Frequency Division Multiple Access Technique (FDMA) in 1G mobile system
First-generation mobile systems had been used for decades. It is an analog system. The example of a 1G cellular system is AMPS (Advanced mobile phone systems) which is based on frequency division multiplexing. FDMA is the first multiple-access technique used in mobile systems and narrow band channels are used. It has the channel capacity of 30 KHz and the frequency range is was 824-894MHz. In this technique, every user is assigned a frequency pair to make a call or to receive. The two frequencies are used for the uplink and downlink purposes. This is frequency division multiplexing (FDD). The pair of frequency which is allocated cannot be used in same or adjacent cells while making a call to avoid the co-channel interference. In FDMA at a time only one phone circuit is carried. If the user is not using his channel, the other users are not allowed to use it or share the capacity. The FDMA mobile systems are less complex as compared to TDMA mobile systems. Tight filtering is needed to decrease the adjacent channel interference. Use can use the allocated spectrum for sending data, voice or for nothing.
Time Division Multiple Access Technique (TDMA) in 2G mobile system
In 2G mobile systems, FDMA is also used but it is mainly for analog systems. So in case of 2G digital phones, Time Division Multiple access techniques are used (TDMA) is best to use. TDMA is a narrow band with a width of 30 KHz and length of 6.7ms. This narrow band is divided into three time slots which enhance the frequency band capacity. Both the frequency bands IS-54 and IS-136 are supported by TDMA. It uses different technologies like IDEN, PDC, and IS-136 but mostly used and common technology is GSM (Global Service Mobile Communication) which is used all over the world.
Narrowband is actually a channel. Transmission space is taken much less because voice data is converted into digital information and then it is compressed. Thus its capacity is 3 times greater as compared to analog systems with the same number of channels.
Code Division Multiple Access Technique (CDMA) in 3G mobile system
In 3G mobile systems, CDMA (Code Division Multiple Access Technique) is used. It allows various signals can occupy the same transmission channel. UHF (Ultra High Frequency) mobile systems use this technology with a frequency range of 800-MHz and 1.9-GHz. Analogue to digital conversion combine with the spread spectrum technology is implemented in CDMA. Processing power is little more in CDMA. It is also called “Code-Division” system. A unique key is used to encode data in every call to transmit the calls at once. Say if a channel has calls like 1, 2 and 3 then they will be transmitted as 666666. Each receiver divides the combined signal to get its call by using their unique key. It is a flexible and more powerful technology. So actually 3G GSM is a CDMA technology known as Wideband CDMA (WCDMA). They use a wider channel with more capacity.
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