GB2179826A - Data waveform generation and detection apparatus - Google Patents

Abstract

The present invention provides method and means of data recovery from non-discrete intersymbol interference waveforms generated by truncating the spectra of pseudo-random partial-response (p.r.) signals by means of a low-pass filter. Data are recovered by synchronously sampling a reconstituted discrete-amplitude p.r. waveform derived by multiplying any one of the aforesaid non-discrete interference waveforms with a locally generated sinusoidal waveform. The invention provides a sinusoidal oscillator source 38 for connection to one input of each of the multiplier circuits 32,37 used in the p.r. waveform reconstitution process: the oscillator frequency being equal to one half the data bit rate 1 /2THz for the reconstitution of Class 1 p.r. diode waveforms and one quarter of the data bit rate 1 /4THz for the reconstitution of Class 4 p.r. waveforms. <IMAGE>

Description

SPECIFICATION Improvements in or relating to data waveform generation and detection apparatus This invention relates in general to the generation and detection of partial response (p.r.) type data waveforms and in particular to the exploitation of the interrelationship between three specific type p.r. waveforms in order to optimise system performance. The three p.r. waveforms heretofore mentioned are: (i) Duobinary, also known as Partial Response Class 1.

(ii) Maximum bandwidth dicode.

(iii) Minimum bandwidth dicode, hereafter in this Application, called duobipolar.

details of (i) and (ii) are given in the attached Appendix.

The basic principle underlying the generation of partial response (p.r.) type waveforms is a predetermined interference pattern manifesting in the transmitted Spectrum: this is achieved by transmitting one or more impulse type waveforms for each data '1' symbol separated in time by an integral multiple of T secs and bandlimited at the Nyquist frequency or a multiple thereof: where 1 T secs represents the basic data transmission rate. For example, in the case of duobinary, two unipolar impulse type waveforms are generated separated in time by T secs and bandlimited at the Nyquist frequency. In the maximum bandwidth dicode case two bipolar impulse type waveforms are transmitted at T sec intervals and bandlimited at twice the Nyquist frequency; bandlimiting at the Nyquist frequency produces a Duobipolar waveform.

Duobinary transmission requires that the transmission filters and channel bandwidth constraints conform to the Nyquist 'equal areas' criterion typified by a raised cosine roll-off characteristic: under these conditions, a baseband operational data capacity of 2 bit/Hz can be realised.

However, the spectral energy of a duobinary baseband signal is concentrated at d.c. tapering to zero energy at the Nyquist frequency: this characteristic renders it unsuitable for direct transmission over A.C. coupled cables and also for DSB-SC and SSB-SC transmission.

On the other hand, maximum bandwidth dicode is eminently suited to transmission over AC coupled cables and also to DSB-SC and SSB-SC transmission due to the zero d.c. component at baseband: the disadvantage being that a minimum bandwidth of - Hz T is required thus halving the operational data capacity compared with duobinary. This disadvantage cannot be overcome, in the present state-of-the-art, by simply truncating the maximum bandwidth dicode spectrum at the Nyquist frequency in order to produce known 'minimum bandwidth dicode': to do so produces a dicontinuity at the Nyquists frequency thereby violating the Nyquist ctirerion and causing unacceptable intersymbol interference; there is, at present, no known method for data recovery at the receiver.

It is the purpose of the present invention to describe method and means whereby known minimum bandwidth dicode (duobipolar) waveforms can-after transmission over a communication channel-be converted to known maximum dicode at a receiver destination by two successive stages of multiplication with a sinusoidal waveform

Cos (e where 0 is an arbitrary phase angle. The first stage of multiplication translates the frequency spectrum of the duobipolar waveform to produce a duobinary spectrum truncated at Hz.

2T The second stage of multiplication translates the truncated duobinary spectrum to produce a dicode spectrum with spectral nulls at zero frequency and Hz.

T For the purpose of this Patent Application the transmission data in duobipolar form with data recovery at the receiver by the aforesaid simple multiplication process together with the appropriate filtering is called 'duobipolar data transmission' i.e. data transmission with an operational data capacity equal to that of duobinary transmission with the added advantages offered by maximum bandwidth dicode transmission.

The present invention provides communication apparatus for the transmission of a dicode data sequence e,(t) of predetermined bit duration, T 2 secs, comprising at the point of transmission: ...a low pass filter for sharply bandlimiting the dicode data sequence at the Nyquist frequency - Hz 2T in order to produce a duobipolar information carrying waveform el(t) ...means for a.c. coupling the aforesaid information carrying waveform e'(t) to a communication medium or channel: this may be simple transformer coupling or means for modulating the aforesaid information carrying waveform el(t) on to an arbitrary sinusoidal (frequency translation) waveform for channel matching purposes: and at the point of reception; ...means for recovering the duobipolar data waveform exit) from the aforesaid communication channel; this may again be simple transformer coupling or coherent detection in the case of amplitude modulation (A.M.) or double-sideband suppressed carrier modulation (DSB-SC) or single sideband suppressed carrier modulation (SSB-SC); a .a local oscillator for generating a sinusoidal waveform

A Cos where 0 is an arbitrary phase angle and A is the peak amplitude.

