METHOD AND ARRANGEMENT FOR MEASURING THE CONDITION OF A RECEIVER ANTENNA

Abstract: The invention relates to a method and an arrangement for measuring the condition of a receiver antenna (26) at a base station of a TDMA radio system. The method comprises: a) transmission (28A) of a radio frequency measuring signal through an antenna line (27) towards the antenna; b) measurement (20) of the strength of a measuring signal component reflected back and passed along a predetermined measuring path; c) transmission (28B) of the measuring signal directly towards the receiver (210) to said measuring path; d) measurement (20) of the strength of the measuring signal transmitted directly along the measuring path. In the invention, the measurement is performed in a time slot allocated for a radio test loop, due to which it is possible to utilize frequencies actually used for traffic.

Description: Method and arrangement for measuring the condition of a receiver antenna The invention relates to a method for measuring the condition of a receiver antenna at a base station of a TDMA radio system, the method comprising a) transmission of a radio frequency measuring signal through an antenna line towards the antenna, b) mea¬ surement of the strength of a measuring signal com- ponent reflected back and passed along a predetermin¬ ed measuring path, c) transmission of the measuring signal directly towards the receiver to said measur¬ ing path, d) measurement of the strength of the mea¬ suring signal transmitted directly along the measur- ing path. A substantial part of radio systems, for in¬ stance cellular radio telephone systems and base sta¬ tions thereof, are receiver and transmitter antennas, the condition of which influences the quality of con- nections. The condition of the antennas can be con¬ trolled e.g. by measuring the standing wave ratio of the antennas, i.e. electrical matching of the anten¬ nas to the rest of the receiver and transmitter sys¬ tem. By means of present methods, the measurement is performed by supplying power through the antenna line to the receiver antenna and by measuring the power reflected back from the antenna along the antenna line by means of a broadband power meter. Due to the broadband power meter, the power used for the mea¬ surement must be high in order that the measurement will not be sensitive to interference, which means that signals received by the antenna do not disturb the measurement. Using high power causes intermodula- tion distortion in the receiver. These problems can be avoided by using a duplex filter separating from each other the measuring signal and the signals in¬ tended to be received. However, the duplex filter shall be installed before the receiver parts, which weakens the sensitivity of the receiver, because the filter causes losses in the antenna signal. A further solution is known, avoiding both above-mentioned problems by using a measuring fre¬ quency outside the receiver band and a narrow-band power measurement adapted to this frequency. European Patent Application 0 261 828 discloses an analyzer of microwave range, measuring a signal supplied directly from a measuring source as well as a signal reflected from the microwave network to be analyzed, for a vector-based relative power measure¬ ment. At measurement of reflected power, a sample signal taken of the output power of a measuring gen¬ erator is brought to a detector along a path differ¬ ent from the path of a sample signal taken of the power reflected from the microwave network to be ana¬ lyzed. Thus the relative measurement of the reference cited would be somewhat insensitive to variations in the output level of the measuring generator, but since the signals to be compared with each other use different paths to the detector, the measurement does not automatically consider non-idealities or changes in the properties of the components present on signal paths. The analyzer shall be calibrated for the mea¬ surement of the reflected power by measuring a known microwave standard at first with the analyzer and by using thus obtained calibration values for later mea¬ surements. For measuring the condition of an antenna, this would mean that the power supply of the antenna should be connected to some microwave standard in- stead of the antenna for the calibration. This problem has been solved in the Finnish Patent Application 904085 in such a way that, in ad¬ dition to the signal reflected from the antenna, also the . strength of the measuring signal sent directly towards the receiver is measured separately. By this second measurement is obtained a reference value for the strength of the measuring signal, which value considers at the measuring moment the transmission power of a measuring signal transmitter and the pro- perties of the components present on the signal path, such as amplifiers and branching elements, and with which value the strength of a measuring signal com¬ ponent reflected from the antenna is compared. By a relative measurement of this kind, it is possible to eliminate the influence of the components present in the measuring circuit and on the measuring signal path between the properties of individual components or the influence of temporal changes in the proper¬ ties of an individual component on the accuracy of measurement. Manual checkings and calibrations during installation and operation can also be reduced and simplified substantially or they can even be entirely avoided. The measuring signal is preferably a narrow¬ band signal, the frequency of which is outside the frequency band allocated for traffic, due to which the measurement does