Predistortion for transmitter with array

In certain aspects of the present disclosure, a method for wireless communication comprises predistorting a first signal based on a set of coefficients to generate a first predistorted signal, outputting the first predistorted signal for a first transmission to a receiving device, and receiving a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The method also comprises determining whether to store the set of coefficients in a memory based on the received first feedback signal, and storing the set of coefficients in the memory based on the determination.

FIELD

The present disclosure generally relates to wireless communications and, more particularly, to methods and apparatuses for predistortion (PD).

BACKGROUND

A transmitting device may transmit a signal using an array of antennas. In this regard, the transmitting device may include a separate power amplifier (PA) for each antenna in the array, in which each PA amplifies the signal with sufficient power for wireless transmission to a remote device via the respective antenna. Each PA is typically a non-linear device with a limited linear dynamic range (DR). For power efficiency, it is desirable to drive each PA as close as possible to saturation. However, driving each PA close to saturation causes each PA to operate outside its linear range, which can lead to significant non-linear distortion if not corrected.

SUMMARY

A first aspect relates to an apparatus. The apparatus comprises a processing system configured to configure a predistorter to predistort a first signal based on a set of coefficients to generate a first predistorted signal, and an interface configured to output the first predistorted signal for a first transmission to a receiving device, and to receive a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The processing system is further configured to determine whether to store the set of coefficients in a memory based on the received first feedback signal, and store the set of coefficients in the memory based on the determination.

A second aspect relates to an apparatus. The apparatus comprises a processing system configured to configure a predistorter to predistort each one of a plurality of signals based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals, and an interface configured to output each one of the plurality of predistorted signals for transmission to a receiving device, and to receive a plurality of feedback signals from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals. The processing system is further configured to select one of the plurality of sets of coefficients based on the plurality of feedback signals, and store the selected one of the plurality of sets of coefficients in a memory.

A third aspect relates to a method for wireless communications. The method comprises predistorting a first signal based on a set of coefficients to generate a first predistorted signal, outputting the first predistorted signal for a first transmission to a receiving device, and receiving a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The method also comprises determining whether to store the set of coefficients in a memory based on the received first feedback signal, and storing the set of coefficients in the memory based on the determination.

A fourth aspect relates to a method for wireless communications. The method comprises predistorting each one of a plurality of signals based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals, outputting each one of the plurality of predistorted signals for transmission to a receiving device, and receiving a plurality of feedback signals from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals. The method also comprises selecting one of the plurality of sets of coefficients based on the plurality of feedback signals, and storing the selected one of the plurality of sets of coefficients in a memory.

A fifth aspect relates to an apparatus for wireless communications. The apparatus comprises means for predistorting a first signal based on a set of coefficients to generate a first predistorted signal, means for outputting the first predistorted signal for a first transmission to a receiving device, and means for receiving a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The apparatus also comprises means for determining whether to store the set of coefficients in a memory based on the received first feedback signal, and means for storing the set of coefficients in the memory based on the determination.

A sixth aspect relates to an apparatus for wireless communications. The apparatus comprises means for predistorting each one of a plurality of signals based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals, means for outputting each one of the plurality of predistorted signals for transmission to a receiving device, and means for receiving a plurality of feedback signals from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals. The apparatus also comprises means for selecting one of the plurality of sets of coefficients based on the plurality of feedback signals, and means for storing the selected one of the plurality of sets of coefficients in a memory.

A seventh aspect relates to a computer readable medium comprising instructions for predistorting a first signal based on a set of coefficients to generate a first predistorted signal, outputting the first predistorted signal for a first transmission to a receiving device, and receiving a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The computer readable medium also comprises instructions for determining whether to store the set of coefficients in a memory based on the received first feedback signal, and storing the set of coefficients in the memory based on the determination.

An eighth aspect relates to a computer readable medium comprising instructions for predistorting each one of a plurality of signals based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals, outputting each one of the plurality of predistorted signals for transmission to a receiving device, and receiving a plurality of feedback signals from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals. The computer readable medium also comprises instructions for selecting one of the plurality of sets of coefficients based on the plurality of feedback signals, and storing the selected one of the plurality of sets of coefficients in a memory.

A ninth aspect relates to a wireless node. The wireless node comprises a transceiver, a processing system configured to configure a predistorter to predistort a first signal based on a set of coefficients to generate a first predistorted signal, and an interface configured to output the first predistorted signal for transmission to a receiving device via the transceiver, and to receive a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The processing system is further configured to determine whether to store the set of coefficients in a memory based on the received first feedback signal, and store the set of coefficients in the memory based on the determination.

A tenth aspect relates to a wireless node. The wireless node comprises a transceiver, a processing system configured to configure a predistorter to predistort each one of a plurality of signals based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals, and an interface configured to output each one of the plurality of predistorted signals for transmission to a receiving device via the transceiver, and to receive a plurality of feedback signals from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals. The processing system is further configured to select one of the plurality of sets of coefficients based on the plurality of feedback signals, and store the selected one of the plurality of sets of coefficients in a memory.

DETAILED DESCRIPTION

As discussed above, power amplifiers (PAs) can become significantly non-linear close to saturation. For applications where power efficiency is not crucial, non-linearity can be avoided by backing off a PA from saturation into the linear region of the PA. However, for wireless communications, power efficiency is often quite important. To improve power efficiency, the input signal applied to a PA is set to a power level to drive the PA as close to saturation as possible.

Although driving the PA close to saturation improves power efficiency, it can also significantly distort the output signal from the PA due to the non-linearity of the PA, particularly when the input signal has a high peak-to-average-power ratio (PAPR) as is common in wireless communication. The resulting signal distortion has two main components: an in-band component and an out-of-band component. The in-band distortion can result in an increase in the error vector magnitude (EVM) of the in-band signal component. The out-of-band distortion can result in pollution or interference to adjacent channel transmission, i.e., adjacent channel interference (ACI).

