Effect of gain expansion of MMIC power amplifier for high order modulation OFDM portable system

We propose a realistic complex output behavior of MMIC HBT power amplifier having the gain expansion effect for high order modulation M-QAM OFDM portable system, and we also prove that it is the effective method for reducing the bias current of power amplifier for OFDM portable system. The optimized gain expansion region is used for the amplification for peak level of OFDM signal. Therefore, bias current is allowed to reduce and it does not affect the degeneration of transmitted signal to noise ratio compared with that of typical power amplifier biased the sufficient current. To investigate, we use OFDM signal based on 802.16 mobile system with 2048(2/sup 11/) IFFT size, 10 dB peak to average power ratio (PAPR) at 0.01% CCDF and 10 MHz overall occupation bandwidth. The commercial fabricated HBT MMIC power amplifier with bias control circuit exhibits the maximum average output power of 26 dBm in both bias condition cases, operated in the 15 dBm(11dB backoff) of average channel output power. The near class B operation improves the power added efficiency by factor of 1.8 times at operation output power level, and even reduces the bias current as much as half of typical biased amplifier without a sacrificing the linearity of OFDM signal.