Requirements for Good Communications Link Performance: IQ Modulation and Demodulation


In the last article, we discussed the quandary of whether it’s better to use digital or analog means to combine and separate I and Q.

Here, we’ll pick up the topic again by characterizing the requirements necessary for good communication link performance in these analog and digital applications.

 

Requirements for Analog IQ Modulator and Demodulator

Table 1 shows the results of a literature survey of articles describing the degradation in communications links due to IQ imbalance. OFDM was the modulation for most of the articles.

For the following, please see the previous article for my list of references.

 

Table 1: Requirements for Analog IQ Modulator and Demodulator

 

Click to enlarge table to full size

 

It is possible to draw a few conclusions.

At the modulator (TX) end, IQ imbalances up to 1 dB and 5º give what might be acceptable degradation; 0.5 to 1.0 dB; even with 64-QAM OFDM subcarriers, at bit error rates of 10-4 to 10-5.

The situation is different at the demodulator (RX) end. For N-QAM subcarriers, IQ imbalances of 1 dB and 5º give degradations of many dB, according to most of the articles. For these imbalances, and N > 4, generally there is a bit error rate floor from 10-3 to 10-1. An imbalance of 0.5 dB and 1º gives degradations of ~8 dB for 16-QAM subcarriers at a bit error rate of 10-4.

There are no results given for imbalances less than 0.5 dB and 1º.  For N > 16, none of the articles show how small the imbalances must be for degradations to give a bit error rate floor less than 10-3.

This is unfortunate because integrated IQ modulators and demodulators are available with typical imbalances of ~0.02 dB and 0.2º. None of the articles explains why the demodulation is so much more sensitive to IQ imbalances than the modulation.

 

Requirements for DACs and ADCs

Requirements for DACs and ADCs Used at Baseband for Analog IQ Approach

The analog IQ modulator and demodulator approaches still require data converters in the baseband I and Q paths in position A in Figures 1A and 1B from our previous article.

 

Figure 1(a). Modulator

 

Figure 1(b). Demodulator

 

 

It is important to know how good these devices must be. The author found there is much less published on this effect than on the IQ imbalance effect. The best that could be deduced is that a Signal-to-Noise+Distortion Ratio (SINAD) of > 38 dB is required.

 

Requirements for DACs and ADCs Used at RF for Digital IQ Approach

The Digital IQ approaches require data converters in position D in Figures 1A and 1B. The author also found very little published on this subject. There have been some results published of the effects of analog amplifier effects such as 3rd Order Intermodulation Products (IP3) on communications systems, and it might be valid to also apply these to data converters, which has been done.

Generally, it seems a Signal-to-Noise+Distortion Ratio (SINAD) of > 50 dB and a Third Order Intermodulation Product of > 44 dBc are required for acceptable performance degradation.

 

Table 2. Baseband Data Converter Requirements

 

Table 3. RF or IF Data Converter Requirements



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