Abstract:
There is disclosed a pre-distortion adjustment circuit for use in an RF transmitter that adjusts the actual adjacent channel power (ACP) noise profile of an RF power amplifier to fully use the ACP profile allowed under the applicable RF communication standard. The pre-distortion adjustment circuit pre-distorts selected components of the input signal to the RF power amplifier so that the actual output ACP profile appears similar to, if not the same as, the ACP profile under the standard. The distortion required is determined based on information extracted from the input signal, the output signal, and the standard ACP profile. The pre-distortion adjustment circuit allows significant overdrive of the RF power amplifier while maintaining the ACP noise in the RF output below the levels allowed under the standard.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a 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  
         [0002]    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  
         [0003]    Every wireless network base station has a 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 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).  
           [0004]    In most cases, these two criteria are contradictory. ACP noise results from no-linear effects, such as over-driving the power amplifier into its nonlinear region (clipping). Spurious spectral components are introduced when a signal peak is sufficiently large 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 station cooling requirements, the overall system volume, weight, and cost.  
           [0005]    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-there is a need for RF power controllers that make RF power amplifiers more efficient by utilizing the available ACP noise margins under the applicable standard ACP profile.  
         SUMMARY OF THE INVENTION  
         [0006]    To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a pre-distortion adjustment circuit for use in an RF transmitter that optimizes the ACP profile of an RF power amplifier to fully use the ACP profile under the applicable RF communication standard. The present invention pre-distorts the RF signal so that the actual output ACP profile appears similar to, if not the same as, the ACP profile under the standard. The pre-distortion required is determined based on information extracted from the input signal, the output signal, and the standard ACP profile. Thus, the present invention allows significant overdrive of the power amplifier while still maintaining its ACP noise under the standard ACP profile.  
           [0007]    Accordingly, in an exemplary embodiment of the present invention, there is provided, for use in an RF transmitter having an RF power amplifier required to transmit an RF output signal within selected limits of an adjacent channel power (ACP) profile specified for the RF transmitter, a pre-distortion adjustment circuit comprising: 1) input sampling means, coupled to an input of a transmit path of the RF transmitter, capable of capturing input samples from a digital input baseband signal, the input samples comprising a first input sample of amplitude X; 2) output sampling means, coupled to an output of the transmit path, capable of capturing output samples of a digital output baseband signal derived from the RF output signal, wherein a first output sample corresponds to the first input sample; and 3) processing means capable of determining from the first input sample and the first output sample a pre-distortion adjustment value capable of adjusting an amplitude of the digital input baseband signal prior to amplification by the RF power amplifier without causing the RF output signal to exceed the selected limits of the ACP profile.  
           [0008]    According to one embodiment of the present invention, the specified limits of the ACP profile are stored in a memory associated with the processing means.  
           [0009]    According to another embodiment of the present invention, the specified limits are specified at discrete frequency points.  
           [0010]    According to still another embodiment of the present invention, the processing means applies the pre-distortion adjustment value to a subsequently received input sample of amplitude X.  
           [0011]    According to yet another embodiment of the present invention, the processing means is capable of determining if the amplitude X is sufficiently small to ensure that an amplification distortion caused by the RF power amplifier is negligibly small and, in response to the determination, is capable of determining a scaling factor for the output samples.  
           [0012]    According to a further embodiment of the present invention, the processing means scales subsequently received input samples of the digital input baseband signal according to a value of the scaling factor.  
           [0013]    According to a still further embodiment of the present invention, the processing means adjusting an amplitude of the scaled digital input baseband signal.  
           [0014]    According to a yet further embodiment of the present invention, the processing means modifies a selected subsequently received input sample according to a value of the scaling factor without regard to an amplitude of the selected subsequently received input sample.  
           [0015]    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.  
           [0016]    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  
       [0017]    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:  
         [0018]    [0018]FIG. 1 illustrates an exemplary wireless network according to one embodiment of the present invention;  
         [0019]    [0019]FIG. 2 illustrates in greater detail an exemplary base station in accordance with one embodiment of the present invention;  
         [0020]    [0020]FIG. 3 illustrates an exemplary RF transmitter for use in the RF transceiver unit in FIG. 2 in accordance with one embodiment of the present invention;  
         [0021]    [0021]FIG. 4 illustrates exemplary input and output synchronization and data acquisition controllers in accordance with one embodiment of the present invention; and  
         [0022]    [0022]FIG. 5 is a flow diagram illustrating the operation of the exemplary RF transmitter in accordance with one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0023]    [0023]FIGS. 1 through 5, 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.  