...a multiplier for multiplying the aforesaid duobipolar data waveform with the aforesaid sinusoidal waveform

2rt \ A Cos +0 | in order to produce a duobinary data waveform e2(t) ... a second multiplier in order to multiply the aforesaid duobinary data waveform e2(t) with the aforesaid sinusoidal waveform

/ gt A Cos +sX T in order to produce a maximum bandwidth dicode data waveform e3(t).

...a low pass filter with a raised cosine roll-off characteristic in order to constrain the spectrum of the aforesaid waveform e3(t) within the bounds 1 0- - Hz T while conforming to the Nyquist 'equal areas' criterion.

Embodiments of the invention for direct duobipolar data transmission over cable, without the use of carrier wave modulation will now be described by way of example only, with reference to the accompanying drawings, wherein: Figure 1, is a block diagram illustrating equipment at the transmitting point for transmission apparatus according to the invention.

Figure 2(a) is a block diagram illustrating equipment at the receiving point complementary to the equipment shown in Fig. 1.

Figure 2(b) is a block diagram illustrating an alternative arrangement if equipment at the receiving point complementary to the equipment shown in Fig. 1.

Figure 3(a), (b), (c) and (d) show typical waveforms (time v amplitude) where the circuits of Figs. 1 and 2 are used for transmitting and receiving digital information and Figures 4(a) and 4(b) show typical frequency spectra (frequency v amplitude) complementary to the waveform shown in Figs. 3(a) and 3(b); Figs. 5(a) and 5(b) show typical frequency spectra compementary to the waveforms shown in Figs. 3(d) and 3(c).

The waveforms and spectra drawings are copies of the print-out obtained from a computer simulation of the processes involved in the present invention.

Fig. 1 shows: (a) A digital signal 9. The signal e,(t) at this input is a 50% duty cycle bipolar digital signal having a bit period of T seconds. The input 9 connects with a low pass filter 10 which band limits the signal input at the Nyquist frequency - Hz 2T to provide a derived duobipolar waveform el(t) in connection 11.

(b) An A.C. coupling circuit 12 in order to match the derived duobipolar waveform to the cable at output 13.

The signal e,(t) at input 9 is shown in Fig. 3(a): this figure has a binary code marked on it; the frequency spectrum is shown in Fig. 4(a).

The derived duobipolar waveform el(t) in connection 11 is shown at Fig. 3(b) and the spectrum of el(t) is shown at Fig. 4(b).

The signal el(t) represented by Fig. 3(b) and Fig. 4(b) is that transmitted from the A.C.

coupling circuit output 13 and is subject to a data recovery process at a receiving station represented by Fig. 2(a).

Fig. 2(a) shows: (a) A signal input 43 at which is received the duobipolar waveform shown in Fig. 3(b) (b) An A.C. coupling circuit 42 for matching the received duobipolar waveform to a first multiplier circuit 37 at the first input connection 40.

(c) A local oscillator for generating the sinusoidal waveform

Ul A Cos +0 T with an output connection 39 connected to the second input 41 connection of the first multiplier 37 and the second input connection 34 of a second multiplier 32.

(d) A low pass filter 35 with a cut-off frequency 1 - Hz 2T inserted between the output 36 of the first multiplier 37 and the first input 33 of the second multiplier 32.

Fig. 2(b) shows: (a) An alternative arrangement of the circuit component shown in Fig. 2(a) in which the low pass filter is inserted between the output 33 of the second multiplier 32 and the output connection 31; in this case, the filter has a cut-off frequency at - Hz T The waveform in Fig. 2(a) at the output connection 33 of the low pass filter 35 is shown in Fig. 3(c) e2(t) and is, in fact, a balanced duobinary waveform with the extreme +Ve and -Ve levels representing binary '1' symbols and the zero levels representing binary '0' symbols; the balanced duobinary spectrum is shown in Fig. 5(b), sharply bandlimited at the Nyquist frequency.

The waveform at the output connection 31 of the second multiplier is shown in Fig. 3(d): this is a maximum bandwidth dicode waveform and the spectrum is shown in Fig. 5(a) bandlimited at - Hz T This signal will also appear at the output connection 31 of the low pass filter 35 shown in the alternative block schematic in Fig. 2(b): the complex waveform and spectra appearing at the input connection 33 of the low pass filter 35 and shown in Fig. 2(b) have been omitted as they have not been the subject of computer simulation.

