GB2390262A - Method and apparatus for fault detection in a radio transceiver - Google Patents

Abstract

The invention relates to a system for fault detection for a radio transceiver 200. The apparatus comprises a circulator 211 connected between a transmitter power amplifier (209) and a transceiver output 213. A load port 219 of the circulator 211 is the output for a reflected signal from an antenna subsystem 213, 221, 223, 225 and is coupled to a receiver unit 231. The receiver unit 231 measures a load port parameter, preferably the power level, of the reflected signal on the load port 219. The fault detection system determines if a fault condition exists in response to the measured load port parameter. Specifically, a fault is determined to exist if the reflected power level is above a given threshold. The system is applicable to for example base stations for a cellular communication system.

Description

METHOD AND APPARATUS FOR FAULT DETECTION IN A RADIO

TRANSCEIVER

Field of the invention

The invention relates to a method and apparatus for fault detection for a radio transceiver, and in particular to a method and apparatus for fault 10 detection in a base station for a cellular communication system.

Background of the Invention

l 5 FIG. 1 illustrates the principle of a conventional cellular communication system 100 in accordance with prior art. A geographical region is divided

into a number of cells 101, 103, 105, 107 each of which is served by base station 109, 111, 113, 115. The base stations are interconnected by a fixed network which can communicate data between the base stations 101, 103, 20 105, 107. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated. In the example if FIG. 1, mobile station 117 is served by base station 109 over radio link 119, mobile station 121 is served by base station 111 over radio link 123 and so on.

As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. For example mobile station 125 is initially served by base station 113 over radio link 127. As it moves towards base station 115, it enters a region of 30 overlapping coverage of the two base stations 111 and 113 and within this overlap region it changes to be supported by base station 115 over radio link 129. As the mobile station 125 moves further into cell 107, it

2 1 ( continues to be supported by base station 115. This is known as a handover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typically 5 an entire country and comprises hundred or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as unlink, and communication from a base station to a mobile station is known as downlink. The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition the fixed network comprises gateway functions for interconnecting to external l 5 networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and I other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for I 20 routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. Currently the most ubiquitous cellular communication system is the 2nd Generation system known as the Global System for Mobile communication 25 (GSM). In GSM, the frequency band is divided into relatively narrow channels of 200 kHz, and each base station is allocated one or more of; these frequency channels. Each frequency channel is divided into eight separate time slots allowing up to eight mobile stations to use each frequency channel. This method of sharing the available resource is known 30 as Time Division Multiple Access (TDMA). Further description of the GSM

TDMA communication system can be found in 'The GSM System for

Mobile Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

Another principle of resource distribution is employed in the 2nd 5 generation system known as IS95, as well as in 3rd Generation systems, such as the Universal Mobile Telecommunication System (UMTS). These systems divide the frequency into one or few wide band channels, which for UMTS has a bandwidth of 5 MHz. Typically, one wide band frequency channel is used for all uplink in all cells, and a different wide band lO frequency channel is used for downlink. In this case, separation between cells is achieved through the use of spread spectrum techniques, where each cell is allocated a cell specific long user spreading code.

In these systems, a signal to be transmitted is multiplied by the spreading 15 code, which has a chip rate typically much larger than the data rate of the signal. Consequently, a narrowband signal is spread over the wideband frequency channel. In the receiver, the received signal is multiplied by the same spreading code thereby causing the original narrowband signal to be regenerated. However, signals from other cells having a different 20 spreading code are not despread by the multiplication in the receiver but remain wideband signals. The majority of the interference from these signals can consequently be removed by filtering of the despread narrowband signal, which can then be received.

25 Separation between mobile stations of the same cell is also achieved by use of spread spectrum techniques. The signal to be transmitted is multiplied by a shorter user specific code. Similarly, the receiver multiplies the received signal with the user specific code, thereby recovering the originally transmitted signal without Respreading signals from any of the 30 other mobile stations. Thus, the interference from all other mobile stations, whether in the same or a different cell, can effectively be reduced by filtering.

( A consequence of the spread spectrum techniques employed is that the amount of the interfering signals that fall within the bandwidth of the narrowband signal cannot be removed by filtering, and will thus reduce 5 the signal to interference ratio of the received signal. Consequently, it is of the outmost importance that the interference between mobile stations is optimised in order to maximise the capacity of the system. The reduction of the interference from an unwanted mobile station is equal to the ratio between the bandwidth of the spread signal and the narrowband despread lO signal, equivalent to the ratio between the chip rate and the symbol rate of the transmitted signal. This ratio is known as the processing gain. The technique is known as Code Division Multiple Access (CDMA), and further description of CDMA and specifically of the Wideband CDMA (WCDMA3

mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), 15 Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

In 3r generation (3G) communication systems, the communication network comprises a core network and a Radio Access Network (RAN).

