Code Division Multiplexing Access (CDMA) is the technique used to enable multiple signals from multiple users to share a single communication channel when CDM is utilized. Individual talks are encoded in a digital sequence, and each group of users is given a shared code. The users connected with a specific code can only access the data that is available on the shared channel.
Walsh-Hadamard codes are designed to provide efficient and orthogonal coding sequences, which allow multiple signals or users to share the same frequency band without interfering with each other
Example: H2 = [1, 1 ; 1, −1)] ;
H4 = [1, 1, 1, 1; 1, −1, 1, −1; 1, 1, −1, −1 ; 1, −1, −1, 1 ]
Each row in the above matrices represents a codeword
The dot product of any two distinct codewords is 0
Consider that there are four stations, w, x, y, and z, that have been assigned the codes cw, cx, cy, and cz and need to transmit data dw, dx, dy, and dz, respectively
The received data by the code of station y, which is dy
data = (dw . cw+ dx . cx+ dy . cy+ dz . cz ) . cy
= dw. cw. cy + dx. cx. cy+ dy. cy. cy+ dz. cz. cy
= 0 + 0 + dy . 4 + 0 = 4dy
The user has received data from only station y while neglecting the other codes