GB2187349A - Data transmission using a transparent tone-in band system - Google Patents

Description

GB2187349A 1 SPECIFICATION the information in the information signal. How

ever since the processes used for forming the Data transmission using a transparent upper and lower portions are not perfect, tone-in band system some of the information will usually be lost 70 unless both portions are demodulated. Prefera The present invention relates to apparatus and bly therefore the demodulation means is con methods for demodulating signals, particularly structed to demodulate the upper and lower data signals, transmitted using a transparent portions using the said demodulation signal tone-in band (TTIB) system. British Patent Ap- and another demodulation signal, respectively.

plication No. 2161661A introduces TTIB sys- 75 These signals are hereinafter referred to as tems and various embodiments are described. the first and second demodulation signals.

Where data signals are to be transmitted The main advantage of the present invention the data signals are used to modulate a carrier is the direct demodulation of at least one of signal before the---notch-in the frequency the upper and lower portions without previ- spectrum is generated. The carrier signal may 80 ously restoring the spectrum to its original then be positioned in the notch during condition in the receiver.

transmission and retrieved at the receiver to Preferably the carrier signal is inserted, after allow demodulation after the original spectrum frequency changing if required, into the notch (without the notch) has been restored. in the transmitter and the generating means According to a first aspect of the present 85 extracts the carrier signal from the signal re invention there is provided a receiver for a ceived.

communication system which employs a The correcting means may be coupled to transmitter comprising receive, as input signals, at least parts of the modulation means for modulating a carrier upper and lower portions. Alternatively the signal with an information signal, 90 correcting means may be coupled to receive, frequency selection and translation means as input signals, demodulated output signals for dividing a frequency band containing the from the demodulation means, these output modulated signal into upper and lower por- signals being dependent on any difference in tions and for carrying out frequency transla- frequency and phase in the upper and lower tion, the output signal of the frequency selecportions. Thus the correction applied in deriv tion and translation means having at least one ing the first and second demodulation signals said portion which is translated in frequency may be---feedforward- or feedback-.

to provide a notch between the lower and Where the feed forward alternative is em upper portions, and ployed, the upper and lower portions must means for transmitting the upper and lower 100 contain at least one signal present at a signifi portions and the said carrier signal, cant level in both portions and the correcting the receiver comprising means may include first mixer means coupled means for demodulating at least one of the to receive at least parts of the first and sec upper and lower portions using a demodula- ond portions containing the said one signal tion signal, 105 and deriving as its output signal a signal hav correcting means for deriving a correcting ing a frequency which is equal to the notch signal dependent on any difference in fre- width. This output signal may then be coupled quency and phase in the lower and upper por- to means for dividing frequency by two to tions on reception but as referred to the origi- provide the output signal of the correcting nal spectrum of the information signal, and 110 means. The generating means may then em generating means for generating the demodu- ploy a second mixer means which receives the lation signal from the said carrier signal, the carrier signal and the output signal from the generating means being coupled to receive the correcting means and provides two output correcting signal and employ it in generating signals corresponding to upper and lower si the demodulation signal. 115 debands which are then used as the first and In the transmitter, the modulation means second demodulation signals and applied to may comprise a mixer receiving the informa- third and fourth mixer means which demodu tion signal and the carrier signal but as an late the upper and lower portions, respec alternative the modulation means may com- tively.

prise a modem employing, for example, binary 120 Where the feedback alternative is used, the phase-shift keying (13PSK) or quadrature phacorrecting means may comprise first mixer seshift keying (O.PSK). means which controls a locally generated ref Where the upper and lower portions corre erence signal and the generating means may spond to the upper and lower sidebands of comprise a second mixer which receives the the output signal of the modulation means and 125 carrier signal and the locally generated refer a suitable simple modulation technique is ence signal, and generates the first and sec used, for example BPSK, then it may some- ond demodulating signals as upper and lower times only be necessary to demodulate only sidebands. The demodulation means may the upper or lower portion at the receiver, comprise third and fourth mixer means and since each sideband theoretically contains all 130 the first mixer means may be coupled to re- 2 GB2187349A 2 ceive the output signals of the third and fourth signal, comprising means for demodulating at mixer means as input signals. least either of the portions above and below According to a second aspect of the pre- the notch in the frequency spectrum without sent invention there is provided a receiver first removing the notch from the spectrum.

