A! Aalto UniversityComnet

Co-primary inter-operator spectrum sharingover a limited spectrum pool using repeatedgames

Bikramjit Singh1, Konstantinos Koufos1, Olav Tirkkonen1 andRandall Berry2

1Aalto University, Finland and 2Northwestern University, USA

IEEE ICC London, 9.6.2015

A! Motivation: Dense Networks⇒ New challenges for flexible, efficient spectrum use

• small cells added to increase capacity in hot spots

• multiple indoor operators in the same geographic area

⇒ Prominent load asymmetry• number of UEs per BS can vary a lot

• orthogonal spectrum sharing between the operators becomes inefficient

IEEE ICC 2015 2 (13) London, 9.6.2015

A! Co-primary Spectrum Sharing• More spectrum is needed for 5G cellular systems⇒ Co-primary sharing: Multiple operators share spectrum

• Closed set of players with similar legal rights to spectrum

• Only these players have access to spectrum considered.

⇒ Limited spectrum pool (LSP)

• Operators have equal legal rights to the shared spectrum

• Baseline: shared spectrum is used by all

⇒ Mutual renting

• Operators have exclusive right to own spectrum, may permit others to use

• Baseline: each operator uses its own spectrum

IEEE ICC 2015 3 (13) London, 9.6.2015

A! System Model I• Operator x constructs a number (utility) Ux to describe QoS of its

user population• proportionally fair, cell edge throughput, ...

• Operators do not have to maintain same utility nor be aware of otheroperator utility

• Operators are self-interested entities• They do not want to reveal their CSI, performance, or load

• Added functionality over state-of-the-art• Operator may ask its UE to mea-

sure aggregate signal level fromopponent network

• UEs able to separate between ownand other operator’s interference

• Simple extension of LTE handovermeasurements

Serving BS

Interfering BS

IEEE ICC 2015 4 (13) London, 9.6.2015

A! Coordination Protocol

• We assume direct connection between spectrum controllingelements in the radio access networks of the operators

• We assume no monetary transactions• Instead, to enable spectrum sharing, coordination protocol

• Operators may ask/grant opponents for spectrum usage favor• Protocol consists of bids and results (agreements, disagreements)• Results hold for a certain fixed time slot.

IEEE ICC 2015 5 (13) London, 9.6.2015

A! Coordination Protocol: Possible States

Network states

Symmetric load

Asymmetric load

IEEE ICC 2015 6 (13) London, 9.6.2015

A! Spectrum Sharing Game based on Favors

• Given a coordination protocol, spectrum sharing becomes anon-cooperative game between operators• The protocol determines the rules

• The players are rational and self-interested

• The game is Bayesian: opponent state is not known• All you know about the value of a favor to the opponent is:

a) sometimes he asks for it, sometimes notb) sometimes he grants it to you, sometimes not

• Spectrum sharing game for LSP with K carriers• Actions: ak—ask opponent to stop using k = 1, . . . K carriers,

gk—propose to stop using k = 1, . . . K carriers,n—do nothing

• Outcome: If one plays gl and other ak with l ≥ k ⇒ k favours exhanged.Otherwise nothing happens.

IEEE ICC 2015 7 (13) London, 9.6.2015

A! Repeated Game

• In one-shot game, rational player always asks, never grants

• Operators share spectrum for a long time• an almost infinite sequence of games

⇒ Repeated game:• Strategy depends on network state, network state statistics, and history of

interactions

• Book-keeping of spectrum usage favors given and taken

• Operators are free to decide whether they play the game or not

⇒ A rudimentary economy on RAN-level• A favor is the currency

IEEE ICC 2015 8 (13) London, 9.6.2015

A! Decision Making Process• Heuristic strategy for long-term reciprocity• Operator has estimated probability distribution of gain/loss when

asking/granting a carrier

0 1 2 3 4 5 6 7 80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Utility

CD

F

Gain, FG2

Gain, FG1

Loss, FL2

Loss, FL1

• In every time slot, Operator x estimates utility gain/loss forasking/granting up to K favors

• Decision to ask/grant a favor on k carriers based onthreshold-based tests• Gain threshold, θX,j; Loss threshold, λX,j

IEEE ICC 2015 9 (13) London, 9.6.2015

A! Optimal Gain/Loss thresholds

• Decision thresholds set to maximize excess utility∼UX over the

one-shot game NE

• Constraint: long-term reciprocity

Maximize :θA,k,λA,k ∀k

∼UA,

Subject to :

K∑k=1

kP askA,kP

grantB,k =

K∑k=1

kP askB,kP

grantA,k ,

• History of interactions taken into account: own ask/grantthresholds depend on opponent ask/grant probabilities

IEEE ICC 2015 10 (13) London, 9.6.2015

A! Simulation Setting• Two operators each with 2 BSs each in a single-story building• Spectrum divided into 4 parts

• Two parts in limited spectrum pool• one private part each

• Users generated according to Poisson point process• Operator mean load 5 UEs• fully overlapping coverage areas

• Operators maximize proportionally fair utility

Operator A

Operator B

120 MHz

IEEE ICC 2015 11 (13) London, 9.6.2015

A! Simulation results• Learning phase

• operators construct the distributions and set decision thresholds

• Steady state distributions are constructed over many network states

• Operation phase• load pdfs assumed constant over both learning and operation phase

• Gain at 50-th percentile of UEs rate CDF is 25%

0 10 20 30 40 50 60 700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Mbps

CD

F

One−shot game

Coordination protocol

IEEE ICC 2015 12 (13) London, 9.6.2015

A! Summary

• Repeated game framework for spectrum sharing fornon-cooperative operators

• Operators share spectrum pool consisting of multiple carriers• Spectrum sharing through spectrum usage favors

• Operators construct utility gain/loss distributions and obtaindecision thresholds

• Based on gain and loss thresholds, operators ask and grantspectrum usage favors

• Favors provide a rudimentary currency to trade losses and gainsin the time domain• Effective when operator load varies in time

IEEE ICC 2015 13 (13) London, 9.6.2015