CODE DIVISION MULTIPLE ACCESS SYSTEM (CDMA)

Mobile communications are rapidly becoming more and more necessary for everyday activities. With so many more users to accommodate, more efficient use of bandwidth is a priority among cellular phone system operators. Equally important is the security and reliability of these calls. One solution that has been offered is a CODE DIVISION MULTIPLE ACCESS SYSTEM.

CDMA is one method for implementing a multiple access communication system. MULTIPLE ACCESS is a technique where many subscribers or local stations can share the use of the use of a communication channel at the same time or nearly so despite the fact originate from widely different locations. A channel can be thought of as merely a portion of the limited radio resource, which is temporarily 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 the channels are allocated to the many users of the system.

Since there are multiple users transmitting over the same channel, a method must be established so that individual users will not disrupt one another. There are essentially three ways to do this.

Code Division Multiple Access is a new technology used in wireless communication devices. This technology made its commercial debut in the early nineties. Significant advantage of the CDMA is the fact that unlike other modulation schemes it does not have to allocate part of the frequency for each user. It allocates whole frequency spectrum to each user, distinguishing each signal with the unique pseudo-random sequence.

CDMA stands for "Code Division Multiple Access." It is a form of spread-spectrum, an advanced digital wireless transmission technique. Instead of using frequencies or time slots, as do traditional technologies, it uses mathematical codes to transmit and distinguish between multiple wireless conversations. Its bandwidth is much wider than that required for simple point-to-point communications at the same data rate because it uses noise-like carrier waves to spread the information contained in a signal of interest over a much greater bandwidth. However, because the conversations taking place are distinguished by digital codes, many users can share the same bandwidth simultaneously.

What is CDMA?

Code Division Multiple Access (CDMA) is a digital cellular spread-spectrum modulation technique that implements distributed voice and data networks.CDMA works by converting speech into digital information, which is then transmitted as a radio signal over a wireless network. CDMA does not assign a specific frequency to each user, but the full available spectrum. Each signal is encoded differently using a unique code. This way CDMA enables a large number of users to share the same frequency band at the same time, without interference.The receiving device is instructed to use the code to extract the data out of the received signal.

ADVANTAGES

Capacity increase

No frequency management or assignment

Best for micro-cell and in building system

Reduce average transmitted power

Reduces number of sites needed to support any  given amount of traffic

Reduce deployment and operating cost because fewer cell sites are needed

Improves the telephone traffic capacity

improves the voice quality and eliminate audible  and effects of multipath fading

Provides reliable transport mechanism for data communication, such as facsimile and internet traffic.

Simplifies site selection

DISADVANTAGE

Multi-user interference or multiple access interference(MAI)

Multi-path fading

near- for problem

CONCLUSION

CDMA is radically new concept in wireless communication.

It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their systems capacity and the service quality. Moreover it spread spectrum technology is both more secure , less probable to intercept and jam, highly private and offer higher transmission quality than TDMA because of its increase  resistance to multipath distortion .The principle type of CDMA systems are direct sequence CDMA, frequency hopping CDMA and multicarrier CDMA. The major problem in CDMA is the multiple Access interference (MAI) which arises due the deviation of the spreading codes from perfect orthogonally. Capacity of CDMA is interference limited .the obvious way to increase capacity of the CDMA is to reduce the level of interference. This is achieved by reducing cross correlation, power control and with antenna arrays.