Code Division Multiple Access
Published on Dec 12, 2015
Code division multiple access (CDMA) is a modulation and multiple-access scheme based on spread-spectrum communication. In this scheme, multiple users share the same frequency band at the same time, by spreading the spectrum of their transmitted signals, so that each user's signal is pseudo-orthogonal to the signals of the other users.
In a CDMA system each signal consists of a different pseudorandom binary sequence (called the spreading code) that modulates a carrier, spreading the spectrum of the waveform. A large number of CDMA signals share the same frequency spectrum. If CDMA is viewed in either the frequency or time domain, the multiple access signals overlap with each other. However, the use of statistically orthogonal spreading codes separates the various signals in the code space.
A CDMA receiver separates the signals by means of a correlator that uses the particular binary sequence to despread the signal and collect the energy of the desired signal. Other users' signals, whose spreading codes do not match this sequence, are not despread in bandwidth and, as a result, contribute only to the noise. These signals represent a self-interference generated by the system.
The output of the correlator is sent to a narrow-bandwidth filter. The filter allows all of the desired signal's energy to pass through, but reduces the interfering signal's energy by the ratio of the bandwidth before the correlator to the bandwidth after the correlator. This reduction greatly improves the signal-to-interference ratio of the desired signal. This ratio is also known as the processing gain. The signal-to-noise ratio is determined by the ratio of the desired signal power to the sum of all of the other signal powers. It is enhanced by the processing gain or the ratio of spread bandwidth to baseband data rate.
CDMA Channel Assignments
A CDMA digital cellular waveform design uses a pseudorandom noise (PN) sequence to spread the spectrum. The sample rate of the spreading sequence (called the chip rate) is chosen so that the bandwidth of the filtered signal is several times the bandwidth of the original signal.
A typical system might use multiple PN sequences. In addition, it might use repeated spreading codes of known lengths to ensure orthogonality between signals intended for different users. The channel assignment is essentially determined by the set of codes that are used for that particular link. Thus, the signal transmitted at any time in a logical channel is determined by:
* The frequency of operation for the base station
* The current symbol
* The specific orthogonal spreading code assigned for the logical channel
* The PN spreading code
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