DEVICE FOR DETECTING BIT PHASE DIFFERENCE

Abstract: A bit phase difference detecting device applicable to, for example, a digital microwave communications system for channel switching. The device detects a difference in phase difference between two identical data sequences each being transmitter over a respective one of two independent transmission paths. Each of the data sequences is serial-to-parallel converted into bit sequences the number of which is equal to a predetermined common measure of the frame period of the data sequences. Comparators and decision circuits associated on-to-one with with the comparators are provided for individually detecting differences in bit phase between those bit sequences derived from one data sequence and those derived from the other data sequence which correspond to each other with r espect to order.

Description: BACKGROUND OF THE INVENTION The present invention relates to a device for detecting a difference in bit phase between two identical data sequences each of which is transmitted through a different transmission path. A device of the kind described is essential in many of electronic equipments which handle data. For example, in a digital microwave communications system having a regular channel and a backup channel, two identical data sequences (referred to as data sequences A and B hereinafter for distinction) which are individually transmitted in parallel over the regular and backup channels do not always coincide in bit phase with each other due to the difference in transmission delay between the two independent channels. Moreover, because the difference in transmission delay fluctuates with time, the deviation in bit phase between the data sequences A and B, too, fluctuates with time. Should the regular and backup channels be switched from one to the other while the bit phases are not coincident, a bit error would be caused at the time of switching. To eliminate such a bit error, there are performed a sequence of steps: determining whether or not the data sequences A and B are matched in bit phase with each other, if they are not matched, forcibly shifting the relative bit phase until the data sequences become coincident, and thereafter switching from one channel to the other. This function is implemented with a bit phase difference detecting device as stated above. A prior art device for the detection of a bit phase difference includes a comparator for detecting a bit phase difference between the two data sequences A and B, i.e., whether or not the bit phases of the data sequences A and B are coincident, and a decision circuit responsive to an output of the the comparator for deciding whether or not the data sequences A and B are matched in bit phase. Such a prior art device, however, leaves the following problem unsolved. Namely, when the data sequences A and B are each loaded with only a small amount of data. i. e., In a so-called light load condition, the number of data bits per predetermined number of bits of each of the data sequences A and B, e. g. frame is decreased to cause the decision circuit to malfunction and, therefore, to fail to accurately detect a bit phase difference. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a bit phase difference detecting device capable of detecting a difference in bit phase between two identical data sequences, which are propagated through individual transmission paths, despite any decrease in the number of data bits per predetermined number of bits of each data sequence, even to zero. It is another object of the present invention to provide a generally improved device for detecting a bit phase difference. The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings. In an embodiment to be described, there is provided a device for detecting a difference in bit phase between two identical data sequences A and B each being transmitted over a respective one of two transmission paths, comprising sequence converting means for serial-to-parallel converting, respectively, the data sequences A and B into bit sequences A₁ to A M and B₁ to B M, M being a positive integer, and a number which is equal to a predetermined common measure of a frame period of the data sequences A and B, and M bit-phase detecting means for detecting, respectively, phase differences between the bit sequences A₁ to A M and the bit sequences B₁ to B M which correspond to each other with respect to order, whereby a bit difference between data sequences A and B is determined based on an output of the M bit-phase detecting means. The invention is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a prior art device for detecting a bit phase difference; Fig. 2 is a block diagram showing a device for detecting a bit phase difference embodying the present invention; Fig. 3 is a block diagram showing a specific construction of a serial-to-parallel (S/P) converter as shown in Fig. 2; Fig. 4A is a block diagram showing a specific construction of a sequence replacing circuit as also shown in Fig. 3; Fig. 4B is a table showing a relationship between inputs and outputs of the sequence replacing circuit of Fig. 4A; Figs. 5A and 5B are timing charts representative of the conversion as performed by the S/P converters of Fig. 2; Fig. 6 is a timing chart demonstrating the operation of the S/P converters and that of comparators as shown