Technical data

Chapter 12 377
GSM and EDGE Analyzer
Interpretation of GSM and EDGE Analyzer Measurement Results
GSM and EDGE Analyzer
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 (C/I)” on page 378.
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