Transcript Slide 1
A Hybrid Spatial Reuse MAC Protocol for Ad-Hoc Underwater Acoustic Communication Networks
By : Roee Diamant, Lutz Lampe University of British Columbia (UBC) 1
Outline
The broadcast scheduling problem (
BSP
) Communication Networks (
UWAC
) in Underwater Acoustic Related work Scheduling using graph coloring Sub-optimal Hybrid TDMA-CDMA scheduling protocol Sea trial results 2
Motivation
Most underwater applications requires networks supporting broadcast communication: Other modems Relay nodes Underwater acoustic modem Relay, AUV collaboration In most cases topological structure of the network is unknown
Centralized solution is difficult. Ad-hoc scheduling is more natural
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Related work Graph coloring BSP Hybrid TDMA-CDMA Sea trial results
Broadcast Scheduling Problem
(
BSP
) We require both high throughput and low transmission delay: Throughput – average number of successful transmissions Transmission delay – average waiting time between transmissions Transmission delay This is the BSP :
How to schedule transmissions such that: optimal tradeoff between throughput and transmission delay is achieved
Related work Graph coloring Hybrid TDMA-CDMA Sea trial results BSP
Scheduling in UWAC – related work
Most scheduling protocols involves hand-shaking techniques adopted from the CA/CSMA protocol [Molins:2006] Advantages spatial reuse high reliability Disadvantage (for high traffic) low throughput high transmission delay Hybrid Aloha-CDMA: Transmitter adapts code length and transmission power [Pompili:2009] Aloha based Header packet CDMA based Information packet Cluster based – TDMA inside cluster, CDMA between clusters [Salva-Garau:2003] Advantages: increased availability; scalable TDMA Disadvantages: Cluster management; spatial reuse between clustered is not managed For small scale high-traffic networks, TDMA outperforms most existing protocols 5
Formalization of BSP
Network is represented by a directed graph
G(V,E)
Consider Spatial TDMA (STDMA) scheduling. The solution is a matrix,
M
such that: Node
i
transmits in slot
t
only if
M it
1 Node
i
and
j
are not scheduled in the same time slot if max
M
General formalization [Menon:2009]: min
L
1
NL i
1
t L N
1
M it
s.t
1 .
t L
1
M it
d i
i
2 .
t
1
i N N L
1
j
1
e ij M it M jt
0 Channel utilization Flow constraints Feasible solution
Not a convex problem!
e ij
1
M
t
1
t
2
t
3 X X X X X 6
Related work BSP Graph coloring Hybrid TDMA-CDMA Sea trial results
Coloring the network
Time-slot in BSP can be generalized as graph-coloring [Ephremides:1990] Minimize number of colors while adjacent vertices (half-duplex) gets different colors Fully connected network Start topology Colors are assigned according to the connection between nodes Performance increase the more sparse the network is General BSP is non-convex and Graph coloring is NP-complete
spatial reuse
Sub-optimal approach is required 7
Topology variations
Consider the following topology:
1 2 4 3 Time slot
1 2
Tx nodes
1 2,3,4 When topology changes, nodes can “see” the network differently, resulting packet collisions
2 1 3 4 Time slot
1 2 3
Tx nodes
1 2,4 3,4
Robustness to topology changes is required
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Overview
Till now we introduced the BSP for UWAC networks and its relation to graph coloring We observed that the optimal solution for BSP is non-convex and thus computationally difficult We also realized the importance of robustness to topology changes in MAC scheduling We now move on to introduce our Hybrid TDMA-CDMA solution,
TDMA)
, based on STDMA scheduling
(HSR-
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Related work Graph coloring Hybrid TDMA-CDMA BSP
The HSR-TDMA protocol
Sea trial results
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The protocol is based on a TDMA skeleton schedule
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Time slot
3
X X Robustness, minimal flow assurance X X Each node is getting a unique DSSS polynomial (CDMA) In each time slot, the pre-assign node is denoted as the “
slot node
”
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The rest of the nodes are divided to “
receiving nodes
” and “
joining nodes
” and are given a unique priority per time slot: Receiving nodes: nodes connected to the slot node Joining nodes: nodes not connected to the slot node and has higher priority than their neighbors which are not receiving nodes 10
BSP Broadcast packets Related work Graph coloring Topology matrix construction Hybrid TDMA-CDMA
The HSR-TDMA protocol
Shortest path algorithm Sea trial results Hop-distance matrix, H
