Abstract:
The present invention provides a technique for canceling spurious noise content in transmitted signals. During transmission, the transmitted signals are fed back to circuitry adapted to provide a predistortion component for signals to be transmitted, as well as to compensate for spurious noise injected into the system. In a first path, the circuitry will predistort the baseband signals to provide a predistorted output. Along a second path, a fast Fourier transform is performed on the feedback signal. The resultant frequency domain signal is then processed to detect spurious noise content above an identified defined threshold. The spurious noise content is used to recreate a replica of the noise content. The replica will have the same phase and frequency and a magnitude sufficient to reduce or eliminate the content to a desired degree. The replicated signal is then negatively added to the predistorted signals to create a final signal for transmission.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to wireless communications, and in particular to canceling spurious signal content from signals to be transmitted. 
     BACKGROUND OF THE INVENTION 
     Wireless communication systems often incorporate predistortion techniques configured to pre-process signals for transmission to compensate for non-linearities or like anomalies inherent to transmission. Unfortunately, the compactness of wireless architectures makes these systems prone to having spurious noise injected into the signals to be transmitted. The source of the spurious noise may take many forms, including the various local oscillators facilitating reception and transmission of wireless communication signals, harmonics and mixing products from mixing circuitry, and the like. Unfortunately, compensating for the unwanted injection of spurious noise content is difficult to address, and can be even more difficult to address in a cost-effective manner in systems incorporating predistortion techniques. 
     Accordingly, there is a need for a technique to cancel spurious noise in wireless communication systems, and a further need for a technique to cancel spurious noise in wireless communication systems incorporating predistortion techniques to compensate for non-linearities or other anomalies in the transmission path. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique for canceling spurious noise content in transmitted signals. During transmission, the transmitted signals are fed back to predistortion and cancellation circuitry adapted to provide a predistortion component for signals to be transmitted, as well as to compensate for spurious noise injected into the system. In a first path, the predistortion and cancellation circuitry will predistort the baseband signals to compensate for non-linearity and like anomalies injected by the transmission circuitry to provide a predistorted output. Along a second path, a fast Fourier transform is performed on the feedback signal. The resultant frequency domain signal is then processed to detect spurious noise content above a defined threshold, throughout the range or within defined bands. The spurious noise content is identified and used to recreate a replica of the noise content. The replica will preferably have the same phase and frequency, and a magnitude sufficient to reduce or eliminate the content to a desired degree. The replicated signal is then negatively added to the predistorted signals to create a final signal for transmission. 
     Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention. 
     FIG. 1 is a block representation of a base station architecture according to one embodiment of the present invention. 
     FIG. 2 is a block representation of the feedback circuitry used for predistortion and cancellation according to one embodiment of the present invention. 
     FIG. 3 is a block representation of the predistortion and cancellation circuitry according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     As illustrated in FIG. 1, the present invention may be incorporated in a base station transceiver  20 , which includes a receiver front end  22 , a radio frequency transmitter section  24 , an antenna  26 , a duplexer or switch  28 , a baseband processor  30 , a control system  32 , and a frequency synthesizer  34 . The receiver front end  22  receives information bearing radio frequency signals from one or more remote transmitters provided by mobile terminals, such as mobile telephones, wireless personal digital assistants, or like wireless communication devices. A low noise amplifier  38  amplifies the signal. A filter circuit  40  minimizes broadband interference in the received signal, while downconversion and digitization circuitry  42  downconverts the filtered, received signal to an intermediate frequency (IF) signal, which is then digitized into one or more digital streams. The receiver front end  22  typically uses one or more mixing frequencies generated by the frequency synthesizer  34 . 
     The baseband processor  30  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, error correction, and interference cancellation operations. As such, the baseband processor  30  is generally implemented in one or more digital signal processors (DSPs), application specific integrated circuits (ASICs), or field programmable gate arrays (FPGAs). Further detail regarding the operation of the baseband processor  30  is described in greater detail below. On the transmit side, the baseband processor  30  receives digitized data, which may represent voice, data, or control information, from the control system  32 , which it encodes for transmission. 
