Symbol based envelope tracking and crest factor reduction

Abstract

Aspects of the present disclosure relate to symbol-based envelope tracking. A voltage modulator circuit can generate an output bias voltage that tracks the root mean square symbol power of a radio frequency signal. In a signal path providing the radio frequency signal, a crest factor reduction circuit can adjust a crest factor reduction threshold such that the crest factor reduction threshold corresponds to the output bias voltage.

Description

Technology Field

[0001] Cross-reference for priority applications

[0002] This application claims the benefit of priority to U.S. application No. 63 / 512,041, filed on July 5, 2023, titled “Symbol-based Envelope Tracking,” the disclosure of which is incorporated by reference in whole and for all purposes.

[0003] Technology field

[0004] Embodiments of the present disclosure relate to a system for generating a bias voltage for a power amplifier using symbol-based envelope tracking. Background Technology

[0005] Radio systems can transmit and receive signals in the form of electromagnetic waves with frequencies in the range of approximately 30 kilohertz (kHz) to 300 gigahertz (GHz). Radio systems can be used for wireless communications, such as cellular communications and / or other wireless network communications.

[0006] Radio systems that transmit signals often include power amplifiers to amplify radio frequency (RF) signals for transmission through one or more antennas. Power amplifiers can consume significant power in these systems. Power-efficient power amplifiers may be desirable for various applications.

[0007] The innovations described in the claims each have various aspects, and no single one is solely responsible for the desirable attributes. Without limiting the scope of the claims, some important features of the present disclosure will now be briefly described.

[0008] One aspect of the present disclosure is a power amplifier system utilizing symbol-based envelope tracking, comprising a voltage modulator circuit, a power amplifier, and a signal path. The voltage modulator circuit is configured to generate an output bias voltage that tracks the root mean square symbol power of a radio frequency signal. The voltage modulator circuit is configured to adjust the output bias voltage over symbol boundaries. The power amplifier is configured to receive the output bias voltage and amplify the radio frequency signal. The signal path is configured to provide the radio frequency signal to the power amplifier. The signal path includes a crest factor reduction block configured to adjust a crest factor reduction threshold such that the crest factor reduction threshold corresponds to the output bias voltage.

[0009] The power amplifier system may include a quantizer configured to determine a symbol-based envelope tracking state based on the transmitted signal power. A voltage modulator circuit may generate an output bias voltage based on the symbol-based envelope tracking state. A crest factor reduction block may adjust a crest factor reduction threshold based on the symbol-based envelope tracking state. The signal path may include a digital pre-distortion system coupled between the crest factor reduction block and a digital-to-analog converter. The digital pre-distortion system may adjust digital pre-distortion based on the symbol-based envelope tracking state. The quantizer may receive a complex transmitted symbol power associated with multiple carriers.

[0010] The signal path may include a digital-to-analog converter coupled between the output of the crest factor reduction block and the input to the power amplifier.

[0011] A power amplifier system may be capable of operating in a discontinuous transmission state. A power amplifier transistor of the power amplifier system may have an output terminal connected to ground in a discontinuous transmission state. A power amplifier transistor of the power amplifier system may have a floating output terminal in a discontinuous transmission state.

[0012] A radio frequency signal may include at least two carriers having different symbol lengths. A voltage modulator circuit can adjust an output bias voltage corresponding to the symbol boundaries of one of the at least two carriers having the shortest symbol period.

[0013] The power amplifier may include a main power amplifier transistor coupled to a voltage modulator circuit and a peaking power amplifier transistor coupled to a fixed voltage supply. The power amplifier may include a main power amplifier transistor coupled to a voltage modulator circuit and a peaking power amplifier transistor coupled to a voltage modulator circuit. The power amplifier may include a main power amplifier transistor coupled to a voltage modulator circuit and a peaking power amplifier transistor coupled to a second voltage modulator circuit.

[0014] The voltage modulator circuit can transition the output bias voltage between discrete voltage levels in a time range between 5 nanoseconds and 200 nanoseconds.

[0015] The voltage modulator circuit can transition the output bias voltage between at least four discrete voltage levels on the symbol boundaries.

[0016] The base station may include a power amplifier system.

[0017] Another aspect of the present disclosure is a method for amplifying a radio frequency signal using symbol-based envelope tracking. The method comprises: determining a symbol-based envelope tracking state based on a transmitted signal power; toggling an output bias voltage between a plurality of discrete voltage levels on the symbol boundaries of a radio frequency signal based on the symbol-based envelope tracking state—the output bias voltage is provided to a power amplifier—; adjusting a crest factor reduction threshold of a crest factor reduction block based on the symbol-based envelope tracking state—the crest factor reduction block is in a signal path providing the radio frequency signal to a power amplifier—; and amplifying the radio frequency signal using the power amplifier.

[0018] The method may include adjusting digital pre-distortion in the signal path based on the symbol-based envelope tracking state.

[0019] The method may include operating a power amplifier system including a power amplifier in a discontinuous transmission state.

[0020] A radio frequency signal may contain two carriers with different symbol periods. Toggling of the output voltage may occur on the symbol boundaries of one of the two carriers, which has a shorter symbol period.

[0021] Another aspect of the present disclosure is a system utilizing symbol-based envelope tracking. The symbol includes means for generating an output bias voltage that tracks the root mean square symbol power of a radio frequency signal on a symbol-by-symbol basis, and a signal path configured to provide the radio frequency signal. The signal path includes a crest factor reduction block configured to adjust a crest factor reduction threshold so that the crest factor reduction threshold corresponds to the output bias voltage.

[0022] Another aspect of the present disclosure is a power amplifier system utilizing symbol-based envelope tracking, comprising a voltage modulator circuit, a power amplifier, and a signal path configured to provide a radio frequency signal to the power amplifier. The voltage modulator circuit is configured to generate an output bias voltage based on a symbol-based envelope tracking state such that the output bias voltage tracks the root mean square symbol power of the radio frequency signal on a symbol-by-symbol basis. The power amplifier is configured to receive the output bias voltage and amplify the radio frequency signal. It includes a digital pre-distortion system configured to adjust digital pre-distortion based on a symbol-based envelope tracking state.

