GB2196514A - Radio communications - Google Patents

Abstract

A radio communication system is operative for transmission and reception relative to a single transmission frequency. Transmission 16 and reception 18 intervals are defined 20 to alternate in time, and reception involves sampling 38, 40 received signals only during reception intervals. A repeater is described, Fig. 1, as is a transceiver (Fig. 4), which is operative as a repeater. <IMAGE>

Description

SPECIFICATION Radio communications This invention relates to communications systems and has particular application to radio communications systems whether as practiced by licensed radio amateurs or as employed by professional operators such as those involved in the well-known Cellnet system and whether of simplex or duplex operation.

Origins of this invention are from problems associated with actual or effective range and achieving out-of-range communication. Thus, problems can arise in setting up temporary communications say to help with disasters/accidents, perhaps particularly in remote areas.

One frequent problem is in relaying communications over or round hills/mountains, especially in maintaining contact with mobile radio operators. Amateur radio practitioners have proved to be extremely useful in such circumstances, at least in providing links to, sometimes between, established communication systems some of which include repeaters permanently installed at suitable sites such as hilltops. A first objective of the inventor was to provide a portable repeater that could be taken to any desired site and set up for operation unattended or with minimum operator attention thereafter.

In addressing that objective, it was appreciated that the intended portable repeater should be capable of handling single frequency transmissions as used by many amateur radio practitioners in a simplex manner, i.e. able to transmit or receive but not both at any one time; and also two-frequency transmissions as conventionally used for duplex communications, i.e. able to transmit and receive at the same time, where one frequency is used for transmission and the other for reception between communicating stations. The solution arrived at not only copes with single frequency transmission for the purposes of a repeater but enables duplex transmissions on a single frequency, which could allow the capacity of conventional two-frequency duplex systems to be doubled.

According to the invention, a radio communication system includes means operative relative to one transmission frequency or channel to define transmission and reception intervals alternating in time, preferably nonoverlapping, and associated means for sampling received signals only during said reception intervals. Such sampling is conveniently after demodulation from carrier signals, i.e. relative to audio content signals.

Intelligible radio voice communications result using a repetition rate of said transmission and reception intervals that is higher than the usually accepted maximum necessary radiated speech frequency (3 KHz), preferably substantially higher, say above 2-1/2 times, for example 10 KHz.

Considering successive transmission and reception intervals as parts of the same transceiver cycle, a convenient mark-space ratio representing transmission and reception intervals is 1:2. It is preferred that the reception interval be spaced in time from the preceding transmission interval sufficiently for transmission to be reduced in an orderly way, and that a sampling interval be defined in the reception interval so that the former starts sufficiently after the latter for signal receiver means to have settled electrically.

Each transmission interval will, of course, accommodate a burst of carrier signal suitably modulated for voice information purposes. It is preferred that such modulation is by frequency rather than amplitude thereby to facilitate receiver "capture" of the strongest frequency and militate against spurious whistles etc. that can result from mixing of different strength signals virtually on the same frequency or channel. However, it is preferred that bursts of carrier signal for transmission are synchronised with the aforesaid transmission intervals in a way that reduces incidence of wide bandwidth clicks, basically by superimposing an envelope on the burst that has appropriately slow rise and fall, for which purpose an amplitude modulation stage is satisfactory.

That can conveniently be via an amplitude modulated transmitter fed with a trapezoidal modulating signal and going to and from 100% modulation for each transmission interval, say rising at the start of that interval and falling from its end to substantially zero at or before effective start of the next reception interval.

For a repeater embodying this invention, a one cycle delay is effectively introduced in that samples taken in reception intervals are each retransmitted in the next following transmission interval. Short term memory means is, of course, required to serve each of successive ones of such samples, and a socalled sample-and-hold circuit is suitable, same conveniently being reset as the next sample is taken. Then, the repeater is entirely transparent (effectively invisible) to parties engaged in single frequency simplex transmission.

For duplex communication using a single frequency or channel, it is arranged for each party's transceiver to be operative so that its reception interval encompasses the transmission bursts arriving from the other party and for those received transmission bursts to encompass the sampling interval of the reception interval concerned. Then, speech signals locally regenerated from that sampling can be applied to a speaker or earphone. Transmission would take place between reception intervals. Having reception intervals substantially longer than transmission intervals themselves substantially longer than sampling intervals is generally advantageous in practising this inven tion.

