Monitoring of an antenna system

The present invention relates to a method and apparatus for monitoring an antenna system comprising at least two antenna branches providing antenna diversity. At least a first signal branch measure and a second signal branch measure are repeatedly generated in response to a first signal branch and a second signal branch received by a respective one of the at least two antenna branches, thus generating a first plurality of second signal branch measure values in a manner so that the first and second pluralities reflect the quality of the first and second signal branches, respectively, at different points in time. The first and second pluralities are analysed in order to distinguish any differences in the performance of the first and second antenna branches.

FIELD OF THE INVENTION

The present invention relates to the field of radio communication, and in particular to the monitoring of antenna systems providing antenna diversity.

BACKGROUND

An antenna system is a most vital part in a mobile radio communications system. Errors in the antenna system give rise to a reduced coverage area and a decrease in the information throughput. Hence, it is important to monitor the state of the antenna system.

One method of monitoring the state of an antenna system is to perform Voltage Standing Wave Ratio (VSWR) measurements, where a transmitted signal and the reflected part of the transmitted signal are determined and compared in order to determine the state of the antenna system. However, this method can only be used for the monitoring of antenna systems in radio base stations which comprise both a transmitter and a receiver.

Another method of monitoring antenna systems is to measure the DC feeder resistance. However, since the feeder resistance varies as equipment, such as filters, amplifiers etc., are introduced to or removed from, the antenna system, this method requires knowledge of which equipment is presently connected to the antenna system.

The above mentioned methods do not explicitly show whether the receiver is affected, but rather that the impedance of the antenna system has changed. A further problem with the above mentioned methods is that manual configuration of the measurement equipment is generally required, since there exist many different types of antenna configurations and transceiver/receiver units.

SUMMARY

A problem to which the present invention relates is how to improve the performance of an antenna system.

This problem is addressed by a method of evaluating an antenna system which are connected to a radio device, where the antenna system comprises at least two antenna branches. The method comprises receiving a radio signal as a first signal branch in a first antenna branch: receiving the radio signal as a second signal branch in a second antenna branch. The method further comprises repeatedly generating a first signal branch measure of the first signal branch and a second signal branch measure of the second signal branch, thus generating a first plurality of first signal branch measure values and a second plurality of second signal branch measure values in a manner so that the first and second pluralities reflect the quality of the first and second signal branches, respectively, at different points in lime; and analysing the first and second pluralities in order to distinguish any differences in the operation of the first and second antenna branches.

The problem is further addresses by a radio device adapted to be connected to at least a first and a second antenna branch on which a first and a second signal branch, respectively, may be received. The radio device is arranged to repeatedly generate a first signal branch measure of the first signal branch and a second signal branch measure of the second signal branch: thus generating a first plurality of first signal branch measure values and a second plurality of second signal branch measure values in a manner so that the first and second pluralities reflect the quality of the first and second signal branches, respectively, at different points in time. The radio device further comprises a signal branch comparator having an input arranged to receive signals indicative of the first and second signal branch measures, and being adapted to analyse the first and second pluralities in order to distinguish any differences in the operation of the first and second antenna branches.

By the inventive method and radio device is achieved that any differences in performance of the antenna branches may be detected. Such differences give an indication that an antenna branch may experience a fault. By ensuring that faults in the antenna system can be detected and attended to, the performance of the radio device will improve, and the overall performance of a radio system of which the radio device forms a part will be improved.

The invention can advantageously be applied to a radio device operating according to code division multiple access, where at least one scrambling code and/or channelisation code are applied to the first and second signal branches in order to de-scramble and/or de-spread the first and second signal branches, respectively. In this context, the first and second signal branch measures can advantageously be generated in response to the de-scrambled and/or de-spread first signal branch and the de-scrambled and/or de-spread second signal branch, respectively. The de-scrambled/de-spread signal branches show a higher signal-to-interference-and-noise-ratio than the signal branches before de-scrambling/de-spreading, and a difference in the quality of the signal branches can therefore more easily be discerned after de-scrambling/de-spreading than before.