[0023] A digital pre-distortion system can adjust digital pre-distortion by adjusting at least the digital pre-distortion coefficients. A digital pre-distortion system can adjust digital pre-distortion by adjusting at least the digital pre-distortion actuator.

[0024] The power amplifier system may include a quantizer configured to determine a symbol-based envelope tracking state based on the transmitted signal power. The quantizer may receive a complex transmitted symbol power associated with multiple carriers. The power amplifier system may include one or more delay blocks coupled between the quantizer and the digital pre-distortion system.

[0025] The signal path may include a wave rate reduction block configured to adjust the wave rate reduction threshold based on the symbol-based envelope tracking state.

[0026] The power amplifier system can operate in a discontinuous transmission state.

[0027] A radio frequency signal may include at least two carriers having different symbol lengths. A voltage modulator circuit can adjust an output bias voltage corresponding to the symbol boundaries of one of the at least two carriers having the shortest symbol period.

[0028] The voltage modulator circuit can transition the output bias voltage between at least four discrete voltage levels on the symbol boundaries.

[0029] The power amplifier can be a Doherty power amplifier.

[0030] Another aspect of the present disclosure is a method for amplifying a radio frequency signal using symbol-based envelope tracking. The method comprises: determining a symbol-based envelope tracking state based on a transmitted signal power; toggling an output bias voltage between a plurality of discrete voltage levels on the symbol boundaries of a radio frequency signal based on the symbol-based envelope tracking state—the output bias voltage is provided to a power amplifier—; adjusting a digital pre-distortion based on the symbol-based envelope tracking state—a digital pre-distortion system performing the digital pre-distortion is in a signal path providing the radio frequency signal to a power amplifier—; and amplifying the radio frequency signal using the power amplifier.

[0031] The method may include operating a power amplifier system including a power amplifier in a discontinuous transmission state.

[0032] A radio frequency signal may contain two carriers with different symbol periods. Toggling of the output bias voltage may occur on the symbol boundaries of one of the two carriers, which has a shorter symbol period.

[0033] Another aspect of the present disclosure is a system utilizing symbol-based envelope tracking. The system includes a voltage modulator circuit and a signal path configured to provide a radio frequency signal. The voltage modulator circuit is configured to generate an output bias voltage based on a symbol-based envelope tracking state such that the output bias voltage tracks the root mean square symbol power of the radio frequency signal on a symbol-by-symbol basis. The signal path includes a digital pre-distortion system configured to adjust digital pre-distortion based on the symbol-based envelope tracking state.

[0034] The system may include a quantizer configured to determine a symbol-based envelope tracking state based on the indication of transmission power. The system may include one or more delay blocks coupled between the quantizer and a digital pre-distortion system. The quantizer may receive a complex transmission symbol power associated with multiple carriers.

[0035] The signal path may include a wave rate reduction block configured to adjust the wave rate reduction threshold based on the symbol-based envelope tracking state.

[0036] The voltage modulator circuit can transition the output bias voltage between multiple discrete voltage levels in a time range between 5 nanoseconds and 200 nanoseconds.

[0037] For the purpose of summarizing the present disclosure, specific aspects, advantages, and novel features of the innovations have been described herein. It will be understood that not all of these advantages may necessarily be achieved according to any specific embodiment. Accordingly, the innovations may be embodied or practiced in a manner that achieves or optimizes one advantage or a group of advantages as taught herein, without necessarily achieving other advantages as taught or suggested herein. Brief explanation of the drawing

[0038] Embodiments of the present disclosure will be described with reference to the accompanying drawings as a non-limiting example. Figure 1 is a bar graph of the probability of cell resource usage for a relatively large number of base stations over several weeks. Figure 2 is a graph of the radio frequency envelope of a 20 megahertz long-term evolution carrier over time with resource usage varying from 100% to 1.6%. Figures 3a, 3b, and 3c are graphs for five symbols of a radio frequency signal. Figure 3a shows a fixed bias voltage. Figure 3b shows a symbol-based envelope tracking bias voltage. Figure 3c shows a continuous envelope tracking voltage. FIG. 4 is a schematic block diagram of a symbol-based envelope tracking system according to an embodiment. Figure 5 is a graph of the error vector magnitude of each symbol for the 20 megahertz long-term evolution carrier corresponding to Figure 2. Figures 6a and 6b are graphs of experimental results showing efficiency and power savings using symbol-based envelope tracking. FIGS. 7a, FIG. 7b, and FIG. 7c are schematic diagrams of Doherty amplifiers connected to symbol-based envelope-tracking voltage modulators according to embodiments. FIG. 8 is a schematic diagram of a power amplifier transistor connected to symbol-based envelope tracking voltage modulators according to an embodiment. Specific details for implementing the invention

[0039] The following detailed description of specific embodiments provides various descriptions of specific embodiments. However, the innovations described herein may be embodied in a number of different ways, for example, as defined and covered by the claims. In this description, reference is made to the drawings, and similar reference numbers herein may represent identical or functionally similar elements. It will be understood that the elements illustrated in the drawings are not necessarily depicted in equal proportions. Furthermore, it will be understood that specific embodiments may include more elements than those illustrated in the drawings and / or a subset of the illustrated elements. Moreover, some embodiments may include any suitable combination of features from two or more drawings. The headings provided herein are for convenience only and do not necessarily affect the scope or meaning of the claims.

[0040] Introduction

[0041] Desirable features of cellular wireless base stations include maximizing data throughput capabilities, minimizing energy consumption, minimizing purchase costs, minimizing the number of sites, and minimizing size and weight.

[0042] Achieving these goals simultaneously is challenging. Over the past 20 years, it has been difficult to implement energy-saving measures that generate a measurable impact on initial equipment costs (capital expenditures). Any increase in equipment costs has generally gone hand in hand with increases in throughput. While the situation has gradually changed, recent significant increases in global energy prices and the continuous, gradual shift toward a greener economy are finally tilting the situation toward supporting reductions in operating costs through reduced energy consumption, even if it results in slightly higher initial purchase costs.

[0043] Radio frequency (RF) power amplifiers (PAs) can be the dominant energy consumers in wireless base stations, and remain so despite evolutions such as massive multiple-input multiple-output (mMIMO). Therefore, PAs can be an important part of base station design when it comes to energy reduction methods.

