Manage Power-Level Difference Slot to Slot

The power levels and power masks are described in GSM 11.10-1. Compliance requires
the first time slot be set to maximum power (PMAX) and the second time slot to minimum
power, with all subsequent slots set to maximum. PMAX is a variable established
by the base station and establishes the maximum power allowed for handset transmit.
The handset uses received power measurements to calculate a second value and transmits
with the lower one.
This open-loop control requires a close, accurate link between received signal and
transmitted power, which in turn requires careful calibration and testing during production.
All TDMA transmissions (handset to base, base to handset) require transmit
burst shaping and power control to maintain RF energy within the allocated time slot.
The simplest form of power control is to use an adjustable gain element in the transmitter
amplifying chain. Either an in-line attenuator is used (for example, PIN diode),
a variable gain driver amplifier, or power rail control on the final PA.

The principal problem with this open-loop power control is the large number of
unknowns that determine the output power—for example, device gains, temperature,
variable loading conditions, and variable drive levels. To overcome some of the problems
in the open-loop system, a closed-loop feedback may be used.
The power leveling/controlling of RF power amplifiers (transmitter output stages)
is performed by tapping off a small amount of the RF output power, feeding it to a
diode detector (producing a DC proportional to the RF energy detected), comparing
the DC obtained with a reference level (variable if required), and using the comparison
output to control the PA and PA driver chain gain.
RF output power control can be implemented using a closed-loop approach. The RF
power is sampled at the output using a directional coupler or capacitive divider and is
detected in a fast Schottky diode. The resultant signal representing the peak RF output
voltage is compared to a reference voltage in an error amplifier. The loop controls the
power amplifier gain via a control line to force the measured voltage and the reference
voltage to be equal.
Power control is accomplished by changing the reference voltage. Although
straightforward as a technique, there are disadvantages:
 The diode temperature variation requires compensation to achieve the required
accuracy. The dynamic range is limited to that of the detector diode (approximately
20 dB—without compensation).
 Loop gain can vary significantly over the dynamic range, causing stability
problems.
 Switching transients are difficult to control if loop bandwidth is not constant.
An alternative control mechanism can be used with amplifiers employing square
law devices (for example, FETs). The supply voltage can be used to control the amplifier’s
output power. The RF output power from an amplifier is proportional to the
square of the supply voltage. Reducing the drain voltage effectively limits the RF voltage
swing and, hence, limits the output power. The response time for this technique is
very fast, and in the case of a square-law device, this response time is voltage-linear, for
a constant load.
The direct diode detection power control system has been satisfactory for analog cellular
systems and is just satisfactory for current TDMA cellular systems (for example,
GSM, IS54 TDMA, and PDC), although as voltage headrooms come down (4.8 V to 3.3
V to 2.7 V), lossy supply control becomes unacceptable.
CDMAand W-CDMArequire the transmitter power to be controlled more accurately
and more frequently than previous systems. This has driven R&D to find power control
methods that meet the new requirements and are more production-cost-effective.
Analog Devices, for example, have an application specific IC that replaces the traditional
simple diode detector with an active logarithmic detector. The feedback includes
a variable gain single pole low pass filter with the gain determined by a multiplying
digital-to-analog converter (DAC) The ADC is removed. A switched RF attenuator is
added between the output coupler and the detector, and a voltage reference source is
added. Power control is achieved by selecting/deselecting the RF attenuator and
adjusting the gain of the LPF by means of the DAC
The system relies on the use of detecting log amps that work at RF to allow direct
measurement of the transmitted signal strength over a wide dynamic range. Detecting log amps have a considerable application history in wide dynamic range signal
measurement—for example, spectrum analyzers. Recently the implementation has
improved and higher accuracy now results from improvements in their key parameters: slope and intercept. 54