minimal cdma recoding strategies in power-controlled ad-hoc wireless networks

© Honglei Miao: Presentation in Ad-Hoc Network course 13.01.2004 1(19)

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

Networks

Honglei Miao

[email protected]

Centre for Wireless CommunicationsUniversity of Oulu, Finland


Introduction Problem statement and previous

work New recoding strategies Simulation Results Conclusions

Outline


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.


• 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.

Introduction (2)


• 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

Problem statement and previous work

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jikjki ccjiEvvvv ,&),(),,(


• 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.

Problem statement and previous work (2)


• 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.

Problem statement and previous work (3)


• Handling Node Join

New recoding strategies


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.


• Algorithm for recoding on a node join



• Example of recoding on a node join

• 1n = {7}, 2n = {1 2 3 6}, 3n = {}, 4n = {4 5}



• Handling Node Power Increase



• 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.




• 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.


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).


Simulation results (2)


• 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.

Conclusions

minimal cdma recoding strategies in power-controlled ad-hoc wireless networks

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