GB1581294A - Elementary character for methods of telegraph - Google Patents

Description

(54) AN ELEMENTARY CHARACTER FOR METHODS OF TELEGRAPH (71) We, LICENTIA PATENT VER WALTUNGS G.m.b.H., of 1 Theodor Stern-Kai, 6 Frankfurt/Main 70, Federal Republic of Germany, a German body corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an elementary character for methods of telegraphy, more particularly those having vestigial side band modulation with a suppressed carrier.

In linear, serial bit telegraphy, a minimum amplitude modulation is always important with the signals to be sent out which have been superimposed on carriers: a) if the mean signal output of the final stage of a transmitter is to be optimised, b) if only small requirement can be made of the linearity of the amplification of this final stage.

With binary serial bit telegraphy with any desired size bandwidth, the elementary character shown, for example, in Fig. la is in use in the form of a rectangular function of the amplitude I and the duration T. If an information with the bit sequence HLHLL is transmitted by this elementary character then the character sequence shown in Fig.

lb is obtained. In a system of carriers-for example in a radio transmission method when using double side band modulation an envelope is observed as shown in Fig. Ic.

The signal in accordance with Fig. Ic is a wide band signal and contains side bands of a fairly high order.

The invention seeks to provide elementary character of the type stated at the outset which is limited in its band, facilitates very high speed telegraphy and produces a minimum amplitude modulation in the code.

According to the invention, there is provided an elementary character fol methods of telegraphy in which the elementary character has a frequency spectrum in its frequency range of a F(f)=cos2-(f B) 2B 2 where: a a 2 2 and where f is the frequency, B the band width and a0.7 and in which the elementary character appears in the first and orthogonal second channel of a vestigial side band modulation as sin 2nh x(h)= coss h 27ch(l-4h2) or sin 27dh y(h)= sin a 27rh( I-4h2) where h=Bt, t being the time The time spacing (in the code) of successive elementary characters may be selected to be 1 T= 2aB An embodiment of the invention is shown in the drawings and is described in greater detail in the following.

Figure 2 shows the spectral curve of an elementary character in accordance with the relationship aB F(f)=cos2 (f- ) (1) 2B 2 where a a -B(l- -) < fB(l±) (2) 2 2 It is a question of a representation in the base band, i.e. the (suppressed) carrier having the carrier frequency fT is at the frequency f=0. The spectrum is displaced by aB 2 with respect to the carrier frequency fT towards positive frequencies. Reference will be made later to the broken line. The spectrum is limited with respect to its band and has a (-6 dB) bandwidth B.

Figs. 3a and 3b show the elementary character in accordance with Fig. 2 after transformation in the time range. As a result of Fourier transformation, a time function is obtained from the spectrum F(f) in accordance with Fig. 2 having the real part: sin 27rh x(h)= cos cr nh (3) 2h( 1 -4h2) which is shown in Fig. 3a and with an imaginary part shown in Fig. 3b: sin 2 7dh y(h)= sin ce 7rh (4) 2nh( 1-4h2) whereby h=B .t (t=time). The functions x(h) and y(h) are orthogonal with respect to each other. They correspond to the time representation of the elementary character having the spectrum in accordance with Fig.

2 in the two orthogonal channels (sine and cosine channel) of a vestigial side band modulator. Fig. 3a moreover, contains the envelope

as well as-shown in broken lines-an adjacent elementary character. The spacing T of adjacent elementary characters (this spacing corresponds to the spacing of the bit steps or telegraphy steps) is selected to be 2aB because this spacing also leads to a minimum amplitude modulation. The telegraphy speed 1 VT= =2aB T increases with an increasing a-at the same telegraphy bandwidth. The range 0.5 < a < L2 is of interest for vestigial side band modulation.While a=0.5 there is hardly any gain in telegraphy speed with respect to the double side band modulation, but, with a=l, twice the telegraphy speed is possible with respect to the double side band modulation.

In Fig. 3a a is selected at 0.714, so that 1 T= =0.7 . h 2B (zero position of x(h)). It is particularly important that the carrier frequency fT should not be tuned to the centre of the Nyquist flank (cf. the broken line in Fig.

2)-in contrast to the known vestigial side band modulation method, this would correspond to a setting a=l, but rather should be tuned to a frequency at the spacing B 2 from the centre of the spectrum (cf.

continuous line) with cur=0.7. Thus in conjunction with the specific form of the elementary character and selection of the bit step T is achieved by 1 2aB that the signal to be transmitted which has been made into a carrier has minimum amplitude modulation with a binary phaseshifted code. Thus the band used by the elementary character of the width 2B is considered to be the information code, whereby the information instruction is then contained in the instantaneous phase of the signal transmitted. The minimum amplitude modulation brings two considerable advantages in that on the one hand (because of the small depth of the amplitude modulation) optimization of the mean signal output of the final stage of the transmitter is permitted and on the other hand use of a final stage is facilitated, only small requirements being set thus with respect to the linearity of the amplification. The latter is important inasmuch as the transmitter then achieves favourable values with respect to the economic expense, to constructional size, weight and output efficiency.

