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| | Saturday, April 25, 2009

Code-Division Multiple Access (CDMA)

Code-division multiple access (CDMA) was conceived several decades ago. Recent advances in electronic technology have finally made its implementation possible. CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link. It differs from TDMA because all stations can send data simultaneously; there is no timesharing.


Let us first give an analogy. CDMA simply means communication with different codes. For example, in a large room with many people, two people can talk in English if nobody else understands English. Another two people can talk in Chinese if they are the only ones who understand Chinese, and so on. In other words, the common channel, the space of the room in this case, can easily allow communication between several couples, but in different languages (codes).


Let us assume we have four stations 1, 2, 3, and 4 connected to the same channel. The data from station 1 are d 1, from station 2 are d 2, and so on. The code assigned to the first station is cl, to the second is c2, and so on. We assume that the assigned codes have two properties.

1. If we multiply each code by another, we get 0.
2. If we multiply each code by itself, we get 4 (the number of stations).
With these two properties in mind, let us see how the above four stations can send data using the same common channel,

data that go on the channel are the sum of all these terms, as shown in the box. Any station that wants to receive data from one of the other three multiplies the data on the channel by the code of the sender. For example, suppose stations 1 and 2 are talking to each other. Station 2 wants to hear what station 1 is saying. It multiplies the data on the channel by c 1, the code of station 1.
Because (c 1 ?? Cl) is 4, but (c 2 ?? Cl), (c. Cl), and (c 4 - c 1) are all Os, station 2 divides
the result by 4 to get the data from station 1.
data = (d 1 - c t + d 2 ?? c 2 +d 3 - c 3 + d 4- c4) ?? c I
=d l.c 1.c l+d 2.c 2.c l+d 3-c 3.c l+d 4-c4.c l=4Xd


CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips.

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