Correction of specific intermodulation products in a concurrent multi-band system

Systems and methods are disclosed herein for selectively compensating for a specific Intermodulation Distortion (IMO) product(s) of an arbitrary order in a transmitter system. In some embodiments, a method of compensating for one or more specific IMO products in a concurrent multi-band transmitter system comprises generating an IMO correction signal for a specific IMO product as a function of two or more frequency band input signals for two or more frequency bands of a concurrent multi-band signal, the IMO product being an arbitrary order IMD product. The method further comprises frequency translating the IMD correction signal to a desired frequency that corresponds to a Radio Frequency (RF) location of the specific IMO product and, after frequency translating the IMO correction signal to the desired frequency, utilizing the IMO correction signal to compensate for the specific IMO product.

This application is a 35 U.S.C. § 371 national phase filing of International Application No. PCT/IB2017/055225, filed Aug. 30, 2017, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a concurrent multi-band transmitter and, in particular, to correction of an Intermodulation Distortion (IMD) product in a concurrent multi-band transmitter.

BACKGROUND

Digital Predistortion (DPD) employs Digital Signal Processing (DSP) techniques to impress an “inverse characteristic” of the Power Amplifier (PA) on the transmitted signal to compensate for the non-linear distortion thereby introduced. Typically, the distortion function is modeled as a sum of output signals produced from (non-orthogonal) basis functions weighted by a corresponding set of complex-valued tap coefficients as in the Generalized Memory Polynomial (GMP) framework of [1].

Recent advanced transmitter architectures target the capability to service signals in multiple bands concurrently as a means to lower cell site cost and complexity. Concurrent dual-band systems require DPD with much higher computational complexity since nonlinear behavior of concurrent dual-band PAs includes both intra-band and inter-band (cross-band) distortion products. Concurrent dual-band DPD requires extension to two dimensions (i.e., Two Dimensional DPD (2D-DPD)) leading to costly increases in computational complexity for GMP schemes [2] or to impractical memory depths for techniques based on Lookup Tables (LUTs) [3]. Recently, a flexible architecture has been proposed based on overlapping splines [4] and a closed-loop Least Mean Square (LMS) adaptation procedure [4] to solve these issues.

In some dual-band (or multi-band) configurations, it is not only necessary to correct the distortion centered around the carriers in each band, but also some of the Intermodulation Distortion (IMD) products. These IMD products fall at integer multiples of the band frequencies, as well as frequencies related to the sum and difference of the band frequencies and their multiples. For a dual-band scenario, the frequency location of these IMD products can be denoted by:

Equation 1—IMD Frequency Location
fIMD=c1f1+c2f2,
where f1and f2are the center frequencies of a first band and a second band, respectively, and c1and c2are signed integer valued coefficients. Note that the order of the IMD product is given by:

As an example, consider a dual-band configuration with a first band centered at f1=759 megahertz (MHz) and a second band centered at f2=958 MHz. The potential IMD product locations are calculated and presented inFIG. 1(up to the fourteenth order). As can be seen fromFIG. 1, several high-order IMD products (represented by bold boxes) fall close enough to the main carrier locations (represented by the bold numbers) that they may require some form of correction.

The predistortion implications of IMD products can be better understood by considering the mathematical formulation of a simple example. Let a simple third order nonlinear (baseband) system be described as:

Equation 3—Simple Third Order Nonlinearity
y(n)=x2(n)x*(n),
where y(n) is the system output and x(n) is the system input. For a dual-band system, the input signal is given by:

Equation 4—Dual-band Input Signal
x(n)=x1(n)ejω1n+x2(n)ejω2n,
where x1(n) and x2(n) are the input signals for a first band and a second band, respectively, and where ω1and ω2are the digital frequency variables that describe the frequency location of each band. Then, the system output in terms of the individual band inputs can be obtained by substituting Equation 4 into Equation 3 in accordance with:

Equation 5—Dual-Band System Output Equation
y(n)=x12(n)x1*(n)ejw1n+2x1(n)x2(n)x2*(n)ejw1n+x12(n)x2*(n)ej(2w1−w2)n+x22(n)x1*(n)ej(−w1+2w2)n+2x1(n)x2(n)x1*(n)ejw2n+x22(n)x2*(n)ejw2n

From Equation 5, it can be observed that the distortion in each band is not only a function of that band's input, but is also a function of the other band. Moreover, third order IMD (IMD3) products are located at 2f1−f2and 2f2−f1and are a function of both band inputs.

