GSM Measurements of Mobile Stations (UE): 3GPP Conformance Tests

GSM measurements provide critical data about network performance, helping operators optimize services and troubleshoot issues. This document describes measurements for GSM mobile stations (i.e., UE) in idle and connected modes in layer 1 (Physical Layer). It also covers conformance level measurements of GSM mobiles, including RSSI and BSIC measurements.

Transmitter measurements include Frequency error, phase error, transmit output power, spectrum, modulation accuracy, and I/Q gain offset. Receiver measurements include Frame Erasure ratio, Block Error measurement, Receiver Sensitivity, Received signal strength, Co-channel rejection, Adjacent channel rejection, Intermodulation rejection, RF Spurious emission, and AM suppression.

A GSM Mobile station comprises various layers; therefore, it requires extensive content to cover measurements for each of these layers (layer 1, RF part, layer 2, layer 3). This document will cover measurements related to the RF (Radio Frequency) and physical layer of the Mobile Station.

RSSI Measurements in IDLE and Connected Mode

• When a user switches ON the GSM mobile, it will be in idle mode until it connects to the network and enters connected mode. There are two ways a mobile can be brought to connected mode: network-initiated and mobile-initiated.
• In idle mode, the mobile performs frequency synchronization using the frequency sync burst transmitted by the BTS over FCCH. It also performs time synchronization with the GSM hyperframe using the synchronization burst transmitted by the BTS over SCH. The SCH carries 25 useful bits, 19 of which are for the reduced frame number (RFN), and 6 bits are for the BSIC. The BSIC is made up of BCC (3 bits) and NCC (3 bits). The RFN provides information on T1, T2, and T3, which in turn provide information on the frame number.
• During Radio Frequency planning in GSM network deployment, each BTS is uniquely allocated a BSIC, other than ARFCN. In a GSM network, each cell will be provided one broadcast downlink frequency and one broadcast uplink frequency. There are one or more downlink and uplink frequencies allocated for traffic channels. Broadcast frequencies are used for signaling, while traffic channels are used for voice/data packets.
• RSSI measurements on ARFCNs (mainly broadcast frequencies) are performed. This is done both in idle and connected modes on active and neighbor cells. BSIC (Base station identification and reconfirmation) is also performed on active and neighbor cells. BSIC helps the mobile differentiate two similar broadcast frequencies from different cells.
• In GSM systems, neighbor cell measurements are mainly performed in idle frames or during unused time slots.

GSM Mobile Baseband/RF Conformance Measurements

The following are the RF (Radio Frequency) and baseband (layer-1) related measurements for GSM mobiles as mentioned in the 3GPP conformance and other documents.

GSM Mobile Transmitter Measurements

1. Frequency Error: Should be less than about 0.1ppm compared to the Base station RF carrier frequency.
2. RMS Phase Error: Should not be greater than about 5 degrees. Read the 3GPP standard specified below for details.
3. Transmit Output power: The instantaneous power versus time depends on the power class of the UE (user equipment or mobile), modulation technique (GMSK, 8PSK), and frequency bands, including GSM850, GSM900, DCS1800, and PCS1900.
4. Output Power control limit: Output power control can be varied based on varying levels of attenuation.
5. RF power spectrum: A power spectrum mask is defined in the standard, which specifies various power limits at different frequency offsets from the RF center frequency.
6. Modulation Accuracy in EVM(rms): Measures the accuracy of the modulated baseband signal (symbol vector).
7. I/Q gain offset: Gain offset between real and imaginary signals in the modem.

• Note: For details on the limits above, read 3GPP standard for conformance specifications.

GSM Mobile Receiver Measurements

1. Frame Erasure ratio or Bad Frame Indication
2. Block Error measurement
3. Receiver Sensitivity
4. Received signal strength
5. Co channel rejection
6. Adjacent channel rejection
7. Intermodulation rejection
8. RF Spurious emission
9. AM suppression

Equipments for GSM Measurements

• GSM/EDGE signal generation and analysis toolkit and measurements suite from NI: https://sine.ni.com/nips/cds/view/p/lang/en/nid/209228
• Wireless communications test Equipment (model-8960, series-10) with GSM/GPRS/EGPRS test application suit (E1968A) from Agilent Technologies: www.agilent.com/
• Test and measurement solutions for GSM, GPRS, and EGPRS (Model MD-8480C) from Anritsu corporation: www.anritsu.com/
• VSG and VSA (various models: SMU200A, CMW-500 and CMU200 available) from R&S: www.rohde-schwarz.com

References

• 3GPP, TS51.010-1, version 10.2.0, Release-10, MS conformance test document.

Conclusion

GSM measurements are indispensable for maintaining high-quality mobile services, offering valuable insights into signal strength, quality, and network efficiency. By monitoring these key metrics, network operators can make informed decisions to optimize performance and address any issues, ensuring seamless communication for users. These measurements play a crucial role in sustaining the reliability and effectiveness of GSM networks.