Technical Field

The present invention relates to a circuit for downlink/uplink operational mode switching in a TDD wireless communication system.

Background Art

In the field of telecommunications, it is known that an efficient and fast switching from/to DL/UL (downlink/uplink) operational mode, i.e. Tx/Rx (transmission/reception), is of outmost importance when dealing with Time Division Duplexed (TDD) signals.

Particularly, when the system is in Tx mode a PA (Power Amplifier) on the DL should be turned on and this can be realized by deploying an LDMOS transistor equipped with an embedded thermal tracking device for thermal compensation. An example of high-level standard TDD RF front end designed with and high- isolation RF switch at the antenna port is showed in Figure 1.

Particularly, it is known that during a RF power amplifier design phase, an important aspect to be considered is performance variation over temperature. In this perspective, one of the main parameters to control is the quiescent current trying to maintain it constant over a large temperature range, which is the real challenging task in this kind of design.

Quiescent current tends to rise with working temperature, whose effect is to reduce linearity. To overcome this difficulty, transistor manufactories have embedded a quiescent current thermal tracking circuit T (a small integrated LDMOS FET located close to the active power LDMOS) in latest RF power integrated circuits.

An example of tracking thermal device T is illustrated in Figure 2.

When a constant current source is applied to gate voltage VG, the thermal tracking RF FET draws a constant gate current IG; when the temperature varies, the gate-source voltage VGS changes to maintain constant the gate current IG. As a consequence, the LDMOS power FET sees a varying gate voltage, VGS, that also maintains its quiescent current, IDQ, constant over temperature.

However, the presence of the thermal tracking device T does not allow for complete transistor shut down within Tx/Rx switching time as defined by the 3 GPP standard (approximatively lps).

This implies that an RF switch located at the antenna port, driven by the TDD synchronism signal, is needed to commute between Tx and Rx mode, and it has to be designed with high isolation capabilities (see Figure 1).

However, several drawbacks are known for the above solution.

A first drawback concerns the high cost of the components. Particularly, the high cost is due to the high-isolation requirements for the RF switch.

A further drawback concerns the loss in the DL path efficiency. Particularly, the introduction of an RF switch after the PA increases DL path insertion loss, hence decreasing overall system efficiency.

Furthermore, the know solution has a reduced system reliability. The reduced reliability is due to the fact that the RF switch deals with high power levels. Finally, the system realized according to the know solution has high power consumption. Particularly, since the PA is never completely turned off it draws quiescent current also during Rx time slots.

Description of the Invention

The main aim of the present invention is to provide a circuit for an efficient and fast switching from/to DL/UL (downlink/uplink) operational mode in a TDD wireless communication system.

Particularly, when the system is in Tx mode and when dealing with Time Division Duplexed (TDD) signals, the circuit according to the invention allows to turn on/off the LDMOS transistor equipped with embedded thermal tracking device in compliancy with the stringent time requirements defined by the standard.

Another object of the present invention is to provide a circuit for downlink/uplink operational mode switching in a TDD wireless communication system that allows to reduce the overall cost of the used components.

Another object of the present invention is to provide a circuit for downlink/uplink operational mode switching in a TDD wireless communication system that allows to reduce the loss in the DL path efficiency.

Another object of the present invention is to provide a circuit for downlink/uplink operational mode switching in a TDD wireless communication system that allows to improve the system reliability.

Another object of the present invention is to provide a circuit for downlink/uplink operational mode switching in a TDD wireless communication system that allows to reduce the power consumption.

The above-mentioned objects are achieved by the present circuit for downlink/uplink operational mode switching in a TDD wireless communication system according to the features of claim 1.

Brief Description of the Drawings

Other characteristics and advantages of the present invention will become better evident from the description of a preferred, but not exclusive embodiments of a circuit for downlink/uplink operational mode switching in a TDD wireless communication system, illustrated by way of an indicative but non-limiting example in the accompanying Figures, in which:

Figure 1 shows a high-level standard TDD RF front end designed with a high- isolation RF switch at the antenna port, according to a known solution;

Figure 2 illustrates a tracking thermal device according to a known solution; Figure 3 shows a high-level standard TDD RF front-end designed with a high- isolation RF switch at the antenna port, according to the invention;

Figure 4 is a high-level electrical scheme of the switching circuit for RF LDMOS activation/deactivation, in the Rx operation mode configuration;

Figure 5 shows an equivalent circuit in the Rx operational mode configuration; Figure 6 is a high-level electrical scheme of the switch circuit for RF LDMOS activation/deactivation, in the Tx operational mode configuration;

Figure 7 shows an equivalent circuit in the Tx operational mode configuration. Embodiments the Invention

With particular reference to such illustrations, globally indicated with reference C is a circuit for downlink/uplink operational mode switching in a TDD wireless communication system.

The circuit C according to the invention allows to remove in a high-level TDD RF front end the RF switch at the antenna port and place it in the UL path before LNA, as shown in Figure 3, hence addressing all issues of the known solutions.

