08-02-2013, 12:01 PM
BASICS OF CDMA
[attachment=50154]
WHAT IS MULTIPLE ACCESS ?
NUMBER OF USERS ACCESS AND SHARE
TRANSMISSION MEDIUM
BANDWIDTH AVAILABLE
FOR COMMUNICATION AT THE SAME TIME.
MULTIPLE ACCESS
MULTIPLE SIMULTANEOUS TRANSMISSIONS.
SHARING FINITE SPECTRUM AMONG LARGE NO. OF SIMULTANEOUS USERS.
NO PRE-ASSIGNED CIRCUIT TO ANY USER.
ANY USER CAN ACCESS ANY CIRCUIT AND MAY ACCESS DIFFERENT CIRCUITS FOR DIFFERENT CALLS.
DEMAND ASSIGNED CIRCUITS ON FIRST COME FIRST SERVE BASIS.
PRIVACY.
TRUNKING EFFICIENCY; THEREBY IMPROVEMENT IN SYSTEM CAPACITY.
Advantages:-
The main advantages of this technology are:
Fast Network deployment.
Reduced service interruptions.
Low Maintenance & operational cost.
Better system coverage flexibility
Higher capacity
Easy transition to mobile services.
Salient Features of CDMA
It is an advanced comm. Technology.
It has Anti-jam and security features.
Large capacity as compared to other Technology.
like FDMA and TDMA.
It uses spread spectrum technology.
Better use of the multipath.
Frequency Reuse.
Frequency Reuse
In CDMA reuse patterns are not required.
Subscriber in every cell can use the same frequency at the same time. Subscriber is discriminated from another by the assignment of a unique code to every conversation.
In GSM freq. Reuse pattern of 7 is used.
Spread spectrum principle:
Originally spread spectrum radio technology was developed for military use to counter the interference by hostile jamming. The broad spectrum of the transmitted signal gives rise to”spread spectrum”. A spectrum signal is generated by modulating the radio frequency (RF) signal with a code consisting of different pseudo random binary sequences, which is inherently resistant to noisy signal environment.
Rake receiver
CDMA mobiles use rake receivers. The rake receiver essentially a set of four or more receivers (or fingers). One of the receivers constantly searches for different multipaths and helps to direct the other three fingers to lock onto strong multipath signals.
Allows combined reception of up to three different paths.
Provides searcher receiver to identify changes in path characteristics/new cells.
Provides both path diversity and frequency diversity.
Closed loop power control
After the traffic channel is assigned, the power control shifts to closed loop control
Reverse channel has got 16 power control groups of 1.25ms in one 20ms frame.
BTS receives the mobile receive power once every 1.25ms and BTS send Power control bit in the 2nd next 1.25 ms cycle to increase(0) or decrease(1) the power by 1db.
[attachment=50154]
WHAT IS MULTIPLE ACCESS ?
NUMBER OF USERS ACCESS AND SHARE
TRANSMISSION MEDIUM
BANDWIDTH AVAILABLE
FOR COMMUNICATION AT THE SAME TIME.
MULTIPLE ACCESS
MULTIPLE SIMULTANEOUS TRANSMISSIONS.
SHARING FINITE SPECTRUM AMONG LARGE NO. OF SIMULTANEOUS USERS.
NO PRE-ASSIGNED CIRCUIT TO ANY USER.
ANY USER CAN ACCESS ANY CIRCUIT AND MAY ACCESS DIFFERENT CIRCUITS FOR DIFFERENT CALLS.
DEMAND ASSIGNED CIRCUITS ON FIRST COME FIRST SERVE BASIS.
PRIVACY.
TRUNKING EFFICIENCY; THEREBY IMPROVEMENT IN SYSTEM CAPACITY.
Advantages:-
The main advantages of this technology are:
Fast Network deployment.
Reduced service interruptions.
Low Maintenance & operational cost.
Better system coverage flexibility
Higher capacity
Easy transition to mobile services.
Salient Features of CDMA
It is an advanced comm. Technology.
It has Anti-jam and security features.
Large capacity as compared to other Technology.
like FDMA and TDMA.
It uses spread spectrum technology.
Better use of the multipath.
Frequency Reuse.
Frequency Reuse
In CDMA reuse patterns are not required.
Subscriber in every cell can use the same frequency at the same time. Subscriber is discriminated from another by the assignment of a unique code to every conversation.
In GSM freq. Reuse pattern of 7 is used.
Spread spectrum principle:
Originally spread spectrum radio technology was developed for military use to counter the interference by hostile jamming. The broad spectrum of the transmitted signal gives rise to”spread spectrum”. A spectrum signal is generated by modulating the radio frequency (RF) signal with a code consisting of different pseudo random binary sequences, which is inherently resistant to noisy signal environment.
Rake receiver
CDMA mobiles use rake receivers. The rake receiver essentially a set of four or more receivers (or fingers). One of the receivers constantly searches for different multipaths and helps to direct the other three fingers to lock onto strong multipath signals.
Allows combined reception of up to three different paths.
Provides searcher receiver to identify changes in path characteristics/new cells.
Provides both path diversity and frequency diversity.
Closed loop power control
After the traffic channel is assigned, the power control shifts to closed loop control
Reverse channel has got 16 power control groups of 1.25ms in one 20ms frame.
BTS receives the mobile receive power once every 1.25ms and BTS send Power control bit in the 2nd next 1.25 ms cycle to increase(0) or decrease(1) the power by 1db.