GB2307050A

GB2307050A - Method and apparatus for measuring antenna performance

Info

Publication number: GB2307050A
Application number: GB9522666A
Authority: GB
Inventors: Simon Everett Pike; Axel Bree
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-11-04
Filing date: 1995-11-04
Publication date: 1997-05-14
Anticipated expiration: 2015-11-04
Also published as: GB2307050B; GB9522666D0

Abstract

Magnitudes of reflection of individual discontinuities (18) in an antenna system (12) are assessed by the apparatus of FIG. 1. A carrier signal is modulated with a symbol sequence, with a resultant modulated signal injected into the antenna system to generate a reflected signal (56) from a discontinuity. At the same time, the symbol sequence is delayed (52). A test signal (60), having a power spectrum, is then produced by modulating the reflected signal (56) with the delayed symbol sequence thereby producing a test signal having a power spectrum. Filtering of the test signal (60) through a filter (32) produces a filtered signal (33) in which a proportion of the power spectrum is passed dependent on an amount of cancellation of modulation in the test signal (60) arising from modulating the reflected signal (56) with the delayed symbol sequence. A power level for the filtered signal is then determined to assess the magnitude of the reflection.

Description

METHOD AND APPARATUS FOR MEASURING ANTENNA PERFORMANCE Field of Invention This invention relates to apparatus for measuring the performance of an antenna system, and is particularly, but not exclusively, applicable to performance measurement apparatus for use with radio receivers.

Background to the Invention In radio communication systems generally, it is often considered necessary to monitor the performance or operational condition of certain (or all) parts of the system, and to generate alarm signals in the event that component failure occurs or system/component performance falls below a predetermined threshold.

A technique for assessing performance of an antenna system in a transmitter is to measure both the forward power from the transmitter into the antenna system and also the reflected power from the antenna system. Consequently, a Voltage Standing Wave Ratio (VSWR) or a "return loss" parameter may be calculated; both of which characterise the performance of the antenna system. Power will however be reflected from any discontinuity in the antenna system, with a discontinuity occurring whenever there is a change in the impedance, such as would occur when two interconnected components have different interface impedances or, perhaps, when a feeder line in the antenna system is inadvertently crushed out of shape.

For an antenna system of a receiver, it is possible to deliberately inject a signal into the antenna system and to measure the VSWR or return loss in a like manner to that immediately described above. However, the injected signal will, in part, be radiated by the receiver antenna. This radiation may cause interference to other radio communication systems using the same frequency, and may also exceed legal limits for spurious emissions from receivers. Furthermore, the receiver may not be able to operate while measurements are being made.

A further limitation in characterising the performance of an antenna system using VSWR or return loss occurs as a consequence of the fact that if there is more than one cause of reflection within the antenna system, a reflected signal at a port of the antenna system will be the result of the complex (vector) addition of the individual reflections from each discontinuity within the system. As such, the VSWR or return loss of an antenna system containing a plurality of reflections may be better than the VSWR or return loss resulting from only one such reflection, albeit that the performance of the former system may be worse.

A time domain reflectometer has an ability to measure separately each reflection within an antenna system. This particular piece of apparatus injects a pulse of very short duration into the system and measures the delays and magnitudes of the pulses reflected from each discontinuity.

Moreover, each pulse has a broad spectrum which may extend beyond an intended operating band for the components being measured within the antenna system. As will be appreciated, once outside the intended operating bandwidth for these components, their interface impedances may not be controlled and reflections may therefore result. Therefore, a time domain reflectometer is inappropriate in assessing such radio communication systems (with narrow operating bandwidths) since substantial power is reflected outside the intended operating bandwidth of the system, while it is the power reflected within the operating bandwidth that is of interest.