To drive the PA as close as possible to saturation for power efficiency purposes, while also reducing distortion of the output signal of the PA, a wireless transmitter may employ a digital predistorter (DPD). A DPD reduces non-linear distortion at the output of a PA by applying an inverse of the non-linear distortion of the PA to the input signal to substantially cancel out the non-linear distortion. In this manner, the amount of back off from saturation can be significantly reduced compared to a transmitter that does not employ a DPD, thus improving power efficiency without introducing significant signal distortion.

FIG. 1Ashows an example of a transmitter100employing predistortion according to certain aspects of the present disclosure. The transmitter100includes a digital predistorter (DPD)104, a digital-to-analog converter (DAC)106, a frequency up-converter108, and a power amplifier (PA)110.

In operation, the DPD104receives a baseband digital signal (generated by a baseband processor or modem of the wireless device in which the transmitter100is incorporated). The DPD104predistorts the baseband digital signal to reduce non-linear distortion at the output of the PA110, as discussed further below. The DAC106converts the predistorted signal into an analog signal. The frequency up-converter108frequency up-converts the analog signal into a radio frequency (RF) signal (e.g., by mixing the analog signal with a high-frequency signal from a local oscillator (LO)). The PA110amplifies the RF signal to generate a transmit output signal, which is output to an antenna114for wireless transmission to a receiving device (not shown). For power efficiency purposes, the RF signal at the input of the PA110may be set to drive the PA110close to saturation. If not for the DPD104, this would result in significant non-linear distortion in the output signal of the transmitter100.

As discussed above, the DPD104is used to reduce non-linear distortion at the output of the PA110. The DPD104is a non-linear device that is placed in the transmit path before the PA110. The non-linear response (transfer function) of the DPD104is configured according to a set of predistortion coefficients such that the non-linear response of the DPD104substantially cancels out the non-linear response of the PA110, resulting in reduced non-linear distortion at the output of the PA110.

In one example, the DPD104may comprise one or more non-linear filters, in which the predistortion coefficients of the DPD104correspond to filter coefficients of the one or more non-linear filters. In another example, the non-linear response of the DPD104may be specified by a polynomial function, in which the predistortion coefficients DPD104correspond to polynomial coefficients of the polynomial function.

In the example shown inFIG. 1A, the transmitter100employs digital predistortion in an open-loop configuration. That is, the predistortion coefficients are static or predetermined and may be set in advance (e.g., at a factory location).

FIG. 1Bshows an example in which the transmitter100employs digital predistortion in a closed-loop configuration. In this example, the transmitter100further includes a frequency down-converter120, an analog-to-digital converter (ADC)122, and a DPD controller124. The frequency down-converter120and ADC122are in a feedback path126that runs from the output of the PA110to the DPD controller124, as shown inFIG. 1B.

In operation, the frequency down-converter120frequency down-converts the output signal of the PA110into a baseband feedback signal. The ADC122converts the baseband feedback signal into a digital feedback signal, which is input to the DPD controller124. The digital feedback signal provides the DPD controller124with feedback of the PA output signal. This allows the DPD controller124to determine the non-linear characteristics of the PA output signal, and to dynamically or adaptively adjust the predistortion coefficients of the DPD104based on the current and/or historical non-linear characteristics of the PA output signal to reduce non-linearity at the output of the PA110. In some aspects, the DPD controller124may determine the non-linear characteristics of the PA output signal by comparing the digital feedback signal of the PA output with the digital signal at the output of the DPD104.

FIG. 2shows an example of a transmitting device200that includes a transmitter202and an antenna array including N antennas114(1)-114(N). The transmitter includes N transmit circuits205(1)-205(N) (one transmit circuit for each of the N antennas114(1)-114(N) of the antenna array). Each transmit circuit205(1)-205(N) includes a respective transmit chain210(1)-210(N) coupled to the respective one of the antennas114(1)-114(N) of the array. Each transmit chain210(1) to210(N) includes a respective DPD104(1)-104(N), a respective DAC106(1)-106(N), a respective frequency up-converter108(1)-108(N), a respective phase shifter220(1)-220(N), and a respective PA110(1)-110(N). As discussed further below, the phase shifters220(1)-220(N) are used to electronically steer the transmit beam of the antenna array by applying a different phase shift to the signal for each antenna114(1)-114(N) of the array.

In operation, the transmitter202receives a baseband digital signal (generated by a baseband processor or modem of the wireless device in which the transmitter is incorporated). The baseband digital signal is input to each of the transmit chains210(1)-210(N), as shown inFIG. 2. In each transmit chain210(1)-210(N), the respective DPD104(1)-104(N) predistorts the baseband digital signal to reduce non-linear distortion at the output of the respective PA110(1)-110(N), and the respective DAC106(1)-106(N) converts the respective predistorted signal into a respective analog signal. In each transmit chain210(1)-210(N), the respective frequency up-converter108(1)-108(N) then frequency up-converts the respective analog signal into a respective radio frequency (RF) signal, the respective phase shifter220(1)-220(N) shifts the phase of the respective RF signal by a respective phase shift, and the respective PA110(1)-110(N) amplifies the respective phase-shifted signal for transmission. Each transmit chain210(1)-210(N) outputs the respective output signal to the respective antenna114(1)-114(N) of the antenna array for wireless transmission.

A beamformer controller250controls the phase shifts (labeled P1to PN) of the phase shifters220(1)-220(N) in the transmit chains210(1)-210(N) according to a selected transmit beam direction for the antenna array. More particularly, the beamformer controller250sets the phase shifts P1to PNof the phase shifters220(1)-220(N) such that the output signals of the transmit chains210(1)-210(N) are phase shifted relative to one another according to the selected transmit beam direction. The selected transmit beam direction may be directed towards a target receiving device to increase transmission power in the direction of the receiving device. For ease of illustration, the individual connections between the beamformer controller250and the phase shifters220(1)-220(N) are not shown inFIG. 2.