         [0024]    [0024]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.  
         [0025]    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.  
         [0026]    In one embodiment of the present invention, BS  101 , BS  102 , and BS  103  may comprise a base station controller (BSC) and a base transceiver station (BTS). Base station controllers and base transceiver stations 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 station, for specified cells within a wireless communications network. A base transceiver station 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. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver station in each of cells  121 ,  122 , and  123  and the base station controller associated with each base transceiver station are collectively represented by BS  101 , BS  102  and BS  103 , respectively.  
         [0027]    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 .  
         [0028]    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.  
         [0029]    As is well know, 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.  
         [0030]    [0030]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 station (BTS)  220 . Base station controllers and base transceiver stations were described previously in connection with FIG. 1. BSC  210  manages the resources in cell site  121 , including BTS  220 . BTS  120  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 .  
         [0031]    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  240  and RF transceiver unit  250 .  
         [0032]    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 multi-sector antenna, such as a three sector antenna in which each antenna sector is responsible for transmitting and receiving in a 120° arc of coverage area. Additionally, 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.  
         [0033]    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 input and output signals, and synchronizes and compares the samples to each other and to the ACP profile of an applicable standard. The present invention then determines the pre-distortion adjustment required to overdrive the power amplifier, while maintaining ACP noise below the ACP profile of the standard. The pre-distortion adjustment is then added to subsequent input samples. The present invention may be implemented in any type of digital modulation scheme, including TDMA, CDMA, GSM, NCDMA, multi-carrier signals, and even modems.  
         [0034]    [0034]FIG. 3 illustrates exemplary RF transmitter  300  for use in RF transceiver unit  250  in accordance with one embodiment of the present invention. RF transmitter  300  contains a transmit path that receives input data and generates an RF output signal that is sent to antenna array  255 . The transmit path elements in RF transmitter  300  comprise pre-distorter circuit  305 , digital-to-analog converter (DAC)  310 , RF modulator  315 , local oscillator  320 , RF power amplifier (PA)  325 , and RF coupler (RFC)  330 .  
         [0035]    RF transmitter  300  also contains a pre-distortion adjustment feedback loop that samples the input data signal and a corresponding part of the RF output signal, compares the samples to each other and to the accepted ACP profile, and generates a pre-distortion adjustment signal that is added to subsequent samples of the input signal data. The pre-distortion correction feedback loop elements in RF transmitter  300  comprise RF demodulator  335 , local oscillator  320 , analog-to-digital converter (ADC)  340 , input synchronization and data acquisition controller  345 , output synchronization and data acquisition controller  350 , processor  355 , and memory  360 , which stores ACP profile data  365 . The ACP profile data  365  varies according to the communications standard to which wireless network  101  must conform. For example, the ACP profile data  365  may comprise the ACP noise limitations (ACP “mask”) under the IS95 CDMA system standard.  
         [0036]    A digital baseband signal, referred to as “INPUT DATA” in FIG. 3, is received by pre-distorter circuit  305 , which may optionally add a pre-distortion error correction retrieved from LUT  306  before sending the INPUT DATA signal to DAC  310 . DAC  310  converts the digital signal to an analog signal that forms the baseband input to RF modulator  315 . The other input to RF modulator is a reference RF carrier signal from local oscillator  320 . The output of RF modulator  315  is an RF signal modulated by the baseband signal. Next, the modulated RF signal is amplified by RF power amplifier  325  to a power level suitable for transmission. The amplified modulated RF output signal is then sent to antenna array  255  via RFC  330 .  
         [0037]    Those skilled in the art will recognize that the above-described modulation and amplification steps are common operations in conventional RF transmitters. If the amplitude of the INPUT DATA signal is relatively low, RF power amplifier  325  operates well within the linear region and little or no distortion is introduced in the RF output signal sent to antenna array  255 . However, when operating in the linear region, RF power amplifier  325  is very inefficient in terms of power consumption.  
         [0038]    As the amplitude of the INPUT DATA signal rises, RF power amplifier  325  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.  