In case of the radio device comprising a rake receiver, the first and second branch measures can advantageously be obtained in response to an output from the rake receiver. In one embodiment of this aspect of the invention, samples are obtained of the first and second signal branches, thus forming a first and second stream of samples obtained from the first signal branch and the second signal branch, respectively. The rake receiver has a set of rake fingers operative to select a set of samples from the first and second streams of samples: and the first signal branch measure is obtained in response to the number of samples selected by the rake receiver from the first stream of samples during a time period; and the second signal branch is obtained in response to the number of samples selected by the rake receiver from the second stream of samples during the time period.

The first and second signal branch measures may alternatively be obtained as signal-to-noise-and-interference-ratio values obtained on the first and second signal branches, respectively. When the radio device comprises a rake receiver, the signal-to-noise-and-interference-ratio values of an antenna branch may advantageously be obtained from signals processed by the rake receiver.

The signal branch measure of a signal branch may be obtained from signals received on any logical channel, such as a random access channel or a traffic channel. The random access channels are often processed on the same circuit board for all antenna branches, and the process of identifying any erroneous antenna branch could easily be centralised if the signal branch measures are performed on the random access channel signals, thus rendering the construction of the signal branch comparator simple.

The comparison of the signal branch measures can be performed on a per-measurement basis, or performed on an average signal branch measure calculated as the average value of a plurality of signal branch values obtained on a signal branch. By performing a per-measurement comparison, the differences in signal-to-noise-and-interference ratio values between signals originating from different radio transmitters experiencing different radio conditions can be compensated for.

The problem is further addressed by a computer program product comprising computer program code operable to execute the inventive method when run on computer means.

DETAILED DESCRIPTION

FIG. 1is a schematic illustration of a radio base station100comprising an antenna system105, and a signal processing part110. In the signal processing part110, radio and base band processing of a signal received by the antenna system105is performed. The processing could for example include demodulation of the signal so that a digital version of the signal is obtained, de-coding, de-interleaving, de-multiplexing and rake receiving. In a mobile radio system operating according to spread spectrum multiple access/code division multiple access (CDMA), the processing performed in the signal processing part110of the radio base station100includes de-scrambling of the signal by applying the appropriate scrambling codes to the digital signal in order to retrieve the signals transmitted by different transmitters (typically, different mobile stations). The signal processing part110of a radio base station operating according to a spread spectrum multiple access/code division multiple access standard typically further applies a channelisation code to the retrieved transmitted signal in order to de-spread the signal.

The antenna system105is connected to the signal processing part110via a feeder115. The radio base station100would typically be connected, via a transport part (not shown), to a base station controller (BSC) or a radio network controller (RNC) of a mobile radio communications system, where the radio base station100would provide radio links for communication with mobile stations roaming in the mobile radio communications system.

Antenna system105can comprise antenna(s) for signal reception, and/or antenna(s) for signal transmission. Furthermore, in most mobile radio communications systems, antenna diversity is employed. An antenna employing antenna diversity comprises two (or more) antenna elements so that a signal can be received and/or transmitted by two (or more) different antenna elements at the same time. Such different antenna elements could e.g. comprise different physical antennas placed at different points in space—so called space diversity—or different antenna elements for receiving signal elements of different polarity—so called polarisation diversity. By introducing antenna diversity, the negative effects of fading are reduced, since the fading conditions are not the same at different points in space (space diversity) or for different polarisations of the signal (polarisation diversity). By receiving the signal at two or more different antenna elements for which the fading conditions are basically uncorrelated, the probability of receiving a version of the signal having sufficient signal strength is greatly increased. The different antenna elements providing antenna diversity to an antenna is often referred to as different antenna branches. InFIG. 1, the antenna system105comprises two antenna branches120A and120B, respectively connected to the signal processing part110via feeders115A and115B. The antenna branches120could be realized by space diversity, as inFIG. 1, by polarisation diversity, a combination of the two, or any other type of antenna diversity.

An antenna system105employing antenna diversity on the receiving signals thus receives different versions of a received signal on the different antenna branches120. According to the invention, the different versions of the received signals can be compared in order to detect any errors that have occurred on one of the antenna branches. In the following, the different versions of a signal that have been received on different antenna branches120will be referred to as different signal branches.

