Abstract:
There is disclosed a pre-distortion correction circuit for use in an RF transmitter having a transmit path for receiving and amplifying an RF input signal. The pre-distortion correction circuit modifies distortion in the RF output signal caused by an RF power amplifier in the transmit path. The pre-distortion circuit comprises: 1) feedback circuitry coupled to an output of the transmit path for demodulating and digitizing the distorted RF output signal to thereby produce a first demodulated digital output signal; and 2) a pre-distortion calculation controller coupled to the feedback circuitry for comparing the first demodulated digital output signal to adjacent channel power (ACP) profile data stored in the pre-distortion calculation controller and generating therefrom pre-distortion correction values. The pre-distortion correction values are used to pre-distort the RF input signal to thereby cause the RF output signal to more closely resemble an ideal RF output signal within the limits of the ACP profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to that disclosed in co-pending U.S. patent application Ser. No. 09/224,193 for “ADAPTIVE DIGITAL PRE-DISTORTION CORRECTION CIRCUIT FOR USE IN A TRANSMITTER IN A DIGITAL COMMUNICATION SYSTEM AND METHOD OF OPERATION,” filed on Dec. 30, 1998. U.S. patent application Ser. No. 09/224,193 is hereby incorporated by reference in the present disclosure as if fully set forth herein. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to wireless networks and, more specifically, to an adaptive digital pre-distortion correction circuit for use in an RF transmitter. 
     BACKGROUND OF THE INVENTION 
     Every wireless network base station has an RF power amplifier for transmitting voice and/or data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, pagers, and the like) and a receiver for receiving voice and/or data signals from the mobile units. The design of an RF power amplifier (PA) for digital radio systems is controlled by two overriding criteria: 1) The RF power amplifier should transmit sufficient RF output power to serve the cell site of the base station in which it is installed, but should also use the minimum amount of input DC power in doing so; and 2) The adjacent channel power (ACP) noise (distortion) should be under certain limits (mask), that are usually defined in a standard (i.e., ACP profile) 
     In most cases, these two criteria are contradictory. ACP noise results from non-linear effects, such as over-driving the power amplifier into its non-linear region (clipping). Spurious spectral components are introduced when a signal peak is sufficiently large to over-drive or to saturate an RF amplifier in the transmitter. In order to meet the ACP profile, the RF transmitters in wireless networks in which digital signals have high peak-to-mean ratios, such as CDMA and multi-carrier systems, are frequently “backed off” from full power (or peak power) to avoid operating the transmitter in non-linear conditions. In these digital systems that have high peak-to-mean signal ratios, the RF power amplifier thus needs a considerable amount of power “headroom” to accommodate the peak power. For example, RF power amplifiers in some CDMA systems need more than  10 dB of headroom space to protect the peak CDMA signal power from clipping. Unfortunately, leaving this much overhead significantly reduces the power efficiency of the RF power amplifier. This increases the DC power consumption, the base transceiver subsystem cooling requirements, the overall system volume, weight, and cost. 
     For a particular digital radio system, such as cellular CDMA or TDMA, the transmitter ACP profile is defined in the system standard. Generally speaking, the actual ACP profile of an RF power amplifier is not the same as the ACP profile required by the standard. The power amplifier ACP profile is determined more or less by the power amplifier device characteristics, operating modes, and signal behaviors. For example, the out-of-band spurious components generated from a CDMA signal appear like white noise: the power density does not change significantly with frequency. However, the ACP profile defined in, for example, the IS 95  CDMA system standard does not require a constant spurious power density over different frequencies. The whole frequency spectrum is divided into a few blocks and the standard ACP profile changes significantly from one block to the next. 
     This may lead to situations in which the power amplifier output power level is dictated by the ACP noise at a few frequency points where the standard ACP profile appears the most stringent. However, there may still be relatively large ACP noise margins at many other frequencies. In a sense, the power amplifier ACP noise is not optimized to make full usage of the ACP profile under the applicable standard. The excess ACP noise margin at most frequencies is not used. 
