GB2111352A - Improvements in or relating to radio communication systems - Google Patents

Abstract

A communication system comprising at least two channels, which are carried by a digital transmission signal, one of said channels having greater noise immunity than the other channel or channels, whereby when operating conditions render communication impossible on one channel, another channel having better noise immunity can be utilised. The channels are distinguished one from another by the extent of frequency or phase excursion or pulse width or amplitude.

Description

SPECIFICATION Improvements in or relating to radio communication systems This invention relates to radio communication systems and more especially it relates to radio communication systems adapted to provide two way communication between at least two transmitter/receiver stations.

The reliability of radio communication between two stations is affected by various factors such as transmission power-interference, the distance between the stations, the terrain between the stations, ionospheric conditions, the carrier frequency used, and the channel bandwidth for example.

The quality or reliability of radio communications might be measured either in terms of signal to noise ratio or in the case of digital data transmission in terms of error rate.

If the signal to noise ratio of signals received by the receiver of two stations in two way speech communication for example, either in.a simplex mode or in a duplex mode, falls below a certain limit level, communication between these stations becomes impossible. It will however be appreciated by those skilled in the art that communication becomes impossible in many cases due partly to constraints imposed by the system itself. Thus although speech communication between two stations may be totally impossible due for example to interference, coded data communication at a relatively slow bit rate may with the same basic system under the same interference conditions be perfectly possible at very low error rates.

It is an object of the present invention to provide a communication system having at least two channels one of which affords better communication reliability than the other whereby communications can be maintained when noise conditions make communication on one of the channels impossible.

According to the present invention a radio communication system comprises a transmitter adapted to transmit a carrier frequency which is modulated to carry a plurality of channels comprising a high priority main channel defined by modulation of the carrier frequency to a first depth and at least one further lower priority channel modulated to a second depth smaller than the said first depth.

The depth of modulation used to define a first of the further lower priority channels may be a predetermined fraction of the modulation depth used for the main channel and the depth of modulation used for other further channels may be smaller fractions of the modulation depth used for the main channel, the fraction used being proportional to the priority required.

The character of the modulation may take any form and is arranged to be tapered from channel to channel so that the channel having the highest priority has the greatest depth of modulation and the channel having the lowest priority has the smallest depth of modulation whereby under progressively worsening noise conditions, the channel having the lowest priority will become unusable first and the channel having the highest priority will subsequently become unusable only when the noise level has increased to a much higher level. If the modulation applied is amplitude modulation, amplitude excursions of the carrier which define the main channel will be large compared with amplitude excursions defining the other channels.Similarly for phase or frequency modulated systems a high priority main channel would be defined by large frequency or phase excursions and lower priority channels would be defined by correspondingly smaller phase or frequency excursions.

For the contemporaneous transmission of two or more channels, the modulation of each channel with the exception of the main channel would be superimposed on the modulation of a channel or channels having higher priority.

At least one channel may comprise a digital transmission signal.

Any number of channels may be carried by a single digital transmission signal wherein the channels have differing priorities.

This arrangement permits the information capacity of the digital transmission signal to be fully exploited since it permits the information transmission rate over a communication link between two stations to be closely adapted to operating conditions.

The total number of information channels allocated to the digital transmission signal (hereinafter described as the channel set) may be either permanently connected or switched in or out of circuit as required. Also, the tolerance of individual channels to reduced performance margins of the communications link may be varied by automatic or manual control to satisfy changing requirements or conditions.

The design of future systems may be adapted in accordance with the present invention and existing systems may be provided with add-on equipment.

Ideally the spectrum of the channel having the greatest noise immunity herein called the priority channel should preferably occupy the full width of a normal channel to avoid narrow band interference, however this is not essential and it may occupy a narrower bandwidth.

The most suitable technique for Coding will depend on the type of modulation to be transmitted.