By acquiring comparison results from the different received signal branches at several points in time, an indication can be obtained as to whether or not the different antenna branches120operate in the same way. If one antenna branch120generally gives rise to a weaker signal branch than the other(s), this is a good indication that the operation of this antenna branch120may be faulty. Advantageously, the signal branches of many received signals are analysed in order to obtain good statistical certainty of the result.

According to the invention, a signal branch measure, indicating the quality of the signal received on a signal branch, is obtained at several points in time for the different antenna branches120that are to be compared. When comparing the signal branch measures for more than two different branches, the comparison could preferably be performed on a two-and-two basis.

An embodiment of the inventive antenna system evaluation process is schematically illustrated inFIG. 2. The antenna system evaluation process ofFIG. 2could be performed continuously, at regular intervals, or on demand.

A signal is received at different antenna branches120. In step205, a value of a signal branch measure, indicative of the signal quality of the signal branch received by an antenna branch120, is collected for the antenna branches120that are to be compared. In step207, it is checked whether enough values of the signal branch measures have been collected. This check could for example involve checking whether a timer, set before the first signal branch measure measurement was taken, has reached a pre-determined time, or whether a counter, which is incremented each time a signal branch measure measurement is taken, has reached a pre-determined number. Such pre-determined lime period could typically be in the order of minutes (e.g. 2 minutes), and a typical pre-determined number of samples could for example be in the order of 1000. If enough samples have not been collected, then step205is re-entered and another signal branch measure is collected from each of the antenna branches120that are part of the comparison. In step215, the values of the signal branch measures are compared for different signal branches, for example by calculating the ratio or the difference between the average values of the signal branch measures of the antenna branches120, in order to determine whether the quality of the different branches differ significantly. Different signal branch measures, as well as different methods of comparing the signal branch measures, are further discussed below.

In the embodiment of the invention illustrated byFIG. 2, the step of collecting of signal branch measure values of steps205and207is completed before any evaluation and comparison of the signal branch measure results take place in step215. Needless to say, the comparison of the collected signal branch measure values could start while new signal branch measure values are still being collected.

When more than two antenna branches120are present, several comparisons of the signal branch measure values could advantageously be made on a two-and-two basis.

If it is found in step215that the signal quality of any of the antenna branches120differ significantly from the other branches, step220is entered wherein appropriate action is taken, such as e.g. the issuing of an alarm or the start-up of further investigation processes. Step230is then entered, in which an instruction to start the antenna system evaluation process is awaited. If the comparison in step220does not indicate any erroneous performance of any of the antenna branches120, step230is entered without first entering step225.

Rather than measuring the signal branch measure of the first and second antenna branches, the signal branch measure of the combined signal from two antenna branches120could be measured/estimated in step205ofFIG. 2, as well as the signal branch measure of one of the signal branches305. The signal branch measure value of the second signal branch305for which a signal branch measure value is desired can then easily be derived as the difference between the signal branch measure value of the combined signal and the signal branch measure value of the first signal branch.

Branch information of the received signal is normally known within the radio part of the signal processing part110, wherein demodulation of the received signal takes place, and the base band part, wherein further processing of the de-modulated signal takes place. The comparison can thus be made in the radio part of the signal processing part110as well as in the base band part of the signal processing part110. In a mobile radio communication system operating according to the Wideband Code Division Multiple Access (WCDMA) standard, the base band part of the signal processing part110is generally implemented in the “Node B uplink user data processing for Uu-interface (User equipment-network layer part 1)”.

When the comparison is made in the radio part of signal processing part110, a useful signal branch measure to be used in the comparison could for example be the received signal strength as measured in the radio part (which can for example be measured as “RXLEV” in a system based on the GSM standard, and as “RSCP” in a system based on the WCDMA standard. The received signal strength is particularly useful as a signal branch measure in the comparison between different signal branches in situations where the signal-to-noise-and-interference ratio is comparatively high, so that the received signal strength effects of interference or noise on the received signal strength is low. This is typically the case in a mobile radio communication system based on the GSM standard.

In a system based on spread spectrum multiple access/code division multiple access (CDMA), such as a mobile radio communications system based on the WCDMA standard, the signal-to-noise-and-interference levels are typically comparatively low, and it is often difficult to discern any difference in the signal-to-noise level of the different antenna branches120. However, the present invention acknowledges that after the received signal has been de-scrambled by applying the appropriate scrambling code and/or has been de-spread by use of appropriate channelisation codes, the signal-to-noise-and-interference-ratio is increased. By performing the comparison between antenna branches120based on signal branch measures obtained after de-scrambling and/or de-spreading of the received signal has been performed, the reliability of the method will be improved.

