COMPENSATION OF A CLOCK OPERATING ERROR

Abstract: The invention relates to a method for compensating a clock operating error. The method can be applied to the compensation of an operating error found in transmission clocks at the base stations of a long-range paging system of any radio transmitter network. At certain intervals, the transmission clocks of base stations are synchronized either to each other or to a reference clock. During the course of synchronization, there is measured an oscillator operating error or a clock operating deviation, which is created during a period equal to the synchronizing interval. The clock frequency is obtained from the frequency of a high-frequency oscillator by dividing it with a constant. During the synchronizing interval, the oscillator frequency is adjusted by means of a controlled variable, such as voltage, based on the measurement of an oscillator operating error. Optionally, a clock phase is shifted by changing said dividing constant by one lower or higher for the duration of a single cycle of a transmission clock at certain correction intervals.

Description: Compensation of a clock operating error. The present invention relates to a method for compen¬ sating an operating error of a clock, particularly that of transmission clocks used in the base stations of a long-range paging system. A problem in long-range paging systems with a dense network of base stations is that a paging receiver may be located within the audible range of two transmitters. Since transmitters operate on the same frequency, they interfere with each other unless synchronized. Synchronization means that the transmission clocks of base stations send out the same information at exactly the same instant, so-called quasi synchronous trans¬ mission. In long-range paging systems with digital information, this means that the same information sym¬ bol is transmitted from various base stations at exact¬ ly the same time. Transmission is quasi synchronous if the phase difference of symbols transmitted by var¬ ious base stations, upon the arrival thereof in a pag¬ ing receiver, does not exceed 1/4 of the time required by transmission of the symbol. As transmission speed increases, the requirement for synchronization will be stricter since the duration of a symbol becomes shorter. Especially in the novel long-range paging system ac¬ cording to ERMES standard there is a set requirement that no more than _ 5 microseconds difference can be accepted in various base stations at the commencement of symbol modulation. In order to attain a synchronization accuracy of five microseconds, a base station must necessarily be pro¬ vided with a high-precision time reference (atomic clock) , which is synchronized to a certain time reference, or with continous reception of time from a high-precision time reference. A drawback with both of these is a high price. High-precision time references are expen¬ sive and, on the other hand, the attainment of a necess¬ ary degree of precision would require the use of a satellite receiver and those are also expensive. A preferred solution, currently used for synchronization, is to provide a base station with a quartz oscillator as time reference and to synchronize the clocks of various base stations periodically to each other by using a radio path for the transmission of a synchronization signal. A problem here is, however, that during the course of synchronization the transmission of pagings is prevent¬ ed. Synchronization, takes about a minute. The synchro¬ nizing accuracy of _ 5 microseconds would require syn- schronization at least at fi- e-minute intervals due to the insufficient accuracy of a quartz oscillator. How¬ ever, such a frequent synchronization takes up too much radio capacity to be practical. An object of the invention is to solve this problem by compensating a clock operating error in a manner that the synchronizing interval can be substantially length¬ ened. This object is achieved on the basis of the character¬ izing features set forth in the annexed claims. A solution of the invention will now be described in more detail. A closk frequency f2 of a base station is obtained from a frequency f of a high-frequency quartz oscillator, by dividing it by a constant K. quartz oscillator f1 f2 = f1/K divider K f2 transmission clock Thus, the frequency deviation of a--.quartz oscillator causes in the transmission clock a phase shift, which increases continuously over a synchronizing interval and drops back to zero after synchronization. Since the maximum acceptable phase shift is constant, the accuracy of a quartz oscillator defines a necessary synchronizing interval. The frequency deviation of a quartz oscillator is a result of the following factors: environmental con¬ ditions, particularly temperature, ageing, a setting error and short-term instability. The most significant of these factors are environ¬ mental conditions, ageing and setting error. The short- term instability is several times better compared to those. All three significant error factors remain con¬ stant (setting error) or change slowly (environmental factors and ageing) . This renders possible the compen- εation of these factors and a categorical improvement of the characteristics of a quartz oscillator. The compensation is effected in a manner that, during synchronization, the oscillator operating error is mea¬ sured and, during a synchronizing interval, the ocil- lator is adjusted so as to minimize the effect of an operating error on the transmission clock. The compensation can be performed in two ways. One way is to adjust the frequency of an oscillator by some controlled variable, for example voltage. The other way is to shift the phase of a base station clock gra¬ dually over a synchronizing interval. Adjustment of the frequency of an oscillator with a controlled variable produces a retroactive control cir¬ cuit, wherein the controlled variable is the frequency of an oscillator. The real frequency of an oscillator is measured during synchronization and the controlled variable is used to correct the frequency of an oscil¬ lator. The control circuit is slew since synchroniza¬ tion is effected at intervals of about 30 minutes to measure the present frequency rate. The best control algorithm for adjusting the frequency would be purely integrating control. Another method is a phase shift of the base station clock over a synchronizing interval. This can be effect¬ ed by way of a program. The unit of a clock phase shift is 1/f1. The phase shift is effected in including in the divider of formula f1/K for the duration of one cycle of a clock either one too high or too low a dividing constant K - 1 , depending on the direction of a required correction. The correction can be effected as follows: 1. Measuring a clock operating deviation, which is created during a period equal to the synchro¬ nizing interval. 2. Calculating the real deviation by adding to the measured deviation a clock phase shift effected during the synchronizing interval. t1 = measured deviation t2 = correction made during synchronizing interval t3 = real deviation t1 is understood to have a positive sign if the base station clock had been fast re¬ lative to reference time and a negative sign if it had been slow. Correction f2 is posi¬ tive if the base station clock has been made slower and negative if the clock has been made faster over a synchronizing interval. t3 = t1 + t2 3. Calculating a deviation per unit time t4. Ts = synchronizing interval t4 = t3/Ts 4. Calculating a necessary correction period Tk. Tk = 1/ | (t4*f1)| 5. Correcting the transmission clock phase at Tk in¬ tervals by varying a dividing constant K of the divider for the duration of a single period by one to be either lower or higher. If t4 were negative, the constant is subtracted by one, and if positive, the constant is increased by one. Thus, the variation - 1 of dividing constant K pro¬ duces a correction in the phase of a clock equal to phase shift unit 1/f1, and correction period Tk deter¬ mines how often this correction is made during a syn¬ chronizing interval. By way of an example it can be appreciated that the oscillator can have a frequency of 4 MHz and constant K can be in the order of 1000.