Each transmit circuit205(1)-205(N) also includes a respective frequency down-converter120(1)-120(N), a respective ADC122(1)-1(N), and a respective DPD controller124(1)-124(N). In each transmit circuit205(1)-205(N), the respective frequency down-converter120(1)-120(N) and the respective ADC122(1)-122(N) are in a respective feedback path126(1)-126(N) that runs from the output of the respective PA110(1)-110(N) to the respective DPD controller124(1)-124(N). In each transmit circuit205(1)-205(N), the respective frequency down-converter120(1)-120(N) frequency down-converts the output signal of the respective PA110(1)-110(N) into a respective baseband feedback signal, and the respective ADC122(1)-122(N) converts the respective baseband feedback signal into a respective digital feedback signal, which is input to the respective DPD controller124(1)-124(N).

Thus, the DPD controller124(1)-124(N) in each transmit circuit205(1)-205(N) receives a respective digital feedback signal that provides feedback of the output signal of the respective PA110(1)-110(N). This allows the DPD controller124(1)-124(N) in each transmit circuit205(1)-205(N) to determine the non-linear characteristics of the respective PA output signal, and to adjust the predistortion coefficients of the respective DPD104(1)-104(N) based on the current and/or historical non-linear characteristics of the respective PA output signal to reduce non-linearity at the output of the respective PA110(1)-110(N).

The predistortion approach illustrated inFIG. 2employs a separate DPD104(1)-104(N) and feedback path126(1)-126(N) for each PA110(1)-110(N) in the transmitter202, in which the set of predistortion coefficients for each DPD104(1)-104(N) is adjusted according to the non-linear characteristics of the respective PA110(1)-110(N). Thus, this approach optimizes predistortion for each PA110(1)-110(N) in the transmitter202, and hence each antenna114(1)-114(N) in the antenna array. However, this approach is expensive in terms of chip area and power consumption since it requires a separate DPD and feedback path for each antenna114(1)-114(N) in the array. For a large antenna array (e.g., an array with thirty-two or sixty-four antennas), the cost of this approach can be prohibitive, making this approach impractical for many applications.

Embodiments of the present disclosure provide a much more cost-effective solution to non-linear distortion for large antenna arrays. Instead of using a separate DPD and feedback path for each antenna of an antenna array, embodiments of the present disclosure employ a single DPD for the antenna array, in which the predistortion coefficients of the DPD may be optimized for the array instead of per antenna (as in the approach illustrated inFIG. 2).

FIG. 3shows an example of a transmitting device300according to aspects of the present disclosure. The transmitting device300includes a transmitter302and an antenna array including N antennas114(1)-114(N). As shown inFIG. 3, the transmitter302includes multiple transmit chains310(1)-310(N) (one transmit chain for each of the N antennas114(1)-114(N) of the antenna array). The output of each transmit chain310(1)-310(N) is coupled to the respective antenna114(1)-114(N) of the array, and includes a respective phase shifter220(1)-220(N) and respective PA110(1)-110(N).

The transmitter302also includes a DPD104, a DAC106and an up-converter108that are shared by the transmit chains310(1)-310(N) of the transmitter302. Thus, in this example, the transmitter302uses a single DPD104for the PAs110(1)-110(N) of the transmit chains310(1)-310(N) instead of a separate DPD for each PA. This significantly reduces the cost of the transmitter302compared with the transmitter202illustrated inFIG. 2.

In operation, the transmitter302receives a baseband digital signal (generated by a baseband processor or modem of the wireless device in which the transmitter is incorporated). The DPD104predistorts the baseband digital signal, the DAC106converts the predistorted digital signal into an analog predistorted signal, and the up-converter108frequency up-converts the analog predistorted signal into an RF predistorted signal. The RF predistorted signal is then input to each of the transmit chains310(1)-310(N). This may be accomplished, for example, by splitting the RF signal into N signals using a power divider (not shown), and inputting each of the N signals to a respective one of the transmit chains310(1)-310(N).

In each transmit chain310(1)-310(N), the respective phase shifter220(1)-220(N) shifts the phase of the respective RF signal by a respective phase shift, and the respective PA110(1)-110(N) amplifies the respective phase-shifted signal for transmission. Each transmit chain310(1)-310(N) outputs the respective output signal to the respective antenna114(1)-114(N) of the antenna array for wireless transmission. The phase shifts P1to PNof the phase shifters220(1)-220(N) are set by the beamformer controller250according to a selected transmit beam direction for the antenna array, as discussed above.

In the example inFIG. 3, the transmitter302uses the DPD104in an open-loop configuration. That is, the predistortion coefficients of the DPD104are predetermined and may be set in advance (e.g., at a factory location). In this regard, the predetermined predistortion coefficients may be stored in a memory430coupled to the DPD controller124. In operation, the DPD controller124retrieves the stored predistortion coefficients from the memory430, and inputs the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved predistortion coefficients. In contrast to the transmitter202illustrated inFIG. 2, the transmitter302inFIG. 3does not include a separate feedback path for each transmit chain, which significantly reduces cost compared with the transmitter202inFIG. 2.

The predistortion coefficients of the DPD104may be determined during a calibration procedure (e.g., performed at a factory) and stored in the memory430on the transmitting device300. In this regard,FIG. 4shows an example of a calibration setup for determining the predistortion coefficients of the DPD104according to aspects of the present disclosure.

As shown inFIG. 4, the setup includes a test receiving device400, a frequency down-converter420, an ADC422, and a DPD training processor450. The test receiving device400includes a receiver402, and one or more antennas414(1)-414(M). Although the frequency down-converter420and the ADC422are shown being outside the receiver402inFIG. 4, it is to be appreciated that the frequency down-converter420and the ADC422may be part of the receiver402. During the calibration procedure, the DPD training processor450may be coupled to the DPD controller124(e.g., via an interface (not shown)).

During calibration, the transmitter302transmits a test signal to the test receiving device400across a wireless channel using the antenna array. In this regard, the beamformer controller250may set the phase shifts P1to PNof the phase shifters220(1)-220(N) to direct the transmit beam of the antenna array towards the test receiving device400. The receiver402of the test receiving device400receives the corresponding transmit signals from the antennas of the antenna array of the transmitting device300via the one or more antennas414(1)-414(M). The receiver402may amplify the signals (e.g., using one or more low-noise amplifiers (LNAs)), and perform beamforming if the one or more antennas414(1)-414(M) include multiple antennas in an array. The receiver402combines the received signals into a combined receive signal415.

