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Why should error vector magnitude be measured on TDMA cellular phones?

Error Vector Magnitude (EVM) is the measurement specified by the EIA/TIA for measuring modulation accuracy on the NADC TDMA pi/4 DQPSK signal. This is specified in section 2.1.3.3.1.3.1 in the EIA/TIA IS-54 specification. The EVM for an NADC TDMA signal is specified to be less than 12.5% rms. Radios not meeting this specification will likely not work properly on the system, and may have poor voice quality or just not work at all. This measurement is in the same class of measurement as the analog modulation quality measurements of FM deviation and FM distortion. If you could not measure EVM you would have no way of knowing if the transmitter is performing correctly.

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