The core network is operable to route data from one part of the RAN to 20 another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link being part of the air interface. The RAN comprises the base stations, which in UMTS are 25 known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.

The most critical interface of a cellular communication system is the air interface between mobile stations and base stations. Base stations (or 30 Node Bs) comprise the necessary circuitry for receiving and transmitting signals over the air interface to all mobile stations served within the cell.

( This circuitry includes data interfaces, demodulators, radio front ends, modulators, power amplifiers, antenna couplers and so on.

One of the most important performance parameters for a cellular 5 communication system is the number of dropped calls, including failed set up of new calls. If a base station malfunctions such that it is completely non-operational, all mobile stations within the cell will be affected and prevented from making or receiving any calls (except for a relatively small percentage that may handover to be served by neighbour cells). A 10 malfunctioning base station will therefore have significant impact on the overall reliability of the entire communication system. As most network operators aim at a down time of much less than 1 per cent, it is of the extreme importance to have very high reliability of the base stations of the system. Therefore, base stations are designed with emphasis on achieving a high mean time between failure (MTBF). Consequently, each unit of the base station is designed to have a high MTBF, but in addition the design minimises the possible single point failures by having redundant units 20 available that can take over from a faulty unit. Various designs include various levels and implementations of redundancy, but most base stations include some form of redundancy for the most critical units.

In order to achieve high reliability, most base stations include means for 25 performance monitoring, self testing and fault checking. These functions are further beneficial for most redundancy implementations, where a new unit may automatically replace a unit found to be faulty during a self test performed by the base station.

30 Self-testing and performance monitoring can be implemented in different ways. Typically, base stations include power detector circuitry which is conductively or inductively coupled to the output of the power amplifier.

This power detector measures the output of the power amplifier, and can therefore detect errors which cause the power output of the final power amplifier to drop.

5 However, a number of errors can occur which cannot be detected by a simple power detector coupled to the power amplifier. For example, envelope detection performed by the power detector is inefficient for signals that have a high dynamic range. As an example, an error resulting in a 3 dB drop in power is indistinguishable from a correctly transmitted 10 signal at half the power of the current signal. The problem is exacerbated as power detectors for high dynamic ranges tend to be inaccurate and therefore unable to distinguish errors that only result in small transmit power changes. Therefore the conventional approach of using power detectors tend to only be efficient for detecting errors resulting in no power 15 output from the power amplifier.

In addition, as the power detector is coupled to the power amplifier only faults in circuitry in or before the power amplifier can be detected. Thus faults developing after the power amplifier in the transmit path will not be 20 detected by the conventional approach. Hence, errors in the output isolator, the output connector, the cabling or the antenna are undetectable.

Some base stations further comprise means for self testing wherein the 25 signal at the output of the power amplifier is coupled back to the receiver.

This is known as RF (Radio Frequency) loop-back and enables the base station to detect some faults using the circuitry if the receiver. However, as for the power detector method, conventional loop-back implementations comprise the output of the power amplifier being coupled to the input of 30 the receiver through switches. As such the loop-back method is incapable of detecting any faults arising in the transmit path after the tap-off point,

( i.e. after the power amplifier, thus leaving a significant portion of the transmit path untested.

An Improved system for fault detection is therefore desired.

Summary of the Invention

The invention seeks to provide a system for fault detecting which 10 mitigates or alleviates one or more of the problems associated with the prior art.

Accordingly there is provided an apparatus for fault detection for a radio transceiver comprising: a circulator unit connected between a transmitter 15 power amplifier and a transceiver output; the circulator having a load port being an output for a reflected signal incident on an output port of the circulator circuit; a receiver unit coupled to the load port of the circulator unit and operable to measure a load port parameter of a load signal at the load port, the load signal being dependent on the reflected signal; and 20 fault determining means for determining if a fault condition exists in response to the measured load port parameter.

Thus the invention provides a number of advantages including the possibility of detecting faults in the external antenna system thereby 25 providing almost 100% test coverage. Having the receiver associated with the fault detection enables existing receiver circuitry and functionality to be used for fault detection. The invention thus provides accurate fault detection with little or no added complexity, and further enables a number of fault diagnostics being generated.