     Prior art solutions for allowing RF power amplifiers to operate more closely to full power in systems having high peak-to-mean digital signal ratios typically use a digital pre-distortion adjustment circuit that uses the input signal, the output signal, and the standard ACP profile to optimize the performance of the RF amplifier to more closely match the desired standard ACP profile. These conventional digital pre-distortion methods sample, digitize, synchronize, and compare input and output signals to determine the signal distortion. The amount of correction is usually based on the difference between the input and output signals. 
     However, comparison of input and output signals requires sophisticated circuits for synchronizing the signals over time and temperature in order to extract correct signal distortion information. In addition, the pre-distortion correction step, which is based upon the difference between the input and output signals, may not yield the optimum correction in terms of efficiency, speed, and amount of required circuitry. 
     There is therefore a need in the art for improved wireless networks that use more efficient RF power amplifiers. In particular, there is a need for improved RF power amplifiers that can operate more closely to full power in systems having high peak-to-mean digital signal ratios. More particularly, there is a need for power control apparatuses that make RF power amplifiers more efficient by utilizing the available ACP noise margins under the applicable standard ACP profile. There is a further need for power control apparatuses that are not limited by circuitry required to synchronize the RF input and RF output signals in order to generate pre-distortion correction signals. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a pre-distortion correction circuit for use in an RF transmitter having a transmit path capable of receiving and amplifying an RF input signal. The pre-distortion correction circuit modifies distortion in the RF output signal caused by an RF power amplifier in the transmit path. In an advantageous embodiment of the present invention, the pre-distortion circuit comprises: 1) feedback circuitry coupled to an output of the transmit path capable of demodulating and digitizing the distorted RF output signal to thereby produce a first demodulated digital output signal; and 2) a pre-distortion calculation controller coupled to the feedback circuitry capable of comparing the first demodulated digital output signal to adjacent channel power (ACP) profile data associated with the pre-distortion calculation controller and generating therefrom pre-distortion correction values capable of being used to predistort the RF input signal to thereby cause the RF output signal to more closely resemble an ideal RF output signal within the limits of the ACP profile. 
     Advantageously, no synchronization circuitry is needed on either the input of the output in order to pre-distort the RF input signal. Instead, the pre-distortion correction values used to generate an ideal RF output signal are derived solely from the actual RF output signal and the applicable ACP profile limits. 
     According to one embodiment of the present invention, the transmit path comprises demodulation circuitry capable of demodulating and digitizing the RF input signal to thereby produce a first demodulated digital input signal. 
     According to another embodiment of the present invention, the pre-distortion correction values are used to pre-distort the first demodulated digital input signal produced from the input RF signal. 
     According to an intermediate frequency (IF) embodiment of the present invention, the feedback circuitry comprises a first intermediate frequency (IF) demodulator and the first demodulated digital output signal comprises a first digital IF output signal. 
     Still according to the IF embodiment of the present invention, the demodulation circuitry in the transmit path comprises a second intermediate frequency (IF) demodulator and the first demodulated digital input signal comprises a first digital IF input signal. 
     Further according to the IF embodiment of the present invention, the pre-distortion correction values are used to pre-distort the first digital IF input signal produced from the input RF signal. 
     According to a baseband embodiment of the present invention, the feedback circuitry comprises a first baseband demodulator and the first demodulated digital output signal comprises a first digital baseband output signal. 
     Still according to the baseband embodiment of the present invention, the demodulation circuitry in the transmit path comprises a second baseband demodulator and the first demodulated digital input signal comprises a first digital baseband input signal. 