Priority coding i.e. coding of the priority channel or channels, may be applied either at baseband e.g. at audio frequency or in the R.F.

modulation. In both cases, it is assumed desirable that the coding technique used should spread the energy of the priority channel(s) substantially uniformly across the whole of the allocated spectrum of the transmission signal in order to avoid vulnerability to narrow band interference.

Baseband coding has the advantage that it permits a priority coding technique to be applied to existing equipment with the minimum of modification. Also, in principle, there is no upper limit on the maximum permissible number of channels that can be provided. Coding may be achieved either by the direct additon of baseband signals or by digital interleaving, e.g. the insertion of one priority channel bit every 20 non-priority bits say.

Priority coding of the R.F. modulation may comprise the use of a multi-level modulation method (e.g. biternary instead of binary) to create an increased data capacity which may be coded as required.

Alternatively, it may comprise the use of orthogonal modulation types (e.g. AM and FM) or the use of separate co-channel transmission signals which are mutually non-interfering (e.g. N Plex, Patent U.K. No. 2064 271).

The use of multilevel modulation involves a fixed penalty upon the path loss capability of the transmission used. Also, for optimum performance with this technique, modem shaping filters and digit reconstitution circuits of equipment utilising binary modulation would require to be modified.

The use of orthogonal modulation types involves theoretically no reduction of path loss capability of the transmission signal. However, this approach is restricted to the provision of a small number of channels only and would also require modification of existing equipment.

The use of co-channel transmission signals permits physically dispersed channels to share a common transmission signal. Differences in received signal to noise ratios between the various channels at the receiver input are generally preserved at the receiver output, i.e. a stronger channel input signal to the receiver gives a stronger output. Therefore a channel may be allocated priority status by giving it a greater reserve of available power than other channels in the set. Alternatively, all channels may be restricted to give equal signal to noise ratios at the input of the receiver and the priority channel allocated by appropriate baseband coding.

According to one embodiment of the present invention, a radio communication system comprises a transmitter adapted to radiate a digital transmission signal, which comprises the combination of at least two digital channel signals which are synchronised at the bit frequency or a multiple of the transmission signal frequency but wherein the modulation depth differential between mark and space bits is different for each channel whereby the number of different modulation depths present in the digital transmission signal is equal to twice the number of digital channel signals, and wherein a digital channel signal having the smallest level differential between mark and space bits is used for carrying digital data each bit of which comprises a predetermined sequence of bits synchronised with the bit frequency of the transmission signal and wherein the digital channel signal having the largest differential between mark and space bits is arranged to carry other digital signals, and comprising a receiver adapted to receive from a remote station a signal corresponding to the digital transmission signal and to provide an output signal corresponding to each of said channel signals.

The modulation depth may be defined in terms of amplitude excursions in the case of amplitude modulation and/or phase excursion in the case of phase modulation, and/or pulse width excursions in the case of pulse width modulation and/or frequency excursion in the case of F.M.

The said other digital signals may comprise delta modulated speech signals or pulse code modulated speech signals.

Thus when due to interference, speech communication is rendered impossible over one channel, data communication over the other channel carried by the same digital transmission signal, may still be maintained.

It may happen that communications between two stations are quite suddenly broken and a station then has no way of knowing the cause of the break which may be due either to an equipment failure, to interference, or even to the destruction of a station. It is highly desirable to be able as far as possible to assess the cause of a sudden break so that appropriate action may be taken.

In a system according to the present invention indicator means may be provided operatively associated with the channel carrying the said digital data and having the greatest noise immunity, to afford an indication that the communication link has not been broken when speech communication for example over the other channel becomes impossible due to noise.

The degree of noise immunity afforded by the channel carrying the said digital data may be made adjustable in accordance with the operating conditions which obtain.

For this purpose means may be provided for reducing the rate of data transmission.

This may be effected by modifying the said predetermined sequence so that the number of bits at the bit frequency occupied by each data bit is increased.

Alternatively the effective modulation depth differential between mark and space bits may be increased so that the channel carrying the said digital data represents a larger proportion of the digital transmission signal.