The frequency down-converter420frequency down-converts the combined receive signal415into a baseband combined receive signal, and the ADC422converts the baseband combined receive signal into a digital combined receive signal, which is input to the DPD training processor450. The digital combined receive signal provides the DPD training processor450with feedback on the combined receive signal at the receiver402. In contrast, the feedback signals in the previous approach illustrated inFIG. 2provide feedback of the output signals of the PAs in the transmitter202.

The DPD training processor450then determines an amount of non-linear distortion in the combined receive signal415based on the digital combined receive signal. For example, the test signal transmitted by the transmitter302may be known by the DPD training processor450. In this example, the DPD training processor450may determine the amount of non-linear distortion in the combined receive signal415based on a comparison of the digital combined receive signal and the known test signal transmitted by the transmitter302. In another example, the DPD training processor450may be coupled to the output of the DPD104. In this example, the DPD training processor450may determine the amount of non-linear distortion in the combined receive signal415based on a comparison of the digital combined receive signal and the test signal at the output of the DPD104. The DPD training processor450may determine the amount of non-linear distortion in terms of error vector magnitude (EVM), adjacent channel leakage ratio (ACLR), and/or or other type of measurement.

The DPD training processor450may determine a set of predistortion coefficients for the DPD104during calibration by determining the amount of non-linear distortion at the receiver402for each one of a plurality of different sets of predistortion coefficients, and selecting the set of predistortion coefficients resulting in the least amount of non-linear distortion at the receiver402. To do this, the DPD training processor450may instruct the DPD controller124to sequentially input each one of the different sets of predistortion coefficients to the DPD104to sequentially configure the non-linear response of the DPD104according to each one of the different sets of predistortion coefficients. For each one of the different sets of predistortion coefficients, the transmitter202transmits a test signal and the DPD training processor450determines the amount of non-linear distortion in the corresponding combined receive signal, as discussed above. Thus, the DPD training processor450determines the amount of non-linear distortion in the combined receive signal for each one of the different sets of predistortion coefficients. This way, the DPD training processor450determines the effect different sets of predistortion coefficients of the DPD104have on the non-linear distortion observed at the receiver402. The DPD training processor450may then select the set of predistortion coefficients resulting in the least amount of non-linear distortion in the combined receive signal at the receiver402.

The DPD training processor450may store the selected set of predistortion coefficients in the memory430for subsequent use by the DPD104. During normal operation, the DPD controller124may retrieve the stored predistortion coefficients from the memory430, and input the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved predistortion coefficients.

In this example, the calibration setup forms a closed-loop system during calibration in which the DPD training processor450adjusts the predistortion coefficients of the DPD104in the transmitter302while observing the effect the adjustment has on the non-linear distortion in the combined receive signal at the receiver402. After calibration, the DPD training processor450may be decoupled from the transmitting device300. During normal operation, the transmitter302operates in an open-loop configuration, in which the DPD controller124retrieves the set of predistortion coefficients determined during calibration from the memory430, and inputs the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved set of predistortion coefficients.

In certain aspects, the DPD training processor450may determine a set of predistortion coefficients for each one of a plurality of different transmit beam directions. In this example, the DPD training processor450may instruct the beamformer controller250to sequentially set the transmit beam direction of the antenna array to each one the different transmit beam directions. As discussed above, the beamformer controller250sets the direction of the transmit beam by setting the phase shifts P1to PNof the phase shifters220(1)-220(N) accordingly.

For each one of the transmit beam directions, the DPD training processor450performs the calibration procedure discussed above to determine a set of predistortion coefficients resulting in the least amount of non-linear distortion at the receiver402. This way, the DPD training processor450determines a set of predistortion coefficients for each one of the transmit beam directions. The DPD training processor450stores the determined set of predistortion coefficients for each transmit beam direction in the memory430for subsequent use by the DPD104. During normal operation, the DPD controller124may receive a signal (e.g., from the beamformer controller250) indicating the current transmit beam direction of the array. The DPD controller124may then retrieve the set of predistortion coefficients corresponding to the current transmit beam direction from the memory430, and input the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved set of predistortion coefficients.

FIG. 5illustrates an exemplary calibration procedure500for determining a set of predistortion coefficients for a selected transmit beam direction. The calibration procedure may be performed using the calibration setup illustrated inFIG. 4. At step510, the phase shifts P1to PNof the phase shifters220(1)-220(N) are set according to the selected transmit beam direction. At step520, the non-linear response of the DPD104is configured according to a set of predistortion coefficients from among a plurality of different sets of predistortion coefficients. At step530, the transmitter302transmits a test signal to the receiver402. At step540, an amount of non-linear distortion in the corresponding combined receive signal415is determined. For example, the DPD training processor450may determine the amount of non-linear distortion from the digital combined receive signal from the ADC422, as discussed above.

At step550, a determination is made whether all of the sets of predistortion coefficients have been tested (i.e., the amount of non-linear distortion at the receiver has been determined for each set of predistortion coefficients). If not all of the sets of predistortion coefficients have been tested, then the procedure500proceeds to step555, in which the non-linear response of the DPD104is configured according to another one of the sets of predistortion coefficients that has not already been tested. In this case, steps530and555are repeated for the other set of predistortion coefficients to determine the amount of non liner non-linear distortion at the receiver402for the other set of predistortion coefficients. If all of the sets of predistortion coefficients have been tested, then the procedure500proceeds to step560, in which the set of predistortion coefficients corresponding to the least amount of non-linear distortion at the receiver402is selected. In this regard, the selected set of predistortion coefficients may be stored in the memory430for subsequent use by the DPD104for the selected transmit direction.

The calibration procedure500illustrated inFIG. 5may be repeated for each one of the plurality of different transmit beam directions to determined a set of predistortion coefficients for each one of the different transmit beam directions. The determined set of predistortion coefficients for each transmit beam direction may be stored in the memory430for subsequent use by the DPD104, as discussed above.

