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 Cyclic Shift Diversity (CSD)
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Cyclic Shift Diversity (CSD)

  • Cyclic shift diversity (CSD) is transmit diversity technique specified in the 802.11n standard.
  • Unlike STBC, a signal from a transmitter that uses CSD can be received by legacy 802.11g and 802.11a devices.
  • For mixed mode deployments, where 802.11n coexists with 802.11g and 802.11a devices, there is a need to have a way of transmitting the symbols in the legacy OFDM preamble over multiple transmit antennas.
  • CSD is used and a cyclic delay is applied to each of the transmitted signals.
  • The delays are calculated to minimize the correlation between the multiple signals.
  • A conventional legacy system would treat the multiple received signals as multipath versions of the same signal.
  • The cyclic delay is chosen to be within the limits of the guard interval (GI) so that it does not cause excessive intersymbol interference (ISI).
  • An 802.11n system has no problem using the multiple signals to improve the overall SNR of the preamble.

Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.

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