Cdma tdd systems for mobile multimedia communications
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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 l.jpg

CDMA/TDD Systems for Mobile Multimedia Communications

Wha Sook Jeon

Mobile Networks Lab.

School of Computer Science and Engineering

Seoul National University


Outline l.jpg
Outline

  • Introduction

    2. Advantages with CDMA/TDD systems

    3. Engineering Issues

    4. Conclusion

Mobile Networks Lab., SNU


Introduction l.jpg
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


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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


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Slot and Frame Format

  • Frequency and time allocation in FDD and TDD systems

Mobile Networks Lab., SNU


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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


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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


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Easy Power Control

Closed loop feedback PC

Open loop PC

Mobile Networks Lab., SNU


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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


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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


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Pre-RAKE Diversity Method

Base

base

Mobile Networks Lab., SNU


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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


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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


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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


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Inter-Cell Synchronization

  • Two- or three-level synchronization

  • Example of two-level synchronization

Mobile Networks Lab., SNU


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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


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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


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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


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