Interpretation of GSM and EDGE Analyzer Measurement Results

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This section contains expected result values and possible causes of error if the expected results are not met.

Frequency Error

The GSM standard specifies very tight frequency error performance. In the GSM 900 band, the 0.05 ppm (parts-per-million) specification translates to only 45 Hz error.
Frequency error measurements indicate synthesizer/phase lock loop performance. This is especially important in a BTS with frequency hopping active. Poor frequency error measurements can show, for example, that a synthesizer is failing to settle quickly enough as it shifts frequency between transmissions. Poor frequency error can cause a multitude of problems, for example, the target receiver may be unable to gain lock, and the transmitter may cause interference with other users.
You will need to use an external reference or GPS (if available) in order to achieve an accurate frequency error reading.

BSIC (Base Station Identity Code)

If the BSIC can not be decoded, dashes will be displayed. If it is not as expected, check the base station settings against the plan.

TS Pwr (Timeslot Power)

Maximum Timeslot Power is the measure of in-channel power for GSM Systems. The purpose of the Maximum Timeslot Power measurement is to determine the power delivered to the antenna system in the radio frequency/channel under test. The Maximum Timeslot Power measurement verifies the accuracy of the transmitted RF carrier power.
If the maximum timeslot power is lower than expected, verify you have a good connection to the RF output of the base station. Also, verify the cable you are using to connect to the base station is not faulty. A low power may also indicate an issue with the power amplifier.
Inaccurate channel power (high or low) may indicate an incorrect power setting at the base station.

Phase Error and I/Q offset

Phase error (not EVM) is the fundamental measure of modulation quality in GSM systems. Since GSM systems use relative phase to transmit information, the phase accuracy of the GSM transmission is critical to the system's performance, and ultimately to its effective range. EVM is not the primary metric for GSM signals because GMSK modulation uses constant amplitude.
The phase error is computed by measuring the test signal and computing the difference between the phase of the transmitted signal and the phase of a theoretically perfect signal.
Poor phase error or I/Q offset could indicate a problem with the I/Q baseband generator, filters, or modulator in the transmitter circuitry. The output amplifier in the transmitter can also create distortion that causes unacceptably high phase error. Poor phase error will reduce the ability of a receiver to correctly demodulate signals, especially in marginal signal conditions. This ultimately affects range.
The Phase Error metrics are only reported for GSM signals with their GMSK modulation. Phase Error is not reported for EDGE signals, with their 8PSK modulation, since this measurement of signal quality is reported as part of the Error Vector Magnitude metric. For more details on Error Vector Magnitude measurements with EDGE signals, see below.
Another measure of quality similar to EVM is Modulation Error ratio (MER), which is sometimes referred to as Carrier to Interference Ratio (C/I). For further details on MER and C/I, see "Modulation Error Ratio (MER) and Carrier to Interference Ratio" (below)
RMS EVM: This is the RMS of the EVM (Error Vector Magnitude). EVM is measured and calculated over the useful part of the burst, and is shown as a percentage. The EVM figure represents the difference, in both phase and magnitude, between a theoretically perfect signal and the actual signal received. The EDGE standard specifies the RMS EVM not to exceed 7.0% during the useful part of the EDGE burst, when under normal operating conditions, and excluding the effect of passive combining equipment. RMS EVM may go as high as 8% under extreme operating conditions, again excluding the effect of passive combining equipment.
The EVM metrics are only reported for EDGE signals with their 8PSK modulation. EVM is not reported for GSM signals, with their GMSK modulation, since this measurement of signal quality is reported as the Phase Error metric.
Pk EVM: This is the RMS of the EVM (Error Vector Magnitude). EVM is measured and calculated over the useful part of the burst, and is shown as a percentage. The EVM figure represents the difference, in both phase and magnitude, between a theoretically perfect signal and the actual signal received. The EDGE standard specifies the Peak EVM not to exceed 11.0% during the useful part of the EDGE burst, under all operating conditions, and excluding the effect of passive combining equipment.
The EVM metrics are only reported for EDGE signals with their 8PSK modulation. EVM is not reported for GSM signals, with their GMSK modulation, since this measurement of signal quality is reported as the Phase Error metric.
95%tile EVM: This is the 95th Percentile of the EVM (Error Vector Magnitude). In other words, it is the EVM value below which 95% of all measurements fall. EVM is measured and calculated over the useful part of the burst, and is shown as a percentage. The EVM figure represents the difference, in both phase and magnitude, between a theoretically perfect signal and the actual signal received. The EDGE standard specifies the 95th Percentile EVM not to exceed 11.0% during the useful part of the EDGE burst, under all operating conditions, and excluding the effect of passive combining equipment.
The EVM metrics are only reported for EDGE signals with their 8PSK modulation. EVM is not reported for GSM signals, with their GMSK modulation, since this measurement of signal quality is reported as the Phase Error metric.

Modulation Error Ratio (MER) and Carrier to Interference Ratio (C/I)

Modulation Error Ratio (also referred to as Carrier to Interference Ratio) is a measure of signal quality that is similar to EVM. A measured signal can be modeled as the sum of an ideal signal and an error signal. MER represents the ratio of the ideal signal's power and amplitude to the error signal's power and amplitude. The result is usually expressed in dB.
Although the Agilent E7495A/B base station test sets do not display the MER (or Carrier to Interference Ratio) results, they can be calculated from the EVM result using the following equation:
Modulation Error Ratio or Carrier to Interference Ratio = -20 * log(EVM/100) dB
Example: If you measure a signal and find its EVM to be 2.56%, the calculation to find the MER or C/I is as follows:
MER (or C/I) = -20 * log(EVM/100) dB
= -20 * log(2.56/100) dB
= -20 * log 0.0256 dB
= -20 * -1.59176 dB
so EVM of 2.56% is equivalent to MER (or C/I) = 31.84 dB