02-05-2011, 02:32 PM
ABHI.ppt (Size: 378.5 KB / Downloads: 60)
Cellular Technology
Introduction
MOBILE PHONE…most popular device
WHY ‘CELL PHONES’?
Cellular Evolution
Concept first tested in Chicago in 1978
First operational in Sweden in 1981
In the US, became operational in 1983
Introduced in Canada in 1986
Until 1991, all systems were analogue
1994, PCS were introduced
Frequency Bands
Operate in the UHF range, either in:
450 MHz: Analogue
800 MHz: Analogue & Digital
900 MHz: Analogue & Digital
1400 MHz: Digital (Japan only)
1800 MHz: Digital PCN
1900 MHz: Digital PCS
What does this tell you?
how is …EXAT MODEL?
Cellular System - Why
Limited Spectrum
The cellular concept solved the problem by replacing a single, high power transmitter (large cell) with many low power transmitters (small cells). Each providing coverage to only a small portion of the service area.
Fundamental Concepts
The fundamental idea behind cellular communications is Frequency Reuse.
Question: Is this TDMA? CDMA?….
Answer: SDMA (space division multiple access)
frequencies used in one area can be reused in another area located some sufficient distance away (to avoid co-channel interference)
NOTE: WE CAN USE ANY TYPE OF MA TECHNIQUE
Geographic Layout
Theoretical 7-cell cluster arrangement
Real World Cell Shape
Each cell has a coverage area which is ~circular
(exact shape is subject to distortions from obstacles and ground topology).
Cell Frequency Distribution
Each subset of frequency pairs are then reused according to a pattern
WHAT depends on cell size?
Changing the Cell Size
Cluster STRUCTURES
Sectorization
Why sectorize?
Results in lower interference due to an increase in D/R ratio (means more freq in a cell)
How it works
IN DETAIL
4 Channels between BS & MS
Where am I
Every live cell phone always knows the location area via the location area identity code (transmitted from BS)
position is entered in the Home Location Register, and if roaming, in the Visitor Location Register of network
BS continually monitors signal strength of all Mobile Stations (MS) in its area.
If strength falls below a specified level, cell phone tells the Mobile Switching Centre (via the nearest Base Station), and a new BS is selected
Current generation
mobile user monitors signal strength of various cells and updates BS of strongest - much faster handoff
Handoffs
Goal: seamless connectivity from cell to cell without dropping or disrupting calls in progress
If required - info is exchanged via FCC & RCC
During the handoff, conversation is interrupted for less than 400 ms (not noticeable to the voice user)
Two Types of Handoffs:
Hard Handoffs
new frequency allocated (FDMA & TDMA systems)
Soft Handoffs
same frequency is retained (CDMA systems)
Conclusion….
Whatever………is was basic!!!
Applications in……
1.CDMA
2.FDMA
3.GSM……[2G , 3G ,4G….]