Code Division Multiple Access (CDMA) is a multiple access and modulation scheme based on the communication of spread-spectrum. CDMA is a new concept in the advanced wireless communications and digital cellular radio communications.
CDMA IN WIRELESS COMMUNICATION:
With CDMA, each signal comprises various pseudo-random binary sequences which set the carrier and spreads the waveform spectrum. CDMA is a system of direct sequence spread spectrum. Numerous CDMA signals share the same spectrum of frequency. If CDMA is viewed either in the time or frequency domain, the signals of multiple access appear to be on topmost of each other. The multiple access signals are separated at the receiver with the help of correlator that permits only signal energy from the selected sequence of binary and despreads its spectrum. The system of CDMA works directly on digital signals like 64 Kbit/Sec. These signals may be ISDN channels, digitized voice, modem data and more (Adachi et al., 1998).
According to Viterbi (1995) CDMA changes the subscriber station nature from an analog to digital device. CDMS receivers do not neglect analog processing completely, but they separate channels of communication by a means of a modulation of pseudo-random which is applied and removed in the domain of digital, non on the frequency basis. Multiple users occupy the band of the same frequency. CDMS is changing the face of PCS and cellular communication by improving the voice quality and removing the audible multipath fading effects, improving the capacity of telephone traffic, minimizing the incidence of dropped calls since handoff failures, providing reliable mechanism of transport for data communications like internet traffic, facsimile, simplifying site selection, reducing the number of sites required to support any provided amount of traffic, reducing the operating and deployment costs since fewer cells sites are required, minimizing average transmitted power, reducing potential health risks and minimizing interference to the other electronic devices (Sari et al., 2000).
It is concluded that CDMA is a radio access system and digital modulation which employs frequency bands in order to arrange continuous and simultaneous access to a wireless network by multiple users.
1. Adachi.F, Sawahashi.M and Suda.H (1998), Wideband DS-CDMA for Next- Generation Mobile Communication Systems”, IEEE Communication, Mag., vol.36, pp. 56-59.
2. Sari.H, Vanhaverbeke.F, Moeneclaey.M (2000), “Multiple access using two sets of orthogonal signal waveforms,” IEEE Communication. Lett., vol. 4, no. 1, pp. 4-6.
3. Viterbi. A (1995), CDMA: Principles of Spread Spectrum Communication Addison-Wesley Wireless Communications Series.