It is important to note that “simple” odd-order IMD products that satisfy the requirement:

Equation 6—Simple Odd-Order Relationship
ci+cj=1,
maintain their frequency position relative to the band frequencies even if the band frequencies are translated by a constant offset (e.g., from Radio Frequency (RF) to baseband). However, this is not true for any even-order products or other odd-order products that do not satisfy Equation 6. In conventional DPD systems, signals are often translated to/from their absolute frequency location to a baseband location (e.g., located around 0 hertz (Hz)). Consequently, only “simple” odd-order IMD products generated in a baseband DPD system will be in the correct frequency location when the DPD output is translated back to the appropriate frequency location for transmission. Other types of IMD products could be individually filtered and translated separately to appropriate absolute frequencies, but this would result in increased computational complexity.

A traditional baseband DPD architecture is illustrated inFIG. 2. In this architecture, the signals for each band are combined into a single composite signal that is used as an input to the DPD function. This composite signal is placed at baseband (centered at 0 Hz). Architectures of this type face a number of challenges in the correction IMD products:They can only address simple odd-order IMD products as described above because they are based on the translation of the input and output signals to baseband (centered at 0 Hz).They must operate at high sampling rates in order to have sufficient bandwidth to cover all the IMD products to be corrected. This results in an increased computational complexity.They cannot focus on a particular IMD product (e.g., 6f1−4f2), but must generate a large number of higher order terms that contribute at the given IMD location. This increases the computational complexity and may generate correction terms that are not required.

Another baseband DPD architecture is a multi-dimensional DPD architecture as illustrated inFIG. 3. Examples of a multi-dimensional DPD architecture are described in [4][5][7]. In this architecture, the overall DPD “problem” is decomposed into separate DPD actuators for each band, as shown inFIG. 3which gives an example for a dual band system. Note that each band actuator has multiple inputs (one per band). Consequently, the underlying basis functions are multi-dimensional, with a dimension per input signal (e.g., a dual band system uses 2D basis functions). The advantage of this type of architecture is that computational resources are focused on the particular bands of interest. However, the conventional application of this architecture does not support the correction of IMD products.

Yet another baseband DPD architecture is a “channel-selective” DPD architecture. An example is described in [6]. This architecture is based on the multi-dimensional DPD architecture described above, but with additional processing blocks after the DPD actuators for each band. This architecture is illustrated inFIG. 4.

In the channel-selective DPD architecture, the cancellation of IMD products is based on injecting a signal, with equal magnitude but 180° degree phase shift compared to the generated IMD3 product terms, into the input of the of the transmitter. As such, the outputs of DPD actuators for the main signal bands (C2and C3inFIG. 4) are tuned and combined at a higher sampling rate to create a composite signal. Then, a nonlinearity is applied to the composite signal in order to generate IMD products. The desired IMD products are selected via filtering and then adjusted by a gain and phase rotation to achieve the desired cancellation term. This is accomplished in processing blocks C1and C4as shown inFIG. 4.

Architectures of this type face several challenges in the correction IMD products:Only the cancellation of IMD3 (third order) products is considered.They must operate at high sampling rates to have sufficient bandwidth to generate the IMD products to be corrected and must also use a large number of nonlinear terms to generate the desired IMD products. This results in an increased computational complexity.This architecture encapsulates the predistortion/correction of the main signal bands and the IMD products. It does not provide a method to only correct the IMD products in support of a preexisting DPD system.

Another similar technology is modeling and suppressing transmitter leakage in a concurrent dual-band system, as described in [7]. This architecture is focused on the cancellation of IMD3 products from a dual-band configuration in the receiver of a radio. A high-level view of the architecture is given inFIG. 5. The IMD3 product is modeled and weighted with an envelope dependent nonlinearity and then subtracted from the receive signals as shown inFIG. 6. Architectures of this type face a few challenges in the correction IMD products:Does not correct distortions in the transmitter path (only the receiver); andOnly the cancellation of simple IMD3 (third-order) products for a dual-band configuration is considered.

Another architecture for DPD is an “augmented” dual-band DPD with predictive injection as described in [8]. In this architecture, the IMD products are addressed using a “predictive injection” technique. A high-level overview of the architecture is given inFIG. 7. Without directly observing the specific IMD products in a feedback loop, approximations are synthesized (in modules Tx2and Tx3) and injected into the transmitter. Architectures of this type face several challenges in the correction IMD products:They only consider simple odd-order IMD products.This architecture predicts, but does not directly observe or adapt, based on observations of the IMD products to be cancelled. Consequently, cancellation performance can be limited.This approach is only intended to work in conjunction with an underlying multi-dimensional DPD system (i.e., cannot be used in conjunction with a traditional baseband DPD system).

Thus, there is a need for a DPD architecture that addresses the shortcomings of the existing DPD architectures described above.

