Arrangement for measuring the condition of a receiver antenna

Arrangement for measuring the condition of a receiver antenna

The invention relates to an arrangement for measuring the condition of a receiver antenna at a base station of a system, comprising a receiver antenna spaced apart from other base station equipment, e.g. a mast; amplifier means disposed in connection with the antenna for amplifying the antenna signal; means for generating a radio frequency measurement signal; first directional switching means 10 for supplying a measurement signal to the antenna line in the direction of the antenna between the amplifier means and the antenna; second directional switching means for supplying a measurement signal to the antenna line in the direction of the receiver between the amplifier means and the antenna; switching means for switching a radio frequency measurement15 signal from the generating means alternately to the first and second directional switching means; means for measuring the strength of the measurement signal component transmitted in the direction of the antenna and reflected back from the antenna and the strength of the measurement signal 20 transmitted directly in the direction of the receiver «

Radio systems, such as cellular radiotelephone systems and their base stations, include, as an integral part, receiver and transmit antennas, the condition of which affects the quality of the connections. The condition monitoring of the antennas can be performed e.g. by measuring their standing wave ratio SWR, i.e. the electrical matching of the antennas to the rest of the receiver and transmitter system.

With currently used methods, the measurement is performed by feeding an antenna line to the receiver antenna. 30 through power and measuring the reflected power back from the antenna to the antenna line with a broadband power meter. Due to the broadband of the power meter, the power used in the measurement must be high so that the measurement does not become interference sensitive, i.e. the signals received by the antenna do not interfere with the measurement. Using high power will cause central modulation distortion in the receiver. These problems can be avoided by using a duplex filter that separates the measurement signal from the signals to be received. However, the duplex filter must be installed before the receiver parts, which reduces the sensitivity of the receiver because the filter causes losses in the antenna signal.

A solution is also known in which both of the above-mentioned problems are avoided by using a measurement frequency outside the receiver band 10 and a narrowband power measurement adapted to this frequency.

EP patent application 0 261 828 discloses a microwave analyzer which measures both the signal fed directly from the measurement source and the signal reflected from the microwave network to be analyzed for vector-based relative power measurement. In the measurement of the reflected power, the sample signal taken from the output power of the measurement generator is applied to the detector by a different route than the 20 sample signals taken from the reflected power of the microwave network to be analyzed. Thus, in the reference, the relative measurement would be somewhat insensitive to variations in the output level of the measurement generator, but because comparable signals travel different paths to the detector, the measurement does not automatically take into account non-idealities or changes in the properties of the components in the signal paths. For the measurement of the reflected power, the analyzer must be calibrated by first measuring the known microwave standard with the analyzer and using the calibration values ​​obtained in the subsequent measurements. In measuring the condition of the antenna, this would mean that the antenna supply would have to be connected to a microwave standard instead of an antenna for calibration.

This problem is solved in patent application 904085 by measuring the strength of the measurement signal transmitted directly to the receiver in addition to the signal reflected from the antenna. This second measurement provides a reference value of the measurement signal strength at the time of measurement, taking into account the transmission power of the measurement signal transmitter and the characteristics of components in the signal path, such as amplifiers and splitters, to which the strength of the measurement signal component reflected from the antenna is compared. Such relative measurement can eliminate the effect on the measurement accuracy of the variance between the properties of different individuals of the components involved in the measurement circuit and the measurement signal path or of the changes in the properties of the same individual over time. The invention can also substantially reduce or simplify manual checks and calibrations during installation and operation, or even avoid them altogether. 15 The measurement signal is preferably a narrowband signal whose frequency is outside the frequency band reserved for communication, whereby the measurement does not interfere with the actual radio traffic but on the other hand the measurement is not performed at the actual frequencies used and thus does not obtain the actual standing wave ratio SWR.

In new digital TDMA (Time Division Multiple Access) type radio systems, signaling is time division including multiple, typically 8, time slots on a single frequency. One TDMA system is the pan-European25 mobile telephone system GSM. GSM Recommendation 12.11,

3.1.0 B 05 Receiver Antenna Fault sets requirements for monitoring the condition of the antenna.

When the antenna is placed on the mast and equipped with a mast preamplifier, the condition of the antenna cannot be measured in the normal way because the antenna line has that preamplifier between the measurement signal input point and the antenna through which the measurement signal should go in the blocking direction. The attenuation in the blocking direction of the amplifier is typically 40 dB. The measurement accuracy is already affected by attenuations of a few decades, such as the receive filter, which is also on the mast. Thus, there is a need to have a connection point for the measurement signal on the mast between the mast preamplifier and the antenna.

It is an object of the invention to implement a condition measurement of a receiver antenna equipped with a mast preamplifier at a TDMA base station at the actual frequencies used.