At this point reference should be made to yet another advantage of the elementary character in accordance with Figs. 2 and 3a and 3b. In order to achieve minimum amplitude modulation with an information code and at the same time optimum utilization of the available bandwidth for the information (bit) rate, the elementary character must decay as rapidly as possible after -reaching the maximum so that the following information bit (or elementary character) is interfered with as little as possible by it. It is apparent from Fig. 3a or 3b that /x(h) or /y(h) drops to negligible values smaller than 2.5 or 1.5 /" for /he0.7 or /h21 .

An additional advantage of the elementary character with a=0.7 lies in the fact that, with its use, the vestigial amplitude modulation of the coded signal contains side bands which lie outside the frequency range of the elementary character (limited with respect to band) and which compensate those frequency components also produced in phase modulation outside the predetermined frequency band.

Vestigial side band modulated signals are distorted linearly because of their mode of formation. This linear distortion can be cancelled again when using the elementary character in accordance with the invention by using a filter matched to the character (a so-called matched filter) on the receiving side without loss of signal energy.

Fig. 4 shows with a broken line the spectrum according to Fig. 2, i.e. the spectrum of the elementary character in the base band which has not been provided with carriers and with a continuous line the frequency characteristic of a receiver side low-pass filter matched to the elementary character. It is a question of a vestigial side band low-pass filter, the throughput characteristics of which are displaced in the direction of positive frequencies corresponding to the frequency characteristics of the elementary character with respect to the frequency f=O by B 2 As can be gathered easily from this Figure a side band cone of the frequency f=fT+aB (cf. continuous vertical towards the right) corresponds to a signal sequence of the form on the other hand the carrier frequency fT (cf. continuous ordinate towards the left) corresponds to a signal frequency . . ++++++ .

or which is in fact at the frequency f=0. The carrier frequency reproduces the same proportion of the signal to be transmitted.

The half telegraphy speed is designated by vet/2 .

Figs. Sa and Sb show the surge reply of the vestigial side band low-pass filter and in fact its real part is as follows in Fig. Sa: sin 2 X ah cos A ah (5) 2 7r ah . exp(h2/2) and its imaginary part is as follows in Fig.

Sb: sin 2 nah sin 7r ah (6) 2 7r ah . exp(h2/2) whereby the parameter a is in turn selected at a=0.714.

The minimum scanning rate is defined by the scanning theorem. In d vestigial side band signal one scan per bit step can be sufficient in the sine channel and one in the cosine channel. It is apparent from Fig. 6 how well this relates to the selected elementary character. The spectrum shown by the continuous line is apparent from the spectral curve of the elementary character in accordance with Fig. 2 after receiver side filtering of this spectrum with a vestigial side band low-pass filter having a throughput characteristic according to Fig. 4 which is shown here by chain lines. For comparison Fig. 6 also contains the unfiltered spectrum of the elementary character-as shown in broken lines. The scanning rate l/T corresponds to the telegraphy speed.As a result of selection of the vestigial side lowpass filter, overlapping of the spectra of adjacent elementary characters is substantially reduced. This is important because the region of overlapping of the folded spectra should be as small as possible in accordance with the scanning theorem.

A decisive criterion as to whether the scanning rate (once per bit step) is sufficient for the selected elementary character, is supplied by the dependence of the autocorrelation function AKF of the scanning values of the elementary character on the selection of the scanning time or of a shift in the scanning raster at constant scanning rate.

Figs. 7a to 7c show this dependence for the elementary character in accordance with the invention (after filtering by the vestigial side band low-pass filter), whereby the scanning time h0 in Fig. 7a is selected at ho=0 and in Fig. 7b it is selected at he=0.2 and in Fig. 7c at ho= 0.4. On the left in Figs.

7a to 7c are the real and imaginary parts of the elementary character (in the time range) with their respective scanning values i.e. the vertical lines under the curves. In the centre of the autocorrelation function AKF of the scanning values of the real part as well as of the imaginary part and on the right-hand side the sum ÀKF of the autocorrelation functions belonging to one and the same scanning time of the real and imaginary part. Per bit step only one scan is undertaken as mentioned in the real and imaginary part. The representation makes it clear that the sum signal ÀKF has practically only the main value (with negligible subsidiary values) independent of the selection of the scanning time and that the amplitude of the main value is almost independent of the scanning time.Thus it has been proved that the linear distortions of the elementary character can be cancelled out with the aid of a vestigial side band filter restricting the band and matched filter on the receiver side following this vestigial side band filter. This filter which limits the band would have a (-6 dB) bandwidth of 3 kHz at a bit speed of 3000 Baud for example when used in the short wave range. The A3J filter in the intermediate frequency band of series short wave receivers is suitable forts this for example. In the base band, the filter limiting the band is a vestigial side band low-pass filter. The matched filter is set on the receiver side with the aid of a test sequence during the adaption phase. The setting procedure is not the subject of this application.

