CDMATDD Systems for Mobile Multimedia Communications

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

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

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

5
Slot and Frame Format

• Frequency and time allocation in FDD and TDD
  systems

6
Slot and Frame Format (contd)

• CDMA / TDD systems
• multiple-switching-point .vs. single-switching-poi
  nt
• symmetric .vs. asymmetric slot allocation

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 mobiles 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

8
Easy Power Control
Closed loop feedback PC
Open loop PC
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
• users 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

10
AdvantagesPre-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.

11
Pre-RAKE Diversity Method
Base
base
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

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

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

15
Inter-Cell Synchronization

• Two- or three-level synchronization
• Example of two-level synchronization

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

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.

18
References

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  Time-division duplex CDMA communications, IEEE
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