Recently, I have done a simulation and debugging of TD-LTE downlink EVM. Some questions about EVM are summarized as follows:
The EVM requirements in the specification are relatively easy to implement, but since the EVM value is the value of the entire digital and RF link. Since the generation of EVM is mainly from the analog domain, the nonlinearity of the RF power amplifier in the analog domain is its main source. Therefore, we generally have a strict EVM performance requirement in all designs of the digital domain. The benefit of doing so is obvious, to alleviate the design pressure required for EVM in the analog domain. It is convenient to compare changes in the digital domain.
My job is to find a way to convert the EVM's demand parameters into filter characteristic parameters, find some empirical values ​​or formulas, analysis methods, and so on. At present, no concrete conclusions have been drawn. Due to too many design factors involved, an obvious analytical method cannot be obtained.
Empirically speaking, the length of the EVM Filter is related. In the actual design, we found that as long as the filter length is long enough, the EVM is easy to satisfy. This conclusion is given by increasing the Group delay of the digital domain. In the system design, if the EVM does not meet the indicator, the only effective and fast method is to adjust the length of the filter.
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