     Further according to the baseband embodiment of the present invention, the pre-distortion correction values are used to pre-distort the first digital baseband signal produced from the input RF signal. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 illustrates an exemplary wireless network according to one embodiment of the present invention; 
     FIG. 2 illustrates in greater detail an exemplary base station according to one embodiment of the present invention; 
     FIG. 3 illustrates selected portions of an exemplary RF transmitter for use in the exemplary base station according to one embodiment of the present invention; 
     FIG. 4 illustrates an exemplary pre-distortion calculation controller in greater detail according to one embodiment of the present invention; 
     FIG. 5 depicts a frequency response diagram, which illustrates pre-distortion adjustments generate by the exemplary pre-distortion calculation controller, according to one embodiment of the present invention; 
     FIG. 6 illustrates selected portions of an exemplary RF transmitter for use in the exemplary base station according to an alternate embodiment of the present invention; and 
     FIG. 7 is a flow diagram illustrating the overall operation of the exemplary RF transmitter according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 7, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless network. 
     FIG. 1 illustrates exemplary wireless network  100  according to one embodiment of the present invention. The wireless telephone network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 - 103  are operable to communicate with a plurality of mobile stations (MS)  111 - 114 . Mobile stations  111 - 114  may be any suitable cellular devices, including conventional cellular telephones, PCS handset devices, portable computers, metering devices, and the like. 
     Dotted lines show the approximate boundaries of the cells sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions. 
     In one embodiment of the present invention, BS  101 , BS  102 , and BS  103  may comprise a base station controller (BSC) and one or more base transceiver sub-systems (BTS). Base station controllers and base transceiver subsystems are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver subsystem, for specified cells within a wireless communications network. A base transceiver subsystem comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces, and RF transmitters and RF receivers. 
     In a typical wireless network, a base transceiver subsystem is at the center of each cell site. Frequently, multiple base transceiver subsystems may be connected to a single base station controller and multiple base station controllers may be connected to a single mobile switching center, such as mobile switching center (MSC)  140 . However, for the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver subsystem(s) and the associated base station controller(s) for each of cells  121 ,  122 , and  123  are collectively represented by BS  101 , BS  102  and BS  103 , respectively. 
     BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public telephone system (not shown) via communications line  131  and mobile switching center (MSC)  140 . 
     Mobile switching center  140  is well known to those skilled in the art. Mobile switching center  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the public telephone system. Communications line  131  may be any suitable connection means, including a T1 line, a T3 line, a fiber optic link, a network backbone connection, and the like. In some embodiments of the present invention, communications line  131  may be several different data links, where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 . 
     In the exemplary wireless network  100 , MS  111  is located in cell site  121  and is in communication with BS  101 , MS  113  is located in cell site  122  and is in communication with BS  102 , and MS  114  is located in cell site  123  and is in communication with BS  103 . The MS  112  is also located in cell site  121 , close to the edge of cell site  123 . The direction arrow proximate MS  112  indicates the movement of MS  112  towards cell site  123 . At some point, as MS  112  moves into cell site  123  and out of cell site  121 , a “handoff” will occur. 
     As is well known, the “handoff” procedure transfers control of a call from a first cell to a second cell. For example, if MS  112  is in communication with BS  101  and senses that the signal from BS  101  is becoming unacceptably weak, MS  112  may then switch to a BS that has a stronger signal, such as the signal transmitted by BS  103 . MS  112  and BS  103  establish a new communication link and a signal is sent to BS  101  and the public telephone network to transfer the on-going voice, data, or control signals through BS  103 . The call is thereby seamlessly transferred from BS  101  to BS  103 . An “idle” handoff is a handoff between cells of a mobile device that is communicating in the control or paging channel, rather than transmitting voice and/or data signals in the regular traffic channels. 
     FIG. 2 illustrates in greater detail exemplary base station  101  in accordance with one embodiment of the present invention. Base station  101  comprises base station controller (BSC)  210  and base transceiver subsystem (BTS)  220 . Base station controllers and base transceiver subsystems were described previously in connection with FIG.  1 . For the purposes of simplicity and clarity in explaining the operation of the present invention, only a single exemplary base transceiver subsystem and a single exemplary base station controller are shown in FIG.  2 . BSC  210  manages the resources in cell site  121 , including BTS  220 . BTS  220  comprises BTS controller  225 , channel controller  235 , which contains one or more representative channel elements  240 , transceiver interface (IF)  245 , RF transceiver unit  250 , antenna array  255 , and channel monitor  260 . 
