life-add: a novel wifi design with battery life, throughput and fairness improvement shengbo chen*,...

Life-Add: A novel WiFi design with battery life, throughput and fairness improvement

Shengbo Chen*, Tarun Bansal*, Yin Sun*,Prasun Sinha and Ness B. Shroff

Dept. ECE & CSE, The Ohio State University

WiOpt 2013

2

Background Battery life is a serious problem for most smartphone users

WiFi, 4G LTE, GPS, Bluetooth, screen, CPU, ... Web browsing via WiFi

Test results in April 2013 by Battery life < 11 hours for most popular smartphones

iPhone 5802.11n

Samsung Galaxy S 4 802.11ac

HTC One802.11ac

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Existing Solutions to Prolong Lifetime

Mobile Charging Additional equipment Solar charger portable battery wireless charger

Reduce power when sensing Lower hardware clock-rate [E-MiLi, Mobicom 11] Broadcom SoC Solution

—802.11 ac

—Used in HTC One and Samsung Galaxy S 4

—Test: 7.8 hours by

Trade bandwidth/throughput for power reduction —Cannot have both benefits

4

Physical layer Significant evolutions towards high throughput

MAC CSMA/CA and its enhancements

—QoS, security, frame aggregation, block ACK

IEEE 802.11 Standard Evolution

WLAN

802.11-1997

2 Mbps, DSSS, FHSS

802.11b11 Mbps, CCK, DSSS

802.11a54 Mbps,

OFDM, 5 GHz

802.11n600 Mbps with

4x4 MIMO, 20/40 MHz BW,

2.4 or 5 GHz

802.11p27 Mbps,10 MHz

BW, 5.9 GHz

802.11afTVWS

802.11g54 Mbps,

OFDM, 2.4 GHz

802.11ac256QAM160MHz

802.11ad

Wireless Access for Vehicular Environment

TV WhiteSpaces

Wireless Gigabit, <6 GHz

Wireless Gigabit, 60 GHz

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Life-Add: An innovative MAC design Battery Lifetime

—Avoid unnecessary sensing Throughput

—Reduce collisions and starvations Fairness

—Near-far effect

Can we do better?

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Contents

Background Life-Add: An innovative MAC design Simulation Results Summary

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Life-Add: Smartphone energy model Power source:

Strong: Wall power, portable battery Weak: Solar charger

Other components 4G LTE, CPU, screen, …

WiFi chip ON: Transmit/receive/sensing

—High power consumption

OFF: Sleep—Very low power consumption

Too much sensing means a significant waste of energy Sleep/wake (asynchronous)

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Life-Add: Sleep/Wake + Channel Contention

Uplink

Device 2

Device 1

APACK

Device 2

Device 1

AP

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Uplink

Device 2

Device 1

APACK

Device 2

Device 1

AP

Device 1 wakes up earlier and senses the channel

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Uplink

Device 2

Device 1

APACK

Data

Device 2

Device 1

AP

Device 1 transmits, Device 2 goes back to sleep

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Uplink

Device 2

Device 1

APACK

Data

ACK

Cycle 1

Device 2

Device 1

AP

AP replies an ACK to Device 1. Cycle 1 completes.

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Uplink

Device 2

Device 1

APACK

Data

ACK

Cycle 1

Device 2

Device 1

AP

Devices 1 and 2 wake up at almost the same time

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Uplink

Device 2

Device 1

APACK

Data

ACK

Data

Data

Cycle 1 Cycle 2

Device 2

Device 1

AP

A collision occurs, followed by a timeout. Cycle 2 completes.

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Uplink

A new renewal process model: each cycle is an i.i.d. period—Requires 2 assumptions:

—Exponential distributed sleep period: Memoryless (independent from last cycle)

—Tdata + TACK≈ Tcollision + Ttimeout (only assumed in analysis,

not in simulations)

Device 2

Device 1

APACK

Data

ACK

Data

Data

Data

Cycle 1 Cycle 2

Device 2

Device 1

AP

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Life-Add

IEEE 802.11

Sleep backoff vs sensing backoff (save energy) Renewal process vs 2D Markov chain [Bianchi 2000] (simplify optimization)

Life-Add vs IEEE 802.11

Device 2

Device 1

APACK

Data

ACK

Data

Data

Data

Cycle 1 Cycle 2

Device 2

Device 1

APACK

Data

ACK

Data Data

Data

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Life-Add: Downlink

Still a renewal process Uplink: sleep + data + overhead (ACK/collision/timeout) Downlink: sleep + data + overhead (ACK/Ps-poll/collision/timeout)

—Additional Ps-poll packet as part of overhead

Can be modeled together

Device 2

Device 1

APData

Cycle 1 Cycle 2Beacon

Ps-poll

Data

ACK

Ps-poll

ACK

Ps-poll

A short Ps-poll packet is used to contend for the channel

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Proportional-fair Utility Maximization max ∑ log E{Throughput of Device i}

s.t. E{Battery Life of Device i} ≥ Tmin,i

Life-Add: Single AP

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s.t.