     The data for transmission is preferably provided in a quadrature format wherein the data is represented by in-phase (I) and quadrature phase (Q) signals. The encoded data is output to predistortion and cancellation circuitry  44 , which will be discussed below in greater detail. In general, the predistortion and cancellation circuitry  44  predistorts each of the in-phase and quadrature phase signals I, Q to compensate for non-linearities introduced by the transmission circuitry  24 , as well as minimizing the impact of spurious signals on the signals to be transmitted. The resultant in-phase (I T ) and quadrature phase (Q T ) signals are sent to the transmitter circuitry  24 , where they are processed by a modulator  46  to modulate a carrier signal that is at a desired transmit frequency. Power amplifier circuitry  48  amplifies the modulated carrier signal to a level appropriate for transmission, and delivers the modulated carrier signal to the antenna  26  through a matching network  50 , coupler  51 , and duplexer  28 . 
     For the present invention, the transmitted signal is fed back from the coupler  51  in the form of a feedback signal for processing by feedback signal processing circuitry  52 , which will generate a feedback IF signal to send to the predistortion and cancellation circuitry  44 . The predistortion and cancellation circuitry  44  facilitates predistortion and cancellation of the spurious signals from the signals to be transmitted based on the feedback IF signal. 
     Turning now to FIG. 2, the in-phase and quadrature phase signals I T  and Q T , which have been predistorted and processed to cancel spurious signals, are respectively presented in a digitized format to digital-to-analog converters (DAC)  54 I and  54 Q. The DACs  54 I,  54 Q will convert the respective digitized in-phase and quadrature phase signals I T  and Q T  to an analog format, wherein the respective signals are filtered by filters  56 I and  56 Q, amplified by amplifiers  58 I and  58 Q, and presented to mixers  60 I and  60 Q. The mixers  60 I and  60 Q are driven in a quadrature arrangement by a radio frequency local oscillator  62  such that the signal provided to mixer  60 I is offset by  90  degrees by a  90  degree shift function  64  in traditional fashion. Accordingly, the mixers  60 I and  60 Q modulate the in-phase and quadrature phase signals I T  and Q T  to generate radio frequency modulated versions of each. The modulated signals are summed at summing circuitry  66 , the output of which is filtered by filter  68 , preamplified by preamplifier  70 , and amplified to a level for transmission by power amplifier  74 . The output of the power amplifier  74  is sent to the coupler  51  and then to the duplexer  28  and transmitted via the antenna  26 . Preferably, the coupling function is located within the transmit circuitry  24  such that the transmitted signal is fed back in the form of the feedback signal to the feedback signal processing circuitry  52 . Alternatively the coupling function could be located within the duplexer  28 . 
     In one embodiment, the feedback signal is sent directly to an attenuator  76  to reduce the level of the feedback signal prior to being presented to mixer  78 , which is driven by radio frequency oscillator  80 . Thus, the attenuated feedback signal is downconverted to an intermediate frequency (IF) signal by the mixer  78  prior to being amplified by preamplifier  82 , filtered by filter  84 , and presented to an analog-to-digital converter (ADC)  86  to provide a digitized feedback IF signal, which is presented to the predistortion circuitry  44  to control predistortion and cancellation of spurious signals. 
     Referring now to FIG. 3, the digitized feedback IF signal is split and provided along two separate paths in the predistortion and cancellation circuitry  44 . Along the first path, the feedback IF signal is provided to a quadrature-based mixer  88  controlled by a numerically controlled oscillator  90 . The quadrature-based mixing circuitry  88  provides in-phase and quadrature phase baseband signals I F  and Q F  derived from the feedback IF signal. The in-phase and quadrature phase baseband signals I F  and Q F  are presented to adaptive predistortion and modulation compensation circuitry  92 . This circuitry receives the actual in-phase and quadrature phase data I and Q from the baseband processor  30  and provides the following processing. First, the adaptive predistortion aspect of the circuitry  92  processes the baseband I and Q signals to compensate for non-linearities injected by the transmission circuitry  24 . In one embodiment, this is done by multiplying the complex numbers represented by the in-phase and quadrature phase signals I and Q with values from a look-up table that are determined by the in-phase and quadrature phase feedback signals. The modulation compensation aspect of the circuitry  92  adjusts for phase and amplitude imbalance as well as DC offset between the in-phase and quadrature signals incurred in the modulation aspect of transmission circuitry  24 . Those skilled in the art will appreciate the various ways to implement predistortion and compensate for I and Q phase and amplitude imbalance as well as DC offset based on the provided feedback. The outputs of the circuitry  92  are the in-phase and quadrature phase signals I P  and Q P , which have been processed to compensate for non-linearities, phase and amplitude imbalances, and DC offsets imposed by the modulation aspect of the transmit circuitry  24 . 
