Interleaving in CDMA

It can be seen from the figure that the data are read row by row into an interleaver at the transmit end,read column by column out (this process is called interleaving) and propagated after other modulation process. Then, the data enter the interleaver at the receive end row by row and are read out column by column (this process is called de-interleaving). 
Currently, we assume that in the course of propagation, when row 2 data are transmitted, consecutive error codes occur to the 2nd, 3rd and 4th bits as a result of fading or other reasons, as shown by the left side of the figure. 
If the original data had been transmitted after the interleaving, the second bit of row 2 ,the second bit of row 3 and the second bit of row 4 would have been  error codes  after being read out column by column at the receive end. Because common error correcting codes can very easily process discrete error codes, the receive end can very easily recover the signals after the anti-interleave into the original signals by means of error correcting,but always cannot recover those signals not interleaved as a result of consecutive error codes. 
Therefore, the interleave can overcome fast fading caused during the signals transmission in air. The interleave code seldom functions in correcting error codes caused by slow fading, because slow fading may result in long consecutive error codes,even the whole frame may be error.Therefore, there will occur consecutive error codes after de-interleaving . The above figure is the simplest interleave and the interleave in an actual application is far more complex.

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