Synchronisation for duplex communications using a single frequency channel is readily achieved, as is setting up of a communication.

A simple synchronisation system involves one station always being considered the master and the other(s), say mobile(s), as slave(s) operative to start transmission intervals only after reception of a pulse/burst of signal from the master whether with or without audio content, i.e. so that a slave ceases to transmit when the master terminates the communication. Setting up by either master or slave is readily achieved by transmission of a coded sequence of transmission pulses/bursts to which the other must respond also by coded transmission pulses, the slave re-trying until the master responds.

It will be appreciated that the well-known cellular radio system could, by use of this invention, have its traffic carrying capacity doubled as only one and (not two) frequencies are required for each communication between the parties.

The range of an existing two-frequency duplex system may be extended by use of two portable repeaters synchronised together, so that both transmit at the same time and receive at the same time, but each one is on different frequencies-one on the frequency which is used for transmission from the existing base station, and one on the frequency of the transmissions from existing mobile stations. That would extend the service area of an existing duplex scheme to cover an area not normally serviced, say during the course of a disaster/accident rescue operation.

Specific implementation of this invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block circuit diagram of a repeater; Figure 2 is a basic logic circuit diagram of a control unit of Figure 1; Figure 3 shows a set of timing pulse signals and other waveforms; Figure 4 is a block circuit diagram of a single frequency duplex transceiver; and Figure 5 is a circuit diagram of a modulation envelope defining circuit.

In the drawings, referring first to Figure 1, the repeater 10 is shown with an aerial connection 12 for an aerial 14, a transmitter section 16, a receiver section 18 and control logic unit 20. The receiver section 18 is for FM detection, see radio frequency preamplifier stage 22, one or more sets of mixer/intermediate frequency amplifier (24/26) stages, and FM detection stage 28, and provides an audio frequency output signal to a suitable amplifier 30. A further amplifier and speaker or earphone are indicated at 32, 34 for monitoring purposes.

The receiver section 18 is selectively enabled and disabled, see line 36, and it is convenient to achieve that by way of a switched type of preamplifier stage 22 and/or by a switched type of frequency amplifier stage 26, both being indicated in the drawing for illustrative purposes. The desired effect is that the receiver section 18 is active only at intervals (reception intervals) prescribed by signals on line 36, and is to be achieved in any convenient way.

During such reception intervals, at respective sampling intervals, signals on line 38 enable sample-and-hold circuitry 40 thus operative periodically to derive amplitude samples of the audio signal from amplifier 30 via branch 42, say as a level of charge on capacitor 40C set by each sample and then available over line 44 until the next sample is taken, i.e. throughout intervals between sampling intervals including what are called herein transmission intervals and actually defined by signals to be described relative to output 46 of the control unit 20.

Line 37 from the FM Detector 28 to the logic control unit 20 is for squelch purposes to inhibit the transmitter section 16 in the absence of a valid received signal. However, this may come from another part of the receiver.

Signals on line 44 corresponding to audio content of received signals will naturally be a step signal corresponding to successive samples taken and held by circuitry 40, and are shown applied to an amplifier 48 which sets the required level of sample for frequency modulation. (In practice, the level produced by a buffer amplifier inside the sample/hold 40 may be adequate, thus rendering amplifier 48 unnecessary.) Such audio signal samples serve as input to the transmitter section 16 operates on an FM basis, see FM modulator stage 52, any required frequency multiplication stage or stages 54, and buffer amplifier stage 56; then, for the purposes of achieving effective transmission intervals in a manner preferred herein, also an amplitude modulated transmitter stage 58. This may be followed by a linear radio frequency amplifier to raise the transmitted output power to the desired level.

Nominally, transmission intervals are defined by pulses on line 46 from the control unit, which pulses comprise a stream at the same repetition frequency as reception interval defining pulses on line 36 and interleaved therewith. Those pulses (46) could be used directly to control one or more output amplifier stages of the transmitter section 16, see dashed line 46A.As preferred herein, however, leading and trailing edges of the pulses on line 46 are adjusted to reduce their rise and fall slopes in a modulation envelope defining circuit 60 (see Figure 5 and associated description for further details thereof) the output of which is taken via buffer amplifier 62 to serve in controlling transitions between what amounts to 100% modulation by the transmitter stage 58 (and effectively suppresses transmitter stage output) and a much lower even zero modulation by the stage 58 to give bursts of frequency modulated carrier signal as output in the transmission intervals. Another use for signals on line 46A, then conveniently as wider pulses than on line 46, would be as enabling signals affording positive blanking of the transmitter section output between shaped transmission pulses.

