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Explain features of CDMA
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Features (attributes) of CDMA are as follows:

  1. Frequency usage – Many users of a CDMA system share the same frequency. Either TDD or FDD may be used.
  2. Soft Capacity Limit – Increasing the number of users in a CDMA system raises the noise level in a linear manner. Thus, there is no absolute limit on the number of users in CDMA. Rather, the system performance gradually degrades for all users as the number of users is increased, and improves as the number of users is decreased. Hence, unlike TDMA or FDMA, CDMA has a soft capacity limit.
  3. Multipath Resistance – In CDMA systems, channel data rates are very high. Hence the symbol (chip) duration is very short and usually much less than the channel delay spread. Since PN sequences have low auto correlation, multipath which is delayed by more than a chip will appear as noise. A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signal.
  4. Soft Handoff – Since CDMA uses co channel cells, it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations. The MSC may choose the best version of the signal at any time without switching frequencies.
  5. Self Jamming – Self Jamming is a problem in CDMA system. Self Jamming arises from the fact that the spreading sequences of different users are not exactly orthogonal. Hence in the de-spreading of a particular PN code, there is some level of cross correlation. There will be non-zero contributions at the receiver from the transmissions of other users in the system.
  6. Near Far effect – The near far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user. Signals close to the receiver are received with less attenuation than signals farther away.
  7. Fading effects – Multipath may be substantially reduced because signal is spread over a large spectrum.
  8. Economies – CDMA is the most cost effective technology that requires fewer cell sites and no costly frequency reuse pattern. The average power transmitted by CDMA mobile station averages 6 to 7 mill watts(mW), which is significantly lower than the average power transmitted by FM and TDMA phones. Transmitting less power means that the average battery life will be longer.
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