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Comparison of FDMA, TDMA, & CDMA | Methods of Transmitting and Receiving ...




Two key multiple access techniques utilized in 2G GSM are TDMA and FDMA. The advantages of modulation techniques have already been explored. TDM and FDM allow several data streams to pass through the channel between transmitter and receiver at the same time. We can figure out what they are based on their names. For example, each GSM channel has a bandwidth of 200 KHz. Furthermore, a single channel can connect up to eight users at the same time via different time slots.
 

1. FDMA:


Frequency Division Multiple Access (FDMA) is the full name for this technique. The entire available bandwidth is subdivided into several sections or frequency bands using this strategy. Each sub-band is assigned to a certain device. It's also feasible to apply TDMA on each of the sub-bands separately.
 

2. TDMA: [↗]


Time Division Multiple Access (TDMA) is the full name for this technique. TDMA is a multiple access technology that allows us to connect many devices to a base station or access point by providing them distinct time slots. Conceptually, we use a timing "rotator" in TDMA to establish distinct slots, and then we use TDMA to link multiple devices. For example, each 2G GSM channel has a bandwidth of 200 KHz, and we connect eight users using TDMA time slots.
 

3. CDMA:[↗]

Code Division Multiple Access (CDMA) is the full name for this technique. 3G technology was the first to widely use this strategy. Different forms of coding are used in Code Division Multiple Access. The term "CDMA" can refer to a variety of communication protocols. The fundamental idea is to give each mobile phone a special code. These codes are all mutually orthogonal to one another. For example, a base station (BS) emits a signal containing many codes, which many devices attempt to decode. The signal will only be fully received by the intended user; it will be discarded by others. Simply put, we can say that there is a conference room and that there are numerous individuals speaking different languages in it. Now that one of the speakers is speaking Chinese, only those who are familiar with the language will be able to understand. A person who does not speak Chinese will hear it only as noise. The same thing happens when users have access to CDMA.

Each user in this scenario has access to the full frequency band and is free to transmit at any moment. In comparison to FDMA and TDMA, CDMA is hence more flexible. Other CDMA plans take advantage of system resources to provide multiple channels.

Spread spectrum techniques include frequency-hopping CDMA technology. Pseudorandom (PN) codes assigned to each user are used to spread the signal that will be broadcast. In frequency-hopping, this is comparable to FDMA because each user will be transmitting at a separate frequency as a result. As the PN code evolves, the user will eventually be broadcasting over a different carrier frequency for each time slot, which is akin to TDMA.
[Click here to read about CDMA in details]


4. Comparison of TDMA & FDMA:


1. In FDMA, users can transmit and receive in different frequency bands at the same time.


2. In TDMA, transmission and reception take place on the same frequency range, but at different time slots.


3. For FDMA, guard frequency bands are necessary to prevent interference, resulting in system overhead.


4. Guard time slots are required for TDMA to prevent overlapping, which affects spectral efficiency.


5. TDMA is generally more efficient in digital systems compared to traditional FDMA in terms of managing multiple users.




We can conclude from the three multiple access techniques mentioned above that we can send multiple data streams utilizing those techniques over a single signal path / route. It is also clear that while using the same transmission medium, desired users can access independent signals.


5. Advantages of CDMA Technique over FDMA and TDMA

The use of a CDMA system has some key benefits. There may be excessive multipath propagation when signals are sent across a random medium. This phenomenon results in small-scale fading. A frequency selective channel is one sort of fading channel that attenuates some frequencies more than others. Because of this, received signal strength inside this kind of channel can fluctuate significantly. 

A user in a poor frequency band will only use that specific band for a brief amount of time in an FH-CDMA scheme. Therefore, CDMA systems can aid in combating fading channels. This is a primary benefit of CDMA.

Another advantage to a CDMA code is the privacy that it can afford a user. Any receiver can pick up the wideband signal, but only the user with the correct code can decode the transmission.






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