Power Control

 

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
.
In addition to the gross path loss dependence on distance, the loss may also vary rapidly due to multipath induced Rayleigh fading. Multipath fading happens when the signal arrives at the receiving antenna after traversing different routes, and from different directions. If the path length difference are small compared to a chip of the spreading, or about 244 m, then the wavelets interfere in a stochastic fashion, the resultant being a Rayleigh fading process. Such fading commonly causes fluctuations of 20 to 30 dB over a distance of one or two wavelengths, 30 to 60 cm. If the mobile is traveling at 30 m/s, a typical freeway speed, then the fade rate can exceed 100 Hz. Fading of this depth, though common in narrowband systems, is less likely in CDMA systems due to their ability to separate the multipath components in the Rake receivers.

Yet another consideration is that the fading processes for transmit and receive, while correlated, are not necessarily identical due to the 45 MHz (cellular) or 80 MHz (PCS) frequency difference between forward and reverse channel frequencies.

Reverse Link Power Control

Open 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:
Rx Power (dB) + Tx Power (dB) = -73 dB (mW²)

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).

Thus, when the mobile received power is, say, -90 dBm, as it might be in a nominal cell, the mobile transmitter power will be +17 dBm. It is perhaps curious that the CDMA mobile will, when close to a cell, actually be transmitting less power than it is receiving.

The commanded adjustments are transmitted to the mobile in the overhead messages. The are used to adjust the open loop power control for different sized cells and different cell ERP and receiver sensitivities.

One might question the wisdom of using the entire front end power for open loop control. However, the tradeoff was made in favor of fast response time. It is indeed true that the open loop estimate is often in error by several dB, but this is easily compensated by the closed loop correction. Nor could the problem be solved by eliminating the closed loop control and doing something more sophisticated by way of forward loop measurements. The closed loop correction would still be needed to handle the loss differences due to the frequency difference.

Closed Loop Control

Closed loop power control is a sort of "fine tuning" on the open loop power estimate. The cell measures the received E_b/N₀ and compares it to a set point (which may itself be adjusted dynamically, but that is a cell function). If the measured E_b/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.

This is a very fast control mechanism (800 dB per second rate-of-change), and has proven to work very well in practice. Measurements of E_b/N₀ errors in a large-scale testbed system showed that the errors were approximately log-normal, with a standard deviation of about 2 dB. One would expect log-normal distributed errors due to the fact that the adjustments are constant deltas in dB units.

While the closed loop control may not be fast enough to quite keep up with the very fastest fading, it is at those higher fading rates that the coding and interleaving are most effective. At lower fade rates the interleaving may be less effective, but then the power control is extremely robust.

The power control bits are sent uncoded and punctured into the forward traffic channel transmission in place of two symbols. The receiver, knowing where the puncturing is taking place, sets the Viterbi decoder metric to zero for those punctured symbols, so that they have only minimal effect on the forward link traffic error rate. The two symbols are coherently combined to make the power control decision, but otherwise unprocessed.

The decision to send the power control bits uncoded was, like the open loop control, also in the interests of fast loop response. Any coding of these bits would result in a processing delay and hence a more sluggish loop.

The dynamic range of the closed loop control is ±24 dB relative to the open loop estimate.

Soft Handoff

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 its power.

A special variation is permitted on this rule. In some circumstances the soft handoff participant sectors may be able to see one anothers' decisions and make a joint decision, that they then both transmit. This is normally the case when the soft handoff is between sectors of one cell. The same processing engine will be handling both branches of the handoff. In this case, the mobile is informed that the power control bits are identical. It can then do diversity combining and a single bit decision, rather than separate decisions and a logical "or".

Forward Link Power Control

The 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.

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Copyright © 1996-1999 Arthur H. M. Ross, Ph.D., Limited