Radio transmitter and method and controller therefor

Abstract

A radio transmitter, a method and a controller therefor are disclosed. According to one embodiment, the radio transmitter includes a power amplifier, a filter unit, a circulator, a switch, and a reflection cancellation unit. The filter unit is capable of filtering the output from the power amplifier during a downlink time division duplex (TDD) time slot and filtering an uplink signal during an uplink TDD time slot. The circulator has a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot and coupling the third port of the circulator to an uplink receiving path during an uplink TDD time slot. The reflection cancellation unit is coupled between the switch and the termination load and is capable of generating a first reflection signal that propagates to the first port of the circulator via the switch and the third port of the circulator during a downlink TDD time slot.

Description

Technical Field

[0001] Embodiments of the present disclosure generally relate to communications, and more particularly to radio transmitters and methods and controllers therefor. Background Art

[0002] This section presents aspects that may facilitate a better understanding of the present disclosure. Accordingly, statements in this section should be read in this sense and should not be construed as an admission as to what is prior art and what is not prior art.

[0003] Efficiency is one of the most important parameters of a power amplifier (PA) in a base station, as it determines the power consumption and cooling requirements of the base station. To achieve better efficiency, the PA typically operates near saturation, which means poor linearity. To meet the spurious emission limit, linearization techniques are often necessary. Currently, digital predistortion (DPD) is one of the most widely used linearization techniques. It captures data from the PA output and constructs an inverse model of the PA to pre-distort the baseband signal before sending it to the PA. Thus, the entire transmitter (TX) link is linearized. With the help of DPD, the PA can operate with higher efficiency while still having good linearity.

[0004] To construct the inverse model of the PA, feedback from the PA output is observed. This signal is a copy of the PA output signal with a much smaller amplitude. Thus, it has a negligible impact on the performance of the PA while still containing all the non-linear information of the PA. By minimizing the difference between the feedback signal and the original baseband signal, the inverse model of the PA can be constructed. Therefore, the accuracy of the feedback signal is important for DPD. However, any interference on the feedback signal will affect the non-linear behavior of the PA as "seen" by DPD, thus making the construction of the inverse model of the PA inaccurate and deteriorating the linearization performance. Summary of the Invention

[0005] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0006] One objective of the present disclosure is to provide an improved radio transmitter. In particular, one of the problems to be solved by the present disclosure is that the traditional PA backend in a time division duplex (TDD) system may have poor isolation performance.

[0007] According to a first aspect of the present disclosure, a radio transmitter is provided. The radio transmitter may include a power amplifier, a filter unit, a circulator, a switch, and a reflection cancellation unit. The filter unit is capable of filtering the output from the power amplifier during a downlink TDD time slot and filtering an uplink signal during an uplink TDD time slot. The circulator may have a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during the downlink TDD time slot and coupling the third port of the circulator to an uplink receiving path during the uplink TDD time slot. The reflection cancellation unit may be coupled between the switch and the termination load and is capable of generating a first reflection signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot.

[0008] In this way, the isolation of the backend of the PA can be improved.

[0009] In an embodiment of the present disclosure, at the first port of the circulator, the first reflection signal may be added to the sum of the reflection signals so as to cancel the sum of the reflection signals at the first port of the circulator.

[0010] In an embodiment of the present disclosure, the reflection signals canceled by the first reflection signal may include: a second reflection signal leaking from the second port of the circulator to the first port; a third reflection signal generated by the switch and propagating to the first port via the third port of the circulator; and a fourth reflection signal generated by the first port of the circulator and propagating backward to the power amplifier.

[0011] In an embodiment of the present disclosure, the reflection cancellation unit may be a passive impedance network having predetermined component values.

[0012] In an embodiment of the present disclosure, the reflection cancellation unit may include an impedance tuner.

[0013] In an embodiment of the present disclosure, the impedance tuner may be one of the following: a single stub impedance tuner; a multi-stub impedance tuner; and a coupler-based impedance tuner formed by a hybrid coupler and a tunable element.

[0014] In an embodiment of the present disclosure, one or more tunable elements in the single stub impedance tuner, or the multi-stub impedance tuner, or the coupler-based impedance tuner may include at least one of the following: a variable capacitor; a variable resistor; a varactor; and a microelectromechanical system (MEMS) switch.

[0015] In an embodiment of the present disclosure, the reflection cancellation unit may further include a controller configured to control the impedance tuner to minimize the total reflection at the first port of the circulator.

[0016] In an embodiment of the present disclosure, the reflection cancellation unit may further include a temperature sensor configured to sense the ambient temperature of the radio transmitter. The controller may be configured to control the impedance tuner based on the sensed ambient temperature.

[0017] In an embodiment of the present disclosure, the impedance tuner may be controlled by using a pre-configured look-up table that indicates the correspondence between the control values for the impedance tuner and the temperature.

[0018] In an embodiment of the present disclosure, the reflection cancellation unit may further include a power meter coupled between the first port of the circulator and the power amplifier and configured to measure the power of the total reflection at the first port of the circulator. The controller may be configured to control the impedance tuner to minimize the measured power of the total reflection.

[0019] In an embodiment of the present disclosure, the radio transmitter may further include: a directional coupler coupled between the power amplifier and the first port of the circulator; and a feedback receiver coupled to the directional coupler and configured to feedback a portion of the output from the power amplifier for digital predistortion.

[0020] In an embodiment of the present disclosure, the reflection cancellation unit may further include a controller configured to control the impedance tuner to optimize the linearization performance of the power amplifier monitored by the feedback receiver.

[0021] In an embodiment of the present disclosure, the linearization performance of the power amplifier may be represented by at least one of the following: adjacent channel leakage ratio (ACLR); and error vector magnitude (EVM).

[0022] In an embodiment of the present disclosure, the impedance tuner may be controlled by using one of the following: a gradient descent process; a Levenberg Marquardt process; a Gauss-Newton process; and a simultaneous perturbation stochastic approximation (SPSA) process.

[0023] In an embodiment of the present disclosure, the radio transmitter may further include an antenna unit coupled to the filter unit.

[0024] According to a second aspect of the present disclosure, there is provided a radio device including the radio transmitter according to the first aspect above.

[0025] In an embodiment of the present disclosure, the radio device may be one of the following: a remote radio unit (RRU); a distributed unit (DU); an active antenna system (AAS); and a base station.

