Abstract:
There is disclosed an adaptive pre-distortion correction circuit for use in an RF transmitter having a transmit path capable of receiving a digital input baseband signal and generating a modulated RF output signal. The adaptive pre-distortion circuit comprises 1) input sampling means coupled to an input of the transmit path for capturing from a first digital input sample of amplitude X; 2) demodulation circuitry coupled to an output of the transmit path for receiving and demodulating the modulated RF output signal to produce a digital output baseband signal; 3) output sampling means coupled to the demodulation circuitry for capturing a first digital output sample corresponding to the first input sample; and 4) processing means for comparing the first digital input sample and the first digital output sample and calculating a pre-distortion correction value corresponding to the amplitude X. The pre-distortion correction value are stored in a look-up table and are continually updated to compensate for circuit changes over time.

Description:
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 
     Wireless networks, and cellular telephone networks in particular, have become ubiquitous in society. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is increased by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. To maximize usage of the available bandwidth in each cell, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof. 
     Every cellular base station has a RF transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the RF power amplifier in a transmitter operate in a highly linear manner, especially when amplifying a signal whose envelope changes in time over a wide range, as in CDMA and multi-carrier systems. It also is important that the RF transmitter operate efficiently under high-power conditions. It also is important that RF amplifiers having good linearity characteristics across a wide range of operating conditions are required because wireless systems cannot tolerate large amounts of signal distortion and may not violate the IS 95 bandwidth requirements regarding spectral spreading effects. 
     Spurious spectral components are introduced when a signal peak is sufficiently large to saturate an RF amplifier in the transmitter. 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 clipping the signal peaks. For example, RF power amplifiers in some CDMA systems need more than 10 dB of “overhead” 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 and increases the power consumption and the cooling requirements of the base transceiver station. 
     A number of techniques are known to try to minimize the amount of overhead an RF power amplifier requires, including feedforward, feedback, and pre-distortion. Each technique has its drawbacks, however. Feedforward systems require a large error power amplifier in the correction loop, which lowers the overall power amplifier efficiency. Feedback systems introduce a delay in the feedback signal, which limits the signal bandwidth to a few MHz. Pre-distortion systems typically exhibit low correction efficiency. 
     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 without generating spurious spectral components when a large signal peak is encountered. More particularly, there is a need for improved RF power amplifiers that require less “overhead” to prevent sudden large peaks from being clipped due to saturation of the RF power amplifier. 
     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 a digital input baseband signal and generating therefrom a modulated RF output signal. The pre-distortion correction circuit adaptively corrects an amplification distortion caused by an RF power amplifier in the transmit path. In an advantageous embodiment of the present invention, the adaptive pre-distortion circuit comprises 1) input sampling means coupled to an input of the transmit path capable of capturing from the digital input baseband signal a first input sample of amplitude X; 2) demodulation circuitry coupled to an output of the transmit path capable of receiving and demodulating the modulated RF output signal to thereby produce a digital output baseband signal; 3) output sampling means coupled to the demodulation circuitry capable of capturing a first output sample from the digital output baseband signal corresponding to the first input sample; and 4) processing means capable of comparing the first input sample and the first output sample and determining therefrom a pre-distortion correction value corresponding to the amplitude X. 
     According to an exemplary embodiment of the present invention, the processing means adds the pre-distortion correction value to a subsequently received input sample of amplitude X. 
     According to another embodiment of the present invention, the processing means comprises a table for storing the pre-distortion correction value. 
     According to still another embodiment of the present invention, the processing means modifies the pre-distortion correction value in response to a subsequent comparison of a second input sample of amplitude X with a second output sample corresponding to the second input sample. 
     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 the amplification distortion caused by an RF power amplifier is negligibly small and, in response to the determination, determines a scaling factor for the output samples. 
     According to a further embodiment of the present invention, the processing means scales the output samples, determines the pre-distortion correction values, and adds the pre-distortion correction values to the look-up table. 
     According to a still further embodiment of the present invention, the processing means modifies subsequently received input samples of amplitude X according to a value in the look-up table that corresponds to the amplitude X. 
     According to a yet further embodiment of the present invention, the processing means modifies subsequently received input samples according to a value in the look-up table regardless of the amplitude of the input samples. 
     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 in accordance with one embodiment of the present invention; 
     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; 
     FIG. 4 illustrates exemplary input and output synchronization and data acquisition controllers in accordance with one embodiment of the present invention; 
     FIG. 5 is an input power-output power diagram illustrating an exemplary pre-distortion error correction operation in accordance with one embodiment of the present invention; and 
     FIG. 6 is a flow diagram illustrating the operation of RF transmitter in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 6, 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 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. 
     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 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. 
     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 representative channel element  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  240  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 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. 
     To increase the efficiency of the RF transmitters in RF transceiver  250 , the present invention by implementing an adaptive digital pre-distortion correction (ADPD) circuit that samples the RF transmitter input and output signals and then synchronizes and compares the samples. The present invention then determines the pre-distortion correction required to correct the input signal and adds the pre-distortion correction to subsequent input samples of similar amplitude. The present invention theoretically can correct any distortion experienced by signals between the input and output sampling points. The present invention may be implemented in any type of digital modulation scheme, including TDMA, CDMA, GSM, multi-carrier signals, and even modems. 
     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-distortion correction controller  305 , look-up table (LUT)  306 , digital-to-analog converter (DAC)  310 , RF modulator  315 , local oscillator  320 , RF power amplifier (PA)  325 , and RF coupler (RFC)  330 . 
     RF transmitter  300  also contains a pre-distortion correction feedback loop that samples the input data signal and a corresponding part of the RF output signal, compares the samples, and generates a pre-distortion correction 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  and comparison and correction controller  355 . 
