CA1254277A

CA1254277A - Digital communication system including an error correcting encoder/decoder and a scrambler/descrambler

Info

Publication number: CA1254277A
Application number: CA000478114A
Authority: CA
Inventors: Goro Oshima; Kotaro Kato
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1984-04-03
Filing date: 1985-04-02
Publication date: 1989-05-16
Also published as: EP0157413A3; EP0157413B1; EP0157413A2; DE3586328D1; DE3586328T2; JPH0376613B2; US4639548A; AU567474B2; AU4071985A; JPS60210044A

Abstract

ABSTRACT
An error correction code data communication system of the present invention has a transmitter and a receiver. The transmitter includes an encoder for adding a correction code to a data signal to be transmitted and a scrambler for randomizing the data signal. The receiver includes a descrambler for descrambling the data signal randomized by the scrambler, and a decoder for correcting a code error by means of the error correction code.
The transmitter in accordance with the present invention comprises a random signal generator for generating a random signal in a repetitive pattern having a predetermined period. The scrambler includes circuitry which modulo 2 adds an output of the random signal generator to both a data signal inputted to the encoder and a data signal outputted from the encoder. The receiver comprises a random signal generator for generating a random signal in the same pattern as the signal generated by the first-mentioned random signal generator. The descrambler includes circuitry which modulo 2 adds an output of the second-mentioned random signal generator to both a data signal inputted to the decoder and a data signal outputted from the decoder. The receiver further comprises moni-toring circuitry for monitoring a code error in the decoder, and circuitry for setting up synchronization of the second-mentioned random signal generator in response to an output of the monitoring circuitry in such a manner as to reduce the code error. The inven-tion dispenses with the need for a special synchronizing bit and permits descrambling without enhancing data errors developed in the transmission path.

Description

1~5~2'7~

DIGITAL COMMUNICATION SYSTEM
INCLUDING AN ERROR CORRECTING ENCODER/DECODER
AND A SCRAMBLER/DESCRAMBLER

Background of the Invention The present invention relates to a digital communica-tion system of the type using error correction codes and, more particularly, to a digital communication system which includes an improved scrambler/descrambler.
In a digital communication system of the type described, a transmitter station passes an input data signal through a scrambler to randomize it and then through an encoder to error-eorrection eneode the serambler output, the eneoder output being sent to a receiver station over a transmission path. The receiver station in turn routes the received data signal through a deeoder to error-eorreetion decode it and then through a descrambler to~randomize the decoder output to thereby reeover the original data signal.
As will be described later in detail, the descrambler comprises a shift register, an inhibit circuit connected to intermediate taps of the shift register, and an Exclusive-OR gate (modulo 2 adder) connected to inter-mediate taps of the shift register and an output of the inhibit eircuit. The incoming data signal is applied to the shift register by way of the Exclusive-OR gate. Also, 125~ 7

-2- 664~6-324 the intermediate tap output of the shift register is fed back to the shift register via the Exclusive-OR gate. The problem en-countered with such a construction is that da-ta errors developed in the transmission path are magnified by the descrambler at the receiver station.
Summary of the Invention It is therefore an object of the present invention to provide a digital communication system capable of synchronizing a scrambler and a descrambler to each other without the need for a special synchronizing bit, and descrambling a signal without enhancing data errors developed in a transmission path.
An error correction code data communication system of the present invention is characterized in that a transmitter scrambles both non-coded data and coded data while a receiver descrambles both non-decoded data and decoded data using the same code process, and that the frequency of occurrence of an error correction pulse in a decoder is monitored to set up synchroniza-tion for descrambling.
Specifically, according to one aspect the invention provides an error correction code communication system having a transmitter and a receiver; said transmitter including an encoder for adding a correction code to a data signal which is to be transmitted, and scrambler means for randomizing the data signal;
said transmitter further comprising a random signal generator for generating a first pseudo random signal in a repetitive pattern having a predetermined period; said scrambler means including ~:54~Z~7