When an antenna branch120receives a signal branch305carrying information relating to several radio links, measurements could be made on all of these links. The signal branch measure values could advantageously be collected at the traffic channel of active radio links, for example as specified in 3GPP TS 25.215, Physical layer measurements: (FDD). Measurements could for example advantageously be performed on the pilot bits of the Dedicated Physical Control Channel (DPCCH). The pilot bits include a well defined pattern of several bits, thus simplifying the measurement procedure. Alternatively, the signal branch measure may be collected on the random access channel (RACH). In particular, the signal branch measure may advantageously be collected on the pre-amble part of the random access channel.

Base Station Including a Rake Receiver

In one aspect of the invention in which the signal processing part110of the radio base station100comprises a rake receiver, the signal branch measures can be obtained from an output signal from the rake receiver.

A rake receiver of a radio base station100typically operates in the base band to compensate for the fact that a received signal has been transmitted over a multi-path channel and has therefore been smeared due to a difference in path length of the different paths. In the rake receiver, signal components that represent the same data but have experienced different path lengths (and are therefore received by the antenna system105at different points in time) are synchronised.

A rake receiver of L rake fingers operates to periodically select the L strongest samples of a number of samples having been sampled from the received signal within a certain time period Trake, and then processes these samples individually prior to adding them into an output signal (often by first multiplying them with a weight factor).

The concept of a rake receiver is schematically illustrated inFIG. 3. A signal300is illustrated to comprise three different signal components which have traveled along three different signal paths. The signal is received by a radio base station100comprising two antenna branches120A and120B, so that a signal branch305A of signal300is received by the antenna branch120A, and a signal branch305B is received by the antenna branch120A. For illustration purposes, the antenna branches120ofFIG. 3are illustrated to be space diversity antenna branches, however, any antenna diversity method could be employed.

The radio base station100ofFIG. 3further comprises two samplers310A and310B, connected to the antenna branches120A and120B, respectively. (The two samplers310A and310B could be implemented in the same physical processor, or in different physical processors. Furthermore, a sampler310could be implemented in one or more physical processors).

The signal branches305A and305B are fed from the antenna branches120A and120B to the samplers310A and310B, where, inter alia, the samplers310operate to sample the signal branches305at regular intervals. The sampling is illustrated inFIG. 3as a number of signal samples315, the exemplary signal samples315which have been sampled by sampler310A inFIG. 1being indicated as samples A1-A13, and the signal samples315which have been sampled by sampler310B being indicated as samples B1-B13. The height of a signal sample315corresponds to the signal strength of the received signal branch305.

A rake receiver320including L different rake fingers325is connected to the samplers310A and310B ofFIG. 3, where L=3 in the example ofFIG. 3. The rake receiver320operates to select the L strongest samples during a time interval Trake, and process these L samples in the L rake fingers325. In the example given byFIG. 3, the L (=3) strongest samples sampled during the time interval Trake, (which in the illustration corresponds to 12 samples), are all derived from the signal branch305A received by antenna branch120A, and these 3 strongest samples, i.e. samples A1, A6and A10, are processed in the 3 rake fingers325of the example.

A reason for a first antenna branch120to receive a weaker signal branch305than the other signal branch(es) of a radio base station100could be that the signal branch305received by the first antenna branch120has experienced more fading than the other signal branches. However, another reason could be that there is something wrong with the first antenna branch120.

Number of Samples Selected by Rake Fingers Used as Signal Branch Measure

In one embodiment of the invention, the number of samples selected by the rake receiver from a signal branch305, or a measure derived from this number, is used as the signal branch measure of the signal branch305. In other words, the number of samples selected by the rake fingers320from a particular antenna branch120is compared to the number of samples selected from the other antenna branch(es)120over a period of time. If the rake receiver is arranged to select the L strongest samples, regardless of which antenna branch120the samples originate from, a comparison between the different antenna branches120of an antenna system105could be based on the number of rake fingers325that select a sample from each of the signal branches within a time interval Trake. By performing this comparison, an indication is given as to whether the antenna branches120operate equally well, or whether there is an erroneous behaviour of one or more antenna branches120. If the rake receiver320only has one rake finger325, the percentage of samples selected by the rake finger325from one signal branch305could be used as a signal branch measure.