[0044] Cellular networks are also becoming dominated by newer wireless standards of the fourth generation (4G) (Long-term Evolution) and fifth generation (5G) (New Radio), while phasing out older technologies of the third generation (3G) (Broadband Code Division Multiple Access) and second generation (2G) (Global System for Mobile Communications). It appears that 2G will be phased out first, with 3G remaining in a limited number of frequency bands on some continents, such as Europe, to support existing machine-to-machine (M2M) communication infrastructure. In any case, most of the new equipment to be installed in the future will be entirely 4G and 5G, using compatible Orthogonal Frequency Division Multiplexing (OFDM) modulation techniques.

[0045] The mode of operation of cellular wireless networks is to provide a good experience to the user. This generally means that if a user suddenly demands network resources, these resources are immediately available. In access networks, these network resources include RF power and bandwidth. This generally means that wireless access networks operate with the capacity to be dedicated most of the time.

[0046] This can be seen in data taken from a large base station network over several weeks, both during the day and at night. Figure 1 is a bar graph of the probability of cell resource usage taken for multiple base stations over several weeks. In Figure 1, the average resource usage is 18% of the maximum capacity. In Figure 1, cell resource usage can correspond to the root mean square (RMS) RF output power. Figure 1 shows 100% resource usage and maximum RMS RF power with low probability.

[0047] Even when considering low nighttime usage, base stations are utilized at a high percentage for a relatively short period. This provides opportunities for energy savings by optimizing the efficiency of the RF PA at lower RF power outputs rather than at its maximum RF power.

[0048] Both Long Evolution (LT) and New Radio (NR) use OFDM modulation for RF signal modulation. This means that both LTE and NR are symbol-based transmission methods. Over the duration of a symbol—approximately 70 µs for LTE and approximately 70 / (2^N) µs for NR (where N is the numerology 0, 1, 2, ... 4)—the RMS power of the waveform is constant because it transmits what can be the sum of thousands of sine waves at different frequencies and relative phases. In the next symbol, there may be different sums of sine waves, which may be just a few sine waves if there is not much data to transmit. Within a symbol, these sine waves are summed to produce a peak waveform with a peak-to-average ratio of about 11 decibels (dB), which can be clipped to about 8 dB by Crest Factor Reduction (CFR) with minimal loss of data integrity. This can be shown in Fig. 2 for a 10-millisecond (ms) long LTE transmission waveform where the percentage of available sine waves used varies from 100 to 1.6%.

[0049] FIG. 2 is a graph of the radio frequency envelope of a 20 megahertz (MHz) LTE carrier over time with resource usage varying from 100% to 1.6%. The y-axis of this graph is the RF envelope (or amplitude). The x-axis of this graph is time at 491.52 MHz clock samples. The graph includes instantaneous amplitude (peaks) (202), a trace at the RMS level (204), a trace at the CFR clipping level (206), and the waveform envelope (208) after clipping at an 8 dB peak-to-average power ratio (PAR).

[0050] Symbol Based Envelope Tracking (SBET) can utilize the fact that the RF peak power associated with lower-power symbols to transmit these symbols is significantly reduced and can be supplied by the same RF transistor operating at a significantly reduced drain voltage. The reduced drain voltage can lead to significantly lower energy consumption. The drain voltage can be rapidly switched from one value to another within 10 nanoseconds (ns) at the symbol boundary.

[0051] Description of specific embodiments

[0052] Figures 3a, 3b, and 3c are graphs of drain voltages for five symbols of a radio frequency signal. Figure 3a illustrates a fixed bias voltage. Figure 3b illustrates a symbol-based envelope tracking bias voltage. Figure 3c illustrates a continuous envelope tracking voltage. Figures 3a, 3b, and 3c graphically illustrate the concept of envelope tracking. The waveform is what a single RF PA transistor intends to generate at its output. The waveform is symbol-based, corresponding to time, with symbol boundaries at 0, 1, 2, 3, 4, and 5 on the x-axis. Therefore, the RMS power of the waveform varies at all symbol boundaries (symbols are typically 10 microseconds long in LTE and NR).

[0053] If the system is limited to having a fixed drain voltage as in FIG. 3a, said drain voltage supports the maximum RF peak power it can generate. A fixed drain voltage line (302) is shown in FIG. 3a. This is how most cellular wireless base station RF power amplifiers are configured. It is possible to achieve slow changes in drain voltage (over a few seconds) to accommodate times when the possibility of high traffic is minimal, such as during nighttime hours.

[0054] In a symbol-based envelope tracing (SBET), the power amplifier transistor output terminal voltages (e.g., drain voltages) can vary only along the symbol boundaries, as shown in FIG. 3b. The drain voltage can support maximum peak power within the symbol. This improves PA efficiency because the same RF power is generated in the symbol but operates at a lower drain voltage, as shown by the symbol-based envelope tracing line (304).

[0055] FIG. 3c illustrates a much higher efficiency mode of operation using full envelope tracing. The drain voltage in FIG. 3c traces all peaks of the waveform envelope. As shown in FIG. 3c, the drain voltage curve (306) traces each peak on the waveform envelope. However, this efficiency gain may be achieved at the sacrifice of changing the drain voltage 1,000 times faster than for an SBET. This full envelope tracing in FIG. 3c is much more complex than the SBET embodiments disclosed herein and involves more sophisticated digital pre-distortion (DPD) and faster analog switches (e.g., gallium nitride (GaN) switches) than an SBET. As the instantaneous bandwidth of the RF signal increases, it may be found that full envelope tracing is difficult or impossible to trace all peaks and troughs. In this case, full envelope tracing may begin skipping troughs to reduce complexity at the cost of losing some efficiency performance. This differs from the disclosed embodiments of the SBET, where the drain voltage merely shifts its value over the symbol boundaries. In the SBET, the drain voltage generally has a constant voltage level over the entire symbol, while the waveform envelope has multiple peaks and troughs over the symbol.

[0056] In the case of NR(5G) mixed numeral carriers, symbol lengths are shorter for higher numerals. In this case, the SBET can switch at the symbol boundaries of the highest numeral carrier in some instances. Alternatively, the SBET can switch at the symbol boundaries of the lowest numeral carrier in some other instances.

