The Broadcast Function in Wireless Ad-Hoc Network

1. The Broadcast Function in Wireless Ad-Hoc Network 2002.9.2 Speaker:peter

2. An overview of the broadcast function Difference between wired and wireless networks The essence of broadcast problem Categorization of present protocols Ultra WideBand technology Broadcast protocol for Ultra WideBand ? Outline

3. Function: paging a particular host sending an alarm signal finding a route to a particular host Two type: Be notified -> topology change Be shortest -> finding route Objective: Reliability (all nodes have received the broadcast packet) Optimization Broadcast

4. The difference of two type 5 forwarding nodes 4 hop time 6 forwarding nodes 3 hop time source source Be notified Be shortest

5. Broadcast property Reliability: CSMA/CD vs. CSMA/CA RTS/CTS/data/ACK procedure is too cumbersome to implement for broadcast Simultaneous transmission and hidden node problem Expensive bandwidth Therefore the flooding which is the simple broadcast mechanism in wired networks is not suitable in wireless networks (broadcast storm problem) Difference between wired and wireless networks

6. Many retransmissions are redundant Because radio propagation is omnidirectional and a physical location may be covered by the transmission ranges of several nodes Heavy contention could exist Because retransmitting nodes are probably close to each other Collisions are more likely to occur Because the RTS/CTS dialogue is inapplicable and the timing of retransmissions is highly correlated Broadcast Storm

7. Reliability Reliable MAC negotiation Collision avoidance Reduce redundant rebroadcasts Avoid Simultaneous transmission Optimization Minimize the forwarding nodes Minimize the power consumption The essence of broadcast problem

8. Extend RTS/CTS for broadcast Waiting for all neighbors to be ready RTS collision ? Forwarding ? Sending broadcast packets anyway Affect other transmission Repeatedly broadcast until all neighbor received Acknowledgement Reliable MAC negotiation A The neighborhood state of mobile nodes is under control by RTS/CTS

9. Reduce redundant rebroadcasts (minimize forwarding nodes) Be shortest  minimum forwarding set Be notified  minimum broadcasting set Avoid Simultaneous transmission Different timing of rebroadcasts Collision avoidance

10. Define: Given a source A let D and P be the sets of k and k+1 hop neighbors of A Find a minimum-size subsetF of D such that every node in P is within the coverage area of at least one node from F In general graph: NP-complete: reduce “Set Cover” to it Approximation ratio: logn In unit disk graph: Unknown Approximation ratio: constant (by reference [1]) Minimum Forwarding Set Problem

11. Define: Given a source A Find a spanning tree T such that the number of internal nodes is minimum In general graph: NP-hard:hard to“Minimum Connected Dominating Set” Approximation ratio: log ( is the maximum node degree) In unit disk graph: NP-hard Approximation ratio: constant (by reference [2]) Minimum Broadcasting Set Problem

12. Minimize the power consumption (Minimum-Energy Broadcast Tree Problem) Define: Given a wireless ad hoc network M = (N,L) A source node s to broadcast a message from s to all the other nodes such that the sum of transmission powers at all nodes is minimized Same as the Steiner Tree problem in directed graph: NP-complete: reduce “3-CNF SAT”to it Approximation ratio: n Optimization

13. Simple flooding Area based method (by reference [3]) Counter based scheme Distance based scheme Location based scheme Neighbor knowledge method Neighborhood base Set cover base MCDS base Categorization of present protocols

14. Each node forwarding the broadcasting packets exactly one time Using Process ID Flooding

15. Area Based Method1

16. Area Based Method2 • Maximum additional coverage of previous transmission: • Average additional coverage: ≈ 0.41r2 • Average additional coverage after having received a broadcast message twice: ≈ 0.19r2

17. Area Based Method3 The expected additional coverage after hearing the message k times, is expected to decrease quickly as k increases.

18. Counter based scheme: Using “Random Assessment Delay” (RAD) The counter is incremented by one for each redundant packet received If the counter is less than a threshold value when the RAD expires, the packet is rebroadcast. Otherwise, it is simply dropped Area Based Method4

19. Area Based Method5 • Distance based scheme: • Using “Random Assessment Delay” (RAD) • Estimating the distance d between sender and receiver by signal strength • Calculate the additional coverage by d (additional coverage = ) • If the additional coverage which is calculated by the minimum distance is more than a threshold value when the RAD expires, the packet is rebroadcast. Otherwise, it is simply dropped

20. Area Based Method6 • Location based scheme: • Using “Random Assessment Delay” (RAD) • Adding location information to the header of the broadcast packets • Calculate the additional coverage by k location information which are received during RAD • Difficult to calculate exactly • Using grid-filling approximation • If the additional coverage is more than a threshold value, the packet is rebroadcast. Otherwise, it is simply dropped

21. Neighborhood information How to decision forwarding nodes Neighborhood base SBA, Self pruning Set cover base Multipoint relaying, Dominant pruning, AHBP MCDS base Neighbor Knowledge Method

22. Scalable Broadcast Algorithm (SBA) • Information: • Hello message (2-hop) • Forwarding node decision: • Node vj who receives the packet from vi checks whether the set N(vj)-N(vi)-{vi} is empty • Node vj schedules the packet for delivery with a RAD (Random Assessment Delay) • Dynamically adjust the RAD to (nodes with the most neighbors usually broadcast before the others)

23. Information: Hello message (1-hop) Piggyback adjacent node list in broadcast packets (2-hop) Store adjacent node list in cache Forwarding node decision: Node vj who receives the packet from vi checks whether the set N(vj)-N(vi)-{vi} is empty Self pruning vj vi

