CDMA phone
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. Code division multiple access (CDMA) is a digital wireless technology that inherently has relatively greater bandwidth capacity, i.e., that inherently permits the servicing of more telephone calls per frequency band, than other wireless communication technologies. Moreover, the spread spectrum principles of CDMA inherently provide secure communications. Code division multiple access (CDMA) is a multiple access technique that allows multiple users to communicate simultaneously over the same frequency. Multiple access is achieved through the use of pseudorandom (PN) sequences called spreading codes that modulate the carrier, spreading the spectrum of the waveform. In a typical code division multiple access (CDMA) system, the signals are distinguished from one another through the use of different codes. In direct-sequence (DS) CDMA, the transmitter multiplies each user's signal by a distinct code waveform. The code waveforms of the different users have the form of "pseudo-noise" (PN) sequences, also known as spreading codes, and are chosen so as to be as much as possible mutually orthogonal. The receiver receives a composite signal made up of the sum of all of the users' signals, which overlap in time and frequency. To recover the individual signal of a given user, the receiver correlates the composite signal with that user's distinct code. In a CDMA system, the spreading codes are orthogonal only when the codes are aligned in time. When the spreading codes are shifted in time, there is some cross-correlation between spreading codes. This cross-correlation results in additional interference at the output of the correlator. The CDMA system is employed in W-CDMA and CDMA2000, which are specifications for the third generation cellular phone device.
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