RF Analog Impairments Modeling for Communication Systems Simulation: Application to OFDM-based Transceivers

3.8 Summary

In this chapter we simulated and studied the impact of the RF analog impairments, theoretically described in Chapter 2, on WiFi and mobile WiMAX zero-IF transceivers. Both use OFDM modulation, but with different subcarrier spacing, which makes them interesting for comparing sensitivity differences to frequency errors.

The WiFi subcarrier spacing is 312.5 kHz versus 10.9375 kHz for WiMAX. Because WiFi only uses 20 MHz channel bandwidth, we considered the 20 MHz channel parameters for WiMAX.

Some RF analog impairment impacts on the OFDM signal are comparable in transmission and reception:

• DAC and ADC resolution and clipping;
• phase noise;
• IQ mismatch.

The DAC/ADC resolution and clipping and IQ mismatch have the same impact on WiFi and WiMAX signals; their specifications are imposed by the 64-QAM EVM requirement. Regarding the EVM introduced by phase noise, because the WiFi subcarrier spacing is larger than typical RF PLL bandwidths used in modern transceivers, the CPE can be tracked and then compensated, improving the EVM. On the other hand, with WiMAX having a subcarrier spacing smaller than classical PLL bandwidths, ICI dominates and the EVM is directly given by the integrated phase noise.

In transmission, the PA distortion is the major bottleneck requiring a tradeoff between maximum output power and low EVM dictated by high-order 64-QAM modulation. Since WiMAX and WiFi signals have the same PAPR statistics, they impose the same linearity constraints for the PA in ...