For this purpose, the base station continuously monitors the reverse link signal quality. This time, the mobile transmit power is controlled by the base station. The power of closed loop control is used to compensate for the rapid Rayleigh discoloration.
This power control fails or too slow for fast Rayleigh fading channels. 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 is used to compensate for the slow variables shading effects. Open loop power control starts when the first mobile attempts to communicate with the base station. The process described above is called open loop power control since it is controlled only by the mobile itself. Similarly, if the signal is weak, the mobile knows the path loss is greater and transmits high power. 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. The process is repeated until the base station responds. If it receives no response, then the second access probe is sent with a higher power. The mobile sends its first access probe, then waits for a response from the base station. 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. On the other hand, if the mobile transmits less power (not to disturb other mobile connections), the power cannot meet the E b/N 0 as required.Īs specified in the IS-95 standards, mobile acts when it wants to get into the system, it sends a signal called access. If it attempts to transmit high power to ensure contact, then it can introduce too much interference. Until the mobile does not come in contact with the base station, it has no idea of the amount of interference in the system.
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. First is the open-loop (Open Loop) control and second is closed-loop (Closed Loop) control. There are two ways to control the transmission power. This control is called the transmission power control (Control Power). We need to control the transmission power of each user. In order to maintain the received level at BS, a suitable power control technique must be employed in CDMA systems. The received level fluctuates quickly due to fading. When all mobile stations transmit the signals at the same power (MS), the received levels at the base station are different from each other, which depend on the distances between BS and MSs. If there is no power control, in other words, the transmission power are the same from both the mobile cells, the signal received from A is much stronger than the signals received from mobile cell B. Therefore, the mobile B should transmit more power to achieve the same Pr to the base station (PB>PA). Pr is the minimum signal level for the performance of the required system. A is closer to the base station and B is far from the base station. In the figure given below, there are two mobile cells A and B. To receive the same power level at the base station, the mobiles those are closer to the base station should transmit less power than the mobiles which are far away from the mobile base station. Each received power must be at least level, so that it allows the link to meet the requirements of the system such that Eb/N0. The main idea to reduce the near-far problem, is to achieve the same power level received by all mobiles to the base station.
Power control is essentially needed to solve the near-far problem. Further, each mobile transmitter power must be controlled to limit the interference.
In CDMA, since all the mobiles transmit at the same frequency, the internal interference of the network plays a critical role in determining network capacity.
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