WO2007073253A1

WO2007073253A1 - Minimizing tx noise in rx for tdd systems

Info

Publication number: WO2007073253A1
Application number: PCT/SE2005/002006
Authority: WO
Inventors: Vimar BJÖRK
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2005-12-22
Filing date: 2005-12-22
Publication date: 2007-06-28

Abstract

The invention refers to a TDD based radio communication system comprising a sender (1) arranged to transmit during a TX period and a receiver (4) arranged to receive during an RX period being a TX off period. The system also comprises a PA (5) arranged to enhance the power of the signal to be transmitted during the TX. The invention is characterised in that the system comprises a bias control (10) arranged to change the bias of a key transistor (8) comprised in the PA (5) to a level below a predetermined threshold value during the TX off period so that noise from the PA (5) is attenuated or inhibited.

Description

TITLE

MINIMIZING TX NOISE IN RX FOR TDD SYSTEMS

ABBREVIATIONS

ACLR Adjacent Channel Leakage Ratio

NF The noise figure of an amplifier is the amount of noise at the output divided by the amount of noise one would have had from an ideal amplifier with the same gain connected to the same source. Noise figures are expressed in dB and they depend on what the source is. If nothing else is explicitly stated the input is connected to a resistor at room temperature, 290K

PA Power Amplifier

RF Radio Frequency

RX Receive, as in receiving mode

TDD Time Division Duplex

TX Transmit, as in transmission mode

VSWR Voltage Standing Wave Ratio

WiMAX is a standards-based wireless technology that provides high- throughput broadband connections over long distances. WiMAX can be used for a number of applications, including "last mile" broadband connections, hotspots and cellular backhaul, and high-speed enterprise connectivity for business TECHNICAL FIELD

The invention refers to a TDD based radio communication system comprising a sender arranged to transmit during a TX period and a receiver arranged to receive during an RX period being a TX off period. The system also comprises a PA arranged to enhance the power of the signal to be transmitted during the TX.

BACKGROUND ART

Wimax is a TDD based radio communication system comprising a sender and a receiver utilizing an antenna for transmission and reception. The sender transmits during a transmission mode TX and the receiver receives during a receiving mode RX. All TDD based radio systems alternates between the TX and the RX such that during TX the RX is off and vice versa.

The sender comprises a power amplifier PA for increasing the output power during the TX. In Wimax and other TDD based radio communication systems there is an increasing demand for larger coverage. To achieve this there is a need for higher output power and better sensitivity. This need demands an increased difference between the output power and the reception noise in the receiver. The importance of isolation between the TX and RX in the RX state is thus increased.

During the TX off period in the TDD communication system, the PA produces a noise that leaks from the sender during the TX off period. The TX noise needs to be minimized to not degrade the receiver performance. Insufficient TX noise attenuation will degrade the RX performance for the following reason.

It is long known to use a circulator for isolation between the different modes. However, when using a circulator, we assume a 4 dB system NF for the TDD mode and a maximum contribution from the TX noise of 0.1 dB then the TX noise should be 16 dB below the system noise floor. If an isolator is used to separate the TX and RX path the isolation in the isolator will be limited to the VSWR at the antenna port. A reasonable VSWR should be 15 dB. This gives that the output NF of the PA should be roughly 3 dB. This is not realistic since the gain of the PA is much greater than 3dB.

In prior art this problem has for a long time been solved by use of a switch after the PA or fast switching of the PA power supply. Both alternatives are difficult to implement and gives the following disadvantages:

After the PA the power is so high that the switch causes too much loss of effect in the system during the TX.

The switch is hard to design as it needs to be highly linear not to violate ACLR and spectrum mask requirements.

Fast switching of the PA power supply causes transient problems when turning the PA on and off and a lot of power is lost due to the charging an discharging of the decoupling capacitors.

It is thus known to use both a circulator and a switch in order to isolate in the different modes, but both are connected to disadvantages. The circulator gives less loss than a switch but less isolation, whereas the switch gives better isolation but greater loss.

Therefore, there is a long felt need for an improved control of a TDD based radio communication system giving an increased difference between the output power during the TX and the reception noise in the receiver during the TX off period.

