Arrangement for measuring the condition of a receiver antenna

(74) Agent

Nokia Telecommunications OY, Makkylån puistotie 1, SF-02600 Espoo, FI Jalo Ahonen, Oulu, FI

Bryn & Aarflot AS, 0104 Oslo (54) Designation Arrangement to measure the condition of a receiving antenna (56) Published publications None (57) Summary

The invention relates to an arrangement for measuring the state of a receiving antenna in a base station which uses an antenna amplifier (15) disposed in connection with an antenna (16) separate from other base station equipment, for example in a mast. According to the invention, a measurement signal in connection (11) with the second base station ion output is generated at a frequency outside the frequency band used for radio traffic and supplied through an antenna line (7) to the antenna amplifier (15) in which the measurement signal is converted to the actual measurement signal frequency within the frequency band intended for radio traffic.

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The invention relates to an arrangement for measuring the condition of a receiving antenna at a base station, the arrangement comprising a receiver antenna disposed separate from other base station equipment, for example by a mast; an amplifier arranged in conjunction with the antenna to amplify an antenna signal; a device for generating a radio frequency measurement signal; a first directional coupler means for supplying the measuring signal to an antenna line against the antenna between the amplifier and the antenna; another directional coupler means for supplying the measurement signal to the antenna line against the receiver between the amplifier and the antenna; a coupling means for coupling the radio frequency signal from the generating means alternatively to the first and second directional coupling means; means for measuring the strength of a measurement signal component transmitted to the antenna and reflected back from the antenna and the strength of the measurement signal transmitted directly to the receiver.

A significant portion of radio systems, such as cellular radio telephone systems and their base stations, are receiver and transmitter antennas whose condition affects the quality of the connections. An adjustment of the condition of the antennas can be performed, for example, by measuring the antenna's standwave ratio SWR, ie electrical fitting between the antennas and the other part of the receiver and transmitter system.

By the present methods, the measurement is performed by delivering power through the antenna line to the receiving antenna, and by measuring the power reflected back from the antenna along the antenna line by means of a broadband power meter. Because of the broadband power meter, the power used in the measurement must be high so that the measurement is not sensitive to interference, which means that signals received by the antenna do not interfere with the measurement. Use of high power causes intermodulation distortion in the receiver. These problems can be avoided by using a duplex filter that separates the measurement signal and the signals to be received. However, the duplex filter must be mounted in front of the receiver parts, which weakens the receiver's sensitivity because the filter causes loss of the antenna signal.

Another solution is known which avoids both of the above problems by using a measurement frequency outside the receiver band and a narrow band power measurement adapted to this frequency.

European Patent Application No. 261,828 discloses a microwave region analyzer that measures a signal delivered directly from a measurement source as well as a signal reflected from the microwave circuit to be analyzed, for a vector-based relative power measurement. In measuring reflected power, a sample signal taken from the output power of a grinding generator is brought to a detector along a path other than the path of a sample signal taken by the power reflected from the microwave circuit to be analyzed. Thus, the relative measurement of said reference will be somewhat insensitive to variations in the output level of the measurement generator, but since the signals to be compared with each other use different paths to the detector, the measurement does not automatically consider non-ideal states or changes in the properties of components that are present. present in the signal paths. The analyzer should be calibrated for the measurement of reflected power by first measuring a known microwave standard with the analyzer and using the calibration values ​​thus obtained for subsequent measurements. To measure the condition of an antenna, this would mean that the antenna's power supply should be connected to some microwave standard instead of the antenna during calibration.

This problem has been solved in Finnish Patent Application No. 904085 in such a way that, in addition to the signal reflected from the antenna, the strength of the measurement signal sent directly to the receiver is also measured separately. By means of this second measurement, a reference value for the strength of the measuring signal is obtained, which value at the moment of measurement takes into account the transmit power of a measuring signal transmitter and the characteristics of the components present in the signal path, such as amplifiers and branching elements, and with which value the strength of a measurement signal component reflected from the antenna is compared. By means of a relative measurement of this kind, it is possible to eliminate the influence of the components present in the measurement circuit and of the measurement signal path between the properties of the individual components, or the influence due to temporary changes in the properties of a single component on the accuracy of the measurement. . Manual controls and calibrations during installation and operation can also be significantly reduced and simplified, or they can be avoided completely. The measurement signal is preferably a narrow band signal whose frequency is outside the frequency band assigned to traffic, so that the measurement for this reason does not interfere with real radio traffic, but so that the measurement on the other hand is not carried out at really used frequencies, and for this reason a truly steady-wave relationship SWR will also not be achieved.