24. Multipoint relaying • Information: • Hello message (2-hop) • Forwarding node decision: • The sending node A selects forwarding nodes from it’s adjacent nodes • A select a minimum node set F N(A) such that: • A node set U = N(N(A)) –N(A) • Piggyback forward list in “Hello” packets

25. Dominant pruning • Information: • Hello message (2-hop) • Forwarding node decision: • The sending node selects forwarding nodes from it’s adjacent nodes • Node vj who receives the packet from vi , vj select a minimum node set F N(vj) - N(vi) such that: • A node set U = N(N(vj)) –N(vi) –N(vj) • Piggyback forward list in broadcast packets

26. vi vj Dominant pruning N(N(vj)) N(vj) N(vi) B(vi,vj) U

27. The drawback of present set cover based protocols1 When a node vireceived the broadcast packet from node vi-1, it will select some forwarding nodes from N(vi)-N(vi-1) to cover all nodes in U. However, some nodes in U are not i+2 level nodes, and some nodes in N(vi)-N(vj) are not i+1 level nodes. vi vi-1 s … i-1 i i+1 i+2

28. The drawback of present set cover based protocols2 When we will select some level i+1 nodes to cover all level i+2 nodes, the number of forwarding nodes selected by distributed algorithm can not be bounded to some ratio of the optimal solution ? vi2 vi-1 s vi1 … i-1 i i+1 i+2

29. Forwarding Set Problem in unit disk graph Fiis the output of the -approximation algorithm which select some nodes in blue area to cover all nodes in Qi OPTi is the optimal solution of Forwarding Set problem and lie on Ai Q1 Q2 A2 A1 A4 A3 Q3 Q4

30. Approximation Algorithm: Definition: A piece is defined as a white node or a black connected component Initialize: all nodes are white Procedure: At each step we pick a node u that gives the maximum (non-zero) reduction in the number of pieces. coloring u black and coloring all adjacent white nodes gray. Recursively connect pairs of black components by choosing a chain of two vertices. Approximation ratio: 3+log(reference [4]) MCDS based algorithm

31. Idea: Any MIS (maximal independent set) is also a DS, and conversely, any independent DS must be an MIS The size of any MIS in a unit disk graph is at most four times of the size of the MCDS The shortest distance between a node in MIS and it’s nearest node in MIS is at most three Algorithm: Find any MIS Spanning all nodes in MIS Approximation ratio: 12 (reference [2]) MCDS based algorithm in unit disk graph1

32. MCDS based algorithm in unit disk graph2 • Lemma: The size of any MIS in a unit disk graph is at most four times of the size of the MCDS • proof: • U is any MIS, T is a spanning tree of MCDS • v1,v2,…,v|T| be an arbitrary preorder traversal of T • Ui is the set of nodes in U that are adjacent to vi but none of v1, v2,…,vi-1 • Then U1,U2,…,U|T| form a partition of U • |U1|5, |Ui|4, 2i|T|

33. MCDS based algorithm in unit disk graph3 at most 240 vj vi j=1~i-1 Ui lie in a sector of at most 240 degree within the coverage range of node vi, this implies that |Ui|4 A node is adjacent to at most five independent nodes in unit disk graph

34. Comparison 350x350 r:100

35. Ultra WideBand Technology(UWB) By Chiang Jui-Hao

36. What is Ultra Wideband? • Originally referred to • “baseband”, “carrier-free”, or impulse • Any wireless transmission scheme • occupies a bandwidth of more than 25% of a center frequency, or more than 1.5GHz

37. Compare with narrowband and wideband UWB systems have two characteristics • Bandwidth is much greater, • Defined by the Federal Communications Commission (FCC), is more than 25% of a center frequency or more than 1.5GHz • Carrierless fashion • “narrowband” and “wideband” use RF • UWB directly modulate an "impulse" that has a very sharp rise and fall time

38. UWB in Short Range Wireless • Spatial capacity : (bps/m2)higher bit rates concentrated in smaller areas • For users gather in crowded spaces, the most critical parameter of a wireless system will be its spatial capacity [1]

39. Compare with IEEE 802.11 and Bluetooth (cont.) • UWB have greater spatial capacity • From the Hartley-Shannon law • Potential • for support of future high-capacity wireless systems

40. Notice of Proposed Rule Making • In May of 2000, the FCC issued a Notice of Proposed Rule Making (NPRM) • limit UWB • transmitted power spectral density for frequencies greater than 2GHz.

41. Ultra-Wideband transceiver • Advantages: • UWB is a “carrierless” system,thus we can remove traditional blocks such as carrier recovery loop,mixer…etc. • High data rate and number of users. • Robustness to multi-path fading. [3]

42. UWB Advantages • Extremely difficult to intercept • Short pulse excitation generates wideband spectra – low energy densities • Low energy density also minimizes interference to other services • Multipath immunity • Time-gated detector can excise delayed returns - time separation

43. UWB Advantages (cont.) • Commonality of signal generation and processing architectures • Communications • LPI/D, High Data Rates, Multipath Tolerance • Radar • Inherent high precision – sub-centimeter ranging • Wideband excitation for detection of complex, low RCS targets • Low cost • Nearly “all-digital” architecture • Ideal for microminiaturization into a chipset • Frequency diversity with minimal hardware modifications

44. UWB Signal in multi-path fading channel • Multi-path fading results from the destructive interference caused by the sum of several received paths that may be out of phase with each other.The very narrow pulses of UWB waveforms result in the multiple reflections being resolved independently rather than combining destructively. [4]

45. UWB Applications

46. UWB operation and technology • Imaging Systems • Ground Penetrating Radar Systems • Wall Imaging Systems • Through-wall Imaging Systems • Medical Systems • Surveillance Systems • Vehicular Radar Systems • Communications and Measurement Systems