This is achieved by an arrangement of the type described in the preamble, which according to the invention is characterized in that the measurement signal to be switched to the directional switching means is at a first frequency in the frequency band reserved for radio communication in the radio system and that the generating means are located in connection with the other base station outside the frequency band reserved for radio communication in the radio system, and that the switching means comprises means connected to an antenna line between the amplifier means and other base station equipment for receiving said measurement signal on said second frequency and means for transmitting said measurement signal from the second frequency to the first frequency before

In the invention, the measurement signal is generated in connection with other base station equipment and is transmitted outside the frequency band used for communication via an antenna line to a preamplifier, where the measurement signal is transmitted to the actual measurement frequency in said frequency band for communication.

In a preferred embodiment of the invention, the radio system is a TDMA system and the measurement is performed in a time slot reserved for a radio test loop, in which case the actual frequencies used for communication can be used. In one embodiment of the invention, the radio frequency measurement 92259 signal is generated as a transmission frequency at the transmitter and looped through the radio test loop as the reception frequency to the reception side, thereby avoiding a separate measurement signal generator. By means of the invention, said signal is obtained in its original form at a switching point in the mast without interfering with the reception signal coming from the antenna in the actual reception band.

The invention will now be described in more detail by means of exemplary embodiments with reference to the accompanying drawing, in which Figure 1 shows a block diagram of a base station receiver apparatus according to the invention, Figure 2 shows a block diagram of subunit 8 of Figure 1.

Fig. 3 shows a block diagram of the mast unit 15 of Fig. 1.

The invention is particularly suitable for use in a base station full duplex type transceiver for TDMA radio systems.

In Figure 1, a TDMA transceiver comprises at least two, preferably four, pairs of transceivers ΙΑ, ΙΟΑ and ΙΒ, vasta, respectively, each pair forming one full duplex connection. The outputs of the transmitter units IA and IB are connected by means of an adder 2 (combiner) to a common antenna line 3 and a common transmitter antenna 4. A receiving antenna 6 located separately from other base station devices, e.g. a mast, is connected via a preamplifier unit 15 (hereinafter referred to as a mast unit) connected to it and an antenna line 7 to a splitter member 9 which distributes the received signal to the receiver units 10A and 10B. In this context, transmitter and receiver units refer primarily to the radio parts of the transceiver. Each transmitter and receiver unit has its own transmission or reception frequency, with the transmission and reception frequencies of the units forming a full duplex pair, e.g. IA and 10A, being spaced apart by a duplex interval, e.g. 45 MHz.

The receiver Rx receives a TDMA signal from the antenna, where each frequency includes several, typically 8, time slots. The signal also has a frame structure in which, for example, eight consecutive time slots form a frame. The frames may further form superframes (e.g., 26 or 52 frames) and the superframes as hyperframes. One TDMA system is the pan-European mobile telephone system GSM. Time slots are mainly used to shift control and traffic channels.

Transceivers often use a radio test loop in which the test frequency of the radio frequency generated by the transmitter section is properly routed back to the receiver section of the same transceiver for reception and analysis. Such a radio test loop for a transceiver is described e.g. in patent FI85080, which is incorporated herein by reference.

For example, in the GSM system, however, at least one time slot in a frame, superframe or hyperframe is reserved for forming a radio test loop from the transmitter to the receiver.

The radio test unit 11 is connected between antenna lines 5 and 7. To this end, the antenna line 5 has a splitter member 3 which branches a part of the transmission frequency RF signal to the input 12 of the radio test unit 11. The radio test unit 11 converts the reception frequency signal into a reception RF signal. The output 13 of the radio test unit 11 is connected to the antenna line 7 by a switching unit 8, which is hereinafter referred to as a subunit. Normally, the radio test loop is designed to test the RF components of the base station with the exception of the receiver antenna and antenna cable.

In a preferred embodiment of the invention, the measurement of the condition of the receiver antenna 1 is performed in the time slot reserved for the radio test loop Kalle. In the embodiment shown in Fig. 1, a radio test signal looped from the transmitter part to the receiver antenna line on the radio channel (frequency) used for normal communication of the radio system is used as the measurement signal during the radio test interval.

Fig. 2 shows the subunit 8 of Fig. 1 in more detail. The reception frequency fr measurement signal received from the radio test unit 11 is mixed in the mixer 20 to a frequency fa outside the reception band by the local oscillator signal LO obtained from the local oscillator 22. For example, when the frequency of the signal 13 is 890-915 MHz and the frequency of the signal LO is 120 MHz, the measurement signal is shifted to an upper or lower frequency of 120 MHz, e.g., the range 770-795 MHz. The upper harmonic of the mixing result is filtered out by a bandpass filter 21, and the downmixed measurement signal is connected by a directional switch 24 to the antenna cable 7 in the direction of the antenna 7. Oscillators 25 and 26 generate control signals having frequencies f1 and f2, respectively. The control signals are connected to the antenna cable 7 by coil L2. The 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 operation and maintenance unit (OMU) 14 and the DC level from line 13 of the test unit 11. When the oscillator 22 is turned on, the oscillator 26 is also in operation and the frequency f2 is transmitted. When the oscillator 22 is off, the frequency f2 is also not transmitted. As will be seen later, with this arrangement, both the oscillator 39 of the mast unit 15 and the oscillator 22 of the subunit 8 are on only during the measurement of the antenna. This prevents possible interference from the oscillators to normal radio traffic. The subunit 8 receives information about the antenna measurement time from the OMU 14 via the line 14B. The DC level of the signal 13, in turn, indicates the direction of connection of the measurement signal, i.e. in the direction of the receiver or antenna. This Direction Information is converted to the frequency fl. When fl is transmitted, there is a switching direction to the antenna. When fl is missing, there is a switching direction to the receiver.

