Share Basics of CDMA

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Hi!
Is anyone interested to share some basics of CDMA technology ?

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CDMA Code Division Multiple Access
BCDMA Broadband CDMA
NCDMA Narrowband CDMA

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MIN --- Mobile Identification Number
ESN ---Eectronic Serial Number
PRL ---Preferred Roaming List
SID ---System Identification Number
SCM ---Station Class Mark
CAI ---Common Air Interface
A Key ---Authentication Key
NAM ---Numeric Assignment Module
SPC ---Service Programming Code

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All About CDMA

Today there are two major standards for cell phone technology. CDMA and GSM. Almost all phones in the United States operate on either of these networks. However, the average person tends to be unfamiliar with these terms. Common cell phone service providers like Verizon and Sprint use CDMA technology, while carriers like T-Mobile, and Cingular operate on the GSM network.

CDMA stands for Code Division Multiple Access, but was originally known as IS-95. Qualcomm was the first to created this technology and by 1993 it was adopted by the Telecommunication Industry Association. Later this technology was enhanced and refined by Ericsson.

Since the development of CDMA technology there has been many new releases and platforms. The original CDMA is now referred to as CDMAone. Several different variants of CDMA technology been developed continuously improving quality and data transfer speeds. Third generation CDMA technology, commonly referred to as CDMA2000 encompasses a wide variety of different standards, each continually improving upon the first including; 1X EV, 1XEV-DO, and MC 3X. CDMA2000 is the current standard used by most US carriers today. The first release of CDMA2000 was refereed to as either 3G1X, 1XRTT, or 1X. Designed to provide data transmissions of ten times faster then the previous technology and double the voice capacity of CDMAone.

As stated above, Verizon Wireless operates on the CDMA network. Depending on the phone you have and its capabilities you will notice symbols in the default screen of your phone reading either 1X, 1XEV-DO or some variation of the two. This symbol defines the CDMA2000 standards your phone is operating on. Newer phones will display EV or EV-DO using the newer faster, more reliable CDMA technology.

Enhanced data transfer provides for the new technologies released by companies like Verizon. Including data transfer for files, music, games and the Internet.

WCDMA technology, standing for Wideband Code Division Multiple access, is the most developed and advanced form of the third generation CDMA2000 technology. It encompasses higher data transfer rates and provides wireless connections in markets world wide. Many existing GSM 2G (GSM/GPRS) operators have slowly began the switch to using WCDMA technology.

Qualcomm the original developer of CDMA owns patents of this technology. They have granted royalty-bearing licenses to over 100 network operators. The chart below displays the number of operators, vendors, subscribers and countries each technology is used in.

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ACCESS SCHEMES
For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.

CODING
CDMA uses unique spreading codes to spread the baseband data before transmission. The signal is transmitted in a channel, which is below noise level. The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals will not be despread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.
See coding process page for more details.

CODES
CDMA codes are not required to provide call security, but create a uniqueness to enable call identification. Codes should not correlate to other codes or time shifted version of itself. Spreading codes are noise like pseudo-random codes, channel codes are designed for maximum separation from each other and cell identification codes are balanced not to correlate to other codes of itself.

THE SPREADING PROCESS
WCDMA uses Direct Sequence spreading, where spreading process is done by directly combining the baseband information to high chip rate binary code. The Spreading Factor is the ratio of the chips (UMTS = 3.84Mchips/s) to baseband information rate. Spreading factors vary from 4 to 512 in FDD UMTS. Spreading process gain can in expressed in dBs (Spreading factor 128 = 21dB gain.

POWER CONTROL
CDMA is interference limited multiple access system. Because all users transmit on the same frequency, internal interference generated by the system is the most significant factor in determining system capacity and call quality. The transmit power for each user must be reduced to limit interference, however, the power should be enough to maintain the required Eb/No (signal to noise ratio) for a satisfactory call quality. Maximum capacity is achieved when Eb/No of every user is at the minimum level needed for the acceptable channel performance. As the MS moves around, the RF environment continuously changes due to fast and slow fading, external interference, shadowing , and other factors. The aim of the dynamic power control is to limit transmitted power on both the links while maintaining link quality under all conditions. Additional advantages are longer mobile battery life and longer life span of BTS power amplifiers
See UMTS power control page for more details.

HANDOVER
Handover occurs when a call has to be passed from one cell to another as the user moves between cells. In a traditional "hard" handover, the connection to the current cell is broken, and then the connection to the new cell is made. This is known as a "break-before-make" handover. Since all cells in CDMA use the same frequency, it is possible to make the connection to the new cell before leaving the current cell. This is known as a "make-before-break" or "soft" handover. Soft handover require less power, which reduces interference and increases capacity. Mobile can be connected to more that two BTS the handover. "Softer" handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B.
See Handover page for more details.

MULTIPATH AND RAKE RECEIVERS
One of the main advantages of CDMA systems is the capability of using signals that arrive in the receivers with different time delays. This phenomenon is called multi path. FDMA and TDMA, which are narrow band systems, cannot discriminate between the multi path arrivals, and resort to equalization to mitigate the negative effects of multi path. Due to its wide bandwidth and rake receivers, CDMA uses the multi path signals and combines them to make an even stronger signal at the receivers. CDMA subscriber units use rake receivers. This is essentially a set of several receivers. One of the receivers (fingers) constantly searches for different multi paths and feeds the information to the other three fingers. Each finger then demodulates the signal corresponding to a strong multi path. The results are then combined together to make the signal stronger.

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Home System Identification Number (Home SID)

It identifies the cellular carrier your phone is registered with.

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Station Class Mark (SCM)

It tells the cell site and the switch, what power level the mobile operates at.

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Preferred Roaming List (PRL)

It specifies the channels and the operators the phone can use.

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SSD A

For authentication, a random challenge number is transmitted from the base station (BS) to the mobile station (MS). The MS performs a calculation using it and an internal secret number called SSD A, with the help of a second internal number called A-Key.