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
Mohamed.siala@supcom.rnu.tn
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
Tunis, Tunisia, 28 January 2014
2
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
3
Overview on Single Carrier
Communications
1/3
Frequency (f)
Symbols
1
w 
T
Power
Bandwidth (w)
Carrier frequency (fc)
Time (t)
Symbol duration (T)
Tunis, Tunisia, 28 January 2014
Symbol rate (R)
1
R
T
4
Overview on Single Carrier
Communications
2/3
Frequency (f)
w 
1
T
Power
w

T

R
1
T
Bandwidth (w)
Time (t)
Symbol duration (T)
Tunis, Tunisia, 28 January 2014
Symbol rate (R)
1
R
T
5
Overview on Single Carrier
Communications
3/3
Frequency (f)
Power
Bandwidth (w)
Time (t)
Symbol duration (T)
Tunis, Tunisia, 28 January 2014
w

T

R
1
T
6
Radio Mobile Channel Characteristics:
Multipath and Delay Spread 1/4
Longest path
Shortest path
Frequency (f)
Power
Received
Received
Received
symbol
replica
symbol
replica
symbol
replica
Time (t)
Transmitted Symbol
Tunis, Tunisia, 28 January 2014
7
Radio Mobile Channel Characteristics:
Multipath and Delay Spread 2/4
Longest path
Shortest path
Frequency (f)
Power
Time (t)
Tunis, Tunisia, 28 January 2014
Delay spread
8
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
9
Radio Mobile Channel Characteristics:
Multipath and Delay Spread 4/4
Frequency (f)
w
fc
Received symbols
Inter-Symbol Interference
(ISI)
Tm
Delay spread
Time (t)
Power
Time (t)
Tunis, Tunisia, 28 January 2014
10
Radio Mobile Channel Characteristics:
Sensitivity to Delay Spread 1/3
Frequency (f)
Frequency (f)
fc
w fc
w
T
T
Power
Tunis, Tunisia, 28 January 2014
Time (t)
Time (t)
Power
Time (t)
Time (t)
11
Radio Mobile Channel Characteristics:
Sensitivity to Delay Spread 2/3
Frequency (f)
Frequency (f)
w
fc
fc
w
Tm
ISI
Power
Tunis, Tunisia, 28 January 2014
Tm
Delay spread
Time (t)
Time (t)
Delay spread
ISI
Power
Algiers, Algeria, 8 September 2013
Time (t)
Time (t)
12
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
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Subcarrier Aggregation: Multicarrier
Systems
Frequency (f)
Frequency (f)
F=1/T
w
fc
T
Tunis, Tunisia, 28 January 2014
T
Time (t)
Time (t)
Delay-Spread ISI Immune
Communications: Guard Interval 1/6
Frequency (f)
Tg ≥ Tm
F
w
fc
Symbol occupancy
FT > 1

Reduced symbol rate
T
Tg
Tunis, Tunisia, 28 January 2014
Guard interval insertion
Time (t)
15
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
16
Delay-Spread ISI Immune
Communications: Guard Interval 3/6
Frequency (f)
F
w
TgTm
T
FT
N=4
Total duration
Tunis, Tunisia, 28 January 2014
Time (t)
Delay-Spread ISI Immune
Communications: Guard Interval 4/6
Frequency (f)
F
w
FT 
TgTm
T
N=8 
Total duration 
Tunis, Tunisia, 28 January 2014
Time (t)
Delay-Spread ISI Immune
Communications: Guard Interval 5/6
Frequency (f)
F
w
FT 
TgTm
T
N=16 
Total duration 
Tunis, Tunisia, 28 January 2014
Time (t)
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
20
Radio Mobile Channel Characteristics:
Doppler Spread 1/3
0
-fd
+fd
Mobile speed
(v)
Power
Transmitted Symbol
Received
symbol
replica
Received
symbol
replica
Received
symbol replica
Time (t)
Frequency (f)
-fd
w
+fd
21
Radio Mobile Channel Characteristics:
Doppler Spread 2/3
Frequency (f)
F
Time (t)
Power
Subcarrier spacing
w
Frequency (f)
Transmitted symbols
Tunis, Tunisia, 28 January 2014
22
Radio Mobile Channel Characteristics:
Doppler Spread 3/3
Frequency (f)
Doppler spread
Time (t)
F+Bd
ICI
Bd = 2 fd
Power
Frequency (f)
Received symbols
Tunis, Tunisia, 28 January 2014
23
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
24
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
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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
26
Quality of Service Evaluation and
Optimization: SINR 1/2
Frequency (f)
F
User 1
IUI
ISI
ICI
User 2
T
SINR: Signal-to-Noise Plus Interference Ratio
Tunis, Tunisia, 28 January 2014
Time (t)
27
Quality of Service Evaluation and
Optimization: SINR 2/2
Signal-to-Interference plus Noise Ratio (SINR):
Useful signal power (S)
ISI  ICI  IUI
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
SINR 
28
Transmit and Receive Waveforms
Optimization Results
1/6
5.9 dB
Channel
spread factor
BdTm  0.01
FT  1.5
SNR  30dB
Waveform
Duration  T
29
Transmit and Receive Waveforms
Optimization Results
2/6
BdTm  0.01
SNR  30dB
Waveform
Duration  T
30
Transmit and Receive Waveforms
Optimization Results
3/6
BdTm  0.01
SNR  30dB
Waveform
Duration  3T
31
Transmit and Receive Waveforms
Optimization Results
4/6
BdTm  0.01
Bd / F  0.1
FT  1.25
Waveform
Duration  3T
32
Transmit and Receive Waveforms
Optimization Results
5/6
Transmit Waveform
BdTm  0.01
Bd / F  0.1
FT  1.25
> 40 dB
Waveform
Duration  3T
33
Transmit and Receive Waveforms
Optimization Results
6/6
Transmit Waveform
BdTm  0.01
Bd / F  0.1
FT  1.25
Waveform
Duration  3T
34