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DIGITAL BASEBAND RECEIVER IN A
MULTI-CARRIER POWER AMPLIFIER
CROSS-REFERENCE TO RELATED
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/273,987, filed Mar. 7, 2001 by Paul E. White et al., which application is incorporated by reference herein.
FIELD OF THE INVENTION
This invention relates generally to radio frequency (RF) power amplifier systems, and more particularly to a method and apparatus for locating and suppressing intermodulation 15 distortion (IMD) products in a multi-carrier power amplifier (MCPA) system.
BACKGROUND OF THE INVENTION
Ideally, RF power amplifiers would act linearly, faithfully reproducing an amplified RF signal at their output with no distortion. Unfortunately, in practice, physical RF power amplifiers can be non-linear and add a certain amount of unwanted distortion to a signal, which distortion is realized
as IMD products. These IMD products cause interference over in the normal operating frequency range of the amplifier, which may impede proper transmission and reception of RF signals. Numerous techniques have been developed to reduce IMD products from amplified RF signals, including feed forward, predistortion, and linear amplification with non-linear components (LINC).
In multi-carrier power amplifier systems, the effects of IMD products such as interference and crosstalk may be compounded as a result of the close proximity of frequency 35 bands. Multi-carrier power amplifiers (MCPA) therefore must operate at high drive levels in order to achieve the high linearity demanded by broadband applications. Energy leakage resulting from one band spilling over into another can undesirably degrade the signal-to-noise (SNR) ratio or bit- 4Q error rate (BER) of the proximate frequency bands.
One common technique to reduce IMD to acceptable levels is feed forward correction, whereby the IMD products are manipulated so that at the final summing point the IMD products substantially cancel out. Classic feed forward 45 amplifiers use what is conventionally known as a pilot tone to assist in the control of the phase and gain of an error amplifier in order to minimize IMD. A pilot tone is generated and injected with the RF signals at the input to simulate an artificial signal whose frequency content is known. The 50 amplifier produces amplified signals and simulated distortion products based on the pilot tone. At the output, a pilot tone receiver detects the simulated distortion, not the actual distortion, and the amplifier is aligned based upon minimization of the simulated distortion. However, because the 55 amplifier is not aligned in accordance with the actual distortion products, they may not be entirely cancelled or may leak into the output, creating unwanted byproducts.
Another technique is to digitize the RF signals to baseband, filter out the desired frequency components, and 60 then analyze the remaining undesired distortion components in a digital signal processor (DSP). This technique does not require the use of a pilot tone. The energy of these distortion components is located and measured in the DSP, and the feed forward loop is adjusted until the undesired compo- 65 nents are eliminated. In one conventional design, for example, a feed forward amplification system uses mask
detection compensation on an RF signal modulated according to a known modulation format. The RF signal is amplified, producing in-band frequency components and undesired out-of-band distortion components. The amplified signal is heterodyned to baseband so as to be centered about DC. A wide passband (1.25 MHz) bandpass filter is used to eliminate the in-band frequency components. A microprocessor queries a DSP for the energy at predetermined offsets (representative of an IMD location), and control signals adjust the gain-phase network of a feed forward network in accordance with the out-of-band distortion components.
The above approach operates in an environment where signals are modulated according to a single known modulation format (CDMA). However, such an approach would not be well suited for detecting narrowband signals such as TDMA and their associated IMD products, in part due to the wide bandwidth of the filter (1.25 MHz).
RF signals can be modulated according to any number of modulation formats which are well known in the art, including, for example, TDMA, GSM, CDMA, WCDMA, QAM, and OFDM, each of which have varying bandwidths. For example, the bandwidth for a WCDMA signal is 3.84 MHz (wideband), and the bandwidth for a CDMA signal is 1.25 MHz. By contrast, a GSM signal has a bandwidth of 250 kHz, and a TDMA signal has a bandwidth of only 30 kHz (narrowband). Thus, the bandwidth of a signal, depending on its modulation format, can vary from 30 kHz to 3.84 MHz. If the signals are located in a PCS frequency band (1930 to 1990 MHz), a narrowband tuner would require too much time to tune across the 60 MHz band, and a wideband tuner would not be able to detect the individual carriers of TDMA or GSM signals or their associated IMD products. In short, there is a tradeoff between the bandwidth of a tuner and the speed with which it can identify and eliminate IMD products.
Therefore, a need exists for a tunable receiver having a dynamic range sufficient to identify and eliminate IMD products from RF signals, particularly multi-carrier signals modulated according to both wideband and narrowband modulation formats.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a functional diagram of a feed forward multicarrier power amplifier circuit with a tunable receiver in accordance with one aspect of the present invention.
FIG. 2 is a functional diagram of a tunable receiver in accordance with one aspect of the present invention.
FIG. 3 is a functional diagram of a frequency synthesizer in accordance with a specific aspect of the present invention.
FIG. 4 is a diagrammatic chart of a carrier search algorithm in accordance with a specific aspect of the present invention.
FIG. 5 is a diagrammatic chart of an IMD suppression algorithm in accordance with a specific aspect of the present invention.
Although the invention will be described next in connection with certain exemplary embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the description of the inven3
tion is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims.
The present invention relates to an apparatus and method 5 for locating and suppressing intermodulation distortion (IMD) products in an amplifier system. A signal path having an input and an output is configured to communicate an RF communications signal disposed in a frequency band. A tunable receiver is coupled to the signal path and configured 1° to downconvert at least a portion of the frequency band for the RF communications signal to an Intermediate Frequency (IF) signal. A circuit arrangement is configured to convert a time domain representation of the IF signal output by the tunable receiver to a frequency domain representation, iden- :5 tify an IMD product from the frequency domain representation of the IF signal, and control an IMD reduction component that is coupled to the signal path to suppress the IMD product at the output of the signal path.
