Abstract:
A synchronization method and apparatus in redundant time-division-multiple-access communication equipment, wherein the synchronization between an active circuit and a standby circuit is carried out using a frequency divided signal from the active transmission frame timing signal instead of using the transmission frame timing signal. At timings other than the output timing of the frequency divided signal, the standby transmission frame counter runs independently. Thus, failure of the active circuit has less effect on the standby circuit and switching between the active circuit and the standby circuit can be carried out smoothly.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a synchronization method and apparatus in redundant time-division-multiple-access (TDMA) communication equipment used for satellite communication and the like. 
     (2) Description of the Prior Art 
     Satellite communication networks and the like make use of the TDMA system. The TDMA system enables TDMA using a single radio frequency, but necessitates synchronization of transmission timing between stations. 
     A redundant transmission (TX) synchronization circuit used in TDMA communication equipment includes an active TX synchronization circuit and a standby TX synchronization circuit of the same construction. If the active circuit fails, the standby circuit takes over the operation. In the prior art, the synchronization between an active TX frame timing signal and a standby TX frame timing signal is frequently checked and corrected. Therefore, there is no problem in switching from the active circuit to the standby circuit when the failure occurs anywhere except in the TX synchronization circuits. If, however, an active TX frame counter fails and the timing of the active TX frame timing signal becomes abnormal, the timing of the standby TX frame timing signal will also become abnormal, thus making the existence of the redundant circuits meaningless. 
     Publications of interest in regard to the above-mentioned prior art are Japanese Unexamined Patent Publications (Kokai) Nos. 55-82550 and 56-89148. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a synchronization method and apparatus in redundant TDMA communication equipment wherein switching from the active TX synchronization circuit to the standby TX synchronization circuit is smoothly carried out even if the active TX frame counter fails. 
     According to one aspect of the present invention, there is provided a synchronization method in redundant TDMA communication equipment having at least an active TX synchronization circuit and a standby TX synchronization circuit, which includes synchronizing a standby TX frame timing signal with a frequency divided signal from an active TX frame timing signal, at timings where the frequency divided signal is supplied. The standby TX frame timing signal is independently run at timings where the frequency divided signal is not supplied. 
     According to another aspect of the present invention, there is provided a synchronization apparatus in redundant TDMA communication equipment having at least an active TX synchronization circuit and a standby TX synchronization circuit, each TX synchronizaion circuit includes a TX frame counter for counting each clock signal and for supplying a TX frame timing signal when the count reaches a predetermined value; a frequency divider for receiving the TX frame timing signal and for generating a frequency divided signal; a TX synchronization correction controller for receiving the TX frame timing signal and synchronization correction signals from a synchronization controller and for delaying the TX frame timing signal in response to the synchronization correction signals; a logic circuit for receiving the TX frame timing signal and the output of the frequency divider of the other TX synchronization circuit, for providing the output of the frequency divider when the output signal of the frequency divider is supplied, and for providing the TX frame timing signal when the output of the frequency divider is not supplied; and a selector for receiving the output of the TX synchronization correction controller, the output of the logic circuit, and signals from a redundant circuit switching controller, for selecting the output of the TX synchronization correction controller when the signal from the redundant circuit switching controller commands the selection of the active side, for selecting the output of the logic circuit when the signal from the redundant circuit switching controller commands the selection of the standby side, and for resetting the TX frame counter by the selected output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a signal diagram explaining burst signals, a frame, and a super-frame in prior art TDMA communication equipment; 
     FIG. 2 is a block circuit diagram of a prior art TX syncronization circuit in redundant TDMA communication equipment; 
     FIG. 3 is a waveform diagram of an active TX frame timing signal TXFTa and a standby TX frame timing signal TXFTb of the circuit of FIG. 2; 
     FIG. 4 is a waveform diagram similar to FIG. 3 when the active TX frame timing signal becomes abnormal; 
     FIG. 5 is a block circuit diagram of a TX synchronization circuit for redundant TDMA communication equipment according to an embodiment of the present invention; 
     FIG. 6 is a waveform diagram of the circuit of FIG. 5; and 
     FIG. 7 is a waveform diagram explaining the relation between TX frame timing signals in a case where the active TX frame counter fails. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing the preferred embodiments of this invention, an explanation will be made with respect to the prior art. 