Preferably, the load port parameter is a load port power level of the load signal. The measurement of a power level of the reflected signal provides

for accurate measurements of the impedance matching throughout the antenna system, thereby enabling faults resulting in impedance mismatches being detected. Most faults in an antenna system will result in impedance mismatch.

According to a second feature of the invention, the apparatus further comprises means for determining a reflected power level of the reflected signal from the load port power level and the fault determining means Is operable to determine if a fault condition exists in response to the reflected 10 power level. This determination provides for an accurate evaluation of the reflected power thereby enabling accurate and reliable fault detection for the system.

According to a third feature of the invention, the apparatus further 15 comprises means for determining a ratio between the reflected power level and an output power level of the transceiver; and wherein the fault determining means is operable to determine if a fault condition exists in response to the ratio. This feature has the advantage of providing an accurate fault detection parameter which is independent of the 20 transmitted power.

According to fourth feature of the invention, the fault detection means are operable to determine that a fault condition exists if the load port power level is above a given threshold. This feature provides the advantage of a 25 simple yet effective criterion for determining if the measured power level corresponds to a fault detection or normal operation of the receiver.

According to a fifth and sixth feature of the invention, the fault condition is an improper termination of the transceiver output or a cable coupled to 30 the transceiver output.

According to a seventh feature of the invention, the fault determining means are operable to determine a distance to the fault in the transmission path from the load port power level. This feature provides the advantage of providing fault diagnostics that can assist in localizing 5 the fault thereby enabling focussed repair or replacement rather than replacement of the entire cable or antenna system.

According to an eight feature of the invention, the apparatus further comprises a variable attenuator coupled between the receiver unit and the 10 load port. This feature provides the advantage of extending the dynamic range for the measurements of the reflected signal made by the receiver unit, thereby allowing a much increased and dynamically adjustable fault detection range.

15 According to a ninth feature of the invention, the invention is part of a base station for a cellular communication system. Accordingly the entire base station system can be self testing with test coverage extending to the antenna system.

20 According to a tenth feature of the invention, the cellular communication system is a GSM (Global System for Mobile communications) communication system, and the fault determining means are operable to determine if a fault detection exists in a single time slot and to compensate the measured load port power level for a transmit power of the 25 base station in the single timeslot. Thus the fault detection system provided has the advantage of enabling accurate fault detection in a GSM environment wherein the transmit power varies significantly between time slots.

30 According to a second aspect of the invention, there is provided a method of fault detection for a radio transceiver having a circulator unit connected between a transmitter power amplifier and a transceiver output,

comprising the steps of: coupling a load port of the circulator unit to a receiver unit, said load port being an output for a reflected signal incident on an output port of the circulator circuit; measuring, by the receiver unit, a load port parameter of a load signal at the load port of the circulator 5 circuit, the load signal being dependent on the reflected signal; and determining if a fault condition exists in response to the measured load port parameter.

10 Brief Description of the Drawings

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which 15 FIG. 1 is an illustration of a cellular communication system in accordance with the prior art;

FIG. 2 is a simplified block diagram of a base station 200 comprising an embodiment of the invention; FIG. 3 is an illustration of an isolator 300 in accordance with prior art;

and FIG. 4 is an illustration of a circulator 400 suitable for an embodiment of 25 the invention.

Detailed Description of a Preferred Embodiment of the Invention

30 The following description focuses on a description of a preferred

embodiment in a base station of a cellular GSM communication system.

However, the invention is equally applicable to other transceivers

( including for example base stations for other cellular communication systems such as UMTS, or transceivers for wireless Local Area Networks (LAN).

5 FIG. 2 is a simplified block diagram of a base station 200 comprising an embodiment of the invention.

The base station 200 is connected to the fixed network 201 through a high rate data line 203 which is typically a 2 Mbps E1 line. The high data line lO 203 is a bidirectional line, which is connected to a network interface 205 in the base station 200. The network interface 205 receives data from the fixed network 201 to be transmitted, and communicates received data to the fixed network 201. In addition, the network interface 201 receives control data enabling network components of the fixed network 201 l 5 controlling the operation of the base station 200. Similarly measurement and status data is transmitted to the fixed network 201 from the base station 200 through the network interface 205. For clarity, control measurement and status circuits of the base station 200 are not included in FIG. 2.