In an exemplary embodiment of the invention, the afore- 20 mentioned apparatus and method are utilized to locate and suppress IMD products in a feed forward multi-carrier power amplifier system. In such an embodiment, an IMD reduction component is disposed in a feed forward path that is coupled to a main signal path over which an RF commu- 25 nications signal is communicated. The RF communications signal is typically located in a predetermined frequency band and may include any number of RF signals modulated according to any combination of wideband and narrowband modulation formats. 30
In the exemplary embodiment, the RF signals are amplified by a main amplifier on the main signal path, which produces both amplified RF signals and undesired IMD products. The amplified RF signals and undesired IMD 3J products are coupled to a tunable receiver. The tunable receiver generally includes a frequency synthesizer, a mixer, a filter, and an analog-to-digital converter (ADC). A processing unit connected to the tunable receiver tunes the frequency synthesizer to a location within the frequency 4Q band, the mixer downconverts the RF signals to IF signals based on the output of the frequency synthesizer, and a filter passes only a portion of the IF signals. The filter has a sufficient bandwidth so as to discern both wideband and narrowband carriers and their associated IMD products. 4J
The analog-to-digital converter digitizes the passed IF signals, and then provides the digitized signals to a digital signal processor (DSP). The DSP converts the digitized signals to the frequency domain for spectral analysis. The DSP may provide the power of a signal at a particular 50 frequency or range of frequencies. During operation, the processing unit typically steps the frequency synthesizer across a selected portion of the frequency band (which may include the entire frequency band), and stores the frequency spectrum produced by the DSP at each step as a map in 55 memory. Once a selected portion of the band has been mapped out, the processing unit detects peaks in the map and identifies those peaks as carrier signals. Optionally, the processing unit may also identify the type of modulation format of a signal based on its bandwidth and signal statis- 60 tics. The processing unit also locates IMD products by detecting the power below a certain threshold (typically 20 to 30 dB below the carrier levels) and drives feed forward control elements to optimally suppress the IMD products in the main signal path. 65
It will be appreciated by one of ordinary skill in the art, however, that the principles of the invention may be applied
to other environments. For example, the principles of the invention may be utilized in connection with single carrier RF signals, as well as in conjunction with other types of amplifier designs, e.g., amplifier designs incorporating analog and/or digital predistortion, or in other amplifier designs that inherently generate IMD products as a result of amplification. Moreover, it will be appreciated that the application of the herein-described concepts to single carrier environments, as well as other amplifier designs, would be well within the ability of one of ordinary skill in the art having the benefit of the instant disclosure. Therefore, while the invention will be described hereinafter in connection with a feed forward multi-carrier power amplifier system, the invention is not limited to the particular embodiments described herein.
FIG. 1 shows a functional diagram of a typical feed forward multi-carrier power amplifier (MCPA) circuit with a tunable receiver 144. It should be understood that feed forward circuits are well known in the art, and that the feed forward circuit shown in FIG. 1 is merely exemplary and that numerous variations of the feed forward circuit provided in FIG. 1 could be employed without departing from the spirit and scope of the present invention. According to one aspect of the present invention, the typical circuit generally includes an input 100, a main signal path 102, a feed forward path 104, and an output 112. The circuit further includes a carrier correction loop (CCL) 106, an error correction loop (ECL) 108, and a tunable receiver loop 110. On the feed forward path 104, there is provided a feed forward delay filter 118, a feed forward attenuator 120, a feed forward phase shifter 122, and a feed forward amplifier 124. On the main signal path 102, there is provided a main attenuator 134, a main phase shifter 136, a main amplifier 138, and a main delay filter 140. Note that the feed forward attenuator 120 and feed forward phase shifter 122 may be incorporated into the feed forward amplifier 124, and the gain and phase of the feed forward amplifier 124 may be controlled by gain and phase control lines (not shown). Similarly, the main attenuator 134 and main phase shifter 136 may be incorporated into the main amplifier 138.
The input 100 receives radio frequency (RF) carrier signals that collectively comprise a multi-carrier RF communications signal, and an input carrier coupler 114 couples the RF carrier signals onto both the main signal path 102 and the feed forward path 104. Alternatively, a splitter (not shown) may be used to provide the RF carrier signals onto the main signal path 102 and the feed forward path 104. The RF carrier signals may lie in any frequency band such as, for example, AMPS, DCS, PCS, UMTS, or MMDS. Furthermore, the RF carrier signals may be modulated according to any modulation format such as, for example, TDMA, GSM, CDMA, WCDMA, QAM, and OFDM, to name a few. Other modulation formats are expressly contemplated by the present invention, so long as their bandwidth is known. For example, bandwidths of the aforementioned modulation formats include: WCDMA, 3.84 MHz; CDMA, 1.25 MHz; GSM, 250 kHz; TDMA, 30 kHz. QAM-modulated signals, for example, have varying bandwidths depending on the data rate.
Referring again to FIG. 1, the RF carrier signals in the main signal path 102 may be attenuated by the main attenuator 134 and phase shifted by the main phase shifter 136, but not necessarily in that order. Optionally, a CCL power detector 150 may be provided on the feed forward path 104 to monitor the power level of the signals after the carriers have been subtracted from the CCL 106. Control of the main attenuator 134 and the phase shifter 136 may be under