     The upper portion of FIG. 1 shows the timing location (map) of communication signals from stations in a TDMA system. In FIG. 1, the horizontal axes indicate time (t). Burst signals A, B, C, D and E from the stations are located with the time sharing arrangement shown by the blocks in FIG. 1. The burst signals A to E, for example, form one frame. The lower portion of FIG. 1 shows a super-frame. A super-frame consists of a plurality of the above frames (e.g., 25), i.e., is an assembly of such frames in a larger unit, and transmits, for example, information regarding the burst signal location map. 
     FIG. 2 is a redundant transmission (TX) synchronization circuit in TDMA communication equipment. TX synchronization circuit 1a includes a TX syncronization correction controller 11a, a selector 12a, a TX frame counter 13a, and a TX super-frame counter 14a. Another TX synchronization circuit 1b includes the same elements as the TX synchronization circuit 1a. The elements of the TX synchronization circuit 1b are referenced with the same numbers but with the suffixes &#34;b&#34;. When one circuit fails, the other circuit takes over the operation. Here, to avoid complication, the circuit shown in the upper portion of FIG. 2 is referred to as the &#34;active circuit&#34; and the circuit shown in the lower portion of FIG. 2 as the &#34;standby circuit&#34;. 
     The TX frame counter 13a counts a clock signal CKa, generated by a clock signal generator (not shown) contained in each TX synchronization circuit, and outputs a TX frame timing signal when the count reaches a predetermined value. The TX frame timing signal is supplied to the TX super-frame counter 14a and the TX synchronization correction controller 11a as a frame trigger signal. The output of the TX synchronization correction controller 11a is fed back to the TX frame counter 13a through the selector 12a and resets the count. 
     In the case of the active circuit 1a, the selector 12a is switched by a switching signal Sa from a redundant circuit switching controller (not shown), and the output of the TX synchronization correction controller 11a is switched to the TX frame counter 13a, as mentioned above. The TX synchronization correction controller 11a receives a synchronization correction signal Ca from a TDMA synchronization controller (not shown) and corrects the deviation between a reference burst signal (e.g. block A in FIG. 1) and the TX frame timing signal from the TX frame counter 13a. 
     In the case of the standby circuit 1b, the same operation is effected, except that the state of the selector 12b differs. The selector 12b is switched so that the output of the TX synchronization correction controller 11a is supplied to the TX frame counter 13b. Therefore, while the TX frame counter 13b counts its own clock signal CKb and outputs a TX frame timing signal, when the count reaches a predetermined value, it is reset by the signal from the active circuit 1a. 
     FIG. 3 is a waveform diagram of the timing relationship between an active TX frame timing signal TXFTa and a standby TX frame timing signal TXFTb. In the above-mentioned synchronization method, the phases of the active and the standby TX frame timing signals are made to coincide at every frame timing signal (shown in FIG. 3 by arrows). Accordingly, synchronization coupling between the active TX frame timing signal and the standby TX frame signal is frequently performed. Therefore, there is no problem in switching from the active circuit 1a to the standby circuit 1b when failure occurs at portions other than the TX synchronization circuits. 
     However, as mentioned earlier, when the active TX frame counter 13a fails and the timing of the active TX frame timing signal TXFTa becomes abnormal, the timing of the standby TX frame timing signal TXFTb also becomes abnormal. Namely, as shown in FIG. 4, while the active TX frame signal TXFTa should have the timing shown by the broken line at time T1, if the active circuit fails and the timing of the active TX frame signal TXFTa becomes that shown by the solid line at time T1, the timing of the standby TX frame signal TXFTb also becomes that shown by the solid line. It takes a long time to correct the error when switching from the active circuit 1a to the standby circuit 1b is carried out. 
     FIG. 5 is a block circuit diagram of a TX synchronization circuit in redundant TDMA communication equipment according to an embodiment of the present invention. A TX synchronization circuit 2a and a TX synchronization circuit 2b comprise the same elements. When one circuit fails, the other circuit takes over the operation. Here, for convenience of explanation, the TX synchronization circuit 2a is referred to as the &#34;active circuit&#34; and the TX synchronization circuit 2b is referred to as the &#34;standby circuit&#34;. 
     The active circuit 2a comprises a TX frame counter 13a for receiving and counting a clock signal CKa, a TX super-frame counter 14a for receiving and counting the output of the TX frame counter 13a and for generating a TX super-frame timing signal TXSFTa when the count reaches a predetermined value, and a TX synchronization correction controller 11a for receiving the output of the TX frame counter 13a and a synchronization correction signal Ca from a TMDA synchronization controller (not shown). The active circuit further comprises a selector 12a for receiving the output of the TX synchronization correction controller 11a at one input terminal thereof, a TX frame timing inhibit gate 21a for receiving the output of the TX frame counter 13a, an OR gate 22a for receiving the output of the inhibit gate 21a at one input terminal thereof and for supplying the output to the other input terminal of the selector 12a, and a delay circuit 23a for supplying the output to the other input terminal of the OR gate 22a. 