In the mast, the mast unit 15 connected to the antenna preamplifier is illustrated in more detail in the figure

3. The unit 15 comprises, as usual, a receive filter 30 and a hot-swappable amplifier unit consisting of a branch 31, parallel amplifier branches 32 and 33 and an adder 34. The actual measuring apparatus comprises a directional switch 15 width e.g. 770-795 MHz) other frequencies, such as actual reception frequencies. The filtered measurement signal is amplified and then mixed back in mixer 38 to the reception frequency fr by the local oscillator signal LO obtained from the oscillator 39. The signal

The LO frequency is the same as the LO frequency of the subunit 8 signal. Frequencies outside the reception band are removed from the scrambling result by a bandpass filter 40 and the filtered signal is applied to a switch S1. Switch 41 selectively switches the measurement signal alternately to the direction switches

42 and 43, which connect the measurement signal to the antenna cable between the mast unit 15 and the antenna 6. The direction switch 42 supplies the measurement signal in the direction of the preamplifier and the receiver and the direction switch 43 in the direction of the antenna 6. In this way, a measurement signal identical to the original signal 13 is connected to the antenna line 7 between the antenna 6 and the unit 15 before the reception filter 30 without interfering with the actual reception signal going in the other direction,

Switch 41 and oscillator 39 are controlled by control logic 44 according to control signals 35 en f1 and f2 received from the antenna line. The control signals are separated from the antenna cable 7 from the subunit by a coil L and filtered by bandpass filters 45 and 46. The filtered control signals are detected in the control logic 44. When the control logic 44 detects the frequency f1, it controls the switch 41 to position I where the measurement signal is connected to the direction switch 43. If the frequency f1 does not occur, the control logic 44 controls the switch 41 to position II, where the measurement signal is connected to the directional switch 42. When the control logic 44 detects the frequency f2, it turns on the oscillator 39. When the frequency f2 does not occur, the control logic 44 turns off the oscillator 39. With this arrangement, both the oscillator 39 of the mast unit 15 and the oscillator 22 of the subunit 8 are on only during the measurement of the antenna. This prevents possible interference from the oscillators to normal radio traffic. The subunit 8 receives information about the antenna measurement time from the OMU 14 via the line 14B.

The measurement takes place at the normal reception frequencies of the radio system during normal operation of the receiver Rx. The test operation is controlled by the base station operation and maintenance unit 14, which instructs the test unit 11 via the control line 14A to form a test loop in a certain test interval and indicates whether the measurement signal is connected in the direction of the antenna or receiver. In a preferred embodiment of the invention, the test unit 11 then controls the subunit 8 at the DC level of its output 13. Direction control can also be provided directly from the OMU 14 to subunit 8 via control line 14D. Outside the test interval, the measurement signal is separated from the output 13 by a isolating switch inside the unit 11.

The OMU 14 also acts as a measuring unit that receives the signal from the splitter 9 and measures the signal strength. Alternatively, the receivers Rx may include measuring equipment whose measurement result is received by the OMU 14 via line 14c.

The measurement procedure is as follows. The measurand is the antenna standing wave ratio SWR.

1) The OMU 14 issues a normal radio test loop command to the test unit 11, as a result of which the measurement signal is looped in the test interval to the subunit 8. The OMU 14 also informs the subunit 8 of the antenna measurement event, the oscillator 22 starts and the depending on the direction information, e.g. to the direction switch 42, i.e. directly in the direction of the receiver. The OMU 14 measures the strength of the measurement signal and uses the RSSI value of the received signal strength 10 formed from the measurement result as the reference value REFSIGN.

2) The OMU 13 issues a new radio test loop command to the test unit 11, which, however, instructs to direct the measurement signal towards the antenna 6. The switch 41 switches the measurement signal to the direction switch 43. The OMU 14 measures the strength of the signal component reflected from the antenna 6 and uses the RSSI value formed from the measurement result as the measurement value ANTSIGN.

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

4) OMU 14 compares the calculated SWR value with the stored alarm limits. If the calculated value exceeds / falls below the alarm limit, an alarm is issued. The database of the OMU 20 contains a calibrated SWR or SRRI value for each frequency used, preferably also for each antenna sector and for both normal and diversity antennas e. When the base station is first set up or the antenna or antenna cable is changed, measurements 1, 2 and 3 for each frequency, sector and antenna are reviewed and the resulting SWR values ​​are stored as calibrated values ​​in the OMU database.

The figure and the related description are only intended to illustrate the present invention. The details of the method and apparatus of the present invention may vary within the scope of the appended claims.