GB2104348A - Improvements in or relating to radio communication data systems - Google Patents

Abstract

In a radio communication system a receiving terminal has a number of traffic receivers, each of which can respectively receive a different communication channel. Each channel is capable of receiving data signals, and in order to assess the quality of reception of a channel in the absence of a received data signal, a locally generated test signal is combined with the output of an aerial which is common to all traffic receivers, and the combined signal is injected into a test receiver which is tuned to each radio channel in turn. The test signal is extracted from the test receiver and after demodulation is compared directly with the locally generated data signal so that a data error can be established for each channel. Thus when a new communication link is to be opened, it can be allocated a channel having an acceptably good reception characteristic. <IMAGE>

Description

SPECIFICATION Improvements in or relating to radio communication data systems This invention relates to radio communication systems in which a number of channels at different frequencies are available for selection. In circumstances in which a number of radio channels are available to a user to provide a communication link between a transmitter and a receiver, it is desirable to choose that channel which has the most satisfactory transmission reception characteristics.

One of the most important factors which prejudices the reliable reception of a radio signal is the presence of noise or interference within the transmision band. Although it is possible in principle to measure noise and/or interference in a communication channel, it can be very difficult to assess the adverse effect which it will have upon a signal under practical circumstances. This is particularly so when the signal is not a voice communication, but is data of some kind.Wide variations in the amplitude characteristics and probability distributions of atmospheric noise and in-band and out-band inerference can be encountered, and even sophisticated systems designd to measure the levels present in a channel do not reliably and consistently allow the correct selection of the best channel to be made from those channels which are available especially since the selection may need to be made prior to the presence of a data signal.

The present invention seeks to provide an improved radio communication system.

According to a first aspect of this invention, a method of evaluating a radio communication channel includes the steps of combining the output of an aerial with a test signal comprising a radio frequency signal having a predetermined carrier frequency corresponding to the channel under evaluation and which carries a locally generated data signal; feeding the combined signal to a radio receiver which is tuned to said predetermined carrier frequency; extracting data from the output of the receiver and comparing it with the locally generated data signal to provide an indication of signal degradation occurring in said channel.

According to a second aspect of this invention, a radio communication system having a plurality of radio channels each at a different predetermined carrier frequency includes means for combining signals received at an aerial with a test signal comprising a radio frequency signal having a selectably predetermined carrier frequency corresponding to one of said channels and which carries a locally generated data signal; a radio receiver arranged to receive said combined signal, the radio receiver being tuned in step with the selection of test signals at different carrier frequencies corresponding to the available channels; means for extracting data from the output of the receiver and for comparing the extracted data with the locally generated data for each of said channels; and means utilising the result of said comparisons to select one of said channels for operational use in said radio communication system.

The invention allows the operational performance of a number of available radio channels at different frequencies to be evaluated, even during quiet periods during which no signals are being transmitted or received on those channels. Then, when it is desired to commence transmision, that channel is selected which is suffering least from the adverse effect of noise or interference. The invention can equally be used in a frequencyhopping (or frequency-agility) system in which during transmission the carrier frequency is repeatedly an rapidly altered, and it is necessary for a receiver to frequency hop in step with the transmitter. Successive carrier frequencies in a frequency-hopping sequence can be selected by using the present invention to exclude those frequencies which currently have inferior reception characteristics.Although the invention is particularly suitable for use in connection with a radio system for receiving data signals, it can also be used to evaluate the reception of voice signals. In this latter case the locally generated data signal is chosen to simulate the characteristics of voice signals.

The invention is further described by way of example with reference to the accompanying drawings in which Fig. 1 shows part of a radio communication system in accordance with the invention, and Fig. 2 is an explantatory diagram.

Referring to the drawings, a radio communication system includes a number of traffic receivers 1, only one of which is shown in detail. In practice, a large number of traffic receivers 1 are coupled to a common receiving aerial 2 via a multicoupler 7 which is used to amplify and divide the input signal from the aerial 2 to the receivers 1. Each traffic receiver 1 is arranged to operate in a different communication channel. The input paths to the other remaining traffic receivers are indicated diagrammatically by the broken lines 3, 4. The output of each traffic receiver 1 is fed to a respective demodulator 5, which extracts the data received in a particular channel and passes the data to a printer 6 for utilisation.

In a large communication system, a number of terminals may be present which are able to commuicate with each other over a large number of individual channels. Each channel corresponds to a particular carrier frequency, and generally a number of channels will be grouped within different frequency bands.

Radio communication links are inevitably affected to a greater or lesser extent by the presence of atmospheric noise, and by interference signals produced by other radio transmitters. Whilst the effect of the noise and interference can be readily determined from the extent to which it degrades data received in a particular channel, it is extremely difficult to predict its effect before the transmission and reception of data traffic commences. Thus in a large communication system a large number of channels may be available for subsequent allocation as and when the need arises, and it is desirable to determine in advance which of those channels will be most free of interference and noise.

