Working of CDMA

The following steps are generally performed in the process of CDMA technique:-

• Two senders ,A and B want to send data.CDMA assigns the following unique and orthogonal key sequences: key Ak =010011 ,key Bk = 110101 for sender B .Sender A wants to send the bit Ad = 1, sender B sends Bd= 0.To illustrate this example ,let us assume that we code a binary 0 = -1 ,a binary value 1 as +1.We can apply the standard addition and multiplication rules.
• Both the senders spread their signals using their key as chipping sequence (the term spreading here refers to the simple multiplication of the data bit with the whole chipping sequence.).In reality ,parts of much longer chipping sequence applied to single bits for spreading .Sender A then sends the signal As = Ad * Ak = +1 *(-1,+1,-1,-1,+1,+1) = (-1, +1,-1,-1,+1,+1).Sender NB does the same with its data to spread the signal with code : Bs = Bd *Bk = -1 *(+1,+1,-1,+1,-1,+1) = (-1, -1,+1,-1,+1,-1).
• Both the signals are then transmitted at the same time using the same frequency ,so the signals superimpose in space (analog modulation is neglected in this example).Discounting interferance from other sender and envoirnmental noise from the simple example,and assuming that the signals have the same strength at the receiver : C = As + Bs =(-2, 0, 0,-2,+2,0).
• The receiver now wants to receive the data from the sender A and , therefore tunes into the code of A, i.e it applies A's codes for de-spreading .As a result is much larger than 0, the receiver detects a binary 1.Tuning in to sender B, i.e applying B's code gives C * Bk = (-2,0,0,-2,+2,0)*(+1,+1,-1,+1,-1,+1) = -2 + 0+0-2-2+0 =- 6.The result is negative, so 0 has been detected.
• This is the way in which one CDMA technique is to be performed ,this example involved several simplifications.