Output Radio Frequency Spectrum (ORFS) Measurement

RFmx GSM 19.1 Help

Edition Date: October 2019

Part Number: 375414H-01

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RFmx GSM 19.1 Help
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Output radio frequency spectrum (ORFS) is a frequency-domain measurement in which the spread of GSM signal power outside the channel is assessed. The spectrum of GSM is influenced by two major factors, namely, modulation in the transmitted signal and the switching transient of the GSM transmitter.

The digital modulation incorporated in the GSM signal, for example, GMSK, 8-PSK with 3π/8 rotation, causes the spectrum of the modulated carrier wave to spread to the adjacent channel. To test the effect of adjacent channel power, you can perform what is known as a spectrum due to modulation measurement.

GSM has bursty signals, so the transmitter quickly ramps signal power up at the beginning of the burst and then down at the end of the burst. Also, the transmit power is switched on and off in accordance with the presence of active and idle time slots. These switching intervals may cause the distribution of transmit power to some unwanted spectral component. You can assess these unwanted spectral components of the spectrum using a spectrum due to switching measurement. For more information on GSM ORFS measurement and best practices refer to the Output RF Spectrum (ORFS) section in the Introduction to GSM Device Testing application note[1].

You can configure RFmxGSM ORFS measurements to use a predefined list of offset frequencies at which the power spectrum of the signal is assessed. These lists include standard and short offset frequency mode. For a spectrum due to modulation measurement, the standard mode contains the offset frequencies specified in section 13.4.4 of 3GPP TS 51.010-1 specifications.  

ListModulation Offsets (kHz)Switching Offsets (kHz)
Standard±100, ±200, ±250, ±400, ±600, ±800, ±1000, ±1200, ±1400, ±1600, ±1800±400, ±600, ±1200, ±1800
Short±200, ±250, ±400, ±600, ±1200, ±1800±400, ±600, ±1200, ±1800

Alternatively, you can specify the set of offset frequencies for which the ORFS measurement is performed. This feature is known as custom frequency mode.

For frequencies greater than or equal to 1800 KHz the measurement bandwidth is 100 KHz, as defined in 3GPP TS 45.005 specifications. For all other frequencies the measurement bandwidth is 30 KHz.

Evaluation Symbols

Evaluation symbols are applicable to spectrum due to modulation measurement. GSM standard specifies that 50% to 90% portion of the burst excluding the midamble is used for measurement. However, RFmx GSM allows you to specify which portion of the burst to use for the spectrum due to modulation measurement. RFmxGSM considers the measurement over evaluation symbols for a single burst as one average.  


[1] NI RF Academy. (2014, February 01). Introduction to GSM Device Testing. Retrieved March 10, 2015, from http://download.ni.com/evaluation/rf/Introduction_to_GSM_Device_Testing.pdf



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