METHOD AND APPARATUS FOR SYNCHRONIZING RADIO TRANSMITTERS FOR SYNCHRONOUS RADIO TRANSMISSION

Abstract: In transmission by radio of personal paging calls synchronous radio transmission from several transmitters is often used, i.e. the transmitters send on the same frequency. The characters of the call must also be sent simultaneously from the transmitters so that they shall be correctly comprehended by the receivers. A method and an apparatus are described here for synchronizing the separate transmitters (2:2, 3:16) such as to transmit on the same frequency.

Description: METHOD AND APPARATUS FOR SYNCHRONIZING RADIO TRANSMITTERS FOR SYNCHRONOUS RADIO TRANSMISSION TECHNICAL FIELD The present invention relates to a method of synchronizing radio trans¬ mitters for synchronous radio transmission and an apparatus for carry¬ ing out a part of the method, in accordance with the preamble in the following independent claims. BACKGROUND ART For transmitting short messages by radio, particularly messages contai¬ ning personal paging calls, it is usual to use a large number of radio transmitters, each with a limited range, these transmitters being adap¬ ted for synchronous radio transmission, i.e. all of them send the same message with the same frequency. The transmission is modulated with bi- nary frequency modulation (frequency shift keying, FSK), and the trans¬ mitters are further adaptedfor sending the message bits simultaneously. In known installations for sending personal paging calls the method of transmission is normally: transmission on a line of a message from a central station to all radio stations simultaneously, transmission of the message by radio, differences in propagation time on different lines first being compensated, so that the message is transmitted simultaneously from all radio transmitters. An example of a system for nation-wide transmission of personal pag- ing calls is described in "Final Report of the Brittish Post Office Code Standardisation Advisory Group (POCSAG)", London 1978. A method of providing simultaneousness in the transmission of the message with use of time signals sent by broadcasting is also descbribed in EP-A- 0042144. When the same message is sent by radio from several transmitters si¬ multaneously, it is unavoidable that some receivers will receive the transmission from two radio transmitters. If the radio transmitters have exactly the same frequency, their field strengths. may be combined OMPI to an increased field strength and good reception obtained, but in another place approximately a quarter wavelength away, their field strengths can counteract each other so that reception is-made impossib¬ le. The disadvantage of fading field strength in certain places, stan- ding waves, is mitigated by the frequencies of two adjacent transmit¬ ters being given a small offset. Instead of quiet zones, beats will then occur with the frequency difference, which can be of the order of magnitude 500 Hz, while the nominal frequency may be 150 MHz, for example. The beats affect the ability of receiving the separate bi- nary characters in the message, for which reason the bit frequency in the transmission should not exceed the beat frequency. The true carrier frequency of the transmitters may deviate from the selected frequency by 50 Hz at most. The frequency stability require¬ ment is thus high, and it has so far been met by using high-stability transmitters or by transmitting signals on a radio link for synchro¬ nizing the carrier frequency of the transmitters. Both methods result in that the installations.will be expensive. In a receiver which is situated such that the transmission from two transmitters is received in it, the separate characters must arrive simultaneously, or otherwise there will be uncertainty as to when the character begins and ends. It is considered that the uncertain part of a character should not exceed 20 X of the character length, and with a character rate of for example 512 bits/s applicable for the mentioned P0CSAG system the uncertainty may be a maximum of 250 micro- seconds. Radio receivers for the reception of coded personal paging calls are descbribed, inter alia, in the patent specification SE-C-365681. DISCLOSURE OF INVENTION It is an object of the invention to state how the radio transmitters shall be synchronized for sending with a small pre-selected frequency difference. The synchronization takes place in each particular radio OMPI transmitter, and it is carried out progressively, so that it begins in the transmitters closest to the central station and is spead like a wave to stations farther and farther away from the central station, a common time signal transmitter being superfluous. The distinguishing features of the invention are disclosed in the cha¬ racterizing portions of the following calsims. BRIEF DESCRIPTION OF