Field of the invention

The invention relates to a radio transmitter arrangement and a method of operation therefor, and in particular, but not exclusively, to radio transmitter arrangements for a base station of a cellular communication system.

Background of the Invention

In a cellular communication system, a geographical region is divided into a number of cells each of which is served by a base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated. Communication from a mobile station to a base station is known as uplink, and communication from a base station to a mobile station is known as downlink.

Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM) . Further description of the GSM TDMA communication system can be found in ^λ The GSM System for Mobile Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

In the past years, 3rd generation systems have been deployed to further enhance the communication services provided to mobile users. An example of such a communication system is the Universal Mobile Telecommunication System (UMTS) , which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ^V WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

In cellular communication systems, it is important to control and minimize interference in order to maximise the capacity of the system. Accordingly, it is very important to reduce the out of band interference caused by a transmitter in a cellular communication system and the technical specifications for a cellular communication system provide stringent requirements for the spectral density of the transmitter signals. Accordingly, the transmitters in a cellular communication system are designed to meet these requirements and in particular this requires that the power amplifiers of the transmitters are predominantly operated in the linear region.

In order to achieve such accurate performance, power amplifiers frequently comprise an internal feedback loop wherein the output signal is measured and used to adjust characteristics of the amplifying circuits.

However, in addition to requiring acceptable performance during normal operation, it is in many communication systems also important to ensure acceptable performance in case of a malfunction. For example, if a power

amplifier of a base station in a cellular communication system develops a fault resulting in an excessive transmit power or excessive distortion occurring, the interference caused to e.g. other cells or sectors may rise to unacceptable levels. Thus, in such case, the fault will not only disrupt communication to the mobile stations served by the power amplifier but may also degrade or prevent communication to mobile stations served by other power amplifiers or other base stations.

Accordingly reliability and fault condition operation of transmitters is very important in communication systems . One method of addressing these issues is to use internal feedback loops in the power amplifier to sense the output signal in order to detect fault conditions. For example, most microwave radio amplifiers rely on an internal pick up sensor to limit and control the output power of the RF (radio frequency) amplification stage in order to ensure that operation is within the desired transmission power specifications.

However, in many applications and scenarios, such operation is not optimal. For example, if an error occurs in the feedback loop itself, this may result in the output power increasing dramatically. As a result, this may increase interference to other communications.

It is known to include an additional internal power control loop in power amplifiers in order to attenuate the signal if a fault condition is detected. However, as the circuitry is physically and electrically integrated with the power amplifier, it is highly possible for such circuitry to fail together with the main feedback loop.

For example, the internal loop could be subject to internal construction errors which may also cause the main feedback circuitry failing in some operating conditions. Furthermore, such an approach requires an integrated design phase which prevents additional circuitry to be added to a power amplifier, which has not originally been designed to include this functionality. It furthermore tends to result in complex and inflexible power amplifier designs which cannot easily be adapted to different applications and requirements.

Hence, an improved power amplifier arrangement would be advantageous and in particular an arrangement allowing increased flexibility, reduced complexity, increased reliability and/or improved fault condition operation would be advantageous .

Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a radio transmitter arrangement comprising: a power amplifier for generating a radio frequency signal to be radiated from an antenna; power supply means for supplying a supply power to the power amplifier; sensor means for generating a radiated power indication by sensing a radiated signal strength of the radio frequency signal; and control means for controlling the supply

power to the power amplifier in response to the radiated power indication.

The invention may provide an improved radio transmitter arrangement. For example, an improved fault condition performance may be achieved. The control of the radiated signal strength may be completely independent of the operation and design of the power amplifier. In particular, control of the output signal of a power amplifier may be achieved by other circuitry than the power amplifier itself. The invention may provide improved flexibility and may allow different output power control characteristics for the same design of the power amplifiers. Addition of control functionality for existing amplifier designs may be facilitated. In some embodiments, improved reliability may be achieved by an increased independence between the control functionality and the power amplifier. The control functionality may be implemented by low complexity circuitry.

The radiated signal strength may be the signal strength of the electromagnetic signal transmitted from the antenna. The radiated power indication may directly be a signal strength measurement associated with the radio frequency signal .

According to an optional feature of the invention, the control means is arranged to disrupt the power supply in response to the radiated power indication.