SIR as a Signal Branch Measure

In a mobile communications system operating according to a CDMA standard, the signal branch measure can advantageously be calculated as a measure of the signal-to-noise-and-interference-ratio (SIR) on the signal after de-scrambling of the signal and/or after de-spreading of the signal has been performed. As discussed above, SIR of the signal is higher after de-scrambling and de-spreading, and the probability of discerning any difference in SIR between the different antenna branches120increases as compared to performing the SIR calculation on the signal prior to de-scrambling and/or de-spreading. Thus, less measurement values are required than if the SIR-comparison were to be made in the radio part of the signal processing part110. This means, inter alia, that statistically accurate results can be obtained also when the traffic is low. i.e. when there are few active mobile stations transmitting to the radio base station100.

Many implementations of a radio base station100employ a mechanism for determining a target value for the SIR value of a radio link. In such implementations, the target SIR value for different radio links need not be the same, and the target SIR value for a radio link may vary with time. Hence, to simply use the SIR measurement values as the signal branch measure may not give a correct result, since the target SIR value may not have been the same at the different measurement occasions or different radio links. In order to account for this, one could for example ensure that only SIR measurement values that have the same SIR target value are included in the comparison. Alternatively, a relation between the SIR measurement value and the corresponding SIR target value, referred to as SIRerror, could be used as the signal branch measure. SIRerrorcould be expressed as:
SIRerror=SIRmeasured−SIRtarget(1).
where SIRtargetand SIRmeasuredhave been measured/averaged over the same period of time. SIRerrorcould alternatively be expressed as a ratio between SIRmeasuredand SIRtarget. In the following, when the use of SIR as a signal branch measure is discussed, this should be construed to include the use of SIRerror. as well as a measure of SIR, as a signal branch measure.

SIR can advantageously be used as a signal branch measure in a radio base station100comprising a rake receiver320, and the signal branch measure of a signal branch305can be calculated as the base band signal-to-interference-and-noise-ratio of the branch at the rake receiver, SIRrake. In a radio base station100operating according the WCDMA standard, the signal received by the rake receiver320has normally been de-scrambled and de-spreaded by the Application of appropriate channelisation codes prior to arriving at the rake receiver320. SIRrakecan be expressed as:

SIRrake=∑k=1m⁢Sknk,(2)
where Skand nkare the signal strength and the estimated noise-and-interference level, respectively, of the part of the signal branch processed by the kthrake finger325during a time interval T, and m is the number of rake fingers325that have selected samples315from signal branch305during the time interval T (m≧1). Alternatively, a SIR measurement can be obtained at the output of the rake receiver320, where the outputs of the different rake fingers325has been combined into one signal.

In a radio base station100operating according to the WCDMA standard, a measurement of SIR can advantageously be obtained on the Dedicated Physical Control Channel (DPCCH) by calculating the ratio between the Received Signal Code Power (RSCP) and the Interference Signal Code Power (ISCP), and multiplying the ratio with the Spreading Factor (SP). The RSCP is the unbiased measurement of the received power on one code, and the ISCP is the interference on the received signal (see e.g. the technical specification 3GPP TS 25.215).

There may be reasons to why a SIR-value is occasionally not obtained for an antenna branch120. Thus, in order to increase the comparability of the SIR-values of the different antenna branches120, one could ensure that for a SIR-measurement of a first signal branch305included in the comparison of step215ofFIG. 2, there exists a corresponding SIR-measurement of the signal branch305B to which the first signal branch305is to be compared. Furthermore, any SIR-measurements which have been collected when the transmitting mobile station was not synchronised with the radio base station100can advantageously be excluded from the comparison of step215.

