ITU Workshop on ICT Innovations in Emerging Economies

ITU Workshop on "ICT Innovations in Emerging Economies" (Geneva, Switzerland, 18 September 2013) OFDMA with Optimized Transmit and Receive Waveforms for Better Interference Immune Communications in Next Generation Radio Mobile Communication Systems Mohamed Siala Professor at Sup’Com Mohamed. siala@supcom. rnu. tn Geneva, Switzerland, 18 September 2013

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 Geneva, Switzerland, 18 September 2013 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 Qo. S optimization through interference reduction Geneva, Switzerland, 18 September 2013 3

Overview on Single Carrier Communications 1/3 Frequency (f) Power Symbols Bandwidth (w) Carrier frequency (fc) Time (t) Symbol duration (T) Geneva, Switzerland, 18 September 2013 Symbol rate (R) 4

Overview on Single Carrier Communications 2/3 Frequency (f) Power Bandwidth (w) Time (t) Symbol duration (T) Geneva, Switzerland, 18 September 2013 Symbol rate (R) 5

Overview on Single Carrier Communications 3/3 Frequency (f) Power Bandwidth (w) Time (t) Symbol duration (T) Geneva, Switzerland, 18 September 2013 6

Radio Mobile Channel Characteristics: Multipath and Delay Spread 1/4 Longest path Shortest path Frequency (f) Power Received symbol replica Time (t) Transmitted Symbol Geneva, Switzerland, 18 September 2013 7

Radio Mobile Channel Characteristics: Multipath and Delay Spread 2/4 Longest path Shortest path Frequency (f) Power Time (t) Geneva, Switzerland, 18 September 2013 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) Geneva, Switzerland, 18 September 2013 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) Geneva, Switzerland, 18 September 2013 10

Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 1/3 Frequency (f) fc w T T Power Geneva, Switzerland, 18 September 2013 Time (t) Power Time (t) 11

Radio Mobile Channel Characteristics: Sensitivity to Delay Spread 2/3 Frequency (f) w fc fc w Tm ISI Tm Delay spread Power Geneva, Switzerland, 18 September 2013 Time (t) Delay spread ISI Power Algiers, Algeria, 8 September 2013 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 Geneva, Switzerland, 18 September 2013 13

Subcarrier Aggregation: Multicarrier Systems Frequency (f) F=1/T w fc T T Geneva, Switzerland, 18 September 2013 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 Guard interval insertion Geneva, Switzerland, 18 September 2013 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 reduced by reducing F, or equivalently increasing N Geneva, Switzerland, 18 September 2013 16

Delay-Spread ISI Immune Communications: Guard Interval 3/6 Frequency (f) F w Tg Tm T FT N=4 Total duration Geneva, Switzerland, 18 September 2013 Time (t)

Delay-Spread ISI Immune Communications: Guard Interval 4/6 Frequency (f) F w FT T Tg Tm N=8 Total duration Geneva, Switzerland, 18 September 2013 Time (t)

Delay-Spread ISI Immune Communications: Guard Interval 5/6 Frequency (f) F w FT Tg Tm T N=16 Total duration Geneva, Switzerland, 18 September 2013 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 Geneva, Switzerland, 18 September 2013 20

Radio Mobile Channel Characteristics: Doppler Spread 1/3 0 -fd +fd Mobile speed (v) Power Transmitted Symbol 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) Geneva, Switzerland, 18 September 2013 Transmitted symbols 22

Radio Mobile Channel Characteristics: Doppler Spread 3/3 Frequency (f) Doppler spread Time (t) F+Bd ICI Bd = 2 fd Power Frequency (f) Geneva, Switzerland, 18 September 2013 Received symbols 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 Geneva, Switzerland, 18 September 2013 25

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 Geneva, Switzerland, 18 September 2013 26

Quality of Service Evaluation and Optimization: SINR 1/2 Frequency (f) User 1 IUI ISI ICI User 2 T SINR: Signal-to-Noise Plus Interference Ratio Geneva, Switzerland, 18 September 2013 Time (t) 27

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 + IUI Better transmission quality Reduced out-of-band emissions Small guard bands required to protect other systems 28

Transmit and Receive Waveforms Optimization Results 1/6 5. 9 d. B Channel spread factor 29

Transmit and Receive Waveforms Optimization Results 2/6 30

Transmit and Receive Waveforms Optimization Results 3/6 31

Transmit and Receive Waveforms Optimization Results 4/6 Geneva, Switzerland, 18 September 2013 32

Transmit and Receive Waveforms Optimization Results 5/6 Transmit Waveform > 40 d. B Geneva, Switzerland, 18 September 2013 33

Transmit and Receive Waveforms Optimization Results 6/6 Transmit Waveform Geneva, Switzerland, 18 September 2013 34