit/Packet error rate Connectivity control Linkcan adapt to maintain connectivity (adapt rate, power,…) Interaction with routing protocol. Power increase may affect other nodes (Bambos technique). How many connected nodes constitute a network Or, take what you can get. 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 Route dessemination Route computed at centralized node Most efficient route computation. Can’t adapt to fast topology changes. Distributed route computation Each node transmits connectivity information to other nodes. Nodes determine end-to-end route 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. Packet Forwarding Overhead information Routing information Packet identifiers Priority/delay information Tradeoffs in overhead size Synergies of routing and packet forwarding with link layer. Other Network Issues Network Capacity Admission Control Interface with wired networks Security Upgrades Software changes Software radios Network Capacity Capacity limits of ad-hoc 3D networks. Data rates per node Number of nodes Assumptions N users uniformly distributed over the interior of a sphere. Each user communicates with another user randomly chosen among all users. Signal power decays based on free space path loss. All users transmit at the same power. No channel separation or diversity. Interference acts as additive white Gaussian noise Capacity Bounds The total number of bits that may be transmitted by all users, per second, is approximately C K N 3 Proportional to the cube root of N Lower Bound Based on deterministic routing scheme. Upper Bound Similar formula Uses convexity Lower Bound Proof Sketch Estimate the effects of interference in the limit of large N. Construct a series of cell tessellations with useful properties. Use the weak law of large numbers to prove the existence of one user in each cell. Specify a routing and transmitting scheme using time sharing. Determine the capacity of this scheme, which lower bounds the capacity of the best scheme. What has changed since 1985? Signal processing is better, cheaper, and lower power. More powerful channel codes. Multiuser detection and smart antennas. Signal strength measuring techniques available in radios. How would we leverage these developments to make better ad-hoc networks? Sensor Networks Sensor Networks Data highly correlated in time and space. Low homogeneous rates. Links typically asymmetric. Data flows to centralized location. Energy is the driving constraint. 1000-100,000 Nodes Have a common mission. Very different from typical ad-hoc networks
"Resource Allocation in Wireless Networks"