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Monday, August 9, 2010

TDMA and CDMA

What is TDMA and CDMA? why do we need them?
Over the years simultaneous cellular calls are increased over networks but no of accessible channels are limited. Hence there is a necessity of techniques which can solve this problem i.e increasing the number of simultaneous calls even though channels are limited. They expand the capacity of the radio channel.

There you got TDMA and CDMA. TDMA is a (Time division multiple access access)multiple access technology in which the channel is divided into sequential time slots.only a single user is using the channel at any given time. Here I would like introduce the term TDM(Time division multiplexing). Oh! then what is the difference between TDM and TDMA.
TDM is multiplexing technique and TDMA is multiple access technology. In TDM cetain time slot is dedicated forever for a given user even though he is not using it. ISDN is the example for this. In TDMA the time slot get freed up once the user finishes using the time. Here the time slots are dynamically allocated and the user may get different time slot each time accessing the network. GSM is an example for this. Of course GSM is not using ONLY TDMA.(FDMA as well clubbed with TDMA).
In TDMA the channel gets divided into 3 to 8 channels=8 simultaneous conversations.
Lets take an example for TDMA.
1) There are two stations and at each station 3 people. station1--A,B,C Station 2--D,E and F. Station1 and station 2 people want to exchange the messages.
2) The train with three carriages stops at station1 for some time.
3) All three people put the messages in three carriages.
4) The train goes to station 2 and the people there receives the messages from the carriages.
5) Again the train goes to station1 and the loop continues till the messages are completed.

Now coming to the CDMA(Code division multiple access) technology, It uses spread spectrum technology i.e a method in which a signal generated in a bandwidth is spread in the frequency domain resulting a signal with a wider bandwidth. In CDMA each caller will be given certain unique code so that others can not understand and allow multiple users use the same physical channel. The very common example for CDMA is In a room(considering as channel) several people want to communicate each other and each user will speak in different languages (Code divided) .
Now apply the same example to TDMA so that we understand the difference between these two. In TDMA all the people will take out certain time and speak one by one till every one finish their speech. In CDMA each user will speak in different languages. In FDMA each user will speak in different pitches.
Don't confuse this CDMA (channel accessing method) with CDMA phones which is a mobile phone standard called cdmaone and CDMA2000(Reliance CDMA phones which are famous in India). GPS is the first application of this CDMA and qualcomm's standard IS-95 published it as cdmaone and IS2000 as CDMA2000

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