The system may comprise means operative in the limit for modifying the system whereby the said digital transmission signal is arranged to consist solely of the said digital data.

Thus it will be appreciated that with the present system, if severe interference is experienced, the noise immunity of the channel carrying the said digital data may be improved until the communications link can be resumed, and this may be effected progressively or in steps.

The receiver may comprise data reconstitution means responsive to the received digital signal for providing an output signal corresponding to the digital channel signal carrying the said other digital signals such as delta modulated speech signals.

The data reconstitution means may simply comprise a limiter or pulse shaper.

The receiver may additionally comprise subtractor means operative to subtract the output signal produced by the data reconstitution means from the received digital signal thereby to provide a resultant signal which is fed to a further data reconstitutor responsive to a recovered clock signal produced by said data reconstitution means for providing an output signal corresponding to the said digital data signal carried by the priority channel.

The receiver may alternatively comprise additionally a full wave rectifier means responsive to the received digital signal for providing data which is fed to further data reconstitutor means responsive also to a recovered clock signal produced by said data reconstitutor means for providing an output signal corresponding to the said digital data signal carried by the priority channel.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a generally schematic block diagram of a signal transmitter modified for the transmission of priority coded signals; Figure 2 is a generally schematic block diagram of a receiver for decoding two priority coded signal channels; Figure 3 is a generally schematic block diagram of a receiver system showing one arrangement for decoding priority coded signal channels; Figure 4 is a generally schematic block diagram of a receiver system showing an alternative arrangement adapted to decode priority coded signal channels and; Figure 5 is a wave form diagram showing wave forms A, B, C, D, E, F, and G appertaining to the radio communication systems hereinafter described.

Referring now to Figure 1, a communication system comprises a standard transmitter 1 adapted to provide a digital transmission signal comprising a 16 K.bit data stream. The transmitter is modulated by signals on a line 3 fed from an amplifier 4. The amplifier 4 is fed with two channel signals on lines 5 and 6 respectively.

The signal on line 5 hereinafter designated channel V1 comprises a non-priority delta modulated audio frequency data stream and the signal on line 6 hereinafter referred to as channel V2 is a priority digital data stream. The signals on the lines 5 and 6 are clocked in phase at 1 6 K.bits to provide the output signal on the line 2. The amplifier 4 serves to combine the channels V1 and V2 in the amplitude ratio 9:1, so that 9/10 of the signal amplitude is contributed by the channel V1 and only 1/1 0 is contributed by the priority channel V2.Thus it will be appreciated that the addition of the priority channel V2 to the digital transmission system signal imposes a 10% reduction on the normal output of the signal link, or a 1 dB penalty on channel V1 and provides a second channel with an output level of 1/10th of normal i.e. 20 dB down). With these particular levels two channels may be demultiplexed at the receiver output using amplitude discrimination since the channel V1 may be digit reconstituted conventionally with a one dB penalty and the channel V2 may be detected by conventional reconstitution after removing the constant amplitude channel V1 signal as will hereinafter be described.

When the digital transmission signal on the line 2 exhibits a 20 dB performance margin beyond that required for normal operation, as might be apparent when ambient noise levels are low, the digital transmission signal will support two full speech channels. If a 10 dB signal noise ratio is assumed necessary for normal operation, then the channel V2 will be operating at dB signal to noise ratio and in this situation and with correlation detection, the channel V2 could support a signalling rate of 1 60 bits at an error probability of no worse than that applicable to the channel V1. By further lowering the data rate of the priority data transmitted on the channel V2 and by using direct sequence coding correlation detection, the priority channel V2 may be given a high degree of protection.When the signal-tonoise ratio at the input of the receiver drops below a level adequate for normal communication, then correlation detection should also be applied to the channel V1 which then becomes the priority channel and an indicator light may be provided which indicates when this situation obtains. In this situation the channel V2 merely acts as an indicator of the status of the channel Vl;that is it indicates that the channel V1 can no longer support a full speech channel and priority data only is being transmitted. The transfer of priority data to the channel V1 may be controlled manually or automatically.

