Controlling Power in CDMA Systems

The effectiveness of CDMA as a method to increase call capacity is partially determined by how effectively the transmitted power of each mobile can be controlled and minimized. The goal of the mobile is to transmit its signal with the least amount of power capable of producing a detectable signal at the base station. This will increase battery life as well as system capacity. Any more power will simply be added noise to the system, which will reduce capacity.

In order to accurately control the mobile's power on the reverse link, two methods are used: open loop power control and closed loop power control.

Open loop power control
Open loop power control assumes that the path loss is the same on the reverse path as it is on the forward path. The mobile determines its correct transmit power by measuring the power it receives from the base station and subtracting that from a constant (-73 dBm for cell band). The open loop power control response for cellular and

Cellular Equation
Mobile Transmit Power = -73 dBm – Power received PCS frequencies are described by the following equations:

from Base Station

PCS Equation
Mobile Transmit Power = -76 dBm – Power received from Base Station

The open loop power control process is most important when the phone is in an idle state (not on a call).

Closed loop power control
Closed loop power control accompanies the open loop power control process when the phone is in the connected state (on a call). The base station monitors the received signal from the mobile and determines if the signal is "just good enough to maintain a link."

Every 1.25 ms the base station commands the mobile to raise or lower its transmit power by a step of 1 dB based on the signal it received from the mobile. Therefore, the power transmitted from the mobile is never constant for more than 1.25 ms (see figure 1).

The base station sends the closed loop power control instruction to the mobile by replacing a bit of voice data with a power control bit. For instance, if a 1 bit is received, the mobile knows to reduce its power by 1 dB. If a zero bit is received the mobile knows to increase its power by 1 dB. This process is called "puncturing."

Note that if all variables are constant (mobile is not moving and nothing happens to change the power measurement), then the mobile's power will increase and decrease by 1 dB every 1.25 ms. Even though you might think this is unusual, it is an acceptable condition for CDMA systems, primarily because the increase/decrease commands are coming at 800 times per second.

CDMA versus analog
Since the mobile's power is controlled to be no more than is needed to maintain the link at the base station, a CDMA mobile typically transmits much less power than an analog phone. Analog cellular phones need to transmit enough power to maintain a link even in the presence of a fade. As a result, analog phones transmit too much power most of the time.

CDMA radios, on the other hand, are kept at a power level to just maintain a quality transmission based on the changing RF environment. This has the benefit of longer battery life and smaller, lower cost amplifier design.

Karen Otto is a Customer Education Engineer at Hewlett-Packard's (HP's) Spokane Division. In her position at HP, Karen is responsible for designing training material for cellular and PCS test systems. Before coming to HP, Karen was a design engineer in the television industry, where she worked on picture-in-picture, IDTV, and MPEG satellite decider projects. She has a BSEE from Purdue University and can be reached at karen_otto@hp.com.