[0026] According to a third aspect of the present disclosure, there is provided a method executed by a controller in a radio transmitter. The radio transmitter may include a power amplifier, a filter unit, a circulator, a switch, and an impedance tuner. The circulator may have a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot and coupling the third port of the circulator to an uplink receive path during an uplink TDD time slot. The impedance tuner may be coupled between the switch and the termination load and is capable of generating a first reflected signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot. The method may include obtaining a metric capable of reflecting the intensity of the total reflection at the first port of the circulator while controlling the impedance tuner using at least one first control value. The method may further include determining at least one second control value for the impedance tuner based on the obtained metric such that the intensity of the total reflection at the first port of the circulator is minimized. The method may further include controlling the impedance tuner using the at least one second control value.

[0027] In an embodiment of the present disclosure, the metric may be one of the following: the power of the total reflection at the first port of the circulator; ACLR; and EVM.

[0028] In an embodiment of the present disclosure, the at least one second control value may be determined by using one of the following: a gradient descent process; a Levenberg Marquardt process; a Gauss-Newton process; and a simultaneous perturbation stochastic approximation (SPSA) process.

[0029] According to a fourth aspect of the present disclosure, a controller in a radio transmitter is provided. The radio transmitter may include a power amplifier, a filter unit, a circulator, a switch, and an impedance tuner. The circulator may have a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot and coupling the third port of the circulator to an uplink receiving path during an uplink TDD time slot. The impedance tuner may be coupled between the switch and the termination load and is capable of generating a first reflected signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot. The controller may include at least one processor and at least one memory. The at least one memory may contain instructions executable by the at least one processor, whereby the controller may be operable to obtain a measure that can reflect the intensity of the total reflection at the first port of the circulator when controlling the impedance tuner using at least one first control value. The controller may also be operable to determine at least one second control value for the impedance tuner based on the obtained measure such that the intensity of the total reflection at the first port of the circulator is minimized. The controller may also be operable to control the impedance tuner using the at least one second control value.

[0030] In an embodiment of the present disclosure, the controller may be operable to execute the method according to the above-mentioned third aspect.

[0031] According to a fifth aspect of the present disclosure, a computer program product is provided. The computer program product may contain instructions. When executed by at least one processor, the instructions cause the at least one processor to execute the method according to the above-mentioned third aspect.

[0032] According to a sixth aspect of the present disclosure, a computer-readable storage medium is provided. The computer-readable storage medium may contain instructions. When executed by at least one processor, the instructions cause the at least one processor to execute the method according to the above-mentioned third aspect.

[0033] According to a seventh aspect of the present disclosure, a controller in a radio transmitter is provided. The radio transmitter may include a power amplifier, a filter unit, a circulator, a switch, and an impedance tuner. The circulator may have a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot and coupling the third port of the circulator to an uplink receiving path during an uplink TDD time slot. The impedance tuner may be coupled between the switch and the termination load and is capable of generating a first reflected signal that propagates from the switch and the third port of the circulator to the first port of the circulator during the downlink TDD time slot. The controller may include an obtaining module for obtaining a metric that can reflect the intensity of the total reflection at the first port of the circulator when controlling the impedance tuner using at least one first control value. The controller may further include a determining module for determining at least one second control value for the impedance tuner based on the obtained metric such that the intensity of the total reflection at the first port of the circulator is minimized. The controller may further include a control module for controlling the impedance tuner using the at least one second control value. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] These and other objects, features and advantages of the present disclosure will become apparent from the following detailed description of illustrative embodiments of the present disclosure, which is to be read in conjunction with the accompanying drawings.

[0035] Figure 1 is a diagram showing a conventional radio transmitter;

[0036] Figure 2 is a diagram showing a circulator;

[0037] Figure 3 is a diagram showing a radio transmitter according to an embodiment of the present disclosure;

[0038] Figure 4 is a diagram showing an impedance tuner that can be used in an embodiment;

[0039] Figure 5 is a diagram showing Figure 4 the simulation effect of the impedance tuner on a Smith chart;

[0040] Figures 6A - 6B is a diagram showing Figure 5 the values of two capacitors in a sweep;

[0041] Figure 7It is a diagram showing a conventional PA backend for simulation;

[0042] Figure 8 It is a diagram showing the simulation isolation performance in a conventional PA backend varying with phase;

[0043] Figure 9 It is a diagram showing a PA backend of an embodiment for simulation;

[0044] Figure 10 It is a diagram showing the simulation isolation performance of an embodiment;

[0045] Figure 11 It is a diagram showing the simulation isolation performance of an embodiment;

[0046] Figure 12 It is a diagram showing the simulation isolation performance of an embodiment;

[0047] Figures 13A - 13E It is a diagram showing an impedance tuner available in an embodiment;

[0048] Figure 14 It is a diagram showing a radio transmitter according to another embodiment of the present disclosure;

[0049] Figure 15 It is a diagram showing a radio transmitter according to another embodiment of the present disclosure;

[0050] Figure 16 It is a diagram showing a radio transmitter according to another embodiment of the present disclosure;

[0051] Figure 17 It is a flowchart showing a method executed by a controller according to an embodiment of the present disclosure;

[0052] Figure 18 It is a block diagram showing a device suitable for use in practicing some embodiments of the present disclosure; and

[0053] Figure 19 It is a block diagram showing a controller according to an embodiment of the present disclosure. Detailed Description

[0054] For purposes of explanation, some details are set forth in the following description to provide a thorough understanding of the disclosed embodiments. However, it will be apparent to those skilled in the art that the embodiments may be practiced without these specific details or with equivalent configurations.

[0055] As described above, any interference on the feedback signal will affect the non-linear behavior of the PA that the DPD "sees", thus making the construction of the inverse model of the PA inaccurate and deteriorating the linearization performance. The present inventors have found that one of the main interferences is the reflected power at the PA backend, especially in TDD systems. Figure 1 FIG. shows a conventional radio transmitter for explaining the PA backend structure of a TDD system. As shown, the radio transmitter 100 includes a PA 101 (e.g., Doherty PA), a directional coupler 109 (to capture data for DPD), a circulator 103, a switch 104 (e.g., high power (HP) switch), a filter unit (FU) 102, and an antenna unit 108. The main difference between TDD and frequency division duplex (FDD) systems in the PA backend is that the circulator 103 is replaced by an isolator in the FDD system. In the TDD system, during the uplink time slot, the HP switch 104 switches the antenna output to the uplink receiving path 106 that includes a low noise amplifier (LNA), and the received signal is transmitted through the circulator 103 to the LNA. During the downlink time slot, it switches to a termination load 105 (e.g., a high power load) to absorb the reflection from the FU102.