SUMMARY

Systems and methods are disclosed herein for selectively compensating for a specific Intermodulation Distortion (IMD) product(s) of an arbitrary order in a concurrent multi-band transmitter system. In some embodiments, a method of compensating for one or more specific IMD products in a concurrent multi-band transmitter system comprises generating an IMD correction signal for a specific IMD product as a function of two or more frequency band input signals for two or more frequency bands of a concurrent multi-band signal, the IMD product being an arbitrary order IMD product. The method further comprises frequency translating the IMD correction signal to a desired frequency that corresponds to a Radio Frequency (RF) location of the specific IMD product and, after frequency translating the IMD correction signal to the desired frequency, utilizing the IMD correction signal to compensate for the specific IMD product.

In some embodiments, the IMD product is a non-simple odd-order IMD product. In some other embodiments, the IMD product is an even-order IMD product.

In some embodiments, generating the IMD correction signal for the specific IMD product comprises generating the IMD correction signal for the specific IMD product in accordance with:
IMD_PRODUCT(n)=AB,
where
A=[Πi=1N{circumflex over (x)}i|ci|(n−di)],
B=[Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2), . . . ,|xN(n−{tilde over (d)}N)|}],
ciare signed integer values that define the specific IMD product, diis a parameter that controls relative delay of the two or more frequency band input signals, {tilde over (d)}iis a parameter that controls relative delay of envelope signals for the two or more frequency band input signals, βjis an N-dimensional basis function set with M members that span a respective N-dimensional input space, φjare complex coefficients for each set member of the N-dimensional basis function set, and

x^i⁡(n)={xi⁡(n)⁢⁢for⁢⁢ci≥0,xi*⁡(n)⁢⁢for⁢⁢ci<0.
Further, in some embodiments, frequency translating the IMD correction signal to the desired frequency that corresponds to the RF location of the specific IMD product comprises frequency translating the IMD correction signal to the desired frequency that corresponds to the RF location of the specific IMD product in accordance with:
FREQ_TRANS_IMD_PRODUCT(n)=ABej(c1ω1+c2ω2+ . . . +cNωN)n,
where ωiare digital frequency variables that define a frequency location of each frequency band and the weighted sum of ciωidefine the desired frequency to which the IMD correction signal is translated.

In some embodiments, generating the IMD correction signal for the specific IMD product comprises generating a plurality of component signals of the IMD correction signal for the specific IMD product, each component signal of the plurality of component signals being generated in accordance with:
IMD_PRODUCT_COMPONENT(n)=AB,
where
A=[Πi=1N{circumflex over (x)}i|ci|(n−di)],
B=[Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2)|, . . . ,|xN(n−{tilde over (d)}N)|}],
ciare signed integer values that define the specific IMD product, diis a parameter that controls relative delay of the two or more frequency band input signals, {tilde over (d)}iis a parameter that controls relative delay of envelope signals for the two or more frequency band input signals, βjis an N-dimensional basis function set with M members that span a respective N-dimensional input space, φjare complex coefficients for each set member of the N-dimensional basis function set,

In some embodiments, the desired frequency to which the IMD correction signal is translated is a baseband frequency that, after subsequent upconversion, results in the IMD correction signal being located at the RF location of the specific IMD product. In some other embodiments, the desired frequency to which the IMD correction signal is translated is an intermediate frequency that, after subsequent upconversion, results in the IMD correction signal being located at the RF location of the specific IMD product. In some other embodiments, the desired frequency to which the IMD correction signal is translated is the RF location of the specific IMD product.

In some embodiments, the method further comprises generating, from the two or more frequency band input signals, two or more predistorted frequency band input signals, respectively, located at desired frequencies for the two or more predistorted frequency band input signals that correspond to RF locations of carriers of the two or more frequency bands of the concurrent multi-band signal. The method further comprises combining the two or more predistorted frequency band input signals and the IMD correction signal to provide a combined signal. In some embodiments, the method further comprises upconverting the combined signal to provide the concurrent multi-band signal.

Embodiments of a concurrent multi-band transmitter system for compensating for one or more specific IMD products in the concurrent multi-band transmitter system are also disclosed. In some embodiments, the concurrent multi-band transmitter system comprises IMD Digital Predistortion (DPD) circuitry operable to generate an IMD correction signal for a specific IMD product as a function of two or more frequency band input signals for two or more frequency bands of a concurrent multi-band signal, the IMD product being an arbitrary IMD product. The concurrent multi-band transmitter system further comprises tuning circuitry operable to frequency translate the IMD correction signal to a desired frequency that corresponds to a RF location of the specific IMD product, wherein the concurrent multi-band transmitter system is operable to, after frequency translation of the IMD correction signal to the desired frequency, utilize the IMD correction signal to compensate for the specific IMD product.