Two alternative techniques will hereinafter be described for the extraction of the lower amplitude channel data from the received baseband signal and both techniques involve subtraction of the channel V1 signal from a combined data stream.

Referring now to Figure 2, a standard receiver 7 may be provided output signals from which on line 8 are fed to data reconstitution means 9. The data reconstitution means 9 may simply comprise a pulse shaper or pulse leveller and output signals from the data reconstitution means 9 are provided on a line 10 and correspond with delta modulated audio signals which are carried by the channel V1. Output signals from the data reconstitution means 9 are fed also via a line 11 to a subtractor 12 which is fed via a line 1 3 from the standard receiver 7 with the digital signal received.The channel V1 signal as reconstituted by the data reconstitution means 9 is subtracted in the subtraction unit 12, from the received digital signal and an output signal on a line 14 is provided which is fed to a further data reconstitutor 1 5 which is fed also with recovered clock pulses derived in the data reconstitution means 9 via a line 16. In this way the channel V2 signal is reconstituted and fed to an output line 17.

Referring now to Figure 5, the channel V1 signal may comprise an audio signal represented by waveform B. The audio signal is delta modulated to provide a digital signal represented by wave form C. The priority channel V2 may comprise a coded data signal wherein a mark or '1' signal comprises a digital signal 18 as shown in waveform D which is synchronised with a clock signal shown in waveform A. A space or 'O' signal may comprise a digital signal 1 9 as shown in waveform D which corresponds to the bits of the mark signal 1 8 but which is phase inverted with respect thereto. The waveform D is combined with the waveform C to produce a digital signal for transmission as shown in waveform E.The waveform E is fed to the data reconstitution means 9 and an output signal on the line 10 is provided corresponding to the delta modulated audio signal shown in waveform C and a reconstituted data signal is provided on the line 1 7 shown in Figure 2 corresponding to the reconstituted channel V2 signal comprising data as shown in waveform D.

In one particular embodiment, a receiver 20 as shown in Figure 3 may be arranged to feed a data reconstitution device 21 which provides on a line 22 an output signal corresponding to the signal carried by the channel Vi. The receiver 20 is arranged to feed a full wave rectification unit 23 via a line 24. The rectification unit 23 feeds a further data reconstitutor 24, via a band pass filter 25, to provide an output signal on line 26 corresponding to the signal carried by channel V2.

When the received signal is full wave rectified the rectification process imposes inversion whenever the high amplitude signal carried by channel V1 is negative and thus the data carried by channel V2 is inverted whenever the signal carried by the channel V1 is negative. Since the 16 K.bit data streams of the channel V1 and the channel V2 are transmitted in phase, the effect of these invertions as aforesaid on the data carried by the channel V2 can be compensated for by precoding the transmitted data. Thus it will be appreciated that if a digit transmitted on the channel V2 is inverted whenever the corresponding digit carried by the channel V1 is a 'space' the required effect will be produced.This is illustrated in waveform G wherein the digital signal for transmission is inverted for the period of the digits 27 whereby after full wave rectification the digital transmission signal as shown in wave form E will once again be produced. It will be appreciated that a.c. coupling of the rectified base band signal removes the mean amplitude level of the data stream in the channel V1 to leave the correct V2 channel data stream plus noise.

In an alternative embodiment, as shown in Figure 4, a standard receiver 28 is provided arranged to feed a data reconstitution unit 29 via a line 30. Output signals from the data reconstitution unit 29 on a line 31 comprise the reconstituted data carried of the channel V1.

Signals from the standard receiver 28 are fed via a level control device 32 and a subtraction unit 33, which is weighted via lines 34, to a line 35 which feeds a data reconstitutor 38 fed with a recovered clock signal on line 37 to provide an output signal on a line 36 corresponding to the signal carried by the channel V2.

