Patent Application
20080260080
The present invention generally relates to narrowband interference cancellation, and particularly relates to narrowband interference cancellation using a Cartesian feedback loop.
A composite signal received at a wireless receiver comprises a signal of interest and interfering signals, one or more of which may be narrowband in nature, i.e., have a narrow frequency spectrum in relation to the signal of interest. Receiver quality is measured at least in part based on the receiver's ability to extract or recover a signal of interest from the composite signal while rejecting interference. One conventional signal recovery approach involves filtering interfering signals from the composite signal, particularly a dominant interfering signal. A dominant interferer may be the strongest interferer, or the one having the most problematic frequency spectrum.
Some applications are prone to narrowband interference. For example, a Wideband Code Division Multiple Access (WCDMA) receiver conventionally includes one or more channel filters for suppressing narrowband interference. Narrowband interference arises in a WCDMA network when the network is collocated with a Global System for Mobile Communications (GSM) network. GSM networks subdivide their total available bandwidth into a multitude of narrowband carrier frequency channels each spaced 200 kHz apart. Accordingly, a WCDMA receiver is subjected to a narrowband GSM interferer spaced approximately 2.7 MHz away from the center frequency of the WCDMA signal of interest (or possibly even closer for prospective wideband cellular standards). The narrowband GSM interferer typically has a power level greater than that of the desired WCDMA signal. As such, a channel filter tuned to the desired WCDMA signal must suppress the narrowband GSM interferer by 30 to 40 dB or more. Absent such suppression, downstream receiver circuits must be able to tolerate or suppress potentially significant levels of narrowband interference, which may be impractical or impossible to achieve.
Further complicating channel filter design is the frequency closeness between a desired WCDMA signal and a narrowband GSM interferer. The channel filter must have a steep cutoff response to suppress the narrowband GSM interferer without adversely affecting the desired WCDMA signal. Of course, narrowband interference cancellation is a concern for applications other than WCDMA where narrowband interferers have large power levels and are spaced closely in frequency to the wideband signal of interest. Typically, conventional filters have one or more high-Q (narrowband) zeroes for providing a steep cutoff response, e.g., such as a band-stop filter. However, steep band-stop filters are complex and difficult to implement. Further, filters having a steep cutoff response tend to consume large amounts of power and semiconductor area. The signal of interest is also subjected to distortion, attenuation or other adverse effects that arise when the signal of interest is passed through a filtering stage to remove narrowband interference.
According to the methods and apparatus taught herein, a Cartesian Feedback-loop Signal Canceller (CFSC) cancels narrowband interference from a composite signal to recover a signal of interest. The CFSC cancels a narrowband interferer by generating an estimate of the interferer and destructively combining the estimate with the composite signal to form an interference-compensated signal. The interference-compensated signal is frequency down-converted to Cartesian components to form a feedback signal. Cartesian components associated with the signal of interest are filtered or removed from the feedback signal. The remaining Cartesian components correspond to any residual difference between the narrowband interferer estimate and interferer that remains after destructive combination. The filtered feedback signal is then modulated and amplified to form the narrowband interferer estimate, thus completing a closed-loop Cartesian interference cancellation feedback loop without subjecting the signal of interest to distortion, attenuation or other adverse effects associated with passing the signal of interest through a filter.
According to one embodiment, narrowband interference cancellation is performed by destructively combining a composite signal including a narrowband interference signal and a signal of interest with an estimate of the narrowband interference signal to generate an interference-compensated signal. A feedback signal is generated having Cartesian components derived from the interference-compensated signal. Cartesian components associated with the signal of interest are removed from the feedback signal to generate the narrowband interference signal estimate. The CFSC may be used to cancel various types of narrowband interference such as GSM interference in a WCDMA receiver, undesired sampling images generated by a digital-to-analog converter, etc.
Of course, the present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.
The interference-compensated signal is fed back through the CFSC 100 via a feedback path 102. The feedback path 102 generates a feedback signal having in-phase (I) and quadrature (Q) Cartesian components derived from the interference-compensated signal. At this point, the feedback signal (IFB/QFB) includes Cartesian components associated with both the signal of interest (IINTEREST/QINTEREST) and any residual difference remaining between the narrowband interferer estimate and interferer after destructive combination (INB/QNB=SNB−ŜNB). The Cartesian components associated with the signal of interest are removed from the feedback signal. The feedback signal is then used to generate the narrowband interference signal estimate based on the remaining interferer Cartesian components, thus completing a closed-loop Cartesian feedback loop.