in Fig. 2; Fig, 7 is a block diagram showing a specific construction of a decision circuit as shown in Fig. 2; and Fig. 8 is a block diagram showing an alternative construction of the decision circuit. DESCRIPTION OF THE PREFERRED EMBODIMENTS To better understand the present invention, a brief reference will be made to a prior art bit phase difference detecting device, shown in Fig. 1. As shown, the prior art device, generally 10, includes a comparator 12 which is implemented with an Exclusive-OR (EX-OR) gate, and a decision circuit 14. While the comparison 12 receives data sequences A and B and produces a comparison signal C, the decision circuit 14 receives the signal C to decide whether the data sequences A and B are coincident with each other in terms of bit phase. The EX-OR 12 produces a (logical) ZERO when the two inputs are coincident and a (logical) ONE when they are not coincident and, in this sense, compares the data sequences bit by bit to produce a result of decision. So long as the bit phases of the data sequences A and B are matched with each other and, yet, the data sequences A and B are tree from transmission bit errors, the output C of the comparator 12 is a string of ZEROs only. However, the signal C sometimes becomes a ONE due to transmission bit errors. Further, when the data sequences A and B are not coincident in bit phase, the signal C usually becomes a ONE with substantial probability. The decision circuit 14 counts the frequency of ZERO and that of ONE, which the signal C assumes, during each predetermined period of time each, thereby deciding whether or not the data sequences A and B are coincident. The result of decision is produced as a decide signal D. However, when the data sequences A and B are in a light load condition as previously stated and, therefore, there is hardly any bit change component, the probability that the comparison signal C becomes a ONE when the bit phases are not coincident is decreased. For example, assuming that each of the data sequences A and B includes a single frame sync bit of ONE in one of its frame which is 2,928 bits long, and that all the other bits are ZEROs (except for transmission bit errors), the probability that the signal C becomes a ONE when the data sequences A and B are different in bit phase is as small as 2/2928 which is nearly equal to 7 x 10⁻⁴. In this instance, as the bit error rate (BER) of the data sequences A and B increases to the order of 7 x 10⁻⁴, it is impossible to determine whether it is the non-coincidence of bit phases or the transmission bit error that made the signal C ONE. The prior art device 10, therefore, is frequently caused to malfunction. Referring to Fig. 2 a bit phase difference detecting device 20 embodying the present invention is shown which in free from the drawback discussed above. In the figure, the device 20 includes two S/P converters 22a and 22b to which are applied, respectively, identical data sequences A and B which are transmitted over independent transmission paths. In response, the S/P converters 22a and 22b produce a bit sequence A₁ to A₄ and a bit sequence B₁ to B₄, respectively. The bit sequences A₁ to A₄ and B₁ to B₄ are applied in pairs to four comparators 24, 26, 28 and 30 which are each implemented with, for example, an EX-OR gate. Comparison signals C₁ to C₄ output by the bit phase comparators 24, 26, 28 and 30, respectively, are fed to four decision circuits 32, 34, 36 and 38, respectively. Further, decide signals D₁ to D₄ output by the decision circuits 32, 34, 36 and 38, respectively, are fed to an OR gate 40 which is adapted to produce a decide signal E, which is the final result of detection. Each of the S/P converters 22a and 22b functions to convert its associated data sequence A or B into a number of bit sequences which is equal to a predetermined common measure M (positive integer) of the frame period of the data sequences A and B. Hence, the S/P converters 22a and 22b each has a (1-M) sequence converting function and may be implemented with a circuit construction which per se is well known in the art. In this particular embodiment, M is assumed to be 4. In detail, the S/P converters 22a and 22b divide, respectively, the data sequences A and B into four with respect to frequency to produce the bit sequences A₁ to A₄ and B₁ to B₄. By this conversion, the bit sequences A₁ to A₄ and B₁ to B₄ are each provided with a time slot which is four times longer than that of the original data sequences A and B. It follows that, assuming that each data sequence A or B includes 2,928 bits in one frame period, each of the bit sequences A₁ to A₄ and B₁ to B₄ includes 737 bits in one frame period, i.e. 2928/4 = 732. The S/P converters 22a and 22b are operated in synchronism with each other such that the bits in the same time slots of the data sequences A and B are located in the same time slots of those of the bit sequences A₁ to A₄ and B₁ to B₄ which correspond to each other with respect to order. Referring to Fig. 3, a specific construction of each of the S/P converters 22a and 22b is shown. Because the S/P converters 22a and 22b are substantially identical in construction, let the construction of Fig. 3 be representative of the S/P converter 22a by way of example. As shown, the S/P converter 22a is made up of four flip-flops (F/Fs) 222, 224, 226 and 228 which are