Code Division Multiple Access, also known as CDMA, is a term for a type of cellphone service or radio communication technology.
It refers to 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 available bandwidth.
CDMA is reasonably distinct from frequency and time multiplexing, and users can temporarily access an entire bandwidth.
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.
Using a group of orthogonal codes makes it possible to choose a signal with a specific code among many other signals with different orthogonal codes.
How Does CDMA Work?
CDMA uses 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 and voice packets are separated using codes and transmitted using a wide range of frequencies.
The provision of additional data space with CDMA has become a standard 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 of 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.
Neighboring cells use similar frequencies, suggesting no reuse, so CDMA capacity calculations should be straightforward.
Centralized Methods
- The band employed in CDMA is between 24MHz and 894MHz (50MHz +20 MHz separation).
- The frequency channel comprises 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 bits 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 the Spread Rate.
- R stands for Data Rate
- For CDMA P (gain) = 10 log (1228800/9600) = 21dB
A normal transmission condition requires a signal-to-noise ratio of 7 dB for good voice quality. When translated into a ratio, the signal must be five times stronger than the noise.
- Actual processing gain = P (gain) / SNR = 21 / 7 = 14Db
- SNR means signal-to-noise ratio
CDMA uses a variable rate coder, and the voice activity factor of 0.4 is considered -4dB.
CDMA has 100% frequency re-use, and using the same frequency in surrounding cells prompts 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 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 need to be provided
- the quality of services decreases with increasing numbers of users
