GB2137456A - Carrier data transmission system with error correcting data encoding - Google Patents

Abstract

A multi-phase communication system for simultaneously transmitting plural binary digits employs one or more error-check bits derived by a convolutional coding procedure over a plurality of digit intervals. The information and error check digits are then communicated by amplitude modulating phase-orthogonal carriers (Fig. 1). Upon reception, the incoming information and error check digits are decoded, and supplied to convolutional error syndrome circuitry (Fig. 5) which corrects errors which may arise from time to time during data transmission. Cascaded syndrome circuitry may be provided. <IMAGE>

Description

SPECIFICATION Carrier data transmission system with error correcting data encoding Disclosure of the invention This invention relates to electronic communications and, more specifically, to an improved data transmission and reception system employing error detection and correction.

It is an object of the present invention to provide an improved data transmission system More specifically, it is an object of the present invention to provide improved data transmission apparatus which coincidentally transmits plural digits via quadrature phase modulation; and which encodes the outgoing digital information to detect and correct errors in the received information data stream.

The above and other objects of the present invention are realized in a specific, illustrative multi-phase communication system for simultaneously transmitting plural binary digits which employs one or more error-check lists to perform an encoding on the conveyed information over a period of time. The information and error check digits are then communicated via a transmission channel - as by amplitude modulating phase-orthogonal carriers.

Upon reception, the incoming information and error check digits are decoded, and supplied to error syndrome circuitry which corrects errors which may arise from time to time during data transmission.

The above and additional features and advantages of the present invention will become more clear from the following detailed description of a specific, illustrative embodiment thereof, presented hereinbelow in conjuction with the accompanying drawing, in which: Figure 1 is a signal space diagram depicting multi-phase transmission of the prior art and of the instant invention; Figure 2 is a block diagram of transmitter circuitry, of the invention; Figure 3 schematically depicts receiver circuitry operative in conjunction with the Figure 2 transmission apparatus; Figure 4 represents a specific, illustrative implementation of transmitter convolutional encoder circuitry 13 of Figure 2; and Figure 5 illustrates a specific, illustrative implementation for decoder 41 and syndrome error correcting apparatus 43 utilized in the Figure 3 receiver.

Referring now to the drawing and more particularly Figure 1, there is shown a signal space diagram for multi-phase, e.g., quadrature, transmission. As is peruse well known (e.g., for QPSK orquadraturephaseshift keying transmission), two carriers are independently amplitude modulated along orthogonal (i.e., 90 phase shifted) axes. Thus, for conventional QPSKtransmission one carrier assumes an orientation along the plus or minus X-axis (0 or 180 phase shift) depending upon one of two bits for the di-bit encoding; while another carrier resides along either the plus or minus Y axis (90 or 270 transmission) in accordance with the binary value ofthe second bit.Accordingly, an outgoing QPSK radiation will assume one ofthe pointsa-d of Figure 1 depending upon the value ofthetwo information bitsto betransmitted. Such QPSK coding is widely employed and basically has a phase error margin of45'. That is, a received QPSKsignal mayvaryfrom its nominal value and still be properly detected as long as the variation does not exceed 45 . Once the 45 limit is exceeded, the received point appears closer to a different QPSK state and thus an error occurs in one of the two information bits.

In QPSKtransmission, all carriers have a like nominal amplitude. For purposes ofthe Figure 1 description assuming a unit circle sample space (shown dashed in Figure 1), the relative amplitude of each carrier for QPSK transmission (permissible transmission states a-d) is always + g1E In accordance with the principles underlying one aspect of the instant invention, eight possible transmission states, given by the points a-h of Figure 1, are utilized. To determine one of eight states (vis-a-vis, one of four for conventional QPSK transmission) a third outgoing phase-specifying digit is developed in addition to the two di-bit outgoing information values (herein deemed X2 and x1). This third digit is an error encoding bit stream and is deemed herein xo.Itwill now be apparentfrom Figure 1 thatthe error correcting additional bit increases the phase margin for properly recovering a di-bittransmission by 22 1/2'to total of 67.5" as shown in Figure 1 (thus effecting approximately a 2 db improvement in error rejection).