As mentioned above, the signal branch measure values could be collected at the traffic channels, or on the random access channels (RACH). The signal processing part110of the radio base station100typically measures the power of the signal branches305of the preambles of the RACH at regular intervals (e.g. even half chip), and/or the power of the combination of the different signal branches305. Interference & noise estimates of the pre-ambles can also be obtained. An appropriate signal branch measure could then be the SIRrakeof the signal branch305as given in equation (2). An advantage of performing the signal branch measure measurements on the random access channels is that the random access channels are often processed on the same circuit board for all antenna branches120, and the process of identifying any erroneous antenna branch120could easily be centralised. To improve the error identification process, one could preferably choose to base the signal branch measure measurements only on the random access preambles that result in an acknowledgment (typically in Layer 1), since the acknowledgement indicates that the signal quality lies above a certain threshold.

Comparison Between Different Antenna Branches

According to the invention, a number of signal branch measure values are collected for at least two antenna branches120, and the signal branch measure values of the different antenna branches120are compared in order to detect any erroneous behaviour of any of the antenna branches120. The comparison between the signal branch measure values of the different antenna branches can be made in different ways.

When a measurement of SIR is used as a signal branch measure, either as measured on traffic channels or on the random access channel, a simple test of the appropriate function of the different antenna branches120could be a check as to whether a SIR-value has been obtained for all antenna branches120—if a branch120does not generate any SIR-values over a longer period of time, this may be an indication of malfunction of this antenna branch120.

The comparison between the signal branch measures performed in step215ofFIG. 2could for example be performed by comparing the average signal branch measures of different signal branches (where the average values may for example be obtained by measuring the signal branch measures over a pre-determined period of time, or by including a predetermined number of signal branch measure values in the average signal branch measure calculation). Over a period of time, the ratio between the average signal branch measures from two different antenna branches120should approach 1, whereas the difference should approach 0, when both antenna branches120operate equally well. If the comparison performed in step215involves calculating the difference between the average value of the signal branch measure of two antenna branches120, a predetermined threshold value could be used in the check for erroneous behaviour performed in step220ofFIG. 2, so that when the magnitude of this difference exceeds the threshold value, step225should be entered and action should be taken. Depending on the signal branch measure used, different threshold values could be applied. When the comparison in step215involves calculating a ratio between signal branch measures of two different signal branches, the check of step220could include a check as to whether the ratio R lies within an interval [Rmin, Rmax], where Rmin<1<Rmax, and where the values of Rminand Rmaxare chosen in accordance with which signal branch measure is used.

FIG. 4is a flowchart illustrating an embodiment of the inventive method wherein the comparison of step215is performed as a comparison between average values of the signal branch measures of two different antenna branches120, and the number of samples315selected by the rake receiver320of a radio base station100from a signal branch305during a time period T is used as a signal branch measure of the signal branch305.

In the example ofFIG. 4, a radio signal300is received in different antenna branches120of the radio base station100as different signal branches305. In step405, the number samples315from a signal branch305that are selected by a rake finger325is counted for the different signal branches120during a time period T, which could for example be the time interval Trake. In step407it is checked whether the number of time periods during which the selected number of samples have been counted are sufficient. If not, step405is re-entered, and if so, step410is entered. In step410, the average number of samples selected from a signal branch305is calculated for the different signal branches305. In step415, the average number of samples of the different signal branches305are compared with each other. In step420, it is checked whether the comparison result indicates any erroneous behaviour of any of the antenna branches120.

The comparison between the number of samples selected from the two different antenna branches120performed in step415could e.g. be made as a ratio of the number of samples selected from the two antenna branches120, or as a difference. If the comparison is performed as a ratio, the step of calculating an average number of selected samples could advantageously be omitted, and instead, the time period T in step405can be set to an appropriate length, typically in the order of minutes (e.g. 2 minutes). Typical values of the end values Rminand Rmaxof the interval within which the ratio R should lie could then e.g. be [0.5:2].

The average value of a signal branch measure (SBM).SBM, can advantageously be calculated by adding signal branch measure values from several or all radio links received by an antenna branch120, and by adding the signal branch measure values obtained during a time period:

SBM_=1M⁢∑l=1M⁢SBM⁢l.(3)
where M is the total number of measurement values of the signal branch measure collected on a signal branch305during the time period. As mentioned above, the total number of measurement values. M, can include measurement values from different radio links. If the total number of measurement values is the same for each antenna branch120to be included in the comparison, an aggregate signal branch measure could be calculated instead of an average signal branch measure value of eq. (3), i.e. the division by M could be omitted from eq. (3).