[0056] Since the FU has total reflection outside the operating frequency band, the reflection always exists, especially for signals having a spectrum outside the operating frequency band, which is called the spectrum spread caused by PA non-linearity. If this reflected signal is not isolated from the PA, it will have a significant impact on the system linearization performance in two aspects. First, when it reaches the PA output, it will modulate the PA output signal, thus generating new intermodulation that is difficult to be corrected by the DPD. The stronger the non-linearity of the PA, the stronger the intermodulation it will generate. Second, it will be reflected again from the PA output because the PA output often has a poor return loss. Then it will enter the feedback path and become interference to the DPD. This may seriously affect the DPD modeling. Therefore, isolating this reflection from the PA output is important for the system linearity.

[0057] In an FDD system, the isolation performance of an isolator is often very good. For example, it is better than 20 dB. However, in a TDD system, due to two reasons, isolation can be a problem. First, for downlink / uplink (DL / UL) multiplexing, the third port of the circulator needs to be connected to an HP switch. So in the downlink, the switch switches to the load, and most of the reflections are absorbed at the load. In an ideal case, the HP switch presents a perfect match to the circulator, i.e., there is no reflection at its input. However, in reality, the reflection at this HP switch is never zero. Sometimes, its return loss is far from "good". If so, the reflected signal at the input of the HP switch will be directly fed to the PA output through the circulator. This lack of isolation will cause interference. Second, the reverse signals can add up at the input (port 1) of the circulator and make the isolation worse, as described below.

[0058] Figure 2 An illustration shows a circulator. As shown, the circulator can be characterized by a three-port scattering matrix, and the input signals (a1, a2, a3) / output signals (b1, b2, b3) are related by the following equation:

[0059]

[0060] Here, it is assumed that port 1 is connected to the PA output, port 2 is connected to the FU, and port 3 is connected to the HP switch that switches between the LNA and the load, which is the same as the connection shown in Figure 1 Assume there is a reflected signal (or reverse wave) a2 propagating towards port 2 (one possible source of a2 is the reflection from the FU). Then, the reflection that the PA "sees" is b1, which can be expressed as:

[0061] b1 = S 11 *a1 + S 12 *a2 + S 13 *a3 = S 12 *a2 + S 13 *a3, (Equation 2)

[0062] where a3 is the reflected signal from the HP switch. If the return loss at the switch input is defined as Γ, then a3 is expressed as:

[0063]

[0064] By substituting Equation 3 into Equation 2, the relationship between b1 and a2 can be obtained as follows, where b1 / a2 is the so-called isolation degree.

[0065]

[0066] For an actual device, |S 21| = | S 13 | = | S 32 is the insertion loss of the circulator and is close to 1 (0 dB), while S nn (n = 1, 2, 3) is the return loss of each port and is often very good (~ -20 dB). Therefore, it is assumed that ( is the phase shift of the signal propagating through the circulator) and S 33 = 0 is reasonable. Then, an approximate expression can be obtained as follows:

[0067]

[0068] This equation shows that the signal reflected back to the PA consists of two parts. The first part S 12 *a2 is related to the isolation of the circulator itself. For practical devices, the amplitude of the first part is about -20 dB. The second part comes from the reflection at the HP switch, as described above. Since the phase relationship between these two parts is not completely under control, inevitably, in some cases, their phases can be close to each other. After they are added in phase, a higher reflection will occur. For example, assume that the return loss at the input of the HP switch is -18 dB, and the isolation of the circulator is -22 dB. Then, if they are added in phase, the PA will "see" a total reflection of -13.7 dB, which will significantly affect the system linearity. This shows that focusing only on the matching of the HP switch itself is not sufficient to avoid poor isolation.

[0069] The present disclosure proposes an improved solution for a radio transmitter, a controller for the radio transmitter, a method executed by the controller, and a radio device including the radio transmitter. Hereinafter, the solution will be described with reference to Figures 3 to 19 in detail.

[0070] Figure 3 is a diagram showing a radio transmitter according to an embodiment of the present disclosure. As shown, compared with the conventional radio transmitter 100 shown in Figure 1 , an impedance tuner 307 is additionally provided in the radio transmitter 300. The impedance tuner 307 is coupled between the switch 104 and the termination load 105, and is capable of generating a first reflected signal that propagates to the first port of the circulator 103 via the switch 104 and the third port of the circulator 103 during the downlink TDD time slot. Due to the introduction of the impedance tuner 307, the isolation at the back end of the PA can be improved.

[0071] For example, at the first port of the circulator 103, the first reflected signal is added to the sum of the reflected signals in order to cancel the sum of the reflected signals. This means that at the first port of the circulator 103, the first reflected signal cancels the sum of the reflected signals that would result in the radio transmitter 300 without the impedance tuner 307 being provided. In this way, the isolation risk in the TDD system can be greatly reduced. It should be noted that the expression "is canceled" used here can cover "is completely canceled" and "is partially canceled", and thus can be used interchangeably with "is eliminated" in the present disclosure. A detailed analysis will be provided as follows.

[0072] As Figure 3 shown, by adding the impedance tuner 307 between the switch 104 and the terminating load 105, an intentional reflection is generated at the impedance tuner 307, and its phase and amplitude can be controlled. Therefore, the PA 101 sees another reflection source. Now, Equation 5 should be modified by adding a new term:

[0073]

[0074] where Γ L is defined as the voltage reflection coefficient at the impedance tuner, is the phase shift of the signal propagating through the circulator and the switch, and is the combination of the first two reverse signals, which is a fixed value for each product. Here, for simplicity and without loss of generality, it is assumed that the loss of the switch is negligible. By tuning the amplitude and phase of Γ L , it is possible to make have a similar amplitude and a phase difference of nearly 180° from Γ′. Then, these two can cancel each other out, and the total reflection can be reduced to a very low value. It should be noted that in order to simplify the analysis, the reflected signal reflected from port 1 (reflection 4 in Figure 1 ) is not considered in the above analysis. However, it does not affect the validity of the analysis because both reflection 4 and a2 are related to the forward transmission signal a1, and thus reflection 4 can be combined into Γ′ in Equation 6.