The method of matched filtering is well known for characterising the dispersion properties of a composite signal that consists of the sum of a number of components (which are different instances of a single stimulus signal) having different amplitudes and different delays. One particular technique of matched filtering is known as Swept Time Delay Cross-Correlation (STDCC) in which a source generates, typically, a continuous wave (CW) radio frequency stimulus signal. This stimulus signal is modulated with a pseudo-random binary sequence (PRBS) of known symbol rate using binary phase-shift keying (BPSK). In what can be considered as physically separate reception apparatus responsive to the source, a signal at an input to the reception apparatus is modulated using BPSK with the PRBS previously used in the source.As such, symbol sequences in the source and the reception apparatus are identical, but the rate and timing of the symbol sequences may vary.

A timing offset between the PRBS in the source and the reception apparatus is then arranged to vary slowly (in terms of either using a symbol rate in the reception apparatus that is slightly different from in the symbol rate used in the source, or by periodically inserting a delay between repetitions of the PRBS in the reception apparatus). An output of a modulator in the reception apparatus is then passed through a bandpass filter and the power measured.

In combination, the source and reception apparatus is generally used to characterise the dispersion of a radio frequency propagation channel between two antennas. In this case, an output from the source is radiated from one antenna and an input to the reception apparatus is connected to the second antenna. When the difference between the timing of the PRBSs in the source and reception apparatus equals a delay of a component of the radio frequency propagation channel, the power measured by the reception apparatus rises. The measured power will then be proportional to the power of that component at the input to the reception apparatus.

As will be appreciated, other forms of modulation may be used to generate the stimulus signal, provided that the modulator in the reception apparatus is appropriate for the type of modulation employed in the source.

Therefore, a need exists for an improved method (and apparatus) of measuring performance in antenna systems, which method (and apparatus) is not subject to the inaccuracies of VSWR or return loss measurement, or the bandwidth limitations imposed by time domain reflectometers.

Summarv of the Invention According to a first aspect of the present invention there is provided a method of determining a magnitude of a reflection from a discontinuity in an antenna system, comprising the steps of: generating a symbol sequence; modulating a carrier signal with the symbol sequence to produce a modulated signal; injecting the modulated signal into the antenna system to generate a reflected signal reflected from the discontinuity; delaying the symbol sequence to produce a delayed symbol sequence; modulating the reflected signal with the delayed symbol sequence thereby producing a test signal having a power spectrum; filtering the test signal to produce a filtered signal, whereby a proportion of the power spectrum is passed through the filter dependent on an amount of cancellation of modulation in the test signal arising from modulating the reflected signal with the delayed symbol sequence; and determining a power level for the filtered signal to assess the magnitude of the reflection.

In a preferred embodiment, the symbol sequence is subject to a variable delay interval, whereby the proportion of the power spectrum passed through the filter varies as a result of a variation in the amount of cancellation of modulation.

In a second aspect of the present invention there is provided a receiver having an antenna system containing a discontinuity causing a reflection having a magnitude in the antenna system, the receiver comprising: symbol generating means for generating a symbol sequence; means for modulating a carrier signal with the symbol sequence to produce a modulated signal; means for injecting the modulated signal into the antenna system to generate a reflected signal reflected from the discontinuity; delay means for delaying the symbol sequence to produce a delayed symbol sequence; means for modulating the reflected signal with the delayed symbol sequence thereby producing a test signal having a power spectrum; a filter for filtering the test signal to produce a filtered signal, whereby a proportion of the power spectrum is passed through the filter dependent on an amount of cancellation of modulation in the test signal arising from modulating the reflected signal with the delayed symbol sequence; and means for determining a power level for the filtered signal to assess the magnitude of the reflection.

Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings FIG. 1 shows a receiver having antenna performance measuring system according to a preferred embodiment of the present invention.

FIG. 2 is an anticipated graph, generated by the antenna performance apparatus of FIG. 1, that illustrates the effects of discontinuities in an antenna system.

FIG. 3 is an alternate embodiment of antenna performance measuring apparatus having an independent path within a receiver, for example.