By way of overview, the ability to correct for single errors is imparted to the apparatus of the instant invention by employing atthetransmitter an additional bit which performs an encoding overthetwo outgoing digital data streams. Moreover, the encoding is effected over a period of time such that each error correcting, encoding digit represents information which is in general descriptive of each of (i) the first (or xa) data bit and the previous such bits over some plural-digittime interval; (ii) the second (orxi) data bit and previous such bits over a predetermined interval; (iii) the coding or error check bit (xo) data stream over the like interval.

At the receiver, the recovered information data streams X'2 and x'. are delayed for the number of digit intervals over which the encoding was performed (as is also the received coding bit stream x'o).The inverse of the coding operation effected atthe transmitter is then accomplished. If the recovered information X', x' and x'0 suffered no errors, the parity will fully check and the xP and x. data is simply passed to an output utilization device or circuit.However, if any error in reception or receiver data processing has occurred, the parity will not check in the receiver decoding apparatus and then the error syndrome processing structure will then operate in the manner more fully described below to overcome or cure the error in the received information.

With the above overview in mind, attention will now be directed to the transmission and receiver apparatus of Figures 2 and 3 which specifically implement the above discussed overall mode of operation. Considering initially the transmitter apparatus of Figure 2, a data source 10 supplies two data serial streams X2 and X1 such that for the instant di-bit-transmission during each digit or bittime, there exists one binary value for each of x.

and X2. The X2 and x. values are then supplied to a convolutional encoder 13 (together with the inernally generated error or check bit stream output xo which is fed back into the circuit). The convolutional encoder includes a number of delay elements and during each bit time provides a specific output value xo which is dependent upon the current and/or past values of X2, xa and Xo over some interval. A specific embodiment of the convolutional encoder 13 is shown in Figure 4 and is discussed below. Again, suffice it for present purposes thatthe error check output bit stream xo performs an error correction encoding on both itself and on the X2 and xi data flow.Specific error detecting and correction codes are per se well known to those skilled in the art - see, e.g., Figure 4 and the discussion below.

During each data interval, the then obtaining X2, xi and xo binary values constitute addressing inputs to a read only memory (ROM) 16 which for each one-of-eight input address conditions supplies three amplitude and one polarity defining input to each of digital-to-analog converters 17 and 18. The outputs of the digital-toanalog converters 17 and 18 supply an analog level of one-of-three bi-polar states sufficientto give rise to one of the eight permissible transmission points a-h of Figure 1 depending upon the input variables X2, xi and xo.

More specifically, with respect to the outgoing quadrature phase transmission, a carriersource 12 supplies a source of carrier directly to a modulator 21 wherein the 0" phase carrier is modulated by the output of the digital to analog converter 18. The output carrier of source 12 is shifted by 90 in a phase shifter 15 and is supplied to the modulator 19. The output amplitude of modulator 19 is controlled by digital-to-analog converter 17.

Accordingly, in aperse straightforward mannerthe input variables X2, Xi and Xo to ROM 16 specify one of the eighttransmission pointsa-h; andthe output of ROM 16 acting in consortwiththe digital to analog converters 17 and 18 and modulators 19 and 21 generate the requisite two modulated quadrature carrier signals to implement the desired transmission point. It is apparent that instead of the shifted amplitude signals of the QPSK situation considered above, the phase modulated carrier signals of the instant transmission system have relative values of "0" ~ g7t and 1 "1" for the assumed unit circle of Figure 1.The two modulated quadrature carriers Z. and Z2 are then combined in a linear summing network 26 and impressed upon a communications channel for distribution to any and all intended recipients. The communications channel may be of any form well known to those skilled in the art, e.g., radio carrier, wire communications or the like.