not disturb the actual radio traffic, but, on the other hand, the measurement is not performed at actually used frequencies either, and for this reason, an actual standing wave ratio SWR will not be obtained. New digital TDMA-type (TDMA = Time Division Multiple Access) radio systems have time-division signalling including several, typically 8, time slots at one frequency. One TDMA system is the pan-European mobile telephone system GSM. The GSM Specification 12.11, 3.1.0 B 05 "Receiver Antenna Fault" states re¬ quirements for the control of the condition of the antenna. The object of the invention is for instance to implement a control of the condition of an antenna at a TDMA base station at actually used frequencies. This is achieved by means of a method of the type presented in the introduction, the method being according to the invention characterized in that said measuring signal is sent at a frequency allocated for the radio traffic of the radio system in a TDMA time slot of the base station intended for an establish¬ ment of a radio test loop. Another aspect of the invention is an arrange- ment for measuring the condition of a receiver anten¬ na at a base station of a TDMA radio system, compris¬ ing a means for generating a radio frequency measur¬ ing signal; a first directional coupler means for applying the measuring signal to an antenna line to- wards the antenna; a second directional coupler means for applying the measuring signal to the antenna line towards the receiver; means for switching the radio frequency measuring signal from the means generating the measuring signal alternately to the first and the second directional coupler means; means for measuring the strength of a measuring signal component sent to¬ wards the antenna and reflected back from the antenna and the strength of the measuring signal sent direct¬ ly towards the receiver. The arrangement is according to the invention characterized in means for permit¬ ting a measurement on a radio channel allocated for the radio traffic of the radio system in a TDMA time slot of the base station intended for an establish¬ ment of a radio test loop. In the invention, the measurement is performed in a time slot allocated for a radio test loop, which makes it possible to utilize frequencies actually used for the traffic. In a preferred embodiment of the invention, a radio frequency measuring signal is generated at transmission frequency in a transmitter and looped through the radio test loop at reception frequency to the receiver antenna line, due to which a separate measuring signal generator is avoided. In the following, the invention will be ex- plained in more detail by means of illustrative embo¬ diments with reference to the attached drawing, in which Figure 1 shows a block diagram of a base sta¬ tion receiver equipment according to the invention, Figure 2 shows a block diagram of a base sta¬ tion transceiver equipment according to the inven¬ tion. The invention is suitable for being used in a full duplex-type transceiver of a base station of TDMA radio systems. Figure 1 illustrates only a base station re¬ ceiver equipment and only to those parts which are essential for explaining and understanding the inven¬ tion. A receiver antenna 1 is connected by means of an antenna cable 2 through a passband filter 7 and a dividing amplifier 5 to a branching element 6, which divides a signal coming from the antenna direction into two or several branches. The branching element 6 is used especially in radio systems comprising many frequency channels, each frequency channel having a receiver of its own after the branching element. The filter 7 may also be located between a directional coupler 4 and the amplifier 5. A measuring signal source 10, e.g. a signal generator, generates a radio frequency measuring sig- nal to be used for the measurement of the condition of the antenna, the frequency of this signal corre¬ sponding to one of the normal reception frequencies of the receiver, preferably within the range 800 to 1000 MHz. To an output of the signal generator 10 can be switched an adjustable attenuator or amplifier 9 for the adjustment of the strength of the measuring signal to be transmitted. The measuring signal is led from the attenuator 9 to a change-over switch SI, switching the measuring signal to a directional coup¬ ler means 3 in a first position I and to the direc¬ tional coupler 4 in a second position II and discon¬ necting the measuring signal from the directional coupler means 3, 4 in a third position III. The di- rectional coupler means 3 and 4 are switched to the antenna line 2 in such a way that the directional coupler means 3 directs the measuring signal to be transmitted along the antenna line towards the anten¬ na and the directional coupler means 4 directs the measuring signal to be transmitted along the antenna line towards the receiver. One output of the branching element 6 is con¬ nected to a measuring unit 8 measuring the strength of the measuring signal coming from the branching element. The measuring unit may be for instance a power meter, to which is connected a logic circuitry for necessary control and calculation operations. For the measurement may also be used measuring equipments possibly included in the receiver already. Further, the measuring unit 8 of the shown em¬ bodiment controls the changeover switch SI through control line 11. The measurement takes place at the normal reception frequencies of the radio system dur¬ ing normal operation of a receiver Rx. The receiver Rx receives from the antenna a TDMA signal, each fre- quency of which comprises several, typically 