This IT  Engineering Seminar Topic deals with the following:

WHAT IS CDMA?
(Code Division Multiple Access) A method for transmitting real time signals over a shared portion of the spectrum. The foremost application of CDMA is the digital cellular phone technology that operates in the 800MHz band and 1.9GHz . Unlike GSM and TDMA, which divides the spectrum into different time slots CDMA uses a spread spectrum technique to assign a code to each conversation. After the speech codec converts voice to digital, CDMA spreads the voice stream over the full 1.25MHz bandwidth of the CDMA channel coding each stream separately so it can be decoded at the receiving end. The rate of the spreading signal is known as the “chip rate,” as each bit in the spreading signal is called a “chip” voice conversations use the full bandwidth at the same time. One bit from each conversation is multiplied into 128 coded bits by the spreading techniques, giving the receiving side an enormous amount of data it can average just to determine the value of one bit.

CDMA phones are noted for their excellent call quality and low current draw CDMA is less costly to implement, requiring fewer cell sites than the GSM and TDMA digital cell phone systems and providing three to five times the calling capacity. It provides more than 10 times the capacity of the analog cell phone system (AMPS). CDMA is also expected to become the third-generation (3G) technology for GSM

CDMA transmission has been used by the military for secure phone calls. Unlike FDMA and TDMA methods, CDMA’s wide spreading signal makes it difficult to detect and jam.

One of the most important concepts to any cellular telephone system is that of “multiple access”, meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of the limited radio resource which is temporary allocated for a specific purpose, such as someone’s phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how channels are allocated to the many users of the system.

MULTIPLE ACCESS COMPARISON
It is easier to understand CDMA if it is compared with other multiple access technologies. The following sections describe the fundamental differences between a Frequency Division Multiple Access Analog technology (FDMA), a Time Division Multiple Access Digital technology (TDMA) and a Code Division Multiple Access Digital technology (CDMA).

FDMA - Frequency Division Multiple Access
FDMA is used for standard analog cellular. Each user is assigned a discrete slice of the RF spectrum. FDMA permits only one user per channel since it allows the user to use the channel 100% of the time. Therefore, only the frequency “dimension” is used to define channels.
TDMA – Time Division Multiole Access

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