Thus, it will be seen that for each digit period, the two then obtaining characters X2 and xi -togetherwith the coincidentally present error bit xo define a specific transmission point of Figure 1 - and the phase and amplitude ofthe outgoing quadrature carriers in general varies from digit period to digit period in accordance with the incoming data streams and in accordance with the history of those streams as reflected by the xo signal.

At each receiver location (Figure 3), a reconstituted carrier source 29 supplies a carrier in phase with that of the carrier source 12 at the transmitter. Carrier reconstitution circuitry is per se well known to those skilled in the art and may be implemented, for example, by simply multiplying the incoming carrier signal by a factor of eight to resolve phase ambiguity; and deviding the eighth harmonic so formed by a factor of eight. The reconstituted carrier 29 is supplied directly to detector (e.g., a synchronous mixer and low pass filter) 31.

Further, the reconsituted carrier from source 29 is shifted in phase by 90 in circuitry 33 and supplied to detector 30. The outputs ofthe detectors 30 and 31 (which correspond to the analog levels of converters 18 and 17) are respectively supplied to analog-to-digital converters 38 and 36. The output of converter 36 is a three bit digital word which corresponds in the absence of transmission errorto the input atthe transmitter (Figure 2) to digital-to-analog converter 18. Similarly, the output of analog-to-digital converter 38 at the receiver corresponds to the three digital bits supplied to the digital-to-analog converter 17.

The read only memory (ROM) 40 at the receiver provides a function inverse to that effected by ROM 16 in the transmitter. In particular, the six bit output of analog-to-digital converters 36 and 38 are address inputs to ROM 40 (and specify thex and y coordinates of the received signal point). The ROM 40 includes a stored pattern which converts these inputs to the equivalent X'2, x'i and X'o received versions of the original X2 and xi and xo outgoing data at the transmitter.The use of the prime designations on thex-underlying variables indicates received versions of the outgoing data. The x'a, x'i and x'o data will identically equal the X2, Xi and Xc variables in the absence of error. Should a transmission error or receiver data processing erroroccurthere of course will be a difference between the two and it is the function of the instant apparatus to detect and correctthat difference and that error.

The received X'2, x'i and x'o signals are supplied to a decoder circuit 41 which provides a decoding which is the inverse ofthateffected by convolutional encoder 13. The decoder 41 includes delay apparatus for delaying between its input and output the X'2, X'i and x'o signals for an interval corresponding to the encoding period for convolutional encoder 13. Assuming that the received signals are identical to the transmitted signals, the decoder 41 supplies on an output lead 42 to a syndrome correcting circuit 43 characterized by an error-free signalling state (e.g., a binary "0") indicating that no errors have occurred. As long as this situation obtains, the delayed x'2 and x'l outputs of decoder 41 pass without change through Exclusive-Or gates 79 and 81 such that the recovered X2 and x. signals are available for any desired output utilization purpose. However, when an error does occur, the output lead 42 switches to its alternate (assumed "1") state. Depending upon the nature ofthe error and the power of the code employed, one or perhaps both ofthe erroneous x'z orx'i variables are corrected in the corresponding Exclusive-Or gates 79 or 81 by "1 " output signal on leads 44 or 45 (since a "1 " input to an Exclusive-Or gate will invert whatever binary level is applied to the other input of an Exclusive-Or gate).A specific implementation for decoder 41 and the syndrome correcting circuit 43 operative with respect to the specific encoder 13 shown in Figure 4 is discussed below with respect to Figure 5.

