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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 Mohamed.siala@supcom.rnu.tn. Presentation Outline.

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Mohamed Siala Professor at Sup’Com Mohamed.siala@supcom.rnu.tn

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  1. 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

  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

  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

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

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

  6. Overview on Single Carrier Communications 3/3 Frequency (f) Power Bandwidth (w) Time (t) Symbol duration (T)

  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

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

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

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

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

  12. Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 2/3 Frequency (f) Frequency (f) w w fc fc ISI ISI Time (t) Time (t) Power Power Tm Delay spread Tm Delay spread Time (t) Time (t) 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

  14. Subcarrier Aggregation: Multicarrier Systems Frequency (f) Frequency (f) F=1/T w fc T T Time (t) Time (t)

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

  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

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

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

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

  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

  21. Radio Mobile Channel Characteristics: Doppler Spread 1/3 0 -fd +fd Mobile speed (v) Power Transmitted Symbol Received symbol replica Frequency (f) Received symbol replica Received symbol replica -fd +fd Time (t) w

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

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

  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

  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

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

  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 Channel spread factor 5.9 dB

  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

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