[0075] As an exemplary example, Figure 4The impedance tuner shown. As shown in the figure, the impedance tuner includes two voltage-controlled capacitors and a section of transmission line. Here, Z0 is the characteristic impedance of the system (e.g., 50 ohms), d is the length of the transmission line between the two tunable capacitors, and it can be any value. The detailed working principle of this impedance tuner can be found in Section 5.3 of "Microwave Engineering, Fourth Edition" (David M. Pozar). The benefit of this impedance tuner is that by electrically tuning the values of the two capacitors, any impedance within a specific voltage standing wave ratio (VSWR) circle can be obtained, as analyzed below.

[0076] To obtain a conductance with arbitrary amplitude and phase at the input of this structure can be defined as follows:

[0077]

[0078] Y L and Γ L The relationship between them is:

[0079]

[0080] where By substituting Equation 8 into Equation 7, the following result can be obtained:

[0081]

[0082] Finally, by using Equations 5.22 and 5.23 in "Microwave Engineering, Fourth Edition" (David M. Pozar), the expected values of the capacitors can be obtained for the target Γ L or Z L :

[0083]

[0084] where t = tanβd and θ = βd are defined as the electrical length of the transmission line. It should be noted that in Equations 5.22 and 5.23, the solution with the "+" sign is selected because the capacitor has a positive susceptance.

[0085] The limitation of this structure is that is not achievable, or equivalently, by tuning the values of C1 and C2, any impedance within the circle in the Smith chart is achievable. Therefore, in order for this impedance tuner to have a large tuning range, the length of the transmission line should not be too long. On the other hand, it is clear from Equation 10 that the expected value of the capacitor is inversely proportional to d. Therefore, for practical applications, a suitable value of d should be selected. For example, if d = λ / 4 is selected, any desired Γ L .

[0086] Figure 5 is a Smith chart showing the simulation results of the impedance tuner shown in Figure 4 . In the simulation, the capacitance values of C1 and C2 were scanned to traverse the impedance within the circle of VSWR = 2. The frequency was 1 GHz and d = λ / 4. The VSWR was swept from 1 to 2 in 10 steps, and the phase step = 10°. Figure 6A and 6B shows the corresponding values of the two capacitors used in this scan for Γ L = 2, 1.6, and 1.4. It can be seen that by tuning the values of the capacitors in this structure, any desired Γ within the target VSWR circle can be obtained. L .

[0087] To verify the effectiveness of this embodiment, the isolation performance will be compared between the conventional PA backend shown in Figure 1 and the new PA backend shown in Figure 3 using simulation. The models of the devices (circulator and switch) used in the simulation are listed below:

[0088] Switch: SKY12207-306LF, SKYWORKS, 0.9 - 4 GHz; and

[0089] Circulator: SKYFR-001163, SKYWORKS, 1.8 - 2.7 GHz.

[0090] In addition, 2635 - 2675 MHz (B41F) was selected in the simulation for illustration. Note that although the parameters from the manufacturer were used for device modeling, this embodiment is generally applicable and is not limited to the detailed models of the devices.

[0091] Figure 7 is a circuit diagram of the conventional PA backend used in the simulation. For simplicity, only the parts affecting the isolation are shown, namely the circulator, HP switch, and load. The port definitions of the circulator and HP switch are the same as in Figure 1 and Figure 3The port definitions are exactly the same. That is, ports 1, 2, and 3 of the circulator are connected to the PA, FU, and HP switches respectively during the downlink TDD time slot. During the uplink TDD time slot, ports 1, 2, and 3 of the HP switch are connected to the circulator, LNA (receiver (RX)), and load respectively. It is assumed that port 2 of the HP switch is terminated because it has little effect on the isolation in the DL time slot. A phase shifter is added between the circulator and the HP switch. In reality, its value depends on the device itself, the length of the transmission line between the two, and the parasitic parameters caused by, for example, a printed circuit board (PCB), and is completely uncontrollable. In the frequency range of 2635 - 2675 MHz, the isolation of the circulator (S12) is about -23 dB, and the return loss (S11) at the input port of the HP switch is about -20.5 dB.

[0092] Figure 8 shows in Figure 7 the isolation (S12) of the structure of. In the simulation, the phase value of the phase shifter is swept from 0° to 90° with a step of 10°. It can be seen that this phase has a great influence on the system isolation performance. At about 10°, the two terms in Equation 5 are close to in-phase, so the addition of these two will result in a poor isolation value, up to about -16 dB. At about 90°, these two terms are close to out-of-phase, so they cancel each other out, and the isolation is about -32 dB, which is more than 16 dB better than the worst case. Therefore, as described above, the uncontrolled phase will have a significant impact on the system isolation. In addition, if imperfect matching and device variations are considered, the results may be worse than those shown in this simple simulation.

[0093] Figure 9 is a circuit diagram of the PA backend showing the embodiment used in the simulation. As shown, an impedance tuner is added between the HP switch and the load. In the simulation, the phase of the phase shifter is fixed at 10° to produce the worst isolation, which is about -16 dB, as Figure 8 shown. This makes it possible to more clearly observe the improvement of the isolation of this embodiment. The simulation results of the isolation performance of this embodiment are shown in Figure 10 As shown, Γ L (which is the voltage reflection at the input of the impedance tuner) is changed in the simulation to observe the isolation performance. The magnitude of Γ L is swept from 0 to 0.15 in 10 steps, and the phase of Γ L is swept from 0 to 360° in 10° steps. If -16 dB is taken as the baseline, it is obvious that by selecting appropriate parameters of the impedance tuner, the isolation can be significantly improved in the new structure of this embodiment.

[0094] Figure 11Illustrates the isolation performance in the new structure of this embodiment. In the simulation, the scanning range is reduced to: Γ L The magnitude of is swept from 0.115 to 0.135 with a step of 0.005, and Γ L The phase of is swept from 150° to 180° with a step of 10°. It can be seen that for all parameter settings within this range, this new structure has at least a 6 dB improvement in the system isolation performance. Therefore, the desired impedance can be flexibly selected to improve the system isolation.

[0095] Figure 12 Also illustrates the isolation performance in the new structure of this embodiment. Figure 12 The simulation results shown are obtained by using Figure 4 The specific structure of the impedance tuner shown (where d = λ / 4 is selected). The value of C1 is swept from 1.8 pf to 2.1 pf with a step of 0.05 pf, and the value of C2 is swept from 1.7 pf to 2 pf with a step of 0.05 pf. It can be clearly seen that all combinations of C1 and C2 within this range can improve the system isolation.