The network interface 205 is connected to a transmitter 207. The transmitter is operable to perform all the necessary functions for transmitting data over the air interface. As such, the transmitter 207 amongst other function comprises data interface circuitry for converting 25 the data from the network data into a format suitable for the transmission. This includes separating transmitter control data from user data. The transmitter 207 also includes a processor for formatting the data to be transmitted over the air interface. This includes dividing the data into blocks of the right size for transmission in time slots, error correcting 30 coding, insertion of training sequences and other functions as is known in the art. The formatted data is GMSK (Gaussian Minimum Shift Keying) modulated in a modulator and the modulated signal is up- converted to the

( appropriate RF frequency by use of a local oscillator signal. The up converted signal is fed from the transmitter to a power amplifier 209, which amplifies the signal to the desired transmit power for transmission over the radio link to a mobile station or other suitable receiving 5 communication terminal.

It will be clear to the person skilled in the art, that the transmitter 207 will comprise further functions necessary for the proper operation in a cellular communication system, including transmit power control to circuitry, calibration circuitry, data multiplexers etc. For the sake of brevity and clarity of description these aspects are not described in further

detail. In a conventional base station, the output of the power amplifier is 5 connected to an antenna output of the base station through an isolator, which isolates the transmit circuitry from the antenna system.

Conventionally, the isolator is directly coupled to an antenna connector, which through an antenna cable is connected to an external antenna.

20 FIG. 3 is an illustration of an isolator 211 in accordance with prior art.

The isolator has an input port 301 and an output port 303. In addition the isolator comprises a magnetic coupler 305 between the input port 301 and the output port 303. The magnetic coupler 305 comprises magnetic 25 material (e.g. ferrite materials) arranged such that they create magnetic fields that allow the signal incident on the input port 301 to be transferred

to the output port 303. However, any signal incident on the output port 303 will not be transferred back to the input port 303 but rather will be coupled through a third port 307 to a termination load 309. The 30 termination load is typically a resistive load having an impedance matching the impedances of the input and output.

( An isolator 300 is frequently used in transmission equipment because it provides isolation of the transmitter circuitry from power signals reflected from any cabling, circuitry and/or antenna external to the transmitter.

5 In practical transmission systems, it is not possible to obtain ideal impedance matching resulting from perfect termination of the transmitter output. For example, typically there will be some antenna mix-match which will result in some transmit power being reflected from the antenna to the power amplifier output. In the absence of any isolator, the reflected lO power will be coupled directly back into the power amplifier output thereby causing distortion or possibly damaging the power amplifier.

Specifically, if a fault occurs, such as for example a break in the antenna cable or antenna connectors, the full power of the signal may be reflected back to the power amplifier output (minus any cable attenuation) . This l 5 may be destructive for the power amplifier.

However, by including an isolator 307 the reflected power will be transferred to the termination load, thereby safeguarding the power amplifier output and preventing distortion caused by the reflector.

In accordance with the preferred embodiment of the invention, the base station 200 rather than an isolator comprises a circulator 211 coupled between the transmitter 209 and an antenna connector 213.

25 FIG. 4 is an illustration of a circulator 400 suitable for an embodiment of the invention. A circulator is similar to an isolator in that it uses magnetic materials to control the flow of radio signals between ports of the circulator. The circulator 400 has three ports; in input port 215, an output port 217 and a load port 219. Similarly to the isolator any signal fed to the 30 input port 215 is coupled to the output port 217. However, in contrast to the isolator any signal incident on the output port is not coupled to a termination load, but is rather coupled to the load port 219, which is an

externally accessible port of the circulator. Further, any signal incident on the load port will be coupled back to the input port, and in order to avoid the incident signal of the output port reaching the input port through the load port, the load port must be properly terminated. Specifically, an 5 isolator can be considered as a form of circulator with a built-in internal termination load.

The base station 200 of FIG. 2 is through the antenna connector 213 connected to an antenna system comprising an antenna cable 221 and an lO external antenna 223.

The antenna system may have many other components than the cable 221 and 223, including beamformers, antenna switches, combiners and duplexers. In the described embodiment, the antenna system further 15 comprises a duplexer 225 operable to combine receive and transmit signals of the base station such that the same antenna can be used for both uplink and downlink transmissions. Duplexers are well known in the art and are in some embodiments integrated with the base station. However, in the described embodiment, the duplexer 225 is connected between the antenna 20 port 213 and the antenna 223. The duplexer 225 is through a cable 227 connected to a receive antenna connector 229 of a base station 200, and the receive antenna connector 229 is connected to the receiver unit 231 of the base station 200. Hence, the signals received by the antenna 223 are through the duplexer 225 fed to the receiver unit 231 which then 25 demodulates and decodes the received signals to provide the received data, as is well known in the art.

