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This experimental study explores wireless scheduling, aiming to schedule simultaneous transmissions and avoid interference among neighboring nodes. The research delves into a simplified interference model, system model, and optimal scheduler. Centralized algorithms and message passing approaches are analyzed for their impact on performance and throughput. Simulation results on interference graphs and admissible traffic patterns are discussed, highlighting the benefits of memory utilization and message averaging in improving convergence time and system performance. Conclusions suggest that the message passing approach with memory and averaging shows promising results for optimizing wireless scheduling efficiency.
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Message-Passing for Wireless Scheduling: an Experimental Study Paolo Giaccone (Politecnico di Torino) Devavrat Shah (MIT) ICCCN 2010 – Zurich August 2nd, 2010
Scheduling in wireless networks • schedule simultaneous transmissions • to avoid interference among neighboring nodes • needs coordination across the communication medium • simplified interference model • a transmission is successful if none of its neighbors are transmitting • neighbors defined simply by the transmission range RT
System model and notation • packet duration is fixed and time is slotted • i is the node • xi=1 if node is transmitting, 0 if not • X=[xi] is the transmission vector • N(i) is the set of neighboring nodes at a distance < RT from node i, i.e. the set of nodes that may eventually interfere • a interference-free X must be
Interference graph • G=(V,E) • V is the set of nodes • edge • an independent set (IS) on G corresponds to a subset of nodes that can transmit simultaneously without interference
Optimal scheduler • Optimal scheduling • for generic constrained resource allocation problem • Tassiulas and Ephremides, IEEE Trans. Automatic Control, 1992 • to maximize throughput, compute the maximum weight independent set (MWIS) at each timeslot • weight wi of a node i is the number of enqueued packets 10 5 5 10
Centralized algorithms for IS • IS is NP-complete • greedy approximations • Rnd-IS: S is a random permutation of nodes • MaxW-IS: S is a sequence of nodes in decreasing order of weights 10 10 9 9 9 9 1 1
Message passing approach • derived from belief propagation to perform inference on graphical models, such as Bayesian networks and Markov random fields • successfully employed in many fields: physics, computer vision, statistics, coding (Viterbi algorithm), generic combinatorial optimization • amenable to parallel implementation • network protocols are based on message passing algorithms
Message passing • update phase • each node sends a message to the neighbors based on the received messages • is the message from node i to j at iteration n • estimate phase • each node takes a local decision
Message Passing for MWIS Derived by Sanghavi, Shah, Willsky, IEEE Transactions in Information Theory, 2009
Computational tree interpretation 1 2 5 3 4
Contribution • for a generic graph with loops, messages may not converge, leading to unfeasible solutions • to improve converge we propose • use of memory • message averaging • we investigate their effects on the performance
Memory • exploit “continuity” in the system state • queue evolution is limited: |wi(t+1)-wi(t)|≤1 • Property: |MWIS(t+1)-MWIS(t)|≤ N • MWIS(t) is also a good candidate for time t+1 • idea: keep the most recent messages from the previous timeslot as the initial value • leads to reduced convergence time
Message averaging • observation: message may oscillate • idea: to average message values with a weighted moving average filtering • filter constant: α=1 no filtering
Asynchronous update • Earlier pseudocode of MP-IS assumes that all the nodes update synchronously their messages in parallel at each iteration • this assumption is not needed • We assume uncoordinated, asynchronous update • each node wakes at some random time • it updates the outgoing messages based the messages received so far • its sends the new updated messages to all its neighbors
Simulation results • given • interference graph • traffic pattern • the simulator estimates • throughput • packet delay • packet loss for the whole network and for each node
Noisy grid as interference graph • random geometric graph • place N nodes on a perfect grid • add some noise to the position (η parameter) • η=0 corresponds to a perfect grid • η very large corresponds to a bidimensional Poisson process • all the nodes with distance < RT are connected η=0.0 η=0.5 η=1.0
Admissible traffic pattern • given G, finding the admissibility rate region is NP-hard • ri is the normalized arrival rate at node i • ρ is the load factor • ρ=1 is such that Rnd-IS will obtain 100% throughput • K is a traffic parameter • K=1 unbalanced traffic • large K balanced traffic
Perfect grid • N=100 nodes • ρ=1.35 • Conclusions • memory boosts performance of MP-IS • one iteration is enough for MP-IS to be optimal
Noisy grid • ρ=1.0 • Conclusions • very little throughput degradation in irregular graphs
Conclusions • MP-IS with just 1 iteration + memory + averaging performs comparable with centralized algorithms • similar result for Tree-Reweighted Message Passing algorithm • promising approach for the limited protocol overhead • belief propagation is taking care of • longer queues -> messages are proportional to wi • graph structure -> messages depend on the graph