WO1995025390A1

WO1995025390A1 - A method and apparatus for measuring digital radio interference

Info

Publication number: WO1995025390A1
Application number: PCT/AU1995/000132
Authority: WO
Inventors: Colin Neill Young; Philip Raymond Casper
Original assignee: Telstra Corporation Limited
Priority date: 1994-03-16
Filing date: 1995-03-15
Publication date: 1995-09-21
Also published as: AUPM451394A0

Abstract

Method and apparatus for measuring digital radio interference. A bit error rate (BER) sender (10) generates a coded signal in bit stream form which is passed to a transmitter modulator and exciter (20) to generate a radio frequency signal which is modulated by the bit stream from the BER sender (10). The modulated RF signal together with radio frequency noise signal (30) received by an antenna (28) are passed via a circulator (26) to an RF receiver (32). Signal from the RF receiver (32) is directed to a BER receiver (34). The BER receiver (34) registers errors occurring in the signal passed to it from the RF receiver (32). The RF signal generated by transmitter (20) may alternatively be directed by an antenna (42) to a receiving antenna (40) arranged to pass that RF signal, as received, together with RF noise signal to the circulator (26).

Description

A METHOD AND APPARATUS FOR MEASURING DIGITAL RADIO

INTERFERENCE

This invention relates to a method and apparatus for measuring digital radio interference, and particularly for assessing the effect of noise on a digital radio transmission system.

Since the introduction of digital radio transmission systems, such as microwave telecommunications relay stations and exchanges, a particular system performance problem manifesting itself as errors in the demodulated digital signal has been recognised.

The errors, it has been found, can be attributed to the effects of sources of impulse noise or interference from power lines, ignition systems of motor vehicles, industrial equipment or radiation from other radio frequency sources, etc., entering the digital radio receiver and corrupting the demodulated signal. Sources of interference have the effect of reducing the standard of performance of the digital radio system by raising the threshold at which errors occur. A reduction in the system fade margin and statistical fade performance is a result, and in severe cases this may cause the radio bearer to be outside of satisfactory performance specifications. These sources of interference are significant in currently used frequency bands up to and including 1800MHz. In addition errors in the digital stream can be caused by poorly performing connectors in the RF feeder lines to the Transmitter or Receiver.

Historically, there has not been a satisfactory method of measuring these introduced errors with the testing equipment available. For example, a spectrum analyser is able to provide basic amplitude information only about the relevant RF spectrum, but in most cases is not able to measure fleeting in band noise or subtle interference buried within the wanted signal envelope. Also, spectral analysis of the received signal is unable to satisfactorily detect bursty or impulse type noise characterised by a fast rise time and is unable, in any event, to judge the effect that such noise has on the desired received signal. In some instances measurement of error threshold at sites prior to equipment installation commencing is desirable to determine the magnitude of the effect of locally originating site noise and interference. In addition, it is sometimes desirable to be able to track down the location of noise and interference sources affecting a site but external to that site. Furthermore, it may also be desired to measure errors caused by faulty or poorly adjusted connectors in the radio equipment coaxial feeder lines.

Unwanted electrical signals can arise from a variety of sources, generally classified as human interference or naturally occurring noise. Human interference comes from other communication systems, ignition and commutator sparking, mains power hum, and so forth, whereas natural noise-producing phenomena include atmospheric disturbances, extraterrestrial radiation, and circuit noise. However, for ease of reference, throughout this specification the term "noise" is used to refer to any form of unwanted electrical or electromagnetic signal, whilst the term "interference" refers to the effect of noise on a digital signal under study.

In accordance with the present invention there is provided a method for measuring interference in a digital radio system, comprising: generating a digital data stream; modulating a radio frequency signal on the basis of the digital data stream; directing an antenna at a possible noise source so as to receive a noise signal; combining the modulated signal with the received noise signal to form a combined signal; demodulating the combined signal; and counting the bit errors in the demodulated signal.

Preferably the digital stream is generated by means of a bit error rate test set.

Preferably the modulated signal is attenuated, before combination with the noise signal, to a level in the range -lOOdBm to -20dBm.

The invention also provides an apparatus for measuring digital radio interference, comprising: an antenna for receiving a noise signal; means for generating a digital data stream; a modulator for modulating a radio frequency (RF) signal on the basis of the digital data stream; coupling means for combining the received noise signal with the modulated RF signal to form a combined signal; a demodulator for demodulating the combined signal; and means for counting the bit errors in the demodulated signal.

In one form of the invention, the antenna comprises a portable and/or directional antenna to enable a source of noise to be more easily determined by correlating the number of bit errors with the direction or location of the antenna.

Preferably the modulator includes a transmitter exciter and a variable attenuator and produces a modulated RF signal having a level adjustable in the range -lOOdBm to - 20dBm before being combined with the received noise signal.

The invention is described in greater detail hereinafter, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a block diagram illustrating a first embodiment of the invention;

Figure 2 is a more detailed block diagram of the first embodiment; and

Figure 3 is a block diagram of a second embodiment of the invention.