Typically, the antenna 223 is situated on top of an antenna tower and the antenna cable can consequently be very long and can in some 30 implementations extend to 100 meters or longer.

( It is therefore typically difficult to access most parts of the antenna system, and detection of faults in the antenna system are difficult and time consuming to detect. In addition detection of fault conditions in the antenna system require on site visits by qualified staff resulting in 5 significant cost and delay in detecting and locating errors. As the antenna system is external to the base station, faults in the antenna system are typically detected indirectly through reduced capacity of the system, e.g. by detection of an unreasonable large number of dropped calls in the cell supported by the base station, lack of handover attempts to the base 10 station etc. In order to improve the speed and accuracy of fault detection in the antenna system of a base station, it is known to use a spare measurement receiver. The spare measurement receiver is a separate measurement unit 15 which measures the power of the transmitted signals of the base station.

Hence, if a fault occurs in the antenna system resulting in the loss of all signal power Further in some situations the power level reflected by the antenna 20 system is measured in a power detector. This power detector is separate functional unit which may be integrated with the base station. However, as the power detector is additional circuitry added to the base station, and consequently adds to the cost, power consumption and complexity of the base station, they tend to be implemented by simple circuitry.

25 Consequently, the power detectors tend to be inaccurate and have limited measurement possibilities. Typically, it is simply measured if the reflected power is above a given level, in which case it is considered that a fault in the antenna system has occurred. However, as for separate measurement receivers a simple power level measurement is often not sufficient to 30 detect if a fault has occurred as the dynamic range of the transmitted signal may extend well beyond the variation in transmit signal Thus a given signal power may correspond to a low power signal being

( transmitted in a well functioning system or a high power signal being detected in a system having a faulty antenna system. The value of measurement receivers or reflected power detectors are thus only suited for detecting faults which result in no power at all being transmitted.

Hence, although use of separate measurement receivers or reflected power detectors improves the fault detection of the antenna system, the inherent trade off between cost and complexity on one hand and measurement reliability and functionality on the other hand, means that these fault lO detection methods are necessarily suboptimal.

In accordance with the preferred embodiment, the load port 219 of the circulator 211 is coupled to the base station receiver unit 231 through an (optional) variable attenuator 220.

In the preferred embodiment, signals from the load port 219 is coupled to the receive input of the receiver unit 231. In this embodiment, thereceiver unit comprises an input switch which can select to couple either the load signal or the receive signal from the receive antenna connector 229 to the 20 receive circuitry. The signal from the load port 219 is in this embodiment coupled to the receive circuitry of the receiver unit 231 as if it was a received signal from the antenna 223.

In this way, the receiver unit 231 is coupled to the load port 219 of the 25 circulator unit 211, and the receiver circuitry can be used to measure a load port parameter of the load signal at the load port 219. As the load port 219 is an output port for signal incident on the output port 217 of the isolator 211, the load signal is dependent on the reflected signal incident on an output port 217 of the circulator circuit 211. As the load port 30 parameter is dependent on the reflected signal, it can be used to determine if a fault condition exists.

( In the preferred embodiment, the load parameter is the power level on the load port 219 and thus the receiver unit 231 measures the power level of the reflected signal returned by the antenna system. In its simplest embodiment, a fault detection is detected if the reflected power level is 5 above a given threshold. However, in contrast to conventional power detection methods, the receiver unit 231, rather than an external power envelope detector, is used to detect the power level. Consequently, much of the circuitry is reused and thus accurate measurement circuitry can be employed without significantly increasing the complexity of the base lo station. Hence, significantly more complex circuitry can be used for the power detection resulting in much improved accuracy of the measurement.

Specifically, the receiver may exclusively (or almost exclusively) utilise existing circuitry and therefore provide the additional fault detection function without any or with very little added complexity.

In the preferred embodiment, the receiver unit 231 is operable to process the reflected signal as a normally received signal. This specifically includes down-converting, demodulating and decoding the signal. As part of this process the receiver unit 231 will generate the same measurement 20 reports as for a received communication signal including signal strength measurements (RSSI - Radio Signal Strength Indication) and error quality measurements. Thus, the highly complex but already required circuitry is used to generate accurate measurements of the reflected signal. Consequently, a fault condition is determined to exist in the output 25 connector or the antenna system, if the RSSI measurements show a reflected signal power level above a given threshold. Hence, the embodiment provides a fault detection system with improved accuracy and reduced complexity.

