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    Introduction:

   As seen in Cellular Communication, multiple users have to accommodated in the given spectrum. Before going in deep, let's understand what is Spectrum.

Spectrum:

       In order to cellularnetworks like Airtel, Vodafone etc to provide service to users, it has to buy frequency range from Department of Telecom. It is in Ultra High Frequency Range.

       Not only for Cellular, even FM radio has spectrum. Each FM service providers has different frequencies eg:100.5, 90.8 like that. It is spectrum. The same applies to TV,Cellular etc.

 

Multiple Access Techniques:

There are different multiple access techniques based on technology. They are

1.Frequency Division Multiple Access:

       In FDMA, each user is allocated with a particular frequency. It is used in BSNL landlines, where each user is allocated with a frequency. The frequency allocated to a particular user can't be reused (can't be allocated to another user unless the user withdraws his connection). This technology is not used in mobile phones. It uses either Horizontal or Vertical polarization.

Time vs Frequency plot of FDMA. It shows that for each call one frequency can be used.

Image courtesy: clear.rice.eduTime vs Frequency plot of FDMA. It shows that for each call one frequency can be used. Image courtesy: clear.rice.edu

 

 2.Time Division Multiple Access:

     This is used in Second Generation (2G) GSM mobile phones. Do you think it is possible to allocate seperate frequency for crores and crores of users?. This is where Time Division Multiple Access Comes in. When a user makes a call, he will be allocated two time slots, one for transmission, one for reception.

In TDMA, you can observe that same frequency is split into timeslots. 3 users can use the same frequency whereas in FDMA, only each user is allocated with one frequency.
Image Reference: clear.rice.eduIn TDMA, you can observe that same frequency is split into timeslots. 3 users can use the same frequency whereas in FDMA, only each user is allocated with one frequency. Image Reference: clear.rice.edu

 

     Activity:

          Call your friend from your mobile, ask your friend to be in another room. It is advised to record the call, so that that you can see what happened after the call ends. Both of you should be in 2G coverage. After the call starts, both of you start speak randomly without even listening to other. After the call ends, hear the recording. What did you hear, you can hear the break in voice from both the end. Do you know why ?

           Even though you can do two way communication, mobile is half duplex which means at the time one can speak and other has to hear and vise versa. Normally when you say hello, other person hears it and then says hello, then you ask question, the other person hears it and after then answers that question (when other person speaks, you will be silent and hearing it).

 

3.Code Division Multiple Access:

     This is used in Reliance CDMA (the one which came in 2002) and Tata Indicom mobiles and Landlines.Reliance Netconnect 1x uses CDMA technologyReliance Netconnect 1x uses CDMA technology

 Reliance CDMA mobileReliance CDMA mobile

 

 

 

 

 

 

 

 

 

 

 

 

 

Tata wireless Landline with CDMA technologyTata wireless Landline with CDMA technology

 

Tata launced CDMA technology mobile in partnership with ZTETata launced CDMA technology mobile in partnership with ZTE

 

 

 

 

 

 

 

 

 

 

Tata Photon using CDMA technologyTata Photon using CDMA technology

 

 

 

 

 

 

 

 

 

     There is a misunderstanding that Reliance completely relies on Satellite for CDMA technology.

     CDMA is initially being used by military initially, then it was out for public use. In India, CDMA uses 700 MHz frequency.

   CDMA can't be blocked by mobile jammer. Jammer blocks GSM not CDMA.

   You will be experiencing slow internet speed while browsing in 1x mode, because it uses narrow band frequency.

    CDMA technology is also used in satellites, but not in India.

 

4.Orthogonal Frequency Division Multiple Access (OFDMA):

      It is used in Long Term Evolution (4G LTE). It is similar to FDMA which is used in First Generation, except that both Horizontal and Vertical Polarization of a frequency can be used in OFDMA. It is not only used in 4G mobiles, but also in Satellite DTH like Airtel DTH, Sun Direct etc.