Particularly, the circuit C is designed to act on the power amplifier LDMOS FET Gate voltage, while the Drain voltage is maintained constant.

From a high-level viewpoint, the circuit C according to the invention comprises:

- a voltage generator for setting the quiescent Drain current of the power amplifier LDMOS;

- a large- value resistor connected in series to the voltage generator;

- a sample- and-hold circuit comprising two analog switches driven by the synchronism signal of the TDD standard and a hold capacitor.

This circuit C allows for LDMOS power amplifier PA complete shut down in a very limited time interval, even if the embedded thermal tracking device needs a high resistor in series at the gate port of LDMOS for its correct functioning.

The resistor limits the current feeding the Gate port to change the LDMOS status (i.e. switching from on to off and vice versa), hence increasing the activation/deactivation time exceeding 3 GPP standard’s limits.

Moreover, a long commutation period potentially causes the reactive parasite effects of surrounding circuit components to be quite heavy, resulting in larger damped oscillations of the Drain current (whose module could be much larger than the maximum LDMOS current).

Focusing on a TDD scenario, RF LDMOS used to realize the power amplifier PA needs to be active only during Tx phase (DL period). One option to increase overall system efficiency, while removing possible source of thermal noise at the output of the power amplifier PA, is to turn off the RF LDMOS during RX phase (UL phase) by minimizing quiescent current, IDQ.

The challenge is to realize it using RF FET equipped with thermal tracking devices while keeping transitions latency from TX to RX phases lower than 1 ps, as required by 3 GPP standard.

The invention proposed realizes the circuit C to address the issue above, its high-level electrical representation is shown in Figure 4 and Figure 6 for Rx and Tx operating mode, respectively.

The circuit C comprises at least a field-effect transistor RF FET operatively connected to a power amplifier PA on the downlink path (DL) of a RF front-end apparatus in a TDD wireless communication system.

Furthermore, the circuit C comprises:

- a first voltage generator VgsoFF connected to at least a large- value first resistor Rhoid;

- a second voltage generator VG _ate connected to a second resistor RG _ate ;

- at least a large- value hold capacitor Choid;

- a sample-and-hold circuit.

Particularly, the sample-and-hold circuit is configured to be switched between:

- a reception (Rx) configuration, wherein the first voltage generator Vgso _FF is connected to the gate G of the field-effect transistor RF FET and the large-value capacitor Choid is connected to the first voltage generator VgsoFF through the first resistor Rhoid, and

- a transmission (Tx) configuration, wherein the gate (G) of the field-effect transistor RF FET is connected to the hold capacitor Choid and the hold capacitor Choid is connected to the second voltage generator VGate through the second resistor RG _ate .

According to preferred embodiment of the circuit C, the sample-and-hold circuit comprises a first electronic switch SW1 connected to the gate G of the field- effect transistor RF FET) and configured to be connected to the first voltage generator VgsoFF in the reception configuration and to the hold capacitor Choid in the transmission configuration.

Furthermore, the sample-and-hold circuit comprises a second electronic switch SW2 connected to the hold capacitor Choid and configured to be connected to the first voltage generator VgsoFF through the first resistor Rhoid in the reception configuration and to the second voltage generator V _Gate through the second resistor RG _ate in the transmission configuration.

Particularly, during the TDD RX phase (Figure 4) the Gate G of the field-effect transistor RF FET is connected to the first voltage generator VgsoFF through the first electronic switch SW1 driven by the TDD Synchronism Signal (TDD CRTL).

The first voltage generator Vgso _FF is maintained to a constant value lower than the gate-source threshold voltage Vgs _th of the field-effect transistor RF FET, which is defined as the voltage level corresponding to the field-effect transistor RF FET to be active (i.e. small amount of current flowing from Drain to Source).

In these conditions the field-effect transistor RF FET Drain current, I _d , is close to zero.

The second electrical switch SW2 connects the large- value capacitor Choid to the first voltage generator VgsoFF through the large- value first resistor Rhoid (hold resistor).

The first resistor Rhoid allows controlling the discharge time of the hold capacitor Choid that results several orders of magnitude larger that TDD frame period. In this way, V hoid=VgsoN is almost constant over entire RX phase when system is stable (i.e. out of transient initial phase), wherein Vchoid is voltage value over the hold capacitor Choid.

Advantageously, the presence of the first (hold) resistor R _hoid assures that even when the system is not fed by any signal (no TDD CTRL to pilot the sample- and-hold circuit) the circuit C will reach idle conditions in a defined amount of time, i.e. when the hold capacitor C _hoid completes its discharge phase, and the system does not remain in the last active state (either Tx or Rx).

During TX phase (see Figure 6) the Gate of the field-effect transistor RF FET is connected to the large-value hold capacitor Choid through the first electronic switch SW1 driven by TDD Synchronism Signal (TDD CRTL). The second electrical switch SW2 connects the hold capacitor Choid to the second voltage generator, VG _ate , through the second resistor, RG _ate , of the proper size.

