OFDMA with Optimized Transmit and Receive Waveforms for Better Interference Immune Communications in...

OFDMA with Optimized Transmit and Receive Waveforms for Better Interference Immune

Communications in Next Generation Radio Mobile Communication Systems

Mohamed SialaProfessor at Sup’Com

Mohamed.siala@supcom.rnu.tn

ITU Workshop on "ICT Innovations in Emerging Economies"

(Geneva, Switzerland, 18 September 2013)

Geneva, Switzerland, 18 September 2013

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

2Geneva, Switzerland, 18 September 2013

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

3Geneva, Switzerland, 18 September 2013

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)

Geneva, Switzerland, 18 September 2013

Bandwidth (w)

Symbol duration (T)

Overview on Single Carrier Communications 2/3

5

Frequency (f)

Time (t)

Power

1

wT

1

w T RT

1

RT

Symbol rate (R)

Geneva, Switzerland, 18 September 2013

Overview on Single Carrier Communications 3/3

6

Frequency (f)

Time (t)

Power

Symbol duration (T) 1

w T RT

Bandwidth (w)

Geneva, Switzerland, 18 September 2013

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

Geneva, Switzerland, 18 September 2013 7

Frequency (f)

Time (t)

Power

Transmitted Symbol

Shortest path

Receivedsymbol replica

Receivedsymbol replica

Receivedsymbol replica

Longest path

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

Geneva, Switzerland, 18 September 2013 8

Frequency (f)

Time (t)

Power

Delay spread

Shortest path

Longest path

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

Geneva, Switzerland, 18 September 2013 9

Transmitted symbolsT

Frequency (f)

Time (t)

w

Time (t)

Power

fc

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

Geneva, Switzerland, 18 September 2013 10

Frequency (f)

Time (t)

w

Received symbols TmDelay spread

Time (t)

Power

Inter-Symbol Interference(ISI)

fc

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

Geneva, Switzerland, 18 September 2013 11

T

Frequency (f)

Time (t)

w

Time (t)

Power

fc

T

Frequency (f)

Time (t)

w

Time (t)

Power

fc

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

Geneva, Switzerland, 18 September 2013 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

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

Geneva, Switzerland, 18 September 2013 13

Subcarrier Aggregation: Multicarrier Systems

T

Frequency (f)

Time (t)

T

Frequency (f)

Time (t)

wfc

F=1/T

Geneva, Switzerland, 18 September 2013

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

Geneva, Switzerland, 18 September 2013 15

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

Geneva, Switzerland, 18 September 2013 16

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

T

Frequency (f)

Time (t)

w

F

TgTm FT N=4Total duration

Geneva, Switzerland, 18 September 2013

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

Frequency (f)

Time (t)

w

F

TgTm N=8 T

FT

Total duration

Geneva, Switzerland, 18 September 2013

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

Frequency (f)

Time (t)

w

F

TgTm N=16 T

Total duration

FT

Geneva, Switzerland, 18 September 2013

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

Geneva, Switzerland, 18 September 2013 20

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)w

Power

Frequency (f)

Transmitted symbolsGeneva, Switzerland, 18 September 2013

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

Geneva, Switzerland, 18 September 2013

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)

Geneva, Switzerland, 18 September 2013 25

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

Geneva, Switzerland, 18 September 2013 26

Quality of Service Evaluation and Optimization: SINR 1/2

Frequency (f)

Time (t)

T

ISI

IUI

User 1

User 2ICI

SINR: Signal-to-Noise Plus Interference Ratio

Geneva, Switzerland, 18 September 2013 27

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 mB T

1.5FT

30SNR dB

Waveform

Duration T

5.9 dBChannelspread factor

Transmit and Receive Waveforms Optimization Results 2/6

30

30SNR dB

Waveform

Duration T

0.01d mB T

Transmit and Receive Waveforms Optimization Results 3/6

31

0.01d mB T

30SNR dB

3

Waveform

Duration T

Transmit and Receive Waveforms Optimization Results 4/6

Geneva, Switzerland, 18 September 2013 32

0.01d mB T

3

Waveform

Duration T

1.25FT

/ 0.1dB F

Transmit and Receive Waveforms Optimization Results 5/6

Geneva, Switzerland, 18 September 2013 33

0.01d mB T

3

Waveform

Duration T

1.25FT

/ 0.1dB F

> 40 dB

Transmit Waveform

Transmit and Receive Waveforms Optimization Results 6/6

Geneva, Switzerland, 18 September 2013 34

0.01d mB T

3

Waveform

Duration T

1.25FT

/ 0.1dB F

Transmit Waveform