In some embodiments, the IMD product is a non-simple odd-order IMD product. In some other embodiments, the IMD product is an even-order IMD product.

In some embodiments, the IMD DPD circuitry is operable to generate the IMD correction signal for the specific IMD product in accordance with:
IMD_PRODUCT(n)=AB,
where
A=[Πi=1N{circumflex over (x)}i|ci|(n−di)],
B=[Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2)|, . . . ,|xN(n−{tilde over (d)}N)|}],
ciare signed integer values that define the specific IMD product, diis a parameter that controls relative delay of the two or more frequency band input signals, {tilde over (d)}iis a parameter that controls relative delay of envelope signals for the two or more frequency band input signals, βjis an N-dimensional basis function set with M members that span a respective N-dimensional input space, φjare complex coefficients for each set member of the N-dimensional basis function set, and

x^i⁡(n)={xi⁡(n)⁢⁢for⁢⁢ci≥0,xi*⁡(n)⁢⁢for⁢⁢ci<0.
Further, in some embodiments, the tuning circuitry is operable to frequency translate the IMD correction signal to the desired frequency that corresponds to the RF location of the specific IMD product in accordance with:
FREQ_TRANS_IMD_PRODUCT(n)=ABej(c1ω1c2ω2+ . . . +cNωN)n,
where ωiare digital frequency variables that define a frequency location of each frequency band and the weighted sum of ciωidefine the desired frequency to which the IMD correction signal is translated.

In some embodiments, in order to generate the IMD correction signal for the specific IMD product, the IMD DPD circuitry is operable to generate a plurality of component signals of the IMD correction signal for the specific IMD product, each component signal of the plurality of component signals being generated in accordance with:
IMD_PRODUCT_COMPONENT(n)=AB,
where
A=[Πi=1N{circumflex over (x)}i|ci|(n−di)],
B=[Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2)|, . . . ,|xN(n−{tilde over (d)}N)|}],
ciare signed integer values that define the specific IMD product, diis a parameter that controls relative delay of the two or more frequency band input signals, {tilde over (d)}iis a parameter that controls relative delay of envelope signals for the two or more frequency band input signals, βjis an N-dimensional basis function set with M members that span a respective N-dimensional input space, φjare complex coefficients for each set member of the N-dimensional basis function set,

In some embodiments, the desired frequency to which the IMD correction signal is translated is a baseband frequency that, after subsequent upconversion, results in the IMD correction signal being located at the RF location of the specific IMD product. In some other embodiments, the desired frequency to which the IMD correction signal is translated is an intermediate frequency that, after subsequent upconversion, results in the IMD correction signal being located at the RF location of the specific IMD product. In some other embodiments, the desired frequency to which the IMD correction signal is translated is the RF location of the specific IMD product.

In some embodiments, the concurrent multi-band transmitter system further comprises DPD circuitry operable to generate, from the two or more frequency band input signals, two or more predistorted frequency band input signals, respectively, located at desired frequencies for the two or more predistorted frequency band input signals that correspond to RF locations of carriers of the two or more frequency bands of the concurrent multi-band signal. The concurrent multi-band transmitter system further comprises combining circuitry operable to combine the two or more predistorted frequency band input signals and the IMD correction signal to provide a combined signal. In some embodiments, the concurrent multi-band transmitter system further comprises upconversion circuitry operable to upconvert the combined signal to provide the concurrent multi-band signal.

In some embodiments, a concurrent multi-band transmitter for compensating for one or more specific IMD products in a concurrent multi-band transmitter system is adapted to generate an IMD correction signal for a specific IMD product as a function of two or more frequency band input signals for two or more frequency bands of a concurrent multi-band signal, the IMD product being an arbitrary order IMD product. The concurrent multi-band transmitter is further adapted to frequency translate the IMD correction signal to a desired frequency that corresponds to a RF location of the specific IMD product and, after frequency translating the IMD correction signal to the desired frequency, utilize the IMD correction signal to compensate for the specific IMD product.

In some embodiments, a concurrent multi-band transmitter system for compensating for one or more specific IMD products in a concurrent multi-band transmitter system comprises a generating module, a frequency translating module, and a utilizing module. The generating module is operable to generate an IMD correction signal for a specific IMD product as a function of two or more frequency band input signals for two or more frequency bands of a concurrent multi-band signal, the IMD product being an arbitrary order IMD product. The frequency translating module is operable to frequency translate the IMD correction signal to a desired frequency that corresponds to a RF location of the specific IMD product. The utilizing module is operable to, after frequency translating the IMD correction signal to the desired frequency, utilize the IMD correction signal to compensate for the specific IMD product.