         [0039]    The pre-distortion adjustment signal is determined by the operation of input synchronization and data acquisition controller  345 , output synchronization and data acquisition controller  350  and processor  355 . RFC  330  sends a copy of the RF output signal to RF demodulator  335 . The other input to RF demodulator  335  is the same carrier reference signal from local oscillator  320  that was used by RF modulator  315  to produce the original RF modulated signal. The output of RF demodulator  335  is a scaled version of the original analog baseband signal generated by DAC  310 , plus possible distortion components. The scaled, distorted analog baseband is converted by ADC  340  to digital values that are read by output synchronization and data acquisition controller  350 .  
         [0040]    [0040]FIG. 4 illustrates exemplary input synchronization and data acquisition controller (ISDAC)  345  and output synchronization and data acquisition controller (OSDAC)  350  in accordance with one embodiment of the present invention. The operations of ISDAC  345  and OSDAC  350  are quite similar, as explained below in greater detail.  
         [0041]    ISDAC  345  comprises data processor  401 , interface (I/F) and control circuit  402 , and RAM  403 . A system clock provides a reference for clocking the input digital baseband signal (i.e., INPUT DATA) into data processor  401  and clocking the acquired data out of interface and control circuit  402 . The INPUT DATA signal samples are stored in RAM  403 . Data processor  401  comprises a signal correlator that analyzes the bits in the INPUT DATA signal to determine the start and stop of N-bit data samples, where “N” is a known system parameter that varies depending on the type of system wireless network  100  is (i.e., CDMA, GSM, TDMA, WCDMA, etc.). The N-bit samples begin with a circuit  402  which transfers the acquired data to processor  355 .  
         [0042]    Processor  355  comprises comparison circuitry for comparing the acquired data received from ISDAC  345  and OSDAC  350  with each other and with ACP profile data  365  stored in memory  360  and for calculating a pre-distortion adjustment value that is used by pre-distorter circuit  305 . ACP profile data  365  specifies the required ACP mask at a set of discrete frequency points, A s (z). Its time domain counterpart, {a s (n), n=1,2,3, . . . }, can be obtained by using a fast Fourier transform (FFT): 
           a   s ( n )= FFT   −1 ( A   s ( z )).  Equation 1: 
         [0043]    Pre-distorter circuit  305  comprises a series combination of: 1) a nonlinear distorter, and 2) a post digital filter. The nonlinear distorter is characterized by its transfer function f 1 ( . . . ). If INPUT DATA samples are defined as {a 0 (n), n=1,2,3, . . . }, then f 1 ( . . . ) is defined by the nonlinear output-input relationship: 
           a   1 ( n )= f   1 ( a   0 ( n ));  n= 1,2,3, . . .   Equation 2: 
         [0044]    The transfer function f 1 ( . . . ) of the nonlinear distorter can be written in a generic function form that contains adjustable parameters {di}, i=1,2,3, . . . . The transfer function f 1 ( . . . ) is then denoted as f 1 {di}. The Z-domain, A 1 (z), of a 1 (n) is given by: 
           A   1 ( z )= FFT ( a   1 ( n ))  Equation 3: 
         [0045]    The output of the non-linear distorter is received by the post digital filter portion of pre-distorter circuit  305 , and is characterized by its transfer function F 2 . The output A 2 (z) of the post digital filter is related to the input A 1 (z) received from the non-linear distorter by: 
           A   2 ( z )= A   1 ( z )· F   2 ( z )  Equation 4: 
         [0046]    F 2 ( . . . ) can be written in a generic function form with adjustable parameters {fi}, i=1,2,3, . . . , denoted as F 2 {fi}.  
         [0047]    By using an inverse FFT, the output of the post digital filter (and pre-distorter circuit  305 ) is given by: 
           a   2 ( n )= FFT   −1 ( A   2 ( z ))  Equation 5: 
         [0048]    The output, a 2 (n), of pre-distorter circuit  305  is received by RF power amplifier  325  (after conversion in DAC  310  and modulation in RF modulator  315 ). RF power amplifier  325  can be modeled as a nonlinear device and characterized by a nonlinear function, f p , as follows: 
           a   p ( n )= f   p ( a   2 ( n ));  n= 1,2,3, . . .   Equation 6: 
         [0049]    The value a p  is the output signal of RF power amplifier  325  and the value a 2  is its input signal. The nonlinear function, f p , can be written in a generic function form that contains a set of adjustable parameters {p i }, i=1,2,3, . . . . Function f p  is then denoted as f p {p i }. The parameter {p i } can be obtained from the measured input signal a 0 (n) and output signal a p (n) using Equations 3 through Equation 6 if the parameters {d i } of the distorter and {f i } of the post digital filter are given.  
