In addition to the near-far effect described above, the immediate problem is to determine the transmit power of the mobile when it first establishes a connection. Until the mobile does not come in contact with the base station, it has no idea of the amount of interference in the system. If it attempts to transmit high power to ensure contact, then it can introduce too much interference. On the other hand, if the mobile transmits less power (not to disturb other mobile connections), the power cannot meet the Eb/N0 as required.
As specified in the IS-95 standards, mobile acts when it wants to get into the system, it sends a signal called access.
In CDMA, each user's transmission power is allocated by the control power to achieve the same power (Pr) which is received by the base station/BTS with access probe with low power. The mobile sends its first access probe, then waits for a response from the base station. If it receives no response, then the second access probe is sent with a higher power.
The process is repeated until the base station responds. If the signal answered by the base station is high, then the mobile gets connected with the base station which is closer to the mobile cell with low transmission power. Similarly, if the signal is weak, the mobile knows the path loss is greater and transmits high power.
The process described above is called open loop power control since it is controlled only by the mobile itself. Open loop power control starts when the first mobile attempts to communicate with the base station.
This power control is used to compensate for the slow variables shading effects. However, since the rear and forward links are on different frequencies, the estimate transmit power does not give accurate solution for the power control because of the path loss to the front of the base station. This power control fails or too slow for fast Rayleigh fading channels.
The power of closed loop control is used to compensate for the rapid Rayleigh discoloration. This time, the mobile transmit power is controlled by the base station. For this purpose, the base station continuously monitors the reverse link signal quality. If the quality of the connection is low, it tells the mobile to increase its power; and if the quality of the connection is very high, the mobile base station controller reduces its power.