DETAILED DESCRIPTION

Systems and methods are disclosed herein for selectively targeting an Intermodulation Distortion (IMD) product(s) for elimination by generating the relevant predistortion products as a function of separate frequency band input signals for a concurrent multi-band transmitter system. The selected IMD product(s) can be even or odd-order products of arbitrary order (i.e., arbitrary order IMD product(s)). Within the context of an adaptive loop that observes the specific IMD product(s), the predistortion terms are adjusted to maximize the effectiveness of the IMD cancellation. After generation, the IMD correction signal(s) is placed at the IMD product frequency location(s) before transmission through a Power Amplifier (PA) of the concurrent multi-band transmitter system.

The embodiments disclosed herein have several distinct advantages. For instance, embodiments of the present disclosure perform Digital Predistortion (DPD) for a specific IMD product(s) that need cancellation or for which cancellation is desired in a targeted manner. This, in turn, leads to certain implementation benefits such as, e.g., potentially lower resource utilization because resources are not wasted on IMD products that do not need cancellation and potentially reduced bandwidth and sample rate requirements, which in turn reduces computational complexity. Embodiments of the present disclosure are applicable to even or odd-order products of any arbitrary order. Further, embodiments of the present disclosure can be extended to an arbitrary number of two or more frequency bands. Further, embodiments of the present disclosure can be employed in a flexible manner, either directly as additional terms in a multi-dimensional DPD system or as a separate DPD subsystem that supports a pre-existing DPD system which cannot correct IMD products. Embodiments of the present disclosure also offer enhanced cancellation performance by incorporating an adaption loop.

In this regard,FIGS. 8 and 9illustrate two non-limiting examples of a concurrent multi-band transmitter system10in which embodiments of the present disclosure may be implemented. In these examples, the concurrent multi-band transmitter system10is a concurrent dual-band transmitter system; however, the embodiments disclosed herein can be extended to any arbitrary number of two or more frequency bands. InFIG. 8, the concurrent multi-band transmitter system10includes a source12that provides frequency band input signals x1(n) and x2(n) for the two frequency bands of the concurrent dual-band signal to be transmitted. In this example, each of the frequency band input signals x1(n) and x2(n) is centered at 0 hertz (Hz). However, the present disclosure is not limited thereto.

A Baseband Digital Predistorter (BB-DPD)14, which may also be referred to herein as a BB-DPD actuator or BB-DPD circuitry, operates to digitally predistort the frequency band input signals x1(n) and x2(n) to provide predistorted frequency band input signals x′1(n) and x′2(n). The BB-DPD14uses, e.g., any conventional BB-DPD scheme. For example, the BB-DPD14may use the dual-band DPD architecture as described in U.S. Pat. No. 9,252,718, entitled LOW COMPLEXITY DIGITAL PREDISTORTION FOR CONCURRENT MULTI-BAND TRANSMITTERS, or in U.S. Pat. No. 9,385,762, entitled LINEARIZATION OF INTERMODULATION BANDS FOR CONCURRENT DUAL-BAND POWER AMPLIFIERS, both of which are hereby incorporated by reference for their teachings on a DPD architecture.

An IMD Digital Predistorter (IMD-DPD)16, an optional upsampler18(also referred to herein as upsampling circuitry), and a tuner20(also referred to herein as tuning circuitry) operate to generate an IMD correction signal for a specific IMD product(s), as described below in detail. As discussed above, the IMD-DPD16generates a baseband IMD correction signal that is optionally upsampled to the sampling rate used for the predistorted frequency band input signals x′1(n) and x′2(n) and tuned, by the tuner20, to a desired frequency. Note that while illustrated separately for clarity and ease of discussion, the tuner20may be implemented within the IMD-DPD16. In this example, the desired frequency is a desired baseband frequency that, after upconversion by upconversion circuitry22, is located at the frequency location of the specific IMD product(s) to be cancelled. Note, however, that in some other embodiments, the predistorted frequency band input signals x′1(n) and x′2(n) are at Intermediate Frequency (IF) and the IMD correction signal is tuned to the appropriate IF frequency. In some other embodiments, the predistorted frequency band input signals x′1(n) and x′2(n) are at Radio Frequency (RF) and the IMD correction signal is tuned to the RF frequency of the specific IMD product(s) being cancelled (in which case the upconversion circuitry22is not needed).

In this example, the predistorted frequency band input signals x′1(n) and x′2(n) and the IMD correction signal are combined (i.e., added) by combining circuitry24to provide a combined signal. Here, the combined signal is a concurrent dual-band signal centered at 0 Hz (i.e., a baseband signal). The combined signal is upconverted to RF by the upconversion circuitry22and amplified by a PA26for transmission.