FIG. 6illustrates another exemplary calibration procedure600for determining a set of predistortion coefficients for a selected transmit beam direction. The calibration procedure may be performed using the calibration setup illustrated inFIG. 4. At step610, the phase shifts P1to PNof the phase shifters220(1)-220(N) are set according to the selected transmit beam direction. At step620, the non-linear response of the DPD104is configured according to an initial set of predistortion coefficients. At step630, the transmitter302transmits a test signal to the receiver402. At step640, an amount of non-linear distortion in the corresponding combined receive signal415is determined. For example, the DPD training processor450may determine the amount of non-linear distortion from the digital combined receive signal from the ADC422, as discussed above.

At step650, a determination is made whether the amount of non-linear distortion for the current set of predistortion coefficients is below a threshold. The threshold may be set, for example, according to an acceptable level of non-linear distortion that can be tolerated at the receiver402(e.g., a level at which the receiving device400is still able to recover data from a received signal). If the amount of non-linear distortion for the current set of predistortion coefficients is above the threshold, then the procedure600proceeds to step655, in which the set of predistortion coefficients used by the DPD to configure its non-linear response is adjusted. The adjustment may include changing one or more of the predistortion coefficients. In this case, steps630and640are repeated for the adjusted set of predistortion coefficients to determine the amount of non-linear distortion at the receiver402for the adjusted set of predistortion coefficients. If the amount of non-linear distortion for the current set of predistortion coefficients is below the threshold, then the procedure600proceeds to step660, in which the current set of predistortion coefficients is used for the selected transmit direction. This is indicative that the current set of predistortion coefficients provide a sufficient amount of non-linear distortion cancellation. In this regard, the current set of predistortion coefficients may be stored in the memory430for subsequent use by the DPD104for the selected transmit direction. If the determined amount of non-linear distortion is equal to the threshold, then the procedure600may adjust the set of coefficients and repeat steps630and640, or proceed to step660. Adjusting the set of coefficients may involve adjusting one or more of the coefficients.

The calibration procedure600illustrated inFIG. 6may be repeated for each one of the plurality of different transmit beam directions to determine a set of predistortion coefficients for each one of the different transmit beam directions. The determined set of predistortion coefficients for each transmit beam direction may be stored in the memory430for subsequent use by the DPD104, as discussed above.

The sets of predistortion coefficients for the different transmit beam directions may be stored in a lookup table in the memory430. In this regard,FIG. 7illustrates an exemplary lookup table710stored in the memory430. The table710includes L sets of predistortion coefficients for L transmit beam directions, where L is the number of different transmit beam directions. In operation, the DPD controller124receives a signal indicating the selected transmit beam direction of the antenna array. The DPD controller124then retrieves the set of predistortion coefficients corresponding to the selected transmit beam direction from the lookup table710, and inputs the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved set of predistortion coefficients.

As discussed above, the beamformer controller250sets the phase shifts P1to PNof the phase shifters220(1) to220(N) to point the transmit beam of the array in a selected direction. The direction may be determined, for example, using beam training. In this example, the beamformer controller250may sequentially point the transmit beam in a plurality of different directions during a training procedure, and the transmitter302may transmit a training signal in each one of the different transmit directions to a target receiving device. The receiving device may measure the receive signal strength (e.g., signal-to-noise ratio (SNR), receive signal strength indicator (RSSI), etc.) of each training signal. The receiving device may then select the transmit direction resulting in the highest measured signal strength, and communicate the selected direction to the beamformer controller250(e.g., via a wireless link). In another example, the receiving device may report its location to the beamformer controller250(e.g., via a wireless link). In this example, the beamformer controller250may determine the direction of the receiving device relative to the transmitting device300based on the reported location of the receiving device and the location of the transmitting device300. It is to be appreciated that the present disclosure is not limited to the above examples, and that the transmit direction may be determining using other techniques. After the transmit direction is determined, the DPD controller124may retrieve the set of predistortion coefficients corresponding to the determined transmit beam direction from the memory430, and input the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved set of predistortion coefficients.

In certain aspects, the transmitter302may employ beamforming to direct the transmit beam towards a receiving device and shape the beam to increase the transmission power directed to the receiving device. In these aspects, the beamformer controller250adjusts both a phase shift and a gain of each transmit chain310(1)-310(N) according to the selected transmit beam direction. In this regard,FIG. 8shows an example in which each transmit chain310(1)-310(N) further includes a respective variable-gain pre-amplifier810(1)-810(N). In this example, the beamformer controller250may independently set the gain (labeled G1to GN) of each variable-gain pre-amplifier810(1)-810(N) according to the selected transmit beam direction. The variable-gain pre-amplifiers810(1)-810(N) allow the beamformer controller250to independently control the output transmission power of each PA according to the selected transmit beam direction. This is because the gain of each variable-gain pre-amplifier810(1)-810(N) controls the power level that is input to the respective PA110(1)-110(N), which in turns controls the output transmission power of the respective PA110(1)-110(N). For ease of illustration, the individual connections between the beamformer controller250and the variable-gain pre-amplifiers810(1)-810(N) are not shown inFIG. 8.

Predistortion coefficients for the transmitter302illustrated inFIG. 8can be determined using the calibration procedures and calibration setup discussed above with reference toFIG. 4. For example, a set of predistortion coefficients may be determined for each one of a plurality of different transmit beam directions by performing the exemplary calibration procedure500illustrated inFIG. 5for each transmit beam direction. In this example, the gains G1to GNof the variable-gain pre-amplifiers810(1)-810(N) are set according to the selected transmit beam direction in step510. In another example, a set of predistortion coefficients may be determined for each one of a plurality of different transmit beam directions by performing the exemplary calibration procedure600illustrated inFIG. 6for each transmit beam direction. In this example, the gains G1to GNof the variable-gain pre-amplifiers810(1)-810(N) are set according to the selected transmit beam direction in step610.

In certain aspects, the saturation levels of the PAs110(1)-110(N) may be independently set such that the PAs110(1)-110(N) have approximately the same backoff value. Having the PAs110(1)-110(N) operate with approximately the same backoff reduces differences between the non-linear characteristics of the PAs110(1)-110(N). In this regard,FIG. 9shows an example in which each PA110(1)-110(N) receives a respective bias voltage (labeled VB1to VBN) that controls the saturation level of the PA110(1)-110(N).