The control means may switch off or disconnect the power supplied to the power amplifier. The control means may for example disrupt the power supply by switching off the

power supplied to the power amplifier e.g. by activating a switch element in the power supply lines between power supply means and the power amplifier or by switching off the power supply means itself. This may e.g. allow an effective and low complexity prevention of excessive power transmission due to a malfunction.

According to an optional feature of the invention, the control means is arranged to limit the power supply in response to the radiated power indication.

The power supplied to the power amplifier may be limited rather than completely switched off. This may allow for a more gradual control and may allow the power amplifier to be restricted to operate within an acceptable operating interval .

According to an optional feature of the invention, the control means is arranged to limit the power supply by reducing a supply current limit.

This may provide an efficient and low complexity means of limiting the power supplied to the power amplifier. The approach may in particular be suitable for applications already using current limiting circuitry.

According to an optional feature of the invention, the control means is arranged to modify the supply power if the radiated power indication is outside a predetermined interval .

For example, the control means may only affect the power supplied to the power amplifier if the radiated power

indication falls below a given threshold value and/or if the radiated power indication increases above a given threshold value. This may allow for a highly efficient and reliable performance while allowing a low complexity implementation.

According to an optional feature of the invention, the sensor means is located remote from the power amplifier.

The sensor may be located optimally for the specific application without being limited by the location of the power amplifier. This approach may allow improved performance and may allow an increased independence between the power amplifier and the sensor. This may increase reliability as e.g. an action affecting the power amplifier need not affect the sensor means.

According to an optional feature of the invention, the sensor means is located proximal to the antenna.

This may allow high performance and may allow an efficient control of the transmitted signal and in particular may allow protection against excessive interference being transmitted in case of a power amplifier malfunction. The sensor means may be located sufficiently close to the antenna for the sensor means to predominantly sense the signal radiated from the antenna. The exact physical and electrical distance may depend on the specific application but may for example be less than 2 meters from the antenna. In some embodiments, the sensor means may be adjacent to the antenna, e.g. within a distance of less than 20 cm.

According to an optional feature of the invention, the sensor means comprises a resonating element for generating an electrical signal from the radiated radio frequency signal .

This may allow a practical implementation and/or efficient performance. The resonating element may generate an electrical signal from the electromagnetic field radiated by the antenna. The resonating element may have a selective frequency response.

According to an optional feature of the invention, the resonating element is a tuned pick-up coil.

This may allow a particularly advantages implementation offering efficient sensing and low complexity/cost. The tuned pick-up coil may generate an electrical signal from the electromagnetic field radiated by the antenna. The tuned pick-up coil may have a selective frequency response.

According to an optional feature of the invention, the resonating element is tuned to a frequency of the radio frequency signal .

The resonating element may specifically be tuned to attenuate signals outside a frequency interval comprising the frequency of the radio frequency signal. This may allow improved performance and may in particular allow increased accuracy of the sensing by attenuating sources of interference.

According to an optional feature of the invention, the radio transmitter arrangement further comprises means for powering the control means from the electrical signal.

This may allow an efficient implementation, increased reliability and/or may obviate or mitigate the requirement for an external power source. The electrical signal may for example be rectified to generate a power source for circuitry implementing the control means.

According to an optional feature of the invention, the radio transmitter arrangement further comprises back-up power supply means for the power amplifier and the control means is arranged to control a power supplied to the power amplifier by the back-up power supply means in response to the radiated power indication.

This may allow improved performance and specifically improved performance during malfunctions .

According to an optional feature of the invention, the back-up power supply means comprises a battery back-up power source and the control means is arranged to control the power supplied to the power amplifier by the back-up power supply by controlling a battery under-voltage control signal .

This may allow a low complexity and practical implementation.

According to a second aspect of the invention, there is provided a base station for a cellular communication

system comprising a radio transmitter arrangement as described above .

According to a third aspect of the invention, there is provided method of controlling a radio transmitter arrangement, the method comprising: a power amplifier generating a radio frequency signal to be radiated from an antenna; supplying a supply power to the power amplifier; generating a radiated power indication by sensing a radiated signal strength of the radio frequency signal; and controlling the supply power to the power amplifier in response to the radiated power indication.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment (s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates a radio transmitter arrangement for a base station of a cellular communication system in accordance with some embodiments of the invention;

FIG. 2 shows an example of a power supply control circuitry in accordance with some embodiments of the invention; and

FIG. 3 illustrates an example of a feedback controller in accordance with some embodiments of the invention.