DRAWING An embodiment of a method in accordance with the invention is descri¬ bed in the following, and with reference to the accompanying drawing, whereon Figure 1 illustrates an installation with a central station and a plu¬ rality of subordinate radio stations, Figure 2 illustrates a block diagram for a radio station, Figure 3 illustrates a plurality .of radio stations connected to a line. Figure 4 illustrates a block diagram for a radio receiver and an auxiliary apparatus for synchronization. EMBODIMENT OF INVENTION It will be described below how the invention is applied to an instal¬ lation, selected as an example, for personal . paging with the aid of ra¬ dio signals. In certain respects, the installation is implemented as described in the mentioned POCSAG report, namely such that the carrying frequency of the radio signals is about 150 MHz, the frequency offset beween transmitters is 500 or 1000 Hz, frequency deviation is permitted to be at most 50 Hz, the transmission is modulated with two frequencies having a difference of 9 kHz, and the time difference for characters sent from different trans¬ mitters is allowed to be at most 250 microseconds. The invention may also be applied to installations for which other specifications than the one illustrated here apply. It is typical for installations for sending personal paging calls, and also applicable to the installation used in the embodiment, that a central station 1 is included in it, as illustrated in Figure 1, the transmission of personal ^paging calls in an extensive area being ad- ministered by the station, from which such calls are sent out by radio to paging receivers within the range of the station and on a line to subordinate radio stations 2, which are to send out calls where the central station radio transmission cannot be comprehended. The subordinate stations 2 are disposed such as to send the same call message as the central station 1, and to send it simultaneously as it is sent from the central station and on the same ' radio frequency, or on a frequency with a preselected offset from this frequency. In the installation where thepresent invention shall be applied asubstation 2, which is illustrated in Figure 2, is equipped, inter alia with a data receiver 5 for receiving a message sent on a line 6 from the cen¬ tral station 1.. The message passes a delaying circuit 7 for delaying by a time Tc, before it is fed into a memory 8, which is connected to the delaying circuit, for transmission by the station radio transmitter, this time Tc specially set for each station such that the message will be transmitted simultaneously from all stations. The message is also fed via a first decoder 9 into a control means 10, which is a micro¬ computer. The station is further equipped with an aerial 11, alternately trans¬ mitting and receiving. A radio receiver 12 can be connected to the ae- rial by a switch 13 for reception of the same message as is received in the data receiver 5. The message received in the radio receiver is fed via a second decoder 14 to the mentioned control means 10. The control means 10 is also connected to the delay circuit 7 by a line for transmitting the necessary correction for the delay time Tc. In accordance with the invention, the setting of the different radio stations on frequency is carried out consecutively, starting with the substation closest to the central station, until setting has been carried out in the most remote station. The central station 1 is schematically illustrated in Figure 3, toget¬ her with a plurality of the subordinate stations. All the stations are provided with the described transmitters and receivers. Some of the subordinate stations, which may be called primary stations 2:1 - 2:3, are placed high so that the message can be sent by radio between them over fairly long distances, while other stations, which may be called secondary stations 3:11 - 3:19 only need to have radio communication with an adjacent primary station. The radio connections between the stations are denoted by full lines and the wire connections by dashed lines in Figure 3. The lay of the wire connections is optional, but such that all the subordinate sta¬ tions are connected to the central station 1. Transmission of personal paging calls by radio from the stations is controlled by the message sent on the line from the central station 1. The propagation time on the line is longest to the most remote station 3:19. If the call message is sent by radio from this station as soon as it has arrived on the line, the message may only be sent after a small delay after arrival at the station 2:3, in order that the message from there will be sent simultaneously. Where the installation for transmitting personal paging calls contains a large number of substations 2, these are connected together into se¬ veral rows