30 Further, in contrast to conventional base stations where the self test functionality is normally limited to the transmit path up to and including the low power amplifier, the described embodiment provides a self test

( system which can detect faults in the entire transmit path up to and - 5 including the antenna 223. Thus any mismatch, break or faulty connection in the antenna system will result in the reflected signal having a higher power level than in a non-faulty system. This increased power level will be 5 accurately detected and it will be reliably determined that a fault in the antenna system has occurred. Thus the test coverage of the base station system is significantly improved and approaches 100% coverage of the base station system including the antenna system. Further, the fault detection is accurate and is not limited to a binary detection of whether lO there is any reflected power. Rather the described embodiment is, for example, able detect degraded system performance due to a mismatch, resulting in a limited increase in the reflected power level.

In the preferred embodiment, the actual power level of the reflected signal l 5 is determined. The reflected signal power level is calculated as the received power level given by the receivers RSSI measurement plus the attenuation of the dynamic attenuator 220 plus the coupling loss between the output port and the load port of the circulator. The exact power level of the reflected power level in e.g. dBm can thus be determined.

However, for most faults a more reliable parameter for fault detection is not the absolute power lever of the reflected signal but rather the ratio of the power amplifier signal that is reflected. This is known as the return loss and is zero for an ideal antenna system with ideal impedance 25 matching throughout. However, in practical systems, ideal impedance matching is not achievable and dependent on how closely the right termination is to ideal the return loss may be smaller or larger.

In the preferred embodiment, the power level of the reflected signal is 30 determined from the receivers RSSI measurements as previously described. In addition because the fault detection system is an integrated part of the base station, information is available on the transmit power -.

( level. In an operational base station for a cellular communication system, the transmit power is closely controlled in order to minimize the interference caused to other cells. As part of the transmission process, the appropriate transmit power level is determined and by supplying this 5 information to the fault detection system, both the transmitted and reflected power levels are known and the return loss can be derived. Since the measurement process performed by the receiver unit is accurate the return loss can be determined with high accuracy. As the return loss is constant for varying transmit powers, the return loss estimation process is 10 not limited to periods of constant power but can be extended to any suitable duration. This further improves the accuracy of the measurement process, and thus the accuracy of the fault detection itself.

Thus in accordance with this embodiment, the fault detection system 15 measures the return loss accurately and determines, if it is below a given threshold level corresponding to an acceptable impedance matching in the system. However, if any part of the antenna system is improperly terminated, the return loss will increase and the fault detection system will detect this increased return loss. The impedance mismatch may occur 20 for example by a fault occurring in the duplexer As a specific example, water ingress in a connector may change the dielectric conditions of the connector resulting in an impedance mismatch in this part of the transmission path. The connector may still be able to pass on the transmit signal further down the transmit path but the impedance mismatch will 25 result in an increased level of the signal being reflected. The accuracy of the fault detection system allows this to be detected.

Due to the high accuracy of the measurement process, the fault detection is not limited to a simply binary determination of whether the reflected 30 power level is above a given threshold. Rather, the high accuracy can be used to perform sophisticated fault diagnostics as well as simple fault detection.

( In one embodiment, the return loss can be used to detect the distance to a disconnect or break in a transmission path. In one specific embodiment, the distance to a break in the cable can be determined. Antenna cables 5 typically have a nominal attenuation per meter. Although the attenuation may vary, it is normally relatively predictable and is known with some accuracy. If a break occurs in a cable, the signal will be reflected at the point of the break. The reflected signal reaching the base station will be identical to the transmitted signal minus the attenuation of the cable.

10 Hence, if the attenuation per meter of the cable is A dB, the reflected power level is PR PT 2X A

15 where x is the distance from the base station to the break in the cable, Pr is the transmit power in dBm and PR is the reflected power in dBm. Hence P. -P. RL

2A 2A where RL is the measured return loss in dB.

For further accuracy, the loss incurring in antenna system components in the transmission path can be determined and compensated for.

