Power control methods in CDMA

Power control are two types: Forward link power control and reverse link power control

Forward link power control: 

Forward link power control is a loop control and the controlled target is the transmit power of base station. Mobile station plays assistant role.

Fast power control can divide into outer loop power control and closed loop power control. This is a slow process and less efficient.  Both loop work together. The base station continually and slowly decreases power to each mobile station (each user forward traffic channel). MS determine FER and based on that forward traffic channel decrease or increase.

Reverse Link power Control:

Reverse link power control is more accurate. It control mobile transmit power and base station plays a assistant role. It included three types of power control:

1)      Open loop power control

2)      Close loop power control

3)      Outer loop power control

1). Reverse link Open loop power control

Reverse open loop power is mobile station controlling it’s transmit power. Estimating how strong the mobile station should transmit based on a coarse measurement of how much power it is receiving from the base station. Problem in open loop control is that It Assumes same exact path loss in both directions; therefore, cannot account for asymmetrical path loss and Estimates are based on total power received; therefore the power received from other cell sites by mobile station introduces inaccuracies. It uses formula Tx+Rx-Tx_adj = -73dBm.

2). Reverse link close loop power control

It compensates for asymmetries between the forward and reverse paths.

It Consists of power up (0) & power down (1) commands sent to the mobile stations, based upon their signal strength measured at the Base Station and compared to a specified threshold(set point).

Each command requests a 1dB increase or decrease of the mobile station transmit power.

It transmitted 800 times per second, always at full power.

It allows compensating for the effects of fast fading.

3). Reverse link outer loop power control

Base station controller (BSC) determines FER on reverse traffic channel and accordingly set point threshold value is changed.  It will increase reverse capacity and improve voice quality.

Why power control required in CDMA system

CDMA is an interference limited system based on the number of user. Here each user is a noise source on shared cannel. Due to this CDMA system practically has a limit of users who can sustain, this is called soft capacity limit.

Near Far effect is basic feature of CDMA. If we assume that mobile transmit power is same for all user then mobile user near a cell jams a user that is distant from the cell. This problem may be present despite high processing gain. So an effective method to eliminate the near-far effect is necessary.

CDMA system uses power control technique to keep each MS at absolute minimum power level necessary to ensure acceptable service quality. Power control is essential for adjusting BTS and MS transmit power instantly according to communication distance (to overcome near far effect).  Ideally the power received at the base station from each mobile station should be same (Minimum signal to interference).