In this example, the transmitting device300also includes a PA bias voltage controller950configured to set the saturation levels of the PAs110(1)-110(N) by setting the respective bias voltages VB1to VBNaccordingly. In one aspect, the bias voltage controller950sets the saturation levels of the PAs110(1)-110(N) to achieve approximately the same backoff value across the PAs110(1)-110(N), as discussed further below.

In operation, the beamformer controller250sets the gains G1to GNof the variable-gain pre-amplifiers810(1)-810(N) according to the selected transmit beam direction of the antenna array. Since the gain of each variable-gain pre-amplifier810(1)-810(N) controls the power level that is input to the respective PA110(1)-110(N), the gain of each variable-gain pre-amplifier810(1)-810(N) also controls the output transmission power of the respective PA110(1)-110(N). Assuming the gains of the variable-gain pre-amplifiers810(1)-810(N) vary across the transmit chains310(1)-310(N), this causes the output transmission powers of the PAs110(1)-110(N) to also vary across the transmit chains310(1)-310(N).

The bias voltage controller950may determine the output transmission powers of the PAs110(1)-110(N) based on the gains set by the beamformer controller250. The bias voltage controller950may then set the saturation levels of the PAs110(1)-110(N) based on the determined output transmission powers of the PAs110(1)-110(N) to achieve approximately the same backoff value across the PAs110(1)-110(N). More particularly, the bias voltage controller950may set the saturation level of each PA above the respective output transmission power by an amount approximately equal to a selected backoff value by setting the respective bias voltage accordingly. For example, if a first one of the PAs has an output transmission power of 20 dBm, a second one of the PAs has an output transmission power of 15 dBm and the selected backoff value is 4 dBm, then the bias voltage controller950may set the saturation level of the first one of the PAs to 24 dBm and set the saturation level of the second one of the PAs to 19 dBm by setting their respective bias voltages accordingly.

In certain aspects, the bias voltages VB1to VBNfor the PAs110(1)-110(N) may be determined in advanced for different transmit beam directions and backoff values using the method discussed above, and stored in a memory (e.g., memory430). In this example, the bias voltage controller950receives signals indicating the selected transmit beam direction and backoff value. The bias voltage controller950then retrieves the bias voltages VB1to VBNfor the selected transmit beam direction and backoff value from the memory and applies the retrieved bias voltages VB1to VBNto the respective PAs110(1)-110(N).

Predistortion coefficients for the transmitter302illustrated inFIG. 9can be determined using the calibration procedures and calibration setup discussed above with reference toFIG. 4. For example, a set of predistortion coefficients may be determined for a selected transmit beam direction and backoff value using the exemplary calibration procedure500illustrated inFIG. 5. In this example, the gains G1to GNof the variable-gain pre-amplifiers810(1)-810(N) are set according to the selected transmit beam direction at step510. In addition, the saturation levels of the PAs110(1)-110(N) are set using the respective bias voltages VB1to VBNsuch that the backoff values of the PAs110(1)-110(N) are approximately equal to the selected backoff value. After the transmit beam is set to the selected transmit beam direction and the backoff values of the PAs110(1)-11(N) are set to the selected backoff value, the calibration procedure500may proceed to step520to determine the set of predistortion coefficients, as discussed above.

The calibration procedure500illustrated inFIG. 5may be repeated for different combinations of transmit beam directions and backoff values to determine sets of predistortion coefficients for the different combinations of transmit beam directions and backoff values. The determined sets of predistortion coefficients may be stored in the memory430for subsequent use by the DPD104, as discussed further below.

In another example, a set of predistortion coefficients may be determined for a selected transmit beam direction and backoff value using the exemplary calibration procedure600illustrated inFIG. 6. In this example, the gains G1to GNof the variable-gain pre-amplifiers810(1)-810(N) are set according to the selected transmit beam direction at step610. In addition, the saturation levels of the PAs110(1)-110(N) are set using the respective bias voltages VB1to VBNsuch that the backoff values of the PAs110(1)-110(N) are approximately equal to the selected backoff value. After the transmit beam is set to the selected transmit beam direction and the backoff values of the PAs110(1)-11(N) are set to the selected backoff value, the calibration procedure600may proceed to step620to determine the set of predistortion coefficients, as discussed above.

The calibration procedure600illustrated inFIG. 6may be repeated for different combinations of transmit beam directions and backoff values to determine sets of predistortion coefficients for the different combinations of transmit beam directions and backoff values. The determined sets of predistortion coefficients may be stored in the memory430for subsequent use by the DPD104, as discussed further below.

The sets of predistortion coefficients for different combinations of transmit beam directions and backoff values may be stored in a lookup table in the memory430. In this regard,FIG. 10illustrates an exemplary lookup table1010stored in the memory430. For each one of K different backoff values, the lookup table1010includes L sets of predistortion coefficients for L transmit beam directions. In the table1010, each set of predistortion coefficients is indexed by two numbers, where the first number indicates the corresponding backoff value and the second number indicates the corresponding transmit beam direction.

In operation, the DPD controller124receives signals indicating the selected transmit beam direction and the selected backoff value. The DPD controller124then retrieves the set of predistortion coefficients corresponding to the selected transmit beam direction and selected backoff value from the lookup table1010, and inputs the retrieved predistortion coefficients to the DPD104to configure the non-linear response of the DPD104according to the retrieved set of predistortion coefficients.

The transmit beam direction can be selected based on the direction of the target receiving device relative to the transmitting device300, as discussed above. The target receiving device is a receiving device that is in communication with the transmitting device300. For a WiFi example where communication is between an access point (AP) and a station (STA), the transmitting device300may be incorporated in the AP and the receiving device may be incorporated in the STA, or vice versa.

FIG. 11illustrates an exemplary method1100for wireless communications according to certain aspects of the present disclosure. The method may be performed by the transmitter302, the DPD controller124, and/or the DPD training processor450.