of stations with several lines, of the kind illustrated in Figure 3. In the installation selected as an embodiment, the subordinate radio stations are all equipped with a previously-mentioned radio receiver 12 of the superheterodyne type, known per se, and here of the double superheterodyne type, i.e. as illustrated in Figure 6, with a first and a second local oscillator 22, 23, a first and second mixer 24, 25 and two intermediate frequencies. OMPI The receiver 21 further contains three bandpass filters 26, 27, 28 for filtering out undesired signal frequencies, a threshold circuit 29 and a demodulator 30, from the output A of which the received signal is fed to the decoder 14 in Figure 2. The second intermediate frequency of the receiver, here about 455 kHz, is taken from the output B after the threshold circuit 29, the signals here having the frequency f -f. n4 - f ιn - Hz, where f is the m LIΠ ιu_; frequency of the received radio signal and f, 01 and f Q2 are the fre¬ quencies of the respective local oscillators. The frequency of the signal at B is to be used for synchronizing the station radio trans¬ mitter to the same frequency as that of the received signal or to a frequency deviating therefrom by a selected amount. A voltage-controlled crystal oscillator VCXO for the transmission fre¬ quency, denoted by 44 in Figure 2, is arranged in the station and is intended for controlling the frequency of the radio transmitter. The oscillator will be most stable and least temperature-dependent when its control crystal is allowed to oscillate with its natural fre¬ quency, which is often lower than the intended transmission frequency. The crystal oscillator 44 is regulated in the installation in question to a frequency f /N, which is the transmission frequency divided by c an integer N, selected within the limits 1 to 9. The output signal of the oscillator, at C in Figure 2, is fed to an auxiliary apparatus for synchronization in Figure 6. The signals of both the local oscillators 22,23 are taken out and fre- quency-divided by said integerNin their individual frequency dividers 34, 35. A-third and a fourth mixer 36, 37 are arranged in the auxil- liary apparatus 31 for mixing the heterodyned frequency of the local osc llators with the frequency of the crystal oscillator. On output D the signal now has the frequency; (f c - _ *LQ2 )N HZ * A lowpass filter 38, 39 is put in after each mixer, and a second thres¬ hold circuit 40 immediately before the output, for filtering the out- put signal D. Only pure frequencies are to be found in the auxiliary apparatus 31, it being sufficient to use lowpass filters here instead of bandpass filters. Comparison of the received frequency with the one generated in the station is arranged such that a number of whole periods of each fre¬ quency are counted in two coacting counters 32, with simultaneous 15 starting. When 2 /N periods have been counted of the signal on out¬ put D, which takes a time of about 72 milliseconds, the count is stop- 15 ped in both counters. If now 2 periods have been counted of the sig- nal on the output B, then f = f ; the frequency f of the received signal is then the same as the frequency f of the internally gene- rated signal. The signal from the crystal oscillator 44 isf.edto the radio transmitter 45 of the station, the signal frequency multiplied by N and the transmitter caused to send with the thus-obtained fre- quency f or with a frequency which has a selected offset from this. A switch 46 is arranged before the transmitter for closing when the transmission is to take place. Should the number of counted periods at the output B be one more than 15 2 , then the frequency f exceeds that intended by 1/0,072 Hz = 15 14 Hz. For each counted period which the number 2 periods is exceed¬ ed or is fallen below, the frequency exceeds or falls below the inten¬ ded one by 14 Hz. The resolution of the frequency measurement is thus sufficient for the frequency error to be kept under 50 Hz, which is the greatest permitted frequency deviation in the example. since the frequencies of the two local oscillators 22, 23 are included in both the frequencies which are compared, their frequencies are without importance in the comparison, no requirement is made of them that they must be accurately stabilized. The error signal fed out from the counters 32, this signal being a mea- sure of the frequency deviation, is fed into the memory 8, Figure 2, where it is converted to a signal for setting the crystal oscillator 44 to the intended frequency. The signal fed from there with the fre¬ quency f C/N Hz has, as mentioned , its frequency multiplied by N to be- come f Hz, which is the sending frequency of the transmitter. Since a one in the message with the synchronization order is transmit¬ ted at one frequency and a zero is transmitted at another