TRANSMISSIONS

Let $S(t)$ a message sent by (A) in the direction of (B).
 
 

The received message is a shift of $S(t)$

$$R(t) = c\times S(t-\Delta)$$

$\Delta$ depends on the distance between (A) and (B) that is is useful to measure distances : synchronization.

In the cellular phones, (A) does not know the position of (B). The signal must be send in all the directions. There are echos and the received message is a linear combinaison of shifts of $S$

$$R(t) = \sum_{\Delta} {c_\Delta} S(t-\Delta)$$
 

    The determination of the $c_\Delta$ is done by means of special sequences with nice autocorrelation properties.

    On an other hand, the families of sequences with nice intercorrelation properties is useful for the Code Division Multiple Access.