[0057] SBET System Architecture

[0058] Aspects of the present disclosure relate to power amplifier systems utilizing symbol-based envelope tracking. A power amplifier system may include a voltage modulator circuit for generating an output bias voltage that tracks the RMS symbol power of a radio frequency signal changing on a symbol-by-symbol basis, and a power amplifier configured to receive the output bias voltage and amplify the radio frequency signal. A power amplifier transmitter system may include a crest factor reduction circuit for adjusting a crest factor reduction threshold on symbol boundaries. A power amplifier transmitter system may include a DPD system that takes into account the output bias voltage at the DPD.

[0059] The SBET state can be determined based on the RMS power of the transmission signal per symbol. The transmission signal may be a composite transmission signal containing multiple carriers in a specific application. A voltage modulator can adjust the output bias voltage based on the SBET state. A crest factor reduction circuit can adjust the crest factor reduction threshold based on the SBET state. A DPD system can adjust the DPD based on the SBET state.

[0060] In applications with composite transmission signals containing two or more carriers with different symbol lengths, the SBET state can transition based on the shortest symbol length. In some other applications with composite transmission signals containing two or more carriers with different symbol lengths, the SBET state can transition based on the longest symbol length.

[0061] In response to detecting that there is no power in the transmission signal, the power amplifier system may operate in a discontinuous transmission state. In the discontinuous transmission state, the output terminal of the power amplifier transistor (e.g., the drain of the field-effect power amplifier transistor) may be connected to ground (short circuit) or to a floating potential (open circuit).

[0062] FIG. 4 is a schematic block diagram of an SBET system (400) according to an embodiment. The SBET system (400) is a power amplifier system comprising a power amplifier. As illustrated, the SBET system (400) includes a voltage modulator (402), a supply voltage circuit (404), and a power amplifier (406). The SBET system (400) can be selected for the drain of the power amplifier transistor and has four SBET voltages in which the waveform is made from three individual carriers. Any other suitable number of SBET voltages and / or any other suitable number of individual carriers may be used in several other applications.

[0063] The voltage modulator (402) selects one of N fixed voltages from the supply voltage circuit (404) to provide the power amplifier (406). In the illustrated power amplifier (406), the selected voltage is provided to the drain of the power amplifier transistor (407). Each of the voltages from the supply voltage circuit (404) corresponds to SBET states 1 through N. There may be any appropriate number of SBET states for a particular application. For example, in certain applications, there may be 2 SBET states, 4 SBET states as illustrated in FIG. 4, 8 SBET states, or 16 SBET states. The voltage modulator (402) can generate an output bias voltage that tracks the envelope of the RF signal on a symbol-by-symbol basis. The output bias voltage can track the envelope of the radio frequency signal for a group of symbols and / or for each individual symbol. The output bias voltage may have a transition time of 10 nanoseconds. In certain applications, the voltage modulator (402) can transition the output bias voltage between discrete voltage levels in a time range between 5 nanoseconds and 200 nanoseconds. In some applications, the voltage modulator (402) can transition the output bias voltage between discrete voltage levels in a time range between 5 nanoseconds and 50 nanoseconds. The output bias voltage can be moved significantly over the symbol boundary (e.g., from 25 volts (V) to 50 V).

[0064] The SBET system (400) also has a state called Discontinuous Transmission (DTX) mode, in which there is no RF power to transmit in the symbol. In this case, no voltage is connected to the common side of the switches (open circuit), or the drain of the power amplifier transistor (407) is connected to ground through the switch. The PA output bias voltage can be discharged so that it reaches 0 volts (e.g., ground) in the DTX state. In the DTX state, the RF PA (406) is off and must draw a negligible amount of power.

[0065] The voltage modulator (402) provides a constant voltage for the entire symbol and can switch the state on the symbol boundaries. The voltage modulator (402) can select from a plurality of discrete voltages (e.g., V1, V2, V3, V4) to provide as an output bias voltage based on the SBET state. The voltage modulator (402) can toggle the output bias voltage among these discrete voltages on the symbol boundaries. In some applications, the voltage modulator (402) provides a constant voltage for a group of symbols and can change the state on the symbol boundaries.

[0066] In certain applications, the voltage modulator (402) may switch the state only on the symbol boundaries of the carrier having the shortest symbol period (highest numerology in NR) when the complex transmission signal contains carriers having different symbol lengths. For example, in an SBET system (400), if one carrier has a symbol length that is half the symbol length of the other two carriers, the quantizer (408) may determine the SBET state at or near the symbol boundaries of the symbols having half the length of the other symbols. The RMS power of the symbols for the other two carriers may maintain the same value for the two different symbols of the shorter symbol length in this example.

[0067] In some applications, the voltage modulator (402) may switch the state only on the symbol boundaries of the carrier having the longest symbol period (lowest numerology in NR) when the composite transmission signal contains carriers having different symbol lengths. For example, in an SBET system (400), if the first carrier has a longer symbol length than the second and third carriers, the quantizer (408) may determine the SBET state at or near the symbol boundaries of the symbols of the first carrier of the composite signal. The maximum RMS power of each individual symbol of the second carrier corresponding to one symbol of the first carrier may be used to determine the SBET state. Similarly, the maximum RMS power of each individual symbol of the third carrier corresponding to one symbol of the first carrier may be used to determine the SBET state. The SBET state is determined accordingly and may be set for the peak RMS power of the composite symbol for the length of the longest symbol in this example.

[0068] Capacitors (C RF and C SNUB ) can be relatively small to reduce and / or minimize the switching / fixing time of the drain voltage of the power amplifier transistor (407) (e.g., having a capacitance of less than 1 nanofarad). Capacitors (C1 to C N )(C1 to C4 in Fig. 4) may be larger (e.g., microfarad size) decoupling capacitors.

[0069] Fixed voltages (V1 to V1) provided by the supply voltage circuit (404) N ) can be selected such that the maximum voltage (V1) supports the maximum peak RF power (typically Ppk(max) = Prms(max) + approximately 8 dB) after applying crest factor reduction (CFR). The minimum voltage (V N)(i.e., V4 in Fig. 4) may be limited by a gain reduction caused by a lower drain voltage. The lowest voltage (V N ) can rely on PA transistor technology. In certain instances, V has an approximate 6 dB gain reduction. N ½V1. With too much gain reduction, the gain of the SBET may start to decrease. Each SBET state is associated with a different RF peak power and can have a different CRF threshold level. In some applications, a subset of SBET states may have different CRF threshold levels to support different RF peak powers.