At step1110, a first signal is predistorted based on a set of coefficients to generate a first predistorted signal. For example, the first signal may be predistorted by a predistorter (e.g., DPD104) based on the set of coefficients (e.g., predistortion coefficients), in which set of coefficients configures a non-linear response of the predistorter. The first signal may comprise a baseband signal.

At step1120, the first predistorted signal is output for a first transmission to a receiving device. For example, the first predistorted signal may be transmitted to the receiving device via multiple transmit chains. In this example, each of the transmit chains (e.g., transmit chains310(1)-310(N)) may include a respective power amplifier (e.g., PA110(1)-110(N)). The first predistorted signal may be converted to an analog predistorted signal and/or frequency up-converted before being output to the transmit chains.

At step1130, a first feedback signal is received from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission. The first receive signal may correspond to a combined receive signal (e.g., combined receive signal415) of a receiver (e.g., receiver402) of the receiving device.

At step1140, a determination is made whether to store the set of coefficients in a memory based on the received first feedback signal. For example, an amount of non-linear distortion in the first receive signal may be determined based on the first feedback signal (e.g., in terms of error vector magnitude (EVM), adjacent channel leakage ratio (ACLR), and/or or other type of measurement). In this example, the determined amount of non-linear distortion may be compared to a threshold.

At step1150, the set of coefficients is stored in the memory based on the determination. For example, the set of coefficients may be stored in the memory if the comparison indicates the determined amount of non-linear distortion is below the threshold. This may be indicative that the set of coefficients provide a sufficient amount of non-linear distortion cancellation for subsequent wireless communications.

The method1100may be repeated for each one of a plurality of different beam directions to determine a set of coefficients for each beam direction. In another example, the method1100may be repeated for each one of a plurality of different combinations of backoff values and beam directions (e.g., to generate the table1010shown inFIG. 10).

FIG. 12illustrates an exemplary method1200for wireless communications according to certain aspects of the present disclosure. The method may be performed by the transmitter302, the DPD controller124, and/or the DPD training processor450.

At step1210, each one of a plurality of signals is predistorted based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals. For example, each signal may be predistorted by a predistorter (e.g., DPD104) at a different time based on the respective set of coefficients (e.g., predistortion coefficients). Each signal may comprise a baseband signal.

At step1220, each one of the plurality of predistorted signals is outputted for transmission to a receiving device. For example, the predistorted signals may be outputted one at a time.

At step1230, a plurality of feedback signals are received from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals. For example, the feedback signals may be received one at a time.

At step1240, one of the plurality of sets of coefficients is selected based on the plurality of feedback signals. For example, for each set of coefficients, an amount of non-linear distortion may be determined based on the corresponding feedback signal. In this example, the set of coefficients corresponding to the lowest determined amount of non-linear distortion may be selected.

At step1250, the selected one of the plurality of sets of coefficients is stored in a memory. The selected one of the plurality of sets of coefficients may be used for subsequent transmissions to predistort a communication signal.

The techniques described herein may be used for various broadband wireless communication systems, including communication systems that are based on a single-carrier or an orthogonal multiplexing scheme. Examples of such communication systems include Spatial Division Multiple Access (SDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA) systems, Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems, and so forth. An SDMA system may utilize sufficiently different directions to simultaneously transmit data belonging to multiple user terminals. A TDMA system may allow multiple user terminals to share the same frequency channel by dividing the transmission signal into different time slots, each time slot being assigned to different user terminal. An OFDMA system utilizes orthogonal frequency division multiplexing (OFDM), which is a modulation technique that partitions the overall system bandwidth into multiple orthogonal sub-carriers. These sub-carriers may also be called tones, bins, etc. With OFDM, each sub-carrier may be independently modulated with data. An SC-FDMA system may utilize interleaved FDMA (IFDMA) to transmit on sub-carriers that are distributed across the system bandwidth, localized FDMA (LFDMA) to transmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA) to transmit on multiple blocks of adjacent sub-carriers. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDMA.

An access point (“AP”) may comprise, be implemented as, or known as a Node B, Radio Network Controller (“RNC”), evolved Node B (eNB), Base Station Controller (“BSC”), Base Transceiver Station (“BTS”), Base Station (“BS”), Transceiver Function (“TF”), Radio Router, Radio Transceiver, Basic Service Set (“BSS”), Extended Service Set (“ESS”), Radio Base Station (“RBS”), or some other terminology.

FIG. 13illustrates an example of a multiple-access multiple-input multiple-output (MIMO) system1300with access points and user terminals in which aspects of the present disclosure may be practiced. Additionally or alternatively, aspects of the present disclosure may be implemented within beamforming systems. For the MIMO example ofFIG. 13, access point1310or user terminals1320may include transmitters with the common DPD configured as described above. For simplicity, only one access point1310is shown inFIG. 13. An access point is generally a fixed station that communicates with the user terminals and may also be referred to as a base station or some other terminology. A user terminal may be fixed or mobile and may also be referred to as a mobile station, a wireless device, or some other terminology. Access point1310may communicate with one or more user terminals1320at any given moment on the downlink and uplink. The downlink (i.e., forward link) is the communication link from the access point1310to the user terminals1320, and the uplink (i.e., reverse link) is the communication link from the user terminals1320to the access point1310. A user terminal may also communicate peer-to-peer with another user terminal. A system controller1330couples to and provides coordination and control for the access points1310.

FIG. 14illustrates various components that may be utilized in a wireless device1402in which aspects of the present disclosure may be practiced and that may be employed within the MIMO system1300. The wireless device1402is an example of a device that may be configured to implement the various methods described herein. The wireless device1402may be an access point1310or a user terminal1320.

The wireless device1402may include a processor1404which controls operation of the wireless device1402. The processor1404may also be referred to as a central processing unit (CPU). Memory1406, which may include both read-only memory (ROM) and random access memory (RAM), provides instructions and data to the processor1404. A portion of the memory1406may also include non-volatile random access memory (NVRAM). The processor1404typically performs logical and arithmetic operations based on program instructions stored within the memory1406. The instructions in the memory1406may be executable to implement the methods described herein. Processor1404may, for example, direct all or some of the operations of the various flowcharts of the drawings to implement the DPD controller124, the digital predistorter104, beamformer controller250, bias voltage controller950and/or other features of the present disclosure.