Referring firstly to Figure 1, there is shown a block diagram of a digital noise and interference tester (DNIT) 2, comprising generally a sending section 4, a receiving section 6, and a noise introduction section 8. The sending section 4 includes a bit error rate (BER) sender 10 which generates a coded digital bit stream. The bit stream from the BER sender 10 is passed to a transmitter modulator and exciter 20 which comprises the low power stages of a standard radio transmitter. The transmitter modulator and exciter 20 generates a radio frequency signal which is modulated on the basis of the bit stream from the BER sender 10. The modulated RF signal is passed through a lOdB fixed attenuator 22, acting as a termination as well as attenuating the modulated signal, to a variable attenuator 24. The attenuated modulated signal output from the variable attenuator 24 is applied to a circulator 26 which inserts an additional attenuation before passing the signal to an RF receiver 32. The RF receiver 32 demodulates the signal output from the circulator 26, and the demodulated signal is passed to a bit error rate (BER) receiver 34 of the receiving section 6. Also coupled to the circulator 26 is an antenna 28 which can be moved or directed to a source of noise or interference so as to receive a noise signal 30. The received noise signal from the antenna 28 is passed to a low loss port of the circulator 26 where it is combined with the modulated and attenuated RF signal, and it is the combined modulated and noise signals which is passed to the RF receiver 32.

The basic operation of the DNIT 2 is as follows.

A radio frequency signal from the low power stages 20 of a transmitter is modulated with a digital stream from the BER sender 10 of a test set. The low level transmitter carrier is connected via an attenuation network 22, 24 and RF circulator 26 into a receiver 32 tuned to the same frequency as the transmitter. The receiver is in turn connected to the bit error rate (BER) receiver 34 of the same test set, thus forming a closed loop condition for the digital signal.

With this arrangement digital errors and error rates occurring within the loop, at any selected RF level, will be registered at the BER test set receiver 34. The RF level emanating from the transmitter can be adjusted by way of the variable attenuator 24 from levels below the receiver mute to levels well above normal signal levels for the receiver, eg. -lOOdBm to -40dBm.

A second input from a test antenna 28 is inserted into the receiver 32 via the second input port on the circulator 26. The test antenna 28 is mounted in such a location at the site being tested, so as to receive any site noise 30 that may be present. The receiver input filter will remove all out of band noise and interference. The remaining signal going to the receiver will consist only of any inband site noise and will not be attenuated significantly as it does not pass through the attenuator 24 as does the input to the receiver coming from the transmitter 20.

The effect of this noise 30 on the modulated BER signal is registered on the BER receiver 34. The RF level from the transmitter can be adjusted through the full range of receiver input to examine the effect of noise and interference for various received levels down to receiver mute. An adjustment can be made to the transmitter RF level in order to counteract the small loss that the inband noise will incur from the circulator. This adjustment is minor (< IdB) is necessary to obtain a corrected ratio of noise and interference to RF carrier. The noise or interference floor for a particular radio site may thus be determined.

The construction of the DNIT 2 is shown in greater detail in Figure 2, which is described below.

The BER sender 10 generates a HDB3 8.448 Mbit/s code complying with CCITT Rec G703, Section 6, which is passed from the BER Tx unit to the input port of an AWA RMD 1808 baseband transmitter 20a. The coded digital signal modulates a carrier (in this case at 1800 MHz) and is amplified to a level of approximately +16dBm in the transmitter exciter unit 20b. The modulated carrier signal thus generated is then applied to a lOdB fixed attenuator 22 acting as a termination as well as attenuating the modulated signal. The modulated signal is passed on to a bandpass filter 12 which allows only the desired inband signal to be passed on to a second fixed attenuator 16, which attenuates the modulated signal by a further 20dB. The signal level at the output of the second fixed attenuator at a level of approximately -14dBm is passed on to a variable attenuator 24 which is used to adjust the signal over the range of -80dBm to -20dBm.

The signal from the output of the variable attenuator 24 is then applied to a circulator 26 which inserts an additional loss of approximately 20dB. From the output of the circulator the signals are adjustable in the range of approximately -lOOdBm to -

40dBm, by way of the variable attenuator 24, and are connected into a receiver 32 via a bandpass filter 14. The signal is thus demodulated by the receiver 32 and appears at the baseband output as HDB3 code. This digital code is processed by the BER Rx equipment 34 and any errors occurring between the transmitted and the received code are counted.

With the arrangement thus far described in connection with Figure 2, the only errors that will be registered are equipment errors and should approximate the BER calibration curves for the receiver equipment. The errors registering will be few down to RF levels in the order of -88dBm after that error levels will rise significantly as per the BER equipment curves. The equipment RF threshold level for an error rate of 1:10^"3 can be determined from the calibration curves, and a level of approximately -89dBm to -90dBm would be anticipated.