[0070] Intermediate drain voltage levels (e.g., V2 and V3 in FIG. 4) can span between maximum and minimum voltages. There is a trade-off in choosing the number of discrete voltages; more discrete voltages generally correspond to higher efficiency improvements but at the expense of increased cost and board area. The typical range of values ​​for N is 2 to 4 in specific applications. The voltage modulator (402) can implement a DTX state. In the DTX state, all switches of the voltage modulator (402) may be off, or one or more switches may be connected to ground.

[0071] The SBET system (400) may include a digital pre-distortion (DPD) block (412) to compensate for the non-linearity of the power amplifier (406). The DPD block (412) is a DPD system. The DPD block (412) may include a DPD actuator and a DPD adaptation circuit. For the power amplifier (406), the RF PA non-linearity response may be a function of the drain voltage. Therefore, the DPD performed in the DPD block (412) may also be a function of the SBET state. In the SBET system (400), the SBET state is supplied to the DPD block (412) after a matching delay. As illustrated in FIG. 4, the quantizer (408) can output an SBET state, and the delay blocks (414 and 416) can add a matching delay so that the SBET state received by the DPD block (412) corresponds to the input signal to the DPD block (412).

[0072] The DPD can be tuned based on the DPD SBET state. Tuning the DPD may involve tuning the DPD coefficients and / or tuning the DPD actuator. In certain embodiments, the DPD model coefficients may be switched on the symbol boundary. Alternatively or additionally, the DPD actuator may be tuned by enabling and / or disabling specific functions and / or by tuning the topology of the DPD actuator. This may tune the DPD model to correspond to the SBET state. The model may be tuned, for example, by tuning the connections of signal paths in the DPD actuator, including non-linear gain blocks (e.g., index tables), and / or by tuning the connections of an artificial neural network in the DPD actuator. Each SBET state or output bias voltage level may have its own DPD model / coefficients, which may be switched simultaneously in one clock cycle as the symbol data boundary passes through the DPD block (412).

[0073] To simplify the DPD implementation example, efforts to accurately correct PA distortion during the switchover transition when the drain voltage is between two set voltages may not be made. However, the switchover duration may be minimized to 10 nanoseconds on average to minimize the contribution of switchover spectral distortion. Although distortion may be relatively high at the transition time, on average it can be reduced by approximately 1:1000, which is an approximate ratio of voltage transition time to symbol period. The output of the DPD block (412) may be provided to a digital-to-analog converter (DAC) (417) to generate an analog transmission signal.

[0074] The analog signal generated by the DAC (417) can be filtered by the filter (452), amplified by the amplifier (453), and split by the hybrid splitter (454). The hybrid splitter (454) can provide the RF signal to the main power amplifier transistor (407) and the peaking power amplifier transistor (462) of the power amplifier (406). The hybrid splitter (454) can also provide a phase shift such that there is a 90-degree phase shift between the output signals provided by the hybrid splitter (454). The power amplifier (406) of the SBET system (400) is a Doherty power amplifier as illustrated.

[0075] The impact of error vector magnitude (EVM) due to drain voltage transitions may be less of a concern as the transitions occur during the cyclic transposition period for most symbols. However, in the case of mixed numerology, it can occur within the symbol.

[0076] The CFR threshold used to limit the peak power of the digital signal may be a function of the SBET state. In the SBET system (400), the SBET state is supplied to the CFR block (418) after a matching delay. As illustrated in FIG. 4, the quantizer (408) may output the SBET state, and the delay block (414) may add a matching delay so that the SBET state received by the CFR block (418) corresponds to the input signal to the CFR block (418). The CFR block (418) may adjust the CFR threshold based on the SBET state. Thus, the CFR threshold may correspond to the output bias voltage provided to the power amplifier (406). The CFR block (418) may be implemented by any suitable circuit. The digital circuit of the digital signal processor may implement the CFR block (418). For example, a transceiver integrated circuit may implement the CFR block (418).

[0077] Adjusting the CFR threshold based on the SBET state can be important for the energy saving performance of the SBET system (400). By having different CFR thresholds for each SBET state (e.g., drain voltage), the peak RF power is defined for each SBET level. The lower the CFR threshold, the greater the energy saving, but the higher the signal distortion and EVM, which will consequently increase the bit error rate (BET). The trade-off is to ensure that the PAR of any symbol in any SBET state is greater than or equal to the threshold. An exemplary threshold is 8 dB.

[0078] Figure 5 is a graph of the EVM of each symbol for a 20 MHz LTE carrier corresponding to Figure 2. The EVM for each of the 140 symbols in the waveform of Figure 2 is shown in Figure 5. The RMS EVM of the reduced power symbols is less than 1% except for one symbol when the CFR clips most of the peaks.

[0079] Referring to FIG. 4, the SBET state is carried by SBET + DTX bits output from an n-level quantizer (408). The n-level quantizer (408) is a 4-level quantizer in FIG. 4 because there are four SBET voltages provided by the voltage modulator circuit (402). The SBET + DTX bits can change the state at each symbol boundary. The bits are appropriately delayed to change their configuration for a new SBET state as these bits arrive at the CRF block (418), DPD block (412), and voltage modulator (402) and as the transmission data passes through the SBET system (400) (in the case of the voltage modulator (402), it is an analog signal that has passed through the field-effect transistors of the power amplifier (406). Delay blocks (414, 416, and 419) provide these delays to the SBET state bits.

[0080] The appropriate SBET state can be determined by effectively calculating the composite transmission RMS signal power of each complete symbol at the input to the CRF block (418). The quantizer (408) can determine the composite transmission RMS signal power. The composite transmission signal can be formed from multiple carriers. In the SBET system (400), each carrier is received as a frequency domain signal; converted to the time domain by each inverse Fast Fourier Transform (iFFT) block (442A, 442B, 442C); a cyclic transpose (CP) is added by each CP block (444A, 444B, 444C) and can be up-converted by each digital up-converter (DUC) (446A, 446B, 446C). The outputs of the DUCs (446A, 446B, 446C) can be combined to form the composite transmission signal provided to the CFR block (418).