Particularly, the second voltage generator VG _ate and the second resistor RG _ate are connected in series and behaves as an equivalent current generator, allowing the transistor RF FET quiescent current, ID, to be kept constant over temperature.

To realize a proper dimensioning of the hold capacitor Choid and of the first hold resistor Rhoid, the first condition to be met is that the variation of capacitor voltage value Vchoid overt time, defined as Vri _PP ie, should be minimized.

At the same time, the capacitor voltage value Vchoid should be as closer as possible to V _gs stady-state value, V _gs _ss, which is defined as the voltage upper limit reached when the system is working in Tx configuration for an infinite period of time.

The voltage variation Vri _PP ie is computed through the equation that describes the discharge phase of the hold capacitor Choid when in Rx operating configuration. In this case, the sample-and-hold circuit realizes a configuration equivalent to the one shown in Figure 5, and the voltage variation Vri _PPie is defined by: where tu _L is the maximum duration of the UL period in the 3 GPP standard for TDD applications, i.e. worst-case condition, and Vchoid is the value taken at the beginning of the UL period.

Given Vchoid and VgsoFF, the aim is to minimize the voltage variation V _riPP ie for an optimized choice of the hold capacitor Choid and of the hold resistor Rhoid. Preferably, the hold capacitor Choid should take a much-larger value (e.g. 1 0 times more) than the transistor RF FET inherent input capacity, to disregard the parasite effects that may take place during the commutation phase. As described above, the voltage variation Vri _PP ie should also meet a second requirement, hence the hold capacitor Choid and the first resistor Rhoid should be optimized to minimize the equation below, describing the charge phase of the hold capacitor Choid during the transmission (Tx) configuration: where t _DL is the minimum duration of the DL period in the 3 GPP standard for TDD applications, and Vchoid is the voltage value taken at the beginning of the DL period.

If Vchoid is close to V _gs _ss, then the quiescent current actually drown by RF FET is very close to the nominal expected I _q .

In the transmission configuration, the sample-and-hold circuit realizes a configuration equivalent to the one shown in Figure 7.

Considering the circuit C layout and functioning principle explained above, during the time intervals when the transistor RF FET is supposed to be ON (i.e. TX periods), the Gate measures a low dynamic impedance due to the large- value capacitor, Choid, connected in parallel.

Similarly, during the time intervals when the transistor RF FET is supposed to be OFF (i.e. RX periods), the Gate still measures a low dynamic impedance, this time due to the connection to the first voltage generator, V _gs oFF.

In this way, transistor RF FET switching latency is short enough to comply with the limits defined by the standard, while assuring a precise thermal compensation over temperature of the quiescent current, ID.

Moreover, achieving a short commutation latency between Tx/Rx phases minimizes the potential damaging impacts of reactive parasite effects of surrounding circuit components.

The TDD wireless communication system S according to the invention, showed in Figure 3, comprises a downlink path DL PATH provided with at least a power amplifier PA, at least an uplink path UL PATH provided with at least a low-noise amplifier LNA, and at least an antenna ANT connected to the downlink path DL PATH and to the uplink path UL PATH.

The system S further comprises the circuit C for downlink/uplink operational mode switching as disclosed above, wherein the circuit C is operatively interposed between said power amplifier PA and said antenna ANT.

Furthermore, the system S comprises an RF switch placed in the uplink path UL PATH and connected to the circuit C, configured to be switched between the reception (Rx) configuration, wherein it is connected to said low-noise amplifier LNA, and the transmission (Tx) configuration, wherein it is disconnected from said low-noise amplifier LNA.

Particularly, in the transmission (Tx) configuration, the RF switch is connected to a resistor R.

With respect to the know system, the benefits introduced in the new system S by the new circuit C proposed allows then to remove the RF switch at the antenna port and place it in the UL path before LNA, as shown in Figure 3.

Therefore, the circuit according to the invention allows to comply with stringent time requirements defined by the 3 GPP standard for TDD operational mode switching latency. The invention proposed realizes a solution a solution to control an LDMOS transistor equipped with an embedded thermal tracking device used in RF front end circuit to realize PA stages for TDD applications.

Particularly, the circuit according to the invention allows to reduce the overall cost of the used components. Requirements for RF switch isolation can be reduced thanks to the introduction of the invention proposed that allows transistor RF FET to be completely turned off.

Furthermore, the circuit according to the invention allows to reduce the loss in the DL path efficiency. Particularly, the introduction of the invention proposed avoids the introduction of an RF switch after the PA in the DL path, hence reducing DL I.L. and increasing overall system efficiency.

The circuit according to the invention further lets to improve the system reliability. The RF switch is placed on the UL path hence dealing with lower power levels and increasing system reliability.

Furthermore, the circuit according to the invention allows to reduce the power consumption. Particularly, the RF FET transistor is completely turned off in Rx phase, with quiescent current maintained almost zero.