Base Station Operation and User Association Mechanisms For Energy-

Delay Tradeoffs in Green Cellular Networks

by Kyuho Son, Hongseok Kim, Yung Yi, Bhaskar Krishnamachari

Haluk Celebi/ hc2489

Outline

• Motivation• Problems to be solved• System model• Solution approaches and algorithms

Main Motivation

• BSs are large underutilized: Traffic is below 10% of peak 45% of the time.

• UMTS BS consumes 800-1500W, and radio 30-40W. Thus, not only turning off the transceivers, we need to turn off BS completely.

Problems to be solved

1. User association: When the set of active BS changes, MT needs to be associated with a new BS

2. Energy efficient BS operation: finding optimal set of active BS

- Paper gives a distributed algorithm to solve 1st problem optimally. For 2nd problem, it offers several greedy algorithms.

System Model and Problem Formulation

( )x• arrival rateTraffic load density

Set of active BS

Transmission rate of user i located at x

Fraction of time required to deliver traffic load from BS i to location x

Feasible solution to the problem: utilizations of BSs

Note that in optimal case p(x) is either 1 or 0

Cost function

•n is the parameter that determines between the flow level performance and the energy level consumption

Flow level performance

is a parameter specifying the degree of load balancing

Each MT prefers a BS that gives the highest rate

Minimizing number of flows, thus delay in M/M/1 queue

Thus as α increases, cost function places more importance on traffic load instead of transmission rate

Cost function of Energy

Portion of fixed power consumption.

Energy that is spend only depends on the radio.

In other case, when q =1, regardless of the utilization amount of energy spend is Pi. That`s where turning BS off becomes more important

We assume femto cells are more energy proportional (smaller q values) macro cells.

Solution approach to the problem

•Solving two cost functions together is hard.•We assume that flow arrival, and departure process is much faster than the period of determining active and passive BSs•Decompose problem into two sub-problems• Solve user association problem. For given active BS

set find which BS each MT should connect to• BS operation problem

Connection policy

It`s proved in the paper that with this policy, each MT is able to choose optimal BS

Energy Efficient BS operation