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Code division multiple access

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Transcript

  • 1. Code Division Multiple Access Prepared by: Christina Chiu Mar 24, 2004
  • 2. Outline
    • Spread Spectrum Communications
    • Multiple Access Technique
      • Direct Sequence DS-CDMA
    • CDMA standards
      • CDMAOne (2G) – IS 95A, (2.5G) IS-95B
      • CDMA 2000 and WCDMA (3G) – IMT2000
    • 4G in the future
    • Conclusions
  • 3. Spread Spectrum Technique
    • Low power spectral density
    • Rejection to jamming signal and interference
    • Pseudorandom sequence
      • Randomness and noise properties
      • Walsh, M-sequence, Gold, Kasami, Z4
      • Provide signal privacy
  • 4. Direct Sequence Spread Spectrum
    • Spreading
      • Source signal is multiplied by a PN signal
    • Processing Gain:
    • Despreading
      • Spread signal is multiplied by the spreading code
  • 5. System Block Diagram
  • 6. Spreading & Despreading
    • Polar {±1} signal representation
  • 7. Spreading & Despreading
  • 8. CDMA
    • Unique code to differentiate all users
    • Sequence used for spreading have low cross-correlations
    • Allow many users to occupy all the frequency/bandwidth allocations at that same time
    • Processing gain is the system capacity
      • How many users the system can support
  • 9. Demodulation (Correlator)
  • 10. 2G: IS-95A (1995)
    • Known as CDMAOne
    • Chip rate at 1.25Mbps
    • Convolutional codes, Viterbi Decoding
    • Downlink (Base station to mobile):
      • Walsh code 64-bit for channel separation
      • M-sequence 2 15 for cell separation
    • Uplink (Mobile to base station):
      • M-sequence 2 41 for channel and user separation
  • 11. 2G: IS-95A (1995) 8~13 kbps Speech Rate Up to 14.4 kbps Data Rate 1.25 Mbps (chip rate) Channel Bit Rate 600 mW Maximum Tx Power 64 Number of Channel BPSK/QPSK Modulation Scheme 1.25 MHz Channel Separation 824-849 MHz Downlink Frequency 869-894 MHz Uplink Frequency CDMA Multiple Access IS-95, ANSI J-STD-008 Standard
  • 12. 2.5G: IS-95B (1998)
    • Increased data rate for internet applications
      • Up to 115 kbps (8 times that of 2G)
    • Support web browser format language
      • Wireless Application Protocol (WAP)
  • 13. 3G Technology
    • Ability to receive live music, interactive web sessions, voice and data with multimedia features
    • Global Standard IMT-2000
      • CDMA 2000, proposed by TIA
      • W-CDMA, proposed by ARIB/ETSI
    • Issued by ITU (International Telecommunication Union)
  • 14. 3G Specficiations
    • Excellent voice quality
    • Data rate
      • 144 kbps in high mobility
      • 384 kbps in limited mobility
      • 2 Mbps in door
    • Frequency Band 1885-2025 MHz
    • Convolutional Codes
    • Turbo Codes for high data rates
  • 15. 3G: CDMA2000 (2000)
    • CDMA 1xEV-DO
      • peak data rate 2.4 Mbps
      • supports mp3 transfer and video conferencing
    • CDMA 1xEV-DV
      • Integrated voice and high-speed data multimedia service up to 3.1 Mbps
    • Channel Bandwidth:
      • 1.25, 5, 10, 15 or 20 MHz
    • Chip rate at 3.6864 Mbps
    • Modulation Scheme
      • QPSK in downlink
      • BPSK in uplink
  • 16. 3G: CDMA2000 Spreading Codes
    • Downlink
      • Variable length orthogonal Walsh sequences for channel separation
      • M-sequences 3x2 15 for cell separation (different phase shifts)
    • Uplink
      • Variable length orthogonal Walsh sequences for channel separation
      • M-sequences 2 41 for user separation (different phase shifts)
  • 17. 3G: W-CDMA (2000)
    • Stands for “wideband” CDMA
    • Channel Bandwidth:
      • 5, 10 or 20 MHz
    • Chip rate at 4.096 Mbps
    • Modulation Scheme
      • QPSK in downlink
      • BPSK in uplink
  • 18. 3G: W-CDMA Spreading Codes
    • Downlink
      • Variable length orthogonal sequences for channel separation
      • Gold sequences 2 18 for cell separation
    • Uplink
      • Variable length orthogonal sequences for channel separation
      • Gold sequences 2 41 for user separation
  • 19. 4G Technology (2010)
    • High data rates => 100 Mbps
    • Multimedia data up to ~10 2 Mbps
    • IP-oriented network => Internet
    • MIMO using Multiple Antennas
    • Spreading code
      • Non-binary sequences
    • Multiple Access Technique
      • MC-CDMA
  • 20. MC-CDMA
  • 21. Conclusions
    • CDMA can increase the capacity of mobile system
    • Goals of next generation cellular
      • Further increase data rate
      • Reduce interference among users
    • Future research:
      • Search for good non-binary sequences
        • Ideal 2-level autocorrelation
        • Low cross correlations