The wireless device1402may also include a housing1408that may include a transmitter1410and a receiver1412to allow transmission and reception of data between the wireless device1402and a remote location. The transmitter1410and receiver1412may be combined into a transceiver1414. A single or a plurality of transmit antennas1416may be attached to the housing1408and electrically coupled to the transceiver1414. The wireless device1402may include multiple transmitters1410, multiple receivers1412, and multiple transceivers1414. The transmitter1410may be equipped or configured as described above to perform the operations of the various flowcharts. In this example, the transmitter1410may implement the transmitter302according to any of the embodiments discussed above.

The wireless device1402may also include a signal detector1418that may be used in an effort to detect and quantify the level of signals received by the transceiver1414. The signal detector1418may detect such signals as total energy, energy per subcarrier per symbol, power spectral density and other signals. The wireless device1402may also include a digital signal processor (DSP)1420for use in processing signals. The various components of the wireless device1402may be coupled together by a bus system1422(interface), which may include a power bus, a control signal bus, and a status signal bus in addition to a data bus.

Examples of means for predistorting a first signal based on a set of coefficients to generate a first predistorted signal include at least one of the DPD controller124, the DPD104, or the processor1404. Examples of means for outputting the first predistorted signal for a first transmission to a receiving device include at least one of the transmit chains310(1) to310(N), the bus system1422, or the transmitter1410. Examples of means for receiving a first feedback signal from the receiving device, wherein the first feedback signal provides feedback of a first receive signal at the receiving device corresponding to the first transmission include at least one of the bus system1422, or the receiver1412. Examples of means for determining whether to store the set of coefficients in a memory based on the received first feedback signal include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for storing the set of coefficients in the memory based on the determination include at least one of the DPD training processor450, the DPD controller124, or the processor1404.

Examples of means for determining an amount of non-linear distortion in the first receive signal based on the received first feedback signal include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for comparing the determined amount of non-linear distortion with a threshold include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for storing the set of coefficients in the memory if the comparison indicates the determined amount of non-linear distortion is below the threshold include at least one of the DPD training processor450, the DPD controller124, or the processor1404.

Examples of means for adjusting the set of coefficients if the comparison indicates the determined amount of non-linear distortion is above the threshold include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for predistorting a second signal based on the adjusted set of coefficients to generate a second predistorted signal include at least one of the DPD controller124, the DPD104, or the processor1404. Examples of means for outputting the second predistorted signal for a second transmission to the receiving device include at least one of the transmit chains310(1) to310(N), the bus system1422, or the transmitter1410. Examples of means for receiving a second feedback signal from the receiving device, wherein the second feedback signal provides feedback of a second receive signal at the receiving device corresponding to the second transmission include at least one of the bus system1422, or the receiver1412. Examples of means for determining an amount of non-linear distortion in the second receive signal based on the received second feedback signal include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for comparing the determined amount of non-linear distortion in the second receive signal with the threshold include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for storing the adjusted set of coefficients in the memory if the comparison indicates the determined amount of non-linear distortion in the second receive signal is below the threshold include at least one of the DPD training processor450, the DPD controller124, or the processor1404.

Examples of means for retrieving the set of coefficients from the memory include at least one of include at least one of the DPD controller124, or the processor1404. Examples of means for predistorting a communication signal based on the retrieved set of coefficients to generate a predistorted communication signal include at least one of the DPD controller124, the DPD104, or the processor1404. Examples of means for outputting the predistorted communication signal for transmission include at least one of the transmit chains310(1) to310(N), the bus system1422, or the transmitter1410.

Examples of means for predistorting each one of a plurality of signals based on a respective one of a plurality of sets of coefficients to generate a respective one of a plurality of predistorted signals include at least one of the DPD controller124, the DPD104, or the processor1404. Examples of means for outputting each one of the plurality of predistorted signals for transmission to a receiving device include at least one of the transmit chains310(1) to310(N), the bus system1422, or the transmitter1410. Examples of means for receiving a plurality of feedback signals from the receiving device, wherein each one of the plurality of feedback signals provides feedback of a respective one of a plurality of receive signals at the receiving device corresponding to the transmission of a respective one of the plurality of predistorted signals include at least one of the bus system1422, or the receiver1412. Examples of means for selecting one of the plurality of sets of coefficients based on the plurality of feedback signals include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for storing the selected one of the plurality of sets of coefficients in a memory include at least one of the DPD training processor450, the DPD controller124, or the processor1404.

Examples of means for determining an amount of non-linear distortion for each one of the plurality of sets of coefficients based on the respective one of the plurality of feedback signals include at least one of the DPD training processor450, the DPD controller124, or the processor1404. Examples of means for selecting the one of the plurality of sets of coefficients corresponding to a lowest determined amount of non-linear distortion include at least one of the DPD training processor450, the DPD controller124, or the processor1404.

Examples of means for retrieving the selected one of the plurality of sets of coefficients from the memory include at least one of the DPD controller124, or the processor1404. Examples of means for predistorting a communication signal based on the retrieved selected one of the plurality of sets of coefficients to generate a predistorted communication signal include at least one of the DPD controller124, the DPD104, or the processor1404. Examples of means for outputting the predistorted communication signal for transmission include at least one of the transmit chains310(1) to310(N), the bus system1422, or the transmitter1410.

As used herein, the term “generating” encompasses a wide variety of actions. For example, “generating” may include calculating, causing, computing, creating, determining, processing, deriving, investigating, making, producing, providing, giving rise to, leading to, resulting in, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “generating” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “generating” may include resolving, selecting, choosing, establishing and the like.

In a hardware implementation, the machine-readable media may be part of the processing system separate from the processor. However, as those skilled in the art will readily appreciate, the machine-readable media, or any portion thereof, may be external to the processing system. By way of example, the machine-readable media may include a transmission line, a carrier wave modulated by data, and/or a computer product separate from the wireless node, all which may be accessed by the processor through the bus interface. Alternatively, or in addition, the machine-readable media, or any portion thereof, may be integrated into the processor, such as the case may be with cache and/or general register files.