-3- 66446-324 means for modulo 2 adding an output of said random signal gener-ator to both a data signal inputted to the encoder and a data signal outputted from the encoder; said receiver including de-scrambler means for descrambling the data signal randomized by said scrambler means, and a decoder for correcting a code error by means of the error correction code; said receiver comprising a random signal generator for generating a second pseudo random signal being substantially the same as and synchronized therewith said first pseudo random signal; said descrambler means including means for modulo 2 adding an output of said second-mentioned random signal generator to both a data signal inputted to the decoder and a data signal outputted from the decoder; said re-ceiver further comprising means for monitoring a code error in the decoder, and means responsive to said code error for setting up a synchronization of the second-mentioned random signal generator in response to an output of said monitoring means, said synchron-ization moving in a direction which reduces the code error.
According to another aspect, the present invention provides an error correcting and scrambling digital communication system comprising a source of digital signals, convolution encoder means having a receive input and output with said receive input coupled to said source and said receive output coupled to a trans-mission path to receive said digital signals from said source to said transmission path, scrambler means including a first random generator means coupled between said source and said receive input and between said receive output and said transmission path whereby said scrambler means is coupled in parallel with said convolutional ~Z5~77 -3a- 66446-324 Pncoder means, convolution decoder means having a receive input and a receive output with said receive input coupled to said transmission path and said receive output coupled to a system output terminal, and descrambler means including a second random generator means coupled between said transmission path and said receive input whereby said descrambler means is coupled in paral-lel with said decoder, said first and second random generator means operating in the same pattern.
According to a third aspect, the present invention pro-vides a method of correcting errors in scrambled digital communi-cation, said method comprising the steps of: (a) providing digital signals for encoding and scrambling; (b) encoding said signals in a convolution encoder means having a transmit input and output with said transmit input coupled to receive said digital signals and said transmi-t output coupled to a transmission path; (c) scram-bling said digital signals by including a first random generator means coupled be-tween a source of said digital signals and said transmit input and between said transmit output and said trans-mission path whereby said scrambler means is coupled in parallel ~0 with said convolutional encoder means; (d) transmitting said digital signals from said transmit output to said transmission path; (e) applying signals transmitted over said transmission path to the convolution decoder means having a receive input and a receive output with said receive input being coupled to said transmission path and said receive output being coupled to a system output terminal; (f) descrambling of said transmitted signals by ~Z5gL~7'7 -3b- 66446-324 including a second random generator means coupled between said transmission path and said receive input whereby a means for accomplishing said descrambling i.s coupled in parallel with said decoder; (g) operating said first and second random generator means operating in the same pattern; and (h) monitoring a code error in said decoder, thereby synchronizing said second random generator.
The invention will now be described in greater detail with reference to the ascompanying drawings, in which:
Fig. 1 is a block diagram of a prior art error correc-tion code data communication system;

4 1;~2'77 Fig. 2 is a circuit diagram showing an example of prior art scramblers or descramblers;
Fig. 3 is a schematic block diagram of an error correction code data communication system in accordance with the present invention;
Fig. 4 is a block diagram representative of a transmitter forming part of the communication system of the present invention; and Fig. 5 is a block diagram of a receiver forming the other part of the system of the present invention.
Rererence will now be made to the accompanying drawings for describing the present invention in detail.

Detailed Description of the Invention Referring to Fig. 1 of the drawings, a prior art data communication system of the kind concerned is shown.
At a transmitter, an input data signal 100 to be transmitted is randomized by a scrambler 1, then provided with an error correction code by a convolutional encoder 2, and then transmitted over a transmission path 3 as a transmit data signal 101. At a receiver station, on the other hand, a received input data signal 102 coming in from the trans-mission path 3 is decoded by a convolutional decoder 4, then turned into the original signal by a descrambler 5 with a code error corrected, and then outputted as a received output data signal 103.