New TDMA (Time Division Multiple Access) digital radio systems have time-division signaling that includes multiple, usually 8, time slots at a frequency. A TDMA system is the pan-European mobile phone system GSM. GSM Specification 12.11, 2.1.0 B 05 Receiver Antenna Fault sets requirements for the antenna condition.

With the antenna positioned at the mast and provided with a mast amplifier, the antenna condition cannot be measured in the usual way because the preamplifier is positioned on the antenna line between the feed point of the measurement signal and the antenna, and the antenna signal should pass through this preamplifier in the opposite direction. Typically, a reverse amplifier attenuation is 40 dB. The accuracy of the measurement is affected by attenuation of even a few decibels, such as a receiver filter, which is also located at the mast. Thus, there is a need to provide the mast with a switching point for the measurement signal between the mast amplifier and the antenna.

The object of the invention is to realize a measurement of the condition of a receiving antenna provided with a mast amplifier at a TDMA base station at really used frequencies.

This is achieved by means of an arrangement of the type set forth in the introduction, wherein the arrangement according to the invention is characterized in that the measuring signal to be coupled to the directional coupler devices has a first frequency in a frequency band assigned to radio traffic in the radio system, and that the generating means is arranged in conjunction with the second base station equipment and comprises means for transmitting the measurement signal through the antenna line to the switching devices at a second frequency outside the frequency band assigned to radio traffic in the radio system, and in that the switching means comprises a device coupled to the antenna line between the amplifiers and the second base station equipment for receiving the measurement signal at the second frequency; and a mixing device for converting the measurement signal from the second frequency to the first frequency before the measurement signal is coupled to the directional coupler means.

According to the invention, the measurement signal is generated in connection with the second base station equipment and supplied outside the frequency band used for traffic through the antenna line of the preamplifier, where the measurement signal is converted to the actual measurement frequency within the frequency band assigned for the traffic.

In a preferred embodiment of the invention, the radio system is a TDMA system and the measurement is performed in a time slot assigned to a radio test loop, whereby frequencies really used for traffic can be used. In one embodiment of the invention, a radio frequency measurement signal is generated at the transmit frequency in a transmitter and looped through the radio test loop at receiver frequency to the receiver side, thereby avoiding a separate measurement signal generator. By means of the invention, the signal in its original form is produced to a switching point at the mast without interfering with the received signal from the antenna within the real receiver band.

In the following, the invention will be described in more detail by means of illustrative embodiments with reference to the accompanying drawings, in which:

Fig. 1 is a block view of the receiving equipment of a base station according to the invention;

FIG. 2 is a block diagram of a ground unit 8 of FIG. 1; and

FIG. 3 is a block diagram of a mast unit 15 of FIG

1.

The invention is particularly suitable for use in a full-duplex type transmitter / receiver in a base station in TDMA radio systems.

In Figure 1, a combined TDMA transmitter / receiver comprises at least 2, preferably 4, pairs of combined transmitters / receivers 1A, 10A and 1B, 10B, respectively, each pair constituting a full duplex connection. The outputs of the transmitter units IA and IB are connected by a combining element 2 (combines) to a common antenna line 3 and to a common transmitter antenna 4. A receiver antenna 6 arranged apart from other base station equipment, for example in a mast, is connected via an associated preamplifier unit 15 (hereinafter called a mast unit) and an antenna line 7 to a branch element 9 which divides the received signal to the receiver units 10A and 10B. In this connection, transmitter and receiver units mainly denote radio parts in a combined transmitter / receiver. Each transmitter and receiver unit has an individual transmitter or receiver frequency so that the transmitter and receiver frequencies of the units, such as IA and 10A constituting a full duplex pair, are at a distance of a duplex space, for example 45 MHz, from each other.

A receiver Rx receives from the antenna a TDMA signal, each frequency of which comprises several, usually 8, time slots.