Unlike conventional filters, the CFSC 100 does not filter the narrowband interferer from the composite signal to recover the signal of interest. Instead, the CFSC 100 filters the signal of interest from a feedback signal. An estimate of the narrowband interferer is then generated from the feedback signal which is then used to cancel the narrowband interference. This way, the signal of interest is not subjected to distortion, attenuation or other adverse effects associated with passing the signal of interest through a filter.
In more detail, the feedback path 102 of the CFSC 100 comprises a demodulator 104 and an optional attenuator 106. The optional attenuator 106 reduces signal distortion when the composite signal has a large voltage margin by attenuating the interference-compensated signal. The demodulator 104 down-converts the interference-compensated signal to in-phase (IFB) and quadrature (QFB) Cartesian components using a local oscillation frequency (LO) as is well known in the art. The down-converted Cartesian components form the feedback signal. In one embodiment, the demodulator 104 includes a switched mixer that down-converts the interference-compensated signal at odd harmonics of the local oscillation frequency. Preferably, the switched mixer is double-balanced to avoid even-order distortion and has a small DC-offset at its output to avoid a large tone.
The local oscillation frequency may be fixed or programmable. Either way, the local oscillation frequency is preferably set near a center frequency of the narrowband interferer. In one embodiment, the local oscillation frequency is fixed and approximately equals the narrowband center frequency. A different embodiment employs a reference clock (not shown) which is programmable to a desired local oscillation frequency for use by the CFSC 100. This way, the reference clock may be shared by the CFSC 100 and other components (not shown). According to this embodiment, the local oscillation frequency is near, but may not identically match the narrowband center frequency.
Regardless, the local oscillation frequency is set near the center frequency of the narrowband interferer to down-convert the residual interferer component of the interference-compensated signal to relatively low frequency Cartesian components, e.g., less than a few MHz.
A filter 108 such as a low pass filter (LPF) included in the CFSC 100 processes the Cartesian components of the feedback signal based on the frequency offset between the signal of interest and the residual narrowband interference. The filter 108 is tuned to pass the Cartesian components associated with the residual narrowband interference and block the Cartesian components associated with the desired signal, e.g., as illustrated by Step 400 of
For example, based on the spectrum illustrated in
The filter 108 may comprise a higher-order filter to achieve a steeper roll-off response at the expense of reduced phase margin for the loop. In one embodiment, the filter 108 has two real-valued poles (e.g., two poles at −2π*570 krads/s or one pole at −2π*350 krads/s and the other at 3× the first pole). In another embodiment, the filter 108 has a Butterworth pole pair (e.g., poles at −2π*380±j2π*380 krads/s). Regardless, filter selection depends on the application supported. Accordingly, any filter having a frequency response that sufficiently passes the residual narrowband interferer while blocking the signal of interest from the CFSC feedback signal may be used. The bandwidth of the filter 108 partly determines the CFSC loop gain and is selected to sufficiently attenuate the narrowband interferer without adversely affecting the signal of interest when the narrowband interferer estimate is combined with the composite signal.
A signal estimator 110 included in the CFSC 100 converts the filter output (ÎNB/{circumflex over (Q)}NB) to the narrowband interferer estimate (ŜNB), e.g., as illustrated by Step 402 of
The amplifier 114 included in the signal estimator 110 amplifies the modulator output to produce an estimate (ŜNB) of the narrowband interferer. A signal combiner 118 such as a summer or a common circuit node destructively combines the interference estimate and the composite signal to recover the signal of interest from the composite signal, e.g., as illustrated by Step 404 of
In more detail, a low-noise amplifier (LNA) stage 502 of the radio/wireless receiver 500 generates an LNA output based on a received analog input signal. The mixer stage 504 frequency down-converts the LNA output. In one embodiment, the mixer 504 directly converts the LNA output to baseband Cartesian components, thus making the receiver 500 a direct-conversion (homodyne) receiver. Two CFSCs 100 may be included for processing the separate Cartesian components output by the direct-conversion mixer stage 504. Alternatively, a single CFSC 100 may be used when the baseband components are accurately matched. The interferer is present in both the I and Q baseband components with approximately the same magnitude, but with a 90° phase shift when matched. Accordingly, a single CFSC 100 may be used to detect the interferer in both baseband components and cancel the interferer from the composite signal.
Regardless, the baseband filter stage 506 filters the mixer output which is then amplified by the amplification stage 508. The ADC stage 510 generates one or more digital outputs for use by a digital domain (not shown) of the radio/wireless receiver 500. Of course, the receiver 500 may comprise other suitable components. For example, the digital domain of the receiver 500 may include digital components for processing the digital output of the ADC stage 510. Additional analog stages (not shown) may also be included in the receiver 500 between which the CFSC 100 may be coupled for cancelling narrowband interference.
With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims, and their legal equivalents.