8, time slots. The signal has additionally a frame structure having e.g. eight successive time slots constituting a frame. Further, frames can form multiframes (e.g. 26 or 52 frames) and multiframes hyperframes. One TDMA system is the pan-European mobile telephone sys¬ tem GSM. Time slots are mainly used for transferring control and traffic channels. In the GSM system, for instance, at least one time slot of a frame, multi- frame or hyperframe is, however, allocated for a for¬ mation of a radio test loop from the transmitter to the receiver. The radio test loop is normally intend¬ ed for testing RF parts of a base station, excluding the receiver antenna and the receiver antenna cable. In the invention, the condition of the receiver antenna 1 is measured in this time slot allocated for the radio test loop. Outside said time slot, the mea¬ suring unit 8 controls a disconnector to position III, in which the measuring signal is disconnected from the antenna line 2. Through control line 12, the measuring unit 8 may alternatively prevent the signal source 10 from generating the measuring signal out¬ side said test time slot and permits it in the test time slot. The control unit 8 receives time slot in- formation in a signal SYNC DATA e.g. from a synchro¬ nization and control unit of the entire receiver equipment. When measuring the condition of the antenna 1 in the test time slot, the switch SI switches the measuring signal from the signal generator 10 to the directional coupler means 4, which directs the mea¬ suring signal towards the receiver, whereby the sig¬ nal passes through the amplifier 5 and the branching element 6 to the measuring unit 8, which measures the strength or power of the signal and stores the mea- sured value as a reference value for later use. Then the switch SI changes its position and switches the measuring signal to the directional coupler means 3, which directs the measuring signal along the antenna line 2 to the antenna 1. A measuring signal component reflected from the antenna 1 back to the antenna line 2 passes through each directional coupler means 3 and 4, the amplifier 5 and the branching element 6 to the measuring unit 8, which remeasures the strength value of the reflected signal. Subsequently, the measuring unit 8 calculates by means of the reference value and the strength of the reflected signal an SWR or SRRI value, which is compared with preset alarm limits, upon exceeding of which the measuring unit gives the alarm in a suitable form. As described previously, a transceiver often utilizes a radio test loop, in which a radio frequen¬ cy test signal generated by the transmitter part is led in a correct manner back to the receiver part of the same transceiver for reception and analysis. Such a radio test loop of a transceiver is described e.g. in the Finnish Patent 85080 incorporated herein as a reference. In a preferred embodiment of the invention shown in Figure 2, a radio test signal looped during a radio test time slot from the transmitter part to the receiver antenna line is used as a measuring sig¬ nal and sent on a radio channel (at a frequency) used for the normal traffic of the radio system. In Figure 2, a TDMA transceiver comprises at least two, preferably 4, pairs of transceivers 21A, 210A and 21B, 210B, respectively, each pair consti¬ tuting one full duplex connection. The outputs of the transmitter units 21A and 21B are connected by means of a combining element 2 (combiner) to a common an- tenna line 23 and to a common transmitter antenna 24. A receiver antenna 26 is connected through an antenna line 27 to a branching element 29, which divides the received signal to the receiver units 210A and 210B. In this connection, transmitter and receiver units primarily signify the radio parts of a transceiver. Each transmitter and receiver unit has a dedicated transmission or reception frequency, whereby the transmission and reception frequencies of the units, such as 21A and 210A, constituting a full duplex pair, are at a distance of a duplex spacing, e.g. 45 MHz, from each other. A radio test unit 112 is connected between the antenna lines 25 and 27. For this purpose, the anten- na line 25 comprises a branching element 23, which branches off a portion of a transmission RF signal to an input 122 of the radio test unit 112. The radio test unit 112 converts the transmission frequency signal into a reception RF signal. An output 132 of the radio test unit 112 is connected to the antenna line 27 by means of a switching unit 28, comprising an RF switch S2 and directional couplers 28A and 28B. Testing operations are controlled by an opera¬ tion and maintenance unit 20 of the base station, which unit commands the test unit 112 through a con¬ trol line 20A to form a test loop in a predetermined test time slot and informs whether the measuring sig¬ nal is switched towards the antenna or the receiver. In a preferred embodiment of the invention, the test unit 112 controls the switch S2 on the DC level of its output 132. The unit 28 contains a detector of the DC level and switches the output 132 on the basis of the detected DC level either to the directional coupler 28A or 28B. The directional coupler 28A di- rects the measuring signal towards the antenna 26 for a reflection measurement of the antenna. The direc¬ tional coupler 28B directs the signal towards the receiver for a conventional test loop or for a refer¬ ence measurement of the antenna measurement. This solution requires fewest additional cables and changes least the previous test loop. The switch S2 may also be controlled directly from the OMU 20, as illustrated by broken lines 20B. Outside the test time slot, the measuring signal is disconnected from the output 132 by means of a disconnector inside the unit 112. The OMU 20 acts also as a measuring unit, which receives a signal from the branching element 29 and measures the strength of the signal. Alternatively, the receivers Rx may contain a measuring equipment, the measuring result of which is received by the OMU 20 through line 20C. The measuring procedure is as follows. The quantity to be measured is the standing wave ratio SWR of the antenna. 