The invention enables an estimate of the quality of each channel to be made even though that channel is not currently operative to handle real traffic. Part of the signal received at the aerial 2 is coupled by the multicoupler 7 to a test receiver 8 via a signal combiner 9. Any noise and interference present at the aerial 2 is combined by the signal combiner 9 with a high frequency radio signal carrying data derived from a local data generator 1 0. The data generator is operative to produce a sequence of digital information, i.e.

a sequence of logic 1 's and O's. This data is fed to a tone generator 11, which is operative to produce one tone whilst a logic 0 is present and a different tone whilst a logic 1 is pre sent--data transmitted in this way is known as two-tone modulation. In this way the data pattern is converted into a two-tone signal, and this signal is used to modulate a radio frequency signal generated at an h.f. drive circuit 1 2. This drive circuit produces a predetermind carrier frequency corresponding to the channel of the radio communication system which is currently under evaluation and it produces what is, in effect, a radio signal of the kind which is to be received by the traffic receivers 1.However, in this case the signal can be of relatively low power since it is fed via an adjustable attenuator 1 3 to the combiner 9.

Thus the test receiver 8 receives an input signal consisting of the locally generated data signal superimposed on any noise and inerference which is received by the aerial 2. The test receiver 8 provides an output which consists of the data signal in combination with any noise and interference lying within the bandwidth of the channel to which it is tuned.

The output of the test receiver 8 is fed to a demodulator 14, which acts to extract the data and to feed it to a digital comparator 15, where it is compared with the original undistorted locally generated data. A delay circuit 1 6 is provided so that both inputs of the comparator 1 5 are synchronised.

It is necessary to evaluate each channel to determine which one is least affected by noise and inerference, and it is therefore necessary to tune the h.f. drive circuit 1 2 and the test ror:wivrr 8 to er.h of the channels in turn.

This is achieved by a controller 1 7 which selects each of the available carrier frequencies in turn and provides the necessary instructions to the receiver 8 and the drive circuit 1 2 over line 1 8. If the effect of the noise and interference is negligible at a particular carrier frequency, then both inputs to the comparator 1 5 will be in agreement and no errors and very little telegraph distortion will occur.In order to provide a reasonable basis for comparing one channel with another, it is desirable to produce a measurable amount of telegraph distortion and possibly some data errors, and accordingly the variable attenuator 1 3 is adjusted under control of the controller 1 7 to progressively reduce the level of the h.f. drive signal unto it is sufficiently low to be comparable within affected by the noise and interference present at the combiner 9.

Each time the variable attenuator 1 3 is adjusted, all of the channels are monitored in turn by sequentially setting the tuning frequency of the test receiver 8 and the h.f.

drive 1 2 to the different channel frequencies.

Once the relative merits of the different channels have been assessed by monitoring the output of the comparator 15, the results are held by a channel selector 20 which selects the channel having the best reception qualities from amongst those channels which are available. When the need next arises to open a communication link, a free traffic receiver is tuned to the selected channel.

In general, the data signal produced by the data generator 10 will be a square wave pulse train since such a signal contains the largest number of signal level transitions for a given data rate. However, in some circumstances, it may be more desirable to generate a code, or a pseudo-random sequence. Telegraph data is usually sent at one of a number of standard rates, e.g. 50, 75, 110 bands etc., and by giving the output of the data generator 10 a different non-standard rate, accidental unwanted interaction can be avoided.

So far it has been assumed that only noise and interference are received at aerial 2 within the frequency band at which each channel is monitored. When a real radio telegraph signal is received at the aerial 2, it will be routed not only to one of the traffic receivers, but also via the multi-coupler 7 and combiner 9 to the test receiver 8. It will be regarded as an interference signal, since it will combine with the output of the data generator and a very high error rate will be registered by the comparator 1 5. Under these circumstances, it is necessary for the traffic receiver 1 to record the fact that a real signal is being received, so that the output of the comparator 1 5 can be ignored whilst the test receiver 8 is tuned to that particular channel.

It will be appreciated that the system iilustrated in Fia. 1 does not attemDt to measure noise or interference, but instead provides a direct indication of the effect which noise or interference has upon a data signal. Thus the same bandwidth used for traffic is used for evaluation purposes. Additionally, all spurious signals both in-band and out-of-band are the sarne for traffic and the reference signal produced by the data generator. Furthermore, the system avoids severe synchronisaton problems which can arise when the quality of data present in a radiated radio signal is evaluated.

