The Access Network is
defined as the setting up the communication between end user terminal and the
transmission Network.
The bandwidth allocated
for a mobile telecom operator is a very expensive resource. Therefore the company must
occupy the frequencies very efficiently.
The widely used moble technology is GSM(Global System for Mobile Communications).GSM networks operate in a number of different carrier frequency ranges.GSM 900MHZ Band is used,
when increasing the coverage. Typically GSM 900MHz can cover wide area compared
with 1800MHz band.When the frequency getting higher and higher air lose increases. Sometimes, 900 Band will not enough to deliver high customer
demand ,in that case we have to use 1800 band to increase the capacity.
We are using different access technologies to address
each user
privately with the limited resource frequencies.TDMA,FDMA and CDMA are the access technologies
which allow to share the limited bandwidth with subscribers.
- FDMA (Frequency Division Multiple Access) In FDMA, signals from various users are assigned to various frequencies. (Just as in an analog system) Cross talk between adjacent channel users is avoided by maintaining a frequency guard band between them. Advantages of FDMA are that the capacity increases can be done by reducing the information rate and efficient digital codes. Also the technological advances required for implementing the system are simple
- TDMA (Time Division Multiple Access) This is the access method used in GSM. In a TDMA system data from each user is conveyed in time intervals called slots. Several slots make up a frame. Each slot is made up of a preamble plus information bits addressed to various (mobile) stations. The functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver. However it is a disadvantage that TDMA requires a large amount of signal processing for matched filtering and correlation detection for synchronization with a time slot.
- CDMA (Code Division Multiple Access) The primary advantage of CDMA is its ability to tolerate a fair amount of interfering signals compared to FDMA and TDMA. So the problems of frequency band assignment and adjacent channel interference is greatly reduced. With CDMA, adjacent micro cells share the same frequencies whereas with FDMA/TDMA it is not feasible for adjacent micro cells to share the same frequencies because of interference.
I will discuss about these access technologies in later posts.
