Welcome to the World of CDMA
The Mobile Propagation Environment
A cellular mobile may be anywhere in the cell from right
under the base station antenna tower to as much as perhaps
30 km distant. In an environment where propagation law is
R-4, as is typical of cellular service areas, the
total dynamic range of path loss is on the order of 80 dB.
With a typical link budget for an IS-95A system, this means
that the mobile transmitter must vary its power from about
2.5 nW to 0.25 W
Reverse Link Power ControlOpen Loop Control The wide dynamic range is handled by an open loop power control technique. The mobile estimates the path loss to the cell by measuring the received signal level. Typically this measurement is based on an analog AGC (automatic gain control) voltage. The receiver AGC loop attempts to hold constant the mean square signal level from its analog-to-digital converters, that is, the total power entering its 1.25 MHz IF passband. This power includes everything entering the receiver front end: signal, thermal noise, and interference. The measured front end power is adjusted by a closed loop correction and then used to control the mobile transmit power in accordance with the relationship, prescribed in the cellular air interface:
Note that this is a reciprocal relationship. As the receive
power increases, the transmit power decreases. Their product,
having dimensions of mWatts2, is numerically equal
to the turnaround constant. In units of dB relative to (1
mW)2, the turnaround constant is -73. The specified
turnaround constant for PCS, at about twice the frequency,
is -76 dB (mW2).
Closed Loop Control
Closed loop power control is a sort of "fine tuning"
on the open loop power estimate. The cell measures the received
Eb/N0 and compares it to a set point
(which may itself be adjusted dynamically, but that is a cell
function). If the measured Eb/N0 is
above the set point, then a "down" command is sent;
if below, an "up" command is sent. The mobile adjusts
its power up or down, relative to the open loop estimate,
by about one dB for each command. There is no "do nothing"
command to keep the commands to one bit. A steady "do
nothing" decision has to be transmitted as alternating
up-down commands. The commands are sent once per 1.25 ms,
or a rate of 800 corrections per second.
During soft handoff it is crucial that the mobile transmit
power be controlled by the cell that is receiving the best
signal, so that the minimum necessary power is transmitted.
This is a key requirement if the maximum overall system capacity
is to be achieved. For this reason, each cell and sector participating
in a soft handoff makes a separate determination of the power
control bit to be sent. The mobile processes them separately,
and performs an "or of the downs" logic operation.
That is, if any of the soft handoff participants (there can
be more than two) says "down" the mobile reduces
Forward Link Power ControlThe requirements on the forward link are less severe than those of the reverse link. While the path loss undergoes the same large variations due to fading and shadowing, as long as the total signal level is adequate, because the own-cell, other user interference arises from the same source, the desired signal and interfering signals tend to fade together.
The same is not true of the foreign cell interference, however, when the potential for harmful interference exists, there will have been a soft handoff initiated, so that an alternative forward link exists.
Support for forward link power control differs between the cellular and PCS air interface standards. IS-95A and Rate Set 1 of J-STD-008 specify only messaging-based forward power control. That is, when the mobile station concludes, because of excessive frame error rate, that its forward signal quality is poor, it sends a report to the base station. This method is relatively slow, being impacted by processing delay in the message parsing by the base station. Rate Set 2, the 14,400 bps set, incorporates a rapid forward power control mechanism. Each reverse traffic frame incorporates a bit that reports erasures with a slight (2 frame) processing delay. This permits a faster, and hence tighter, forward power control.