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    Why is a spreading code used in a CDMA radio system?

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    A terrestrial radio link transmits a CDMA signal from a base station using a spreading code with a chip rate of 1.0 Mchips per seconds. The message data rate of the signal is 10 kbps.

    a. Why is a spreading code used in a CDMA radio system?

    b. What is the coding gain of the correlator in a CDMA receiver when it is synchronized to the 1.0 Mchips/s signal?

    c. A receiver of the CDMA signal has a C/(N+I) ratio of -8.0 dB at its input. What is the CNR at the output of the correlator, assuming ideal operation?

    d. A second CDMA base station is located 2 km away from the first station and transmits a different spreading code. As a receiver synchronized to the first base station signal moves close to the second base station it loses synchronization with the CDMA signal from the first base station. Why?

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    Solution Preview

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    a. CDMA is used as a multiple access technique as it effectively spreads any noise and interference over a very large bandwidth thus enabling multiple users to be able to access the channel simultaneously. As unwanted users (potential sources of interference) are not correlated (do not have the correct de-spreading codes) in the CDMA receiver any sources of interference are rejected; only the required signal being processed and de-spread from the received energy. Indeed CDMA also has the advantage of providing rejection from multipath transmissions if the received multipath signal is received delayed by more than one chip period in ...

    Solution Summary

    The expert determines why spreading code used in a CDMA ratio system. The coding gain used in the radio system is determined.