What is CDMA (Code-Division Multiple Access)?

What is CDMA

Code Division Multiple Access, also knowns as CDMA, is a term used to describe a type of cellphone service technology or a radio communication technology.

It refers to any of the numerous protocols used in second-generation (2G) and third-generation (3G) wireless communication.

CDMA is a form of multiplexing that permits different signals to occupy a single transmission channel and optimize the use of available bandwidth.

CDMA is reasonably distinct from frequency and time multiplexing, and the users have access to an entire bandwidth for some time.

The primary standard is that different CDMA codes are employed to distribute the signals among the users.

Methods employed include:

  • Frequency hopping
  • Direct sequence spread spectrum modulation (DS-CDMA)
  • Mixed CDMA detection (JDCDMA)

A signal is produced, which then extends over a broad bandwidth and the code used to perform this action is known as spreading code.

Making use of a group of codes, which are orthogonal to each other makes it possible to choose a signal that has a specific code amidst many other signals with different orthogonal codes.

 How does CDMA work?

CDMA make use of a spread-spectrum method whereby electromagnetic energy is distributed to obtain a signal with a larger bandwidth.

This method permits multiple people on different cell phones to be multiplexed on a similar channel to share a bandwidth of frequencies.

With CDMA technology, data, as well as voice packets, are separated, making use of codes and are then transmitted utilizing a wide range of frequency.

The provision of additional space for data with CDMA has become a standard that is suitable for 3G high-speed mobile Internet usage.

CDMA capacity

The factors that determine the CDMA capacity include:

  • Processing Gain
  • Signal to Noise Ratio
  • Voice Activity Factor
  • Frequency Re-use Efficiency

The capacity in CDMA is low, and it has every user on each frequency. Users are distinguished using codes, which implies that CDMA can operate regardless of noise and interference.

Neighbouring cells use similar frequencies, suggesting that there is no re-use, so CDMA capacity calculations should be straightforward.

Centralized Methods

  • The band employed in CDMA is between 24MHz – 894MHz (50MHz +20 MHz separation).
  • Frequency channel comprises of different code channels.
  • 1.25 MHz of FDMA channel can be divided into 64 code channels.

Processing Gain

CDMA is a spread spectrum method that distributes data bit using a code sequence. This implies that energy per bit is increased, allowing us to get a gain of the following:

  • P (gain) = 10log (W/R)
  • W represents Spread Rate
  • R stands for Data Rate
  • For CDMA P (gain) = 10 log (1228800/9600) = 21dB

A normal transmission condition requires a signal to the noise ratio of 7 dB for good voice quality. When translated into a ratio, the signal needs to be five times stronger than the noise.

  • Actual processing gain = P (gain) / SNR = 21 / 7 = 14Db
  • SNR means signal to noise  ratio

CDMA makes use of variable rate coder, and the voice activity factor of 0.4 is considered to be -4dB. CDMA has 100% frequency re-use, and the use of the same frequency in surrounding cells prompts some additional interference.

In CDMA frequency, re-use efficiency is 0.67 (70% efficiency), which equals -1.73dB.

Advantages of CDMA

The benefits of CDMA include the following:

  • CDMA does not need any synchronization
  • It has a significant number of users that share the same bandwidth
  • It is well-matched with the other cellular technologies
  • Because of the codeword allocated to each user, interference is minimized
  • Efficient, practical utilization of fixed frequency spectrum

 Disadvantages of CDMA

  • The system is complex
  • Guard band and guard time are needed to be provided
  • the quality of services decreases with increasing numbers of users
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