Site noise and interference 30 is added into the DNIT 2 from the site antenna or a test antenna 28 via an operate, calibrate/terminate switch 36 and low loss (0.4db) port on the circulator 26. Inband noise is passed through the receiver bandpass filter 14 and will affect the Bit Error Rate of the digital signal demodulated by the RX unit 32.

Thus the site noise received by the antenna 28 and combined with the modulated RF signal will register as an increase in the error rate measured by the BER receiver 34. The RF signal level coming into the receiver 32 from the variable attenuator 24 may be adjusted in order to determine the new level of the RF threshold for an error rate of 1:10^"3 for the site. Experimental results indicate site threshold levels will range approximately from -70dBm to -88dBm, which will of course depend on the duration of measurement and the nature of the error source.

Figure 3 is a block diagram which illustrates another embodiment of the invention, wherein the receiving portion 32, 34 of the DNIT is coupled to an antenna 40 of a digital radio station, and the transmitting portion 10, 20 is coupled to a portable test antenna 42. In this embodiment the digital code generated by the BER sender 10 is modulated and transmitted from the test antenna 42 to the system antenna 40. The signal received by the system antenna 40 will include any ambient noise or interference. The level of noise or interference received at the system antenna 40 will thus be reflected in the BER count measured by the BER receiver 34, as described above. Where the digital radio station under test has already been installed and is operational, it may not be possible to halt operation of the station for testing procedures. Therefore it may be necessary for the system transmitter 44 to be operational during the testing procedure in accordance with the invention so that the telecommunications link to which the station belongs is not broken.

If the working transmitter 44 is required to be operational during the test, then the DNIT test can be carried out on an adjacent radio channel which would be subject to the site noise in a similar manner to the operational channel, provided that the site noise is wideband in nature, such as vehicle ignition etc. The test will then determine the site noise floor. However, if the noise/interference is due to a discrete carrier falling within (or partly within) the operational channel bandwidth, the operating transmission system will need to be set to another channel or turned off while the test takes place, to avoid having the DNIT interfere with the operating transmission system.

The DNIT system described herein is able to make accurate real time measurements of the errors directly resulting from site noise and interference. The error measurements can be made at a variety of RF levels thus providing the flexibility to examine the error rate caused by very low levels of noise and interference. The DNIT system is therefore significantly more sensitive than a spectrum analyser which is in any case not capable of measurement of error rates and in many instances will not register the noise or interference of interest in digital radio systems.

The DNIT includes its own transmitter and receiver circuitry and noise insertion system comprising an antenna and circulator. The error measurement capability of the unit provides a significant improvement over the prior art in the measurement of the effects of noise and interference on digital systems and the detection of noise and interference sources. In particular, since the DNTT measures the actual affect of noise on a RF signal, rather than simply measuring the noise itself, the resulting measurements give a good indication of how to implement or alter a digital radio station at the measurement site so as to overcome the noise and interference difficulties. Furthermore, with the noise insertion system comprising a portable and/or directional test antenna, the sources of noise affecting a digital radio system can be pin-pointed, to also aid in dealing with the noise source. For example, it has been found that automobile ignition systems can generate impulse noise which interferes with digital radio reception. Therefore, if the DNIT is utilised at the time of selecting a site of a digital radio relay station, a location can be chosen in relation to nearby roads at which the noise is minimal.

The specific implementations and embodiments of the invention described hereinabove have been put forward by way of example only, and is not intended to be limiting to the invention, which includes every novel feature and novel combination of features herein disclosed.

Claims

CLAIMS:

1. A method for measuring interference in a digital radio system, comprising: generating a digital data stream; modulating a radio frequency signal on the basis of the digital data stream; directing an antenna at a possible noise source so as to receive a noise signal; combining the modulated signal with the received noise signal to form a combined signal; demodulating the combined signal; and counting the bit errors in the demodulated signal.

2. A method as claimed in claim 1 wherein the digital stream is generated by means of a bit error rate test set.

3. A method as claimed in claim 1 or claim 2 wherein the modulated signal is attenuated, before combination with the noise signal, to a level in the range -lOOdBm to -20dBm.

4. An apparatus for measuring digital radio interference, comprising: an antenna for receiving a noise signal; means for generating a digital data stream; a modulator for modulating a radio frequency (RF) signal on the basis of the digital data stream; coupling means for combining the received noise signal with the modulated RF signal to form a combined signal; a demodulator for demodulating the combined signal; and means for counting the bit errors in the demodulated signal.

5. An apparatus as claimed in claim 4 wherein the antenna comprises a portable and/or directional antenna.

6. An apparatus as claimed in claim 4 or claim 5 wherein the modulator includes a transmitter exciter and a variable attenuator and produces a modulated RF signal having a level adjustable in the range -lOOdBm to -20dBm before being combined with the received noise signal.

7. A method for measuring interference in a digital radio system substantially as hereinbefore described with reference to the accompanying drawings.

8. An apparatus for measuring digital radio interference substantially as hereinbefore described with reference to the accompanying drawings.