method for processing signals received in a 70 The invention also includes a method equi communication system which employs a valent to the fourth aspect of the invention.

transmission method comprising modulating an The above mentioned means and methods information signal with a carrier signal, dividing steps may be implemented by operations, or the frequency spectrum containing the modugroups of operations, programmed into an in- lated signal into lower and upper portions with 75 tegrated circuit containing a microprocessor.

a frequency notch between the lower and up- For example a digital signal processor inte per portions, and transmitting the upper and grated circuit can be programmed in this way lower portions and the carrier signal, to form the various demodulators, mixer the receiver method comprising demodulat- means and filters required.

ing the upper and lower portions using first 80 Certain embodiments of the invention will and second demodulation signals respectively, now be described by way of example with deriving a correcting signal dependent on reference to the accompanying drawings, in any difference in frequency and phase in the which:

lower and upper portions on reception but as Figure 1 is a block diagram of a TTIB referred to the original spectrum of the infor- 85 transmitter used in an embodiment of the in mation signal, and vention, generating the first and second demodula- Figure 2 shows the frequency spectrum at tion signals from the carrier signal and em- the output of Figure 1, ploying the correcting signal in generating at Figure 3 is a block diagram of a receiver least one of the first and second demodulation 90 according to the invention employing feed for signals. ward control, According to a third aspect of the present Figure 4 is a block diagram of a receiver invention there is provided a communication according to the invention employing feedback system including a transmitter and a receiver, control, the transmitter comprising modulation 95 Figure 5 is a block diagram of a digital sig means for modulating a carrier signal with an nal processor circuit which may be used to information signal, replace the transmitter of Figure 1 or the re frequency selection and filtering means for ceiver of Figure 3 or 4, dividing the frequency spectrum containing the Figures 6 and 7 are flow charts for modulated carrier signal into upper and lower 100 transmitter and receiver programs, respec portions and for carrying out frequency tran- tively, and slation, the output signal of the frequency se- Figure 8 is a block diagram of another TTIB lection and translation means having at least transmitter used in an alternative embodiment one said portion which is translated in fre- of the invention.