Turning now to Figure 4, there is shown a specific convolutional encoder 13 used in the transmitter portion of the instant communication apparatus (Figure 2). The apparatus includes an array of cascaded unit delay circuits 50, 53, 56, 58, 62, 64, 66, 72 and 73, i.e., delays of one digit period for the data streams Xe and x.. A plurality of Exclusive-Or gates 52,55,60,63,65 and 69 are disposed between illustrated ones ofthe unit delay elements. As above discussed, it is the purpose of the encoder to perform an encoding over the x2, x1 and to digits over a predetermined number of prior bit intervals.Thus, forthe Figure 4 encoder, the error bit encoding xo will include a measure of the x2 signal from two bittimes prior (effected by delays 72 and 73 through the flow combining effect of Exclusive-OR gate 69), of the X2 digit four digits prior (delays 64, 66, 72 and 73 with the combining and pass through effects of Exclusive-OR gates 63,65 and 69); and five bits prior(via delays 62,64, 66, 72 and 73 and their associated Exclusive-OR gates. Similarly, xo is also a function of the Xi variable occurring 3, 4, 5 and 7 bit times earlier; and of its own error check-encoding character xo stream 7, 8, and 9 times earlier.Expressed in mathematical terms, Xo = x2(D+D4+D5)+x1 . (D +D4+D5+D7)+xo . (D7+D8+D9) Equation 1 where "D" is an operator representing one unit of delay, "D" represents two units of delay, and so forth.

Rearranging Equation 1, xc-Xc(D7+ D8+ D) = xe(D'+ D4+D')+xi (D3+ D4= D+ D7) Equation 2 and solving for xo, x2(D+ D4+ D6) + x1(D + D4+ D5+ D7) Xo = ------------------------------------------- Equation 3 1 - (D7+D8+D9) Letting operators G2, G1 and Go represent the functional dependence of x0 on x2, x1 and x0, respectively, Equation 3 may be rewritten, x2G2+x1G1 xo = ------------- Equation 4 1 - Go For purposes which will become more clear below when operation of the Figure 5 decoder 41 is considered, Equation 4 may be rewritten, 3 = XeGe+XiGi+Xc(Gc-1).Equation 5 The negative sign in the last parenthetical expression of Equation 5 may be changed to a positive yielding Equation 6, 0 = XeGe+XiGi+Xc(Ge+1), Equation 6 since, for the modulo-2 purpose effected by the Exclusive-OR gates of the instant apparatus, subtraction and addition yield identical results. Thus it will be clear from the foregoing in Figure 4 that the expression for the output encoding bit stream xo is given by Equation 3 (or 4) above and that xo in fact is dependent upon events occurring as many as nine digit times earlier.

A specific implementation for decoder 41 and syndrome correcting circuit 43 operative in conjunction with the Figure 4 implementation of transmitter encoder 13 is shown in Figure 5. The received versions of the transmitted data x2, x1 and x0, viz., x'2, x'1 and x'0, are respectively loaded into nine stage shift registers 80, 82 and 85. Nine stages are employed for the shift registers since nine digit times of data must be examined to recheckthe parity effected by encoder 13 (Figure 4) which was operative in conjunction with Xo digits occurring nine digit intervals earlier. The shift registers 80,82 and 85, operative in conjunction with cascaded Exclusive OR gates 84,83,87 to recheckthe accuracy or parity of the received digits in essence employing the algorithm or expression of Equation 6 above.That is, if the received digits X'e, X'i and x'O accurately and indentically correspond to x2, x1 and x0, Equation 6 will be satisfied and the output ofthe final Exclusive-OR gate 87 will be a binary "0" indicating that no error has occurred. Thus, for example, during encoding the Xe variable was sampled with unit delays of D2, D4 and D5. Accordingly, the output of the second, fourth and fifth shift register 80 stages 802, 804 and 805 are utilized.Similarly examining shift registers 82 and 85 it will be seen that the X'1 received signal is sampled with unit delays of d2, D4 and D7 (corresponding to G1 in Equation 36) while x'0 is sampled with unit delays D7, D3 and D9 as well as a direct ("1 ") connection into the final Exclusive-OR gate 87 ("1 +Go" in Equations 3-6). Again, if no transmission errors were incurred, the output of gate 87 is and will remain a "0".Accordingly, the output of syndrome error correction circuit 43 and more specifically a error correction pattern storing read only memory (ROM) 89 will have at its output terminals Eo-E2 and array of all "0's" thus not inverting the outputs of the Exclusive-OR gates 79 and 81. Under thins condition, the correct x2 and Xi signals flow out of the Exclusive-OR gates 79 and 81 and appear as the desired output data Xe and Xi.

