1. Mohamed Siala Professor at Sup’Com Mohamed.siala@supcom.rnu.tn
ITU Workshop on "ICT Innovations in Emerging Economies"
(Tunis, Tunisia, 28 January 2014)
OFDMA with Optimized Waveforms for Interference Immune Communications in Next Generation Cellular Systems
Mohamed Siala
Professor at Sup’Com
Tunis, Tunisia, 28 January 2014

2. 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
Tunis, Tunisia, 28 January 2014

3. 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
Tunis, Tunisia, 28 January 2014

4. Overview on Single Carrier Communications 1/3
Frequency (f)
Symbols
Bandwidth (w)
Power
Carrier frequency (fc)
Symbol duration (T)
Time (t)
Symbol rate (R)
Tunis, Tunisia, 28 January 2014

5. Overview on Single Carrier Communications 2/3
Frequency (f)
Bandwidth (w)
Power
Symbol duration (T)
Time (t)
Symbol rate (R)
Tunis, Tunisia, 28 January 2014

6. Overview on Single Carrier Communications 3/3
Frequency (f)
Bandwidth (w)
Power
Symbol duration (T)
Time (t)
Tunis, Tunisia, 28 January 2014

7. Radio Mobile Channel Characteristics: Multipath and Delay Spread 1/4
Longest path
Shortest path
Frequency (f)
Power
Received
symbol replica
Received
symbol replica
Received
symbol replica
Time (t)
Transmitted Symbol
Tunis, Tunisia, 28 January 2014

8. Radio Mobile Channel Characteristics: Multipath and Delay Spread 2/4
Longest path
Shortest path
Frequency (f)
Power
Time (t)
Delay spread
Tunis, Tunisia, 28 January 2014

9. Radio Mobile Channel Characteristics: Multipath and Delay Spread 3/4
Frequency (f) w fc T
Transmitted symbols
Time (t)
Power
Time (t)
Tunis, Tunisia, 28 January 2014

10. Radio Mobile Channel Characteristics: Multipath and Delay Spread 4/4
Frequency (f) w
Inter-Symbol Interference
(ISI) fc Tm
Delay spread
Received symbols
Time (t)
Power
Time (t)
Tunis, Tunisia, 28 January 2014

11. Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 1/3
Frequency (f)
Frequency (f) w w fc fc T T
Time (t)
Time (t)
Power
Power
Time (t)
Time (t)
Tunis, Tunisia, 28 January 2014

12. Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 2/3
Frequency (f)
Frequency (f) w
ISI
ISI w fc fc Tm
Delay spread Tm
Delay spread
Time (t)
Time (t)
Power
Power
Time (t)
Time (t)
Tunis, Tunisia, 28 January 2014
Algiers, Algeria, 8 September 2013

13. Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 3/3
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
Tunis, Tunisia, 28 January 2014

14. Subcarrier Aggregation: Multicarrier Systems
Frequency (f)
Frequency (f) w
F=1/T fc T T
Time (t)
Time (t)
Tunis, Tunisia, 28 January 2014

15. Delay-Spread ISI Immune Communications: Guard Interval 1/6
Frequency (f)
Guard interval insertion F w
Tg ≥ Tm fc
Symbol occupancy
FT > 1 
Reduced symbol rate T Tg
Time (t)
Tunis, Tunisia, 28 January 2014

16. Delay-Spread ISI Immune Communications: Guard Interval 2/6
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
Tunis, Tunisia, 28 January 2014

17. Delay-Spread ISI Immune Communications: Guard Interval 3/6
Frequency (f) F w T
TgTm
N=4 FT
Total duration
Time (t)
Tunis, Tunisia, 28 January 2014

18. Delay-Spread ISI Immune Communications: Guard Interval 4/6
Frequency (f) F w
FT 
T 
TgTm
N=8 
Total duration 
Time (t)
Tunis, Tunisia, 28 January 2014

19. Delay-Spread ISI Immune Communications: Guard Interval 5/6
Frequency (f) F w
FT 
TgTm
T 
N=16 
Total duration 
Time (t)
Tunis, Tunisia, 28 January 2014

20. 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
Tunis, Tunisia, 28 January 2014

21. Radio Mobile Channel Characteristics: Doppler Spread 1/3
Received
symbol replica
Received
symbol replica
+fd
Received
symbol replica
-fd
Mobile speed
(v)
Power
Transmitted Symbol
Frequency (f)
Time (t) w

22. Radio Mobile Channel Characteristics: Doppler Spread 2/3
Frequency (f) F
Subcarrier spacing
Time (t) w
Power
Frequency (f)
Transmitted symbols
Tunis, Tunisia, 28 January 2014

23. Radio Mobile Channel Characteristics: Doppler Spread 3/3
Frequency (f)
F+Bd
Doppler spread
Time (t)
Bd = 2 fd
ICI
Power
Frequency (f)
Received symbols
Tunis, Tunisia, 28 January 2014

24. 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 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

25. Sensitivity to Multiple Access Frequency Synchronization Errors 1/2
Farthest mobile
Perfect synchronization
 No Inter-User Interference (IUI)
Power
Nearest mobile
Large
Power gap
Frequency (f)
Received symbols: Perfect user synchronization
Tunis, Tunisia, 28 January 2014

26. Sensitivity to Multiple Access Frequency Synchronization Errors 2/2
Farthest mobile
Imperfect synchronization
 Large Inter-User Interference (IUI)
Power
Nearest mobile
Large
Power gap
Large IUI
Frequency (f)
Received symbols: Imperfect user synchronization
Tunis, Tunisia, 28 January 2014

27. Quality of Service Evaluation and Optimization: SINR 1/2
Frequency (f) F
User 1
IUI
ICI
User 2
ISI T
Time (t)
SINR: Signal-to-Noise Plus Interference Ratio
Tunis, Tunisia, 28 January 2014

28. 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

29. Transmit and Receive Waveforms Optimization Results 1/6
5.9 dB
Channel
spread factor

30. Transmit and Receive Waveforms Optimization Results 2/6

31. Transmit and Receive Waveforms Optimization Results 3/6

32. Transmit and Receive Waveforms Optimization Results 4/6

33. Transmit and Receive Waveforms Optimization Results 5/6
Transmit Waveform
> 40 dB

34. Transmit and Receive Waveforms Optimization Results 6/6
Transmit Waveform