Detailed Description of Some Embodiments of the Invention

The following description focuses on embodiments of the invention applicable to a base station of a cellular communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other transmitter arrangements and communication systems.

FIG. 1 illustrates a radio transmitter arrangement for a base station of a cellular communication system in accordance with some embodiments of the invention.

The base station comprises a transmit unit 101 which generates signals to be transmitted to mobile stations over the air interface of the cellular communication system. The transmit unit 101 is coupled to a power amplifier 103 which amplifies the signal to be transmitted to a suitable power level required to communicate with the mobile stations. The power amplifier is coupled to an antenna 105 which receives the signal and radiates this as a radio frequency electromagnetic signal .

It will be appreciated that the transmit unit 101, power amplifier 103 and antenna 105 are arranged such that the requirements and desires for the specific cellular communication system is met. For example, it will be appreciated that the exact power levels for transmissions to individual mobile stations are controlled by power control loops as are well known to the person skilled in

the art. Specifically, it will be appreciated that the transmit unit 201, the power amplifier 203 and the antenna 205 may be conventional elements as well known from conventional cellular base stations which will be well known to the person skilled in the art.

It will also be appreciated that the base station 100 typically will comprise many other functional elements required or desired for the operation in the specific cellular communication system. For example, the illustrated transmitter arrangement may be specific to a single sector of the cell served by the base station 100 and the base station 100 may comprise other transmitter arrangements, including separate power amplifiers and antennas, for other sectors.

The power amplifier 103 is coupled to a power supply controller 107 which is further coupled to a power source 109. The power amplifier 103 is supplied with power from the power source 109 through the power supply controller 107. The power source may for example be the public electricity supply or a local battery back-up or a combination thereof. The power source 109 and/or the power supply controller 107 further comprises means for conditioning the supplied power to meet the requirements of the power amplifier 103 including rectification, voltage stabilisation etc as will be well known to the person skilled in the art.

In the example of FIG. 1, the base station 100 further comprises a sensor element 111 which is capable of sensing a radiated signal from the antenna 105. The

sensor element 111 specifically generates an electrical signal from the electromagnetic field around the antenna.

In the preferred embodiment, the sensor element 111 is located proximal to the antenna such that the electromagnetic field at the sensor element 111 is predominantly caused by electromagnetic radiation from the antenna 105. Thus, the electrical signal generated by the sensor element 111 is predominantly caused by the transmissions of the radio frequency signal amplified by the power amplifier 103. The sensor thus generates a measurement which is an indication of the radiated power from the antenna of the signal amplified by the power amplifier 103.

The sensor element 111 is coupled to a feedback controller 113 which is further coupled to the power supply controller 107. The feedback controller 113 receives the electrical signal from the sensor element 111 and controls the power which is supplied to the power amplifier 103 through the power supply controller 107.

The arrangement may for example allow improved reliability and performance of the cellular communication system as a whole. In particular, a decreased sensitivity to fault conditions may be achieved.

Specifically, in conventional systems, a fault in a power amplifier, for example in an internal feedback power control loop, may result in the power amplifier outputting signals at the maximum available power level. This will result in excessive transmit levels and thus excessive interference being generated. Also, the power

amplifier will in such cases tend to operate in a nonlinear region thus causing high distortion resulting in interference in other frequency channels.

In the base station 100 of FIG. 1, such an excessive radiation of power will result in a radiated power indication of the electrical signal generated by the sensor element 111 clearly indicating that the radiated power is outside the expected values. Accordingly, the fault condition may be detected by the feedback controller 113 which may control the power supply controller 107 to reduce the power to the power amplifier 103 thereby limiting the output power and the radiated power from the antenna. This results in a limitation of the interference and may in many embodiments ensure that a fault may only affect the mobile stations served by this power amplifier 103 but will not prevent other mobile stations to be served from their respective transmit arrangements and/or base stations.

The sensor element 111 is in the example of FIG. 1 located close to the antenna 105 in order to increase the electromagnetic coupling between the antenna 105 and the sensor element 111. In some embodiments, the sensor element 111 may for example be placed at the base of the antenna or may be attached to the antenna structure. In other embodiments, the sensor element 111 may be placed at a slightly larger distance and may for example be 0-5 meters away from the antenna 105. The specific location of the sensor element 111 may be varied depending on the specific characteristics and requirements of the individual embodiment including consideration of

practical physical limitations, transmit power levels etc .