In another (or additionally in the same) specific embodiment, the distance 25 to a break is used to detect which unit is faulty. If the fault that occurs in a unit of the antenna system results in a very significant impedance mismatch, almost all of the signal will be reflected back to the base station. In the extreme, but frequently occurring situation of a total disconnect in a unit (for example broken connector), the entire signal is 30 reflected and thus the fault condition is from the fault detection system indistinguishable from a break in the cable at that spot. Thus, if the

algorithm used for detecting breaks in the cable indicates a break at roughly the cable distance to a unit of the antenna system, it Is determined that this unit is faulty. As typically the physical (and thus electrical) transmission path distance to each of the different units of the 5 antenna system (including the output connector 213, the duplexer 225 and the antenna 223) are very different, a faulty unit can be determined with high reliability.

In a different embodiment, the fault detection system is operable to to relatively accurately measure the return loss from a faulty unit. In many implementations the only unit of the antenna system that is situated remotely from the base station is the antenna which typically is at the top of a radio mast. Hence, other units of the base station system including the output connector and the duplexer can easily be manually checked. If l5 they are found not to be faulty, it can be assumed that a degraded return loss is due to faults in the antenna. If the cable attenuation to the antenna is known to be B dB, the return loss for the antenna can be determined as: RL = Pi - PR - 2 B where RL is the return loss in dB, PT is the transmit power in dBm and PR is the reflected power in dBm In one embodiment of the invention, the base station is a GSM base 25 station. In GSM the transmit power may vary substantially from time slot to time slot depending on how many mobile stations are supported in a given time slot and on the radio conditions for each of these time slots. In accordance with this embodiment, the fault detection system determines If a fault detection exists in a single time slot by compensating the measured 30 load port power level for a transmit power of the base station in the single timeslot. Thus in this embodiment the fault detection system is able to

( accurately determine e.g. the return loss and therefrom detect if a fault condition occurs, despite the very high variation in transmit power.

In the preferred embodiment, the load port of the circulator is connected to 5 the receiver unit through a variable attenuator 220. The reflected signal can vary substantially in power level and will often be too high for the receiver. Hence, an attenuator is included between the circulator and the receiver thereby reducing the level of the reflected signal to a level which is appropriate for the receiver unit. Specifically, in the preferred 10 embodiment the attenuator is variable and is adjusted in response to the expected reflected power level. This effectively increases the dynamic range of the receiver. Specifically, the attenuation can be adjusted in response to the transmitted power level, such that for higher transmitted power levels, the introduced attenuation is increased.

In one embodiment, more advanced use is made of the receiver unit functionality. In this embodiment, the receiver completely demodulates the reflected signal to generate a base band signal. From the base band signal, a number of different parameters can be determined and used for 20 evaluating and testing the base station performance. In accordance with one embodiment, the demodulated base band signal is compared to an ideal expected signal representation, and if the deviation is too large a fault condition is determined to exist. The ideal expected signal representation is derived from knowledge of the data which is transmitted 25 and knowledge of the modulation format, filtering etc used in the transmitter. In this way, the fault detection system is able to monitor a range of performance parameters thereby improving the fault detection.

Specifically, uncharacteristic levels of noise, non linearities and drift in the transmitter can be detected.

In order for the receiver to be able to receive and specifically demodulate the reflected signal, it must be able to retune to the transmit frequency of

( the transmitter. In most cellular systems the transmit and receive frequency bands for the base stations are different. In GSM the transmit frequencies are 45 MHz higher than the receive frequencies.

Consequently, the local oscillator, input filters etc of the receiver unit 5 must be modified, such that the receiver is operable to cover this extended area. However, the additional circuitry represents a minimal complexity increase, which is significantly less than that associated with the fault detection systems of the prior art. For example, this can be implemented

using a mixing circuit which translates the transmit band to the receive 10 band using a 45 or 95 MHz fix local oscillator, as required.

Further, the ability of the receiver to process transmit frequencies enables an additional range of features to be easily implemented in the base station. Thus in accordance with one embodiment of the invention, the 15 receiver further evaluates the received signal in the transmit frequency band during times when the transmitter of the base station is inactive. In other words the base station is operable to scan the transmit band during periods of non-transmission. This can be used to determine the downlink interference conditions, which are useful for cell planning and even auto 20 configuration of the base station.

In the previous description, a preferred embodiment and a number of

alternative embodiments have been described. However, it is within the contemplation of the invention that these and other embodiments are not 25 generally mutually exclusive and that any suitable combination or permutation of the various embodiments may be employed at the same time. It will thus be clear that the invention tends to provide a number of 30 advantages, including the following: It provides a fault detection system operable to detect faults throughout the base station transmitter path including the antenna

( system. Consequently a fault detection system as described can provide almost 100% test coverage of a base station system.