Claims: s- AME DED CLAIMS [ receive d b y the International Bureau on 6 January 1992 (06.0192 ) - original claims 2 and 3 cancelled; claim 1 amended; new claim 2 a dd e d * original claim 4 renumbered as claim 3 (2 pages)] 1. A method for compensating a clock operating error, said method comprising the steps of: producing a clock frequency (f2) from a frequency (fl) of a high-frequency oscillator by dividing it with a constant (K) and; at certain intervals synchronizing a clock to an external reference frequency, c h a r a c t e r i z e d in that during the course of clock synchronization, there is measured a clock operating deviation, which is created during a period equal to the synchronizing interval , by comparing the phase of a. clock to that of said reference frequency and, during the synchronizing interval, i.e. the interval between the synchronization steps, the oscillator frequency (fl) is adjusted by means of a controlled variable, such as voltage, based on the measurement of an oscillator operating error, in a manner that the effect of the oscillator operating error on the clock frequency is minimized . s 2. A method for compensating a clock operating error, said method comprising the steps of: producing a clock frequency (f2) from a frequency (fl) of a high-frequency oscillator by dividing it with a constant (K); at certain intervals synchronizing a clock- to an external reference frequency by altering the said dividing constant (K) according to. the need, c h a r a c t e r i z e d in that during the course of clock synchronization, there is measured a clock operating deviation, which is created during a period equal to the synchronizing interval , by comparing the phase of a clock to that of said reference frequency and on the basis of the said phase comparison the clock phase is shifted by varying said dividing constant (K) by one lower or higher (± 1) for the duration of a single cycle of a transmission clock (K/fl) at certain correction intervals (Tk), said correction interval (Tk) being calculated on the basis ©f a measured clock operating deviation created during the synchronizing interval . 3. The application of a method as set forth in claim 1 or 2, c h a r a c t e r i z e d in that the method is applied to effect the compensation of transmission clocks in the base stations of a radio transmitter network, the transmission clocks of base stations being synchronized at certain intervals either to each other or to a common time reference and said compensation of an operating error is effected during the synchronizing interval.

Inventors: HAAPANEN SAKARI, HAAPANEN, SAKARI
Applicants: TECNOMEN OY
Application year: 1991
Main classification: H04L7
IPC codes: H04L7