~Z~4~7 Desirably, an error correction code transmission system constructed as described above should readily set up synchronization between the scrambler 1 at the transmitter and the descrambler 5 at the receiver without resorting to a special synchronizing bit.
Referring to Fig. 2, there is shown in a block diagram a prior art scrambler or a descrambler.
The circuitry shown in Fiq. 2 represents a scrambler or a descrambler presented in CCITT Recommendation V-35.
The circuitry in a scrambler mode operation receives an input data signal at a terminal A and produces a scrambled data signal at a terminal B, while in a descrambler mode operation receiving an input data signal at the terminal B
and producing a descrambled data signal at the terminal A.
Outputs of a twenty-stage shift register 10 appearing at intermediate taps T3 and T20 are applied to an adder 12 to be modulo 2 added thereby. Meanwhile, outputs of the twenty-stage shift register 10 appearing at taps Tl and Tg are delivered to an inhibit circuit 11. An output of the inhibit circuit 11 and that of the adder 12 are routed to an adder 13 also for modulo 2 addition, thereby inhibiting an occurrence that the data signal become all ZEROs.
An output of the adder 13 and the transmit input data signal 100 or a receive input data signal are summed modulo 2 by an adder 14 the output of which is applied - 6 - ~ 2 5~ Z7t7 to the terminal B (or A). The output of the adder 14 is also fed to the twenth-stage shift register 10 for a synchronizing function.
With the construction shown and described, the scrambler or the descrambler allows a transmitter and a receiver to be automatically brought into synchronization and, therefore, needs no special bit or timing otherwise required for synchronization. Due to such a nature, this type of circuitry is generally referred to as a self-synchronizing scrambler/descrambler and extensively usedwith ordinary data transmission systems.
The prior art circuitry, however, has left the following problem unsolved. Since the input data signal is applied to the shift register and an output signal of intermediate taps of the shift register is fed back to the shift register, any error introduced into the data while the latter propagates through the transmission path 3 is undesirably magnified by the descrambler. In the construction shown in Fig. 2, the twenty-stage shift register 10 has two intermediate taps and, hence, the error probability of the data output from the descrambler is at maximum three times greater than that associated with the transmission path.
Referring to Fig. 3, a digital communication system embodying the present invention which is free from the _ 7 _ ~ Z77 problems discussed above is shown in an extremely skeletonized diagram.
In a transmitter, a scrambler la comprises a pseudo random generator 23, a convolutional encoder 33, and

5 modulo-2 adders 21 and 22. The pseudo random generator 23 and convolutional encoder 33 are commonly interconnected to the adders 21 and 22. An output of the adder 22 is coupled to a transmission path 3. A data signal received over the transmission path 3 is applied to an adder 24 of a descrambler 5a. A convolutional decoder 4a and a pseudo random generator 26 of the descrambler 5a are commonly interconnected to modulo-2 adders 24 and 25. An output 110 of the decoder 4a is adapted to control the descrambler 5a.
First, the construction of the transmitter will be described. A transmit input data signal 100 and an output of the pseudo random generator 23 are applied to the adder 21 which then sums them modulo 2. An output of the adder 21 in turn is fed to a convolutional encoder 2 which then produces an error correction code. Specifically, an output of the encoder 2 includes a data signal and a parity signal. The output of the enc~der 2 and that of the pseudo random generator 23 are modulo 2 added by the adder 22, while an output of the adder 22 is sent to the transmission path 3 as a transmit output data signal lOla.

- 8 - ~ 2 Next will be described the construction of the receiver. A receiver input data signal 102a coming in from the transmission path 3 and an output of the pseudo random generator 26 are routed to the adder 24 to be summed modulo 2. An output of the adder 24 is demodulated by the convolutional decoder 4a. An output of the decoder 4a and that of the pseudo random generator 26 are subjected to modulo 2 addition at the adder 25 the output of which is delivered as a receive output data signal 103. The 10 synchroni~ing timing for the generator 26 is accomplished ~
by monitoring the output 110 of the decoder 4a, i.e. an - ~:
error correction pulse signal 110.
Details of the error correction code data communication system of the present invention will be presented. In the ~-illustrative embodiment, the encoder 2 comprises a convolutional encoder having an encoding/decoding rate (R) of 3/4, while the decoder 4a comprises a convolutional decoder with R = 3/4.
Referring to Fig. 4, a construction of the transmitter in accordance with the illustrative embodiment is shown in a block diagram.
As shown in Fig. 4, the transmitter includes a serial-to-parallel (S/P) converter 31 which is connected to three adders 32a, 32b and 32c which in turn are connected to a convolutional encoder 33 with R = 3/4 ':

