CDMA Technology

In usual TDMA, the entire frequency spectrum available is shared among the users. Each user is granted an access for a particular time, then the next gains access, after which another. After all currently using subscribers had a chance, the cycle repeats itself. The above case can be represented pictorially as:


TDMA requires synchronization among the users to achieve good performance.

In FDMA the entire frequency spectrum is divided into a number of units and allocated to each user. Hence, each user can use the allocated frequency spectrum for entire time period. It could be compared to AM or FM radio stations for which a particular operating frequency is assigned.

CDMA (Code Division Multiple Access) is different in that it neither assigns a particular time slot for each user nor it splits the frequency among the users. It allows all the users to use the entire frequency spectrum for all the time. This is achieved by what is called spreading. Each user is assigned a unique code, which allows them to distinguish between others. The base band data is spread with this unique code and a receiver having the same code can only receive the data. The code spreads the bandwidth of the user.

Spread Spectrum

Spread spectrum is a method of spreading the bandwidth such that the data occupies much more bandwidth than actually needed. Bandwidth spreading is achieved by using the above mentioned code. This code is independent of the base band data. The same code is used to despread the data at the receiver. The signal is transmitted in a channel which lies below the noise channel. The receiver then uses a correlator to despread the data and the resulting signal is passed through a narrow band pass filter. Unwanted signals are not despread and hence will not pass through the filter. Bits of spreading signal are known as chips. The rate of spreading is referred to as chip rate rather than bit rate.

Codes

CDMA codes are not supposed to provide call security, rather they are meant to provide a uniqueness which facilitates call identification. Codes should not correlate among themselves nor should they be time shifted versions of each other. Spreading codes should be like noise i.e. they should be random. If the narrow band interference is to be overcome, then the signal should have a balanced number of ones and zeros and it should have a single peak auto correlation function. Channel codes are designed to be maximum separated from each other and cell information codes are to be balanced not to correlate to other codes of itself. Codes are generated using modulo 2 adders, shift registers and feedback loops.

CDMA Advantage

Besides the above mentioned factors, it has the capability of using signals that arrive in the receivers, at different time. This phenomenon is called multipath. This is a problem for TDMA and FDMA. But in the case of CDMA, it is a blessing in disguise. CDMA combines multipath signals and make them stronger to provide a better signal. CDMA user units use rake receivers. This is a set of receivers which work together to provide a much better signal. One of the receivers (fingers) constantly searches for multipath signals and feeds the information back to the other fingers. Each receiver then demodulates the signal corresponding to a strong multipath. The combined results are then processed to get a better quality base band data.