The sensor element 111 is in the example located adjacent to the antenna rather than to the power amplifier 103. Thus, in the system of FIG. 1, the sensor element 111 is located proximal to the antenna 105 but at a distance to the power amplifier 103. This may provide a further separation of the power amplifier 103 and the sensor element 111 thereby reducing the probability of a fault simultaneously affecting both the sensor element 111 and the power amplifier 103.

However, it will be appreciated that in other embodiments, the sensor element 111 may be located elsewhere. For example, the sensor element 111 may be located adjacent a transmission line from the power amplifier 103 to the antenna 105. In such an example, the sensor element 111 may generate a radiated power indication by sensing the radiated signal strength from the transmission line. This may be particularly advantageous for e.g. balanced transmission lines and may for example be useful for detection of the individual performance of one power amplifier 103 out of a plurality of power amplifiers using the same antenna structure for transmission .

The sensor element 111 may specifically be a resonating element such as a tuned pick-up coil or a waveguide antenna element. Thus, the sensor element 111 may in effect be a receive antenna picking up the radiated signal from the antenna 105. The sensor element 111 may be a relatively broadband sensor element 111 picking up

radiation in a wide frequency band. However, in many embodiments, the sensor element 111 may be tuned to a specific frequency and particularly to the frequency band supported by the power amplifier 103.

For example, a base station may support a GSM cell in both the 900 MHz frequency band and the 1800MHz frequency band allocated to GSM in most parts of Europe. The power amplifier 103 may specifically be a power amplifier 103 for the 900MHz band and the sensor element 111 may be a tuned pick-up coil which is tuned to 900 MHz. This may attenuate signals outside the frequencies supported by the power amplifier 103 and thereby provide for a more reliable measurement and estimate.

It will be appreciated that filtering may alternatively or additionally be performed by other functional blocks . For example, the feedback controller 113 may comprise a filter for filtering the signal received from the sensor element 111.

It will be appreciated that any suitable method of controlling the power supplied to the power amplifier 103 in response to the radiated power indication may be used.

In a low complexity embodiment, the power supplied to the power amplifier 103 may simply be switched off if the feedback controller 113 detects that the radiated power indication is indicative of performance outside an acceptable interval .

However, in other embodiments, a more gradual control may be used. Specifically, the power supplied to the power

amplifier 103 may be limited by the feedback controller 113. Thus, rather than switching the power to the power amplifier 103 completely off, this may be reduced to ensure that the power output is reduced.

As a specific example, must power supplies for power amplifiers comprise some form of supply current limitation circuitry. Thus, the power supply controller 107 will in some embodiments comprise a current limiter which ensures that the current drawn by the power amplifier 103 does not exceed a predetermined value. In the example embodiment, the current limit may not be fixed but may be controlled by the feedback controller 113. Specifically, if the radiated power indication is indicative of a transmit power outside the acceptable range, the feedback controller 113 may control the power supply controller 107 to reduce the current limitation value. In such an embodiment, the feedback controller 113 may thus cause the current (and thus power) supplied to the power amplifier 103 to be reduced until the transmitted power falls to an acceptable level.

The feedback controller 113 may implement any control algorithm or criteria for controlling the power supplied to the power amplifier 103 in response to the radiated power indication.

For example, the feedback controller 113 may simply detect if the electrical signal, corresponding to the measured signal strength, is below a first threshold and/or above a second threshold. If not, no action may be performed but otherwise the power supply to the power amplifier 103 may be switched off.

FIG. 2 shows an example of a power supply control circuitry in accordance with some embodiments of the invention.

In the example, the sensor element 111 is coupled to a rectifier 201 which is further coupled to a capacitor 203. A resistor 205 is coupled in parallel to the capacitor 203. The electrical signal received from the sensor element 111 is thus rectified by the rectifier 201 and smoothed by the RC circuit comprising the capacitor 203 and resistor 205. The rectifier 201, capacitor 203 and resistor 205 thus form a peak detector with a given time constant. The voltage level over the capacitor is thus a smoothed DC value corresponding to the measured signal strength of the signal transmitted from the antenna 105. The values of the capacitor 203 and the resistor 205 may be selected to provide the required dynamic performance of the control circuitry and specifically the peak detection.

