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What is CDMA?

by Sonali

CDMA is a multiple access system. This is also known as Code Division Multiple Access. This scheme uses single channel by multiple senders to send the information simultaneously.

  •  In this entire available spectrum is used by all users instead of blocks allocated.
  •  For voice and data communication CDMA is recommended.
  •  As compared to TDMA, CDMA has more air space capacity.
  • CDMA very finely handles hands –on base station.
  • CDMA systems spread symbols. Before spreading symbols it uses distinctive code before transmission.
  •  On the receiver side all symbols accepted by spreading with help of correlating symbols. A signal which is not required is neither correlated nor de-spread to accept.
  •  CDMA signals are sequences of 0’s and 1’s and sequences are generated from code. Symbol per second means original data rate and chip rates means code chipping rate.
  • Every code belongs to a distinct set of data.
  • CDMA allows space, time frequency signals transmission.

Advantages of CDMA

The number of users in this depends upon the number of codes. The few advantages are

  • It has a near-far effect and hence needs power control. If the power of all signals at the receiver is not the same then the user present near the base station will drown the signal later.
  • For improving signal reception we can use a rake receiver. We can collect the delayed version of the signal and can use it to make decisions at bit level.
  • We can change the mobile station without changing the operator. Hence, the transmission is flexible. Two base stations receive mobile signals and the mobile receives signals from the two base stations.
  • There is reduction in interference due to transmission burst.

Disadvantages of CDMA

The disadvantages of using CDMA are as follows −

  • The length of code selected is of major concern because if the code length is more it will increase delay and can also cause interference.
  • There should be synchronization in time.
  • The capacity gets reduced because of gradual transfer.
  • In order to reduce several handovers it is necessary to keep the total base power received and transmitted is not under strict control.

Different coding method used are:

(a)  Autocorrelation codes

(i)   Barker code.                                         

(ii)  Pseudo-noise code.

(b)  Other important codes

(i)   Orthogonal codes.                   

(ii)  Walsh codes.

(iii) Scrambling codes.               

(iv) Channelization codes.

(v)  Carrier modulation codes.

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