     BTS controller  225  comprises processing circuitry and memory capable of executing an operating program that controls the overall operation of BTS  220  and communicates with BSC  210 . Under normal conditions, BTS controller  225  directs the operation of channel controller  235 , which contains a number of channel elements, including channel element  240 , that perform bi-directional communications in the forward channel and the reverse channel. A “forward” channel refers to outbound signals from the base station to the mobile station and a “reverse” channel refers to inbound signals from the mobile station to the base station. Transceiver IF  245  transfers the bi-directional channel signals between channel controller  235  and RF transceiver unit  250 . 
     Antenna array  255  transmits forward channel signals received from RF transceiver unit  250  to mobile stations in the coverage area of BS  101 . Antenna array  255  also sends to transceiver  250  reverse channel signals received from mobile stations in the coverage area of BS  101 . In a preferred embodiment of the present invention, antenna array  255  is one sector of a multi-sector antenna, such as a three sector antenna assembly in which each antenna sector is responsible for transmitting and receiving in a 120° arc of coverage area. Additionally, RF transceiver  250  may contain an antenna selection unit to select among different antennas in antenna array  255  during both transmit and receive operations. In one embodiment of the present invention, antenna array  255  may comprise an adaptive antenna array or “smart” antenna array. 
     To increase the efficiency of the RF transmitters in RF transceiver  250 , the present invention implements an adaptive digital pre-distorter (ADPD) circuit that samples the RF transmitter output signal and compares the output samples to the ACP profile of an applicable standard. The present invention then determines the pre-distortion adjustments required to drive the power amplifier in order to maintain an ideal ACP output profile within the ACP profile of the applicable standard. The adaptive pre-distortion circuit according to the present invention does not require samples of the input signal for synchronization purposes or for calculation of pre-distortion adjustments. The present invention may be implemented in any type of digital modulation scheme, including TDMA, CDMA, GSM, NCDMA, multi-carrier signals, and even modems. 
     FIG. 3 illustrates selected portions of exemplary RF transmitter  300  for use in RF transceiver unit  250  according to one embodiment of the present invention. RF transmitter  300  contains a transmit path that receives an RF input signal from the RF power amplifier (PA) driver, not shown, and generates an RF output signal that is sent to antenna array  255 . The transmit path elements in RF transmitter  300  comprise demodulator  305 , analog-to-digital converter (ADC)  310 , pre-distortion circuit  315 , digital-to-analog converter (DAC)  320 , RF modulator  325 , RF power amplifier (PA)  330 , and RF coupler (RFC)  340 . 
     RF transmitter  300  also contains a pre-distortion adjustment feedback loop that samples the RF output signal, compares the samples to the required ACP profile, and generates a pre-distortion adjustment signal that adjusts subsequent samples of an intermediate frequency (IF) input signal. The pre-distortion correction feedback loop elements in RF transmitter  300  comprise RF demodulator  345 , ADC  350 , and pre-distortion calculation controller  355 . The applicable ACP profile varies according to the communications standard to which wireless network  101  must conform. For example, the ACP profile may comprise the ACP noise limitations (ACP “mask”) under the IS95 CDMA system standard. 
     Demodulator  305  demodulates an RF input signal using a reference RF carrier signal from a local oscillator (LO) (not shown) to produce an intermediate frequency (IF) analog signal. ADC  310  converts the IF analog signal from demodulator  305  to a fourteen bit IF digital signal using the system clock signal (Clock) as reference. 