[0081] However, calculating the transmission power directly at the input of the CRF block (418) may not be ideal, as it may take at least (and ideally the entire symbol) many microseconds to obtain an accurate result. This calculation may be too late, as the complex transmission RMS signal symbol power must be known at the beginning of the symbol rather than at the end. A delay of 10 microseconds may be added to the transmission signal path to calculate the transmission power at the input of the CFR block (418), but such a delay is generally undesirable.

[0082] Another approach is to calculate the symbol power of each carrier by multiplying it by the individual carrier-dependent gains from the iFFT input to the CFR input, and then sum the individual carrier powers. In the SBET system (400), the iFFT gain blocks (432A, 432B, and 432C) and multipliers (434A, 434B, and 434C) can generate a composite transmission signal (symbolPwr) at the input to an n-level quantizer (408) having power corresponding to the composite transmission signal at the input to the CRF block (418). The iFFT gain blocks (432A, 432B, and 432C) can provide gains corresponding to the iFFT blocks (442A, 442B, and 442C). The gain applied in each multiplier (434A, 434B, and 434C) can correspond to the gain of the corresponding DUC of the DUCs (446A, 446B, and 446C) that form the composite transmission signal input to the CFR block (418). Processing frequency domain data rather than time domain data means that the symbol boundaries of the composite transmission signal may be available before being provided as input to the CFR block (418).

[0083] The quantizer (408) can determine the SBET state based on the composite transmission signal (symbolPwr) it receives. The N-level quantizer (408) is pre-programmed with N thresholds in a thermometer manner and can output the SBET state as a number from 1 to N. The DTX bit is a special condition when the symbol power is 0 (or close to 0) and can be used to disable all switches, so that the common switch node of the voltage modulator (402) is open circuit or the drain of the power transistor (407) is shorted to ground using an additional switch.

[0084] SBET performance

[0085] The purpose of adding SBETs is to reduce the energy consumption of RF PAs in wireless base stations, which will reduce operating costs. With this reduced energy consumption, the performance of the RF PA can be minimized with respect to transmitted signal purity (e.g., EVM) and spectral emissions. It is also desirable to achieve these goals with minimal increase in equipment costs (capital expenditures).

[0086] Figures 6a and 6b are graphs of experimental results showing efficiencies and power savings using SBETs. Figure 6a illustrates enhanced drain efficiency using SBETs with off-the-shelf GaN Doherty PA modules targeting small cells and high-capacity MIMO base stations. The SBETs are compared to using a 31 V fixed voltage supply for these PAs. Multiple transmission waveforms were generated for a single 20 MHz LTE carrier. Different waveforms were generated such that the number of resource blocks (RBs) occupied for data (Physical Downlink Shared Channel (PDSCH)) varied from 1 to 100 RBs. The other physical channels (RS, Physical Broadcast Channel (PBCH), Physical Control Format Indicator Channel (PCFICH), and Physical Downlink Control Channel (PDCCH)) were the same for all waveforms. This roughly mimics different cell resource usage from 1% to 100%, as a 20MHz LTE carrier has a maximum of 100 RBs.

[0087] For the SBET in Figures 6a and 6b, four voltages are used with a maximum of 31 V and a minimum of 15.5 V. The transition time for the drain voltage from one symbol to the next was ~20 nanoseconds.

[0088] In Fig. 6b, the RMS RF output power (averaged over all symbols of a 1ms frame) changed from 0.9 watts (W) at 1 RB to 7.5 W at 100 RBs, where most of the power at 1 RB is consumed by non-data physical channels in the waveform. As the number of data RBs decreases from 100%, the RF output power decreases proportionally, and there is no power saving using the SBET until about 60% when the drain voltage first drops to a lower voltage. From this point on, the SBET provides significant savings in DC power from the drain supplies.

[0089] In Fig. 6a, the SBET maintains PA drain efficiency in the 45% to 50% range over a relatively wide range of RF output powers. Efficiency improvement is maximum at lower powers, but less power is saved there. Fig. 6b shows that ~2W of DC power is saved with approximately 1% to 60% RB utilization without power savings compared to approximately 70% RB utilization. These efficiency figures include relatively small power losses in the modulator. The modulator is ~99% efficient in ohms, with relatively low on-resistance (Rds) of the switch, where the frequency of switch changes is relatively low (low tens of kHz) and switch losses are mainly also relatively low.

[0090] The aforementioned efficiency enhancements of the SBET result in energy savings at the base station, depending on a portion of the time the base station spends at a specific percentage of resource usage. If the base station is always 100% in use, there will be no savings from the SBET. If the base station usage is always 1%, the savings can be enormous.

[0091] The statistics on base station resource usage in Figure 1 allow us to estimate power savings for an ensemble of base stations in a network averaged over day / night and week. High resource usage occurs for only a small portion of the time. If we assume a 64 transmit 64 receive (64T64R) high-capacity MIMO base station capable of generating 320 W RMS power (max), it will use approximately 64 PAs corresponding to Figure 6a. In the above network, an average 320 W capable base station will consume about 9 kWh / day of energy at the output terminals of its RF PAs, whereas if SBETs are used, it will consume only about 6.2 kWh / day of energy. These are significant energy savings.

[0092] Other features

[0093] Additional features of SBET systems will be discussed. Any suitable principles and advantages of these features may be implemented together and / or with any of the embodiments disclosed herein.

[0094] 3-state logic to save the number of logic lines to the modulator

[0095] In FIG. 4, the number of logic lines to drive the voltage modulator (402) is ceiling(log2(n)), where n is the number of voltages (or SBET states). Adding a DTX state may add additional logic lines. Therefore, implementing the DTX state by making the SBET logic lines three-states can save the need for additional logic lines. The three-state driver may have a third state that is floating or open circuit with respect to the DTX state. The three-state driver can be implemented in an SBET system having a DTX state.

[0096] Wave height reduction

[0097] A typical CFR block may have three successive engines that first detect peaks and then correct the peaks. During detection, the CFR block (418) may record the SBET / threshold level with the peak. The threshold can then be passed to correction hardware. The correction hardware can reduce the peaks to a desired level. There may be a central block that delays updates for each engine so that the threshold changes at the same relative sample for each engine. This can also be extended to non-SBET uses, where we can now align the threshold and pulse updates to symbol boundaries. The updates may have these features and be continuous, where the previous engine is disabled at once to update the thresholds. These features can be implemented in the CFR block (418) of FIG. 4.