In addition, the signal has a frame structure which has, for example, eight consecutive time slots that make up a frame. Furthermore, frames can form multi-frames (for example, 26 or 52 frames) and multi-frames can be hyper-frames. A TDMA system is the pan-European mobile phone system GSM. Time slots are mainly used for transmission of control and traffic channels.

Combined transmitters / receivers often use a radio test loop where a radio frequency test signal generated by the transmitter part is properly returned to the receiver part of the same combined transmitter / receiver for reception and analysis. Such a radio test loop in a combined transmitter / receiver is described, for example, in Finnish Patent No. 85080, which is hereby incorporated by reference.

However, in the GSM system, for example, at least one time slot in a frame, multi-frame, or hyperframe is assigned to establish a radio test loop from the transmitter to the receiver.

A radio test unit 11 is connected between the antenna lines 5 and 7. For this purpose, the antenna line 5 comprises a branching element 3 defining a portion of a radio frequency transmit signal to an input 12 of the radio test unit 11. The radio test unit 11 converts the transmit frequency signal to a radio frequency receiver signal. An output 13 from the radio test unit 11 is connected to the antenna line 7 by means of a switching unit 8, hereinafter called a ground unit. The radio test loop is usually intended for testing the base station's radio frequency parts, except for the receiver antenna and receiver antenna cable.

In a preferred embodiment of the invention, the state of the receiving antenna 6 is measured in a time slot assigned to the radio test loop. In the embodiment of Figure 1, a radio test signal looped during a time slot for a radio test from the transmitter portion to the receiver antenna line is used as a measurement signal and transmitted on a radio channel (at a frequency) used for the radio traffic's regular traffic.

Figure 2 shows the sub-unit 8 of Figure 1 in more detail. A measuring signal at the receiver frequency obtained from the radio test unit 11 is mixed in a mixer 20 to a frequency fa outside the receiver band by means of a local oscillator signal.

LO is generated from a local oscillator 22. When, for example, the signal 13 has a frequency of 890 to 915 MHz and the signal LO has a frequency of 120 MHz, the measurement signal can be converted to a frequency higher or lower than 120 MHz, for example to a range on from 770 to 795 MHz. The upper harmonic of the mixer result is filtered off by means of a passband filter 21, and the down-converted measuring signal is connected by means of a directional coupler 24 to the antenna cable 7 against the antenna

6. Similarly, oscillators 25 and 26 generate control signals having frequencies f1 and f2. The control signals are connected to the antenna cable 7 by means of a coil L2. A logic unit 23 controls the operation of the oscillators 22, 25 and 26 by switching them on and off according to the control of line 14B from the maintenance unit (OMU) 14 and the direct current level produced on line 13 from the test unit 11. When the oscillator 22 is connected on, the oscillator 26 is also in operation and the frequency f2 is transmitted. When the oscillator 22 is switched off, the frequency f2 is also not transmitted. As described in more detail later, this arrangement allows both an oscillator 39 for the mast unit 15 and the oscillator 22 for the ground unit 8 to be switched on only during the measurement of the antenna. Thus, possible interferences caused by the oscillators in conventional radio traffic are prevented. The ground unit 8 obtains information about the moment when the antenna is measured from OMU 14 via line 14B. On the other hand, the direct current level of signal 13 indicates the direction of coupling to the measurement signal, i.e. towards the receiver or antenna. This directional information is converted to the frequency fl. When fl is transmitted, the coupling direction is towards the antenna. When fl is missing, the switching direction is towards the receiver.

The mast unit 15 in connection with the antenna preamplifier in the mast is described in more detail in Figure 3. The unit 15 conventionally comprises a receiver filter 30 and an amplifier unit secured by the live state by-principle which constitutes a branch element 31, parallel amplifier limits 32 and 33 and a combining element 34. The actual measuring equipment comprises a directional coupler 35 which receives from the antenna cable 7 a measurement signal transmitted by the ground unit 8 at the frequency fa, the other frequencies such as the actual receiver frequencies around the frequency fa being filtered by means of a passband filter 36 (bandwidth for example 770 to 795 mHz. The filtered measurement signal is amplified and then converted in a mixer 38 back to the receiver frequency fr by the local oscillator signal LO provided from the oscillator 39. The signal LO has the same frequency as the signal LO to the ground unit 8. Frequencies outside the receiver band are eliminated from the mixing result by by means of a passband filter 40, and the filtered signal is applied to a switching device S1. A coupling device 41 selectively and alternately connects the measurement signal to directional couplers 42 and 43 connecting the measurement signal to the antenna cable 7 between the mast unit 15 and the antenna 6. The directional coupler 42 supplies the measurement signal to the preamplifier and receiver, and the directional coupler 43 to the antenna 6. In this way, a measurement signal identical to the original signal 13 can be connected to the antenna line 7 between the antenna 6 and the unit 15 in front of the receiver filter 30 without interfering with the actual receiver signal going in a different direction.