     Pre-distortion circuit  315  adds a pre-distortion error correction signal to the IF digital signal before sending the adjusted or pre-distorted IF digital signal to DAC  320 . The pre-distortion distortion error correction is based on the amplitude and phase differences between the desired ACP profile and the distorted output signal as generated by pre-distortion calculation controller  355  and described in greater detail below. Pre-distortion circuit  315  adjusts the digitized IF output from ADC  310  according to the pre-distortion adjustment signals (A m , ΔA m , and Δφ) from pre-distortion calculation controller  355 , providing an adjusted output which is within the desired ACP profile. More specifically, pre-distortion circuit  315  uses the amplitude, A′ m , of the newly received input signal and the reference amplitude A m , amplitude correction ΔA m , and phase correction, Δφ, signals from pre-distortion calculation controller  355 , to identify appropriate entries in an internal look-up table (LUT) for distortion compensation adjustments. Pre-distortion circuit  315  then modifies its input signal according to the distortion adjustments to generate the desired pre-distorted signal for output to DAC  320 . 
     DAC  320  samples the digital bit stream from pre-distortion circuit  315  using Clock reference signal and converts the digital bit stream to an analog signal for input to RF modulator  325 . The LO input to RF modulator  325  is the RF reference carrier signal. Thus, RF modulator  325  outputs an RF signal modulated by the IF signal. Next, RF PA  330  amplifies the RF signal from RF modulator  325  to a power level suitable for transmission. The amplified modulated RF output signal is then sent to antenna array  255  through RFC  340 . 
     Excluding pre-distortion circuit  315 , those skilled in the art will recognize that the above-described demodulation, modulation, and amplification steps are common operations in conventional RF transmitters. When the amplitude of the RF input signal is relatively low, RF PA  330  operates well within its linear region and introduces little or no distortion in the RF output signal sent to antenna array  255 . However, when operating in the linear region, RF PA  330  may be very inefficient in terms of power consumption. Thus, one goal is to have the input signal to RF PA as high as possible in order to improve the efficiency of the power amplifier. 
     As the amplitude of the RF input signal rises, PA  330  begins to saturate (i.e., operates in a non-linear manner) and distortion is introduced in the RF output signal sent to antenna array  255 . This distortion includes adjacent channel power (ACP) noise that must be limited at the frequencies of interest to an amount less than the ACP profile (i.e., “mask”) specified in the applicable system standard. 
     The pre-distortion adjustment signal is determined by the operation of the feedback path comprising demodulator  345 , ADC  350 , and pre-distortion calculation controller  355 . RFC  340  sends a copy of the RF output signal to RF demodulator  345 . The other input to RF demodulator  345  is the same LO reference signal used by RF demodulator  305  and RF modulator  325 . RF demodulator  345  provides a scaled version of the IF analog signal generated by DAC  320 , plus possible distortion introduced by RF PA  330 . ADC  350  converts the scaled, distorted IF analog signal to digital values that are read by pre-distortion calculation controller  355 , described in greater detail below. 
     FIG. 4 illustrates exemplary pre-distortion calculation controller  355  in greater detail according to one embodiment of the present invention. At every amplitude, A m , pre-distortion calculation controller  355  determines the signal distortion, represented by an amplitude difference, ΔA m , and a phase difference, Δφ, introduced by PA  330 . Pre-distortion calculation controller  355  comprises two signal paths for digitized IF signals from ADC  350 . One path comprises digital filter block  405 , amplitude detector  410 , and phase discriminator  420 . The second path comprises amplitude detector  415  and phase discriminator  420 . Phase discriminator  420  provides pre-distortion adjustment signals for input to pre-distortion circuit  315 . 