DISCLOSURE OF INVENTION

The invention refers to a TDD based radio communication system comprising a sender arranged to transmit during a TX period and a receiver arranged to receive during an RX period being a TX off period. The system also comprises a PA arranged to enhance the power of the signal to be transmitted during the TX. The invention is characterised in that the system comprises a bias control arranged to change the bias of a key transistor comprised in the PA to a level below a predetermined threshold value during the TX off period so that noise from the PA is attenuated or inhibited.

One advantage of the invention is that, during the TX off period, noise from the PA is attenuated or inhibited to such an extent that a circulator can be used as an isolator to separate the TX and RX path.

The circulator is a device comprising a magnet creating a magnetic field that allows signals to pass through from the PA to an antenna during to the TX. The magnetic field is also arranged to deflect signals travelling in the opposite direction so that the signals are guided to a receiver instead of back to the PA.

The advantage of using a circulator is that the circulator gives minimum loss to the signal from the PA compared to a switch.

In one embodiment of the invention the PA comprises a driver positioned before a number of final transistors, the driver being connected to the key transistor, and in that the bias control is connected to the key transistor at the same position as the driver.

The bias control changes the bias to a level above the threshold level during the TX period.

A transistor is arranged to hinder a current to run through the transistor should the bias be below a threshold level and to allow current to flow through the transistor should the bias be above the threshold level.

The present invention intends to use the bias control to change the bias of the key transistor in a PA during the transmit mode TX and the receive mode RX. During the TX period the PA is biased as an ordinary amplifier. One benefit of the invention is that the bias control turns off the key transistor in the PA which has the effect that no noise will be generated in the PA by transistors following the key transistor (hereinafter called final transistors) in the PA nor will noise propagate through the final transistors from an input unit positioned before the key transistor. Such an input unit may be a driver and/or a pre-driver that is arranged to forward and enhance an input signal intended to be enhanced by the PA and transmitted by an antenna connected to or comprised in the sender.

The bias control bias the key transistor and thereby the final transistors to such an extent that they are in an off state, during the TX off period, when they are exposed to the noise from the pre-driver and driver transistors. The key transistor thus acts as a switch controlled by the bias control.

One advantage of the invention is that a switch after the PA can be omitted. As mentioned before, such switch is very hard to design since it should be able to handle very high power, have a fast response time and a low loss.

Since the loss after the PA can be reduced with the proposed solution by not using the above mentioned switch, the output power requirement of the PA can also be reduced.

The invention is suitable for all classes of transistors, i.e. class A, AB, B and class C, if they produce or carries noise. However, class C transistors normally behave in a proper manner, but should be completely turned off in cases where noise is produced or carried by the class C transistor. In all classes the transistors are biased according to the invention by use of the bias control.

BRIEF DESCRIPTION OF DRAWINGS

The invention will below be described in connection to a number of a drawings where; Fig. 1 schematically teaches PA noise path in a TDD based system according to prior art being in RX;

Fig. 2 schematically teaches basic components in a PA according to the invention, and where;

Fig. 3 schematically teaches a circuit diagram for a TDD based system comprising a bias control according to the invention implemented in a TDD based system according to figs. 1 or 2.

EMBODIMENT OF THE INVENTION

Fig. 1 schematically teaches PA noise path in a TDD based system according to prior art being in RX.

In figure 1 the system comprises a sender 1 , a circulator 2 and, an antenna 3 and a receiver 4. The sender 1 comprises a Pa 5 connected to the circulator 2. The circulator 2 is connected to the antenna 3 and to the receiver 4. In figure 1 the PA 5 leaks noise during the TX off period, i.e. the RX period, which is shown with a sketched arrow pointing in the direction of propagation. In figure 1 a band pass filter 11 is arranged between the circulator 2 and the antenna 3 in order to filter PA noise during TX and to create selectivity during RX. In figure 1 a switch 12 is arranged between the circulator 2 and the receiver 4. The switch 12 switches off the RX path during TX in order to protect the receiver 4.

Fig. 2 schematically teaches basic components in a PA according to the invention. The PA comprises a pre-driver 6 connected to a driver 7 connected to a key transistor 8 connected to final transistor 9. The PA is part of a sender comprising a signal generating unit. The signal generating unit is connected to the PA which is intended to amplify a signal from the signal generating unit.