Since both inputs to the comparator are derived from the same locally generated data signal excellent synchronisation can be achieved by adjusting the delay circuit 1 6 (which can be under control of the controller 1 7 if necessary). To facilitate detection of the data signal at the output of the test receiver 8 under conditions of severe noise and interference, the locally generated data signal can be coded in accordance with a predetermined pattern.

The amount of distortion and number of errors detacted by the comparator 1 5 can be used to provide a very good indication of the reliability of a particular communication channel. Once the delay circuit 1 6 has been correctly set then transitions of the data signal generated at the data generator 10 should occur at precisely predetermined instants in time. Variation in these edge transitions occurring at the output of the demodulator 14 are indicative of the level of noise and interference. In a real system the data signal may approximate to a Gaussian distribution centred on the occurrence of data transitions received directly from the data generator 10 via the delay circuit 16.

This is illustrated in Fig. 2 in which a square wave signal representing the output of the delay circuit 1 6 is used to identify the time to at which undistored data signals produced by the demodulator 14 will occur.

Actual tansitions occurring within a time period t1 are considered to be acceptable and this period is used to define a guard band centred on t,. Transitions occurring in the time interval t2 would not actually cause real errors in a radio signal, but would provide an indication that the quality of reception is deteriorating, whereas transitions occurring in the periods t3 may result in actual errors occuring in the detection of data. Thus by analysing the distortion with the comparator 15, a quantitive indication of channel performance can be derived an used by the channel selector 20 in choosing a channel from those which are currently free and available for selection.

The three time slot periods t,, t2, t3 shown Fig. 2 illustrate the principle in simple terms, and in practice a greater number of gradations representing different error levels can be used, and the number of signal transitions occurring in th different time slots can be weighted according to a predetermined scale. In this way, considerable flexibility is available for defining the error thresholds of the comparator 1 5 which represent quality levels for the data channels, as various forms of interfering signals will have different effects on the extent and distribution of telegraph distortion. Although in Fig. 2, analysis is shown as being performed only on positive going signal transitions, negative going transitions can also be treated in a similar manner.

The operation of the comparator 1 5 as shown in Fig. 2 can be simplified if desired.

For example, the total number of signal transitions at the output of the demodulator 1 2 occurring in a specified time period (typically 1 second) can be compared with the known number of transitions actually generated by the data generator 10 in the same time period. In this case the comparator need be little more than a resettable counter. Alternatively, since the data generator 10 produces a signal having transitions which are spaced apart in time by a predetermined interval, the output of the demodulator 1 2 can be monitored for the presence of "short elements"; that is to say, a pair of transitions spaced apart by less than the predetermined interval which is used as a reference period. The existence of short elements represents the effect of interference, and the like, in the radio channel under test, as under ideal reception conditions no short elements will be detected.

Claims (9)

1. A method of evaluating a radio communication channel including the steps of combining the output of an aerial with a test signal comprising a radio frequency signal having a predetermined carrier frequency corresponding to the channel under evaluation and which carries a locally generated data signal; feeding the combined signal to a radio receiver which is tuned to said predetermined carrier frequency; extracting data from the output of the receiver and comparing it with the locally generated data signal to provide an indication of signal degradation occurring in said channel.

2. A radio communication system having a plurality of radio channels each at a different predetermined carrier frequency including means for combining signals received at an aerial with a test signal comprising a radio frequency signal having a selectably predetermined carrier frequency corresponding to one of said channels and which carries a locally generated data signal; a radio receiver arranged to receive said combined signal, the radio receiver being tuned in step with the selection of test signals at different carrier frequencies corresponding to the avialable channels; means for extracting data from the output of the receiver and for comparing the extracted data with the locally generated data for each of said channels; and means utilising the result of said comparisons to select one of said channels for operational use in said radio communication system.

3. A radio communication system as claimed in claim 1, and wherein a plurality of traffic receivers are coupled to said aerial, so that said radio receiver receives the same noise and/or interference signals as the traffic receivers.

4. A radio communication system as claimed in claim 2, and wherein the selected one of said channels is used to carry data traffic.

5. A radio communication system as claimed in claim 4, and wherein a traffic receiver is allocated to a particular radio channel in dependence on the result of said comparisons.

6. A radio communication system as claimed in any of the preceding claims and wherein means are provided for adjusting the level of the test signal which is fed to said combining means.

7. A radio communication system as claimed in claim 6 and wherein the means for adjusting the level of the test signal comprises a variable attenuator, the attenuation of which is set in dependence on the result of said comparisons.

8. A radio communication system as claimed in claim 7 and wherein the attenuation of the test signal is adjusted until measureable data distortion occurs on all channels.

9. A radio communication system substantially as illustrated in and described with reference to Fig. 1 of the accompanying drawings.