supplied with a data sequence DATA to convert it into four bit sequences, F/Fs 230 and 232 for retiming in response to a quartered clock CLK, and a sequence replacing circuit 234 supplied with outputs of the four F/Fs 222, 224, 226 and 228 and a control signal CONT for replacing the sequences and, thereby, producing the bit sequences A₁ to A₄. As shown in Fig. 4A, the sequence replacing circuit 234 comprises a binary counter 236 supplied with the control pulse signal CONT for converting it into a quaternary number, and a selector 238 to which an output of the binary counter 236 is applied. The inputs and outputs of the selector 238 are related with each other as shown in Fig. 4B. Referring again to Fig. 2, the bit phase comparators 24, 26, 28 and 30 individually comparison on a bit basis those of the bit sequences A₁ to A₄ and B₁ to B₄ which are the same in order, thereby producing the compare signals C₁ to C₄, respectively. If the data sequences A and B are coincident in bit phase, the bit sequences A₁ to A₄ and the bit sequences B₁ to B₄ which are the same in order with the bit sequences A₁ to A₄ coincide with each other even in terms of the bit phase, except for transmission bit errors. In this condition, all the comparison signals C₁ to C₄ appear as a string of ZEROs except for transmission bit errors. On the other hand, if the data sequences A and B are not coincident in bit phase, those o he bit sequences A₁ to A₄ and B₁ to B₄ which are the same in order do not agree with each other, resulting that the comparison signals C₁ to C₄ become ONEs. Receiving the comparison signals C₁ to C₄, respectively, the decision circuits 32, 34, 36 and 38 decide whether or not those of the bit sequences A₁ to A₄ and B₁ to B₄ which share the same orders are coincident. These circuits 32 to 38 individually produces the decide signals D₁ to D₄ each of which is a ZERO if they are coincident and a ONE if they are not. When the data sequences A and B are coincident, all the decide signals D₁ to D₄ become a ZERO so that the final decide signal E output by the OR gate 40 becomes ZERO. When they are not coincident, at least two of the decide signals D₁ to D₄ become a ONE and, hence, the final decide signal E also becomes a ONE. Referring to Figs. 5A and 5B, there are shown timing charts which are representative of the (1 - 4) sequence conversion as performed by the S/P converters 22a and 22b. Specifically, Fig. 5A shows a case in which the data sequences A and B are coincident in bit phase and one frame period is converted into four bit sequences, i. e. M= 4. In Figs. 5A and 5B, frame sync pulses are indicated by hatching. As shown in Fig. 5A, four time slots A11, A12, A13 and A14 which constitute the first frame of the data sequence A are individually extended with respect to time axis to have the same period as the one frame period of the data sequence A. Consequently, the time slots A11, A12, A13 and A14 are converted into four parallel bit sequences A1, A2, A3 and A4, respectively. This is also true with the second frame and onward of the data sequence A and the data sequence B. In Fig. 5B, a case wherein a phase difference corresponding to the frame sync pulse bit is developed between the data sequences A and B is shown by way of example. In this condition, the four parallel data A1 to A4 associated with the data sequence A are exactly the same as those shown in Fig. 5A. However, as regards the data sequence B, B₁ is equal to Bn4, B₂ is equal to B₁₁, B₃ is equal to B12, and B₄ is equal to B13, i. e., the corresponding ones of the sequences which are derived from the data sequences A and B are each deviated by one time slot from each other. Taking account of the above occurrence, assume a condition wherein each of the data sequences A and B has a frame sync bit only (no load state) and different in phase by one bit from each other, as represented by A and B in Fig. 6. In Fig. 6, the frame sync pulse is shown to comprise one bit to facilitate an understanding. The bit sequences produced by converting the data sequences A and B are represented by, respectively, A1 to A4 and B1 and B4 in Fig. 6. Further, EX-ORs produced by the comparators 24, 26, 28 and 30 are respectively represented by C₁, C₂, C₃ and C₄ in Fig. 6. In this case, although the EX-ORs C₃ and C₄ become all-ZERO each indicating bit-phase coincidence, EX-ORs C₁ and C₂ each includes a ONE indicating bit-phase non-coincidence. Because the frame period is an integral multiple of the number of sequence conversions M (= 4), the individual bits of the frames are necessarily converted into particular conversion output sequences on a frame-by-frame basis, in the example shown in Fig. 6, ONE in the outputs C₁ to C₄ of the comparators 24, 26, 28 and 30, respectively, appears only in the outputs C₁ and C₂ for every 732 bits, i. e. 2928/4 = 732. By the above construction, phase differences between four data sequences are detected within one frame period, offering four times greater amount of phase difference information than conventional. Especially, in a no load condition, the frame bit appears in two sequences so that twice greater amount of phase different information is achievable. As described above, in the de vice 20 of this embodiment, a bit phase difference is detected by the comparators on a 732-bit basis even in a no load condition. In