         [0050]    Equation 6 can also be written in its inverse form: 
           a   2 ( n )= f′   p ( a   p ( n )).  Equation 7: 
         [0051]    where f′ p ( . . . ) is the inverse function of f p ( . . . ).  
         [0052]    The data processing procedure used to determine the distortion adjustment parameter may be summarized as follows:  
         [0053]    1) For given input data, a 0 (n), and given output data, a p (n), processor  355  calculates the scaling value, k. Assuming signals with low magnitude experience no distortion, the scaling value k can be obtained by comparing signals with low magnitudes.  
         [0054]    2) Next, processor  355  multiply the value a p (n) by the scaling value k: k*a p (n)&gt;a p (n).  
         [0055]    3) From the input a 0 (n), processor  355  calculates a 1 (n) using Equation 2 with current parameter {d i } for function f 1 {d i }.  
         [0056]    4) From the value a 1 (n), processor  355  calculates A 1 (z) using Equation 3.  
         [0057]    5) From the value A 1 (z), processor  355  calculates A 2 (z) using Equation 4 with current parameter {f i } for function F 2 (z).  
         [0058]    6) From the value A 2 (z), processor  355  calculates a 2 (n) using Equation 5.  
         [0059]    7) Next, processor  355  compares a 2 (n) with a p (n), and determines the function f′ p ( . . . ). The function f′ p ( . . . ) is the inverse function of f p ( . . . ) defined in Equation 7. This provides the updated parameters {p i } for the function f′ p ( . . . ).  
         [0060]    8) Next, processor  355  constructs the optimization function ∥a 2 (n)−a s2 (n)∥, where a s2 (n)=f′ p (a s (n)). Processor  355  may use the current {pi} or an updated value of {p 1 } for function f′ p ( . . . ). Optimization function ∥a 2 (n)−a s2 (n)∥ is a function of parameter {d i } and {f i }. Processor  355  seeks new values of {d 1 } and {f 1 } at which ∥a 2 (n)−a s2 (n)∥ reaches a minimum value.  
         [0061]    9) Next, processor  355  updates {d i } and {f 1 } in the pre-distorter circuit  305  with the new {d i } and {f 1 }. Pre-distorter circuit  305  uses these values to adjust to adjust the received INPUT DATA signal so RF power amplifier  325  may be saturated to the point where the adjacent channel power noise is driven close to, but not over the limits allowed in the ACP profile. This increases the efficiency of RF power amplifier  325  by transmitting the RF output signal, albeit with ACP noise distortion, using a lesser amount of DC power.  
         [0062]    10) Finally, input synchronization and data acquisition controller  345 , output synchronization and data acquisition controller  350 , and processor  355  start the next round of data synchronization and data acquisition.  
         [0063]    [0063]FIG. 5 depicts flow diagram  500 , which illustrates the overall operation of RF transmitter  300  in accordance with one embodiment of the present invention. First, during routine operation, pre-distorter circuit  305  receives N-bit samples of the digital baseband input signal and makes a pre-distortion adjustment, if any (process step  501 ). The adjusted digital baseband signal is converted to an adjusted analog baseband signal, which is used to modulate an RF carrier signal. The modulated RF signal is then amplified in RF power amplifier  325  (process step  502 ).  
         [0064]    In the pre-distortion correction loop, the RF output signal is demodulated in RF demodulator  335  to recover the analog baseband output signal. The analog baseband output signal is converted to a digital signal and sampled (process step  503 ). Next, the original digital baseband input signal samples are aligned with and compared to the digital baseband output signal samples. A scaling factor, k (small signal closed loop-gain), is determined (described above in greater detail) by comparing digital baseband input signals having small amplitudes with their corresponding digital baseband output signals. Also, processor  355  calculates the parameters {di} and {fi} and updates them in pre-distorter circuit  305  as described above (process step  504 ). Thereafter, the process repeats by looping back to process step  501 , thereby giving the present invention its adaptive nature. The pre-distortion adjustment values are constantly updated to compensate for changes in RF transmitter  300  over time.  
         [0065]    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.