It provides a fault detection system with significantly improved accuracy and thus reliability.

5 It provides the possibility of additional parameter measurements enabling a more exhaustive fault detection.

It provides the possibility of improved fault diagnostics.

It provides an efficient low complexity fault detection system.

Claims (22)

( Claims

1. An apparatus for fault detection for a radio transceiver comprising: 5 a circulator unit connected between a transmitter power amplifier and a transceiver output; the circulator having a load port being an output for a reflected signal incident on an output port of the circulator circuit; a receiver unit coupled to the load port of the circulator unit and operable to measure a load port parameter of a load signal at the load 10 port, the load signal being dependent on the reflected signal; and fault determining means for determining if a fault condition exists in response to the measured load port parameter.

2. An apparatus as claimed in claim 1 wherein the load port 15 parameter is a load port power level of the load signal.

3. An apparatus as claimed in claim 2 further comprising means for determining a reflected power level of the reflected signal from the load port power level, and wherein the fault determining means is operable to 20 determine a fault condition in response to the reflected power level.

4. An apparatus as claimed in claim 3 further comprising means for determining a ratio between the reflected power level and an output power level of the transceiver; and wherein the fault determining means is 25 operable to determine if a fault condition exists in response to the ratio.

5. An apparatus as claimed in claim 2 wherein the fault detection means are operable to determine that a fault condition exists if the load port power level is above a given threshold.

G. An apparatus as claimed in any of the previous claims wherein the fault condition is an improper termination of the transceiver output.

7. An apparatus as claimed in any of the previous claims wherein the fault condition is a fault in a cable coupled to the transceiver output.

5

8. An apparatus as claimed in any previous claim 2, wherein the fault determining means are operable to determine a distance to a fault in a transmission path from the load port power level.

9. An apparatus as claimed in any of the previous claims further 10 comprising a variable attenuator coupled between the receiver unit and the load port.

10. A base station for a cellular communication system comprising the apparatus of any of the previous claims.

11. A base station for a cellular communication system as claimed in claim 10 as dependent on claim 2 wherein the cellular communication system is a GSM (Global System for Mobile communications) communication system, and the fault determining means are operable to 20 determine if a fault condition exists in a single time slot and to compensate the measured load port power level for a transmit power of the base station in the single timeslot.

12. A method of fault detection for a radio transceiver having a 25 circulator unit connected between a transmitter power amplifier and a transceiver output, comprising the steps of: coupling a load port of the circulator unit to a receiver unit, said load port being an output for a reflected signal incident on an output port of the circulator circuit; 30 measuring, by the receiver unit, a load port parameter of a load signal at the load port of the circulator circuit, the load signal being dependent on the reflected signal; and

( determining if a fault condition exists in response to the measured load port parameter.

13. A method of fault detection as claimed in claim 12 wherein the load 5 port parameter is a load port power level of the load signal.

14. A method of fault detection as claimed in claim 13 further comprising the step of determining a reflected power level of the reflected signal from the load port power level and wherein the step of determining 10 a fault condition exists determines a fault condition in response to the reflected power level.

15. A method of fault detection as claimed in claim 14 further comprising the step of determining a ratio between the reflected power 15 level and an output power level of the transceiver; and wherein the step of determining if a fault condition exists determines if a fault condition exists in response to the ratio.

16. A method of fault detection as claimed in claim 13 wherein the step 20 of determining if a fault condition exists determines that a fault condition exists if the load port power level is above a given threshold.

17. A method of fault detection as claimed in any of the previous claims 12 to 16 wherein the fault condition is an improper termination of the 25 transceiver output.

18. A method of fault detection as claimed in any of the previous claims 11 to 17 wherein the fault condition is a fault in a cable coupled to the transceiver output.

(

19. A method of fault detection as claimed in any of the previous claims 12 to 18, further comprising the step of determining a distance to a fault in a transmission path from the load port power level.

5

20. A method of fault detection as claimed in any of the previous claims 12 to 19 further comprising the step of coupling a variable attenuator between the receiver unit and the load port.

21. A method of fault detection substantially as hereinabove described lO with reference to or as shown in FIG. 2 of the drawings.

22. An apparatus for fault detection substantially as hereinabove described with reference to or as shown in FIG. 2 of the drawings.