     A digital baseband signal, referred to as “INPUT DATA” in FIG. 3, is received by pre-distortion correction controller  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 . 
     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, 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 . To compensate for this condition, a pre-distortion signal is added to the INPUT DATA signal by pre-distortion correction controller  305 . 
     The pre-distortion correction signal is determined by the operation of input synchronization and data acquisition controller  345 , output synchronization and data acquisition controller  350  and comparison and correction controller  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 . 
     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. 
     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, etc.). The N-bit samples begin with a recognizable marker that denotes the start of the N-bit sample. When an entire N-bit sample has been detected and captured (acquired), data processor  401  sends a signal to interface and control circuit  402  which transfers the acquired data to comparison and correction controller  355 . 
     Similarly, OSDAC  350  comprises data processor  401 , interface (I/F) and control circuit  402 , and RAM  403 . A system clock provides a reference for clocking the distorted output digital baseband signal into data processor  401  and clocking the acquired data out of interface and control circuit  402 . The distorted output digital baseband signal samples are stored in RAM  403 . Data processor  401  comprises a signal correlator that analyzes the bits in the distorted output digital baseband signal to determine the start and stop of the N-bit data samples. The N-bit samples are the same N-bit samples that are contained in the INPUT DATA signal. Even though the output digital baseband signal may be distorted, enough of the bits remain unchanged to enable the signal correlator in data processor  401  to recognize the marker that denotes the start of the N-bit sample. When an entire N-bit sample has been detected and captured (acquired), data processor  401  sends a signal to interface and control circuit  402  which transfers the acquired data to comparison and correction controller  355 . 
     Comparison and correction controller  355  comprises comparison circuitry for comparing the acquired data received from ISDAC  345  and OSDAC  350 . Comparison and correction controller  355  can therefore perform a bit-by-bit comparison of an N-bit input sample and the corresponding distorted N-bit output sample. Once the amount of distortion has been determined comparison and correction controller  355  generates a pre-distortion error correction value that is sent to pre-distortion correction controller  305  and stored in look-up table (LUT)  306 . Thereafter, as pre-distortion correction controller  305  receives sub-sequent N-bit samples of the INPUT DATA signal, pre-distortion correction controller  305  can look-up the pre-distortion error correction corresponding to the amplitude of the N-bit sample and add the pre-distortion error correction. 
     FIG. 5 depicts an input power-output power diagram  500  which illustrates an exemplary pre-distortion error correction operation in accordance with one embodiment of the present invention. Lines  501 - 503  in FIG. 5 are intended only to help in the explanation of the error correction operation and are not intended to be drawn to scale. Those skilled in the art will recognize that the relative slopes, curvatures and separations of lines  501 - 503  will necessarily vary according to the RF power amplifier type and according to environmental conditions. 
     Line  501  depicts the input/output response of RF power amplifier  325  under ideal linear operating conditions. As the amplitude of the input signal (horizontal axis) rises, the amplitude of the output signal (vertical axis) rises according to a steady slope, indicating constant amplifier gain. Line  502  depicts the input/output response of RF power amplifier  325  under real-world non-linear operating conditions. As the amplitude of the input signal rises, the amplitude of the output signal rises according to a steady slope only up to a certain point, at which time RF power amplifier  325  being to saturate and amplifier gain becomes non-linear. 
     Line  503  indicates the pre-distortion correction values stored in LUT  306  and added by pre-distortion correction controller  305  as the input signal rises to the point where saturation occurs. The pre-distortion correction values compensate for the fall-off of line  502  from the ideal line  501  to thereby make the output of RF power amplifier  325  more like the ideal linear output of line  501 . In an advantageous embodiment of the present invention, the pre-distortion correction circuitry of RF transmitter  300  operate in an iterative manner, such that the pre-distortion correction values in LUT  306  are constantly updated and refined over time. Thus, the pre-distortion correction value for an input peak of amplitude X is calculated the first time an input peak of amplitude X is encountered and is stored in LUT  306 . The second time an input peaks of amplitude X is encountered, the pre-distortion correction value is added to amplitude X, the corrected output is measured, and the pre-distortion correction value is re-calculated to determine if further correction is needed. This process constantly repeats, thereby making the pre-distortion correction values in LUT  306  more accurate and modifying the pre-distortion correction values as temperature and operating frequency change and as RF power amplifier  325  ages. 
     FIG. 6 depicts flow diagram  600 , which illustrates the operation of RF transmitter  300  in accordance with one embodiment of the present invention. First, during routine operation, pre-distortion correction controller  305  receives N-bit samples of the digital baseband input signal and adds a pre-distortion correction value, if any (process step  601 ). The corrected (or “pre-corrected”) digital baseband signal is converted to a corrected 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  602 ). 
     In the pre-distortion correction loop, the RF output signal is demodulated in RF demodulator  335  to recover the analog baseband output signal, which may be distorted. The analog baseband output signal is converted to a digital signal and sampled (process step  603 ). Next, the original digital baseband input signal samples are aligned with and compared to the digital baseband output signal samples. A scaling factor (small signal close loop-gain) is determined by comparing digital baseband input signals having small amplitudes with their corresponding digital baseband output signals. The digital baseband output signals are then divided by this scaling factor and compared to the digital baseband input signals and new pre-distortion correction values are calculated (process step  604 ). Finally, the new pre-distortion correction samples are stored in LUT  306  for use by pre-distortion correction controller  305  (process step  605 ). Thereafter, the process repeats by looping back to process step  601 , thereby giving the present invention its adaptive nature. The pre-distortion correction values are constantly updated and corrected 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.