How important the independence from the scanning time is, can be seen from the fact that a control circuit (feedback from the processor) is superfluous in order to set the scanning moment, which is an inalienable precondition for example-very important for rapid adaption to the transmission channel-for an off-line operation with subsequent equalization.

Finally it should be noted that possible phase difference between demodulation carriers and a carrier signal does not play any part because of the matched filter process described.

The elementary characteristic in accordance with the invention and described in detail above does not require any special technical measures with respect to what is known and may be used advantageously in various telegraphy methods already known having residual side band modulation.

Arrangements for.producing this type of elementary characteristic are known for example for Voelcker, H.B. "Generation of Digital Signalling Waveform", IEEE Trans.

com-16, No. 1(1968). Transmission methods for telegraphy with residual side band modulation are generally known.

WHAT WE CLAIM IS: 1. An elementary character for methods of telegraphy in which the elementary character has a frequency spectrum in its frequency range of n a F(f)=cos2 (f- B) 2B 2 where a -B(l- )Sf < B(l+ .) 2 2 and where f is the frequency, B the bandwidth and a0.7 and in which the elementary character appears in the first and orthogonal second channel of a vestigial side band modulation as sin 2 7th xCh)= cos a 7sh 2h( 1 -4h2) or sin 2 Kh y(h)= a 7th 2h( 1-4h2) where h=Bt, t being the time 2, An elementary character according to claim 1, wherein the time spacing of successive elementary characters amounts to 1 2aB 3. An elementary character according to claim 1 or 2, wherein it is used in a vestigial side band modulation method in which a filter matched to the elementary character at the receiving side, a so-called matched filter, is provided, a filter restricting the band being coP elected before it.

4. An elementary character according to any one of claims 1 to 3, wherein small sensltivity with respect to non-linear distortion of the transmitter is ensured in amplitude.

5. An elementary character for methods of telegraphy substantially as described herein with reference to the drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. scanning time of the real and imaginary part. Per bit step only one scan is undertaken as mentioned in the real and imaginary part. The representation makes it clear that the sum signal ÀKF has practically only the main value (with negligible subsidiary values) independent of the selection of the scanning time and that the amplitude of the main value is almost independent of the scanning time. Thus it has been proved that the linear distortions of the elementary character can be cancelled out with the aid of a vestigial side band filter restricting the band and matched filter on the receiver side following this vestigial side band filter. This filter which limits the band would have a (-6 dB) bandwidth of 3 kHz at a bit speed of 3000 Baud for example when used in the short wave range.The A3J filter in the intermediate frequency band of series short wave receivers is suitable forts this for example. In the base band, the filter limiting the band is a vestigial side band low-pass filter. The matched filter is set on the receiver side with the aid of a test sequence during the adaption phase. The setting procedure is not the subject of this application. How important the independence from the scanning time is, can be seen from the fact that a control circuit (feedback from the processor) is superfluous in order to set the scanning moment, which is an inalienable precondition for example-very important for rapid adaption to the transmission channel-for an off-line operation with subsequent equalization. Finally it should be noted that possible phase difference between demodulation carriers and a carrier signal does not play any part because of the matched filter process described. The elementary characteristic in accordance with the invention and described in detail above does not require any special technical measures with respect to what is known and may be used advantageously in various telegraphy methods already known having residual side band modulation. Arrangements for.producing this type of elementary characteristic are known for example for Voelcker, H.B. "Generation of Digital Signalling Waveform", IEEE Trans. com-16, No. 1(1968). Transmission methods for telegraphy with residual side band modulation are generally known. WHAT WE CLAIM IS:

1. An elementary character for methods of telegraphy in which the elementary character has a frequency spectrum in its frequency range of n a F(f)=cos2 (f- B) 2B

2 where a -B(l- )Sf < B(l+ .) 2 2 and where f is the frequency, B the bandwidth and a0.7 and in which the elementary character appears in the first and orthogonal second channel of a vestigial side band modulation as sin 2 7th xCh)= cos a 7sh 2h( 1 -4h2) or sin 2 Kh y(h)= a 7th 2h( 1-4h2) where h=Bt, t being the time 2, An elementary character according to claim 1, wherein the time spacing of successive elementary characters amounts to 1 2aB

3. An elementary character according to claim 1 or 2, wherein it is used in a vestigial side band modulation method in which a filter matched to the elementary character at the receiving side, a so-called matched filter, is provided, a filter restricting the band being coP elected before it.

4. An elementary character according to any one of claims 1 to 3, wherein small sensltivity with respect to non-linear distortion of the transmitter is ensured in amplitude.

5. An elementary character for methods of telegraphy substantially as described herein with reference to the drawings.