quency to provide a notch between the lower 105 In Figure 1 binary data is applied to a mixer and upper portions, and 10 which also receives a carrier signal f,, from means for transmitting the upper and lower an oscillator 11. The binary data is usually portions and the carrier signal, and received as rectangular pulses but it is shaped the receiver comprising means for demodu- by passage through a low-pass filter (not lating the upper and lower portions using first 110 shown) before reaching the mixer 10. Both and second demodulation signals, respectively, sidebands from the mixer 10 covering, for means for deriving a correcting signal de- example, a frequency spectrum from 600 Hz pendent on any difference in frequency and to 3.6 kHz pass through a low pass filter 12 phase in the lower and upper portions on re- before reaching a low pass filter 13 and a ception but as referred to the original spec- 115 mixer 14. A lower portion of the spectrum, trum of the information signal, and for example from 600 Hz to 1.8 kHz is se generating means for generating the first lected by the filter 13 and applied to a com and second demodulation signals from the bining circuit 15, The mixer 14 also receives a said carrier signal, the generating means being reference signal at a frequency fl and the coupled to receive the correcting signal and to 120 lower portion of the lower sideband of the employ the correcting signal in generating at output signal from the mixer 14 is selected least one of the first and second demodulation; using a low pass filter 16. The output from signals. the filter 16 corresponds to the upper portion The means of modulating a carrier signal of the band from the mixer 10 which is not may comprise a mixer receiving the carrier and 125 passed by the filter 13 (in the example this the information signal, or it may comprise a output corresponds to that part of the original modem employing BPSK or QPSK. band extending from 1.8 Hz to 3.6 Hz). A According to a fourth aspect of the present further step in frequency translation now takes invention there is provided a receiver for a place in a mixer 17 which receives a reference TTIB system carrying a modulated information 130 signal f2 which results in an output signal with 3 GB2187349A 3 two sidebands each corresponding to the up- a signal similar to that shown in Figure 2a per portion of the spectrum from the mixer except that the notch width is now 2f 10. This upper portion is combined with the After transmission to remote location the lower portion in the summing circuit 15 and conventional process of SSB demodulation or the upper sideband of the output from the 70 other demodulation takes place at the receiver mixer 17 is removed by a low pass filter 18. and then the resulting spectrum which is nom The resultant spectrum at the output from the inally as shown in Figure 2b is applied to a filter 18 is shown in Figure 2 and includes a circuit shown in Figure 3. The lower and up lower portion 19, an upper portion 21 and a per portions, below and above the notch re frequency---notch-22 whose width is equal 75 spectively, are separated by means of low to f, - fl. In the present example the notch is and high pass filters 30 and 31 respectively 600 Hz wide and therefore the upper portion and applied to mixers 32 and 33 (which may 21 extends from 2.4 Hz to 4.2 Hz. be mirror filters) where demodulation takes The carrier signal for the mixer 10 is also place. The outputs from the mixers 32 and 33 applied to a mixer 23 which receives a refer- 80 are combined in a combining circuit 34 whose ence signal f3 and the upper sideband of the output forms the data signal and passes by signal from the mixer 23 is placed in the way of a low pass filter 35.

notch by choice of the frequency f3. Preferably In order to obtain correct reference signals the upper sideband is placed at the centre of for demodulation in the mixers 32 and 33, the the notch by making f3 equal to (f, - fj/2. 85 signal 25 in the notch requires frequency tran The frequency f. and the lower sideband are slation and phase correction to appropriate removed by a high pass filter 24 before appli- positions in the received spectrum. Assuming cation to the combining circuit 15. The signal the original modulated carrier f. was at the in the notch is designated 25 in Figure 2. The centre of the band where the frequency notch process of generating a frequency notch as 90 was introduced it must be restored to the described above is similar to that described in position 37 shown in Figure 2(b) and equally British Specification 2161661 mentioned the demodulating signal for the upper portion above. must be positioned at the lower end of this The output signal from the circuit of Figure portion as shown at 38 in Figure 2(b). The 1 can now be applied to a single sideband 95 signal 25 in the notch of the received spec (SSB) radio link with a pilot tone for the SSB trum is selected by a band pass filter 36 and system also inserted into the notch. Alterna- applied to a mixer 40 which nominally shifts tively another radio or line link may be em- the frequency of the signal 25 up and down ployed. by the frequency f The two resulting signals Before considering the receiver an alternative 100 form the upper and lower sidebands of the transmitter shown in Figure 8 is described. output of the mixer 40 and are applied by Incoming data passes to a data shaping filter way of a low pass filter 42 to the mixer 33 (not shown in Figure 1) and then to a and by way of a high pass filter 43 to the mixer 76 which receives the carrier frequency mixer 32.

