1 Post Lunch Session Cooperative Strategies and Optimal Scheduling for Tree Networks Alexandre de Baynast †, Omer Gurewitz ‡, Edward W. Knightly ‡ † RWTH Aachen, Germany ‡ Rice University, Houston, TX

2 To develop and analyze a low-complexity cooperative protocol for wireless tree networks that significantly increases the average throughput for any user independently of its position in the tree. Objective

3 Motivation Network approach In a conventional multi-hop routing, each node relays (forwards) its neighbors’ traffic in addition to its own traffic In an upstream traffic each node forwards its successors’ (children’s) traffic along with its own traffic to its predecessor (parent in the tree) e.g., N. Ben Salem and J.P. Hubaux, “A Fair Scheduling for Wireless Mesh Networks”, Proceedings of WiMesh 2005 C. Jaeweon and Z.J. Haas, “On the throughput enhancement of the downstream channel in cellular radio networks through multihop relaying”, IEEE Journal on Selected Areas in Communications, 2004 SD

4 Motivation Information Theory approach Each node broadcasts its traffic, any other node collects the data from all transmissions, decodes it and rebroadcasts it (decodes and forwards). e.g., P. Gupta and P.R. Kumar, “Towards an information theory of large networks: an achievable rate region", IEEE Transactions on Information Theory, 2003 SD

5 Our Solution “Cheap” cooperative strategy When node S is transmitting, nodes R1 AND R2 are listening though R1 only can decode the message. In order to exploit the overheard information, our protocol is designed such that R1 transmits only some extra information to help R2 to decode the message. At least 50% throughput gain! S R1 D R2

6 The Model Network Model m-ary Regular Tree Topology Single Gateway L Layers S-TDMA (block synchronization) All nodes are fully backlogged Interference Model Spatial Reuse Factor F Collision Free

7 The Model Physical Model Half Duplex (Single Radio) Fix Channel Gain for all links No sample synchronization No multi-user detection Decode threshold Interference threshold OSI Model L 2-3 L 1 Channel attenuation vs. distance 6 dB 4 dB

8 Flow Achievable Rate A flow achievable rate is measured by the rate granted to the flow on its bottleneck link S D

9 Flow Achievable Rate A flow achievable rate is measured by the rate granted to the flow on its bottleneck link S D

10 Achievable Rate vs Fairness The achievable rate is determined by the flow that gets the lowest rate S S S D

11 Parking Lot (m=1): Non Cooperative Case Achievable Rate: Single Link Capacity: F hops apart = optimal spatial reuse First F=4 layers fully utilized = Optimal Schedule FN time slots

12 Parking Lot (m=1): Optimal Power Allocation For a given fixed total power P, higher rate can be achieved if the closer nodes transmit with higher power than the farther nodes Achievable Rate: Gain < 25% for any chain size

13 Parking Lot (m=1): Cooperative Case 1 α 1 α 1 1 RELAY (TRP)  =rate of two-hop link / rate of one-hop link < 1 F-1 F F+1

14 Turbo Relay Protocol (TRP) ~ Hop by Hop

15 TRP vs. Conventional Hop by Hop Relaying TRP Parking Lot (m=1): Cooperative Case Achievable Rate: At least 50% throughput gain with TRP

16 m-ary Tree (m>1): Cooperative Case TRP Hop-by-Hop

17 m-ary Tree (m>1): Cooperative Case TRP vs. Conventional Hop by Hop Relaying SNR (dB) Throughput gain (%) m=5 m=4 m=3 m=2 Pathloss exponent = 2; Spatial Reuse = 5 4-QAM Gaussian throughput gain > 90% throughput gain > 60%  100% throughput gain with TRP

18 Conclusion We proposed the Turbo-Relaying Protocol (TRP) to increase each node’s throughput of upstream transmission in tree topologies. We showed that our approach can achieve up to 100% throughput gain over the basic multihop hop-by-hop transmission, for any regular tree with any connectivity degree.

19 Questions

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21 Turbo Relay Protocol (TRP) ~ Hop by Hop In turbocharged engines, the combustion air is already pre-compressed before being supplied to the engine. The engine aspirates the same volume of air, but due to the higher pressure, more air mass is supplied into the combustion chamber. Consequently, more fuel can be burnt, so that the engine’s power output increases related to the same speed and swept volume