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

3.4 WiFi OFDM and Mobile WiMAX Signals PAPR

As we have seen in Section 2.8.2, OFDM signals used in communication systems look like Gaussian noise in the temporal domain because they are composed of a large number of modulated subcarriers; as a result, their instantaneous PAPR can be very high. The direct impact of this large PAPR concerns the transceiver linearity, which has to be very good in order to limit the signal distortions which degrade the EVM and generate out-of-band emissions. Consequently, in transmission the PA may require a large back-off in order to operate in its linear range, leading to poor efficiency, and in reception we have to carefully control the gain settings and limit or prevent ADC clipping.

Figure 3.7 presents the PAPR complementary cumulative distribution function (CCDF) for WiFi and WiMAX signals estimated from the simulation bench previously described. Although WiMAX employs many more subcarriers than WiFi, 2048 vs. 64, we can see that their PAPR statistics are similar because, in both cases, the number of subcarriers is large enough to apply the central limit theorem approximation. This result is quite interesting because it shows that, for the same EVM performance, WiFi and WiMAX transceivers will have the same relative linearity requirement in terms of back-off. From these curves we can see that the probability of having a PAPR higher than 8 dB is around 10⁻³, and is lower than 10⁻⁷ for a PAPR superior to 12 dB. It is important to keep in mind that ...