1 ) The OMU 20 gives the test unit 112 a normal radio test loop command, in consequence of which the measuring signal is looped in the test time slot to the switch S2 switching the measuring signal on the basis of the DC level it has detected to the direc¬ tional coupler 28B sending the measuring signal di¬ rectly towards the receiver. The OMU 20 measures the strength of the measuring signal and uses an RSSI value formed from the measurement result as a refer- ence value REFSIGN. 2) The OMU 20 gives the test unit 112 a new radio test loop command to direct the measuring sig¬ nal towards the antenna 6, however. The test unit 112 changes the DC level of its output, in consequence of which the switch S2 switches the measuring signal to the directional coupler 28A. The OMU 20 measures the strength of the signal component reflected from the antenna 26 and uses the RSSI value of the strength of the received signal, formed from the measurement re- suit, as a measured value ANTSIGN. 3) The OMU 20 calculates the SWR of the antenna from these two values ANTSIGN and REFSIGN. 4) The OMU 20 compares the calculated SWR value with the stored alarm limits. If the calculated value is higher/lower than the alarm limit, the alarm is given. The data base of the OMU 20 contains a cali¬ brated SWR value for each used frequency, preferably also for each antenna sector and both for a normal antenna and a diversity antenna. During a setup of the base station or when the antenna or the antenna cable is changed, the measurements 1, 2 and 3 of each frequency, sector and antenna to be used are gone through and the SWR values obtained as a result are stored as calibrated values in the data base of the OMU. The figure and the description attached thereto are only intended to illustrate the present inven¬ tion. As to the details, the method and the arrange¬ ment according to the present invention may vary within the scope of the attached claims.

Claims: Claims: 1. A method for measuring the condition of a receiver antenna at a base station of a TDMA radio 5 system, the method comprising a) transmission of a radio frequency measuring signal through an antenna line towards the antenna, b) measurement of the strength of a measuring signal component reflected back and passed along a 10 predetermined measuring path, c) transmission of the measuring signal direct¬ ly towards the receiver to said measuring path, d) measurement of the strength of the measuring signal transmitted directly along the measuring path, 15 c h a r a c t e r i z e d in that said measuring signal is sent at a frequency allocated for the radio traffic of the radio system in a TDMA time slot of the base station intended for an establishment of a radio test loop. 20 2. A method according to claim 1, c h a r a c¬ t e r i z e d in that the radio frequency measuring signal is sent towards the antenna and towards the receiver in separate TDMA time slots. 3. A method according to claim 1 or 2, 25 c h a r a c t e r i z e d in that the radio frequen¬ cy measuring signal is generated at transmission fre¬ quency in a transmitter and looped through the radio test loop at reception frequency to the receiver an¬ tenna line. 30 4. A method according to claim 1, 2 or 3, c h a r a c t e r i z e d in that it comprises a determination of standing wave ratio or reflection attenuation on the basis of the measured signal strengths. • 35 5. An arrangement for measuring the condition of a receiver antenna at a base station of a TDMA radio system, comprising a means (10, 112) for generating a radio fre¬ quency measuring signal, a first directional coupler means (4, 28A) for applying the measuring signal to an antenna line to¬ wards the antenna, a second directional coupler means (3, 28B) for applying the measuring signal to the antenna line towards the receiver, means (SI, S2) for switching the radio frequen¬ cy measuring signal from the means generating the measuring signal alternately to the first and the second directional coupler means, means (8, 20) for measuring the strength of a measuring signal component sent towards the antenna and reflected back from the antenna and the strength of the measuring signal sent directly towards the receiver, c h a r a c t e r i z e d in means (8, 20) for permitting a measurement on a radio channel allocated for the radio traffic of the radio system in a TDMA time slot of the base station intended for an estab¬ lishment of a radio test loop. 6. An arrangement according to claim 5, c h a r a c t e r i z e d in that the means for gen¬ erating the radio frequency measuring signal comprise a radio test loop (112) for looping the measuring signal from the transmitter of the base station to said switching means (SI, S2) . 7. An arrangement according to claim 5, c h a r a c t e r i z e d in that the means for gen¬ erating the radio frequency measuring signal com¬ prises a measuring signal generator (10).

Inventors: AHONEN JALO, AHONEN, JALO
Applicants: AHONEN JALO, NOKIA TELECOMMUNICATIONS OY, AHONEN, JALO
Application year: 1993
Main classification: G01R27
IPC codes: G01R27, G01R29, H04B17