     Digital filter block  405  receives the distorted digitized IF signal from ADC  350  and adjusts it with regard to the applicable ACP profile to provide an ideal reference signal as its output. The ACP profile data that applies to BS  101  is stored in digital filter block  405 . Digital filter block  405  generates the ideal reference signal by maintaining a buffer region between the allowed ACP profile power levels and the actual output of digital filter block  405 . Digital filter block  405  filters out spurious emissions in the distorted signal, deriving an ideal reference signal that is below the ACP profile levels by a system selected buffer region. The ideal reference signal from digital filter block  405  is supplied as an input to amplitude detector  410 . Amplitude detector  410  determines the amplitude (A M ) of the ideal reference signal and then transfers A M  and the ideal reference signal to phase discriminator  420  for further processing. 
     In a similar manner, amplitude detector  415  directly processes the distorted digitized IF signal from ADC  350  to provide a distorted signal and distorted amplitude signal (A m ′) for input to phase discriminator  420 . 
     Phase discriminator  420  determines the amplitude and phase distortion (or differences) between the distorted signal and the ideal reference signal, to provide pre-distortion signals for input to pre-distortion circuit  315 . Phase discriminator  420  generates an amplitude difference signal, ΔA m , which represents the amplitude difference between the ideal reference signal and the distorted signal, ΔA m =A m −A m ′. Phase discriminator  420  determines the phase difference, Δφ, between the ideal reference signal and the distorted signal using any known circuit or technique. Pre-distortion calculation controller  355  provides A m , ΔA m , and Δφas inputs to pre-distortion circuit  315 , as previously described. 
     FIG. 5 depicts frequency response diagram  500 , which illustrates pre-distortion adjustments generated by exemplary pre-distortion calculation controller  355  according to one embodiment of the present invention. The center 1.25 MHz band represents the full power signal within the designated traffic channel. The adjacent channels on both sides of the traffic channel are represented by reduced power level signals of −10 dB, −15 dB, and −20 dB. In all cases, the RF output signal is not to exceed the ACP profile. Thus, the ideal output signal is obtained by filtering the RF power level in the adjacent channels according to the digital filtering profile defined by the solid lines at the bottom of the shaded regions as shown in FIG.  5 . It is noted that the digital filtering profile depicted in FIG. 5 is illustrative and other filtering profiles may readily be used. 
     Frequency response diagram  500  indicates that the reference or target signal has 10 dB, 15 dB, or 20 dB lower emissions than the distorted signal at frequency offsets of 885 kHz, 1.25MHz, or 2.24 MHz from the center traffic channel for a single CDMA carrier case, respectively. In other words, the new input signal will be distorted to produce an output signal that will have 10 dB, 15 dB, and 20 dB improvements in its spurious emission at the respective referenced frequency offset when compared to the output taken in the previous run. 
     FIG. 6 illustrates selected portions of exemplary RF transmitter  600  for use in RF transceiver unit  250  according to an alternate embodiment of the present invention. RF transmitter  600  contains a transmit path that receives an RF input signal from the RF power amplifier driver (not shown) and generates an RF output signal that is sent to antenna array  255 . Transmitter  600  is a baseband implementation of the direct digital pre-distortion technique according to the principles of the present invention. The transmit path elements in RF transmitter  600  comprise RF demodulator  605 , IF demodulator  610 , ADC  612 , ADC  614 , pre-distortion circuit  620 , DAC  622 , DAC  624 , RF modulator  625 , IF modulator  627 , RF power amplifier (PA)  630 , and RF coupler (RFC)  635 . 
     RF transmitter  600  also contains a pre-distortion adjustment feedback loop that analyzes the RF output signal, compares the signal to the required ACP profile, and generates pre-distortion adjustment signals that are added to subsequent received baseband input signals. The pre-distortion correction feedback loop elements in RF transmitter  600  comprise RF demodulator  640 , IF demodulator  645 , ADC  647 , ADC  649 , and pre-distortion calculation controller  650 . 
     RF demodulator  605  demodulates the received RF input signal using an RF local oscillator (LO) reference signal to thereby produce an analog IF signal. IF demodulator  610  demodulates the IF analog signal using an IF sine carrier signal and an IF cosine carrier signal to thereby produce an analog in-phase (I) baseband signal and an analog quadrature (Q) baseband signal. 
