Chapter 3 3-3
Making GSM Base Station Measurements
Making the Output RF Spectrum (ORFS) Measurement
Making the Output RF Spectrum (ORFS)
Measurement
Purpose
The Output RF Spectrum (ORFS) measurement is GSM’s version of
adjacent channel power (ACP). Either a single offset is measured with
the corresponding trace visible or multiple (up to 15) offsets are
measured and a table is displayed. It is also possible to measure output
RF spectrum as a swept measurement.
The output RF spectrum measurements determine the spectral energy
emitted into the adjacent channels. Since GSM is a TDMA format, RF
power is being switched on and off depending on whether the actual
burst is being transmitted. The switching of power causes spectral
splatter at frequencies other than that being transmitted by the carrier.
Fast transitions in the time domain causes switching transients that
have high frequency content associated with them.
Excessive amounts of energy spilling into an adjacent frequency
channel could interfere with signals being transmitted to other MS or
BTS. The measurements are divided into two main groups: spectrum
due to the 0.3 GMSK modulation and noise, and spectrum due to
switching transients (burst ramping).
NOTE The default output RF spectrum measurements do not perform tests at
frequency offsets greater than 1800 kHz from the carrier.
Measurement Method
In this measurement, the transmitter (source) is set to transmit a GSM
frame at a given channel (frequency). The instrument acquires a time
record at a particular offset from the channel being transmitted. When
the offset is zero, the instrument is said to be measuring the carrier. For
a given offset frequency from the carrier, the transmitter must not
exceed a certain power level relative to the carrier. The GSM
specification defines the offsets and their maximum absolute and
relative power levels.
The general steps in making the measurement are as follows:
1. Acquire time record.
2. Measure power of the carrier.
3. Synchronize for gating on the carrier - finds 50% and 90% portion of
burst for Spectrum Due to Modulation portion of the test
4. Compare each offset power to reference to get relative power level.
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