Interference-aware Routing and Scheduling in WiMAX ... speci es a MAC protocol but leaves scheduling algorithm to implementation. The trivial solution mentioned in WiMAX standard performs

Interference-aware Routing

andSchedulingin WiMAXBackhaulNetworks

with SmartAntennas

CS 440 —ComputerNetworks

Introduction

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ProblemDefinition

Algorithms

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Interference-aware Routing and Schedulingin WiMAX Backhaul Networks

with Smart Antennas

CS 440 — Computer Networks

Montana State University

Fall 2008

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IntroductionWiMAX

Worldwide Interoperability for Microwave Access, isdescribed by the WiMAX forum as “a standards-basedtechnology enabling the delivery of last mile wirelessbroadband access as an alternative to cable and DSL”.

IEEE 802.16 standards

Base Station vs Subscriber Station

Point-to-Multipoint (PMP) mode vs Mesh mode

Time Division Multiple Access (TDMA)

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IntroductionWiMAX






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IntroductionWiMAX






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IntroductionWiMAX vs WiFi

WiMAX WiFi

802.16 802.11Licensed/unlicensed spectrum Unlicensed spectrum

WMAN WLANLonger range Shorter range

Higher throughput Lower throughtputRequest/Grant (TDMA) Contention based (CSMA/CA)

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IntroductionAn example WiMAX backhaul network

Internet

SS

SSSS

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BS

SS

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Introductionsmart antenna

Adapt to the environment.

Longer transmission range.

Lower power consumption.

Digital Adaptive Array (DAA) antennas

Multiple antenna elements (a.k.a, Degree Of Freedom,DOF)

Beam forming

Spatial Division Multiple Access (SDMA)

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Beam forming


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Beam forming


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Beam forming


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Beam forming


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Beam forming


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Beam forming


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IntroductionAn example of smart antenna beam pattern (from www.wtec.org)

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IntroductionOur problem

We consider routing and scheduling in WiMAX backhaulnetworks with smart antennas.

802.16 specifies a MAC protocol but leaves schedulingalgorithm to implementation.

The trivial solution mentioned in WiMAX standardperforms very poorly.

Schedule with smart antennas and spatial reuse is quitedifferent from traditional link scheduling withomni-directional antennas.

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System ModelAntenna Model

Each node has an antenna with K DOFs. (K is usually aconstant. But we do not make this assumption in ourwork.)

Each antenna can tune its K DOFs to point at arbitrarydirections dynamically.

In order to activate a link, one DOF needs to be assignedfor communication at each end of the link.

In order to cancel interference from one node to another,only one DOF needs to be assigned to form a null at eithernode.

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System ModelScheduling Model

BS schedules the entire frame in a centralized manner,and broadcast the schedule to all SSs.

Bandwidth requests do not change within a schedulingperiod.

How to split downlink subframe and uplink subframe isout of the scope of our discussion.

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System ModelNetwork Model

A static, single channel, multi-hop WiMAX backhaulnetwork.

All traffic demands are unicast and between BS and SS.No traffic from SS to another SS.

Radios are homogeneous, half-duplex, and equipped withDAA antennas.

Protocol interference model

Primary interferenceSecondary interference

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Problem DefinitionRouting (Tree Construction) Problem

If an SS can directly connect to the BS or another SS, norelay needed.

Thus, the communication graph G can firstly be layered.

Then the routing problem becomes to pick a node in layerh− 1 as the parent node for each node in layer h.

We want the routing tree be balanced and have lowinterference.

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Problem DefinitionInterference aware Tree Construction Problem (ITCP)

Define the secondary interference value of vi asIs(vi) = |Ni| − 1, where |Ni| is the set of nodes that canpotentially interfere with vi.

Define the secondary interference value of link e = (vi, vj)as Is(e) = max{Is(vi), Is(vj)}.

Definition

ITCP seeks a spanning tree rooted at the BS, such that in eachlayer h > 1, the maximum node degree of layer h− 1 isminimized subject to the constraint that the maximumsecondary interference value of all links picked between layerh− 1 and layer h is minimized (or less than K), withoutcausing disconnection of the two layers.

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Definition


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Problem DefinitionScheduling Problem

A schedule is composed of both transmission schedulingand DOF assignment.

The spanning tree is given as input, together with thebandwidth demands of each SS.

Nodes need to transmit not only their own demands, butalso the aggregated demands from their descendants inthe spanning tree.

We want to promote the throughput of all nodes whilemaintaining fairness.

We only discuss uplink scheduling. Downlink is symmetric.

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Problem DefinitionUplink Scheduling Problem (USP)

A schedule and DOF assignment is feasible if:

1 For each link scheduled, DOFs have been assigned at bothend for communication.

2 There does not exist primary or secondary interference inevery minislot.

3 Each node assigns at most K DOFs.

Definition

USP seeks a feasible uplink schedule and DOF assignment suchthat the bandwidth demand satisfaction ratio of the leastsatisfied node is maximized.

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A polynomial time optimal algorithm for ITCPIdeas

Construct auxiliary graph and use Ford-Fulkersonalgorithm to check whether it is possible to bound thenode degree of layer h− 1 while keep layer h and h− 1connected.

Use binary search to find the minimum satisfying nodedegree.

The algorithm has been proved to optimally solve ITCP inO(mnhmax log δmax) time.

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A polynomial time optimal algorithm for ITCPExample Auxiliary Graph

C D E F

A B Layer h-1

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A polynomial time optimal algorithm for a specialcase of USPIdeas

Link scheduling problems in a multihop wireless networkhave been shown to be NP-hard.

A special case: when every node has enough DOFs toeiminate all potential secondary interferece.

Find the bottleneck node in the routing tree.

Schedule for the bottleneck node and its descendants, thenremove them and their demands from the routing tree.

Repeat until the tree becomes empty, and all nodesscheduled.

Our algorithm is polynomial in the number of nodes,irrelevant to the number of minislots.

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A heuristic algorithm for USP in the general caseIdeas

Slot-by-slot scheduling.

Greedily tries to pack as many links as possible in aminislot, consider the least satisfied link first.

Use auxiliary graph to decide whether a set of links can beactive concurrently and compute a feasible DOFassignment.

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Simulation ScenariosScenario Parameters

Parameter SettingScenario Area 4× 8km2

Transmission Range 1kmInterference Range 3km

Number of DOFs per Node 3BS Placement Top-left cornerSS Placement Uniform within the area

Number of SSs [25, 50, 75, 100, 125, 150]Minislots per Frame 1024

Uplink demand of each SS Discrete uniform in [5, 10]Downlink demand of each SS Discrete uniform in [10, 20]

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Tree Construction AlgorithmsExample Trees

BFS ITCP MST

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Tree Construction + SchedulingEnd-to-end throughput

25 50 75 100 125 1500

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USP + ITCPFirst−Fit + BFSFirst−Fit + MST

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Tree Construction + SchedulingMinimum satisfaction ratio

25 50 75 100 125 1500

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Tree Construction + SchedulingFairness index

25 50 75 100 125 1500

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Conclusions

We define the Interference-aware Tree ConstructionProblem for routing and present a polynomial-timeoptimal algorithm to solve it.

The trees constructed by our algorithm outperform MSTand BFS trees.

We present a polynomial-time optimal algorithm for aspecial case of the scheduling problem and an effectiveheuristicd algorithm for the general case.

Compared with other solutions, our routing and schedulingscheme can greatly improve the throughput and fairness.

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Thank You

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Interference-aware Routing and Scheduling in WiMAX ... speci es a MAC protocol but leaves scheduling algorithm to implementation. The trivial solution mentioned in WiMAX standard performs