In this embodiment, the concurrent multi-band transmitter system10includes separate training loops for the BB-DPD14and the IMD-DPD16. In this regard, a coupler28couples a transmit observation receiver to the output of the PA26. The transmit observation receiver includes downconversion and digitization circuitry30that downconverts and digitizes the feedback signal from the coupler28to provide a baseband feedback signal. Training signal processing circuitry32operates to process the frequency band input signals and the baseband feedback signal to provide error signals that are provided to the Baseband (BB) training circuitry34and IMD training circuitry36, respectively. In general, the training signal processing circuitry32time-aligns the frequency band input signals and the baseband feedback signal and generates error signals for the BB training circuitry34and the IMD training circuitry36based on a difference between the frequency band input signals or a combined version of the frequency band input signals and the baseband feedback signal. Based on the error signals, the BB training circuitry34updates complex coefficients provided as input to the BB-DPD14, and the IMD training circuitry36updates complex coefficients provided as input to the IMD-DPD16, as will be appreciated by one of skill in the art. The BB training circuitry34and the IMD training circuitry36operate in accordance with any suitable training scheme such as, e.g., Least Mean Square (LMS) or least squares. Note that, while separate error signals are provided to the BB training circuitry34and the IMD training circuitry36in the illustrated example ofFIG. 8, a single error signal may alternatively be provided to both the BB training circuitry34and the IMD training circuitry36in some other implementations.

In the example ofFIG. 8, the IMD-DPD16and the IMD training circuitry36are separate from the BB-DPD14and the BB training circuitry34. As one example implementation, the BB-DPD14and the BB training circuitry34are implemented on one ASIC, and the IMD-DPD16and the IMD training circuitry36are implemented on another ASIC. This may be desirable when the IMD-DPD16is provided as an add on feature for an existing transmitter system.

FIG. 9illustrates another example of the concurrent multi-band transmitter system10in which the BB-DPD14and the IMD-DPD16are implemented in a single DPD system38and training of the BB-DPD14and the IMD-DPD16is performed by a single training circuit40. Otherwise, the operation is the same.

Now, the description turns to the details of the IMD-DPD16and the tuner20and, in particular, to the generation of the IMD correction signal for cancelling a specific IMD product(s).

As shown in the Background, the IMD products can be defined in terms of the separate band signals that make up the composite input signal. This can be extended for an arbitrary order nonlinear term (with envelope dependence) of the form:

Equation 7—General Arbitrary Order Nonlinearity
y(n)=xP(n)x*Q(n),
where P and Q are integers, P>Q, and the order of the nonlinear term is given by P+Q. For a multi-band configuration with N bands, the composite input signal is given by:

Equation 8—Multi-Band Input Signal
x(n)=x1(n)ejω1n+x2(n)ejω2n+ . . . +xN(n)ejωNn,
where x1(n), x2(n), . . . , xN(n) are the input signals for band “1,” band “2,” . . . , band “N” respectively, and ω1, ω2, . . . , ωNare the digital frequency variables that describe the frequency location of each band. Then, by substituting Equation 8 into Equation 7, one can obtain all the IMD products at all frequency locations for a given P, Q, and N. If only a specific IMD product is to be addressed, then one will only be concerned with the distortion products that occur at a specific frequency, where the arbitrary order IMD frequency location is:

Equation 9—Arbitrary Order IMD Frequency Location
fIMD_Target=c1f1+c2f2+ . . . +cNfN,
where c1, c2, . . . , cNare signed integer valued coefficients as before. Then, when considering different values for P and Q, the general form of the IMD product located at fIMD_Targetcan be derived to be of the form:

x^i⁡(n)={xi⁡(n)⁢⁢for⁢⁢ci≥0,xi*⁡(n)⁢⁢for⁢⁢ci<0.
Note that the piterms are shown to have an infinite upper bound in the summations in Equation 10, but when considering a practical PA implementation, the upper bounds will be finite and limited by the effective nonlinearity order of the PA.

Regarding the architecture of the IMD-DPD16and the tuner20, in order to predistort to compensate for the IMD product given in Equation 10, the IMD-DPD16and the tuner20need to synthesize terms of a similar form (and their corresponding inverse). The IMD-DPD16and the tuner20implement terms of the general form:

Equation 11—General DPD Correction Signal Terms
IMD_DPD_TERM(n)=[Πi=1N{circumflex over (x)}i|ci|(n−di)][Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2)|, . . . ,|xN(n−{tilde over (d)}N)|}]ej(c1ω1+c2ω2+ . . . +cNωN)n,
where the parameters dicontrol the relative delay of the frequency band input signals xi, the parameters {tilde over (d)}icontrol the relative delay of the frequency band input envelope signals |xi|, βjis an N-dimensional basis function set with M members that spans the N-dimensional input space, and where the parameters φjare the corresponding complex coefficients for each set member. The N-dimensional basis function set can be simply and efficiently formed from the tensor products of traditional one dimensional basis functions such as polynomials or splines. An example of a polynomial based two-dimensional basis function set is given by:

Typically, a collection of W memory taps will be used to correct a specific IMD product. The outputs of these memory taps42are summed together and then frequency translated to the appropriate (relative) baseband frequency.FIG. 11illustrates one example embodiment of the IMD-DPD16and the tuner20, where the IMD-DPD16includes memory taps42-1through42-W, summation circuitry44that sums the outputs of the memory taps42-1through42-W, and an absolute function circuit46that generates the envelope signals from the frequency band input signals. Note that the upsampler18is omitted for clarity. Each memory tap42is configured with a separate tap configuration. Referring to Equation 11 above, the tap configuration includes parameters diand {tilde over (d)}i.

Note that one possible method of frequency translation that has an efficient hardware is a Coordinate Rotation Digital Computer (CORDIC) tuner. In other words, in some embodiments, the tuner20is a CORDIC tuner.

Also note that the tuner20tunes the IMD correction signal to a desired frequency that corresponds to the frequency location of the IMD product to be cancelled. In some embodiments, the desired frequency to which the tuner20tunes the IMD correction signal is a baseband frequency that, after upconversion by the upconversion circuitry22, results in the IMD correction signal being located at the RF frequency location of the IMD product to be cancelled. In some other embodiments, the desired frequency to which the tuner20tunes the IMD correction signal is an IF that, after upconversion by the upconversion circuitry22, results in the IMD correction signal being located at the RF frequency location of the IMD product to be cancelled. In some other embodiments, the desired frequency to which the tuner20tunes the IMD correction signal is the RF frequency location of the IMD product to be cancelled.

Returning briefly toFIG. 8, separate adaptation loops are used to adapt the BB-DPD14and the IMD-DPD16. For this scenario, the BB-DPD14is, at least in some embodiments, realized using conventional techniques. The BB-DPD14may not have any IMD correction capability. The IMD-DPD16operates to compensate for a specific IMD product(s). The IMD-DPD16only contains IMD specific terms as shownFIGS. 8 and 9. Note that the IMD-DPD16can potentially operate at a lower sampling rate than the BB-DPD14, so an additional upsampling operation may be required to convert the IMD-DPD output to the same sampling rate as the output of the BB-DPD14. Prior to combining with the output of the BB-DPD actuator, the IMD-DPD output is tuned to the appropriate frequency (relative to baseband) by the tuner20.

Now, returning briefly toFIG. 9, if using a multi-dimensional DPD system, then the IMD correction terms can be directly included with the conventional memory taps within the same actuator. In other words, the DPD system38can be implemented by a number of memory taps where the IMD correction terms can be directly included in the memory taps along with the conventional DPD correction terms. Consequently, the training circuit40can be implemented as a training subsystem from a typical feedback loop (e.g., LMS or least squares), which can be employed by simply generating additional basis function inputs. In this way, one can still use the conventional feedback loop architecture as shown inFIG. 3.

FIG. 12is a flow chart that illustrates a process for selectively generating an IMD correction signal for a specific IMD product and using the IMD correction signal to compensate for the specific IMD product according to some embodiments of the present disclosure. This process is performed by a concurrent multi-band transmitter system such as, e.g., the concurrent multi-band transmitter system10illustrated inFIGS. 8 and 9. As such, the concurrent multi-band transmitter system10will be used for this discussion.

As illustrated, the concurrent multi-band transmitter system10, and in particular the IMD-DPD14, generates an IMD correction signal for a specific IMD product as a function of the frequency band input signals for the frequency bands of the concurrent multi-band signal to be transmitted, as described above (step100). In particular, prior to frequency translation, the IMD correction signal is, at least in some embodiments, generated in accordance with Equation 11 and, in particular, in accordance with:
IMD_PRODUCT(n)=AB,
where
A=[Πi=1N{circumflex over (x)}i|ci|(n−di)],
B=[Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2)|, . . . ,|xN(n−{tilde over (d)}N)|}],
ciare signed integer values that define the specific IMD product, diis a parameter that controls relative delay of the two or more frequency band input signals, {tilde over (d)}iis a parameter that controls relative delay of envelope signals for the two or more frequency band input signals, βjis an N-dimensional basis function set with M members that span a respective N-dimensional input space, φjare complex coefficients for each set member of the N-dimensional basis function set, and

