Mohamed Siala Professor at Sup’Com [email protected]
description
Transcript of Mohamed Siala Professor at Sup’Com [email protected]
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
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
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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
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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)
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
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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
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
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Subcarrier Aggregation: Multicarrier Systems
T
Frequency (f)
Time (t)T
Frequency (f)
Time (t)
wfc
F=1/T
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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
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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
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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
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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
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Subcarrier spacingF
Frequency (f)
Time (t) wPower
Frequency (f)
Transmitted symbolsTunis, Tunisia, 28 January 2014
Radio Mobile Channel Characteristics: Doppler Spread 3/3
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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
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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
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
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Transmit and Receive Waveforms Optimization Results 1/6
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0.01d mB T
1.5FT
30SNR dB
WaveformDuration T
5.9 dB Channelspread factor
Transmit and Receive Waveforms Optimization Results 2/6
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30SNR dB
WaveformDuration T
0.01d mB T
Transmit and Receive Waveforms Optimization Results 3/6
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0.01d mB T
30SNR dB
3WaveformDuration T
Transmit and Receive Waveforms Optimization Results 4/6
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0.01d mB T
3WaveformDuration T
1.25FT
/ 0.1dB F
Transmit and Receive Waveforms Optimization Results 5/6
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0.01d mB T
3WaveformDuration T
1.25FT
/ 0.1dB F
> 40 dB
Transmit Waveform
Transmit and Receive Waveforms Optimization Results 6/6
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0.01d mB T
3WaveformDuration T
1.25FT
/ 0.1dB F
Transmit Waveform