[0096] Based on the above description, the new structure of this embodiment can significantly improve the isolation performance of the PA backend in the TDD system mainly from two aspects. First, it can improve the poor isolation caused by the poor matching of devices (such as HP switches) and the in-phase addition of multiple reverse signals. Second, it can handle the isolation problems caused by device / PCB variations that often exist in reality because the tuner can be electrically calibrated. Therefore, the linearity of the radio transmitter can be improved through this new structure.

[0097] Although the example of the impedance tuner shown in Figure 4 has been described above for explaining the principle of this embodiment, it should be noted that various types of impedance tuners can be used. For example, the impedance tuner can have a single-stub or multi-stub structure. Examples of tunable elements in a stub can include but are not limited to variable capacitors (such as voltage-controlled capacitors, MEMS switched capacitors), variable resistors, varactors (such as diode varactors), MEMS switches, and so on.

[0098] As a first exemplary example, as an alternative to the double-stub impedance tuner shown in Figure 4 , the single-stub impedance tuner shown in Figure 13A can be used. By tuning the distance from the load to the stub and the susceptance or reactance value of the stub, this tuner can achieve any desired impedance. The single-stub impedance tuner can also take the form shown in Figure 13B . Regarding Figure 13BMore details of the impedance tuner can be found in T. et al., “A 20-50GHz RF MEMS Single-Stub Impedance Tuner” (IEEE Microwave and Wireless Components Letters, Vol. 15, No. 4, April 2005). As a second exemplary example, a different double-stub impedance tuner as shown in Figure 13C can be used. More details of the impedance tuner in Figure 13C can be found in R. Quaglia et al., “A Double Stub Impedance Tuner with SiC Diode Varactors” (Proceedings of the Asia-Pacific Microwave Conference 2011). As a third exemplary example, a multi-stub impedance tuner as shown in Figure 13D can be used. More details of the impedance tuner in Figure 13D can be found in Zhen Zhou et al., “Frequency Agility of Broadband Antennas Integrated With a Reconfigurable RF Impedance Tuner” (IEEE Antennas and Wireless Propagation Letters, Vol. 6, 2007). As a fourth exemplary example, a coupler-based impedance tuner can be used, which is formed (at least) by a hybrid coupler and a variable (or tunable) element, such as the impedance tuner as shown in Figure 13E More details of the impedance tuner in Figure 13E can be found in Milad Kalantari et al., “A Tunable Reflection / Transmission Coefficient Circuit Using a 45° Hybrid Coupler With Two Orthogonal Variables” (IEEE Transactions on Microwave Theory and Techniques, Vol. 67, No. 4, April 2019). It should be noted that, in addition to variable resistors, variable elements can also be implemented by variable capacitors, p-i-n diodes, MOSFET transistors biased in the triode region, and digital potentiometers.

[0099] It should also be noted that the present disclosure is not limited to Figure 3The above-described structure of the radio transmitter. As another example, the antenna unit 108 can be an optional component of the radio transmitter 300, since some radio transmitters (e.g., RRU) may not have an antenna unit. As yet another example, the directional coupler 109 and the feedback path can be omitted from the radio transmitter 300. In this case, the improved isolation brought by the impedance tuner can still benefit the performance of the power amplifier. As yet another example, the impedance tuner can be replaced with a passive impedance network having predetermined component values at the cost of a certain loss in flexibility and isolation performance. For example, radio transmitters manufactured in the same batch may have substantially the same characteristics of each component. In this case, each of these radio transmitters manufactured in the same batch can have the same passive impedance network with predetermined component values.

[0100] Based on the above description, at least one aspect of the present disclosure provides a radio transmitter. The radio transmitter includes a power amplifier, a filter unit, a circulator, a switch, and a reflection cancellation unit. The filter unit is capable of filtering the output from the power amplifier during a downlink TDD time slot and filtering an uplink signal during an uplink TDD time slot. The circulator has a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot and coupling the third port of the circulator to an uplink receiving path during an uplink TDD time slot. The reflection cancellation unit (e.g., the impedance tuner 307 or the above-mentioned passive impedance network) is coupled between the switch and the termination load and is capable of generating a first reflection signal that propagates to the first port of the circulator via the switch and the third port of the circulator during a downlink TDD time slot.

[0101] For example, at the first port of the circulator, the first reflection signal is added to the sum of the reflection signals in order to cancel the sum of the reflection signals. The reflection signals cancelled by the first reflection signal (e.g., Figure 3 reflection 1 in) can include: a second reflection signal (e.g., Figure 3 reflection 2 in) leaking from the second port of the circulator to the first port, a third reflection signal (e.g., Figure 3 reflection 3 in) generated by the switch and propagating to the first port via the third port of the circulator, and a fourth reflection signal (e.g., Figure 3 reflection 4 in) generated or reflected by the first port of the circulator and propagating backward to the power amplifier.

[0102] Optionally, the radio transmitter may further include a directional coupler coupled between the power amplifier and the first port of the circulator, and a feedback receiver coupled to the directional coupler and configured to feedback a portion of the output from the power amplifier for digital predistortion. The feedback receiver may monitor the linearization performance of the power amplifier based on the received portion of the output from the power amplifier. Optionally, the radio transmitter may further include an antenna unit coupled to the filter unit. In this case, the second reflected signal may include a reflected signal (or reverse wave) from the antenna unit. It should be noted that the terms "reflected signal" and "reverse wave" may be used interchangeably in the present disclosure, because the reverse wave generated by the mutual coupling between different antenna units may also be regarded as a kind of reflected signal.

