How Cdma Works

[Ananya 0]

How CDMA worksThe communication between 2 users in a CDMA happen by allocating codes to users.To prevent 2 users in a CDMA system to interfare guard spaces are used.Guard spaces are realized by using codes having proper distance e.g orthogonal codes. 2 vectors in a 3D space is orthogonal if their cross product is 0.e.g (2,1,0) and (4,-8,0) are orthogoanal vectors.Steps how CDMA assigns codes and works:-1) Assigning codes to users- Suppose user A has been assigned a key sequence A(k)=010011 and user B like B(k)=110101. Now suppose A has to send a binary '1' and B has to send a binary '0'. Now let us assume a '1' as '+1' and '0' as '-1'. So a bit sent by A as A(d)= +1 and B(d)= -12) Spreading of the signals sent by users - Spreading is to multiply the bit sent by user with the code allocated to him i.e data sent by A will be A(s)= (+1)*(-1,+1,-1,-1,+1,+1)=(-1,+1,-1,-1,+1,+1) and B(s)=(-1)*(+1,+1,-1,+1,-1,+1)= (-1,-1,+1,-1,+1,-1)3) Transmition of all signals in space- All the signals sent by users is then superimposed in space so in this case the transmitted signal becomes C(s)= A(s) + B(s)= (-2,0,0,-2,+2,0)4) Despreading of the signals at the receiver end - Now at receiving end it has to detected which data has been sent by which user so despreading is done using the key sequence allocated to them. So signal sent by A becomes (-2,0,0,-2,+2,0)*(-1,+1,-1,-1,+1,+1)= 2+2+2=6 and signal sent by B becomes (-2,0,0,-2,+2,0)*(+1,+1,-1,+1,-1,+1)=(-2,0,0,-2,-2,0)=-2-2-2 = -6 The value 6>0 is detected as a binary 1 at the receiving end and -6< 0 is detected as a binary 0. In the case noise has been ignored but in practical scenario when noise comes in the results aren't that accurate and causes problem to detect whether there had been a 1 or 0. So the biggest problem CDMA systems face is power control.Note: For better understanding on CDMA refer the book on Mobile Communication by Jochen Schiller.

Edited July 10, 2012 by Ananya (see edit history)