Conversely, if an error does occur, the output of the Exclusive-OR gate 87 will be a "1" rather than a "0".

Further, as each error occurs, it will flow through a shift register 87 also having nine stages each formed of an Exclusive-OR gate and following unit delay. The Particular pattern of "1's" and "0's" atthe output of the shift register 87 stage form address inputs to the correction storing ROM 89 and, for each address input, the appropriate stored correction signals are recovered and impressed on the output ports Ee-Ec. where a "1" appears on the corresponding ROM 89 output, the output of the respective gate 79 or 81 is inverted thus correcting the desired error.The particular contents of ROM 89 depends upon the correspondence between the pattern of detected errors (if any) stored in the shift register 87 - and the inversion(s) required to correct that error condition. Again, where the shift register 87 stores all "0's" indicating no errors, a "0" appears at each of the outputs E2-Eo. The pattern for E2-Eo for any non-zero address input depends on the particular coding and the like used and may be empirically orcomputationally derived. Thus, for example, single or multiple errors may simply be postulated for the received variables x'p, x'i and x'o-andthe pattern in shift register87 computed as the error(s) ripple through register 87.The necessary correction outputs Ee-Ec are then computed for each state of register 87 - and define the stored three-bit contents of the corrective word stored at the corresponding address in ROM 89. The process continues for all errors-and error combinations of interest to define all used address locations in ROM 89.

Accordingly,the apparatus of Figures 2 and 3 using the specific implementations of Figures 4 and 5 includes data formed over nine digit intervals upon transmission; and rechecks the transmitted parity upon reception, correcting any detected errors. Many forms of codes may be employed depending upon the hardware, complexity and code pattern desired for any particular application, In accordance with one aspect of the present invention, stored errors in the shift register stages 87i may be cleared (after output correction) by employing a feed back path from the outputs Ea-Eoto the approprite inputs of the stages 871. The particular connection patterns of Figure 5 correspond to that for the assumed encoding and decoding apparatus 13 and 41, and effectively comprises the inverse of the delays effected by the shift registers 80,82 and 85.Thus, for example, the Eo correction signal (co rrespondi ng to x'o errors) is connected to the first, second and third stages 871, 872 and 873 rather than to the last three stages of shift register 85 in decoder 14. Similarly, the feed back connections for the Ei and Ee outputs arethe mirror inverse forthose ofthe shift registers 82 and 80. Exclusive-OR gates (e.g., gates 92-94) are utilised where more than one output resets any particular stage of shift register 87. Thus, for example, the third stage 873 in reset by both the Et and Eo signals (corresponding to connections from shift register stages 827 and 857 in the decoder 41.Note also the corresponding use of the exclusive-OR gates 93 and 94forthe Xe and xt stage 804, 805 and 824, 82s connections of decoder 41. The above feed back connection is optional and when employed will increase system performance.

The maximum error correction capability of the code employed can be achieved if the functions of the decoder 41 and the syndrome correcting circuit 43 are cascaded, i.e., if the structure of Figure 5 is replicated with outputs X2,j, Xld and XO,j for each stage are supplied as inputs to a 1+1st stage. For this purpose, and Exclusive-OR gate 86 is added (shown dashed in Figure 5) to correct x"o bit priorto the x"a-x"o bits being fed into a cascaded, following Figure 5-type arrangement. This cascade may be repeated as many times as is needed.

In practice, the full capability is achieved in three or four sections.

The storage pattern for ROM 89 is different for each stage ofthe casade. The first ROM corrects only the most easily corrected errors. Later ROM's correct the more difficult to correct errors i.e., multiple clustered errors.

This may be thought of as having the earlier ROM's "clear the field" for the final correction stage. The final stage does not require the Exclusive-OR gate 86.