f,. Both side bands are passed to a low-pass 105 The required corrected value of f, is ob fitter 77 and a high-pass filter 78 (which may tained as follows. For this embodiment of the be mirror filters) so that the lower sideband invention to function the sides of the notch appears at the output of the filter 77 and the must not be perfect as shown but have a upper sideband appears at the output of the finite roll-off imparted by the filters 13 and 16 filter 78. A frequency downconverter 79 re- 110 of Figure 1. The upper and lower portions are ceives the lower sideband and two reference applied to a mixer 44 and since signals in the signals of frequency f. in phase quadrature two portions which correspond in that they with one another from an oscillator 81 and a originate from the same frequency in the 90' phase shift circuit 82. The downconverter spectrum before the notch was inserted (and 79 comprises an ---inphase- mixer 83 and a 115 are present by virtue of the above mentioned 11 quadraturemixer 84, the latter receiving the roll-off portions) are separated by the notch lower sideband by way of a 90' phase shift width, the lower sideband of the output of the circuit 85. When the outputs of the mirrors mixer 44 is a signal whose frequency is equal 83 and 84 are summed in a summing circuit to the notch width; that is f2 - fl which the resulting new lower sidebands add and 120 equals 2f3 when the carrier signal is at the the new upper sidebands cancel. A frequency centre of the notch. This frequency is halved upconverter 86 operates in a similar way on in a frequency divider 45 and applied as the the upper sideband signal from the filter 78 reference signal to the mixer 40 so that any except, as indicated by the plus and minus distortion in the notch due for example to fre signs adjacent to a summing circuit 87, the 125 quency drift in the transmitter oscillators is quadrature input is subtracted from the in automatically compensated.

phase input so that the upper sidebands add A block diagram of an alternative circuit to and the lower sidebands cancel. The outputs Figure 3 which uses feedback control is from the downconverter 79 and the upconver- shown in Figure 4. Filters 30, 31, 35 and 36, ter 86 added by a summing cirucit 88 to give 130 mixers 32 and 33 and the combining circuit 4 GB2187349A 4 34 perform the same functions as in Figure 4. above or similar processors. It is well known However the control signals for correcting the that digital filters are implemented by succes demodulation signals applied to the mixers 32 sive multiplication and addition operations and and 33 are derived from signals at the output the processors mentioned are specially of these mixers, the output of the mixer 33 70 adapted to carry out such multiplications at being applied directly to a mixer 50 and that high speed. The various mixers shown are from the mixer 32 being applied by way of a also ideal for implementation as multiplication quadrature phase-shift circuit 51 to the mixer processes in a programmed signal processor.

50. The output of the mixer 50 is a magni- An example of a microprocessor circuit tude signal which is used to vary the fre- 75 which can be used to replace the transmitter quency and phase of an oscillator 52 which is of Figure 1 or the receiver of Figures 3 or 4 nominally at f3. As for Figure 3 the mixers 32 is shown in Figure 5. Analogue signals are and 33 require reference signals positioned as applied to a terminal 60 and then pass by shown at 37 and 38 in Figure 2(b) and these way of an amplifier 61 and an antialiasing fil signals are obtained as lower and upper side- 80 ter 62 to an analogue-to- digital converter ' bands of the output from a mixer 53, the (A/D) 63. A digital signal processor (DSP) 64 appropriate sidebands being selected by a low (that is a specialised microprocessor) such as pass filter 54 and a high pass filter 55, re- the Texas TMS 320-10 or 320- 20 receives spectively. The outputs of the mixers 32 and the output from the A/D 63 and provides out- 33 in Figure 4 (and Figure 3) should theoreti- 85 put digital samples to a digital-to-analogue cally be identical since the upper and lower converter (D/A) 65. A read- only memory 66 portions correspond to the upper and lower contains the program for the DSP and a ran sidebands of the modulator 10, and each sidom-access memory 67 for use during the deband contains all the information of the data operation of the program is also connected to input. The signals applied to the circuit 51 90 the DSP. Analogue signals from the D/A 65 from the mixer 32 and to the mixer 50 from pass through an antialiasing filter 68 and an the mixer 33 are therefore nominally identical amplifier 69 to an output terminal 70.