     ADC  612  and ADC  614  convert the analog I and Q baseband signals to digital I and Q baseband signals for subsequent processing by pre-distortion circuit  620 . Pre-distortion circuit  620  adds pre-distortion error correction signals from distortion calculation controller  650  to the digital I and Q baseband signals to produce pre-distorted digital I and Q baseband signals for input to DAC  622  and DAC  624 , respectively. 
     DAC  622  and DAC  624  convert the pre-distorted digital I and Q baseband signals to pre-distorted analog I and Q signals for input to IF modulator  625 . IF modulator  625  uses cosine and sine IF carrier signals to generate a pre-distorted IF analog signal for input to RF modulator  627 . RF modulator  627  modulates the predistorted distoreted IF analog signal to produce a pre-distorted RF output signal that is sent to PA  630 . PA  630  amplifies the pre-distorted RF output signal to a power level suitable for transmission. The amplified RF output signal is then sent to antenna array  255  via RFC  635 . 
     The pre-distortion adjustment signal for this baseband embodiment is generated in the feedback path comprising RF demodulator  640 , IF demodulator  645 , ADC  647 , ADC  649 , and pre-distortion calculation controller  650 . A two stage demodulation is used to produce digital analog I and Q signals from the distorted RF output signal received from RFC  635 . RF demodulator  640  demodulates the distorted RF output signal to produce an analog IF signal. IF demodulator  645  then demodulates the analog IF signal to produce an analog in-phase (I) baseband signal and an analog quadrature (Q) baseband signal. 
     Pre-distortion calculation controller  650  extracts the signal distortion introduced by PA  630 , as represented by ΔI and ΔQ, at every amplitude A m , where A m is represented by the sum of the squares of I and Q. Pre-distortion calculation controller  650  comprises two signal paths, one each for the digitized I and Q baseband signals from ADC  647  and  649 , respectively. Pre-distortion calculation controller  650  is similar to pre-distortion calculation controller  355 , except that the I and Q baseband signals are filtered separately. Each filter path generates an ideal reference I or Q baseband signal for comparison with the distorted I or Q baseband signal. Pre-distortion calculation controller  650  provides the resultant A m , ΔI, and ΔQ signals as inputs to pre-distortion circuit  620 . 
     Pre-distortion circuit  620  uses A m , ΔI, and ΔQ signals to locate corresponding distortion values in an internal look up table. Pre-distortion circuit  620  adjusts the digital I and Q baseband signals received from ADC  612  and ADC  614  according to the identified pre-distortion values retrieved from the look-up table (LUT) to generate pre-distorted digital I and Q baseband signals for transfer to DAC  622  and DAC  624 . 
     FIG. 7 depicts flow diagram  700 , which illustrates the overall operation of RF transmitter  300  according to one embodiment of the present invention. During routine operation of RF transmitter  300 , demodulator  305  and ADC  310  demodulate and digitize the RF input signal to produce a digital IF signal. Pre-distortion circuit  315  makes pre-distortion adjustments to the digital IF signal, if needed (process step  705 ). The pre-distorted digital IF signal is converted to a pre-distorted analog IF signal. The analog IF signal is then modulated to produce a pre-distorted RF signal. The pre-distorted RF signal is then amplified is then amplified in RF power amplifier  320  (process step  710 ). 
     The resultant RF output signal is demodulated in demodulator  345  to recover the analog IF output signal which is then converted to a digital IF output by ADC  350  (process step  715 ). Pre-distortion calculation controller  355  compares samples of the resultant digital IF output signal with ACP profile data and generates new pre-distortion adjustment values which are output to pre-distortion circuit  315  (process step  720 ). Thereafter, the process repeats by looping back to process step  705 , thereby giving the pre-distortion adjustment of transmitter  300  its adaptive nature. The pre-distortion adjustment values are constantly updated to compensate for changes in RF transmitter  300  over time. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.