The invention refers to a bias control 10 connected to the key transistor 8 such that the key transistor 8 upon being biased below a threshold level, will not produce an output signal. Since there is no output signal, the final transistors 9 are biased to such an extent that they are turned off and is kept in a turned off state when they are exposed to or should be exposed the noise from the Pre-driver 6 and Driver 7 transistors during the TX off period. The key transistor 8 thus acts as a switch controlled by the bias control 10. Furthermore, since the final transistors are in a turned off state, the final transistors 9 cannot produce any noise themselves.

Fig. 3 schematically teaches circuit diagram for a PA in a TDD based system comprising a bias control according to the invention implemented in a TDD based system according to figs. 1 or 2. The circuit diagram is a simplification used only for the facilitation of the understanding of the invention.

The signal generating unit forwards a signal RFj_n to an input matching network comprising capacitors C5 and C6 coupled in series and in parallel respectively and to a transmission line T3. The input matching network is connected to the key transistor TR1 via the transmission line T3. The key transistor is connected to ground and an output matching network and a drain voltage supply. The input signal RF_in is the actual signal intended to be transmitted during TX or noise from the driver and/or pre-driver during RX.

The bias control comprises a bias network comprising a voltage source V1 producing a voltage V1. The voltage source is coupled in series to a resistor

R1. The resistor and the voltage source are coupled in parallel to a capacitor

C1 and a capacitor C2. The capacitors C1 and C2 are both connected to ground. The capacitor C2 is connected to a resistor R2 connected to a transmission line T1 connected to the key transistor TR1 at the same position as the transmission line T3. The bias network is thus coupled in parallel to the input matching network.

The key transistor TR1 is also connected to ground and to an output matching network coupled in parallel to a drain voltage supply. Between the ground and the gate of the key transistor TR1 there is a gate source voltage V2.

The drain voltage supply comprises a transmission line to isolate the supply from the RF path. The drain supply voltage VDD is decoupled and filtered by C3, L1 and C4.

The output matching system comprises a transmission line connected to the key transistor and coupled in parallel to capacitor C8 and in series to a capacitor C7. The output matching network produces an output signal RF_0Ut which is also the output from the PA.

During the TX period the voltage, V1 , is set to an ordinary bias voltage giving a drain voltage V2 above a threshold level for the key transistor.

When the RX mode is entered the RF_in signal is switched off. After this the voltage, V1 , is changed in such a way that the drain voltage, V2, will pinch off the key transistor TR1 and the drain current, Id, will be forced to zero. The voltage V2 is below the threshold level which causes the key transistor not to trigger any Id current due to input noise from the input unit, i.e. the signal generating unit, the driver or the pre-driver. In this mode the TR1 will provide isolation in the RF path and will not generate any noise since the current, I_D, is zero. The only noise will originate from the supply voltage of the PA.

When the TX period is entered again the voltage, V1 , should be raised to the proper bias voltage again and when the voltage V2 and current ID has stabilized to the ordinary bias condition the RFj_n signal can be turned on

Claims

1. A TDD based radio communication system comprising a sender (1) arranged to transmit during a TX period and a receiver (4) arranged to receive during an RX period being a TX off period, the system also comprises a PA (5) arranged to enhance the power of the signal to be transmitted during the TX, characterized in that the system comprises a bias control (10) arranged to change the bias of a key transistor (8) comprised in the PA (5) to a level below a predetermined threshold value during the TX off period so that noise from the PA (5) is attenuated or inhibited.

2. A communication system according to claim 1, characterized in that the PA (5) comprises a driver (7) positioned before a number of final transistors (9), the driver (7) being connected to the key transistor (8), and in that the bias control (10) is connected to the key transistor (8) at the same position as the driver (7).

3. A communication system according to claim 1 or 2, characterized in that the bias control (10) is arranged to change the bias to a level above the threshold level during the TX.

4. A communication system according to any one of the previous claims, characterized in that a circulator (2) is used as an isolator to separate the TX and RX path.

5. A method for a communication system according to any one of claims 1-4, wherein the method comprises the steps of:

- a bias control comprised in the system changes the bias of a key transistor comprised in the PA to a level below a predetermined threshold value during the TX off period so that noise from the PA is attenuated or inhibited.

6. A method according to claim 5, characterized in that the bias control changes the bias to a level above the threshold level during the TX.