HANDBOOK ON GREEN INFORMATION AND COMMUNICATION SYSTEMS Chapter 15 Energy Efficient MIMO-OFDM Systems Zimran Rafique and Boon-Chong Seet Auckland University

HANDBOOK ON GREEN INFORMATION AND COMMUNICATION SYSTEMS

Chapter 15

Energy Efficient MIMO-OFDM Systems

Zimran Rafique and Boon-Chong Seet

Auckland University of TechnologyNew Zealand

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Table of Contents

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INTRODUCTION

Due to multimedia applications, wireless systems with higher data rate are required

Higher data rates necessitate more energy per bit for a given bit error rate (BER)

Thus, overall system energy consumption will increase

Corresponding increase in CO2 emission: threatens climate change and contributes to global warming

Energy efficient designs for high data-rate wireless systems is a crucial issue to be addressed

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Multi-Input-Multi-Output (MIMO) systems In late 1990s, MIMO techniques were proposed to

achieve higher data rates and smaller BER with the same transmit power and bandwidth required by single antenna system

Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a multi carrier modulation technique which has the capability to mitigate the effect of inter-symbol-interference (ISI) at the receiver side

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INTRODUCTION

Fourier based OFDM (FOFDM)

In conventional OFDM, complex exponential Fourier bases are used to generate orthogonal subcarriers consist of a series of orthogonal sine/cosine functions

Wavelet based OFDM (WOFDM)

In WOFDM, wavelet bases are used to generate orthogonal carriers. These bases are generated using symmetric or asymmetric QMF structure of delay or delay-free type

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INTRODUCTION

MIMO-OFDM MIMO techniques are used with OFDM (MIMO-OFDM) to

enhance the system performance

MIMO-OFDM systems are capable of increasing the channel capacity even under severe channel conditions Provide two dimensional space-frequency coding (SFC) in space

and frequency using individual subcarriers of an OFDM symbol or three dimensional coding called space-time-frequency coding (STFC) to achieve larger diversity and coding gains

OFDM can also be used in multi-user cooperative communication system by assigning subcarrier to different users for overall transmit power reduction

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INTRODUCTION

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MULTIPLE ANTENNA SYSTEM

More than one antennas are used on transmitting and/or receiving side

By using spatial multiplexing, data rate can be increased

By using spatial diversity, BER can be improved

SNR can be improved at the receiver and co-channel interference (CCI) can be eliminated along with beam forming techniques

MIMO wireless communication system

MULTIPLE ANTENNA SYSTEMSpatial Multiplexing Techniques

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The number of users, or data rate of a single user, can be increased by the factor of number of transmitting antennas (Nt) for the same transmission power and bandwidthIndividual transmitter antenna power is scaled by 1/ Nt, thus the total power remains constant and independent of number of Nt

At the receiver, the transmitted signals are retrieved from received sequences (layers) by using detection algorithms

Spatial multiplexing system architecture with Nt transmitting and Nr receiving antennas

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D-BLAST

,

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D-BLAST

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D-BLAST

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V-BLAST

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Spatial Multiplexing TechniquesV-BLAST

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Spatial Multiplexing TechniquesTurbo-BLAST

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Space Time Coding Techniques

By using space and time (two-dimensional coding), multiple antenna setups can be used to attain coding gain and diversity gain for the same bit rate, transmission power and bandwidth as compared single antenna system

Information bits are transmitted according to some pre-defined transmission sequence

At the receiver, the received signals are combined by using optimal combining scheme followed by a decision rule for maximum likelihood detection

Space-time coding system architecture with Nt transmitting and Nr receiving

antennas

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MULTIPLE ANTENNA SYSTEMSpace Time Coding Techniques

Alamouti STC Technique

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Alamouti STC Technique

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Space-Time Trellis Coding ( STTC) Technique

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Space-Time Trellis Coding ( STTC) Technique

Time-delay diversity with 2 antennas

PSK 4-state space-time code with two transmitting antennas

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Orthogonal Space-Time Block Coding ( OSTBC) Technique

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Orthogonal Space-Time Block Coding ( OSTBC) Technique

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Space-Time Vector Coding ( STVC) Technique

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Space-Time Vector Coding ( STVC) Technique

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MULTIPLE ANTENNA SYSTEMBeam-Forming

Multiple antennas capable of steering lobes and nulls of antenna beam

Co-channel interference cancellation can be done to improve SNR and to reduce delay spread of the channel