The method heretofore described using a bipolar code sharply bandlimited at the Nyquist frequency has been called 'duobipolar data transmission' as it provides a method of transmitting and recovering bipolar data at twice the normai rate i.e. at 2 bits/Hz with zero amplitude D.C.

content in the frequency spectrum. A special frequency translation technique has been described that enables data recovery within a bandwidth of twice the Nyquist frequency i.e. maximum bandwidth dicode detection.

It will Occur to those skilled in the art of partial response transmission that the circuit configurations represented by the block schematic diagrams shown in Figs. 1 and 2 could, conceptually, be used for the transmission of partial response Class IV signals bandlimited at 1 - Hz 4T with data recovery at the signal destination within a bandwidth of - Hz 2T thereby enabling an operational data capacity of 4 bits/Hz: i.e. 'duobiternary' data transmission.

Partial response Class IV signals-sometimes called 'modified duobinary'-are characterised by a half sine wave spectral envelope for long pseudo random sequences, with spectral nulls occurring at zero frequency and - Hz 2T with a spectral energy maximum at Hz.

4T In fact, the line spectrum for the data sequence used in this Patent Application will closely approximate that shown in Fig. 5(a) but compressed by a factor of two; the truncated spectrum (at - Hz 4T will closely approximate the spectrum shown in Fig. 4(b), also compressed by a factor of two.

In the partial response Class IV case, the low pass filter shown in Fig. 1 would have a cut-off frequency just beyond - Hz 4T and the sinusoidal waveform generator 38 in Fig. 2 would operate at a frequency of - H, 4T APPENDIX Duobinary And Dicode Sequences.

(a) Duobinary Binary input an 100110101111000 Precoding dn=(an+dn )mod 2 0111011001010000 Duobinary bn=dn+dn--1 122112101111000 Balanced Duobinary c,=b, 1 0110010-10000-1-1-1 (invert 1 's following an odd number of 's) (b) Dicode Binary input an 100110101111000 Precoding d,-(a,+d,,)mod 2 0111011001010000 Dicode b,=d,-d, 100-110-101-1-11000 (Invert alternate 1 's)

Claims (9)

1. Telecommunications apparatus including low-pass filter means for truncating the spectrum of pseudo-random Class 1 p.r. 'dicode signals at the Nyquist frequency i.e. 1/2T Hz.

2. Telecommunications apparatus including first multiplier means with two inputs: one input being connected to a source of pseudo-random Class 1 p.r. dicode signals bandlimited at 1/2T Hz such as would appear at the low-pass filter output of the apparatus described in Claim 1, a second input being connected to means for supplying a sinusoidal waveform of frequency equal to half the data bit rate 1/2T Hz a low-pass filter circuit means for band-limiting the first multiplier output signals at 1/2T Hz including means for applying the bandlimited signals to one input of a second multiplier and means whereby a second input of the second multiplier is connected to the heretofore mentioned sinusoidal waveform source.

3. Telecommunications apparatus as claimed in Claim 2 in which there is a direct connection between the output of the first multiplier and one input of the second multiplier a second input of the second multiplier being connected to the heretofore mentioned sinusoidal waveform source, low-pass filter means connected to the output of the second multiplier for bandlimiting the output signals at 1/T Hz.

4. Telecommunications apparatus including low-pass filter means for truncating the spectrum of pseudo-random Class 4 p.r. signals at half the Nyquist frequency i.e. 1/4T Hz.

5. Telecommunications apparatus as claimed in Claim 2 in which the low-pass filter means for bandlimiting the output signals from the first multiplier has a cut-off frequency at 1/4T Hz and the heretofore mentioned sinusoidal waveform source provides a sinusoidal waveform of frequency equal to 1 /4T Hz.

6. Telecommunications apparatus as claimed in Claim 3 in which the low-pass filter means for bandlimiting the output signals from the second multiplier has a cut-off frequency of 1/2T Hz and the heretofore mentioned sinusoidal waveform source provides a sinusoidal waveform frequency equal to 1/4T Hz.

7. Telecommunications apparatus as claimed in Claim 5 in which there is direct access to the output 36 of the first multiplier M, as an alternative system output.

8. Telecommunications apparatus as claimed in Claim 6 in which there is direct access to the output 33 of the second multiplier M2 as an alternative system output.

9. Telecommunications apparatus substantially as hereinbefore described and with reference to Figs. 1, 3 and 4 and also to Figs. 2, 3, 4 and 5.