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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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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.

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See Also This Related BrainMass Solution

TDMA cellular system, Power efficiency and spectral efficiency for a modulation scheme, Propagation index, Multipath, Fast fading, Direct sequence spread spectrum, Outgoing traffic intensity

I need the answers to the questions so that my study group can have the correct information in preparation for a group assignment. This is just for reference and will not be submitted for a grade to any school.

Answer the following questions. Explain your answer and show your work in a logical manner. Use any needed formula -- show the formula, the substitutions made, and explain why.

1. A TDMA cellular system uses a 200 KHz bandwidth carrier, which carries 8 channels. What defines or determines each channel, and how much bandwidth does each channel occupy and why?

2. Define what is meant by power efficiency and spectral efficiency for a modulation scheme? What is the measure used for each?

3. Order the following modulation types in terms of spectral efficiency (best to worse) and explain why (be specific, show a number and how you got it): 64-QAM, QPSK, 8-PSK, and BPSK. Assume that the r=0, perfect filtering.

4. Propagation in a wireless channel is sometimes modeled using the propagation index n. Explain how the propagation index n varies and why. If there are two channels, where n=2 for the first one and n=3 for the second one (as for both the same power is transmitted and the same modulation/coding is used), in which channel will the signal be detectable further away and why?

5. Describe why multipath occurs in wireless. Describe fast fading, and some technique to counter it.

6. Describe how direct sequence spread spectrum (DSSS) suppresses interference from other transmissions in the same frequency band.

7. A telephone switching board can handle 120 phones. Assuming the following, determine the outgoing traffic intensity and the number of channels:
* On average 5 calls/hour per phone
* 60% of all calls made are external
* Average call duration time is 4 minutes
* QoS = 0.9%

Please refer to the attachment for multiple choice questions.

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