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CDMA/TDD Systems for Mobile Multimedia Communications

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  1. CDMA/TDD Systems for Mobile Multimedia Communications Wha Sook Jeon Mobile Networks Lab. School of Computer Science and Engineering Seoul National University

  2. Outline • Introduction 2. Advantages with CDMA/TDD systems 3. Engineering Issues 4. Conclusion Mobile Networks Lab., SNU

  3. Introduction • 4G mobile communications systems • multimedia services (e.g., voice and video telephony, high-speed Internet, mobile computing) • International Mobile Telecommunications-2000 (IMT-2000) • IMT-2000 Radio Transmission Technology (RTT) • 3GPP(3rd Generation Partnership Project) vs 3GPP2 • Wideband Code Division Multiple Access (WCDMA) • Transmission Modes • Frequency division duplex (FDD) • Downlink and uplink use two separate frequency channel • IS-95 (digital cellular, PCS) • Time division duplex (TDD) • two links use the same frequency channel Mobile Networks Lab., SNU

  4. Introduction • Multimedia services • High capacity  wideband CDMA (WCDMA) • Various requirements on quality of service (QoS) and bandwidth  flexible resource allocation  TDD • CDMA + TDD  CDMA/TDD • IMT-2000 • 3GPP: WCDMA/TDD, WCDMA/FDD • 3GPP2: Multicarrier CDMA/WCDMA • Objective of this talk • Overview of CDMA/TDD systems • Advantages of CDMA/TDD systems • Engineering issues in CDMA/TDD systems Mobile Networks Lab., SNU

  5. Slot and Frame Format • Frequency and time allocation in FDD and TDD systems Mobile Networks Lab., SNU

  6. Slot and Frame Format (cont’d) • CDMA / TDD systems • multiple-switching-point .vs. single-switching-point • symmetric .vs. asymmetric slot allocation Mobile Networks Lab., SNU

  7. Advantage: Easy Power Control • CDMA systems • Interference limited system  received powers at base should be equal to each other • Near/far effect • Fading depends on the mobile’s speed and frequency  Uplink power control is essential • Power control in CDMA/FDD • uplink and downlink utilize different frequency bands • mobile can only estimate the downlink channel status • closed-loop feedback control: base commands through downlink Mobile Networks Lab., SNU

  8. Easy Power Control Closed loop feedback PC Open loop PC Mobile Networks Lab., SNU

  9. Easy Power Control • Power control in CDMA/TDD • uplink and downlink utilize the same frequency band • two channel characteristics are highly correlated • open-loop power control • performance depends on: • the number of switching points • user’s moving speed • Merits of TDD open-loop power control • Increment (or decrement) in transmission power can vary greatly • No need for additional power control channel in downlink • Little processing delay Mobile Networks Lab., SNU

  10. Advantages:Pre-RAKE Diversity • RAKE receiver • multipath diversity gain • commercial CDMA systems: both the base and mobiles use RAKE • It requires complex signal processing and power-consumption • pre-RAKE diversity method • The channel characteristics of two link are very similar, because two links use the same frequency band • The base estimates downlink channel impulse response using uplink channel impulse response • pre-RAKE module for downlink • pre-RAKE filter has inverse impulse response of uplink channel • mobile can receive signal equivalent to the output of RAKE • the RAKE receiver function and circuitry are removed from the portable unit and included to the base, while preserving multipath diversity gains. Mobile Networks Lab., SNU

  11. Pre-RAKE Diversity Method Base base Mobile Networks Lab., SNU

  12. Solution for Unpair Frequency Assignment • IMT-2000 frequency allocation TDD FDD(up) FDD(down) • FDD mode: a portion of sub-band may not be used • TDD mode: all portion of sub-band can be used Mobile Networks Lab., SNU

  13. Flexible Resource Allocation • Current CDMA systems • major traffic is voice • downlink and uplink utilize equal bandwidth • FDD does not cause any problem • Next generation mobile communications systems • provide multimedia services • utilization is strongly biased toward the downlink : Internet access • FDD results in bandwidth waste and capacity degradation • Asymmetric slot allocation: TDD can maximize the frequency utilization in any traffic asymmetry Mobile Networks Lab., SNU

  14. Engineering Issues : Cell Size • Guard time for a slot should contain the propagation delay to accomplish the slot synchronization • The maximum coverage of a cell is limited by guard time • Longer guard time • wider coverage, but lower efficiency • Example • CDMA/TDD systems are more appropriate for the micro/pico cell architecture Mobile Networks Lab., SNU

  15. Inter-Cell Synchronization • Two- or three-level synchronization • Example of two-level synchronization Mobile Networks Lab., SNU

  16. Time Slot Allocation • Single-cell model • How to allocate slots to uplink and downlink • Appropriate slot allocation can improve the performance • Multi-cell model • cellular environments • The constitution of traffic classes may be different from cell to cell • Strategy I • same time slot allocation for all cells • Strategy II • cell-by-cell different allocation Mobile Networks Lab., SNU

  17. Conclusion • CDMA/TDD • Reciprocity between uplink and downlink channels  small and light portable mobile units • Flexible Resource allocation  good for multimedia services • CDMA/TDD (along with CDMA/FDD) will become the very important system technology for mobile multimedia communication systems. Mobile Networks Lab., SNU

  18. References • R. Esmailzadeh, M. Nakagawa, and E. A. Sourour, “Time-division duplex CDMA communications,” IEEE Personal Commun., vol. 4, no. 2, pp. 51-56, April 1997. • D. G. Jeong and W. S. Jeon, “CDMA/TDD system for wireless multimedia services with traffic unbalance between uplink and downlink,” IEEE J. Select. Areas Commun., vol. 17, no. 5, pp. 939-946, May 1999. • W. S. Jeon and D. G. Jeong, “Comparison of time slot allocation strategies for CDMA/TDD systems,” IEEE J. Select. Areas Commun., vol. 18, no. 7, pp. 1271-1278, July 2000. • W. Wong and E. S. Sousa, “Frequency selection strategies for hybrid TDD/FDD-CDMA cellular networks,” in Proc. IEEE ICC '99, June 1999. • Z. Pu, X. You, S. Cheng, H. Wang, “Transmission and reception of TDD multicarrier CDMA signals in mobile communications system,” in Proc.IEEE VTC '99, May 1999. • H. Ho;ma, G. J. R. Povey, and A. Toskala, “Evaluation of interference between uplink and downlink in UTRA/TDD,” in Proc. IEEE VTC 1999-Fall, Sept. 1999. Mobile Networks Lab., SNU