When a signal-to-noise-and-interference measure is used as a signal branch measure, the comparison made in step215ofFIG. 2could advantageously be performed as a calculation of the difference between the logarithm of the average value of SIR,SIRof a first antenna branch120A and the logarithm ofSIRof a second antenna branch120B. An exemplar) expression for obtaining a comparison result between the two branches120A and120B is provided by equation (4):
SIRA-B=10 log10SIRA−10 log10SIRB(4)

The magnitude ofSIRA-Bof equation (4) is a measure of how similar the signal branches305, received by the two antenna branches120A and120B, are. If the magnitude ofSIRA-Bis below a certain threshold, then both antenna branches120A and120B would be considered to operate normally. If not, depending of the sign ofSIRA-B, an error would be suspected in either signal branch120A or in signal branch120B.

In alternative embodiment of the comparison step215ofFIG. 2, the comparison between different signal branches could be performed on a per measurement basis rather than on an average signal branch measurement basis. The result of such per-measurement-comparison will in the following be referred to as the per-measurement-comparison-result. The per-measurement-comparison-result could be obtained by calculating the ratio or difference between two signal branch measure values, obtained at the same time, on two different antenna branches120(needless to say, a measurement value of a branch measure does not necessarily reflect an instantaneous value of the branch measure, but could be an average value of the branch measure over a period of time). By analysing a set of such per-measurement-comparison results, an indication of whether the operation of any of the antenna branches120is erroneous can be obtained. The analysis could e.g. include calculation of the average value of the per-measurement-comparison results:

SBMdiff_=1M⁢∑l=1M⁢(SBMAl-SBMBl),(5)
where M is the number of per-measurement-comparison results. If the magnitude ofSBMdiffexceeds a threshold value, this indicates possible malfunction of one of the antenna branches120A or120B. The sign ofSBMdiffindicates which of the antenna branches120experiences the problems. When using SIR as a signal branch measure (SBM), an advantage of calculatingSBMdiffinstead of calculating the difference between the average values of the signal branch measures according to eq. (3) is that any differences in target values of the signal branch measures included in the averaging will be compensated for. The average value of the per-measurement-comparison results can alternatively be expressed as

This expression can be particularly useful when using SIR as a signal branch measure, since appropriate resolution can be obtained. However, expression (6) does not give any indication of which of the two antenna branches120experiences problems. Further analysis of the set of per-measurement-comparison results would be required in order to obtain such information.

The distribution of the per-measurement-comparison results often provide further information regarding any possible erroneous behaviour of any of the antenna branches120. By analysing the distribution of such per-measurement-comparison-results, for example by plotting the number-of-measurement-results vs. the per-measurement-comparison-results in a distribution graph, information regarding the status of the antenna branches120can be obtained. If the antenna branches120operate ideally, the distribution of the difference between the measurement results should be centred around zero (the ratio should be centred around 1). The standard deviation of the distribution can give additional information. For example, if the feeder of a first antenna branch120is connected to signalling part110serving a first cell, and the feeder of a second antenna branch120is connected to a signalling part110serving a second cell, the standard deviation of the distribution will be large. Problems with the antenna equipment or losses in the radio frequency path would give a ratio of the standard deviation and the absolute value of the difference that is smaller than would be obtained if there is an antenna diagram mismatch, etc.

An illustration of a per-measurement distribution graph is provided inFIG. 5, in an embodiment in which SIRrakeis used as the signal branch measure. The difference in SIRrakehas been calculated for two different antenna branches A and B for a number of measurements, and the distribution of SIRrakeA−SIRrakeBfor these measurements is plotted in the graph ofFIG. 5. The distribution illustrated inFIG. 5indicates that the antenna branch A is functioning less well than antenna branch B, since the average value is less than zero. Alternatively, the comparison could be a visual comparison, where the per-measurement-values are plotted in the same graph for the antenna branches120to be compared, rather than the difference or ratio between the signal measures of the different antenna branches120.