Detailed Description of a Preferred Embodiment Referring to FIG. 1, a receiver (or transceiver) 10 having antenna performance measuring apparatus according to a preferred embodiment of the present invention is shown. More particularly, a receiver 10 for a radio communication system is connected to an antenna system 12 comprising an antenna portion 14 and an antenna feeder 16 connecting the antenna portion 14 to the receiver 10. Usually, the antenna system 12 communicates received signals (through the antenna feeder 16) to a receiver signal processor 26 of the receiver 10. The antenna system 12 may include a discontinuity 18 caused from mis-matching of impedances within the antenna feeder 16, for example. With specific regard to the figure, the discontinuity 18 can be seen to arise as a consequence of interconnected blocks 20 and 22 having different impedances Z1 and Z2, respectively.

A first phase modulator 28 receives a first input from a base of the antenna feeder 16, with an output from the first modulator 28 providing an input signal 30 to the receiver signal processor 26. More specifically, the input signal 30 is coupled to a filter 32 that selectively (i.e. bandpass) filters the input signal 30 to produce a filtered signal 33, with a demodulator 34 arranged to demodulate the filtered signal 33. A signal power measuring circuit 36, such as a received signal strength indicator (RSSI), taps the filtered signal 33 to determine and indicate a signal power (or strength) thereof. The indication of signal power is provided on output 38. As will be appreciated, the receiver signal processor 26 will typically comprise other components and circuits apparent to the skilled addressee, and is generally controlled by a control processor 40 of the receiver 10.

An antenna performance monitoring system 42 (of the receiver 10) is positioned between the antenna system 12 and the receiver signal processor 26. More particularly, a radio frequency (RF) generator 44 and a pseudo-random binary sequence (PRBS) signal generator, both under the operational control of the control processor 40, each provide an input signal to a second phase modulator 48. An output signal 49 from the second phase modulator 48 is coupled, ultimately, to the base of the antenna feeder 18 through a coupler 50. An adjustable delay circuit 52 provides a second input to the first phase modulator 28, and is arranged to delay symbols, generated by the PRBS signal generator 46 and provided to the first phase modulator 28, in time. The adjustable delay circuit 52 is also responsive to the control processor 40. The first 28 and second 48 phase modulators are similar.

With respect to the operation of the antenna performance monitoring system 42, the RF generator 44 generates a carrier (RF) signal 45 within the bandwidth of the filter 32, which RF signal may be either modulated or unmodulated. As will be understood, the necessity and type of modulation is solely dependent upon the manner to the signals which the receiver signal processor 26 is intended to receive. The RF signal 45 is applied to the input of the second phase modulator 48 which typically applies binary phase shift keying (BPSK) with data (symbols) 47 generated from the pseudo-random binary sequence (PRBS) generator 46. The data rate of the binary sequence of the PRBS generator 46 is generally greater than the bandwidth of the filter 32, but less than half of an operating bandwidth of the components of the antenna system 12.Furthermore, a bandwidth occupied by the modulated signal at an output of the second phase modulator 48 will be approximately double the data rate of the binary sequence. A proportion of the output signal 49 is injected towards both the antenna system 12 and the first phase modulator 28. In a preferred embodiment, the coupler 50 is a directional coupler, whereby a greater proportion of output signal 49 is injected in the direction of the antenna system 12 than in the direction of the first phase modulator 28.

An injected signal provided from the coupler 50 will propagate along the antenna feeder 16 towards the antenna portion 14. However, at any discontinuity (within the antenna feeder 16, for example), a reflected signal 56 will propagate towards the first phase modulator 28. Therefore, knowing the velocity of propagation of radio signals within the receiver generally (i.e. the antenna feeder and a RF path to the first phase modulator 28), and bearing in mind a distance between the receiver 10 and any cause of reflection, e.g. discontinuity 18, a propagation time for the reflected signal 56 to propagate from the coupler 50 to the discontinuity 18 (and back) may be determined.