[0098] digital dictionary distortion

[0099] DPD can measure and subsequently correct the non-linear response of an RF power amplifier. As noted above, the non-linear response of an RF PA can be a function of drain voltage. Therefore, the non-linear compensation block in the DPD can be a function of the SBET state, which means that non-linear coefficients / models can vary depending on the SBET state and drain voltage at symbol boundaries. An additional issue with using specific transistors (e.g., GaN RF transistors) in an RF PA is that their charge trapping behavior can alter the PA's non-linear characteristics over long-term time scales of microseconds to milliseconds. Neural Predistortion (NPD) can be implemented for long-term tracking of RF PA non-linearity. Using an SBET can amplify long-term dynamic non-linear behavior compared to a fixed drain voltage, which can be particularly evident in GaN RF power transistors.

[0100] Within the NPD input data path, the PA drain voltage (SBET level) can be tracked. Each of the N SBET levels (e.g., the four SBET levels of the SBET system (400) in FIG. 4) can be scaled up to a binary level, and thus there are N binary levels passed to the neural network (NN). Certain NNs use 8-bit MACs. For these NNs, there may be 8-bit programmable values ​​to represent 0 or 1. This allows the NN to learn optimal 8-bit values ​​for all levels. These features can be implemented in the DPD block (412) of FIG. 4.

[0101] Radio frequency power amplifier connections

[0102] Doherty RF amplifiers can be used in wireless infrastructure base stations at RF frequencies below 7 gigahertz (GHz). There are multiple connection options for SBET voltage modulators with two (or more) transistors of the RF Doherty amplifier. FIGS. 7a, 7b, and 7c are schematic diagrams of Doherty power amplifiers (406) connected to SBET voltage modulators (402) according to embodiments. These Doherty power amplifiers (406) include a main power amplifier transistor (407) and a peaking power amplifier transistor (462).

[0103] In FIG. 7a, one voltage modulator (402) is connected to and used with the main power amplifier transistor (407), while the peaking power amplifier transistor (462) has a fixed supply voltage. In FIG. 7b, the drains of the main power amplifier transistor (407) and the peaking power amplifier transistor (462) are connected to the same voltage modulator (402) at a common node or common switch point. It may be desirable to reduce and / or minimize the physical distance between the output node of the voltage modulator (402) and the power amplifier transistor drains in the physical layout. In FIG. 7c, there are two voltage modulators (402A and 402B), one voltage modulator (402A) is connected to the drain of the main power amplifier transistor (407), and the other voltage modulator (402B) is connected to the drain of the peaking power amplifier transistor (462).

[0104] There are price / performance trade-offs between the three configurations shown in FIGS. 7a, 7b, and 7c. Each of the three configurations is advantageous for specific applications.

[0105] Using a Doherty amplifier with an SBET offers benefits in efficiency at lower RMS output power as well as at maximum RMS power output.

[0106] SBETs can also enhance the efficiency of non-Doherty RF amplifier stages, whether used standalone or in other configurations such as a Load Modulated Balanced Amplifier (LMBA). FIG. 8 illustrates an example of a voltage modulator (402) having an SBET connected to a single power amplifier transistor (802).

[0107] Power amplifier transistors

[0108] Although the embodiments disclosed herein may be discussed with reference to power amplifier field-effect transistors and modulating the drain voltage for symbol-based envelope tracking, any suitable principles and advantages disclosed herein may be implemented with power amplifiers having different types of power amplifier transistor output stages. In some applications, power amplifiers in a symbol-based envelope tracking system according to any suitable principles and advantages disclosed herein may, without limitation, include field-effect transistors, complementary metal-oxide-semiconductor (CMOS) transistors, bipolar transistors, heterojunction bipolar transistors, etc. If the power amplifier includes a bipolar power amplifier transistor, symbol-based envelope tracking may modulate the bias voltage at the current collector of the bipolar power amplifier transistor and / or any other suitable teachings applied to the drain may be applied to the current collector. The power amplifier transistor may be implemented by any suitable transistor technology for a particular application. Exemplary power amplifier transistors may include, but are not limited to, GaN transistors, silicon transistors, silicon-on-insulator transistors, SiGe transistors, GaAs transistors, SiC transistors, InP transistors, etc. Furthermore, any of the power amplifier transistors disclosed herein may be implemented by two or more power amplifier transistors.

[0109] Wireless communication standards

[0110] Although the embodiments disclosed herein may be discussed with reference to specific communication standards, any suitable principles and benefits disclosed herein may be implemented with radio frequency systems that transmit radio frequency signals through one or more antennas according to any suitable wireless communication standard. In some applications, a radio frequency system with symbol-based envelope tracking according to any suitable principles and benefits disclosed herein may generate signals according to any suitable wireless communication standard, such as Wi-Fi standards, other local wireless area network standards, personal wireless area network standards, future cellular technology standards, etc.

[0111] conclusion

[0112] In the embodiments described above, devices, systems, and methods for power amplifier envelope tracers are described in relation to specific embodiments. However, it will be understood that the principles and advantages of the embodiments may be applied to any other systems, devices, or methods depending on the requirements for envelope tracing.

[0113] The principles and benefits described herein may be implemented in various devices. Examples of such devices may include, but are not limited to, communication infrastructure such as wireless or wired communication infrastructure, consumer electronics, components of consumer electronics, electronic test equipment, automotive electronics, industrial electronics, etc. Electronic products may include, but are not limited to, base stations such as cellular base stations, access points, repeaters, relays, wireless communication devices, mobile phones (e.g., smartphones), handheld computers, tablet computers, laptop computers, wearable computing devices, automotive electronic systems, radios, wearable health monitoring devices, Internet of Things (IoT) devices, etc. In addition, devices may include unfinished products.