In connection with what is described above, it will be appreciated that one or more demodulating frequency signals to mixer stages of the receiver section 18, see line 64 to mixer 24 and dashed line 66, can readily be provided to suit normal FM radio practice, as can an appropriate frequency signal on line 68 to the FM modulator stage 52 of the transmitter section 16 and appropriately related to such multiplier stage or stages 54 as may be required or desired. Moreover, such various frequency signals may all be derived from a single fast clock pulse source, usually of crystal controlled type, or from several independent sources as desired.

Figure 3 shows idealised basic control signal waveforms for the repeater of Figure 1. Signals C and E correspond to lines 36 and 46 from the control unit, i.e. define basic reception and transmission intervals respectively, and signal D corresponds to sample-and-hold enable via line 38, i.e. its pulses are within the 'on' levels of signal 36 and shown near their ends to allow the receiver section 18 to settle before samples are taken. Signal F represents actual bursts of transmitted FM signals according to on-off and transmission enveloping developed in circuitry 60 from signal E and shown with a slowed rise time from the start of each signal E pulse and a slowed fall time from the end of the same pulse.A suitable repetition rate for the C to F signals is found to be 10 KHz which is more than three times the normally accepted maximum frequency for speech signals and permits bursts of over 4000 and over 13000 cycles of radio carrier in the 2 metre and 70 centimetre bands. A ratio of transmission and reception intervals in each cycle of repeater operation of about 1:2 makes adequate allowance for all settling requirements of the alternatingly operative repeater sections 16, 18 as also indicated in Figure 3. That Figure further shows such cycle of 10 KHz frequency as signal A and a master clock signal B at 100 KHz from which it is convenient to derive all required signals in the control unit 20.

Figure 2 shows logic outline of a suitable control unit 20 using a 1 MHz clock source 70 of crystal oscillator type decimal frequency divider 72 to get the 100 KHz clock pulses on line 74, and decimal dividing counter 76 effectively defining the basic 10 KHz cycles of the repeater, OR logic gate 78 connected to deci mal digit outputs 7 to 9 provides the transmission control signal E for the last three-tenths of the repeater cycle, i.e. from 70% thereof to its end, branch 80 from decimal digit output 5 provides sample-and-hold control signal D for the fifth tenth of the repeater cycle, i.e. from 50% to 60% thereof.

Logic gates 82 (OR for counter digits 2-5), 84 (AND for counter digit 6 and master clock) and 86 (OH for outputs of gates 82 and 84) provide the reception interval control signal C for the second to between the sixth and seventh tenths of the repeater cycle, i.e. ffrom 20% to 65% thereof. In addition, Figure 2 indicates output 88 from the digit (1) stage of the counter divider 76 to be used for active clamping should aerial ringing be found to cause any problem. Receive squelch can also be provided for via a latch connected between counter digit (3) and ANDing of squelch-in with output of gate 84 (to validate received signals), and serve to control a transmit inhibit gate also receiving output of gate 78 and a sample valid gate also receiving bit '0' of the divider 76.Buffer amplifiers are also shown feeding all output lines of the control unit, which can conveniently further provide a 1 MHz output, say for derivation of other frequency signals for receiver and transmitter sections or control circuits within the repeater, or synchronised with it. In practice, of course, NOR and NAND gates can be used to get required signal levels by such additional gates as may be required acting as inverters.

Referring to Figure 5 and the amplitude modulation envelope generator diagram, a timer integrated circuit 250 is used to generate a shaped pulse whose period is determined by the trigger voltage on pin 2 via line 46 which is derived from the transmit enable logic line.

The capacitor on pin 6 of the IC 250 is charged via the constant current circuit consisting of a field effect transistor FET1 and 1 resistor (R15). This produces an approximately linear ramp of voltage which starts when pin 2 goes low (logic 0) and continues until the capacitor 100p reaches the maximum available voltage from the supply.

The charge on the capacitor is maintained until the logic level on pin 2 goes high, whereupon the capacitor is linearly discharged via the other FET2 and resitor via a transistor within the integrated circuit 250.

