ofdma with optimized waveforms for interference immune communications in next generation cellular...
TRANSCRIPT
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OFDMA with Optimized Waveforms for Interference Immune Communications in Next Generation
Cellular Systems
Mohamed Siala
Professor at Sup’[email protected]
ITU Workshop on "ICT Innovations in Emerging Economies"
(Tunis, Tunisia, 28 January 2014)
Tunis, Tunisia, 28 January 2014
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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
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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
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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)
Tunis, Tunisia, 28 January 2014
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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)
Tunis, Tunisia, 28 January 2014
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Overview on Single Carrier Communications 3/3
6
Frequency (f)
Time (t)
Power
Symbol duration (T) 1
w T RT
Bandwidth (w)
Tunis, Tunisia, 28 January 2014
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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
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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
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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
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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
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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
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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
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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
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Subcarrier Aggregation: Multicarrier Systems
T
Frequency (f)
Time (t)
T
Frequency (f)
Time (t)
wfc
F=1/T
Tunis, Tunisia, 28 January 2014
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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
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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
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Delay-Spread ISI Immune Communications: Guard Interval 3/6
T
Frequency (f)
Time (t)
w
F
TgTm FT N=4Total duration
Tunis, Tunisia, 28 January 2014
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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
Tunis, Tunisia, 28 January 2014
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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
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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
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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
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Radio Mobile Channel Characteristics: Doppler Spread 2/3
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Subcarrier spacingF
Frequency (f)
Time (t)w
Power
Frequency (f)
Transmitted symbolsTunis, Tunisia, 28 January 2014
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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
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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
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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)
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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
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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 Ratio27Tunis, Tunisia, 28 January 2014
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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
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Transmit and Receive Waveforms Optimization Results 1/6
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0.01d mB T
1.5FT
30SNR dB
Waveform
Duration T
5.9 dBChannelspread factor
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Transmit and Receive Waveforms Optimization Results 2/6
30
30SNR dB
Waveform
Duration T
0.01d mB T
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Transmit and Receive Waveforms Optimization Results 3/6
31
0.01d mB T
30SNR dB
3
Waveform
Duration T
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Transmit and Receive Waveforms Optimization Results 4/6
32
0.01d mB T
3
Waveform
Duration T
1.25FT
/ 0.1dB F
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Transmit and Receive Waveforms Optimization Results 5/6
33
0.01d mB T
3
Waveform
Duration T
1.25FT
/ 0.1dB F
> 40 dB
Transmit Waveform
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Transmit and Receive Waveforms Optimization Results 6/6
34
0.01d mB T
3
Waveform
Duration T
1.25FT
/ 0.1dB F
Transmit Waveform