and any difference is derived by the mixer 50 An example of a program for the DSP 64 and used to correct the frequency of the oscil- when used to replace the transmitter of Figure lator 52. The circuits 32, 33 and 50 to 54 95 1 is given in the form of the flow chart of can be regarded as forming a phase locked Figure 6. Since this flow chart is analogous to loop, and the phase-shift circuit 51 ensures Figure 1 it will not be described except to that the error correction signal is zero when point out that the various low pass filtering frequency and phase are correct. subroutines may or may not be the same but Some of the data information will almost 100 where they are the same their frequency char certainly be absent from the output of at least acteristics depend on the parameters supplied one of the demodulators 32 and 33 due to to the subroutine so that the required different the roil-off of the various filters, and it is pre- characteristics are obtained. The flow chart of ferable to combine both outputs as shown us- Figure 7 is an example of a program for the ing the combining circuit 35, although there 105 use of the DSP 64 as the receiver of Figure 3.

may be some circumstances when the output Although some ways of putting the inven of one demodulator only is sufficient. tion into effect have been specifically de The mixer 10 and 76 of Figures 1 and 8 scribed it will be clear that there are many may be replaced by a modem employing ways of doing so. For example the notch may BPSK or QPSK, the carrier signal then being 110 be so positioned that no frequency translation generated within the modem. If a more com- of the carrier signal f, is required in the plicated modulation system than BPSK is transmitter. Also the above mentioned British used, for example QPSK, then it is necessary Patent Application and the corresponding Un to demodulate both upper and lower side- ited States Application No. 617733 show bands and use a combining circuit to derive 115 other methods of providing and employing the data output. Where more complicated mo- TTIB as do British Patent Applications Nos.

dulation takes place in the transmitter, the de- 8520580, 8519545 and 8430319, and many modulation in the receiver must, of course, of these methods may be used with the pre correspond. At least one demodulation signal sent invention, in particular shifting both upper of appropriate frequency and phase is derived 120 and lower portions of the spectrum in forming and correction of frequency and phase, which the notch.

is dependent on a signal present in the upper Two forms of receiver locking system have and lower portions is required for a low error been specifically described but others may be data output. used, for example receivers in which the refer- The circuits shown above can be put into 125 ence signal for one of the demodulators 32 operation using discrete filters and mixers, and 33 only is corrected.

either analogue or digital and other discrete

Claims (1)

1. component circuits or they can be wholly or CLAIMS partially formed by

   programming the digital 1. A receiver for a communication system signal processor integrated circuits mentioned 130 which employs a transmitter comprising GB2187349A 5 modulation means for modulating a carrier the following techniques: binary phase-shift signal with an information signal, keying and quadrature phase-shift keying.

   frequency selection and translation means 5. A receiver according to Claim 1 wherein for dividing a frequency band containing the the generating means is arranged to generate, modulated signal into upper and lower por70 in addition to the said demodulation signal tions and for carrying out frequency transla- (that is a first demodulation signal), a second tion, the output signal of the frequency selec- demodulation signal using the said correcting tion and translation means having at least one signal, and the means for demodulating is ar said portion which is translated in frequency ranged to demodulate the upper and lower to provide a notch between the lower and 75 portions using the said first and second demo upper portions, and dulation signals, respectively.

   means for transmitting the upper and lower 6. A receiver according to Claim 1 or 5 for portions and the said carrier signal, a communication system in which the the receiver comprising transmitter includes means for inserting the means for demodulating at least one of the 80 carrier signal into the notch, the receiver in upper and lower portions using a demodulacluding means for extracting the carrier signal tion signal, from the notch, on reception.

   correcting means for deriving a correcting 7. A receiver according to Claim 1, 5 or 6 signal dependent on any difference in fre- wherein the upper and lower portions each quency and phase in the lower and upper por85 contain at least one signal which originated tions on reception but as referred to the origi- from the same frequency in the spectrum be nal spectrum of the information signal, and fore the notch was inserted and the correcting generating means for generating the demo- means is arranged to receive, as input signals, dulation signal from the said carrier signal, the at least parts of the upper and lower portions generating means being coupled to receive the 90 containing the said one signal.

