presentation at itu workshop on "ict innovations in emerging economies", ofdma with...

OFDMA with Optimized Waveforms for Interference Immune Communications in Next Generation

Cellular Systems

Mohamed Siala

Professor at Sup’Com

[email protected]

ITU Workshop on "ICT Innovations in Emerging Economies"

(Tunis, Tunisia, 28 January 2014)

Tunis, Tunisia, 28 January 2014

Presentation Outline

Problem statement and proposed solution

Overview on single carrier communications

Radio Mobile Channel Characteristics:

Multipath and Delay Spread

Sensitivity to Delay Spread

Subcarrier Aggregation: Multicarrier Systems

Delay-Spread ISI Immune Communications: Guard Interval

Radio Mobile Channel Characteristics: Doppler Spread

Considerations on Subcarrier Number

Sensitivity to Multiple Access Frequency Synchronization Errors

Quality of Service Evaluation and Optimization: SINR

Transmit and Receive Waveforms Optimization Results

2Tunis, Tunisia, 28 January 2014

Problem statement and proposed solution

Next generation mobile communication systems will operate on highly dispersive channel environments:

Very dense urban areas High multipath delay spreads

Very high carrier frequencies + high mobile velocities High Doppler spreads

OFDMA/OFDM rely on frequency badly localized waveforms

High sensitivity to Doppler spread and frequency synchronization errors due to multiple access Increased inter-carrier and -user interference

Significant out-of-band emissions Requirement of

large guard bands with respect to other adjacent systems

Optimization of transmit and receive waveforms for QoS

optimization through interference reduction


Bandwidth (w)

Carrier frequency (fc)

Overview on Single Carrier Communications 1/3

4

Frequency (f)

Time (t)

Power

Symbols

Symbol duration (T)

1

wT

1

RT

Symbol rate (R)


Bandwidth (w)

Symbol duration (T)


5

Frequency (f)

Time (t)

Power

1

wT

1

w T RT

1

RT

Symbol rate (R)



6

Frequency (f)

Time (t)

Power

Symbol duration (T) 1

w T RT

Bandwidth (w)


Radio Mobile Channel Characteristics: Multipath and Delay Spread 1/4

7

Frequency (f)

Time (t)

Power

Transmitted Symbol

Shortest path

Receivedsymbol replica



Longest path



8

Frequency (f)

Time (t)

Power

Delay spread

Shortest path

Longest path



9

Transmitted symbolsT

Frequency (f)

Time (t)

w

Time (t)

Power

fc



10

Frequency (f)

Time (t)

w

Received symbolsTm

Delay spread

Time (t)

Power

Inter-Symbol Interference(ISI)

fc


Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 1/3

11

T

Frequency (f)

Time (t)

w

Time (t)

Power

fc

T

Frequency (f)

Time (t)

w

Time (t)

Power

fc



12

Frequency (f)

Time (t)

w

TmDelay spread

Time (t)

Power

ISI

fc

Algiers, Algeria, 8 September 2013

Frequency (f)

Time (t)

w

TmDelay spread

Time (t)

Power

ISI

fc



The channel delay spread Tm is independent of the transmission symbol period T

Reduced bandwidth w Pro: Increased T Better immunity (reduced

sensitivity) to ISI

Con: Reduced symbol rate R

Aggregate together as many reduced bandwidth F

subcarriers as needed to cover the whole transmission bandwidth w:

Reduced subcarrier bandwidth F Increased symbol period T = 1/F Reduced sensitivity to ISI

Unchanged global bandwidth w Unchanged

transmission rate


Subcarrier Aggregation: Multicarrier Systems

T

Frequency (f)

Time (t)

T

Frequency (f)

Time (t)

wfc

F=1/T


Delay-Spread ISI Immune Communications: Guard Interval 1/6

T

Frequency (f)

Time (t)

wfc

F

Tg Guard interval insertion

Tg ≥ Tm

Symbol occupancyFT > 1Reduced symbol rate



No guard interval insertion F = 1/T Symbol occupancy FT = 1 No symbol rate

loss

Still some ISI which can be reduced by

reducing F,

or equivalently, increasing T = 1/F

or equivalently, increasing the number of subcarriers N = w/F

ISI immune communications

Perfectly ISI immune communications

T = 1/F+Tg FT > 1 Symbol rate loss

Symbol rate loss reduced by reducing F, or equivalently increasing N



T

Frequency (f)