The reconstituted channel V1 data is multiplied by a suitable weighting function and subtracted from the received baseband waveform to leave channel V2 data plus noise. The amplitude of the signal input must be accurately controlled to ensure removal of the data carried by the channel Vi. With this system no precoding is required in the transmitter and it can be seen that the priority data carried by the channel V2 is recovered by means of the clock signal in the line 37 derived from the high amplitude V1 data. This technique can be extended to remove predictable intersymbol interference by including in the subtraction term suitably weighted functions of the preceding and following digits carried by the channel V1.

Although the waveforms of Figure 5 show amplitude modulated signals, it will be appreciated that this kind of modulation has been used by way of illustration only and that any other kind of modulation such as pulse width modulation, phase modulation, frequency modulation or combination thereof may alternatively be used without departing from the spirit of the invention.

Claims (14)

Claims

1. A radio communication system comprising a transmitter adapted to transmit a carrier frequency which is modulated to carry a plurality of channels comprising a high priority main channel defined by modulation of the carrier frequency to a first depth and at least one further lower priority channel modulated to a second depth.

2. A radio communication system as claimed in claim 1 wherein the further lower priority channel is modulated to a second depth which is smaller than the said first depth.

3. A radio communication system as claimed in claim 1 or claim 2 wherein the depth of modulation used to define the first of the said further lower priority channels is a predetermined fraction of the modulation depth used for the main channel and wherein the depth of modulation used for other further channels is a smaller fraction of the modulation depth used for the main channel, the fraction used being proportional to the priority required.

4. A radio communication system as claimed in any preceding claim wherein the modulation applied is frequency modulation and wherein the depth of the modulation is determined in dependence upon frequency excursion.

5. A radio communication system as claimed in any of claims 1 to 3 wherein the modulation applied is phase modulation and wherein the depth of modulation applied is determined in dependence upon phase excursion.

6. A radio communication system as claimed in any of claims 1 to 3 wherein the modulation applied is pulse width modulation wherein the depth of modulation is determined in dependence upon pulse width.

7. A radio communication system as claimed in any of claims 1 to 3 wherein the modulation applied is amplitude modulation and wherein the depth of modulation is determined in dependence upon amplitude excursions.

8. A radio communication system as claimed in any preceding claim wherein for the contemperaneous transmission of two or more channels the modulation which defines each channel with the exception of the main channel is superimposed on the modulation defining a channel or channels having higher priority.

9. A radio communication system as claimed in any preceding claim wherein at least one channel comprises a digital transmission signal.

10. A radio communication system as claimed in any preceding claim comprising a transmitter adapted to radiate a digital transmission signal which comprises the combination of at least two digital channel signals which are synchronized at the bit frequency or a multiple of the transmission signal frequency but wherein the modulation depth differential between mark and space bits is different for each channel whereby the number of different modulation depths present in the digital transmission signal is equal to twice the number of digital channel signals and wherein the digital channel signal having the smallest modulation depth differential between mark and space bits is used for carrying digital data each bit of which comprises a predetermined sequence of bits synchronised with the bit frequency of the transmission signal and wherein the digital channel signal having the largest differential between mark and space bits is arranged to carry other digital signals, and comprising a receiver adapted to receive from a remote station a signal corresponding to the digital transmission signal and to provide an output signal corresponding to each of said channel signals.

11. A radio communication system as claimed in claim 10 wherein the said other digital signals comprise delta modulated speech signals or pulse code modulated speech signals.

12. A radio communication system as claimed in claim 10 or claim 11, comprising means for modifying the said predetermined sequence so that the number of bits at the bit frequency occupied by each data bit is increased.

13. A radio communication system as claimed in claim 10 or claim 11 wherein the effective modulation depth differential between mark and space bits can be increased so that a channel carrying the said digital data represents a larger proportion of the digital transmission signal.

14. A radio communication system as claimed in any of claims 10 to 1 3 wherein the receiver comprises data reconstitution means responsive to the receive digital signal for providing an output signal corresponding to the digital channel signal carrying the said other digital signals.

1 5. A radio communication system substantially as hereinbefore described with reference to the accompanying drawings.