The reflected signal 56 is then modulated with a delayed PRBS in the first phase modulator 28 using a modulation technique corresponding to that used in the second phase modulator 48. Now, provided that the delay of the adjustable delay circuit 52 equals the propagation delay of the reflected signal 56 at the input to the first phase modulator 28, the first phase modulator 28 cancels out the modulation applied by the second phase modulator 48. Thus, all of the power of this test signal will pass through the filter 32 and be measured by RSSI 36.Alternatively, when the delay of the adjustable delay circuit does not equal (offset) the propagation delay of a reflected signal applied at the input to the first phase modulator 28, modulation of the signal will not be cancelled and the bandwidth (and power spectrum) of an "alternate" test signal 60 generated by the first phase modulator 28 will be wider than the passband of filter 32. Thus, only a proportion of the power of this alternate test signal 60 will pass through filter 32 and be measured by RSSI 36.

As will be understood, the reflected signal 56 applied to the first phase modulator 28 may be a composite signal arising from the combination of a signal propagating directly from the coupler and a plurality of reflections (each caused by a discontinuity in the antenna system). However, the magnitude and delay of each component of the signal reflected from the antenna may be determined by varying (sweeping) the delay of the adjustable delay circuit 52 to equal inherent propagation delays from discontinuities and then determining the distribution of the power measured by RSSI 36 as a function of the delay. FIG. 2 is an anticipated graph, generated on output 38 of RSSI 36, that illustrates the effects of such discontinuities in an antenna system. Referring to this graph, many peaks in power are illustrated, with each peak indicating reflections from individual discontinuities.Particularly, a delay for the reflection is determined by a peak in the power level. As will be appreciated, a first peak in the series typically represents a propagation path directly from the coupler 50 and therefore may provide a power reference peak.

Since simultaneous operation of the antenna performance monitoring system 42 may affect the ability of the demodulator 34 of the receiver signal processor 26 te provide an accurate demodulated output, it is preferable that the antenna performance monitoring system 42 is enabled (by the control processor 40) only when it is desired to perform measurements.

Furthermore, in a receiver 10 intended to receive signals employing time division multiplexed (TDM) communication, it is possible to perform measurements using timeslots which are not currently in use by the communication system. In such a system, the duration of the PRBS may be made equal to the duration of the timeslot, and the delay of the adjustable delay circuit 52 incremented for each successive timeslot used for the measurement sequence.

Additionally, in any measurement environment, the power of the signal injected by coupler 50 must be sufficiently high to prevent unwanted signals received by the antenna 14 during a period of measurement from adversely affecting the result of the measurement. In this respect, the power measured by RSSI 36 when the antenna performance monitoring system 42 is not enabled may be used to determine whether it is possible to perform a measurement, or to adjust a level of the signal from RF generator 44 to be sufficiently high that the result of the measurement is not significantly affected by the presence of the unwanted signals.

With regard to the selection of the number of symbols in the PRBS, the bandwidth of filter 32 and a rate of change of the delay provided by adjustable delay circuit 52 are selected such that the power which passes through the filter 32 when the delay does not equal the propagation of a reflected signal is minimised, i.e. the range of peak-to-residual power measurement (of FIG. 2) is maximised.

FIG. 3 is an alternate embodiment of antenna performance measuring apparatus having an independent path within, for example, a receiver. In this figure, elements common with FIG. 1 are identified by like reference numerals. Now, from review of FIG. 3, it can be seen that the receiver includes the receiver signal processor 26, the control processor 40 and the adjustable delay circuit 52 of FIG. 1, with the receiver signal processor 26 and the adjustable delay circuit 52 subject to operational control by the control processor 40.

In a similar manner to the preferred embodiment of FIG. 1, data (symbols) 47 generated by the PRBS signal generator are delayed in time by the adjustable delay circuit 52, and then appropriately modulated in phase modulator 80, as previously described. Unlike the embodiment of FIG. 1, a signal splitter 82 (such as a divider or a switch) is located in the path of reflected signal 56, and provides a mechanism for routing signals (either reflected signals produced by discontinuities during antenna performance assessment or signals received during normal receiver operation) from the antenna system 12 to one of either the receiver signal processor 26 or to a second input of phase modulator 80. Typically, the splitter is controlled by the control processor 40, as illustrated.