frequency, a plurality of ones are inserted in sequency in the message, so that a constant frequency is received for a short time when the synchroniza- tion is carried out. The description of synchronization to the right frequency is also ap¬ plicable to radio stations where the receiver is of the superhetero¬ dyne type, thus with only one local oscillator. The description is also applicable where the receiver is of the homodyne type, i.e. its inter- mediate frequency is zero Hz. It is considered sufficient if the crystal oscillator 44 has a fre¬ quency stability, which is as good as can be achieved with a control crystal in a temperature-controlLed oven or with a temperature-compen¬ sated crystal. In order to keep the frequency drift of the oscillator within permitted limits, the synchronization to correct transmission frequency is repeated with an hourly interval in the installation to which the embodiment has been applied, the length of interval which should be selected in other cases depends on the implementation of the oscillator and the operating conditions. The synchronization only decreases the availability of the transmitter insignificantly, since it is carried out in about 10 seconds. Synchronization to the right transmission frequency is carried out im¬ mediately after a setting for synchronousness in the transmission. Both settings are contained in an order included in the message. This message has the same format as a message transmitted for personal pag¬ ing , but with a somewhat different content so that it is not confused with a personal paging call.

Claims: CLAIMS 1. A ehtod of adjusting, before transmission over a plurality of radio stations for synchronous operation on a radio channel, inten¬ ded for radio transmission of a message in binary coded form, e.g. a personal paging call, received on a line from a central station <1), transmission frequency of a radio transmitter arranged in one of the stations (3:16) into conformity with, or with a selected offset from, the frequency of a calibration transmission received by radio from a second radio station (2:2), characterized by the procedural steps: - sending en the line from the central station ( 1 ) to both stations (2:2,3:16) a message containing an order to adjust the transmission frequency and receiving the message in both stations, - sending the received message by radio from the second station (2:2), - receiving the transmitted radio signal in a receiver in the station (3:16) which is to be set, - comparing in the station (3:16) the frequencies of the received ra¬ dio transmission with the oscillation generated locally by a control¬ lable oscillator (44), and,for lack of conformity adjusting the oscil¬ lator (44) to agreement in frequency. 2. A method as claimed in claim 1, characterized by the procedu¬ ral steps: - counting and comparing the periods of the final intermediate fre¬ quency signal of the superheterodyne-type receiver on one hand, and on the other hand the periods of the frequency (f /N) of the controllable oscillator, which is the intended transmission frequency divided by an integer (N), mixed with the Local receiver oscillator frequency, which is also divided by the integer (N). 3. A method as claimed in claim 1, characterized by adjusting the radio stations consecutively, from the one nearest the central station (1 ) to the one farthest there from. 4. Apparatus, in carrying out the method of claim 1, of adjusting the transmission frequency of a radio station arranged in one of a plurality of stations (3:16) into confirmity with, or with a selected offset from, the frequency of a calibration transmission received by radio from a second radio station (2:2), for comparing the calibration transmission frequency (fm) with the transmitter frequency (f ) of the first station (3:16), the receiver of which is of the doub¬ le superheterodyne-type,and is provided with a first and a second local oscillator (22, 23), and has a first and a second intermediate frequency, characterized by an auxiliary means for synchronizing (31), equipped with two dividing circuits (34, 35 ) for dividing by an integer (N) the frequency σf a signal taken from each of the local oscillators (22, 23) in the receiver (12), and with two mixers (36,37) for mixing the signal fed out from the dividing circuit with a signal having a frequency which is the transmission frequency divided by the same integer (N), and by a counter (32) for counting the number of periods during a selected time of the second intermediate frequency and of the heterodyned transmission frequency, and feeding out the difference in the number of periods obtained during the count. OMFI

Inventors: AOKERBERG DAG E SON, AOKERBERG, DAG, E:SON
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1983
Main classification: H04H3
IPC codes: G08B3, H04B7, H04H20, H04Q7