[0114] Unless the context otherwise clearly requires, throughout the description and claims, words ("comprising," "comprising," "including," "including," etc.) shall be interpreted in an inclusive sense, that is, "comprising, but not limited to." As generally used herein, words ("combined" or "connected") indicate two or more elements that may be directly connected or connected by one or more intermediate elements. Additionally, words ("in this application," "above," "below") and words of similar meaning, when used in this application, represent this application as a whole rather than any specific part of this application. Where the context permits, words in the detailed description using singular or plural numbers may also each include plural or singular numbers. With respect to a list of two or more items, the words "or" are intended to cover all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. All numerical values ​​provided herein are intended to include similar values ​​within the measurement error.

[0115] Furthermore, among other things, conditional language used herein, such as “can, could,” “might, may,” “e.g.,” “for example,” “e.g.,” “like,” etc., is generally intended to convey that certain embodiments include certain features, elements, and / or states, while other embodiments do not include them, unless otherwise specifically stated or otherwise understood in context as used.

[0116] The teachings of the inventions provided herein may be applied to systems other than those described above. The elements and operations of the various embodiments described above may be combined to provide additional embodiments. The operations of the methods discussed herein may be performed in any order where appropriate. Furthermore, the operations of the methods discussed herein may be performed sequentially or simultaneously where appropriate.

[0117] Although specific embodiments of the inventions have been described, these embodiments are provided merely as examples and are not intended to limit the scope of the disclosure. In fact, the novel methods and systems described herein may be embodied in various other forms. Furthermore, various omissions, substitutions, and variations in the forms of the methods and systems described herein may be made without departing from the spirit of the disclosure. For example, while the disclosed embodiments are provided in defined arrangements, alternative embodiments may have different components and / or circuit topologies and perform similar functions, and some elements may be deleted, moved, added, subdivided, combined, and / or modified. Each of these elements may be implemented in various different ways if appropriate. Any suitable combination of elements and operations of the various embodiments described above may be combined to provide additional embodiments. The accompanying claims and equivalents are intended to cover these forms or modifications as falling within the scope and spirit of the disclosure. Accordingly, the scope of the inventions is defined by reference to the claims.

Claims

Claim 1 A power amplifier system utilizing symbol-based envelope tracking, comprising: a voltage modulator circuit configured to track the envelope of a radio frequency signal by generating an output bias voltage that tracks the root mean square symbol power of the radio frequency signal—the voltage modulator circuit being configured to adjust the output bias voltage based on a symbol-based envelope tracking state on symbol boundaries, wherein the output bias voltage is selected from a plurality of discrete voltages—; a power amplifier configured to receive the output bias voltage and amplify the radio frequency signal; and a signal path configured to provide the radio frequency signal to the power amplifier—the signal path comprising a crest factor reduction block configured to adjust the crest factor reduction threshold such that the crest factor reduction threshold corresponds to the output bias voltage. Claim 2 A power amplifier system according to claim 1, further comprising a quantizer configured to determine a symbol-based envelope tracking state based on the root mean square power of the transmitted signal power per symbol, wherein the crest factor reduction block is configured to adjust the crest factor reduction threshold based on the symbol-based envelope tracking state. Claim 3 A power amplifier system according to paragraph 2, wherein the signal path comprises a digital pre-distortion system coupled between the crest factor reduction block and the digital-to-analog converter, and the digital pre-distortion system is configured to adjust the digital pre-distortion based on the symbol-based envelope tracking state. Claim 4 In paragraph 2, the power amplifier system is configured such that the quantizer receives complex transmission symbol power associated with a plurality of carriers. Claim 5 A power amplifier system according to claim 1, wherein the signal path comprises a digital-to-analog converter coupled between the output of the crest factor reduction block and the input to the power amplifier. Claim 6 In claim 1, the power amplifier system is a power amplifier system capable of operating in a discontinuous transmission state. Claim 7 In claim 6, the power amplifier transistor of the power amplifier system has an output terminal connected to ground in the discontinuous transmission state, the power amplifier system. Claim 8 In claim 6, the power amplifier transistor of the power amplifier system has an output terminal that floats in the discontinuous transmission state, the power amplifier system. Claim 9 A power amplifier system according to claim 1, wherein the radio frequency signal comprises at least two carriers having different symbol lengths, and the voltage modulator circuit is configured to adjust an output bias voltage corresponding to the symbol boundaries of one of the at least two carriers having the shortest symbol period. Claim 10 A power amplifier system according to claim 1, wherein the power amplifier comprises a main power amplifier transistor coupled to the voltage modulator circuit and a peaking power amplifier transistor coupled to the fixed voltage supply. Claim 11 A power amplifier system according to claim 1, wherein the power amplifier comprises a main power amplifier transistor coupled to the voltage modulator circuit and a peaking power amplifier transistor coupled to the voltage modulator circuit. Claim 12 A power amplifier system according to claim 1, wherein the power amplifier comprises a main power amplifier transistor coupled to the voltage modulator circuit and a peaking power amplifier transistor coupled to the second voltage modulator circuit. Claim 13 A power amplifier system according to claim 1, wherein the voltage modulator circuit is configured to transition the output bias voltage between discrete voltage levels in a time range between 5 nanoseconds and 200 nanoseconds. Claim 14 A power amplifier system according to claim 1, wherein the voltage modulator circuit is configured to transition the output bias voltage between at least four discrete voltage levels on the symbol boundaries. Claim 15 A base station comprising the power amplifier system of claim 1, as a base station. Claim 16 A method for amplifying a radio frequency signal using symbol-based envelope tracking, comprising: determining a symbol-based envelope tracking state based on the root mean square power of the transmitted signal power per symbol; toggling an output bias voltage between a plurality of discrete voltage levels on the symbol boundaries of the radio frequency signal based on the symbol-based envelope tracking state, wherein the output bias voltage is provided to a power amplifier; adjusting a crest factor reduction threshold of a crest factor reduction block based on the symbol-based envelope tracking state, wherein the crest factor reduction block is in a signal path providing the radio frequency signal to the power amplifier; and amplifying the radio frequency signal using the power amplifier. Claim 17 A method according to claim 16, further comprising the step of adjusting digital pre-distortion in the signal path based on the symbol-based envelope tracking state. Claim 18 A method according to claim 16, further comprising the step of operating a power amplifier system including the power amplifier in a discontinuous transmission state. Claim 19 In paragraph 16, the radio frequency signal comprises two carriers having different symbol periods, and the toggling occurs on the symbol boundaries of one of the two carriers having a shorter symbol period. Claim 20 delete

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