   correcting signal and employ it in generating 8. A receiver according to Claim 7 wherein the demodulation signal. the correcting means comprises first receiver 2. A communication system including a mixer means arranged to receive the said in transmitter and a receiver, put signals and to derive a notch width signal the transmitter comprising modulator means 95 having a frequency equal to the notch width, for modulating a carrier signal with an informa- and means for deriving the said correcting sig tion signal, nal from the notch width signal.

   frequency selection and filtering means for 9. A receiver according to Claim 8 insofar dividing the frequency spectrum containing the as dependent on Claims 5 and 6 wherein the modulated carrier signal into upper and lower 100 generating means comprises a second receiver portions and for carrying out frequency tran- mixer means arranged for receiving and mixing slation, the output signal of the frequency se- the carrier signal and the correcting signal and lection and translation means having at least for generating the first and second demodula one said portion which is translated in fre- tion signals as upper and lower sidebands re quency to provide a notch between the lower 105 sulting from the said mixing, and the receiver and upper portions, and also includes third and fourth mixer means ar means for transmitting the upper and lower ranged to receive the first and second demo portions and the carrier signal, and dulation signals, and to demodulate the upper the receiver comprising means for demodu- and lower portions, respectively.

   lating the upper and lower portions using first 110 10. A receiver according to Claim 1, 5 or 6 and second demodulation signals, respectively, wherein the correcting means is arranged to means for deriving a correcting signal de- receive, as input signals, demodulated output pendent on any difference in frequency and signals from the demodulation means, these phase in the lower and upper portions on reoutput signals being dependent on any differ- ception but as referred to the original spec- 115 ence in frequency and phase in the upper and trum of the information signal, and lower portions.

   generating means for generating the first 11. A receiver according to Claim 10 insofar and second demodulation signals from the as dependent on Claims 5 and 6 wherein the said carrier signal, the generating means being correcting means comprises first receiver coupled to receive the correcting signal and to 120 mixer means for deriving the correcting signal, employ the correcting signal in generating at and the generating means comprises a local least one of the first and second demodulation generator for generating a reference signal signals. having a frequency controlled by the correct 3. A communication system according to ing signal, and second receiver mixer means Claim 2 wherein the transmitter modulation 125 arranged to receive the carrier signal and the means comprises means for mixing the infor- said reference signal and to generate the first mation signal and the carrier signal. and second demodulation signals.

   4. A communication system according to 12. A receiver according to Claim 11 Claim 2 wherein the transmitter modulation wherein the demodulation means comprises means comprises a modem employing one of 130 third and fourth receiver mixer means and the 6 GB2187349A 6 first receiver mixer means is arranged to receive the output signals of the third and fourth receiver mixer means as input signals.

   13. A receiver according to any of Claims 1 or 5 to 12 wherein at least some of the said means are formed by a programmed computer.

   14. A communication system according to Claim 2, 3 or 4 wherein at least some of the said means of the transmitter and/or the receiver are formed by a programmed computer.

   15. A receiver method for processing signals received in a communication system which employs a transmission method com- prising modulating an information signal with a carrier signal, dividing the frequency spectrum containing the modulated signal into lower and upper portions with a frequency notch between the lower and upper portions, and transmitting the upper and lower portions and the carrier signal, the receiver method comprising demodulating the upper and lower portions using first and second demodulation signals respectively, 25 deriving a correcting signal dependent on any difference in frequency and phase in the lower and upper portions on reception but as referred to the original spectrum of the information signal, and 30 generating the first and second demodulation signals from the carrier signal and employing the correcting signal in generating at least one of the first and second demodulation signals. 35 16. A communication system substantially as hereinbefore described with reference to Figure 1, and Figure 3 or 4. 17. A receiver for a commun)cation system substantially as hereinbefore described with reference to Figure 3 or 4.

   Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.