Long term evolution (LTE) and WiMAX are the two emerging technologies dominating for the fourth generation (4G) of mobile networks. The in-band and out-of-band emission limits are specified in these two systems, however these relatively new standards still lack mathematical descriptions on spectrum regrowth. Previously, the authors analysed and predicted the spectrum regrowth caused by the nonlinearity of RF power amplifiers related to their intermodulation parameters for two OFDM based signals: Wi-Fi and digital broadcasting. In this reported work, this technology is advanced further to LTE and WiMAX signals and verified by the validity of the theoretical result derived by real experiments. This provides insight for designing power amplifier and digital predistortion in terms of out-band spectrum regrowth.