     The second path in the predistortion and cancellation circuitry  44  processes the feedback IF signal to provide in-phase and quadrature phase components configured to cancel spurious signals that are injected into signals being transmitted. These spurious signals might be injected into the system by the transmitter or receiver&#39;s local oscillator fundamental or harmonic signals, mixing products of these signals, and the like. Spurious signals injected into the system may also originate from oscillation in amplifiers or from digital circuit clocks, or from harmonics or mixing products of these signals. Since the feedback IF signal is in the time domain, it is first presented to fast Fourier transform (FFT) circuitry  94  to perform a fast Fourier transform or like Fourier transform. The resultant signal is sent to a spurious signal level detector  96 . The spurious signal level detector  96  calculates the composite power from the FFT signal or receives it from other circuitry in the base station and also calculates the power in one or more narrow band frequency ranges. The entire spectrum may be divided into multiple narrow bands, or select bands may be defined at or around frequency ranges known to have spurious content. If a spurious signal level within the transmission spectrum exceeds a threshold relative to the composite power, the spurious signal level detector  96  will trigger the controller  98  to attempt to cancel the spurious noise component or otherwise reduce it to an acceptable level. Preferably, the spurious signal level detector  96  will identify the frequency or frequency band, as well as a relative signal level, of spurious content. Accordingly, the controller  98  will signal phase and frequency adjust circuitry  100  to control the numerically controlled oscillator  102  to create in-phase and quadrature phase signals I′ and Q′ at the appropriate frequency and phase of the spurious content. The in-phase and quadrature phase signals I′ and Q′ replicating the spurious noise content are respectively sent to in-phase and quadrature phase attenuators  104 I and  104 Q. For spurious signals detected outside of the transmission bandwidth but within the predistortion and spurious cancellation band, the potential spurious signal will be compared to an absolute level rather than a level relative to the composite power. 
     In addition to controlling the phase and frequency of the replicated in-phase and quadrature phase signals I′ and Q′, the controller  98  provides a control signal to the attenuators  104 I and  104 Q to effectively adjust the magnitude of the replicated in-phase and quadrature phase signals I′ and Q′ to have the same magnitude as the spurious content. Thus, the output of the attenuators  104 I and  104 Q are in-phase and quadrature phase signals I″ and Q″, which have the same frequency, phase, and magnitude as the spurious content. These in-phase and quadrature phase signals I″ and Q″ are combined or summed with the predistorted in-phase and quadrature phase signals I P  and Q P  at summation circuitry  106 I and  106 Q to create the in-phase and quadrature phase signals for transmission I T  and Q T  , which are provided to the digital-to-analog converters  54 I and  54 Q (of FIG.  2 ). Notably, the summation circuitry  106 I and  106 Q will actually add the negative of the in-phase and quadrature phase signals I″ and Q″ representing the spurious noise content to effect cancellation of the components in the in-phase and quadrature phase signals for transmission I T  and Q T . Alternatively, the controller  98  will operate to control the numerically controlled oscillator  102  and attenuators  104 I and  104 Q to create signal replicas, which are 180 degrees out of phase from the spurious content or being the negative thereof. Those skilled in the art will recognize the variations in effectively subtracting the replicas of the spurious content from the predistorted in-phase and quadrature phase signals I T  and Q T . The present invention is particularly beneficial in code division multiple access (CDMA) and wideband CDMA (WCDMA) architectures. Further, the techniques are equally applicable to mobile terminals and wireless LANs in addition to base stations. 
     Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.