     The standby circuit 2b includes the same elements as the active circuit 2a. These elements are referenced by the same numbers but having suffixes &#34;b&#34;. The output of the TX super-frame counter 14a is connected to the control terminal of the inhibit gate 21b and the input terminal of the delay circuit 23b, and the output of the TX super-frame counter 14b is connected to the control terminal of the inhibit gate 21a and the input terminal of the delay circuit 23a, as interconnections between the active circuit 2a and the standby circuit 2b. Further, switching signals Sa and Sb from a redundant circuit switching controller (not shown) are connected to control terminals of the selector 12a and 12b, respectively. As a result, at the active circuit 2a,  the output of the TX synchronization correction controller 11a is supplied to the TX frame counter 13a, and, at the standby circuit 2b, the output of the OR gate 22b is supplied to the TX frame counter 13b. 
     The TX frame counter 13a counts the clock signal CKa from a clock signal generator (not shown) included in each TX synchronization circuit and outputs the TX frame timing signal TXFTa when the count reaches a predetermined value. The TX frame timing signal TXFTa acts as a trigger signal for the TX super-frame counter 14a. When the TX super-frame counter 14a counts a predetermined number of trigger signals, it outputs the TX super-frame timing signal TXSFTa. 
     In the case of the active circuit 2a, the TX frame timing signal acts as a trigger signal to reset the TX frame counter 13a through the TX synchronization correction controller 11a and the selector 12a. The TX synchronization correction controller 11a includes a delay circuit having a plurality of stages of flip-flop circuits. The delay time is controlled by the selection of the stages by the synchronization correction signal Ca. As a result, synchronization between the reference burst signal and the TX frame timing signal is accomplished. 
     In the case of the standby circuit 2b, the output of the active TX super-frame counter 14a, namely, the TX super-frame timing signal, is supplied as a trigger signal to the TX frame counter 13b through the delay circuit 23b, the OR gate 22b, and the selector 12b. The delay circuits 23a and 23b have delay times corresponding to the delay times of the TX synchronization correction controllers 11a and 11b. When the active super-frame timing signal TXSFTa is not being supplied, the TX frame counter 13b runs independently by its own TX frame timing signal TSFTb in a &#34;flywheel&#34; operation. Namely, when the active TX super-frame timing signal TXSFTa is not being supplied to the control terminal of the inhibit gate 21b, the inhibit gate 21b is in a conductive state. The inhibit gates 21a and 21b include delay circuits having delay times corresponding to the delay times of the TX synchronization correction controllers 11a and 11b. 
     The relation between the active TX super-frame timing signal TXSFTa and the standby TX frame timing signal TXFTb is shown in FIG. 6. The phase of the standby TX frame timing signal TXFTb coincides with the phase of the active TX super-frame timing signal TXSFTa when the active TX super-frame timing signal TXSFTa is being supplied. When the active TX super-frame timing signal TXSFTa is not being supplied, the TX frame counter 13b counts its own clock signal CKb. 
     In the above-mentioned embodiment, the standby TX frame counter 13b is triggered only for each super-frame. Almost all other times, the standby TX frame counter 13b effects a flywheel operation. This means less frequent coupling with the active circuit 2a compared with the prior art and, consequently, less likelihood of the standby circuit 2b being affected by failure of the active circuit 2a. When failure is detected in the active circuit 2a, synchronization of transmission signals is maintained by switching to the standby circuit 2b. 
     This aspect is shown in FIG. 7. Even if the active TX frame timing signal TXFTa, which should have the timing shown by the broken line, has the timing shown by the solid line at time T2, the standby TX frame timing signal TXFTb continues at the normal timing since it is independently running except when the active TX superframe signal TXSFTa is supplied to the standby TX frame counter 13b. Therefore, a normal synchronized TX frame timing signal can be obtained continuously by switching to the standby circuit 2b at the timing shown by the chain line. 
     In this embodiment, the TX superframe counters are used both for their inherent function and for triggering the standby TX frame counter. It is also possible, however, to replace the TX superframe counters with frequency dividers or to provide separate frequency dividers in addition to the counters to supply a frequency divided signal of one TX frame counter as a trigger signal to the other TX frame counter. The above-mentioned frequency divided signal may be selected so that the timing is appropriate (longer) for the switching from the active circuit to the standby circuit.