PN Sequences - Part 2 How PN Codes are used in IS-95 CDMA Systems By Dr. Ernest Simo 2.0 Using the Long PN Code How the Long PN Code is Used Let us examine the simplified reception processes at cell site A. The received or input to cell A is: Input = di(t) • LC(t - t A) + dj(t) • LC(t - t B) Cell A is assumed to have synchronized with Mobile A. So, it is tuned to Mobile A with its local Long PN Code set to LC(t - t A). Cell A will then "multiply" its local code LC(t - t A), with the received or incoming input signal. The resulting output signal is: Output = di(t) • LC(t - t A) • LC(t - t A) + dj(t) • LC(t - t B) • LC(t - t A) As noted earlier, the normalized auto-correlation LC(t - t A) • LC(t - t A) is 1. And the normalized cross-correlation LC(t - t B) • LC(t - t A) is -1/L. For large values of L therefore, the second term can be ignored. This simply reduces the output to di(t). (Note that the signal from Mobile B is nicely ignored!) CODE: AN INTRODUCTION By Dr. Ernest Simo © 1999 DR. ERNEST SIMO - SPACE2000