[0103] Figure 14 is a diagram showing a radio transmitter 1400 according to another embodiment of the present disclosure. As shown, Figure 14 The embodiment of Figure 3 differs from the embodiment of Figure 3 in that the reflection cancellation unit 307 (which is an impedance tuner) in

[0104] Figure 15 is replaced by a reflection cancellation unit 1407. As shown, the reflection cancellation unit 1407 includes an impedance tuner 1408, a temperature sensor 1410, and a controller 1409. The temperature sensor 1410 may be configured to sense the ambient temperature of the radio transmitter. The controller 1409 may be configured to control the impedance tuner based on the sensed ambient temperature to minimize the total reflection at the first port of the circulator. For example, a look-up table may be prepared in advance during factory calibration, which indicates the correspondence between different temperatures and different control values (e.g., the control voltage of the impedance tuner) optimized at different temperatures. The controller 1409 may use this look-up table to determine the optimized control value for the impedance tuner according to the sensed ambient temperature. In this way, the temperature-dependent changes of the components can be compensated in real time. Figure 15 The embodiment of Figure 3 differs from the embodiment of Figure 3 in that a power meter 1511 is additionally provided, and the reflection cancellation unit 307 (which is an impedance tuner) in

[0105] Figure 16 FIG. is a diagram showing a radio transmitter 1600 according to another embodiment of the present disclosure. As shown, Figure 16 The embodiment of Figure 3 differs from the embodiment of Figure 3 in that the reflection cancellation unit 307 (which is an impedance tuner) in Figure 16 is replaced by a reflection cancellation unit 1607. It should be noted that in Figure 3 a feedback receiver 110 is clearly shown, which is actually included in the feedback path of Figure 15 and Figure 16 but is omitted for simplicity. As described above, the feedback receiver 110 can be configured to monitor the linearization performance of the power amplifier. For example, the linearization performance of the power amplifier can be represented by at least one of the ACLR and EVM of the forward traveling wave. As shown, the reflection cancellation unit 1607 includes an impedance tuner 1608 and a controller 1609. The controller 1509 can be configured to control the impedance tuner to optimize the monitored linearization performance of the power amplifier. In the embodiments shown in Figure 17 the controller 1509 / 1609 can determine the optimized control value by using one of a gradient decent process, a Levenberg Marquardt process, a Gauss-Newton process, a simultaneous perturbation stochastic approximation (SPSA) process, and any other similar process, which will be described later with reference to

[0106] Based on the above description, in at least one embodiment of the present disclosure, the reflection cancellation unit may include a controller (e.g., controller 1409 / 1509 / 1609), which is configured to control the impedance tuner to minimize the total reflection at the first port of the circulator.

[0107] In addition, at least one aspect of the present disclosure provides a radio device including the above radio transmitter. Examples of radio devices may include, but are not limited to, RRU, distributed unit (DU), active antenna system (AAS), and base station. The base station may be, for example, Node B (NodeB), evolved Node B (eNodeB or eNB), next generation Node B (gNodeB or gNB), relay, integrated access backhaul (IAB), low power nodes such as femto base station (femto), pico base station (pico), etc. Other constituent components of the radio device other than the radio transmitter may be well known in the art, and their details are omitted herein.

[0108] Figure 17It is a flowchart showing a method executed by a controller according to an embodiment of the present disclosure. The controller can be used in a radio transmitter including a power amplifier, a filter unit, a circulator, a switch, and an impedance tuner. The circulator has a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot and coupling the third port of the circulator to an uplink receiving path during an uplink TDD time slot. The impedance tuner is coupled between the switch and the termination load and is capable of generating a first reflected signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot.

[0109] At block 1702, with the impedance tuner being controlled using at least one first control value, the controller obtains a metric that can reflect the intensity of the total reflection at the first port of the circulator. For ease of understanding, the method will be explained below using the SPSA algorithm. Figure 17 Regarding more details of the SPSA algorithm, they can be found in the following literature: for example, J.C. Spall and J.A. Cristion, "Model-free control of nonlinear stochastic systems with discrete-time measurements" (IEEE Transactions on Automatic Control, Vol. 43, No. 9, pp. 1198 - 1210, September 1998). Assume that the impedance tuner is an n-section impedance tuner. Then, in the case of the SPSA algorithm, at least one first control value can be determined through the following steps. At the first step, control values (e.g., the control voltage of the impedance tuner) v1, v2, …, vn are initialized. For example, the initialized control voltage can be the control voltage optimized before leaving the factory. At the second step, random vectors d1, d2, …, dn with a Bernoulli distribution are generated. At the third step, at least one first control value can be determined to include: control voltages v1 + d1, v2 + d2, …, vn + dn; and control voltages v1 - d1, v2 - d2, …, vn - dn. Then, the two sets of control voltages can be applied to the impedance tuner respectively.

[0110] To evaluate the effect brought by at least one control value (e.g., two sets of control voltages), the metric can be the power of the total reflection at the first port of the circulator, which directly reflects the intensity of the total reflection. The power of the total reflection can be measured by the above-mentioned power meter. Alternatively, the metric can be at least one of ACLR and EVM, which indirectly reflects the intensity of the total reflection. ACLR / EVM can be monitored by the above-mentioned feedback receiver. It is also possible to use a combination of the above two types of metrics.

[0111] At block 1704, the controller determines at least one second control value for the impedance tuner based on the obtained metric such that the intensity of the total reflection at the first port of the circulator is minimized. In the case of the SPSA algorithm, at least one second control value can be determined through the following steps. At the first step, the cost function cp can be evaluated based on the metrics obtained under the control voltages v1 + d1, v2 + d2, …, vn + dn. At the second step, the cost function cn can be evaluated based on the metrics obtained under the control voltages v1 - d1, v2 - d2, …, vn - dn. At the third step, at least one second control value can be calculated as:

[0112] [v1, v2, …, vn] = [v1, v2, …, vn] - 0.5 * (cp - cn) / [d1, d2, …, dn].

[0113] At block 1706, the controller controls the impedance tuner using at least one second control value. For example, the control voltage calculated at the third step of block 1704 above can be applied to the impedance tuner. Note that any other similar algorithms (such as the gradient descent algorithm, Levenberg Marquardt algorithm, Gauss-Newton algorithm, etc.) can be used as an alternative. Optionally, blocks 1702 to 1706 can be iteratively executed to control the impedance tuner in real time. It is also possible to use Figure 17 methods to handle the aging of the radio transmitter (which can be optimized after deployment, calibrated occasionally, or tuned in real time), and / or antenna / cable reflections (which can be tuned after deployment, either once or tracked in real time because the cable may also have temperature changes).

[0114] It is also possible to use Figure 17Method. For example, at block 1702, a signal generator can be used to generate a backward traveling wave to the second port of the circulator. It is also assumed that the impedance tuner is an n-section impedance tuner. Then, a set of initialized control voltages can be used as at least one first control value to control the impedance tuner. The power of the backward traveling wave can be measured at the feedback path or the power meter as a metric. At block 1704, the measured metric can be evaluated to calculate a new control voltage (as at least one second control value) to reduce the power of the backward traveling wave. At block 1706, the new control voltage can be applied to the impedance tuner. In this way, process variations (component variations, soldering tolerances, etc.) can be optimized before leaving the factory.

