As wireless communication speeds increase, the bandwidth and the dynamic range of transmission signals become larger. To minimize the quality degradation of the signal, high linearity is required for a transmitting apparatus. Moreover, what is required in terms of the downsizing of the apparatus, reductions in operational costs, environmental issues and the like is a power amplifier that operates in a highly efficient manner.
However, in a typical power amplifier, the linearity goes against the power conversion efficiency. When the power amplifier is operated in a linear region that sufficiently backs off from saturated power, the out-of-band distortion can be made smaller. This, however, decreases the power conversion efficiency significantly, leading to an increase in the power consumption of the apparatus. A distortion compensating circuit is therefore used to eliminate the nonlinear distortion that emerges when the power amplifier is operated in a high-efficiency nonlinear range.
A predistortion method, one of the distortion compensating methods, is a technique of multiplying the transmission signals by the inverse of the nonlinear distortion of the power amplifier in advance to improve the linearity of the output from the power amplifier.
A predistortion method that uses digital signal processing is referred to as a digital predistortion (DPD) method. A LUT-based DPD, one of the DPD methods, is widely known: For the LUT-based DPD, distortion compensation coefficients are kept on a look-up table (LUT) in a memory. What is stored at each LUT address assigned to the LUT is a distortion compensation coefficient that is based on the amplitude of the transmission signal. According to the LUT-based DPD, a LUT address is determined based on the amplitude of the transmission signal, a distortion compensation coefficient stored at the LUT address is read out, and the distortion compensation coefficient is applied to the transmission signal to produce a predistortion signal. Moreover, according to the LUT-based DPD, the distortion compensation coefficient is updated based on a signal (feedback signal) that is generated after a portion of the output from the power amplifier is fed back, thereby making the distortion compensation coefficient vary according to changes in the characteristics of the power amplifier as well as changes over the years.
What is known as a related technique is a nonlinear distortion compensation transmitting apparatus, which reduces the convergence time of distortion compensation coefficients, as well as a distortion compensating apparatus, which makes corrections in such a way that transmission signals do not go beyond the dynamic range of a DA converter.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2002-223171    [Patent Document 2] Japanese Laid-open Patent Publication No. 2001-251148
However, according to the LUT-based DPD, when noise components of a feedback loop are large or when large changes of the characteristics occur instantaneously as a low back-off operation of the power amplifier is performed, the accuracy of the updated distortion compensation coefficients decreases. It may take longer time for the distortion compensation coefficients to converge into the most appropriate value.