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Bandwidth Scaling in UWB Communications

Bandwidth Scaling in UWB Communications. Dana Porrat and David Tse University of California, Berkeley. Capacity of Fading Channels. UWB over Multipath channel. Spreading Modulations DSSS no Duty Cycle  zero capacity Telatar & Tse, Médard & Gallager, Subramanian & Hajek. Theory

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Bandwidth Scaling in UWB Communications

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  1. Bandwidth Scaling in UWB Communications Dana Porrat and David Tse University of California, Berkeley

  2. Capacity of Fading Channels • UWB over Multipath channel • Spreading Modulations • DSSS no Duty Cycle zero capacity Telatar & Tse, Médard & Gallager, Subramanian & Hajek • Theory • Kennedy, Gallager 1968 FSK + Duty Cycle achieves • Combine Duty Cycle and spreading modulations?

  3. The UWB Channel [Volt] Delay [nsec] Measurement Bandwidth ~1 GHz

  4. Multipath Channel Model • Block constant: coherence time • Delay spread • L independent paths

  5. How many paths in channel? • Antennas separation up to 10 m, LOS and NLOS • Strong paths counted: 60-90% of power • Number of paths increases with Bandwidth

  6. DSSS Capacity in Multipath Channel • No Duty Cycle Telater & Tse: • With Duty Cycle: Capacity depends on scaling of number of paths with W

  7. Direct Sequence Spread Spectrum

  8. DSSS Capacity with Duty Cycle • Theorem 1: DSSS systems with duty cycle achieve for if • Theorem 2: DSSS systems with duty cycle, where the receiver knows the path delays achieve for if

  9. Why does duty cycle make a difference? Parameters: W=10 GHz Tc=0.1 msec Td=200 nsec C(AWGN) DSSS with Coherence Time Duty Cycle (LB) Data Rate [bits/sec] DSSS with No Duty (UB) Cycle Number of Paths Because of the channel uncertainty penalty

  10. Duty Cycle for Spread Spectrum Parameters: W=10 GHz Tc=0.1 msec Td=200 nsec L=100 C(AWGN) DSSS Capacity (LB) Data Rate [bits/sec] Not so Bursty: Transmit on 1 coherence time of 500 Extremely Bursty: Infrequent Transmission Duty Cycle

  11. Duty Cycle for Spread Spectrum Parameters: W=10 GHz Tc=0.1 msec Td=200 nsec L=100 C(AWGN) DSSS Capacity (LB) Data Rate [bits/sec] Not so Bursty: Transmit on 1 coherence time of 500 Extremely Bursty: Infrequent Transmission Delay Uncertainty Penalty (UB) Gain Uncertainty Penalty (UB) Duty Cycle

  12. Duty Cycle for Spread Spectrum C(AWGN) Parameters: W=10 GHz Tc=0.1 msec Td=200 nsec L=100 DSSS with Perfect Channel Knowledge DSSS Capacity (LB) Data Rate [bits/sec] Not so Bursty: Transmit on 1 coherence time of 500 Extremely Bursty: Infrequent Transmission Delay Uncertainty Penalty (UB) Gain Uncertainty Penalty (UB) Duty Cycle

  13. Pulse Position Modulation - PPM

  14. DS Spread Spectrum vs. PPM Parameters: W=10 GHz Tc=0.1 msec Td=200 nsec C(AWGN) DSSS with Coherence Time Duty Cycle (LB) Data Rate [bits/sec] PPM with Coherence Time Duty Cycle (UB) Number of Paths

  15. Duty Cycle for Two Modulations Parameters: W=10 GHz Tc=0.1 msec Ts=Td=200 nsec L=100 C(AWGN) PPM (UB) DSSS (LB) Not so Bursty: Transmit on 1 coherence time of 100 Extremely Bursty: Infrequent Transmission Data Rate [bits/sec] Duty Cycle DS Spread Spectrum allows a lower duty cycle because it is more efficient spectrally.

  16. DS Spread Spectrum vs. PPM PPM: Low data rate per Tc  has to transmit often  Large Penalty DSSS: High data rate per Tc  Infrequent transmission  Small Penalty

  17. PPM Capacity with Duty Cycle • Theorem 3: PPM systems with duty cycle, where the receiver knows the path delays achieve for if • Theorem 4 (inverse): PPM systems with duty cycle, where the receiver knows the path delays have for if

  18. DS Spread Spectrum vs. PPM • Spectral efficiency determines max number of paths that can be handled by system • Delays known, gains unknown DS Spread Spectrum PPM L 0 L ? W W

  19. The Message • Direct sequence spread spectrum and PPM achieve the channel capacity for if there are not too many channel paths, because duty cycle reduces the channel uncertainty penalty • In situations that require low duty cycle spectral efficiency is key

  20. Extra Slides

  21. How Often to Transmit? C(AWGN) Calculated With L=100 Paths Data Rate [bits/sec] Not so Bursty: Transmit on 1 coherence time of 500 Extremely Bursty: Infrequent Transmission Duty Cycle

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