_ 9 _ ~ ~54Z~7 (corresponding to the encoder 2 of Fig. 3). The encoder 33 consists of a twenty-stage shift registers 38a, 38b and 38c, modulo-2 adders 39a, 39b and 39c, and an inverter 40, which are interconnected as illustrated. The construction and operation of the encoder 33 per se are well known in the art and, therefore, detailed description thereof will be omitted for simplicity. Outputs of the encoder 33 are applied to four modulo-2 adders 34a, 34b, 34c and 34d which are connected to a parallel-to-serial (P/S) converter 35. A 1/3 frequency converter 36 has an output connecting to the S/P converter 31, pseudo random generator 23 and convolutional encoder 33. A 4J3 frequency converter 37 has an output connecting to the P/S converter 35. The pseudo random generator 23 comprises, for example, a twenty-stage shift register, an inhibit circuit and an Exclusive OR gate and forms part of the scrambler la;
outputs (e.g. first to third stage outputs and seventeenth to twentieth stage outputs) of the twenty-stage shift register are connected to three adders 32a, 32b and 32c and four adders 34a-34d, respectively.
A data signal flows through the transmitter described above as will be explained hereinbelow.
A transmit input data signal 100 is applied to the P/S converter 31 to be thereby converted to a three parallel streams of data. The parallel data streams - lo ~ 4Z77 respectively are coupled to the three adders 32a-32c to be thereby modulo 2 added each to an output of the pseudo random generator 23. The outputs of the adders 32a-32c are applied in parallel to the convolutional encoder 33 which then error-correction codes them to provide four streams of parallel data. The four parallel data streams respectively are routed to the four adders 34a-34d to be thereby summed modulo 2 each with an output of the pseudo random generator 23. The parallel data output from the adders 34a-34d are converted by the P/S
converter 35 to a serial transmit output data signal lOla.
The various sections of the transmitter are operated at individual timings which will be described.
Transmit input pulses 104, an input sync signal associated with the data signal 100, have their frequency reduced to 1/3 by the 1/3 frequency converter 36 in order to set up synchronization between the output of the S/P
converter 31 and that of the pseudo random generator 23.
Simultaneously, the clock pulses 104 are applied to the 4/3 frequency converter 37 to have their frequency reduced to 4/3, whereby a transmit output data signal 105 is provided. The signal 105 serves as a sync signal for synchronizing the output of the P/S converter 35 and, also, for sending it to the transmission path 3.
Referring to Fig. 5, the receiver in accordance with ~I~Z54Z77 the system of the present invention is shown in a block diagram.
As shown, the receiver includes a serial-to-parallel (S/P) converter 41 which is interconnected to four modulo-2 adders 42a, 42b, 42c and 42d, which in turn are connected to a convolutional decoder 43. The decoder 43, corresponding to the decoder 4a of Fig. 3, comprises twenty-stage shift registers 51a, 51b and 51c, Exclusive OR gates 52a, 52b and 52c, Exclusive-OR gates 53a, 53b and 53c, an Exclusive-OR gate 54, an inverter 55, an OR
gate 56, a syndrome register 57, and a threshold detector 58, which are interconnected as illustrated. The convolutional decoder 43 is of the well known type and, therefore, details thereof will not be described for simplicity. The encoder 43 is connected to three modulo-2 adders 44a, 44b and 44c which in turn are connected to a parallel-to-serial (P/S) converter 45.
A 1/4 frequency converter 46 is connected to the S/P
converter 41, and a 3/4 frequency converter 47 to the P/S converter 45. A pseudo random generator 43 is connected to the four adders 42a-42d and three adders 44a-44c. A sync monitor counter 49 is connected to the OR gate 56 of the convolutional decoder 43. The input -of a logic circuit 50 is connected to the output of the sync monitor counter 49 and that of the 1/4 frequency - 12 - ~ ~54~77 convertex 46. The output of the logic circuit 50 is connected to a clock input terminal CLK of the pseudo random generator 48. The pseudo random generator 48, sync monitor counter 49, and adders 42a-42d and 44a-44c 5 constitute a descrambler 5a in combination. -The flow of a data signal through the receiver constructed as above will be described.
A receive input data signal 102a is applied to the S/P converter 41 to be converted to four parallel streams of data. The four adders 42a-42d modulo 2 add respectively the four parallel data to outputs of the pseudo random generator 48 (e.g. twentieth to seventheenth outputs of a twenty-stage shift register built in the generator 48).
Then, the convolutional decoder 43 error-correction ~-15 decodes the outputs of the pseudo random generator 48, -thereby producing three parallel streams of data. These three parallel data streams are summed modulo 2 with outputs of the pseudo random generator 48 (e.g. third to first outputs of the twenty-stage shift register inside the ~:
20 generator 48), the resulting parallel data being turned ~
by the P~S converter 45 to a serial receive output signal i -103. It will be noted that the pseudo random generator 48 is identical with the pseudo random generator 23 of the previously described transmitter.
The various sections of the receiver will be operated at individual timings presented hereinbelow.