The output from the capacitor is buffered by a high input impedance CMOS operational amplifier, so that an output can be taken to drive the PIN diode modulator without distorting the waveform. The capacitors, C1, C2 are for decoupling purposes. The input line 46 is taken from the transmit enable line after gate 78 (see Figure 2). Additional logic gates may be included in the Transmit Enable line in order that the transmitter output stage may be left enabled for a short period after the amplitude modulator has commenced its downward ramp, so that an abrupt termination of the transmit pulse will be avoided.

Reverting to Figure 1, the aerial coupling is shown with a suitable band-pass filter 94.

Turning to Figure 4 for a single frequency duplex transceiver suitable for radio telephone use, parts that are the same or similar to what is shown in Figure 1 have references advanced by 100. It is basic to this transceiver that its transmitted pulse bursts are timed to take place between received pulse bursts.

Pulse bursts for transmission are shown produced in a manner wholly analogous to Figure 1 but relative to speech signals from telephone hand set 200 (which may be local or remote, e.g. telephone subscriber's hand set via telephone exchange) shown going via amplifier 202 to sample-and-hold circuitry 140, thence via buffer amplifer 148 to the transmitter section 116, all under control of signals from control unit 120 over lines 138 and 146 via envelope shaper 160 and amplifier 162. However, it is feasible for speech signals from hand set 200 to go straight to the transmitter section 116, i.e. relying on envelope circuitry 160 for final stage amplitude modulation in the transmitter section 116 and any transmission inhibit, if used, on line 146A to define output pulse bursts.One reason for showing sample-and-hold ciruitry will be appreciated in relation to dual operation of the Figure 4 circuitry as a repeater or transceiver.

Received pulse bursts go from aerial connection 112 to the receiver section 118 for demodulation to give speech-content signals at its output but effectively as successive segments on steps thereof corresponding to the sampling for transmitting purposes. Those signals could then be reconstituted into a continuous audio-envelope signal in amplifier 130 for application to the hand set 200. However, it is preferred to subject the output of amplifier 130 to sample-and-hold techniques, see circuitry 210 (including input and output amplifiers), which will serve in removing any distortion effects that might otherwise result from the shape of the transmitted pulse imposed by its relative modification for selective compression when prepared for transmission, see also control line 212 from control unit 120.

Relative timing of transmission and reception intervals require their intercalation in time and can be done by detection of presence of radio carrier in the receiving section, though a variant on that is shown via circuitry 214, which may be of phase-locked loop type, and operating on the basis that each received pulse burst or superimposed modulation thereon produces detectable output from the de-modulator 118, e.g. as a pedestalling subsequently removed by amplifier 130. It will, of course, be appreciated that an envelope-dependent signal can readily be used via circuitry 214 to control timing within the unit 120. A further variant would be via a phase-locked loop type synchroniser operating off the output of the amplifier 130.

Additional elements of the units 120 are indicated at 212A and 214A for supplying the line 212 and responding to the circuitry 214.

Such circuitry readily permits operation on a basis where an established communication between two stations continues effectively under the control of one station (master) with the other (slave) timing its cycles relative to reception of pulse bursts, i.e. transmission intervals following reception intervals, and that can continue to be the case, after the first pulse, where the slave may initiate the call. Initiation can be by way of coded transmissions and responses, conveniently with a delay, say of two transceiver cycles, between re-trys by the slave in the absence of master response. Each communication will be on a single frequency for both directions and be full-duplex. The usual choice of frequency channels can be made after the manner of Cellnet systems with appropriate controller operation via connection to the receiver and transmitter sections.Using the preferred 1:2 transmission/reception interval ratio, channel spacings of about 12 1/2KHz should be achievable, which would improve upon current UHF specification of 25KHz particularly bearing in mind that adjacent channels would not be used in the same cell, but in more remote cells.

Compared with Figure 1, hand set 200 and amplifier 202, sample-and-hold circuitry 210, and synchronising circuitry 214 are additional in Figure 4,. and it is considered that dualpurpose simplex repeater and duplex radio telephone apparatus would be usefui. Same can readily be achieved via a mode switch operative both to break the connection to the circuitry 214 and to bring in a circuit path paralleling the circuitry 210, 200, 202, see 'X' marks and phantom line 220.

It will be appreciated that, as a simplex repeater, operation will be effectively by disabling the receiver section 118 and enabling the transmitter section 116 alternately with enabling the receiver section 118 and disabling the transmitter section 116, so that transmission occurs only in bursts during which received signals from a normal transceiver are effectively ignored, but that the transmitted bursts will be intelligible to normal transceivers.

