Adaptive predistortion system and a method of adaptively predistorting a signal

#The present invention is directed, in general, to electronic circuits and, more specifically, to an adaptive predistortion system, a method of adaptively predistorting a signal and a transmitter employing the adaptive predistortion system and the method.

#As the number of users of wireless communication systems increases, the requirements on these systems for reliability and efficiency increase. Among other things, there are two conflicting requirements in designing a radio transmitter, especially when the envelope of the transmitted signal varies. These requirements are high linearity in the transmitter characteristics and high power efficiency. A power amplifier is one of the key components of the transmitter that determines the linearity and power efficiency of the transmitter. Usually, high power amplifier efficiency is obtained at the expense of reduced power amplifier linearity and vice versa.

#In modern wireless communication systems, a spectrally efficient transmitted signal is preferable to support high data rate services using a given RF spectrum. If the transmitter characteristics are not linear enough, out-of-band emission can increase beyond the spectrum mask imposed by a standardization or regulatory body. Further, portable radio transceivers should be highly efficient in power consumption so as to provide extended operation time with a battery. A large portion of the total power available is consumed by the power amplifier. Therefore, it is important to use a power efficient power amplifier to conserve battery power. Linearization techniques may be considered as a possible solution to overcome tightened spectral mask requirements with acceptable power amplifier efficiency.

#For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

#FIG. 1 illustrates a system diagram of an embodiment of a transmitter having an adaptive predistortion system constructed in accordance with the principles of the present invention;

#FIG. 2 illustrates a block diagram of an embodiment of an adaptive predistortion system constructed in accordance with the principles of the present invention; and

#Referring initially to FIG. 1, illustrated is a system diagram of an embodiment of a transmitter, generally designated 100, constructed in accordance with the principles of the present invention. The transmitter 100 receives a transmit input signal Txin and provides a transmit output signal Txout. Further, the transmitter 100 includes a feedforward section having a raised root cosine filter RRCF coupled to an adaptive predistortion system APS, a digital to analog converter DAC, a low pass filter LPF, a quadrature modulator QM and a power amplifier PA having a nonlinear amplification characteristic. The transmitter 100 also includes a feedback section having a quadrature demodulator QDM, an anti-alias filter AAF and an analog to digital converter ADC coupled to the adaptive predistortion system APS. In an alternative embodiment, the transmitter 100 may also include in-phase and quadrature-phase paths for implementing a complex baseband signal model.

#The power amplifier PA produces an output signal that is not a linear function of its input signal due to its nonlinear amplification characteristic. This nonlinear amplification characteristic thereby causes distortion of the output signal. Predistortion may be a generally useful technique that may be employed to compensate for such a nonlinearity. A feedback path may be employed for tracking changes in an output parameter of the power amplifier PA that is associated with the nonlinear amplification characteristic. However, a dedicated feedback path may substantially increase the overall level of complexity.

#One solution for overcoming the complexity of a dedicated feedback path is to take advantage of a compressed mode of protocol, such as a Universal Mobile Telecommunications System (UMTS) protocol. In the UMTS protocol, a transmission gap may be inserted periodically or on a requested basis during normal transmission. This transmission gap may then be employed by a receive channel associated with the transmitter 100 wherein the receive channel may then serve temporarily as the required feedback path. In one embodiment of the present invention, the transmitter 100 may use either a periodic or requested transmission gap as a temporary feedback path.