contrast, as represented by a data sequence C in Fig. 6, the prior art device 10 discussed with reference to Fig. 1 cannot detect a bit phase difference except on 2928 bits at a time. Generally, when any bit error is developed on a transmission path, an incorrect result appears on the output of a bit phase comparator. In such a condition while the prior art device 10 cannot detect a bit phase difference except for BERs of up to approximately 3 ×10⁻⁴ (nearly equal to 1/2928), the device 20 in accordance with this embodiment is capable of detecting a bit phase difference up to BERs of approximately 1.4 × 10⁻³, enhancing the protection characteristic. Referring to Fig. 7, a specific construction of the bit phase decision circuit which is applicable to the device as shown in Fig. 2 is shown. In the following description, M is assumed to be 16, i. e. Ci (i = 1 to 16) and Di (i = 1 to 16). In Fig. 7 a decision circuit comprises a NOT gate 322 to which a comparison signal Ci is applied, a first NO counter 324 responsive to the comparison signal Ci for producing a pulse P₁, a first YES counter responsive to an output of the NOT gate 322 for producing a pulse P₂, and NAND gates 328 and 330. The NAND gate 328 produces a decide signal Di in response to the pulse P₁ and the output of the NAND gate 330, and the NAND gate 330 are supplied with the pulse P₂ and an output of the NAND gates 328. The first NO counter 324 counts the occurrences that the comparison signal Ci becomes a ONE (on the basis of the time slot of the bit sequences Ai and Bi) every five-frame period (every 183 × 5 bits of the bit sequences Ai and Bi), producing the negative pulse P₁ every time the count reaches 4. On the other hand, the first YES counter 326 counts the occurrences that the output signal of the NOT gate 322 becomes a ONE (i. e. the comparison signal Ci becomes a ZERO) every sixteen bits of the bit sequences Ai and Bi, producing the negative pulse P₂ every time the count reaches 12. The NAND gates 328 and 330 are so interconnected as to constitute an R-S F/F which is set by the pulse P₁ and reset by the pulse P₂, so that the decide signal Di becomes a ONE in response to the pulse P₁ and a ZERO in response to the pulse P₂. If the transmission bit error of the bit sequences Ai and Bi is negligible, the comparison signal Ci becomes all-ZERO so long as the bit sequences Ai and Bi are coincident. In this condition, the pulse P₁ is not produced and, instead, the pulse P₂ is produced to make the decide signal D i ZERO. Conversely, if the bit sequences Ai and Bi are not coincident in bit phase and the comparison signal Ci becomes a ONE at least once within one frame period, the pulse P₁ appears once for very five frame periods resulting that the decide signal Di also becomes a ONE. Considering the fact that the pulse P₂ sometimes appears even if the bit sequences Ai and Bi are not identical, after the pulse P₂ has appeared, the decide signal Di remains to be ZERO till the appearance of the next pulse P₁. Fig. 8 shows another specific construction of the decision circuit. This embodiment is essentially similar to that of Fig. 7 except for the addition of a second NO counter 332 and a second YES counter 334. The second NO counter 332 receives the pulse P₁ and applies a pulse P₃ to the NAND gate 328. The second YES counter 334 receives the pulses P₁ and P₂ and applies a pulse P₄ to the NAND gate 330. The operation of the circuit shown in Fig. 8 will be described on the assumption that the two bit sequences from which the comparison signal Ci is derived have frames each comprising 183 bits and, if the bit sequences do not agree with each other, the comparison signal Ci becomes a ONE at least once in one frame period, as has been the case with the embodiment of Fig. 7. The second NO counter 332 counts the occurrences that the pulse P₁ appears every 5 × 16 frame period and, each time the count reaches 14, produces the negative pulse P₃. The second YES counter 334, on the other hand, counts the arrivals of the pulse P₂ and, when the count reaches 92″, produces the negative pulse P₄ while, upon arrival of the pulse P₁, being cleared (the count being reduced to zero). As stated earlier, the period for which the first NO counter 324 continues counting is five consecutive frames, and the maximum number of pulses P₂ which may appear during such an interval is 5 × 183/16 which is smaller than 58. The threshold count of the second YES counter 334 is selected to be 92 which is greater than 58. Assuming that the error of the comparison signal Ci with respect to probability is negligible, so long as the bit sequences from which the comparison signal Ci is obtained are coincident, the pulse P₁ and, therefore, the pulse P₃ does not appear. At this instant, because the pulse P₂ appears every sixteen bits and the pulse P₄ appears every 16 × 92 bits (about eight frames), the decide signal Di becomes a ZERO and never becomes a ONE. Conversely, when the bit sequences are not non-coincident, the pulse P₁ appears every five frames and, therefore, sixteen times in 5 × 16 frames resulting that the pulse P₃ is produced to turn the decide signal Di to a ONE. At the same time, each pulse P₁ clears the second YES counter 334 and, because the maximum number of pulses P₂ which may appear before the appearance of the next pulse P₁ is less than 58 which