What is CDMA?

CDMA which stands for Code Division Multiple Access is a cellular digital technology that is used for both voice and data communications on mobile devices. CDMA is the foundation on which various access methods such as cdmaOne, CDMA2000, and the latest WCDMA are built on. CDMA cellular systems are considered to be far superior to TDMA and FDMA, which is the reason why CDMA plays a very important role in developing practical, robust, and more secure radio communication systems.

CDMA is an example of the Multiple Access, where various transmitters can transmit information concurrently over a single channel of communication. This allows lots of users to share the same band of frequencies. To allow this without undue hindrance between the users, CDMA makes use of the Spread Spectrum technology and a special programming scheme in where each transmitter is assigned a code.

Each user in a CDMA system makes use of different codes to regulate their signals. Determining the codes to be used to modulate the signals is very important in the performance of CDMA systems. The best performance with CDMA happens when there is a good division between the signal of the desired user and the signals of other users in it. The division of these signals is made possible by comparing the received signal with the locally generated code of the desired user. If the received signal matches the desired user's code, then the correlation function will be high and the system can then extract that signal. If the desired user's generated code has nothing in common with the signal, the correlation should be as close to zero as possible thereby discarding the signal, and this is referred to as cross-correlation.

Features of CDMA

There are lots of features of CDMA and some of them are:

1. In CDMA, every channel makes use of the full spectrum that is available.

2. Individual conversations in CDMA are primarily encoded with a pseudo-random digital sequence, after that, they would be transmitted using a wide frequency range.

3. CDMA unfailingly presents better capacity for both voice and data communications, by allowing more subscribers to connect at any given time.

4. CDMA is the primary platform on which 3G technologies are built on. For 3G, CDMA makes use of 1x EV-DO and EV-DV.History of CDMA.

History of CDMA

CDMA is built around a type of transmission known as Direct Sequence Spread Spectrum. The CDMA history can be strictly connected back to the 1940s when this type of transmission was first conceived. As electronics technology got better, it began to be used for confidential military transmissions in view of the fact that the transmissions appear like noise, it was extremely hard to decipher without the knowledge of the correct codes, and it is quite difficult to jam. 

With the revolution in cellular telecommunications that happened in the 1980s a then little known small company named Qualcomm working on DSSS transmissions began to look at this as the fundamental for a cellular telecommunications multiple access scheme-CDMA Code division multiple access.

Key Elements of CDMA

CDMA has a number of unique key elements that are important to the spread spectrum transmission technologies:

1. Use of wide bandwidth: Like other spread spectrum technologies, CDMA uses a wider bandwidth than would be needed for the transmission of the data packets. This gives rise to a number of advantages including increased protection against interference or jamming and multiple user access

2. Spreading codes used: For CDMA to achieve the increased bandwidth, the data packets have to be spread by the use of a code that is independent of the data.

3. Level of Security: For CDMA to receive the data packets, the receiver has to know the spreading code, without this it is not possible to decipher the transmitted data packets, and this gives a count of the measure of security. 

4. Multiple Access: The use of the spreading codes that are independent for each user together with synchronous reception allows multiple users to have access to the same channel simultaneously

Benefits and Advantages of CDMA

The following are the benefits or advantages of CDMA:

1. In CDMA, the signal to be transmitted is spread across the wide bandwidth due to spreading. Therefore CDMA is more robust against fading and noisy environments.

2. As the transmitted information is lesser than the noise floor, it is very difficult to interfere with the CDMA spectrum. Besides, it is very difficult for hackers to crack the CDMA code used over the traffic channels. Therefore CDMA is a more secure system.

3. CDMA allows for the use of the entire bandwidth at the same time and consequently there is no limit on a number of users per cell, unlike the FDMA and TDMA access schemes. This depends on a number of codes that are supported by the CDMA compliant base station. Besides, it is easier to add users.

4. During the switching between the cells, due to the soft handoff feature, the initial connection is being set up with a new cell and therefore, it minimizes chances of call drop or call disconnection.

5. CDMA networks can easily work across other cellular networks such as GSM and LTE. Therefore nationwide roaming is possible without any issues.

6. CDMA networks are flexible in the allocation of resources.

7. CDMA networks carry out efficient practical utilization of the fixed frequency spectrum.

8. Using CDMA it is possible for a hub to communicate with two stations at once. 

9. CDMA has significant improvements in network capacity.

Factors That Determine CDMA Capacity

1. Processing Gain

2. Signal to Noise Ratio

3. Voice Activity Factor

4. Frequency Reuse Efficiency

CDMA Code Types

1. PN Codes: PN is the abbreviation for Pseudo-random number codes ( Psuedo-noise or PN code). They can be generated with ease, these codes will sum to zero over a certain period of time. Although the sequence is determined because of the capped length of the linear shift register used to produce the sequence, they give a PN code that can be used within a CDMA system to generate the spreading code required

2.Truly Orthogonal Codes: Two spreading codes are said to be orthogonal if when they are multiplies together the resulting value is added over a specific period of time they sum to zero. An example of an orthogonal code set is the Walsh codes that were used in the IS95 / CDMA2000 system.

CDMA Course Outline

CDMA - Home

CDMA - Introduction

CDMA - Channels

CDMA - Multiple Access Methods

CDMA - FDMA Technology

CDMA - TDMA Technology

CDMA - Technology

CDMA - Network

CDMA - Techniques

CDMA - Spread Spectrum

CDMA - Fading

CDMA - Near-Far Problem

CDMA - Power Control

CDMA - Frequency Allocation

CDMA - Handoff

CDMA - Interferences

 


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