Table 1 illustrates examples of threshold values of the distribution that could be used in notifying an operator of the antenna system105of a possible error. Threshold values are given for the magnitude of the average value of the distribution, the standard deviation of the distribution as well as the ratio of the standard deviation and the absolute value of the average. Table 1 relates to the situation where the SIRrakeis used as the signal branch measure, and the comparison between two different antenna branches is performed as a difference (cf.FIG. 5). The actual thresholds given in Table 1 are not to be seen as absolute numbers, but as a guidance for which thresholds could be used in an implementation of the invention.

TABLE 1Threshold values for indication of errors in the antenna system 105.ErrorStd|average|std|average|Swapped feederstd > 20 dBLosses in RF path>3 dB<3 dBAntenna diagram mismatch≦3 dB>3 dBNone of the above≦20 dB≦3 dB

The above described evaluation of an antenna system105could advantageously be implemented so that the evaluation result, either in the form of all or some of the collected signal branch measure values, and/or in the form of error indication(s), are transmitted over a control channel to an operations & maintenance centre. A mobile radio communications system comprises a large number of radio base stations which are distributed over a vast geographical area, and the possibility of efficient remote monitoring of the antenna systems105of the system will greatly improve the efficiency of the operations & maintenance of the mobile radio system.

FIG. 6is a schematic illustration of an exemplary signal processing part110of a radio base station100, operating according to spread spectrum multiple access/code division multiple access, in which an embodiment of the invention has been implemented. The exemplary signal processing part110comprises a radio part605, a de-scrambler607, a de-spreader610, a rake receiver320and a signal branch comparator615. The signal processing part110has an input connected to the feeder115, on which signal branches305are received from the antenna system105. The signal processing part110converts the signal branches305into digital signal branches, which are de-scrambled in de-scrambler605, de-spread in de-spreader610and processed by the rake receiver320as described in relation toFIG. 3. The output620from the rake receiver325delivers an output signal, where the contribution from the different signal branches305has been taken into account in the manner described in relation toFIG. 3. An input623of the signal branch comparator615is connected to an output625of the rake receiver320in a manner so that the signal branch comparator615receives data indicative of the signal branch measures of the different signal branches305.

Depending on the implementation of the rake receiver325and the signal branch comparator615, outputs620and625could be implemented as the same output, or different outputs. The signal branch comparator615comprises software and hardware for performing an analysis of the received data according to any of the embodiments described above. Furthermore, the signal branch comparator615has an output630for outputting the result of the performed analysis.

When no de-scrambling or de-spreading of the signal is required, the de-scrambler607or the de-spreader610, respectively, could be omitted. The input623of the signal branch comparator615need not be connected an output625of the rake receiver320. In an embodiment of the invention where the signal branch measure is not obtained from an output from the rake receiver320(e.g. when the radio base station100does not comprise a rake receiver320), the input623of the signal branch comparator615is connected to receive a signal indicative of the respective signal branch measures of the different antenna branches120.

In implementations where the evaluation of the antenna system105can be performed on a on-demand basis, the comparator615, or another appropriate entity of signal processing part110, could comprise an input for receiving instructions to perform an evaluation of the antenna system105.

In the embodiment illustrated inFIG. 6, the signal branch comparator615is implemented in the radio base station100. However, the signal branch comparator615could alternatively be implemented in a monitoring system separate from the radio base station100.

In the above, the invention has been described in terms of the antenna system105of a radio base station100. However, the invention is equally applicable to any digital radio apparatus employing antenna diversity. Furthermore, although the invention has been described in relation to receiving antennas, it could also be used for monitoring the status of transmitting antennas by utilising the inherent receiving properties that most transmitting antennas have. The invention may also be used to check the status of an antenna system105where different antenna elements are used for receiving signals on different channels from transmitters within the same coverage area. In such a situation, the different antenna branches120are not for creating antenna diversity, but for increasing the number of channels that can be active at the same time. By making an antenna branch120eavesdropping on the other antenna branch(es)120covering the same area, an evaluation according to the invention would indicate whether any errors in any of the antennas exists, for covering the same area on different channels receiving.

In order to increase the reliability of the inventive evaluation process, a combination of the different embodiments described above could be employed. For example, an analysis of the performance of antenna branches120could be based on the use of two different signal branch measures, or more. Furthermore, the comparison could include both a comparison of the average signal branch measure values, and an analysis based on per-measurement-comparison-results.