Mumbai University > Electronics and Telecommunication > Sem8 > Wireless Networks

Marks: 10M

Year: May2012

Power Control in CDMA Environment

1. CDMA is an interference limited system

i. The maximum utilization of available radio spectrum in CDMA is to have maximum number of simultaneous mobile users served with acceptable signal quality.

ii. CDMA is an interference limited system. This implies that the number of users in the system are not limited by Channel parameters such as fading and receiver noise, but seriously limited by interference. Subsequently, to achieve improvement in spectral efficiency and increase in system capacity requires minimizing the interference.

iii. In a CDMA system, it is not co-Channel and adjacent Channel interference which pose serious bottlenecks, but rather the interference from other mobile users transmitting in the same frequency band at the same time in the system.

iv. It is desirable for maximum efficiency that power received at base station from all mobile users served by it must be nearly equal. If the received signal power is too low, there is a high probability of bit errors or frame errors. If the received signal power is too high, interference increases.

v. On one hand, it is important to implement good power control in order to minimize the near-far effect. On the other hand, the transmit power should be adequate enough so as to encounter the effects of fading and shadowing in order to maintain a good link quality.

vi. Hence, CDMA is an interference limited system.

2. There are two types of power control in CDMA: Open loop and closed loop power control.

Open loop power control

i. In open-loop power control at the mobile user, the mobile user senses the received signal strength of the pilot Channel and can adjust it’s transmit power based on that. If the signal level of the pilot Channel is very strong, it can be assumed that the mobile user is very close to the base station. Therefore, the mobile-user transmit power level should be reduced.

ii. In open-loop power control at the base station, the base station decreases its transmit power level gradually and waits to receive the Frame Error Rate (FER) message from the mobile user. If the FER is beyond a specified level, the base station increases its transmitting power level on the corresponding forward traffic Channel.

iii. When the mobile user is switched on for the first time, it measures the received power from the cell-site assuming that the signal losses on the forward and reverse Channels are equal, and sets the transmitter power accordingly.

iv. The fundamental advantage of open-loop power control is that its operation is quite fast.

v. Open-loop power control is good for tracking median power levels and slow variations such as those due to shadowing. It can provide good coarse correction factors in the mobile transmit levels for these types of signal variations.

vi. For fast Rayleigh fading conditions, open-loop power control is not effective. The reverse Channel and forward Channel transmissions usually occur in separate frequency bands and there is virtually no correlation between fast fading effects on these transmissions.

Closed loop power control

i. With closed-loop power control of the user, the cell-site measures its received signal strength and then sends a corresponding command to adjust its transmit power to the desired level. At the mobile user, power-control information is received from the cell-site. This message indicates either an increment or decrement in the transmit power.

ii. The delay between measurements of the received signal level and implementation of power-control is a critical parameter in closed-loop power control. This delay can occur due to a number of reasons. To provide a reasonably accurate measurement of the received signal level, the measurement must be averaged over several symbol periods. Moreover, the power-control adjustment message must be multiplexed with the ongoing transmission.

iii. At the mobile receiver, the power-control message data should also be averaged over several symbols because the power-control adjustments are uncoded and thus less reliable.

iv. Typically, power-control adjustments are sent as a single bit command, for example, a logical 1 to decrease and a logical 0 to increase the transmit power by a predetermined value of about 1 dB. Sending these power-control bits on a frequent basis minimizes the delay between received signal measurement and power control implementation. In some systems, power control bits are sent at the rate of 800 Hz to 1500 Hz.

v. Due to the delay in the closed power-control loop, power control will not be able to compensate for the fast Rayleigh fading effects that occur at normal vehicle speeds. In fact, at high vehicle speeds, closed-loop power control may degrade performance. But interleaving provides a significant benefit at these faster fading rates, which may in turn compensate for the shortcomings of closed-loop power control mechanism.

vi. There may be a slower closed-loop power control that is based on frame error rate measurements. The FER measurements are often based on CRC included in each frame and are used for error detection after error correction decoding. Since the system is design to tolerate a specified FER, the cell-site monitors the frame rate to ensure that it is within the predetermined threshold value. If the error rate is very high then the mobile user is not getting the acceptable quality. If it is very low then the mobile user is generating an unacceptably high level of interference. If the error rate not in the desired range then the appropriate power-control adjustments are carried out taking into account this information.