12S~Z77 Receive input pulses 106, an input sync signal associated with the receive input data signal 102a, are reduced in frequency to 1/4 by the 1/4 frequency converter 46 in order to provide synchronization between an output of the S/P converter 41 and that of the pseudo random generator 48. The frequency of the receive input pulses is also reduced to 3/4 by the 3/4 frequency converter 47 to provide a receive output data signal 107, which functions to synchronize the output of the P/S converter 45.
Now, synchronization between the transmitter and the receiver in accordance with the present invention will be presented.
As a received input signal 102a begins to come in the receiver and while the pseudo random generator 48 is not synchronized, the threshold detector 58 of the convolutional decoder 43 generates a number of error correction pulses Ci, Cj and Ck and applies them to the sync monitor counter 49. The counter 49 in turn gènerates an inhibit pulse if the number of the correction pulses is generater than a predetermined threshold value. The logics 50 provides AND of an inverted version of the inhibit pulse and an output of the 1/4 frequency converter 46, while the pseudo random generator 48 shifts the sync timing in response to an output of the logics 50. As a result, synchronization of the pseudo random generator 48 is - 14 - ~ 25~77 set up only when the number of the error correction pulses has decreased below the threshold value, desirably to the minimum.
In the illustrative embodiment, the output stages of the pseudo random generators 23 and 48 to be selected are open to choice so long as they satisfy a random condition of a data sequence required of the associated generator.
While the present invention has been shown and described as employing convolutional error codes with R = 3/4, it is readily applicable to any other convolutional error codes, generally with R = n/m, as well as to block codes.
It will be seen from the above that the present invention provides an error correction code data communication system which allows a scrambler of a transmitter and a descrambler of a receiver to be synchronized without resorting to the transmission of a special sync signal and, in addition, prevents an input data signal from being applied to the generator 23 or 48 and, thereby, prevents a data error developed in a transmission path from being magnified at the receiver.
With the above advantageous features, the present invention simplifies a digital communications system of the type using error correction codes while eliminating the need for transmitting additional control signals or sync signals.

