Load-Aware Spectrum Distributionin Wireless LANs

Thomas Moscibroda, Ranveer Chandra, Yunnan Wu, Sudipta Sengupta, Paramvir Bahl, Yuan Yuan

Microsoft Research

ICNP 2008

OutlineIntroductionMotivationDesign ApproachProblem FormulationAlgorithmsEvaluationConclusions

IntroductionIn IEEE 802.11, the entire available

spectrum is divided into smaller channels of equal channel-width (bandwidth)

Introduction

Previous Approaches - 1Change associations between clients and

access points (APs)

Previous Approaches - 1Change associations between clients and

access points (APs)

Previous Approaches – 1IUse transmission powers for load

balancing

Previous Approaches – 1IUse transmission powers for load

balancing

Previous Approaches – III Coloring: Assign best (least-congested) channel

to most-loaded Aps, Reuse some of the channels, weighted coloring

Previous Approaches – III Coloring: Assign best (least-congested) channel

to most-loaded Aps, Reuse some of the channels, weighted coloring

Spectrum assignmentThe problem with existing approaches is

fundamental - Demand at APs very different - But, every AP assigned the same

amount of spectrum! (one 20 MHz channel)

Our approach:

IntroductionIn this paper, we argue that by moving

beyond this fixed channelization structure, the network capacity, spectrum utilization and fairness can be greatly increased◦Wider channels for heavily-loaded APs◦Narrower channels for lightly-loaded

APs

Load-Aware Spectrum Allocation

Problem definition:

1) Assignment withoptimal spectrum utilization:All spectrum to leafs!

Load-Aware Spectrum Allocation

Problem definition:

2) Assignment with Optimal per-load fairness:Every AP gets halfthe spectrum

Problem DefinitionAP1, … , APnConflict graph G=(V,E) describes which

APs interfereEvery APi has a load Li (e.g. #assigned

clients)

Spectrum Assignment Problem:Input: 1) AP1,…,APn

2) L1,…,Ln

3) Conflict graph G

Output: Assign a channel Ii=[Si, Si+Bi] to every AP

Problem Definition

Non-Overlapping ChannelNon-interfering assignment:

An assignment is non-interfering if the channels of no two neighboring APs overlap.

Why Non-Overlapping?◦ Lesser contention overhead, no rate

anomaly

Why is this problem interestingTraditional channel assignment /

frequency assignment problems map to graph coloring problems

We must assign contiguous bands to each node

Solution 1: ILPProblem is clearly NP-hard in general

conflict graphsThe problem can be formulated as an

ILP

Solution 2: Heuristics 1) Determine an ordering O = (AP1,…,APn) over all

APs 2) For a given R, …

a) … assign each node a channel width

b) … check whether these (B1,…,Bn) can be feasibly

allocated when greedily packed in the order of O 3) Find largest R, for which greedy packing is

feasible (binary search).

We looked at several possible orderings O

(most-congested first, smallest-last )

Evaluations Extensive trace-driven simulations using two-

data sets: 1) Small WLAN, monitoring information of 6 APs

on floor at MSR2) Large WLAN, 177 APs in 3 buildings

[Balazinska, Castro, Mobisys 2003]

Evaluations

ConclusionsAdaptive Channel Width

Simple technology (deployable today!)

Provides new knob for optimization

Huge potential for performance improvement!