Time (t)

w

F

TgTm FT N=4

Total duration



Frequency (f)

Time (t)

w

F

TgTm N=8 T

FT

Total duration



Frequency (f)

Time (t)

w

F

TgTm N=16 T

Total duration

FT



Increasing the number of subcarriers N, or equivalently, reducing the subcarrier spacing F:

(Pro) Increases spectrum efficiency (FT ) for a given tolerance to channel delay spread (Tg Tm)

(Pro) Increases tolerance to multiple access time synchronization errors (Tg ) for a given spectrum

efficiency (FT unchanged)

(Con) Increases sensitivity to propagation channel Doppler spread Bd Increase Inter-Carrier Interference

(ICI)

(Con) Increase sensitivity to multiple access frequency synchronization errors


Radio Mobile Channel Characteristics: Doppler Spread 1/3

21

Frequency (f)

Time (t)

PowerTransmitted Symbol

Mobile speed(v)

w


-fd

-fd


0


+fd

+fd


22

Subcarrier spacingF

Frequency (f)

Time (t)w

Power

Frequency (f)

Transmitted symbolsTunis, Tunisia, 28 January 2014


23

F+Bd

Frequency (f)

Time (t)

Power

Frequency (f)

Received symbols

ICI Bd = 2 fd

Doppler spread


Considerations on Subcarrier Number

The Doppler spread Bd is proportional to the mobile speed v and the carrier frequency fc Any increase in carrier

frequency leads to an increase in Doppler spread

Any increase in the number of subcarriers:

Increases the guard interval Tg and the symbol period Tfor a constant spectrum efficiency 1/FT

(Pro) Better tolerance to channel delay spread Reduced ISI

(Pro) Slight decrease in spectrum efficiency due to

the insertion of a guard interval

Decreases the subcarrier spacing F

(Con) Increased sensitivity to the Doppler spread Bd Increased ICI

(Con) Reduced tolerance to multiple access

frequency synchronization errors 24

Sensitivity to Multiple Access Frequency Synchronization Errors 1/2

Farthest mobile

Nearest mobile Power

Frequency (f)

Received symbols: Perfect user synchronization

LargePower gap

Perfect synchronization No Inter-User Interference (IUI)


Sensitivity to Multiple Access Frequency Synchronization Errors 2/2

Farthest mobile

Nearest mobile Power

Frequency (f)

Received symbols: Imperfect user synchronization

Large IUI

Imperfect synchronization Large Inter-User Interference (IUI)

LargePower gap


Quality of Service Evaluation and Optimization: SINR 1/2

Frequency (f)

Time (t)

F

T

ISI

IUI

User 1

User 2ICI

SINR: Signal-to-Noise Plus Interference Ratio


Quality of Service Evaluation and Optimization: SINR 2/2

Signal-to-Interference plus Noise Ratio (SINR):

Conventional multicarrier use badly frequency localized waveforms:

(con) High sensitivity to Doppler spread and

frequency synchronization errors

(con) Out-of-band emissions Large guard band to

protect other systems

Transmit and receive waveforms optimization through

SINR maximization:

(pro) Minimized ISI + ISI + IUI Better

transmission quality

Reduced out-of-band emissions Small guard bands

required to protect other systems

Useful signal power ( )SSINR

ISI ICI IUI

28

Transmit and Receive Waveforms Optimization Results 1/6

29

0.01d m

B T

1.5FT

30SNR dB

Waveform

Duration T

5.9 dBChannelspread factor


30

30SNR dB

Waveform

Duration T

0.01d m

B T


31

0.01d m

B T

30SNR dB

3

Waveform

Duration T


32

0.01d m

B T

3

Waveform

Duration T

1.25FT

/ 0.1d

B F


33

0.01d m

B T

3

Waveform

Duration T

1.25FT

/ 0.1d

B F

> 40 dB

Transmit Waveform


34

0.01d m

B T

3

Waveform

Duration T

1.25FT

/ 0.1d

B F

Transmit Waveform

presentation at itu workshop on "ict innovations in emerging economies", ofdma with...

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