[0115] Figure 18 is a block diagram showing an apparatus suitable for use in practicing some embodiments of the present disclosure. For example, the above-described controller can be implemented by apparatus 1800. As shown, apparatus 1800 can include a processor 1810, a memory 1820 storing a program, and an optional communication interface 1830 for data communication with other external devices via wired and / or wireless communication.

[0116] The program includes program instructions that, when executed by the processor 1810, enable the apparatus 1800 to operate according to embodiments of the present disclosure, as discussed above. That is, embodiments of the present disclosure can be implemented at least in part by computer software executable by the processor 1810, or by hardware, or by a combination of software and hardware.

[0117] The memory 1820 can be of any type suitable for the local technical environment and can be implemented using any suitable data storage technology, such as semiconductor-based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory, and removable memory. The processor 1810 can be of any type suitable for the local technical environment and, by way of non-limiting example, can include one or more of a general-purpose computer, a special-purpose computer, a microprocessor, a digital signal processor (DSP), and a processor based on a multi-core processor architecture.

[0118] Figure 19is a block diagram showing a controller according to an embodiment of the present disclosure. The controller can be used in a radio transmitter including a power amplifier, a filter unit, a circulator, a switch, and an impedance tuner. The circulator has a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to the switch. The switch is capable of coupling the third port of the circulator to a termination load during a downlink TDD time slot, and coupling the third port of the circulator to an uplink receive path during an uplink TDD time slot. The impedance tuner is coupled between the switch and the termination load and is capable of generating a first reflected signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot.

[0119] As shown in the figure, the controller 1900 includes an obtaining module 1902, a determining module 1904, and a control module 1906. The obtaining module 1902 can be configured to obtain a metric that can reflect the intensity of the total reflection at the first port of the circulator when controlling the impedance tuner using at least one first control value, as described above with respect to block 1702. The determining module 1904 can be configured to determine at least one second control value for the impedance tuner based on the obtained metric such that the intensity of the total reflection at the first port of the circulator is minimized, as described above with respect to block 1702. The control module 1906 can be configured to control the impedance tuner using at least one second control value, as described above with respect to block 1706. The above modules can be implemented by hardware (e.g., field programmable gate array (FPGA), application specific integrated circuit (ASIC), etc.), or software, or a combination of both.

[0120] In general, various exemplary embodiments can be implemented in hardware or dedicated circuits, software, logic, or any combination thereof. For example, some aspects can be implemented in hardware, while other aspects can be implemented in firmware or software executable by a controller, microprocessor, or other computing device, although the present disclosure is not limited thereto. Although various aspects of the exemplary embodiments of the present disclosure can be shown and described as block diagrams, flowcharts, or using some other graphical representation, it should be well understood that, by way of non-limiting example, the blocks, devices, systems, techniques, or methods described herein can be implemented in hardware, software, firmware, dedicated circuits or logic, general hardware or controllers, or other computing devices, or some combination thereof.

[0121] Thus, it should be understood that at least some aspects of the exemplary embodiments of the present disclosure may be practiced in various components such as integrated circuit chips and modules. Accordingly, it should be understood that the exemplary embodiments of the present disclosure may be implemented in an apparatus embodied as an integrated circuit, where the integrated circuit may include circuitry (and possibly, firmware) for embodying at least one or more of a data processor, a digital signal processor, a baseband circuit, and a radio frequency circuit that can be configured to operate in accordance with the exemplary embodiments of the present disclosure.

[0122] It should be understood that at least some aspects of the exemplary embodiments of the present disclosure may be embodied in computer-executable instructions executed by one or more computers or other devices, such as embodied in one or more program modules. Generally, program modules include routines, programs, objects, components, data structures, etc., which perform specific tasks or implement specific abstract data types when executed by a processor in a computer or other device. The computer-executable instructions may be stored on a computer-readable medium such as a hard disk, an optical disk, a removable storage medium, a solid-state memory, a RAM, etc. Those skilled in the art will understand that the functions of the program modules may be combined or distributed as needed in various embodiments. Additionally, the functions may be embodied wholly or in part in firmware or hardware equivalents such as integrated circuits, field-programmable gate arrays (FPGAs), etc.

[0123] References in this disclosure to "an embodiment", "one embodiment", etc. mean that the described embodiment may include a particular feature, structure, or characteristic, but not every embodiment necessarily includes that particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Additionally, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of those skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described.

[0124] It should be understood that although terms such as "first", "second", etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element may be termed a second element, and similarly, a second element may be termed a first element, without departing from the scope of the present disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed terms.

[0125] The terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the terms "comprises", "comprising", and / or "having", when used herein, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components, and / or combinations thereof. The term "connected" as used herein covers both direct and / or indirect connections between two elements.

[0126] The disclosure includes any novel feature or combination of features disclosed herein either explicitly or in any generalised form thereof. Various modifications and adaptations of the above-described exemplary embodiments of the disclosure will become apparent to those skilled in the relevant art in view of the above description when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of the disclosure.

Claims

1. A radio transmitter (300, 1400, 1500, 1600), comprising: A power amplifier (101); A filter unit (102) capable of filtering the output from the power amplifier (101) during a downlink time division duplex (TDD) time slot and filtering an uplink signal during an uplink TDD time slot; A circulator (103) having a first port coupled to the power amplifier (101), a second port coupled to the filter unit (102), and a third port coupled to a switch (104); The switch (104) capable of coupling the third port of the circulator (103) to a termination load (105) during the downlink TDD time slot and coupling the third port of the circulator (103) to an uplink receiving path (106) during the uplink TDD time slot; and A reflection cancellation unit (307, 1407, 1507, 1607) coupled between the switch (104) and the termination load (105) and capable of generating a first reflection signal that propagates from the switch (104) and the third port of the circulator (103) to the first port of the circulator (103) during the downlink TDD time slot, wherein the reflection cancellation unit (307, 1407, 1507, 1607) includes an impedance tuner.

2. The radio transmitter (300, 1400, 1500, 1600) according to claim 1, wherein, The first reflection signal is added to the sum of reflection signals to cancel the sum of reflection signals at the first port of the circulator (103).

