sedighe,
Late in the design of the HPRF (High Power RF, the RF PA deck) board for our P4 WiMAX product, we discovered insufficient capacitance for low frequency bypass on the Vcc (Drain voltage or Bias supply) which required re-work to add capacitance ... after adding additional Vcc bypass capacitance the 'spectral mask' performance improved, including EVM. It is critical to provide adequate capacitance to the drain bias supply as the 'peak' current draw at various points in time can be quite high with a multi-carrier OFDM signal.
Please let us know the result if this works to improve the situation.
BTW, adding capacitance affected the series 'switch' that was used to switch the DC to each of the HPRF boards, and necessitated the incorporation of "slow-start" circuitry to ease the rush of current when the fairly high-valued Vcc bypass caps charged during HPRF turn on otherwise our power supply output voltage would 'dip' and present other system-level problems (like uC reset).
Jim