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Mean transmitted RF carrier power

Purpose of measurement—what it proves

Output power is a fundamental transmitter characteristic and is linked directly

to range. GSM systems use dynamic power control to ensure that each link is

maintained sufficiently with a minimum of power. This gives two fundamental

benefits: overall system interference is kept to a minimum and, in the case of

mobile stations, battery life is maximized.

Therefore, output power is controlled within tight limits. If a transmitter

produces too little power, link performance is compromised; too much, and

interference to others might be too high and battery life too short.

Common practical transmitter implementations require output power calibration

in manufacturing to meet the GSM specifications (this allows low-cost

components to be used). This calibration process involves the construction of

a table of calibration factors for power steps and frequency. Power calibration

corrects for the effects of component variation.

Out-of-specification power measurements indicate a fault, usually in the power

amplifier circuitry or the calibration tables. They can also provide early indication

of a fault with the power supply.

Theory in pictures

Conceptually, the mean power measurement in GSM is straightforward. It is

defined as the mean power during the useful part of the GSM burst. The ETSI

and ANSI specifications define that in type approval (at least) test equipment

must be capable of deriving the correct timing reference by demodulating the

incoming signal, and gating over the useful part only.

Most base transceiver stations implement dynamic power control. This makes it

necessary to make multiple power measurements at several power levels and

several frequencies in order to test for proper operation.

147 useful bits

542.8µs

Time (µs)

not to scale

Power

(dBc)

Measure over "useful part"

Figure 7. Theory of mean transmitted RF carrier power

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 35 36

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Agilent Technologies Switch/Measure Manual de usuario Pagina 10