3. The radio transmitter (300, 1400, 1500, 1600) according to claim 2, wherein, The reflection signals canceled by the first reflection signal include: A second reflection signal leaking from the second port of the circulator (103) to the first port; A third reflection signal generated by the switch (104) and propagating from the third port of the circulator (103) to the first port; and A fourth reflection signal generated by the first port of the circulator (103) and propagating backward to the power amplifier (101).

4. The radio transmitter (300, 1400, 1500, 1600) according to any one of claims 1 to 3, wherein, Replace the impedance tuner with a passive impedance network having a predetermined component value.

5. The radio transmitter (300, 1400, 1500, 1600) according to any one of claims 1 to 3, wherein, The impedance tuner is one of the following: A single stub impedance tuner; A multi-stub impedance tuner; and A coupler-based impedance tuner formed by a hybrid coupler and a tunable element.

6. The radio transmitter (300, 1400, 1500, 1600) according to claim 5, wherein, One or more tunable elements in the single stub impedance tuner, or the multi-stub impedance tuner, or the coupler-based impedance tuner include at least one of the following: A variable capacitor; A variable resistor; A varactor; and a microelectromechanical system (MEMS) switch.

7. The radio transmitter (1400, 1500, 1600) according to any one of claims 1-3, 6, wherein, The reflection cancellation unit (1407, 1507, 1607) further includes: A controller (1409, 1509, 1609) configured to control the impedance tuner (1408, 1508, 1608) to minimize the total reflection at the first port of the circulator (103).

8. The radio transmitter (1400) according to claim 7, wherein, The reflection cancellation unit (1407) further includes: A temperature sensor (1410) configured to sense the ambient temperature of the radio transmitter (1400); and wherein the controller (1409) is configured to control the impedance tuner (1408) based on the sensed ambient temperature.

9. The radio transmitter (1400) according to claim 8, wherein, The impedance tuner (1408) is controlled by using a pre-configured look-up table that indicates the correspondence between the control values for the impedance tuner and the temperature.

10. The radio transmitter (1500) according to any one of claims 8 to 9, wherein, The reflection cancellation unit (1507) further comprises:[[]] A power meter (1511) coupled between the first port of the circulator (103) and the power amplifier (101) and configured to measure the power of the total reflection at the first port of the circulator (103); and wherein the controller (1509) is configured to control the impedance tuner (1508) to minimize the measured power of the total reflection.

11. The radio transmitter (300, 1400, 1500, 1600) according to any one of claims 1-3, 6, 8-9 further comprises:[[]] A directional coupler (109) coupled between the power amplifier (101) and the first port of the circulator (103); and A feedback receiver (110) coupled to the directional coupler (109) and configured to feedback a portion of the output from the power amplifier (101) for digital pre-distortion.

12. The radio transmitter (1600) according to claim 11, wherein, The reflection cancellation unit (1607) further comprises:[[]] A controller (1609) configured to control the impedance tuner (1608) to optimize the linearization performance of the power amplifier (101) monitored by the feedback receiver (110).

13. The radio transmitter (1600) according to claim 12, wherein, The linearization performance of the power amplifier (101) is represented by at least one of the following:[[]] Adjacent channel leakage ratio ACLR; and Error vector magnitude EVM.

14. The radio transmitter (1500, 1600) according to any one of claims 12-13 controls the impedance tuner (1508, 1608) by using one of the following:[[]] Gradient descent process; Levenberg Marquardt process; Gauss-Newton process; and Simultaneous perturbation stochastic approximation SPSA process.

15. The radio transmitter (300, 1400, 1500, 1600) according to any one of claims 1-3, 6, 8-9, 12-13 further comprises:[[]] An antenna unit (108) coupled to the filter unit (102).

16. A radio device comprising the radio transmitter (300, 1400, 1500, 1600) according to any one of claims 1 to 15.

17. The radio device according to claim 16, wherein, The radio device is one of the following: Remote Radio Unit RRU; Distributed Unit DU; Active Antenna System AAS; and Base station.

18. A method performed by a controller in a radio transmitter, wherein, The radio transmitter includes: a power amplifier; a filter unit; a circulator having a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to a switch; the switch capable of coupling the third port of the circulator to a termination load during a downlink time division duplex (TDD) time slot and coupling the third port of the circulator to an uplink receive path during an uplink TDD time slot; and an impedance tuner coupled between the switch and the termination load and capable of generating a first reflected signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot. The method includes: obtaining (1702) a metric that can reflect the intensity of the total reflection at the first port of the circulator, with the impedance tuner being controlled using at least one first control value; determining (1704) at least one second control value for the impedance tuner based on the obtained metric such that the intensity of the total reflection at the first port of the circulator is minimized; and controlling (1706) the impedance tuner using the at least one second control value.

19. The method according to claim 18, wherein, The metric is one of the following: the power of the total reflection at the first port of the circulator; adjacent channel leakage ratio (ACLR); and error vector magnitude (EVM).

20. The method according to claim 18 or 19, wherein The at least one second control value is determined by using one of the following: a gradient descent process; a Levenberg Marquardt process; a Gauss-Newton process; and a simultaneous perturbation stochastic approximation (SPSA) process.

21. A controller (1800) in a radio transmitter, wherein, The radio transmitter includes: a power amplifier; a filter unit; a circulator having a first port coupled to the power amplifier, a second port coupled to the filter unit, and a third port coupled to a switch; the switch capable of coupling the third port of the circulator to a termination load during a downlink time division duplex (TDD) time slot and coupling the third port of the circulator to an uplink receive path during an uplink TDD time slot; and an impedance tuner coupled between the switch and the termination load and capable of generating a first reflected signal that propagates to the first port of the circulator via the switch and the third port of the circulator during the downlink TDD time slot. The controller (1800) includes: at least one processor (1810); and at least one memory (1820), the at least one memory (1820) containing instructions executable by the at least one processor (1810), whereby the controller (1800) is operable to: obtain a metric that can reflect the intensity of the total reflection at the first port of the circulator, with the impedance tuner being controlled using at least one first control value; Determine at least one second control value for the impedance tuner based on the obtained metric such that the intensity of the total reflection at the first port of the circulator is minimized; and Control the impedance tuner using the at least one second control value.

22. The controller (1800) according to claim 21, wherein, The controller (1800) is operable to perform the method according to claim 19 or 20.

23. A computer-readable storage medium comprising instructions that, when executed by at least one processor, cause the at least one processor to perform the method according to any one of claims 18-20.

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