If multiple memory taps are used as, e.g., in the embodiment ofFIG. 11, then the IMD-DPD14generates multiple components of the IMD correction signal (i.e., multiple memory tap outputs) (step100A) and then combines these components to provide the IMD correction signal (step100B). Each component is generated in accordance with Equation 11 above and, in particular, in accordance with:
IMD_PRODUCT(n)=AB,
where
A=[Πi=1N{circumflex over (x)}i|ci|(n−di)],
B=[Σj=1Mφjβj{|x1(n−{tilde over (d)}1)|,|x2(n−{tilde over (d)}2)|, . . . ,|xN(n−{tilde over (d)}N)|}],
ciare signed integer values that define the specific IMD product, diis a parameter that controls relative delay of the two or more frequency band input signals, {tilde over (d)}iis a parameter that controls relative delay of envelope signals for the two or more frequency band input signals, βiis an N-dimensional basis function set with M members that span a respective N-dimensional input space, φjare complex coefficients for each set member of the N-dimensional basis function set, and

x^i⁡(n)={xi⁡(n)⁢⁢for⁢⁢ci≥0,xi*⁡(n)⁢⁢for⁢⁢ci<0.
Note that values of diand {tilde over (d)}iare different (or at least separately configurable) for each component of the IMD correction signal.

The concurrent multi-band transmitter system10, and in particular the tuner20, frequency translates the IMD correction signal to a desired frequency that corresponds to the RF location of the specific IMD product to be cancelled (step102). As discussed above, in some embodiments, the desired frequency to which the IMD correction signal is tuned is a baseband frequency that, after upconversion by the upconversion circuitry22, results in the IMD correction signal being located at the RF frequency location of the IMD product to be cancelled. In some other embodiments, the desired frequency to which the IMD correction signal is tuned is an IF that, after upconversion by the upconversion circuitry22, results in the IMD correction signal being located at the RF frequency location of the IMD product to be cancelled. In some other embodiments, the desired frequency to which the IMD correction signal is tuned is the RF frequency location of the IMD product to be cancelled.

The concurrent multi-band transmitter system10then utilizes the IMD correction signal to compensate for the specific IMD product (step104). In general, the IMD correction signal is combined into the main signal path either prior to or after upconversion but prior to amplification by the PA26such that the IMD correction signal cancels the specific IMD product at the output of the PA26. As an example, in the embodiments ofFIGS. 8 and 9, the concurrent multi-band transmitter system10generates predistorted and frequency-translated versions of the frequency band input signals (step104A). Note that the predistortion of the BB-DPD14is optional in which case the frequency band input signals are frequency translated to the appropriate frequencies without predistortion. The (predistorted) frequency-translated frequency band inputs are combined with the IMD correction signal to provide a combined signal (step104B). The combined signal is a concurrent multi-band baseband (or alternatively IF) signal in which the frequency band input signals and the IMD correction signal have all been placed appropriate frequencies relative to one another. The combined signal is then upconverted (if needed) to provide a concurrent multi-band signal at RF that is then amplified for transmission (step104C). Note that the upconversion step is optional, as indicated by the dashed lines, in embodiments in which the frequency band input signals and the IMD correction signal are combined at RF.

FIG. 13illustrates the concurrent multi-band transmitter system10according to some other embodiments of the present disclosure. In this example, the concurrent multi-band transmitter system10includes a number of modules48, each of which is implemented in software. In particular, the concurrent multi-band transmitter system10includes a generating module48-1, a frequency translating module48-2, and a utilizing module48-3. The generating module48-1is operable to generate an IMD correction signal for a specific IMD product as a function of two or more frequency band input signals for two or more frequency bands of a concurrent multi-band signal, the IMD product being an arbitrary order IMD product, as described above. The frequency translating module48-2is operable to frequency translate the IMD correction signal to a desired frequency that corresponds to a RF location of the specific IMD product, as described above. The utilizing module48-3is operable to, after frequency translating the IMD correction signal to the desired frequency, utilize the IMD correction signal to compensate for the specific IMD product, as described above. Note that, while not illustrated, the concurrent multi-band transmitter system10may include additional modules such as, for example, a DPD module that operates to digitally predistort the frequency band input signals as described above, one or more training modules for training BB-DPD and IMD-DPD as described above, etc.

The following acronyms are used throughout this disclosure.2D-DPD Two Dimensional Digital PredistortionASIC Application Specific Integrated CircuitBB BasebandBB-DPD Baseband Digital PredistorterCORDIC Coordinate Rotation Digital ComputerDPD Digital PredistortionDSP Digital Signal ProcessingGMP Generalized Memory PolynomialHz HertzIC Integrated CircuitIF Intermediate FrequencyIMD3 Third Order Intermodulation DistortionIMD Intermodulation DistortionIMD-DPD Intermodulation Distortion Digital PredistorterLMS Least Mean SquareLUT Lookup TableMHz MegahertzPA Power AmplifierRF Radio Frequency

LIST OF REFERENCES