Claims

EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

An error correction code communication system having a transmitter and a receiver; said transmitter including an encoder for adding a correction code to a data signal which is to be trans-mitted, and scrambler means for randomizing the data signal; said transmitter further comprising a random signal generator for gen-erating a first pseudo random signal in a repetitive pattern hav-ing a predetermined period; said scrambler means including means for modulo 2 adding an output of said random signal generator to both a data signal inputted to the encoder and a data signal outputted from the encoder; said receiver including descrambler means for descrambling the data signal randomized by said scram-bler means, and a decoder for correcting a code error by means of the error correction code; said receiver comprising a random signal generator for generating a second pseudo random signal being substantially the same as and synchronized therewith said first pseudo random signal; said descrambler means including means for modulo 2 adding an output of said second-mentioned random signal generator to both a data signal inputted to the decoder and a data signal outputted from the decoder; said receiver further comprising means for monitoring a code error in the decoder, and means responsive to said code error for setting up a synchroniza-tion of the second-mentioned random signal generator in response to an output of said monitoring means, said synchronization moving in a direction which reduces the code error.
2. An error correcting and scrambling digital communica-tion system comprising a source of digital signals, convolution encoder means having a receive input and output with said receive input coupled to said source and said receive output coupled to a transmission path to receive said digital signals from said source to said transmission path, scrambler means including a first random generator means coupled between said source and said receive input and between said receive output and said transmission path whereby said scrambler means is coupled in parallel with said convolution encoder means, convolution decoder means having a receive input and a receive output with said receive input coupled to said transmission path and said receive output coupled to a system output terminal, and descrambler means including a second random generator means coupled between said transmission path and said receive input whereby said descrambler means is coupled in parallel with said decoder, said first and second random generator means operating in the same pattern.
3. The system of claim 2 and first modulo 2 adding means driven by said first random generator means and being coupled between said source and said receive input and between said receive output and said transmission path, and other modulo 2 adding means driven by said second random generator means and being coupled between said transmission path and said receive input and between said receive output and said system output terminal.
4. The system of claim 3 and means responsive to a threshold detection for developing a code error signal responsive to dif-ferences in the synchronism of said first and second random gen-erator means, means responsive to said code error signal for synchronizing said first and second random signal generator means.
5. The system of claim 4 and clock means operating at a given frequency, divider means associated with each of said random generators for dividing said clock frequency and means for driving said random generators responsive to said divided frequencies from the divider means associated therewith.
6. The system of claim 5 and a serial-to-parallel converter coupled between each of said inputs and the modulo 2 adding means associated therewith and a parallel-to-serial converter coupled between said outputs and the modulo 2 adding means associated therewith, said dividing means being coupled to drive each of said converter means.
7. The system of claim 6 wherein said frequency divider associated with said first random generator means applies a 1/3-frequency to the serial-to-parallel converter at said receive input and to said first random generator and applies a 4/3-fre-quency to the parallel-to-serial converter at said receive output.
8. The system of claim 6 wherein said frequency divider as sociated with said first random generator means applies a 1/4-frequency to the serial-to-parallel converter at said receiver input and to said first random generator and applies a 3/4-fre-quency to the parallel-to-serial converter at said receive output.
9. The system of claim 7 wherein said frequency associated with said first random generator means applies a 1/3-frequency to the serial-to-parallel converter at said receive input and to said first random generator and applies a 4/3-frequency to the paral-lel-to-serial converter at said receive output.
10. A method of correcting errors in scrambled digital communication, said method comprising the steps of: (a) providing digital signals for encoding and scrambling; (b) encoding said signals in a convolution encoder means having a transmit input and output with said transmit input coupled to receive said digital signals and said transmit output coupled to a transmission path;
(c) scrambling said digital signals by including a first random generator means coupled between a source of said digital signals and said transmit input and between said transmit output and said transmission path whereby said scrambler means is coupled in parallel with said convolutional encoder means; (d) transmitting said digital signals from said transmit output to said transmis-sion path; (e) applying signals transmitted over said transmission path to the convolution decoder means having a receive input and a receive output with said receive input being coupled to said transmission path and said receive output being coupled to a sys-tem output terminal; (f) descrambling of said transmitted signals by including a second random generator means coupled between said transmission path and said receive input whereby a means for accomplishing said descrambling is coupled in parallel with said decoder; (g) operating said first and second random generator means operating in the same pattern; and (h) monitoring a code error in said decoder, thereby synchronizing said second random generator.