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CDMA/TDD Systems for Mobile Multimedia Communications Wha Sook Jeon Mobile Networks Lab. School of Computer Science and Engineering Seoul National University Outline Introduction 2. Advantages with CDMA/TDD systems 3. Engineering Issues 4 . Conclusion Introduction

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cdma tdd systems for mobile multimedia communications

CDMA/TDD Systems for Mobile Multimedia Communications

Wha Sook Jeon

Mobile Networks Lab.

School of Computer Science and Engineering

Seoul National University

  • Introduction

2. Advantages with CDMA/TDD systems

3. Engineering Issues

4. Conclusion

Mobile Networks Lab., SNU

  • 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

  • Multimedia services
    • High capacity  wideband CDMA (WCDMA)
    • Various requirements on quality of service (QoS) and bandwidth

 flexible resource allocation  TDD

  • IMT-2000
    • 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

slot and frame format
Slot and Frame Format
  • Frequency and time allocation in FDD and TDD systems

Mobile Networks Lab., SNU

slot and frame format cont d
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

advantage easy power control
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

easy power control
Easy Power Control

Closed loop feedback PC

Open loop PC

Mobile Networks Lab., SNU

easy power control9
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

advantages pre rake diversity
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

pre rake diversity method
Pre-RAKE Diversity Method



Mobile Networks Lab., SNU

solution for unpair frequency assignment
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

flexible resource allocation
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

engineering issues cell size
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

inter cell synchronization
Inter-Cell Synchronization
  • Two- or three-level synchronization
  • Example of two-level synchronization

Mobile Networks Lab., SNU

time slot allocation
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

    • 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

  • 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