Maximal device-ON probability: bi

Variables: average sleep period 1/Ri

Life-Add: Single AP

Pr{Device i’s RF is ON}≤ bi

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s.t.

Maximal device-ON probability: bi

Variables: average sleep period 1/Ri

Non-convex—Asynchronous network with collisions

—Channel access probabilities of the devices are coupled

We propose a solution: Life-Add

Theorem: Asymptotically optimal, as Tsensing /(Tdata + TACK)0

—E.g., 802.11b: Tsensing= 4us, Tdata + TACK=511us~1573us

Life-Add: Single AP


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Problem formulation

where , is a scaling constant is the transmission success probability is the device-ON probability

Proof idea: Problem structure, KKT necessary conditions

Upper and lower bounds converge to the same value

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Life-Add: Single AP Implementation procedure:

Each device reports bi to the AP

The AP computes , and broadcast them to the devices—If ,

—If ,

Device n uses and to compute

Use to generate the sleeping period

Low complexity, easy to implement


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NS-3 simulation for a homogeneous scenario

Red curve: simulated performance with no approximation Blue point: closed form solution of Life-Add Observation: Life-Add is near optimal

The renewal process model is reasonably accurate

Life-Add: Single AP

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Life-Add: general multiple APs Too complicated interference model

Global optimization is very difficult

Near-far effect

Device 1 can access the channel all the time Device 2 is in starvation

Hidden terminal problem

Two devices cannot sense each other and cause collisions

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Life-Add: general multiple APs Near-far effect

Node collaboration —Device 1 computes the two values of average sleep

period suggested by AP 1 and AP 2

—Device 1 chooses the longest average sleep period to reduce collisions with Device 2, which is vulnerable

To care for the vulnerable

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Life-Add: general multiple APs Hidden terminal problem

Increase average sleep period after a collision Reset average sleep period after a successful transmission

—Similar idea to 802.11 MAC

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Life-Add: general multiple APs Implementation procedure:

Each device reports bi to nearby APs

Each AP computes and broadcasts and—If ,

—If ,

Device n uses and to compute suggested by nearby APs

Choose to use the smallest value Reduce at collision, reset after receiving ACK Use to generate the sleeping period

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NS-3 simulation results: Uplink: 4 APs, 30 smartphones, randomly located in a 500×500 m

field, UDP saturation

bi = 1 no lifetime (power-ON prob.) constraints

1/3 with battery, 1/3 with battery + solar panel, 1/3 to wall power Battery level: uniform distribution within 200~1000 mAh

Lifetime and throughput benefits

Life-Add: general multiple APs

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Life-Add: general multiple APs NS-3 simulation results:

Per-device performance:

Battery life improvement for all 5 devices Significant throughput increase for the low-rate device

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Life-Add: general multiple APs Average performance gain

Battery Life: —Sleep/Wake

Throughput:—Node collaboration (reduce collisions and starvations)

—Parameter optimization

Fairness:—Node collaboration (to care for the vulnerable)

—Proportional-fair utility

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Life-Add: general multiple APs Coexisting with IEEE 802.11

AP 1,2 and their users upgrade from IEEE 802.11 to Life-Add Battery life

—Longer if you use Life-Add

Throughput—Higher no matter you use Life-Add or not, due to less collisions

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Summary A novel renewal process model for energy efficient WiFi design Proportional-fair utility maximization problem

Non-convex

Life-Add MAC design Near optimal for single AP cases Alleviate “near-far effect” and “hidden terminal problem” in general cases Easy to implement

Ns-3 simulations Battery life, throughput, and fairness improvement Coexists harmoniously with IEEE 802.11

Not just WiFi: Last-hop decentralized access Internet of Things, Military,…

US patent filed

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Tasks to do… More simulations for joint uplink and downlink Practical traffics

Web browsing, video streaming, email, searching

Hardware testing

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Thank you

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life-add: a novel wifi design with battery life, throughput and fairness improvement shengbo chen*,...

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