Why Power Control Essential in CDMA ?
Power control is essential for the smooth operation of a cdma system. Because all users share the same rf band through the use of pn codes Each user looks like random noise to other users.
The power of each individual user therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.
Near mobiles must transmit at lower power Than distant ones to balance link
Need for power control in a cdma arises because of the near-far problem.
All the handsets in a cdma system transmit and receive on the same radio frequency Signals form one mobile appear as noise to the other mobile
If two mobiles at different distances form the base station transmit at same power than the mobile which is nearer to the base station increases the noise floor for the mobile which is far from the base station In other words if the power of the mobile which is near to the base station is not controlled than it increases interfences at the reciever for the other mobiles which are far from the base station.
The mobiles far from the base station in this case have to increase there transmit power to overcome this interference level Therefore power control is implemented in cdma system to overcome this near-far problem
Handset measures data errors and sends signal quality to bs
Bs makes minor changes in power level (+- 3 db)
Base station measures data errors from handset Bs commands the mobile to increase or decrease power BY 1 db
Power control occurs 800 times per second Values for initial power on access or traffic channels are sent on overhead message on paging channel