11-01-2012, 03:22 PM
3g technology
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Simplex and Duplex
In a simplex transmission, information can only flow one way at one time, this is because there is only one frequency being used to communicate on. The easiest way of explaining this is to use walkie-talkies as an example. With a set of walkie-talkies, only one person can talk to the other at any given time, for the other person to transmit, they must wait until the other person has stopped.
In a duplex transmission, two data transmissions can be sent at any one time, this is how mobile phones work, it allows both people to speak at the same time, without any delay. If more than two data transmissions can happen at any one time, this is called multiplex.
TDD and FDD
Up until the recent developments of mobile phones, FDD (frequency division duplex) was used, this is where several frequencies are used, one for the upstream (signals going from the phone to the base station), and one for the downstream (the opposite, from the base station to the phone). A “guard band” is also needed, which sits in between the frequencies to separate them and provide isolation.
Although FDD works, it is very wasteful, as it uses several frequencies in total, and not to there full potential. This is why TDD was developed.
Symmetric and Asymmetric Transmission
A symmetric transmission is where the upstream, and downstream are the same speed, or data rate. Things such as voice on mobile phones use symmetric transmission, as the data rate needed to transmits your voice is the same as receiving another persons.
TDMA vs. CDMA
We have considered how a mobile phone can send and receive calls at the same time (via an uplink and a downlink). Now we will examine how many users can be multiplexed into the same channel (i.e., share the channel) without getting interference from other users, a capability called multiple access. For 3G technology, there are basically two competing technologies to achieve multiple access: TDMA and CDMA
