OFDMA with Optimized Waveforms for Interference Immune Communications in Next Generation

Cellular Systems

Mohamed Siala

Professor at Sup’ComMohamed.siala@supcom.rnu.tn

ITU Workshop on "ICT Innovations in Emerging Economies"

(Tunis, Tunisia, 28 January 2014)

Tunis, Tunisia, 28 January 2014

Presentation Outline

Problem statement and proposed solutionOverview on single carrier communicationsRadio Mobile Channel Characteristics:

Multipath and Delay SpreadSensitivity to Delay Spread

Subcarrier Aggregation: Multicarrier SystemsDelay-Spread ISI Immune Communications: Guard IntervalRadio Mobile Channel Characteristics: Doppler SpreadConsiderations on Subcarrier NumberSensitivity to Multiple Access Frequency Synchronization ErrorsQuality of Service Evaluation and Optimization: SINRTransmit 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 spreadsVery 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

3Tunis, Tunisia, 28 January 2014

Bandwidth (w)

Carrier frequency (fc)

Overview on Single Carrier Communications 1/3

4

Frequency (f)

Time (t)

Power

Symbols

Symbol duration (T)

1wT

1RT

Symbol rate (R)

Tunis, Tunisia, 28 January 2014

Bandwidth (w)

Symbol duration (T)

Overview on Single Carrier Communications 2/3

5

Frequency (f)

Time (t)

Power

1wT

1w T RT

1RT

Symbol rate (R)

Tunis, Tunisia, 28 January 2014

Overview on Single Carrier Communications 3/3

6

Frequency (f)

Time (t)

Power

Symbol duration (T) 1w T RT

Bandwidth (w)

Tunis, Tunisia, 28 January 2014

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

7

Frequency (f)

Time (t)

Power

Transmitted Symbol

Shortest path

Receivedsymbol replica

Receivedsymbol replica

Receivedsymbol replica

Longest path

Tunis, Tunisia, 28 January 2014

Radio Mobile Channel Characteristics: Multipath and Delay Spread 2/4

8

Frequency (f)

Time (t)

Power

Delay spread

Shortest path

Longest path

Tunis, Tunisia, 28 January 2014

Radio Mobile Channel Characteristics: Multipath and Delay Spread 3/4

9

Transmitted symbolsT

Frequency (f)

Time (t)

w

Time (t)

Power

fc

Tunis, Tunisia, 28 January 2014

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

10

Frequency (f)

Time (t)

w

Received symbols TmDelay spread

Time (t)

Power

Inter-Symbol Interference(ISI)

fc

Tunis, Tunisia, 28 January 2014

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

Tunis, Tunisia, 28 January 2014

Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 2/3

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

Tunis, Tunisia, 28 January 2014

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

The channel delay spread Tm is independent of the transmission symbol period TReduced bandwidth w

Pro: Increased T Better immunity (reduced sensitivity) to ISICon: 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 ISIUnchanged global bandwidth w Unchanged transmission rate

13Tunis, Tunisia, 28 January 2014

Subcarrier Aggregation: Multicarrier Systems

T

Frequency (f)

Time (t)T

Frequency (f)

Time (t)

wfc

F=1/T

Tunis, Tunisia, 28 January 2014

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

15Tunis, Tunisia, 28 January 2014

Delay-Spread ISI Immune Communications: Guard Interval 2/6

No guard interval insertion F = 1/T Symbol occupancy FT = 1 No symbol rate lossStill some ISI which can be reduced by

reducing F,or equivalently, increasing T = 1/For equivalently, increasing the number of subcarriers N = w/F

ISI immune communications Perfectly ISI immune communicationsT = 1/F+Tg FT > 1 Symbol rate lossSymbol rate loss reduced by reducing F, or equivalently increasing N

16Tunis, Tunisia, 28 January 2014

Delay-Spread ISI Immune Communications: Guard Interval 3/6

T

Frequency (f)

Time (t)

w

F

TgTm FT N=4Total duration

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Delay-Spread ISI Immune Communications: Guard Interval 4/6

Frequency (f)

Time (t)

w

F

TgTm N=8 T

FT

Total duration

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Delay-Spread ISI Immune Communications: Guard Interval 5/6

Frequency (f)

Time (t)

w

F

TgTm N=16 T

Total duration

FT

Tunis, Tunisia, 28 January 2014

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

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

20Tunis, Tunisia, 28 January 2014

Radio Mobile Channel Characteristics: Doppler Spread 1/3

21

Frequency (f)

Time (t)

PowerTransmitted Symbol

Mobile speed(v)

w

Receivedsymbol replica

-fd

-fd

Receivedsymbol replica

0

Receivedsymbol replica

+fd

+fd

Radio Mobile Channel Characteristics: Doppler Spread 2/3

22

Subcarrier spacingF

Frequency (f)

Time (t) wPower

Frequency (f)

Transmitted symbolsTunis, Tunisia, 28 January 2014

Radio Mobile Channel Characteristics: Doppler Spread 3/3

23

F+Bd

Frequency (f)

Time (t)Power

Frequency (f)

Received symbols

ICI Bd = 2 fd

Doppler spread

Tunis, Tunisia, 28 January 2014

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 spreadAny increase in the number of subcarriers:

Increases the guard interval Tg and the symbol period T for 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)

25Tunis, Tunisia, 28 January 2014

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

26Tunis, Tunisia, 28 January 2014

Quality of Service Evaluation and Optimization: SINR 1/2

Frequency (f)

Time (t)

T

ISIIUI

User 1

User 2ICI

SINR: Signal-to-Noise Plus Interference Ratio27Tunis, Tunisia, 28 January 2014

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 ( )SSINRISI ICI IUI

28

Transmit and Receive Waveforms Optimization Results 1/6

29

0.01d mB T

1.5FT

30SNR dB

WaveformDuration T

5.9 dB Channelspread factor

Transmit and Receive Waveforms Optimization Results 2/6

30

30SNR dB

WaveformDuration T

0.01d mB T

Transmit and Receive Waveforms Optimization Results 3/6

31

0.01d mB T

30SNR dB

3WaveformDuration T

Transmit and Receive Waveforms Optimization Results 4/6

32

0.01d mB T

3WaveformDuration T

1.25FT

/ 0.1dB F

Transmit and Receive Waveforms Optimization Results 5/6

33

0.01d mB T

3WaveformDuration T

1.25FT

/ 0.1dB F

> 40 dB

Transmit Waveform

Transmit and Receive Waveforms Optimization Results 6/6

34

0.01d mB T

3WaveformDuration T

1.25FT

/ 0.1dB F

Transmit Waveform