Again, in a similar manner to FIG. 1, phase modulator 80 modulates a reflected signal (routed through splitter 82) with the delayed symbol sequence to produce a test signal 60 having a power spectrum. The test signal 60 is then filtered through a filter 84, whereby a proportion of the power spectrum is passed through the filter dependent on an amount of cancellation of modulation in the test signal arising from modulating the reflected signal with the delayed symbol sequence. A power detector 86 (such as a RSSI), responsive to an output from filter 84, then determines a power level for the filtered signal to assess the magnitude of the reflection.

In this respect, operation of the alternate embodiment is identical to that for FIG. 1 and FIG. 2.

It will therefore be appreciated that, in the alternate embodiment, the receiver signal processor 26 operates to demodulate and assess, for example, the signal quality of information signals received by the antenna system 12 during a receive mode for the receiver 10, and that an independent path is provided for measurements of reflections arising during antenna performance assessment.

The present invention therefore advantageously provides an improved method and apparatus for assessing discontinuities in antenna systems since each discontinuity may be assessed individually.

It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the invention, for example, although it is envisaged that filter 84 will have similar (if not equivalent) characteristics to filter 32 of the receiver signal processor 26, improved resolution (with respect to the graph of FIG. 2) is obtained when filter 84 has a narrower bandwidth.

Claims

1. A method of determining a magnitude of a reflection from a discontinuity in an antenna system, comprising the steps of: generating a symbol sequence; modulating a carrier signal with the symbol sequence to produce a modulated signal; injecting the modulated signal into the antenna system to generate a reflected signal reflected from the discontinuity; delaying the symbol sequence to produce a delayed symbol sequence; modulating the reflected signal with the delayed symbol sequence thereby producing a test signal having a power spectrum; filtering the test signal to produce a filtered signal, whereby a proportion of the power spectrum is passed through the filter dependent on an amount of cancellation of modulation in the test signal arising from modulating the reflected signal with the delayed symbol sequence; and determining a power level for the filtered signal to assess the magnitude of the reflection.

2. The method of determining a magnitude of a reflection according to claim 1, wherein step of delaying the symbol sequence further comprises the step of varying a delay interval, whereby the proportion of the power spectrum passed through the filter varies as a result of a variation in the amount of cancellation of modulation.

3. The method of determining a magnitude of a reflection according to claim 2, wherein a delay for the reflection is determined by a peak in the power level.

4. The method of determining a magnitude of a reflection according to claim 1, 2 or 3, wherein there are a plurality of reflections from a plurality of discontinuities.

5. The method of determining a magnitude of a reflection according to any preceding claim, further comprising the step of: determining a power level for unwanted signals received by the antenna system; and varying a power level of the carrier signal such that the power level of the carrier signal is relatively high compared with the power level of the unwanted signals.

6. The method of determining a magnitude of a reflection from a discontinuity in an antenna system for a time division multiplexed communication system having a plurality of timeslots available for communication, the method comprising the steps of any preceding claim and wherein determining the magnitude of the reflection occurs in a timeslot not assigned for communication.

7. A receiver having an antenna system containing a discontinuity causing a reflection having a magnitude in the antenna system, the receiver comprising: symbol generating means for generating a symbol sequence; means for modulating a carrier signal with the symbol sequence to produce a modulated signal; means for injecting the modulated signal into the antenna system to generate a reflected signal reflected from the discontinuity; delay means for delaying the symbol sequence to produce a delayed symbol sequence; means for modulating the reflected signal with the delayed symbol sequence thereby producing a test signal having a power spectrum; ; a filter for filtering the test signal to produce a filtered signal, whereby a proportion of the power spectrum is passed through the filter dependent on an amount of cancellation of modulation in the test signal arising from modulating the reflected signal with the delayed symbol sequence; and means for determining a power level for the filtered signal to assess the magnitude of the reflection.

8. A method of determining a magnitude of a reflection from a discontinuity in an antenna system substantially as hereinbefore described with reference to the accompanying drawings.

9. A receiver having an antenna system containing a discontinuity causing a reflection having a magnitude in the antenna system substantially as hereinbefore described with reference to the accompanying drawings.