The capacitor 203 is coupled to a comparator 207 which is furthermore provided with a reference voltage from a voltage generator 209. The comparison voltage of the voltage generator 209 is set to provide an upper threshold for the measured signal level above which a fault is deemed to have occurred. The output of the comparator 207 is fed to a latching relay 211. The relay 211 operates a switch 213 of a power supply line 215 feeding power to the power amplifier 103.

Thus, as long as the signal level measured by the sensor element 111 is sufficiently low for the comparison

voltage to exceed that of the capacitor voltage, the comparator 207 provides a low voltage resulting in the relay remaining in the position where the switch 213 is closed thus allowing power to be supplied to the power amplifier 103.

However, if a fault occurs in the power amplifier 103, for example in the internal power feedback control circuit, the output power level may increase to the maximum level causing excessive interference. This will result in the capacitor voltage increasing and when this exceeds the comparison voltage, the comparator 207 detects this and outputs an output voltage causing the relay 211 to switch position thereby causing the switch 213 to break the power supply to the power amplifier 103.

Thus, a simple external circuited is provided ensuring that a power amplifier fault does not unacceptably affect other cells or- sectors. The circuit is achieved completely externally to the power amplifier resulting in increased reliability. Furthermore, no design changes are required for the power amplifier and the circuit may for example easily be added to existing transmit arrangements and power amplifiers thereby providing improved performance of the communication system as whole.

In some embodiments the base station 100 may further comprise means for powering the control means from the electrical signal from the sensor element 111. An example of an embodiment with a self powering feedback controller 113 is illustrated in FIG. 3. In the example, the feedback controller 113 comprises a power supply circuit 301 which is coupled to the sensor element 111. The power

supply circuit 301 rectifies, smoothes and possible stabilizes the electrical signal from the sensor element 111. The resulting DC voltage may then be used to drive a comparator circuit such as for example the one illustrated in FIG. 2. The comparator circuit is in such embodiments preferably a simple circuit having low power consumption.

In some embodiments, the radio transmitter arrangement may comprise a main power supply as well as a back up power supply for the power amplifier 103. For example, the base station 100 may under normal operation draw power from the public electricity supply. However, in addition the base station 100 may comprise a back-up battery from which power is drawn if the main supply is interrupted. In such embodiments, the power supplied to the power amplifier 103 from the battery back-up may be controlled in response to the radiated power indication.

For example, the feedback controller 113 may control the supplied power to the power amplifier 103 independently of whether this power is supplied by the main power source or by a back-up power source. For example, the switch 213 of FIG. 2 may be inserted in a power line 215 which is common for both power sources.

In other embodiments, the feedback controller 113 may independently control the power supplied from the different power sources or may control only one of these.

As a specific example, many battery back-up circuits comprise an under-voltage protection which switches off the battery back-up when the battery voltage falls below

a given threshold value. This protects the batteries from discharging to a level which may cause damage to the batteries. Such under-voltage protection circuitry may specifically comprise an under-voltage detection circuit which generates a battery under-voltage control signal when the battery voltage drops below the threshold. In response to the activation of this under-voltage control signal, the under-voltage protection circuit switches off the power supply. In some embodiments, the feedback controller 113 may simply control the power supplied to the power amplifier by the back-up power supply by controlling the battery under-voltage control signal. Specifically, if a signal level above a given threshold is detected, an under-voltage control signal may be generated resulting in the back-up power supply being disconnected or switched off.

It will be appreciated that the described embodiments tend to provide one or more advantages including e.g. one or more of the following individually or in combination:

^■ Low complexity but reliable protection circuit.

^■ Possibility of self powering.

^■ Easy to make different versions of e.g. sensor elements matching different power levels and frequencies .

^■ Easy to implement with battery back-up systems.

^■ Independence of power amplifier design and main RF control system. ^■ Compatible with existing designs and power amplifiers without requiring redesign.

^■ Cheap to make, install and operate.

■ Easy to fit to existing systems and power amplifiers .

■ Improved performance of communication systems and in particular improved performance and reduced interference in fault conditions.

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and

functionally distributed between different units and processors .

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous . Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to

"a", "an", "first", "second" etc do not preclude a plurality.