Power Management of MIMO Network Interfaceson Mobile Systems

-- A.ASHWINI

AspirationsHigh data rate wireless communications links with

transmission rates nearing 1 Gigabit/second (will quantify a “bit” shortly)

Provide high speed links that still offer good Quality of Service (QoS) (will be quantified mathematically)

Aspirations of a System Designer

High data rate

Quality

Achieve “Channel Capacity (C)”

Minimize Probability of Error (Pe)

Real-life Issues

Minimize complexity/cost ofimplementation of proposedSystemMinimize transmission powerrequired (translates into SNR)Minimize Bandwidth (frequencyspectrum) Used

IntroductionConventional (SISO) Wireless Systems

Conventional “Single Input Single Output” (SISO) systems were favored for simplicity and low-cost but have some shortcomings:Outage occurs if antennas fall into null

Switching between different antennas can helpEnergy is wasted by sending in all directions

Can cause additional interference to othersSensitive to interference from all directionsOutput power limited by single power amplifier

channelBits RadioDSPTX

Radio DSPBitsRX

MIMO Wireless Systems

Multiple Input Multiple Output (MIMO) systems with multiple parallel radios improve the following:

MIMO networks interfaces are high speed wireless networks.

Outages reduced by using information from multiple antennas

Transmit power can be increased via multiple power amplifiers

Higher throughputs possibleTransmit and receive interference limited by some

techniques

channelRadio

DSP

Bits

TX

Radio

Radio

DSP

Bits

RX

Radio

MIMO Hardware RequirementsMIMO Transmitter (parallelism and data rate scaling)

FEC StreamSplit

MOD

MOD

SpatialMapping

IFFT

IFFT

RF

RF

1 *O(Bw*Es*Ns)

Ns *O(Bw*Es)

1*O(Bw*Es*Ns*NT)

NT*O(Bw*Es)

NT*Analog RF

7

AntennasAn antenna is an electrical conductor or system of

conductors to send/receive RF signalsTransmission - radiates electromagnetic energy into

spaceReception - collects electromagnetic energy from

spaceIn MIMO, each antenna can be used for transmission

and reception.widley used is micro tip antenna in MIMO

Omnidirectional Antenna (lower frequency)

Directional Antenna (higher frequency)

Types of Channels

MIMO Hardware RequirementsMIMO Receiver (parallelism and data rate scaling)

1*O(Bw*Es*Ns)

DECStreamMerge

Demod

Demod

MIMOEqualizer

FFT

FFT

RF

RF

NR*Analog RF

1*O(Bw*Es*NR*Ns2)

NR*O(Bw*Es)

Ns*O(Bw*Es)

Ns*O(Bw*Es)

MIMO System Model

y = Hs + n

User data stream

.

.

User data stream

.

...

ChannelMatrix H

s1

s2

sM

s

y1

y2

yM

yTransmitted vector Received

vector

.

.

h11 h1

2

Where H =

h11 h21 …….. hM1 h12 h22 …….. hM2

h1M h2M …….. hMM

. . …….. .

MT

MR

hij is a Complex Gaussian random variable that models fading gain between the ith transmit and jth receive antenna

MIMO Channel Capacity

The instantaneous channel capacity C Increases in MIMO networks due to multi paths for transmission.

The instantaneous channel capacity C can be calculated as C= min (MT,MR) log 2 (P/MT) + constant b/s/Hz

The capacity expression presented was over one realization of the channel. Capacity is a random variable and has to be averaged over infinite realizations to obtain the true ergodic capacity. Outage capacity is another metric that is used to capture this

MIMO Design CriterionThere are two basic types of MIMO technology:

Beam forming MIMO Standards-compatible techniques to improve the range of

existing data rates using transmit and receive beamforming

Also reduces transmit interference and improves receive interference tolerance

Spatial-multiplexing MIMO Allows even higher data rates by transmitting parallel

data streams in the same frequency spectrum Fundamentally changes the on-air format of signals

Requires new standard (11n) for standards-based operation

Proprietary modes possible but cannot help legacy devices

MIMO ScalabilityMoore’s law

Doubling transistors every couple of years

MIMO Increases number of streamsHigher performance/speedHigher complexity

MIMO is the bridge to allow us to exploit Moore’s law to get higher performance

Need of power management in MIMO Networks

In static mimo configuration all antennas are active so the power consumption is very high.

To avoid this we present a novel management solution for mimo network interfaces on mobile systems , called “ Antenna management”.

Antenna management It adaptively disable a subset of antennas and their RF chains to

reduce power consumption. Antenna management dynamically determines the number of

active antennas to minimize energy per bit while satisfying data rate requirement.

Antenna management can save one-end and two-end power consumption to the front end of the MIMO network interface by 21% and 13% compared to a static MIMO link that always uses all antennas.

We employ both MATLAB-based simulation and prototype-based experiment to validate the energy efficiency benefits of antenna management.

We first present an algorithm that solves the problem of minimizing energy per bit.

After that antenna management can be realized with little change to the wireless standards like 802.11n to maximize energy efficiency.

MIMO based 802.11n System Design802.11n is a specification for wireless LAN (WLAN)

communications. It consist only physical layer & MAC layer of OSI model

Main Features• Extended bandwidth (40MHz)• Power saving• Advanced codingHigher-speed standards -- under developmentSeveral competing and non-compatible technologies; often

called "pre-n“ It provides synchronization & also power management.

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

802.11 - infrastructure network Station (STA)

terminal with access mechanisms to the wireless medium and radio contact to the access point

Basic Service Set (BSS)group of stations using

the same radio frequency

Access Pointstation integrated into

the wireless LAN and the distribution system

Portalbridge to other (wired)

networksDistribution System

interconnection network to form one logical network based on several BSS

STA1

STA2 STA3

ESS

Experimental setup for the prototype-based evaluation of antenna management

• In the above diagram One WARP node with antenna management emulates the mobile node, the other WARP node with legacy 802.11n emulates the access point, and one laptop controls both nodes as well as collects data.

19

802.11 Market Evolution

802.11

CampusNetworking

Mobile userpopulation without anyoffice space

Enterprise

Freedom fromwires for laptopusers;productivity enhancement

IndustryVerticals

Medical

Factory floors

Warehouses

Remote data entry; business process efficiency improvement

Public hotspotsMobile Operators

Revenue generationopportunity;low cost alternativeto GPRS

Broadband accessto home

Untested proposition;attempts are on-going

Thank you !