PREDISTORTION Linear Power Amplifier Uses Mirror Predistortion

Current wireless c o m m u n i c a t i o n standards have been established to carry voice, data and video with high data rates. These high data rates require efficient modulation techniques such as OFDM 64 QAM, to allow transmission in a given finite bandwidth. With these types of modulation, there are probabilities that at certain times, the voltages of the multiple carriers are in phase and their amplitudes add up to create a very high peak to average ratio (PAR) that may reach 15 dB. This high PAR will drive a conventional PA into saturation; causing signal distortion and generating out-band interference. One way to solve this problem is to use a high power PA in Class A mode, with average power set at a back off (BO) of 10-12 dB. Unfortunately, this technique results in a very inefficient PA. There are various linearization techniques that are used to increase the linearity of the PA allowing the use at lower BO values and consequently having higher efficiency. These techniques include feedback, feedforward, digital predistortion or analog predistortion [1, 2]. Amongst all, the analog predistortion (Figure 1) tends to provide a good compromise between complexity, bandwidth and linearity improvement. Several methods are available to implement an analog predistorter circuit. One method is to use a simple diode circuit in front of the PA and to choose its size and bias to compensate for its AM-PM nonlinearity. Although this technique is simple and could be easily integrated with a PA in one package, the linearity improvement reported is very limited and does not track well with higher output power levels [3, 4]. Another way is to use Cuber Predistortion [5, 6] where higher improvement in IMD3 could be achieved, but with an increase in the fifth and higher order nonlinearities. In [7, 9] the IMD3 and IMD5 are independently improved by using two Cuber Predistorter circuits but with an increased level of complexity. A simpler technique is suggested in [10] to improve both of IMD3 and IMD5 by using a new Cuber PD design. All of the predistortion techniques require a full nonlinear characterization of the phase and magnitude of the IM3 and IM5 for both the predistorter circuit and the power amplifier. This characterization requires a special measurement techniques like those reported in [11, 12]. The feedforward linearization method provides higher levels of IMD improvement up to the amplifier’s P1dB but requires the use of an additional Error Amplifier (EA) that is required to be very linear and consequently This article describes a hybrid power amplifier linearization method that combines analog predistortion and feedforward design concepts