is smaller than 92, no pulse P₄ appears. Hence, if the bit sequences are different in bit phase, the decide signal Di remains to be a ONE. As the error rate of the comparison signal Ci increases to the order of 5.5 × 10⁻³, the pulse P₁ does appear even if the bit sequences are coincident. Nevertheless, the probability that the pulse P₃ appears in response to the erroneous appearance of the pulse P₁ is low and, yet, the probability of erroneous appearance of the pulse P₃ sharply decreases with the decrease in the probability of that of the pulse P₁. It follows that even a slight decrease in the error rate of the comparison signal Ci to lower than 5.5 × 10⁻³ serves to reduce the probability of erroneous generation of the pulse P₃ and, therefore, that of erroneous change of the decide signal Di to a ONE to a significant degree. As described above, in accordance with this embodiment, the non-coincidence of two bit sequences can be detected even in a light load condition, i. e., even if the comparison signal Ci becomes a ONE only once in one frame (183 bits). In addition, the probability that coincidence is erroneously decided to be non-coincidence is negligibly low until the error rate of the comparison signal Ci increases up to substantially 5. 5 × 10⁻³. If desired, an arrangement may be made such that upon the appearance of the pulse P₂ the first YES counter 326 is cleared to start on the next counting immediately. This would shorten the period of time necessary for decision. In such a case, too, it is a prerequisite that the maximum number (5 × 183/12 (77) of pulses P₂ which may appear during the counting time (5 × 183 bits) of the first NO counter 324 be smaller than the threshold count (92) of the second YES counter 334. Further, the second NO counter 332 may be cleared by the pulse P₄ to cause the second NO counter 332 to start counting always from "0", thereby enhancing the certainty of the pulse P₃. In summary, it will be seen that the present invention provides a bit error difference detecting device which, even though two data sequences which are transmitted over individual transmission paths may be lightly loaded, is capable of determining a phase difference between those data sequences with accuracy and, yet, improving the phase difference decision characteristic under a degraded path condition. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure, which has been given by way of example, without departing from the scope of the invention as defined in the appended claims.

Claims: A device (20) for detecting a difference in bit phase between two identical data sequences A and B, each being transmitted over a respective one of two transmission paths, characterised by sequence converting means (22a, 22b) for serial-to-parallel converting, respectively, the data sequences A and B into M bit-sequences A₁ to A M and B₁ to B M , M being a positive integer and a number which is equal to a predetermined value such that the frame period of the data sequences A and B is an integral multiple of this number M; and M phase-detecting means (24, 26, 28, 30, 32, 34, 36, 38) for detecting phase differences between each of the M bit-sequences A₁ to A M and the corresponding one of the M bit-sequences B₁ to B M ; whereby a bit difference between the data sequences A and B is determined on the basis of an output of the M bit-phase detecting means (24, 26, 28, 30, 32, 34, 36, 38). A device as claimed in claim 1, wherein the bit-phase detecting means includes M bit-phase comparators (24, 26, 28, 30) for individually comparing in terms of bit phase and bit by bit, the corresponding ones of the bit-sequences A₁ to A M and of the bit-sequences B₁ to B M for producing comparison signals C₁ to C M , and M decision-circuits (32, 34, 36, 38) individually responsive to the comparison signals C₁ to C M , for deciding the coincidence/non-coincidence in the bit phase of corresponding ones of the bit-sequences A₁ to A M and the bit-sequences B₁ to B M so as to produce M decide-signals D₁ to D M . A device as claimed in claim 2, wherein each of the phase comparators comprises an exclusive-OR gate. A device as claimed in claim 2, wherein each of said decision-circuits comprises a first NO counter responsive to one of the comparison signals C₁ to C M for producing a first pulse, a NOT gate responsive to said one of the comparison signals C₁ to C M , a first YES counter responsive to an output signal of the NOT gate for producing a second pulse, and NAND gates responsive to, respectively, the first and second pulses for producing the respective of the decide-signals D₁ to D M . A device as claimed in claim 4, wherein each of said decision-circuits further comprises a second NO counter responsive to the first pulse output by said first NO counter for producing a third pulse, and a second YES counter responsive to the first pulse and the second pulse output by said first YES counter for producing a fourth pulse, the third and fourth pulses being applied to said NAND gates. A device as claimed in claim 2, wherein said bit-phase detecting means further comprise an OR gate for producing a final decide-signal E in response to the M decide-signals D₁ to D m output by said decision-circuits.

Inventors: NAKAZAWA, SATOSHI C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: G01R25
IPC codes: G01R25, H04L1, H03K5