A beam-former with Nt transmitting and Nr receiving antennas

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Delay-Sum Beam-Former

A Simple Delay-Sum Beam-Former

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V-BLAST MIMO System with Beam-Former

V-BLAST MIMO system with beam-former

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MULTIPLE ANTENNA SYSTEMMulti-Functional MIMO Systems

Capable for achieving multiplexing gain, diversity gain and beamforming gain

Has Nt transmit antenna arrays (AAs) which are sufficiently apart to experience independent fading

LAA numbers of elements of each AA are spaced at a distance of λ/2 for achieving beamforming gain

Receiver is equipped with Nr receiving antennas

Multi-functional MIMO system

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MULTIPLE ANTENNA SYSTEMVirtual MIMO (V-MIMO) Systems

Also known as cooperative MIMO systems

Proposed primarily for energy and physically constrained wireless nodes (e.g. sensor nodes) to realize the advantages of MIMO techniques, which is otherwise not possible

V-MIMO systems are distributed in nature because multiple nodes are placed at different physical locations to cooperate with each other

V-MIMO systems may also have problems such as time and frequency asynchronism

Virtual-MIMO system models

Models

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MULTIPLE ANTENNA SYSTEMVirtual MIMO Systems

Models

Virtual-MIMO system models

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MULTIPLE ANTENNA SYSTEMVirtual MIMO Systems

Transmission-Delay for Model-d

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MULTIPLE ANTENNA SYSTEMEnergy Efficiency of MIMO Systems

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Transmitter and receiver architecture (In-Phase/Quadrature-Phase) for FOFDM and QAM (analog)

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OFDM & WOFDMOFDM

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OFDM & WOFDMOrthogonality Principle of OFDM

Comparison of the bandwidth utilization for FDM and OFDM

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OFDM & WOFDMFourier based OFDM (FOFDM)

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A basic FOFDM based communication system

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FOFDM modulator and demodulator with filter bank structure

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OFDM & WOFDMWavelet based OFDM (WOFDM)

Constellation Diagram of WOFDM

-5 -4 -3 -2 -1 0 1 2 3 4 5-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

In-Phase

Qua

drat

ure-

Phas

e

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WOFDM modulator and demodulator using symmetric QMF filter bank structure

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MULTIPLE ANTENNA OFDM SYSTEMS

Most of the MIMO techniques have been developed with the assumption of flat fading channel

For broadband frequency selective wireless channel, the combination of MIMO and OFDM (MIMO-OFDM) was proposed to mitigate the effect of ISI and ICI

In MIMO techniques, CSI is usually required at transmitter and/or receive side, thus OFDM is also used in MIMO systems to estimate CSI

MIMO-OFDM system with Nt transmitting and Nr receiving Antennas

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MULTIPLE ANTENNA OFDM SYSTEMSMIMO Techniques with FOFDM

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Co-operative communication in a multi user scenario using FOFDM

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MULTIPLE ANTENNA OFDM SYSTEMSMIMO Techniques with WOFDM

Transmitter and receiver architecture for WOFDM (analog)

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CONCLUSION

The underlying principles and techniques of MIMO-OFDM systems for energy efficient wireless communications are presented

Multi-antenna systems with spatial multiplexing, space-time coding and beamforming techniques are introduced

To improve BER, SNR, throughput, and energy efficiency, multi-functional MIMO and virtual MIMO systems are discussed along with energy efficiency analysis

The basic principles of FOFDM and WOFDM and their applications in true (co-located) and virtual (cooperative) MIMO wireless systems are described

MIMO-OFDM is a promising solution for energy efficient high data rate wireless networks

WOFDM can be used for SFC, STFC, as well as cooperative communication systems

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CONCLUSION

Potential directions for future work: • New wavelet basis can be designed according to wireless channel conditions to improve the overall system performance

• Multifunctional MIMO performance can be evaluated using WOFDM/FOFDM

• True and virtual MIMO-OFDM systems can be implemented to verify the theoretical results

• Physical layer architecture performance of MIMO-OFDM system along with medium access control (MAC) layer protocols can be explored

• New MAC layer protocols can be proposed for true and virtual MIMO-OFDM systems

Documents

HANDBOOK ON GREEN INFORMATION AND COMMUNICATION SYSTEMS Chapter 15 Energy Efficient MIMO-OFDM Systems Zimran Rafique and Boon-Chong Seet Auckland University