The above described arrangement is merely illustrative ofthe principles ofthe present invention. Numerous modifications and adaptations thereof may be readily apparent to those skilled in the art. Thus, for example, any number of concurrent data streams xi may be encoded by any number of error correction digits xoi, it simply requiring a signal space having sufficienttransmission conditions to reflect each ofthe differentxi, xoj.

Further, as above discussed, the specific coding and decoding implemented may be as elaborate or straightforward as desired by the user depending upon the economics of any application, considering cost, signal delay time and the like.

Claims (12)

1. In combination in multiphase data communictions apparatus for transmitting digital information, transmitter means including convolutional encoding means for generating a digital stream performing an encoding on the digital information to be conveyed by said communications apparatus over a period of time exceeding one information digit period, and signalling means modulated by the output of said convolutional encoding means and by the digital information conveyed for disseminating plural different-phase carriers; and receiving means for recovering the digital information and digital encoding stream from the output of said signaliing means, said receiver means including decoding means, including delay means for performing the inverse of the encoding effected by said encoding means at said transmitter means, and syndrome error detecting and correcting means enabled responsive to the output of said decoding means, said syndrome error detecting and correcting means including a memory supplying error correcting stored output signals.

2. A combination as in claim 1 wherein said convolutional encoding means includes a plurality of cascaded Exclusive-OR gates and delay units, and means for energizing inputs of said convolutional encoding means with the digital information and with the output stream of said convolutional encoding means.

3. A combination as in claim 1 or 2 wherein said signalling means includes means for generating amplitude modulated quadrature phase carriers.

4. A combination as in claim 3 wherein said signailing means further comprises a first additional memory, first and second modulators supplied with carriers having a phase shifttherebetween, and digital-to-analog converter means connecting said first additional memory and said modulators.

5. A combination as in claim 1 or 2 wherein said receiver decoding means includes plural shift registers, and means for checking the encoding effected at the transmitter by said convolutional encoding means responsive to the stored contents of predetermined stages of said registers.

6. A combination as in claim 1, 2 or 5 further comprising Exclusive-OR means connected to the outputs of said decoding means and said syndrome error detecting/correcting means.

7. A combination as in claim 1,2 or 5 wherein said memory of said syndrome error detecting and correcting includes address input ports, and an additional plural stage shift register having an input connected to the output of said decoding means and outputs connected to said address input ports of said syndrome detecting and correcting means.

8. A combination as in claim 1, 2 or 5 further comprising source means for supplying di-bit digital information to said encoding means and to said signalling means.

9. In combination in multiphase data communications apparatus for transmitting digital information, transmitter means for supplying at least one digital information data stream xi, convolutional encoding means having an output bit stream xo and performing an error correcting encoding on said data stream xi and on its own output xo given by Xo = xiGi + xoGo Where Gi and Go are independent Boolean expressions relating values over plural digit intervals of xi and Xo respectively to Xo, and signalling means modulated by the output of said convolutional encoding means and by the digital information xi supplied by said source thereof for disseminating plural carriers of different phases; and receiving means for receiving the digital information from the output of said signalling means, said receiver means including decoder means, including delay means, for providing a first output signal if the expression 0 = xiGi + xo(l+Go) is satisfied and a different signal if such expression is not satisfied, and syndrome error detecting and correcting means enabled responsive to the output of said decoding means for selectively inverting the outputs of said received information depending upon the error state reported thereto by said decoding means.

10. A combination as in claim 1 or 9, further comprising at least one additional decoding means cascaded with said decoding means in said receiver means, and at least one additional syndrome error detecting and correcting means connected to said additional decoding means, said additional syndrome error and detecting means including additional memory means for supplying error correcting stored output signals.

11. The transmitter means or the receiving means of the combination claimed in any of the proceding claims.

12. A transmitter, or a receiver, or a combination of a transmitter and a receiver, substantially as herein described with reference to the accompanying drawings.