Q.
Why is there an EVM difference between the 10MHz BW upstream WiMAX signal at 2.6GHz and a WCDMA signal at 1900MHz? The difference is about 10dB, -50dB vs. -40dB.
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A.
The WiMax signal has a bandwidth of 10MHz the WCDMA bandwidth is 3.84 MHz. In both cases, the IQ Demodulator outputs are ac-coupled, creating a high-pass corner. In the case of the wider band WiMax signal, a smaller fraction of the signal's energy is filtered out compared to the WCDMA carrier. In addition, there is no sub-carrier at the center of a WiMax carrier. So there is less energy to be lost at the center of the carrier. This makes the WiMax modulation scheme conducive demodulation via Zero-IF direct conversion. This is not the case for WCDMA which is a spread spectrum signal not composed of sub carriers.
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Q.
In Figure 83 (WiMax), the EVM degrades at approximately -5 dBm; in Figure 84, the EVM remains flat up to a much higher level. Why is this?
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A.
As the input power to the IQ demodulator increases, increased distortion degrades EVM. An OFDMA WiMAX signal has an 11dB Crest Factor. So although the modulated power is -5dBm, the carrier will have peaks that can be as large as +6 dBm. In contrast to this, a reverse link WCDMA carrier (i.e. from mobile to basestation) will have a much lower crest factor, resulting in a flat EVM profile up to higher input power levels.