For duplex operation relative to a single frequency, two communicating transceivers will operate relative to transmission bursts with operation timed so that respective transmission enablement is between received transmission bursts with alternating respective receiver enablement spanning in time the received transmission burst.

Amongst possible refinements of the circuitry described with reference to the drawings, is means for negating unwanted radio signal components produced by the individual or combined effects of various forms of modulation on the radio frequency wave for transmission. Figure 1 shows such provision at 59 in dashed lines and only symbolically as though of filter type. In practice an active circuit is likely to be used to produce appropriate nulling signals, for example for side bands of the transmission signal, and that may be done in association with modulation, perhaps on the output stage of modulator 58. Alternatively, nulling could be by way of a radiated signal.

Moreover, it may be advantageous during reception intervals for transmission frequency signals to be shifted in frequency so as to be sufficiently off-channel not to interfere with reception, say prior to retransmitting (on a repeater) at the actual, i.e. unshifted, frequency.

That is also symbolically indicated dashed in Figure 1 at 53 for master transmission frequency source and 53A for frequency shifting at reception intervals, see line 36 for appropriate signals. In practice, a variable capacitance diode might be used, then subject to a change of applied D.C. voltage derived or controlled by the receive enable signal (36).

Also, intelligible recovery of weak received signal, at sampling might be enhanced by using plural sample-and-hold circuit provisions operative sequentially within each sampling interval and having their contents summed for each sampling interval, such techniques being, of course, known in themselves. Up to ten such sample-and-hold provisions appear to be practical herein, according to signals derived from a fast pulse source, say 1 MHs as envisaged herein for dividing down to the frequencies of Figure 3.

It will also be appreciated that repeater operation can be relative to a normal incoming signal, i.e. continuous; and that it could be advantageous in terms of increasing average transmitted power of a repeater for transmission intervals to be longer than reception intervals.

Claims (19)

1. A radio communication system including means operative relative to one transmission frequency to define transmission and reception intervals alternating in time, and associated means for sampling received signals only during said reception intervals.

2. A radio communication system according to claim 1, wherein the repetition rate of said transmission and reception intervals is substantially higher than usually accepted maximum necessary for speech (3Kz).

3. A radio communication system according to claim 2, wherein said repetition rate is about three time said usually accepted maximum.

4. A radio communication system according to any preceding claim, wherein successive transmission and reception intervals are of different durations.

5. A radio communication system according to claim 4, whrein the reception intervals are longer than the transmission intervals.

6. A radio communication system according to claim 5, wherein the reception intervals are up to about twice as long as the transmission intervals.

7. A radio communication system according to any preceding claim, wherein each reception interval is spaced in time from the immediately preceding transmission interval.

8. A radio communication system according to any preceding claim, wherein the sampling means is operative directly as audio content of received signals after their demodulation from carrier signals.

9. A radio communication system according to any preceding claim, wherein the means for sampling is operative in a sampling interval within and starting after each reception interval.

1Q. A radio communication system according to any preceding claim, wherein transmission involves frequency modulation of a carrier signal.

11. A radio communication system according to any preceding claim, wherein carrier signal is effectively limited to bursts synchronised with transmission intervals by means of envelope-defining means for the carrier signals.

12. A radio communication system according to claim 11, whrein the envelope-defining means comprises amplitude modulation means operative to produce slow rise and fall times by way of a substantially trapezoidal said envelope rising and falling between substantially 100% and zero modulation during each transmission interval.

13. A transceiver of a radio communication system according to any preceding claim and operative in duplex mode using a single carrier frequency, the tranceiver comprising means for assuring that its reception intervals encompass transmission intervals of the party in communication therewith.

14. A radio communication system according to any one of claims 1 to 12 and comprising transceivers according to claim 13, wherein one of the transceivers is operative as a master and the or each other is operative as a slave to start transmission intervals only after reception of a burst of tansmission from the master.

15. A radio communication system according to claim 14, wherein the or each slave transceiver includes means for re-trying transmission of a coded transmission until the master transceiver responds.

16. A repeater of a radio communication system according to any one of claims 1 to 12, 14 or 15 wherein transmissions received during each reception interval are retransmitted during the next following transmis sion interval.

17. A repeater according to claim 16, whereon the last-mentioned means comprises sample-and-hold circuitry.

18. A radio communication system operative using two frequencies wherein operation for each one of those frequencies is in accordance with the preceding claims.

19. A radio communication system substantially as herein described with reference to and as shown in the accompanying drawings.