Two key modulation techniques utilized in 2G GSM are TDM and FDM. 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.
1. FDMA:
Frequency division multiplexing access (FDMA) is an acronym for frequency division multiplexing. The entire available bandwidth is subdivided into several sections 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 multiplexing access (TDMA) is an acronym for time division multiplexing. TDMA is a modulation technology that allows us to connect many devices to a base station or access point by providing them distinct time slots. We use a rotator in TDMA to establish distinct time 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 or various time slots.
3. CDMA:[↗]
Code division multiplexing access (CDMA) is the abbreviation for code division multiplexing access. 3G technology was the first to use this strategy. Different forms of coding are used in code division multiplexing access. So, 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 code, which many devices attempt to decode. The signal will only be 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 be unable to comprehend a single word. The same thing happens when users or linked devices have access to code division multiplexing.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 make advantage of system resources to provide multiple channels.
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4. Comparison of TDMA & FDMA:
1.In FDM, you can transmit and receive in different bands at the same time.
2.In TDM, transmission and reception take place on the same frequency range, but at different times.
3.For FDM, guard frequency bands are necessary, resulting in system overhead.
4.Spectral inefficiency is required for TDM guard time slots.
5.TDM outperforms FDM in terms of noise resistance.
We can conclude from the three multiplexing techniques mentioned above that we can send multiple data streams utilizing those multiplexing techniques over a single signal path / route. It is also clear that while using the same transmission line, 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 band for a brief amount of time in an FH-CDMA scheme. Therefore, CDMA systems can aid in combating fading channels. It is a benefit of a CDMA
Another advantage to a CDMA code is the privacy that it can afford a user. Any receiver can pick up the same signal that a user is transmitting and receiving when the user has a stable frequency band.
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[1] Click here to read about CDMA in details
[2] CDMA Simulator