1 2 3
Scheduling matrix construction
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Pre-defined skeleton; Weight vector Scheduling algorithm: For each possible joining node, n
k
,
k
n
if
e n
,
k
1 ,
h n
,
s
h k
,
s
If then
n
,
s
0 , n is a joining node
h n
,
s
mod 2 0 , 0 ,
W n W k
k
then n is a joining node i.i.d random variables, sampled afresh with same seed for all nodes Only nodes at even odd distance to the slot node are candidate to join its transmissions 11
Related work Graph coloring Hybrid TDMA-CDMA BSP
The near-far problem
The near-far phenomenon occurs when close node transmissions interrupts transmissions from distance nodes The result is a non-symmetric network connectivity matrix Sea trial results 12 Overcoming the near-far problem: For every set and every node ,check if both m , n are either both connected or both not-connected to p If not (a non-symmetric connection exists) : only one of them is chosen as a joining node, based on their current priority
13 Related work Graph coloring Hybrid TDMA-CDMA BSP
HSR-TDMA: example
Sea trial results
Sea trial results Related work Graph coloring BSP
Sea Trial results
Hybrid TDMA-CDMA Since accurate propagation model are hard to acquire, we preformed a sea trial at the Haifa harbor in May2009 to validate our assumptions: Slow time varying topology structure Ability to decode packets transmitted simultaneously from different users Symmetric channel (at the long run)
1 (full connectivity) 2 1 3 4 4 (chain) 2 1 3 1 2 (mixed) 2 5 (near-far) 4 2 4 1 4 3 1 3 (star) 2 3 6 (island) 4 2 1 4 3 3
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Sea trial results Related work Graph coloring BSP
Sea Trial results
Hybrid TDMA-CDMA Average throughput Per node availability Corresponds to transmission delay TDMA availability and throughput fixed on 25% for four nodes 15
Related work Graph coloring BSP
Sea Trial results
Hybrid TDMA-CDMA Average availability vs. time Sea trial results 16
Summery
Required by most underwater applications Maximize throughput, Minimize transmission delay Robust scheduling is requires High traffic broadcast network communication BSP Topology variations resulting packet collisions Majority of existing protocols deals with low-traffic communication In general, non-convex Colors = time slots Edges = interferences Graph coloring generalize BSP Graph coloring is NP-complete Sub-optimal solution; Hybrid STDMA-CDMA protocol Introducing HSR-TDMA protocol Sea trials confirmed assumptions Considerable improvement in both throughput and transmission delay Polynomial time Robustness to topology variations 17
Reference list
[1] W. Burdic, “Underwater Acoustic System Analysis,” Los Altos, CA, USA: Peninsula Publishing, 2002 [2] J. Partan, J. Kurose, and B. Levine, “A survey of practical issues in underwater networks,” in International Conference on Mobile Computing and Networking (MobiCom), Los Angeles, CA, USA, Sept. 2006.
[3] M. Stojanovic, “On the relationship between capacity and distance in an underwater acoustic communication channel,” in ACM Intaternational Workshop on UnderWater Networks (WUWNet), Los Angeles, CA, USA,Sept. 2006.
[4] C. Sherman and J. Butler, “Transducers and Array for Underwater Sound,” Springer Science + Business Media, LLC,2007 [5] J. Heidemann, W. Ye, J. Wills, A. Syed, and Y. Li, “Research challenges and applications for underwater sensor networking,” in IEEE Wireless Communications and Networking Conference (WCNC),Las-Vegas,NV,USA,Apr. 2006.
[6] I. Akyildiz, D. Pompili, and T. Melodia, “State of the art in protocol research for underwater acoustic sensor networks,” in ACM International Workshop on UnderWater Networks (WUWNet), Los Angeles, CA, USA, Sept. 2006.
[7] M. Molins and M. Stojanovic, “Slotted FAMA: a MAC protocol for underwater acoustic networks,” in IEEE Oceans Conference, Singapore, May 2006, pp. 1–7.
[8] D. Pompili, T. Melodia, and I. Akyildiz, “A CDMA-based medium access control for underwater acoustic sensor networks,” IEEE Trans. Wireless Commun., vol. 8, no. 4, pp. 1899–1909, Apr. 2009.
[9] F. Salva-Garau and M.Stojanovic, “Multi-cluster protocol for ad hocmobile underwater acoustic networks,” in IEEE Oceans Conference, San Diego, CA, USA, Sept. 2003.
[10] T. Cormen, C. Leiserson, R. Rivest, and C. Stein, Introduction to Algorithms, 2nd ed. MIT Press and McGraw-Hill, 2001.
[11] R. Diamant and A. Sinai, “A novel architecture for multi-hops ad-hoc underwater acoustic sensor networking,” in Acoustics 2008, Paris, France, June-July 2008.
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Thank you
Questions?
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