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Minimal CDMA Recoding Strategies in Power-Controlled Ad-Hoc Wireless Networks. Honglei Miao honglei.miao@ee.oulu.fi Centre for Wireless Communications University of Oulu, Finland. Outline. Introduction Problem statement and previous work New recoding strategies Simulation Results

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slide1

Minimal CDMA Recoding Strategies in Power-Controlled Ad-Hoc Wireless Networks

Honglei Miao

honglei.miao@ee.oulu.fiCentre for Wireless CommunicationsUniversity of Oulu, Finland

slide2

Outline

  • Introduction
  • Problem statement and previous work
  • New recoding strategies
  • Simulation Results
  • Conclusions
introduction
Introduction
  • Transmitter Oriented Code Assignment (TOCA) in CDMA based Ad-Hoc wireless network
    • Each node is assigned one code to be used to transmit it’s message.
    • Two kinds of collisions can be happened to damage the transmission.
      • Primary collision where an incoming transmission is damaged by a simultaneous outgoing transmission from the receiving mobile.
      • Secondary collision where two incoming transmissions garble each other.
    • Correct and efficient TOCA algorithms should be:
      • Eliminate all the collisions including primary and secondary collisions.
      • Minimize the maximum code index assigned to any network node.
    • Several centralized and distributed heuristics have been proposed for static multihop networks.
  • Why recoding in Ad-Hoc network?
    • In a dynamic ad-hoc network, nodes are free to
      • move about.
      • connect or disconnect from the network.
      • Increase or decrease transmission ranges.
    • These events may introduce new collisions, Recoding is needed to eliminate these new collisions.
slide4

Introduction (2)

  • Existed code assignment algorithms are inappropriate for recoding
    • Centralized code assignment algorithms determine a new code assignment for every node on each event. (costly)
    • Distributed heuristics assume a static network. (inappropriate)
  • Minimum recoding algorithms are proposed in this paper.
    • Distributed, only need communication local to the event.
    • Minimal recoding, minimize the number of the nodes to be recoded on any network.
    • Least increase in the maximum code index assigned to the network.
slide5

Problem statement and previous work

  • A power controlled ad-hoc network is modelled as a dynamic directed graph G=(V,E).
    • V = {v1,v2,….,vn} is set of nodes in the network. ri is the transmission range of node vi. ci is the code assigned to node vi.
    • E = {(vi, vj): i !=j, and dij<=ri} is the set of the directed edges.
  • TOCA is to assign a code to each node in the network so that the following two constraints are satisfied.
    • CA1-(Primary) collision avoidance 1: For every edge
    • CA2-(Secondary) collision avoidance 2: For every pair of edges
slide6

Problem statement and previous work (2)

  • Assumption of the events or reconfiguration in the dynamic ad-hoc network
    • Events occur one after another and not simultaneously.
    • Nodes move and change their ranges in discrete steps.
    • Minimal connectivity: A node v can change its configuration iff it has both from-neighbour and to-neighbour.
  • The goals of an efficient recoding strategy
    • Minimize the maximum code index used by any node in the network. (hardware consideration)
    • Minimize the number of nodes that change their codes.
    • Minimize the overhead of the recoding
    • Keep the recoding strategy distributed and local.
slide7

Problem statement and previous work (3)

  • Previous strategy: CP strategy
    • The new node and its 1-hop neighbours exchange the information about their old codes and constraints.
    • Ordering by identities
      • The new nodes and it’s 1-hop neighbours need to be recoded continuously check if they are the highest (or lowest)-identity node in its vicinity that has not been assigned a code.
    • Respect for the constraints
      • If it is the highest (or lowest)-identity node. The lowest available code (not taken by any 1-hop and 2-hop neighbours) is selected.
slide8

New recoding strategies

  • Handling Node Join
slide9

New recoding strategies (2)

  • From CA1 and CA2, all nodes in 1n, 2n, {n} each need to have codes different from each other. Nodes in 3n need not change their codes since n will be assigned a new code anyway and this will need to be different from any of the codes in 3n.
  • If a K-sized subset of nodes in 1n U 2n have the same old code, only K-1 nodes need to be changed.
  • More generally, if they are K nodes in 1n U 2n , and m different codes in 1n U 2n, then only K-m nodes need to be changed to different codes.
slide10

New recoding strategies (3)

  • Algorithm for recoding on a node join
slide11

New recoding strategies (4)

  • Example of recoding on a node join
  • 1n = {7}, 2n = {1 2 3 6}, 3n = {}, 4n = {4 5}
slide12

New recoding strategies (5)

  • Handling Node Power Increase
slide13

New recoding strategies (6)

  • No new constraints are induced among 1n U 2n U 3n U 4n.
  • All constraints due to CA1 and CA2 added by the new edges involve node n.
  • Minimum recoding only change the code of n if the old code of n can not satisfy the new constraints.
  • However, the proposed algorithm may not be the optimal among all minimal recoding strategies. For example, n only have one new constraint with another node m. If n has lots of old constraints and m very few, recoding only m might be more optimal in terms of maximum code index assigned to the network while achieving the minimal recoding bound.
slide14

New recoding strategies (7)

  • Handling Node Leaves and Power Decreases
    • No recoding since no new conflicts are introduced.
  • Handling Node Movement
    • Node movement is treated as a pair of consecutive events where the moving node n leaves and joins the network.
    • Recoding strategy on a node move is similar to that on a node join.
slide15

Simulation results

  • The different algorithms are simulated for a long sequence of events.
  • The proposed algorithms are compared to
    • BBB algorithm: centralized colouring heuristic, recolor all the nodes at every event.
    • CP strategy
  • The performance metrics to be concerned
    • Maximum code index assigned in the network (the lower the better).
    • The number of nodes recoded (recoded with a new code different from its old one).
slide19

Conclusions

  • A set of recoding strategies Minim for TOCA CDMA recoding in a dynamic ad-hoc network are proposed.
  • Given an event, the strategy change the codes of the minimum number of mobiles needed to eliminate all collisions in the network.
  • Simulation results reveal that the Minim approaches trade off a relatively small loss in terms of maximum code index assigned in the network to obtain a significant gain in terms of the total number of instances where a node has to change its code.
  • The proposed strategies can be very practical in scenarios such as hard real-time systems and high data rate applications running on an ad-hoc network.