The coupling device 41 and the oscillator 39 are controlled by a control logic 44 according to control signals f1 and f2 received from the antenna line. The control signals are separated from the antenna cable 7 coming from the ground unit 8 by the coil L and filtered by the passband filter 45 and 46. The filtered control signals are detected in the control logic 44. Upon detection of frequency f1, control logic 44 controls switching device 41 to position I where the measurement signal is coupled to directional coupler 43. If frequency f1 is not present, control logic 44 controls switching device 41 to position II where the measurement signal is coupled to directional coupler 42. At detection of frequency f2 the control logic 44 switches on the oscillator 39. When the frequency f2 is not present, the control logic 44 disconnects from the oscillator 39. By means of this arrangement, the oscillator 39 in the mast unit 15 as well as the oscillator 22 in the ground unit 8 are switched on only during the measurement of the antenna. Thus, possible interferences caused by the oscillators in ordinary radio traffic are prevented. The ground unit 8 receives information about the time of measurement of the antenna from the OMU 14 through wire

14B.

The measurement takes place at the regular receiver frequencies of the radio system during normal operation of the receiver Rx. The test operations are controlled by the operating and maintenance unit 14 of the base station, which unit commands the test unit 11 through the control line 14a to form a test loop within a predetermined time slot and inform whether the measurement signal is coupled to the antenna or to the receiver. In the preferred embodiment of the invention, the test unit 11 then controls the ground unit 8 by means of the DC level of the output.

13. The directional control can also be provided directly from the OMU 14 to the sub-unit 8 through the control line 14D. Outside the test time slot, the measurement signal is disconnected from the output 13 by means of a disconnect switch inside the unit 11.

Furthermore, OMU 14 acts as a unit of measurement which receives a signal from the branching element 9 and measures the strength of the signal. Alternatively, the receivers Rx may contain measurement equipment whose measurement result is received by means of OMU14 through line 14C.

The measurement procedure is as follows. The size to be measured is the standwave ratio SWR to the antenna.

1) OMU 14 provides the test unit 11 with a standard radio test loop command, after which a measurement signal is looped in a test time slot for the ground unit 8. OMU 14 also informs the ground unit 8 about the antenna measurement, which causes the oscillator 22 to start and the measurement signal be converted to frequency fa upper unit 15 where it is converted to the frequency fr and switched, depending on the directional information, for example to the directional coupler 42, ie directly to the receiver. OMU 14 measures the strength of the measurement signal and uses an RSSI value of the strength of the received signal, generated from the measurement result, as a reference value REFSIGN.

2) OMU 14 gives the test unit 11 a new radio test loop command to direct the measurement signal to the antenna 6. The switching device 41 connects the measurement signal to the directional coupler 43. The OMU 14 measures the strength of a signal component reflected from the antenna 6 and uses the RSSI value generated from the measurement result, as a measurement value ANTSIGN.

3) OMU 14 calculates the antenna standwave ratio SWR from these two values ​​ANTSIGN and REFSIGN.

4) OMU 14 compares the calculated SWR value with stored alarm limits. If the calculated value is higher / lower than the alarm limits, an alarm is issued. The database of OMU 14 contains a calibrated SWR or SRRI value for each frequency used, preferably also for each antenna sector and both for a normal antenna and a diversity antenna e. During the setup of the base station or when the antenna or antenna cable is changed, measurements 1, 2 and 3 for each frequency, sector and antenna are reviewed and the SWR values ​​obtained as a result are stored as calibrated values ​​in the OMU database.

The figure and description related thereto are intended to illustrate the present invention only. In terms of details, the arrangement of the present invention may vary within the scope of the appended claims.